THE PRINCIPLES OF
                                BIOLOGY


                                  BY
                            HERBERT SPENCER


                           _IN TWO VOLUMES_
                                VOL. II

                     REVISED AND ENLARGED EDITION
                                 1899

                               NEW YORK
                        D. APPLETON AND COMPANY
                                 1900




                        COPYRIGHT, 1867, 1899,
                      BY D. APPLETON AND COMPANY.




                                PREFACE

            TO THE REVISED AND ENLARGED EDITION OF VOL. II.


To the statements made in the preface to the first volume of this
revised edition, there must here be added a few having special
reference to this second volume.

One of them is that the revision has not been carried out in quite the
same way, but in a way somewhat less complete. When reviewing the first
volume a friendly critic, Prof. Lloyd Morgan, said:--

   “But though the intellectual weight has also been augmented,
   it is an open question whether it would not have been wiser
   to leave intact a treatise, &c... relegating corrections and
   additions to notes and appendices.”

I think that Prof. Morgan is right. Though at the close of the preface
to volume I, I wrote:--“in all sections not marked as new, the
essential ideas set forth are the same as they were in the original
edition of 1864,” yet the reader who has not read this statement,
or does not bear it in mind, will suppose that all or most of the
enunciated conceptions are of recent date, whereas only a small part of
them are. I have therefore decided to follow, in this second volume, a
course somewhat like that suggested by Prof. Morgan--somewhat like, I
say, because in sundry cases the amendments could not be satisfactorily
made by appended notes.

But there has been a further reason for this change of method. An
invalid who is nearly eighty cannot with prudence enter upon work which
will take long to complete. Hence I have thought it better to make the
needful alterations and additions in ways requiring relatively moderate
time and labour.

The additions made to this volume are less numerous and less important
than those made to the first volume. A new chapter ending Part V, on
“The Integration of the Organic World,” serves to round off the general
theory of Evolution in its application to living things. Beyond a new
section (§ 289_a_) and the various foot-notes, serving chiefly the
purpose of elucidation, there are notes of some significance appended
to Chapters I, III, IV, and V, in Part IV, Chapters V and VIII, in Part
V, and Chapters IX, X, and XII in Part VI. Moreover there are three
further appendices, D^2, F, and G, which have, I think, considerable
significance as serving to make clearer some of the views expressed in
the body of the work.

Turning from the additions to the revisions, I have to say that the
aid needed for bringing up to date the contents of this volume, has
been given me by the gentlemen who gave me like aid in revising the
first volume: omitting Prof. Perkin, within whose province none of the
contents of this volume fall. Plant-Morphology and Plant-Physiology
have been overseen by Mr. A. G. Tansley. Criticisms upon parts dealing
with Animal Morphology I owe to Mr. J. T. Cunningham and Prof. E. W.
MacBride. And the statements included under Animal Physiology have been
checked by Mr. W. B. Hardy.

For reasons like those named in the preface to the first volume, I
have not submitted the proofs of this revised second volume to these
gentlemen: a fact which it is needful to name, since one or other of
them might else be held responsible for some error which is not his but
mine. It is the more requisite to say this because while, in respect
of matters of fact, I have, save in one or two cases, accepted their
corrections as not to be questioned, I have not always done this in
respect of matters of inference, but in sundry places have adhered to
my own interpretations.

Perhaps I may be excused for expressing some satisfaction that I
have not been obliged to relinquish the views set forth in 1864–7.
The hypothesis of physiological units--or, as I would now call them,
constitutional units--has been adopted by several zoologists under
modified forms. So far as I am aware, the alleged general law of
organic symmetry has not called forth any manifestations of dissent.
The suggested theory of vertebrate structure appears to have become
current; and from the investigations of the late Prof. Cope, has
received verification. The conclusions drawn in Part VI on “The Laws
of Multiplication,” have not, I believe, been controverted. And though
only some works on botany have given currency to the doctrine set forth
in Appendix C, “On Circulation and the Formation of Wood in Plants,”
yet I have met with no attempt to disprove it. The only views contested
by certain of the gentlemen above named, are those concerning the
origin of the two great phænogamic types of plants, and the origin of
the annulose type of animals. I have not, however,--perhaps because
of natural bias--found myself compelled to surrender these views. My
reasons for adhering to them will be found in notes to the ends of
Chapters III and IV in Part IV, and in Appendix D^2.

On now finally leaving biological studies, it remains only to say that
I am glad I have survived long enough to give this work its finished
form.

    BRIGHTON,
        _October, 1899_.




                          PREFACE TO VOL. II.


The proof sheets of this volume, like those of the last volume, have
been looked through by Dr. Hooker and Prof. Huxley; and I have, as
before, to thank them for their valuable criticisms, and for the
trouble they have taken in checking the numerous statements of fact on
which the arguments proceed. The consciousness that their many duties
render time extremely precious to them, makes me feel how heavy is my
obligation.

Part IV., with which this volume commences, contains numerous figures.
Nearly one half of them are repetitions, mostly altered in scale and
simplified in execution, of figures, or parts of figures, contained
in the works of various Botanists and Zoologists. Among the authors
whom I have laid under contribution, I may name Berkeley, Carpenter,
Cuvier, Green, Harvey, Hooker, Huxley, Milne-Edwards, Ralfs, Smith.
The remaining figures, numbering 150, are from original sketches and
diagrams.

The successive instalments which compose this volume, were issued
to the Subscribers at the following dates:--No. 13 (pp. 1–80) in
January, 1865; No. 14 (pp. 81–160) in June, 1865; No. 15 (pp. 161–240)
in December, 1865; No. 16 (pp. 241–320) in June, 1866; No. 17 (pp.
321–400) in November, 1866; and No. 18 (pp. 401–566) in March, 1867.

   LONDON, _March 23rd, 1867_.




                         CONTENTS OF VOL. II.


                 PART IV.--MORPHOLOGICAL DEVELOPMENT.

    CHAP.                                                          PAGE

       I.--THE PROBLEMS OF MORPHOLOGY                                 3

      II.--THE MORPHOLOGICAL COMPOSITION OF PLANTS                   17

     III.--THE MORPHOLOGICAL COMPOSITION OF PLANTS--_Continued_      37

      IV.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS                  85

       V.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS--_Continued_    111

      VI.--MORPHOLOGICAL DIFFERENTIATION IN PLANTS                  128

     VII.--THE GENERAL SHAPES OF PLANTS                             134

    VIII.--THE SHAPES OF BRANCHES                                   145

      IX.--THE SHAPES OF LEAVES                                     152

       X.--THE SHAPES OF FLOWERS                                    161

      XI.--THE SHAPES OF VEGETAL CELLS                              175

     XII.--CHANGES OF SHAPE OTHERWISE CAUSED                        178

    XIII.--MORPHOLOGICAL DIFFERENTIATION IN ANIMALS                 183

     XIV.--THE GENERAL SHAPES OF ANIMALS                            186

      XV.--THE SHAPES OF VERTEBRATE SKELETONS                       209

     XVI.--THE SHAPES OF ANIMAL CELLS                               228

    XVII.--SUMMARY OF MORPHOLOGICAL DEVELOPMENT                     231


                  PART V.--PHYSIOLOGICAL DEVELOPMENT.

       I.--THE PROBLEMS OF PHYSIOLOGY                               239

      II.--DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES
             OF PLANTS                                              244

     III.--DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS       251

      IV.--DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS       272

       V.--PHYSIOLOGICAL INTEGRATION IN PLANTS                      292

      VI.--DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES
             OF ANIMALS                                             299

     VII.--DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS      309

    VIII.--DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS      323

      IX.--PHYSIOLOGICAL INTEGRATION IN ANIMALS                     373

       X.--SUMMARY OF PHYSIOLOGICAL DEVELOPMENT                     384

     X^A.--THE INTEGRATION OF THE ORGANIC WORLD                     396


                   PART VI.--LAWS OF MULTIPLICATION.

       I.--THE FACTORS                                              411

      II.--_À PRIORI_ PRINCIPLE                                     417

     III.--OBVERSE _À PRIORI_ PRINCIPLE                             424

      IV.--DIFFICULTIES OF INDUCTIVE VERIFICATION                   432

       V.--ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS            439

      VI.--ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS             448

     VII.--THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS,
             ASEXUAL AND SEXUAL                                     461

    VIII.--ANTAGONISM BETWEEN EXPENDITURE AND GENESIS               467

      IX.--COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS           475

       X.--SPECIALITIES OF THESE RELATIONS                          486

      XI.--INTERPRETATION AND QUALIFICATION                         497

     XII.--MULTIPLICATION OF THE HUMAN RACE                         506

    XIII.--HUMAN POPULATION IN THE FUTURE                           522


                              APPENDICES.

    A.--SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS           541

    B.--A CRITICISM ON PROF. OWEN’S THEORY OF THE VERTEBRATE
          SKELETON                                                  548

    C.--ON CIRCULATION AND THE FORMATION OF WOOD IN PLANTS          567

    D.--ON THE ORIGIN OF THE VERTEBRATE TYPE                        599

  D^2.--THE ANNULOSE TYPE                                           602

    E.--THE SHAPES AND ARRANGEMENTS OF FLOWERS                      608

    F.--PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS                     612

    G.--THE INHERITANCE OF FUNCTIONALLY-CAUSED MODIFICATIONS        618




                               PART IV.

                      MORPHOLOGICAL DEVELOPMENT.




                              CHAPTER I.

                      THE PROBLEMS OF MORPHOLOGY.


§ 175. The division of Morphology from Physiology, is one which may
be tolerably-well preserved so long as we do not carry our inquiries
beyond the empirical generalizations of their respective phenomena; but
it is one which becomes in great measure nominal, when the phenomena
are to be rationally interpreted. It would be possible, after analyzing
our Solar System, to set down certain general truths respecting the
sizes and distances of its primary and secondary members, omitting
all mention of their motions; and it would be possible to set down
certain other general truths respecting their motions, without
specifying their dimensions or positions, further than as greater
or less, nearer or more remote. But on seeking to account for these
general truths, arrived at by induction, we find ourselves obliged to
consider simultaneously the relative sizes and places of the masses,
and the relative amounts and directions of their motions. Similarly
with organisms. Though we may frame sundry comprehensive propositions
respecting the arrangements of their organs, considered as so many
inert parts; and though we may establish several wide conclusions
respecting the separate and combined actions of their organs, without
knowing anything definite respecting the forms and positions of these
organs; yet we cannot reach such a rationale of the facts as the
hypothesis of Evolution aims at, without contemplating structures and
functions in their mutual relations. Everywhere structures in great
measure determine functions; and everywhere functions are incessantly
modifying structures. In Nature the two are inseparable co-operators;
and Science can give no true interpretation of Nature without keeping
their co-operation constantly in view. An account of organic evolution,
in its more special aspects, must be essentially an account of the
interactions of structures and functions, as perpetually altered by
changes of conditions.

Hence, when treating apart Morphological Development and Physiological
Development, all we can do is to direct our attention mainly to the
one or to the other, as the case may be. In dealing with the facts of
structure, we must consider the facts of function only in such general
way as is needful to explain the facts of structure; and conversely
when dealing with the facts of function.


§ 176. The problems of Morphology fall into two distinct classes,
answering respectively to the two leading aspects of Evolution. In
things which evolve there go on two processes--increase of mass
and increase of structure. Increase of mass is primary, and in
simple evolution takes place almost alone. Increase of structure is
secondary, accompanying or following increase of mass with more or
less regularity, wherever evolution rises above that form which small
inorganic bodies, such as crystals, present to us. As the fundamental
antagonism between Dissolution and Evolution consists in this, that
while the one is an integration of motion and disintegration of matter,
the other is an integration of matter and disintegration of motion; and
as this integration of matter accompanying disintegration of motion,
is a necessary antecedent to the differentiation of the matter so
integrated; it follows that questions concerning the mode in which the
parts are united into a whole, must be dealt with before questions
concerning the mode in which these parts become modified.[1]

This is not obviously a morphological question. But an illustration or
two will make it manifest that fundamental differences may be produced
between aggregates by differences in the degrees of composition of the
increments: the ultimate units of the increments being the same. Thus
an accumulation of things of a given kind may be made by adding one
at a time. Or the things may be tied up into bundles of ten, and the
tens placed together. Or the tens may be united into hundreds, and a
pile of hundreds formed. Such unlikenesses in the structures of masses
are habitually seen in our mercantile transactions. Articles which
the consumer recognizes as single, the retailer keeps wrapped up in
dozens, the wholesaler sends in gross, and the manufacturer supplies
in packages of a hundred gross. That is, they severally increase
their stocks by units of simple, of compound, and of doubly-compound
kinds. Similarly result those differences of morphological composition
which we have first to consider. An organism consists of units.
These units may be aggregated into a mass by the addition of unit
to unit. Or they may be united into groups, and the groups joined
together. Or these groups of groups may be so combined as to form a
doubly-compound aggregate. Hence there arises respecting each organic
form the question--is its composition of the first, second, third,
or fourth order?--does it exhibit units of a singly-compounded kind
only, or are these consolidated into units of a doubly-compounded
kind, or a triply-compounded kind? And if it displays double or triple
composition, the homologies of its different parts become problems.
Under the disguises induced by the consolidation of primary, secondary,
and tertiary units, it has to be ascertained which answer to which, in
their degrees of composition.

Such questions are more intricate than they at first appear; since,
besides the obscurities caused by progressive integration, and those
due to accompanying modifications of form, further obscurities
result from the variable growths of units of the different orders.
Just as an army may be augmented by recruiting each company, without
increasing the number of companies; or may be augmented by making up
the full complement of companies in each regiment, while the number
of regiments remains the same; or may be augmented by putting more
regiments into each division, other things being unchanged; or may be
augmented by adding to the number of its divisions without altering
the components of each division; or may be augmented by two or three
of these processes at once; so, in organisms, increase of mass may
result from additions of units of the first order, or those of the
second order, or those of still higher orders; or it may be due to
simultaneous additions to units of several orders. And this last mode
of integration being the general mode, puts difficulties in the way of
analysis. Just as the structure of an army would be made less easy to
understand if companies often outgrew regiments, or regiments became
larger than brigades; so these questions of morphological composition
are complicated by the indeterminate sizes of the units of each
kind: relatively-simple units frequently becoming more bulky than
relatively-compound units.


§ 177. The morphological problems of the second class are those having
for their subject-matter the changes of shape which accompany changes
of aggregation. The most general questions respecting the structure
of an organism, having been answered when it is ascertained of what
units it is composed as a whole, and in its several parts; there come
the more special questions concerning its form--form in the ordinary
sense. After the contrasts caused by variations in the process of
integration, we have to consider the contrasts caused by variations
in the process of differentiation. To speak specifically--the shape
of the organism as a whole, irrespective of its composition, has to
be accounted for. Reasons have to be found for the unlikeness between
its general outlines and the general outlines of allied organisms. And
there have to be answered kindred inquiries respecting the proportions
of its component parts:--Why, among such of these as are homologous
with one another, have there arisen the differences that exist? And how
have there been produced the contrasts between them and the homologous
parts of organisms of the same type?

Very numerous are the heterogeneities of form presenting themselves
for interpretation under these heads. The ultimate morphological
units combined in any group, may be differentiated individually, or
collectively, or both: each of them may undergo changes of shape;
or some of them may be changed and others not; or the group may be
rendered multiform by the greater growth of some of its units than of
others. Similarly with the compound units arising by union of these
simple units. Aggregates of the second order may be made relatively
complex in form, by inequalities in the rates of multiplication of
their component units in diverse directions; and among a number of such
aggregates, numerous unlikenesses may be constituted by differences in
their degrees of growth, and by differences in their modes of growth.
Manifestly, at each higher stage of composition the possible sources of
divergence are multiplied still further.

That facts of this order can be accounted for in detail is not to
be expected--the data are wanting. All that we may hope to do is to
ascertain their general laws. How this is to be attempted we will now
consider.


§ 178. The task before us is to trace throughout these phenomena
the process of evolution; and to show how, as displayed in them, it
conforms to those first principles which evolution in general conforms
to. Two sets of factors have to be taken into account. Let us look at
them.

The factors of the first class are those which tend directly to
change an organic aggregate, in common with every other aggregate,
from that more simple form which is not in equilibrium with incident
forces, to that more complex form which is in equilibrium with them.
We have to mark how, in correspondence with the universal law that
the uniform lapses into the multiform, and the less multiform into
the more multiform, the parts of each organism are ever becoming
further differentiated; and we have to trace the varying relations
to incident forces by which further differentiations are entailed.
We have to observe, too, how each primary modification of structure,
induced by an altered distribution of forces, becomes a parent of
secondary modifications--how, through the necessary multiplication of
effects, change of form in one part brings about changes of form in
other parts. And then we have also to note the metamorphoses constantly
being induced by the process of segregation--by the gradual union
of like parts exposed to like forces, and the gradual separation of
like parts exposed to unlike forces. The factors of the second class
which we have to keep in view throughout our interpretations, are
the formative tendencies of organisms themselves--the proclivities
inherited by them from antecedent organisms, and which past processes
of evolution have bequeathed. We have seen it to be inferable from
various orders of facts (§§ 65, 84, 97–97_g_), that organisms are
built up of certain highly-complex molecules, which we distinguished as
physiological units [or constitutional units as they might otherwise
be called]--each kind of organism being built up of units peculiar to
itself. We recognized in these units, powers of arranging themselves
into the forms of the organisms to which they belong; analogous
to the powers which the molecules of inorganic substances have of
aggregating into specific crystalline forms. We have consequently
to regard this proclivity of the physiological units, as producing,
during the development of any organism, a combination of internal
forces that expend themselves in working out a structure in equilibrium
with the forces to which ancestral organisms were exposed; but not in
equilibrium with the forces to which the existing organism is exposed,
if the environment has been changed. Hence the problem in all cases is,
to ascertain the resultant of internal organizing forces, tending to
reproduce the ancestral form, and external modifying forces, tending
to cause deviations from that form. Moreover, we have to take into
account, not only the characters of immediately-preceding ancestors,
but also those of their ancestors, and ancestors of all degrees of
remoteness. Setting out with rudimentary types, we have to consider
how, in each successive stage of evolution, the structures acquired
during previous stages have been obscured by further integrations
and further differentiations; or, conversely, how the lineaments of
primitive organisms have all along continued to manifest themselves
under the superposed modifications.


§179. Two ways of carrying on the inquiry suggest themselves. We may
go through the several great groups of organisms, with the view of
reaching, by comparison of parts, certain general truths respecting
the homologies, the forms, and the relations of their parts; and then,
having dealt with the phenomena inductively, may retrace our steps with
the view of deductively interpreting the general truths reached. Or,
instead of thus separating the two investigations, we may carry them on
hand in hand--first establishing each general truth empirically, and
then proceeding to the rationale of it. This last method will, I think,
conduce to both brevity and clearness. Let us now thus deal with the
first class of morphological problems.


[NOTE.--In preparation for treating of morphological development,
sundry other general considerations should have been included in the
foregoing chapter when originally published. This seems the most
appropriate place for now naming them. Some were implicitly contained
in the first volume, but it will be well definitely to state these, as
well as the others not yet implied.

Interpretation of the forms of organisms and the forms of their parts,
must depend mainly on the conclusions previously drawn respecting
their phylogeny; and the drawing of such conclusions must be guided
by recognition of the various factors of Evolution, as well as by
recognition of certain extremely general results of Evolution and
certain concomitants of Evolution.

A primary one among these is that no existing species can exhibit
more than approximately the ancestral structure of any other existing
species. As all ancestors have disappeared, so, in a greater or less
degree, the traits, specific, generic, or ordinal, which distinguished
the earlier of them have disappeared. Setting out with the familiar
symbol, a tree, let us regard its peripheral twigs as representing
extant species; let us assume that the interior of the tree is filled
up with some supporting substance, leaving only the ends of the
living twigs projecting; and let us suppose the trunk, main branches,
secondary branches, tertiary branches, &c., have decayed away.
Then if we take these decayed parts to stand for the divergent and
re-divergent lines of evolution which are represented by fossils in the
Earth’s crust, it will be manifest, first, that no one of the living
superficial twigs (or species) exhibits the ancestral organization
whence any other of the living superficial twigs (or species) has been
developed; it will be manifest, second, that the generic structure
inherited by any existing species must be a structure out of which
came sundry allied species--the fork, as it were, at which adjacent
twigs diverged; and third, that the ancestor of an order must, in like
manner, be sought at some point deeper down in the symbolic tree--a
place of divergence of the sub-branches representing allied genera.
Similarly with the ancestral types of classes, still deeper down in
the tree or further back in time. So that phylogeny becomes more and
more speculative as its questions become more and more radical. And
the difficulty is made greater by the deficiency of palæontological
evidence.

One obvious corollary is that an ancestral type from which sundry
allied types now existing diverged, was, speaking generally, simpler
than these; since the divergent types became different by the
superposing of modifications, adding to their complexities. There is
a further reason for inferring that the least specialized member of
any group is more like the remote ancestor than any of the others; for
every adaptation stands in the way of subsequent re-adaptations: it
presents a greater amount of structure to be undone. To get some idea
of the ancestral type where no extant member of the group is manifestly
simpler than the rest, the method must be to take all its extant
members and, after letting their differences mutually cancel, observe
what remains common to them all.

But there are difficulties standing in the way of phylogeny, and
consequently of morphology, much greater than these. Returning to
our symbolic tree, it is clear that it would be far from easy to say
of any one twig which extinct sub-branch, branch, and main branch it
belonged to, even supposing that the growths of all parts had been
uniformly outwards. Immensely more perplexing, then, must be the
affiliation if various of the branches, sub-branches, &c., have sent
out backward-growing shoots which have come to the surface only after
prolonged retrograde courses, and if other branches have sent shoots
into regions occupied by alien branches--shoots bearing twigs which
come to the surface along with those to which they are but remotely
allied. The problems of origin and of structure which organisms
present, are met by both of the difficulties thus symbolized.

One of them arises from the prevalence of retrograde metamorphoses.
Throughout the animal world these are variously displayed by parasites,
multitudinous in their kinds; for most of them belong to types
much higher in organization. Changed habits and consequent changed
structures have so transferred them that only by study of their
embryonic stages can their kinships be made out. And these retrograde
metamorphoses, conspicuous among parasites, have, in the course of
evolution, affected some members of all groups; for in all groups the
struggle for existence has compelled some to adopt careers less trying
but less profitable.

Not only by forcing on many kinds of organisms simpler ways of living,
and consequent degeneracy, has the universal competition caused
obscuring transformations. It has done this also by tempting many other
kinds of organisms to adopt ways of life not simpler than before but
merely different. Pressure continually prompts every type to intrude on
other types’ spheres of activity; and so causes it to assume certain
structural characters of the types whose spheres it invades, masking
its previous characters. Modifications hence arising have, in the great
mass of cases, been superposed one on another time after time. The
aquatic animal becomes through several transitions a land-animal, and
then the land-animal through other transitions becomes now an aërial
animal like the bat and now an aquatic animal like the whale. Certain
kinds of birds furnish extreme illustrations. There was the change from
the fish to the water-breathing amphibian and then to the air-breathing
amphibian; thence to the reptile living on the Earth’s surface; thence
to the flying reptile and the bird; then came the diving birds, joining
with their aërial life a life passed partly in the water; and finally
came a type like the penguin, in which the power of flight has been
lost and the water has again become the almost exclusive medium, except
for breathing. Of course the mouldings and re-mouldings of structure
resulting from these successive unlike modes of life, in many cases put
great difficulties in the way of ascertaining which are the original
corresponding parts. Some parts have become abnormally large; others
have dwindled or disappeared; and the relative positions of parts
have often been greatly changed. A bat’s wing and a bird’s wing are
analogous organs, but their frameworks are but partially homologous.
While in the bird the terminal parts of the fore-limb do little towards
supporting the wing, in the bat the wing is mainly supported by
enormously-developed terminal parts.

The effects of the struggle to survive, which here prompts a simpler
life with resulting degeneracy and there a different life with
resulting new developments, are far from being the only causes of
morphological obscurations. Fulfilment of certain highly general
requirements gives certain common traits to plants of widely divergent
classes; and fulfilment of certain other highly general requirements
gives certain common traits to animals of widely divergent classes.
It was remarked in the first volume (§ 54_f_) that the cardinal
distinction between the characters of plants and animals arises from
the fact that while the chief food of plants is universally present the
food of animals is scattered. Here it has to be added that to utilize
the universally distributed food the ordinary plant needs the aid of
light, and has to acquire structures enabling it to get that aid;
while the ordinary animal, to utilize the scattered food, must acquire
the structures needful for locomotion. Let us contemplate separately
the traits hence resulting in the vegetal world and the traits hence
resulting in the animal world.

The familiar plantain meets the requirement by growing stiff leaves
enabling it to press down the competing grasses around which would
else shade it; but the great majority of ordinary plants meet the
requirement by raising themselves into the air. Hence the need for a
stem, and hence the fact that plants of widely unlike natures similarly
form stems which, in achieving strength enough to support the foliage
and resist the wind, acquire certain adaptive structures having a
general similarity. Here from the edge of a pool is a reed, and here
from the adjacent copse is a hemlock: the one having grown tall in
escaping the shade of its companions and the other in escaping the
shade of the surrounding brushwood. On being cut across each discloses
a tube, and each exhibits septa dividing this tube into chambers. In
either case by the tubular structure is gained the greatest strength
with the least material; but there is no morphological kinship between
the tubes nor between the septa. Still more marked is the simulation
of homology by analogy in another plant which the adjacent ditch may
furnish--the common Horsetail. In this, again, we see an elongated
vertical-growing part, raising the foliage into the air; and, as
before, this is tubular and divided by septa. A type utterly alien from
the other two has, by survival of the fittest, been similarly moulded
to meet mechanical needs.

Passing now to the obscurations in the animal world caused by
alterations favouring locomotion, we note first that the locomotive
power is at the outset very slight. Among many orders of _Protozoa_,
as also among many low types of _Metazoa_, vibratile cilia are the
most general agents of locomotion--necessarily feeble locomotion.
Regarded in the mass, the _Cœlenterata_, when not stationary like
the _Hydra_ or higher types in the hydroid stage, usually possess
only such small self-mobility as the slow rhythmical contractions of
their umbrella-disks effect, or else such as is effected by bands of
cilia or of vibratile plates, as in the _Beroe_. Even among these low
tpes of _Metazoa_, however, in which ordinarily the radial structure
is conspicuous, or but slightly obscured by an ovoid form as in the
_Ctenophora_, we find, in the _Cestus veneris_, extreme obscuration
caused by an elongation which facilitates movement through the water;
alike by the actions of its vibratile plates and by its undulations,
which simulate those of sundry higher animals.

And here we come upon the essential fact to be recognized. Elongation
favours locomotion in various ways that are severally taken advantage
of by different types of creatures. (1) To a given mass of moving
matter the resistance of the medium decreases along with decrease
in the area of its transverse section, and this implies increase of
length: a given force will move the lengthened mass along with greater
facility. (2) Reaching a certain point the elongated form enables an
animal to progress by undulations, as in the water fish do, and even
some cœlenterates and turbellarians do, and as on land snakes do:
lateral resistances serving in either case as fulcra. (3) Lengthening
of the body serves otherwise to aid locomotion in the creeping or
burrowing worm, which, utilizing the statical resistance of its hinder
part thrusts onwards its fore part, and then, holding fast its fore
part by the aid of minute _setæ_, draws the hinder part after it. But
elongation, doubly advantageous at first, while the body is itself the
chief instrument of locomotion, gradually loses its advantageousness
as special instruments of locomotion are developed. (4) This we
see in that locomotive action effected by limbs, which, many and
small in the lower _Arthropoda_ and becoming few and larger in the
higher, at length give great activity to a shortened and consolidated
body: a stage reached only through stages of decreasing elongation
accompanying increase of limb-power. (5) In the _Vertebrata_ locomotion
by undulations comes, along certain lines of evolution, to be replaced
by that limb locomotion which accompanies the rise from water-life
to land-life: the evolution of Amphibians exhibiting the transition.
(6) Further, we see among mammals that as limbs become efficient the
elongated body ceases to be itself instrumental in locomotion, but that
still some elongation remains a characteristic. (7) Finally, where
limb locomotion reaches its highest degree, as in birds, elongation
disappears.

These classes of familiar facts I have recalled to show that, in
the course of evolution, achievement by plants of the all-essential
elevation into the air and by animals of the all-essential power of
movement have developed this trait of elongation in various types;
and that in each kingdom acquisition of the common trait has had a
tendency now to obscure morphological equivalence, and now to give the
appearance of kinship where there is none. A further purpose has been
to prepare the way for a question hereafter to be discussed--whether,
in the various types of either kingdom, the elongation is effected in
the same ways or in different ways. We shall have to ask whether the
vertically-growing part is always, like that of _Lessonia_, a simple
individual, or whether, as possibly in Phænogams, it is a united series
of individuals; and similarly whether the elongated body is always
single, like that of a mollusc, or whether, as possibly in annulose
animals, it is a series of united individuals.]




                              CHAPTER II.

               THE MORPHOLOGICAL COMPOSITION OF PLANTS.


§ 180. Evolution implies insensible modifications and gradual
transitions, which render definition difficult--which make it
impossible to separate absolutely the phases of organization from
one another. And this indefiniteness of distinction, to be expected
_à priori_, we are compelled to recognize _à posteriori_, the moment
we begin to group morphological phenomena into general propositions.
Thus, on inquiring what is the morphological unit, whether of plants
or of animals, we find that the facts refuse to be included in any
rigid formula. The doctrine that all organisms are built up of cells,
or that cells are the elements out of which every tissue is developed,
is but approximately true. There are living forms of which cellular
structure cannot be asserted; and in living forms that are for the
most part cellular, there are nevertheless certain portions which
are not produced by the metamorphosis of cells. Supposing that clay
were the only material available for building, the proposition that
all houses are built of bricks, would bear about the same relation to
the truth, as does the proposition that all organisms are composed
of cells. This generalization respecting houses would be open to
two criticisms:--first, that certain houses of a primitive kind are
formed, not of bricks, but out of unmoulded clay; and second, that
though other houses consist mainly of bricks, yet their chimney-pots,
drain-pipes, and ridge-tiles, do not result from combination or
metamorphosis of bricks, but are made directly out of the original
clay. And of like natures are the criticisms which must be passed
on the generalization, that cells are the morphological units of
organisms. To continue the simile, the truth turns out to be, that the
primitive clay or protoplasm out of which organisms are built, may be
moulded either directly, or with various degrees of indirectness, into
organic structures. The physiological units which we are obliged to
assume as the components of this protoplasm, must, as we have seen, be
the possessors of those proclivities which result in the structural
arrangements of the organism. The assumption of such structural
arrangements may go on, and in many cases does go on, by the shortest
route; without the passage through what we call metamorphoses. But
where such structural arrangements are reached by a circuitous route,
the first stage is the formation of these small aggregates which, under
the name of cells, are currently regarded as morphological units.

The rationale of these truths appears to be furnished by the hypothesis
of evolution. We set out with molecules some degrees higher in
complexity than those molecules of nitrogenous colloidal substance
into which organic matter is resolvable; and we regard these very much
more complex molecules as having the implied greater instability,
greater sensitiveness to surrounding influences, and consequent
greater mobility of form. Such being the primitive physiological
units, organic evolution must begin with the formation of a minute
aggregate of them--an aggregate showing vitality by a higher degree
of that readiness to change its form of aggregation which colloidal
matter in general displays; and by its ability to unite the nitrogenous
molecules it meets with, into complex molecules like those of which
it is composed. Obviously, the earliest forms must have been minute;
since, in the absence of any but diffused organic matter, no form
but a minute one could find nutriment. Obviously, too, it must have
been structureless; since, as differentiations are producible only
by the unlike actions of incident forces, there could have been no
differentiations before such forces had had time to work. Hence,
distinctions of parts like those required to constitute a cell were
necessarily absent at first. And we need not therefore be surprised
to find, as we do find, specks of protoplasm manifesting life, and
yet showing no signs of organization. A further stage of evolution
is reached when the imperfectly integrated molecules forming one of
these minute aggregates, become more coherent; at the same time as
they pass into a state of heterogeneity, gradually increasing in its
definiteness. That is to say, we may look for the assumption by them,
of some distinctions of parts, such as we find in cells and in what
are called unicellular organisms. They cannot retain their primordial
uniformity; and while in a few cases they may depart from it but
slightly, they will, in the great majority of cases, acquire a decided
multiformity: there will result the comparatively integrated and
comparatively differentiated _Protophyta_ and _Protozoa_.
The production of minute aggregates of physiological units being
the first step, and the passage of such minute aggregates into more
consolidated and more complex forms being the second step, it must
naturally happen that all higher organic types, subsequently arising
by further integrations and differentiations, will everywhere bear the
impress of this earliest phase of evolution. From the law of heredity,
considered as extending to the entire succession of living things
during the Earth’s past history, it follows that since the formation
of these small, simple organisms must have preceded the formation
of larger and more complex organisms, the larger and more complex
organisms must inherit their essential characters. We may anticipate
that the multiplication and combination of these minute aggregates or
cells, will be conspicuous in the early developmental stages of plants
and animals; and that throughout all subsequent stages, cell-production
and cell-differentiation will be dominant characteristics. The
physiological units peculiar to each higher species will, speaking
generally, pass through this form of aggregation on their way towards
the final arrangement they are to assume; because those primordial
physiological units from which they are remotely descended, aggregated
into this form. And yet, just as in other cases we found reasons for
inferring (§ 131) that the traits of ancestral organization may,
under certain conditions, be partially or wholly obliterated, and the
ultimate structure assumed without passing through them; so, here,
it is to be inferred that the process of cell-formation may, in some
cases, be passed over. Thus the hypothesis of evolution prepares us for
those two radical modifications of the cell-doctrine which the facts
oblige us to make. It leads us to expect that as structureless portions
of protoplasm must have preceded cells in the process of general
evolution; so, in the special evolution of each higher organism, there
will be an habitual production of cells out of structureless blastema.
And it leads us to expect that though, generally, the physiological
units composing a structureless blastema, will display their inherited
proclivities by cell-development and metamorphosis; there will
nevertheless occur cases in which the tissue to be formed, is formed by
direct transformation of the blastema.[2]

Interpreting the facts in this manner, we may recognize that large
amount of truth which the cell-doctrine contains, without committing
ourselves to the errors involved by a sweeping assertion of it. We
are enabled to understand how it happens that organic structures are
usually cellular in their composition, at the same time that they are
not universally so. We are shown that while we may properly continue to
regard the cell as the morphological unit, we must constantly bear in
mind that it is such only in a qualified sense.


§ 181. These aggregates of the lowest order, each formed of
physiological units united into a group that is structurally single and
cannot be divided without destruction of its individuality, may, as
above implied, exist as independent organisms. The assumption to which
we are committed by the hypothesis of evolution, that such so-called
unicellular plants were at first the only kinds of plants, is in
harmony with the fact that habitats not occupied by plants of higher
orders, commonly contain these protophytes in great abundance and great
variety. The various species of _Pleurococcaceæ_, of _Desmidiaceæ_,
and _Diatomaceæ_, supply examples of morphological units living and
propagating separately, under numerous modifications of form and
structure. Figures 1, 2, and 3, represent a few of the commonest types.

  [Illustration: Figs. 1, 2, 3.]

  [Illustration: Figs. 4, 5, 6.]

Mostly, simple plants are too small to be individually visible
without the microscope. But, in some cases, these vegetal aggregates
of the first order grow to appreciable sizes. In the mycelium of some
fungi, we have single cells developed into long branched filaments,
or ramified tubules, that are of considerable lengths. An analogous
structure characterizes certain tribes of _Algæ_, of which _Codium
adhærens_, Fig. 4, may serve as an example. In _Botrydium_, another
alga, Fig. 5, we have a structure which is described as simulating a
higher plant, with root, stem, bud, and fruit, all produced by the
branching of a single cell. And among fungi the genus _Mucor_, Fig. 6,
furnishes an example of allied kind.[3] Here, though the size attained
is much greater than that of many organisms which are morphologically
compound, we are compelled to consider the morphological composition
as simple; since the whole can no more be separated into minor wholes,
than can the branched vascular system of an animal. In these cases we
have considerable bulk attained, not by a number of aggregates of the
first order being united into an aggregate of the second order, but by
the continuous growth of an aggregate of the first order.


§ 182. The transition to higher forms begins in a very unobtrusive
manner. Among these aggregates of the first order, an approach towards
that union by which aggregates of the second order are produced, is
indicated by mere juxtaposition. Protophytes multiply rapidly; and
their rapid multiplication sometimes causes crowding. When, instead
of floating free in the water, they form a thin film on a moist
surface, or are imbedded in a common matrix of mucilage; the mechanical
obstacles to dispersion result in a kind of feeble integration, vaguely
shadowing forth a combined group. Somewhat more definite combination is
shown us by such plants as _Palmella botryoides_. Here the members of a
family of cells, arising by the spontaneous fission of a parent-cell,
remain united by slender threads of that jelly-like substance which
envelops their surfaces. In some _Diatomaceæ_ several individuals,
instead of completely separating, hold together by their angles; and
in other _Diatomaceæ_, as the _Bacillaria_, a variable number of units
cohere so slightly, that they are continually moving in relation to one
another.

This formation of aggregates of the second order, faintly indicated
in feeble and variable unions like the above, may be traced through
phases of increasing permanence and definiteness, as well as increasing
extent. In the yeast-plant, Fig. 7, we have cells which may exist
singly, or joined into groups of several; and which have their shapes
scarcely at all modified by their connexion. Among the _Desmidiaceæ_,
it happens in many cases that the two individuals produced by division
of a parent-individual, part as soon as they are fully formed; but in
other cases, instead of parting they compose a group of two. Allied
kinds show us how, by subsequent fissions of the adherent individuals
and their progeny, there result longer groups; and in some species,
a continuous thread of them is thus produced. Figs. 8, 9, 11, exhibit
these several stages. Fig. 10 represents a _Scenedesmus_ in which the
individuation of the group is manifest. Instead of linear aggregation,
many protophytes illustrate central aggregation; as shown in Figs. 12,
13, 14, 15. Other instances are furnished by such forms as the _Gonium
pectorale_, Fig. 16 (_a_ being the front view, and _b_ the edge view),
and the _Sarcina ventriculi_, Fig. 17. Further, we have that spherical
mode of aggregation of which the _Volvox globator_ furnishes a familiar
instance.

  [Illustration: Figs. 7–17.]

  [Illustration: Figs. 18–23.]

Thus far, however, the individuality of the secondary aggregate is
feebly pronounced: not simply in the sense that it is small; but also
in the sense that the individualities of the primary aggregates are
very little subordinated. But on seeking further, we find transitions
towards forms in which the compound individuality is more dominant,
while the simple individualities are more obscured. Obscuration of one
kind accompanies mere increase of size in the secondary aggregate.
In proportion to the greater number of the morphological units held
together in one mass, becomes their relative insignificance as
individuals. We see this in the irregularly-spreading lichens that
form patches on rocks; and in such creeping fungi as grow in films or
laminæ on decaying wood and the bark of trees. In these cases, however,
the integration of the component cells is of an almost mechanical
kind. The aggregate of them is scarcely more individuated than a lump
of inorganic matter: as witness the way in which the lichen extends
its curved edges in this or that direction, as the surface favours;
or the way in which the fungus grows round and imbeds the shoots and
leaves that lie in its way, just as so much plastic clay might do.
Though here, in the augmentation of mass, we see a progress towards the
evolution of a higher type, we have as yet none of that definiteness
required to constitute a compound unit, or true aggregate of the
second order. Another kind of obscuration of the morphological units,
is brought about by their more complete coalescence into the form of
some structure made by their union. This is well exemplified among
the _Confervoideæ_ and _Conjugatæ_. In Fig. 18, there are represented
the stages of a growing _Mougeotia genuflexa_, in which this merging
of the simple individualities into the compound individuality, is
shown in the history of a single plant; and in Figs. 19, 20, 21, 22,
23, are represented a series of species from this group, and that of
_Cladophora_,[4] in which we see a progressing integration. While,
in the lower types, the primitive spheroidal forms of the cells
are scarcely altered, in the higher types the cells are so fused
together as to constitute cylinders divided by septa. Here, however,
the indefiniteness is still great. There are no specific limits to
the length of any thread thus produced, and there is none of that
differentiation of parts required to give a decided individuality to
the whole.

To constitute something like a true aggregate of the second order,
capable of serving as a compound unit that may be combined with others
like itself into still higher aggregates, there must exist both mass
and definiteness.


§ 183. An approach towards plants which unite these characters, may be
traced in such forms as _Bangia ciliaris_, Fig. 24. The multiplication
of cells here takes place, not in a longitudinal direction only, but
also in a transverse direction; and the transverse multiplication being
greater towards the middle of the frond, there results a difference
between the middle and the two extremities--a character which, in
a feeble way, unites all the parts into a whole. Even this slight
individuation is, however, very indefinitely marked; since, as shown by
the figures, the lateral multiplication of cells does not go on in a
precise manner.

  [Illustration: Fig. 24.]

From some such type as this there appear to arise, through slight
differences in the modes of growth, two closely-allied groups of
plants, having individualities somewhat more pronounced. If, while
the cells multiply longitudinally, their lateral multiplication goes
on in one direction only, there results a flat surface, as in the
genus _Ulva_ (Sea-lettuce) or in the upper part of the thallus of
_Enteromorpha Linza_, Fig. 25; or where the lateral multiplication is
less uniform in its rate, in types like Fig. 26. But where the lateral
multiplication occurs in two directions transverse to one another, a
hollow frond may be produced--sometimes irregularly spheroidal, and
sometimes irregularly tubular; as in _Enteromorpha intestinalis_,
Fig. 27. And often, as in _Enteromorpha compressa_, Fig. 28, and
other species, this tubular frond becomes branched. Figs. 29 and 30
are magnified portions of such fronds, showing the simple cellular
aggregation which allies them with the preceding forms.

  [Illustration: Figs. 25–30.]

In the common _Fuci_ of our coasts, other and somewhat higher stages
of this integration are displayed. We have fronds preserving something
like constant breadths and dividing dichotomously with approximate
regularity. Though the subdivisions so produced are not to be regarded
as separate fronds, but only as extensions of one frond, they
foreshadow a higher degree of composition; and by the comparatively
methodic way in which they are united, give to the aggregate a more
definite, as well as a more complex, individuality. Many of the higher
lichens exhibit an analogous advance. While in the lowest lichens, the
different parts of the thallus are held together only by being all
attached to the supporting surface, in the higher lichens the thallus
is so far integrated that it can support itself by attachment to such
surface at one point only. And then, in still more developed kinds,
we find the thallus assuming a dichotomously-branched form, and so
gaining a more specific character as well as greater size.

Where, as in types like these, the morphological units show an inherent
tendency to arrange themselves in a manner which is so far constant
as to give characteristic proportions, we may say that there is a
recognizable compound individuality. Considering the Thallophytes which
grow in this way apart from their kinships, and wholly with reference
to their morphological composition, we might not inaptly describe them
as pseudo-foliar.


§ 184. Another mode in which aggregation is so carried on as to produce
a compound individuality of considerable definiteness, is variously
displayed among other families of _Algæ_. When the cells, instead
of multiplying longitudinally alone, and instead of all multiplying
laterally as well as longitudinally, multiply laterally only at
particular places, they produce branched structures.

Indications of this mode of aggregation occur among the _Confervoideæ_,
as shown in Figs. 22, 23. Though, in some of the more-developed
_Algæ_ which exhibit the ramified arrangement in a higher degree, the
component cells are, like those of the lower _Algæ_, united together
end to end, in such way as but little to obscure their separate
forms, as in _Cladophora Hutchinsiæ_, Fig. 31; they nevertheless
evince greater subordination to the whole of which they are parts,
by arranging themselves more methodically. Still further pronounced
becomes the compound individuality when, while the component cells of
the branches unite completely into jointed cylinders, the component
cells of the stem form an axis distinguished by its relative thickness
and complexity. Such types of structures are indicated by Figs. 32,
33--figures representing small portions of plants which are quite
tree-like in their entire outlines. On examining Figs. 34, 35, 36,
which show the structures of the stems in these types, it will be
seen, too, that the component cells in becoming more coherent,
have undergone changes of form which obscure their individualities
more than before. Not only are they much elongated, but they are so
compressed as to be prismatic rather than cylindrical. This structure,
besides displaying integration of the morphological units carried on
in two directions instead of one; and besides displaying this higher
integration in the greater merging of the individualities of the
morphological units in the general individuality; also displays it in
the more pronounced subordination of the branches and branchlets to the
main stem. This differentiation and consolidation of the stem, brings
all the secondary growths into more marked dependence; and so renders
the individuality of the aggregate more decided.

  [Illustration: Figs. 31–36.]

We might not inappropriately call this type of structure pseud-axial.
It simulates that of the higher plants in certain superficial
characters. We see in it a primary axis along which development may
continue indefinitely, and from which there bud out, laterally,
secondary axes of like nature, bearing like tertiary axes; and this is
a mode of growth with which Phænogams make us familiar.


§ 185. Some of the larger _Algæ_ supply examples of an integration
still more advanced; not simply inasmuch as they unite much greater
numbers of morphological units into continuous masses, but also
inasmuch as they combine the pseudo-foliar structure with the
pseud-axial structure. Our own shores furnish an instance of this in
the common _Laminaria_; and certain gigantic _Laminariaceæ_ of the
Antarctic seas, furnish yet better instances. In _Necrocystis_ the
germ develops a very long slender stem, which eventually expands into
a large bladder-like or cylindrical air-vessel; and the surface of
this bears numerous leaf-shaped expansions. Another kind, _Lessonia
fuscescens_, Fig. 37, shows us a massive stem growing up through water
many feet deep--a stem which, bifurcating as it approaches the surface,
flattens out the ends of its subdivisions into fronds like ribands.
These, however, are not true foliar appendages, since they are merely
expanded continuations of the stem. In _Egregia_ branches of the
thallus not only take the form of leaves, but these are differentiated
into several categories in accordance with a division of labour. In
any of these _Laminariaceæ_ the whole plant, great as may be its size,
and made up though it seems to be of many groups of morphological
units, united into a compound group by their marked subordination to a
connecting mass, is nevertheless a single thallus, which is added to by
intercalary growth at the “transition place,” at the junction of the
stem-like and leaf-like portions. The aggregate is still an aggregate
of the second order.

  [Illustration: Fig. 37.]

  [Illustration: Figs. 38–40.]

But among certain of the highest _Algæ_, we _do_ find something more
than this union of the pseud-axial with the pseudo-foliar structure.
In addition to pseud-axes of comparative complexity; and in addition
to pseudo-folia that are like leaves, not only in their general shapes
but in having mid-ribs and even veins; there are the beginnings of
a higher stage of integration. Figs. 38, 39, and 40, show some of
the steps. In _Rhodymenia palmata_, Fig. 38, the parent-frond is
comparatively irregular in form, and without a mid-rib; and along with
this very imperfect integration, we see that the secondary fronds
growing from the edges are distributed very much at random, and are by
no means specific in their shapes. A considerable advance is displayed
by _Phyllophora rubens_, Fig. 39. Here the frond, primary, secondary,
or tertiary, betrays some approach towards regularity in both form
and size; by which, as also by its partially-developed mid-rib, there
is established a more marked individuality; and at the same time,
the growth of the secondary fronds no longer occurs anywhere on the
edge, in the same plane as the parent-frond, but from the surface at
specific places. _Delesseria sanguinea_, Fig. 40, illustrates a much
more definite arrangement of the same kind. The fronds of this plant,
quite regularly shaped, have their parts decidedly subordinated to the
whole; and from their mid-ribs grow other fronds which are just like
them. Each of these fronds is an organized group of those morphological
units which we distinguish as aggregates of the first order. And in
this case, two or more such aggregates of the second order, well
individuated by their forms and structures, are united together; and
the plant composed of them is thus rendered, in so far, an aggregate of
the third order.

Just noting that in certain of the most developed _Algæ_, as the
_Sargassum_, or common gulf-weed, this tertiary degree of composition
is far more completely displayed, so as to produce among Thallophytes a
type of structure closely simulating that of the higher plants, let us
now pass to the consideration of these higher plants.


§ 186. Having the surface of the soil for a support and the air for a
medium, terrestrial plants are mechanically circumstanced in a manner
widely different from that in which aquatic plants are circumstanced.
Instead of being buoyed up by a surrounding fluid of specific gravity
equal to their own, they have to erect themselves into a rare fluid
which yields no appreciable support. Further, they are dissimilarly
conditioned in having two sources of nutriment in place of one. Unlike
the _Algæ_, which derive all the materials for their tissues from
the water bathing their entire surfaces, and use their roots only
for attachment, most of the plants which cover the Earth’s surface,
absorb part of their food through their imbedded roots and part through
their exposed leaves. These two marked unlikenesses in the relations
to surrounding conditions, profoundly affect the respective modes of
growth. We must duly bear them in mind while studying the further
advance of composition.

  [Illustration: Figs. 41–44.]

  [Illustration: Figs. 45–49.]

The class of plants to which we now turn--that of the
_Archegoniatæ_--is nearly related by its lower members to the classes
above dealt with: so much so, that some of the inferior liverworts are
quite licheniform, and are often mistaken for lichens. Passing over
these, let us recommence our synthesis with such members of the class
as repeat those indications of progress towards a higher composition,
which we have just observed among the more-developed _Algæ_. The
_Jungermanniaceæ_ furnish us with a series of types, clearly indicating
the transition from an aggregate of the second order to an aggregate
of the third order. Figs. 41 and 42, indicate the structure among the
lowest of this group. Here there is but an incomplete development of
the second order of aggregate. The frond grows as irregularly as the
thallus of a lichen: it is indefinite in size and outline, spreading
hither or thither as the conditions favour. Moreover, it lacks the
differentiations required to subordinate its parts to the whole: it
is uniformly cellular, having neither mid-rib nor veins; and it puts
out rootlets indifferently from all parts of its under surface. In
Fig. 43, _Pellia epiphylla_, we have an advance on this type. There
is here, as shown in the transverse section, Fig. 44, a thickening
of the frond along its central portion, producing something like an
approach towards a mid-rib; and from this the rootlets are chiefly
given off. The outline, too, is much less irregular; whence results
greater distinctness of the individuality. A further step is displayed
in _Metzgeria furcata_, Fig. 45. The frond of this plant, comparatively
well integrated by the distribution of its substance around a decided
mid-rib, and by its comparatively-definite outlines, produces secondary
fronds. There is what is called proliferous growth; and occasionally,
as shown in Fig. 46, representing an enlarged portion, the growth is
doubly-proliferous. In these cases, however, the tertiary aggregate, so
far as it is formed, is but very feebly integrated; and its integration
is but temporary. For not only do these younger fronds that bud out
from the mid-ribs of older fronds, develop rootlets of their own; but
as soon as they are well grown and adequately rooted, they dissolve
their connexions with the parent-fronds, and become quite independent.
From these transitional forms we pass, in the higher _Jungermanniaceæ_,
to forms composed of many fronds that are permanently united by a
continuous stem. A more-developed aggregate of the third order is
thus produced. But though, along with increased definiteness in
the secondary aggregates, there is here an integration of them so
extensive and so regular, that they are visibly subordinated to the
whole they form; yet the subordination is really very incomplete.
In some instances, as in _Radula complanata_, Fig. 47, the leaflets
develop roots from their under surfaces, just as the primitive frond
does; and in the majority of the group, as in _J. capitata_, Fig. 48,
roots are given off all along the connecting stem, at the spots where
the leaflets or frondlets join it: the result being that though the
connected frondlets form a physical whole, they do not form, in any
decided manner, a physiological whole; since successive portions of
the united series, carry on their functions independently of the rest.
Finally, the most developed members of the group, whether lineally
descended from the less developed or from an early type common to the
two, present us with tertiary aggregates which are physiologically as
well as physically integrated.[5] Not lying prone like the kinds thus
far described, but growing erect, the stem and attached leaflets become
dependent upon a single root or group of roots; and being so prevented
from carrying on their functions separately, are made members of a
compound individual: there arises a definitely-established aggregate of
the third degree of composition.

The facts as arranged in the above order are suggestive. Minute
aggregates, or cells, the grouping of which we traced in § 182, showed
us analogous phases of indefinite union, which appeared to lead the
way towards definite union. We see here among compound aggregates, as
we saw there among simple aggregates, the establishment of a specific
form, and a size that falls within moderate limits of variation. This
passage from less definite extension to more definite extension,
seems in the one case, as the other, to be accompanied by the result,
that growth exceeding a certain rate, ends in the formation of a new
aggregate, rather than an enlargement of the old. And on the higher
stage, as on the lower, this process, irregularly carried out in the
simpler types, produces in them unions that are but temporary; while in
the more-developed types, it proceeds in a systematic way, and ends in
the production of a permanent aggregate that is doubly compound.

Must we then conclude that as cells, or morphological units, are
integrated into a unit of a higher order, which we call a thallus
or frond; so, by the integration of fronds, there is evolved a
structure such as the above-delineated species possess? Whether this
is the interpretation to be given of these plants, we shall best see
when considering whether it is the interpretation to be given of
plants which rank above them. Thus far we have dealt only with the
Cryptogamia. We have now to deal with the Phanerogamia or Phænogamia.




                             CHAPTER III.

               THE MORPHOLOGICAL COMPOSITION OF PLANTS,
                             _CONTINUED_.


§ 187. That advanced composition arrived at in the _Archegoniatæ_, is
carried still further in the Flowering Plants. In these most-elevated
vegetal forms, aggregation of the third order is always distinctly
displayed; and aggregates of the fourth, fifth, sixth, &c., orders are
very common.

Our inquiry into the morphology of these flowering plants, may be
advantageously commenced by studying the development of simple leaves
into compound leaves. It is easy to trace the transition, as well as
the conditions under which it occurs; and tracing it will prepare us
for understanding how, and when, metamorphoses still greater in degree
take place.


§ 188. If we examine a branch of the common bramble, when in flower
or afterwards, we shall not unfrequently find a simple or undivided
leaf, at the insertion of one of the lateral flower-bearing axes,
composing the terminal cluster of flowers. Sometimes this leaf is
partially lobed; sometimes cleft into three small leaflets. Lower down
on the shoot, if it be a lateral one, occur larger leaves, composed
of three leaflets; and in some of these, two of the leaflets may be
lobed more or less deeply. On the main stem the leaves, usually still
larger, will be found to have five leaflets. Supposing the plant to be
a well-grown one, it will furnish all gradations between the simple,
very small leaf, and the large composite leaf, containing sometimes
even seven leaflets. Figs. 50 to 64, represent leading stages of the
transition. What determines this transition? Observation shows that
the quintuple leaves occur where the materials for growth are supplied
in greatest abundance; that the leaves become less and less compound,
in proportion to their remoteness from the main currents of sap; and
that where an entire absence of divisions or lobes is observed, it is
on leaves within the flower-bunch: at the place, that is, where the
forces which cause growth are nearly equilibrated by the forces which
oppose growth; and where, as a consequence, gamogenesis is about to
be set in (§ 78). Additional evidence that the degree of nutrition
determines the degree of composition of the leaf, is furnished by
the relative sizes of the leaves. Not only, on the average, is the
quintuple leaf much larger in its total area than the triple leaf;
but the component leaflets of the one, are usually much larger than
those of the other. The like contrasts are still more marked between
triple leaves and simple leaves. This connection of decreasing size
with decreasing composition, is conspicuous in the series of figures:
the differences shown being not nearly so great as may be frequently
observed. Confirmation may be drawn from the fact that when the leading
shoot is broken or arrested in its growth, the shoots it gives off
(provided they are given off after the injury), and into which its
checked currents of sap are thrown, produce leaves of five leaflets
where ordinarily leaves of three leaflets occur. Of course incidental
circumstances, as variations in the amounts of sunshine, or of rain,
or of matter supplied to the roots, are ever producing changes in the
state of the plant as a whole; and by thus affecting the nutrition of
its leaf-buds at the times of their formation, cause irregularities
in the relations of size and composition above described. But taking
these causes into account, it is abundantly manifest that a leaf-bud of
the bramble will develop into a simple leaf or into a leaf compounded
in different degrees, according to the quantity of assimilable matter
brought to it at the time when the rudiments of its structure are
being fixed. And on studying the habits of other plants--on observing
how annuals that have compound leaves usually bear simple leaves at
the outset, when the assimilating surface is but small; and how, when
compound-leaved plants in full growth bear simple leaves in the midst
of compound ones, the relative smallness of such simple leaves shows
that the buds from which they arose were ill-supplied with sap; it
will cease to be doubted that a foliar organ may be metamorphosed into
a group of foliar organs, if furnished, at the right time, with a
quantity of matter greater than can be readily organized round a single
centre of growth. An examination of the transitions through which
a compound leaf passes into a doubly-compound leaf, as seen in the
various intermediate forms of leaflets in Fig. 65, will further enforce
this conclusion.

  [Illustration: Figs. 50–64.]

  [Illustration: Fig. 65.]

Here we may advantageously note, too, how in such cases the leaf-stalk
undergoes concomitant changes of structure. In the bramble-leaves above
described, it becomes compound simultaneously with the leaf--the veins
become mid-ribs while the mid-ribs become petioles. Moreover, the
secondary stalks, and still more the main stalks, bear thorns similar
in their shapes, and approaching in their sizes, to those on the stem;
besides simulating the stem in colour and texture. In the petioles of
large compound leaves, like those of the common _Heracleum_, we see
still more distinctly both internal and external approximations in
character to axes. Nor are there wanting plants whose large, though
simple, leaves, are held out far from the stems by foot-stalks that
are, near the ends, sometimes so like axes that the transverse sections
of the two are indistinguishable; as instance the _Calla palustris_.

One other fact respecting the modifications which leaves undergo,
should be set down. Not only may leaf-stalks assume to a great degree
the characters of stems, when they have to discharge the functions
of stems, by supporting many leaves or very large leaves; but they
may assume the characters of leaves, when they have to undertake the
functions of leaves. The Australian Acacias furnish a remarkable
illustration of this. Acacias elsewhere found bear pinnate leaves;
but the majority of those found in Australia bear what appear to be
simple leaves. It turns out, however, that these are merely leaf-stalks
flattened out into foliar shapes: the laminæ of the leaves being
undeveloped. And the proof is that in young plants, showing their
kinships by their embryonic characters, these leaf-like petioles bear
true leaflets at their ends. A metamorphosis of like kind occurs in
_Oxalis bupleurifolia_, Fig. 66. The fact most deserving of notice,
however, is that these leaf-stalks, in usurping the general aspects and
functions of leaf-blades, have, to some also usurped their structures:
though their venation is not like that of the leaf-blades they replace,
yet they have veins, and in some cases mid-ribs.

  [Illustration: Fig. 66.]

Reduced to their most general expression, the truths above shadowed
forth are these:--That group of morphological units, or cells, which
we see integrated into the compound unit called a leaf, has, in
each higher plant, a typical form, due to the special arrangement
of these cells around a mid-rib and veins. If the multiplication of
morphological units, at the time when the leaf-bud is taking on its
main outlines, exceeds a certain limit, these units begin to arrange
themselves round secondary centres, or lines of growth, in such ways as
to repeat, in part or wholly, the typical form: the larger veins become
transformed into imperfect mid-ribs of partially independent leaves;
or into complete mid-ribs of quite separate leaves. And as there goes
on this transition from a single aggregate of cells to a group of
such aggregates, there simultaneously arises, by similarly insensible
steps, a distinct structure which supports the several aggregates thus
produced, and unites them into a compound aggregate. These phenomena
should be carefully studied; since they give us a key to more involved
phenomena.[6]


§ 189. Thus far we have dealt with leaves ordinarily so-called: briefly
indicating the homologies between the parts of the simple and the
compound. Let us now turn to the homologies among foliar organs in
general. These have been made familiar to readers of natural history
by popularized outlines of _The Metamorphosis of Plants_--a title, by
the way, which is far too extensive; since the phenomena treated of
under it, form but a small portion of those it properly includes.

Passing over certain vague anticipations which have been quoted from
ancient writers, and noting only that some clearer recognitions
were reached by Joachim Jung, a Hamburg professor, in the middle
of the 17th century; we come to the _Theoria Generationis_, which
Wolff published in 1759, and in which he gives definite forms to the
conceptions that have since become current. Specifying the views of
Wolff, Dr. Masters writes:--“After speaking of the homologous nature
of the leaves, the sepals and petals, an homology consequent on their
similarity of structure and identity of origin, he goes on to state
that the ‘pericarp is manifestly composed of several leaves, as in the
calyx, with this difference only, that the leaves which are merely
placed in close contact in the calyx, are here united together’; a
view which he corroborates by referring to the manner in which many
capsules open and separate ‘into their leaves.’ The seeds, too, he
looks upon as consisting of leaves in close combination. His reasons
for considering the petals and stamens as homologous with leaves, are
based upon the same facts as those which led Linnæus, and, many years
afterwards, Goethe, to the same conclusion. ‘In a word,’ says Wolff,
‘we see nothing in the whole plant, whose parts at first sight differ
so remarkably from each other, but leaves and stem, to which latter
the root is referrible.’” It appears that Wolff, too, enunciated the
now-accepted interpretation of compound fruits: basing it on the same
evidence as that since assigned. In the essay of Goethe, published
thirty years after, these relations among the parts of flowering plants
were traced out in greater detail, but not in so radical a way; for
Goethe did not, as did Wolff, verify his hypothesis by dissecting buds
in their early stages of development. Goethe appears to have arrived
at his conclusions independently. But that they were original with
him, and that he gave a more variously-illustrated exposition of them
than had been given by Wolff, does not entitle him to anything beyond
a secondary place, among those who have established this important
generalization.

Were it not that these pages may be read by some to whom Biology, in
all its divisions, is a new subject of study, it would be needless to
name the evidence on which this now-familiar generalization rests. For
the information of such it will suffice to say, that the fundamental
kinship existing among all the foliar organs of a flowering plant, is
shown by the transitional forms which may be traced between them, and
by the occasional assumption of one another’s forms. “Floral leaves, or
bracts, are frequently only to be distinguished from ordinary leaves
by their position at the base of the flower; at other times the bracts
gradually assume more and more of the appearance of the sepals.” The
sepals, or divisions of the calyx, are not unlike undeveloped leaves:
sometimes assuming quite the structure of leaves. In other cases, they
acquire partially or wholly the colours of the petals--as, indeed, the
bracts and uppermost stem-leaves occasionally do. Similarly, the petals
show their alliances to the foliar organs lower down on the axis, and
to those higher up on the axis. On the one hand, they may develop into
ordinary leaves that are green and veined; and, on the other hand, as
so commonly seen in double flowers, they may bear anthers on their
edges. All varieties of gradation into neighbouring foliar organs may
be witnessed in stamens. Flattened and tinted in various degrees, they
pass insensibly into petals, and through them prove their homology with
leaves; into which, indeed, they are transformed in flowers that become
wholly foliaceous. The style, too, is occasionally changed into petals
or into green leaflets; and even the ovules are now and then seen to
take on leaf-like forms. Thus we have clear evidence that in Phænogams,
all the appendages of the axis are homologues: they are all modified
leaves.

Wolff established, and Goethe further illustrated, another general law
of structure in flowering plants. Each leaf commonly contains in its
axil a bud, similar in structure to the terminal bud. This axillary bud
may remain undeveloped; or it may develop into a lateral shoot like
the main shoot; or it may develop into a flower. If a shoot bearing
lateral flowers be examined, it will be found that the internode, or
space which separates each leaf with its axillary flower from the leaf
and axillary flower above it, becomes gradually less towards the upper
end of the shoot. In some plants, as in the fox-glove, the internodes
constitute a regularly-diminishing series. In other plants, the series
they form suddenly begins to diminish so rapidly, as to bring the
flowers into a short spike: instance the common orchis. And again, by
still more sudden dwarfing of the internodes, the flowers are brought
into a cluster; as they are in the cowslip. On contemplating a clover
flower, in which this clustering has been carried so far as to produce
a compact head; and on considering what must happen if, by a further
arrest of axial development, the foot-stalks of the florets disappear;
it will be seen that there must result a crowd of flowers, seated
close together on the end of the axis. And if, at the same time, the
internodes of the upper stem-leaves also remain undeveloped, these
stem-leaves will be grouped into a common involucre: we shall have
a composite flower, such as the thistle. Hence, to modifications in
the developments of foliar organs, have to be added modifications in
the developments of axial organs. Comparisons disclose the gradations
through which axes, like their appendages, pass into all varieties
of size, proportion, and structure. And we learn that the occurrence
of these two kinds of metamorphosis, in all conceivable degrees
and combinations, furnishes us with a proximate interpretation of
morphological composition in Phænogams.

I say a proximate interpretation, because there remain to be solved
certain deeper problems; one of which at once presents itself to be
dealt with under the present head. Leaves, petals, stamens, &c., being
shown to be homologous foliar organs; and the part to which they are
attached, proving to be an indefinitely-extended axis of growth, or
axial organ; we are met by the questions,--What is a foliar organ? and
What is an axial organ? The morphological composition of a Phænogam is
undetermined, so long as we cannot say to what lower structures leaves
and shoots are homologous; and how this integration of them originates.
To these questions let us now address ourselves.


§ 190–1. Already, in § 78, reference has been made to the occasional
development of foliar organs into axial organs: the special case there
described being that of a fox-glove, in which some of the sepals were
replaced by flower-buds. The observation of these and some analogous
monstrosities, raising the suspicion that the distinction between
foliar organs and axial organs is not absolute, led me to examine into
the matter; and the result has been the deepening of this suspicion
into a conviction. Part of the evidence is given in Appendix A.

Some time after having reached this conviction, I found on looking
into the literature of the subject, that analogous irregularities had
suggested to other observers, beliefs similarly at variance with the
current morphological creed. Difficulties in satisfactorily defining
these two elements, have served to shake this creed in some minds.
To others, the strange leaf-like developments which axes undergo in
certain plants, have afforded reasons for doubting the constancy of
this distinction which vegetal morphologists usually draw. And those
not otherwise rendered sceptical, have been made to hesitate by such
cases as that of the Nepaul-barley, in which the glume, a foliar
organ, becomes developed into an axis and bears flowers. In his
essay--“Vegetable Morphology: its History and Present Condition,”[7]
whence I have already quoted, Dr. Masters indicates sundry of the
grounds for thinking that there is no impassable demarcation between
leaf and stem. Among other difficulties which meet us if we assume that
the distinction is absolute, one is implied by this question:--“What
shall we say to cases such as those afforded by the leaves of _Guarea_
and _Trichilia_, where the leaves after a time assume the condition
of branches and develop young leaflets from their free extremities,
a process less perfectly seen in some of the pinnate-leaved kinds of
_Berberis_ or _Mahonia_, to be found in almost every shrubbery?”

A class of facts on which it will be desirable for us here to dwell
a moment, before proceeding to deal with the matter deductively, is
presented by the _Cactaceæ_. In this remarkable group of plants,
deviating in such varied ways from the ordinary phænogamic type, we
find many highly instructive modifications of form and structure. By
contemplating the changes here displayed within the limits of a single
order, we shall greatly widen our conception of the possibilities of
metamorphosis in the vegetal kingdom, taken as a whole. Two different,
but similarly-significant, truths are illustrated. First, we are shown
how, of these two components of a flowering plant, commonly regarded
as primordially distinguished, one may assume, throughout numerous
species, the functions, and to a great degree the appearance, of the
other. Second, we are shown how, in the same individual, there may
occur a re-metamorphosis: the usurped function and appearance being
maintained in one part of the plant, while in another part there is a
return to the ordinary appearance and function. We will consider these
two truths separately. Some of the _Euphorbiaceæ_, which simulate
Cactuses, show us the stages through which such abnormal structures are
arrived at. In _Euphorbia splendens_, the lateral axes are considerably
swollen at their distal ends, so as often to be club-shaped: still,
however, being covered with bark of the ordinary colour, and still
bearing leaves. But in kindred plants, as _Euphorbia neriifolia_,
this swelling of the lateral axes is carried to a far greater extent;
and, at the same time, a green colour and a fleshy consistence have
been acquired: the typical relations nevertheless being still shown
by the few leaves that grow out of these soft and swollen axes. In
the _Cactaceæ_, which are thus resembled by plants not otherwise
allied to them, we have indications of a parallel transformation.
Some kinds, not commonly brought to England, bear leaves; but in the
species most familiar to us, the leaves are undeveloped and the axes
assume their functions. Passing over the many varieties of form and
combination which these green succulent growths display, we have to
note that in some genera, as in _Phyllocactus_, they become flattened
out into foliaceous shapes, having mid-ribs and something approaching
to veins. So that here, and in the genus _Epiphyllum_, which has this
character still more marked, the plant appears to be composed of fleshy
leaves growing one upon another. And then, in _Rhipsalis_, the same
parts are so leaf-like, that an uncritical observer would regard them
as leaves. These which are axial organs in their homologies, have
become foliar organs in their analogies. When, instead of comparing
these strangely-modified axes in different genera of Cactuses, we
compare them in the same individual, we meet with transformations no
less striking. Where a tree-like form is produced by the growth of
these foliaceous shoots, one on another; and where, as a consequence,
the first-formed of them become the main stem that acts as support
to secondary and tertiary stems; they lose their green, succulent
character, acquire bark, and become woody. In resuming the functions
of axes they resume the structures of axes, from which they had
deviated. In Fig. 71 are shown some of the leaf-like axes of _Rhipsalis
rhombea_ in their young state; while Fig. 72 represents the oldest
portion of the same plant, in which the foliaceous characters are
quite obliterated, and there has resulted an ordinary stem-structure.
One further fact is to be noted. At the same time that their
leaf-like appearances are lost, the axes also lose their separate
individualities. As they become stem-like, they also become integrated;
and they do this so effectually that their original points of junction,
at first so strongly marked, are effaced, and a consolidated trunk is
produced.

  [Illustration: Figs. 71–72.]

Joined with the facts previously specified, these facts help us to
conceive how, in the evolution of flowering plants in general, the
morphological components that were once distinct, may become extremely
disguised. We may rationally expect that during so long a course of
modification, much greater changes of form, and much more decided
fusions of parts, have taken place. Seeing how, in an individual
plant, the single leaves pass into compound leaves, by the development
of their veins into mid-ribs while their petioles begin to simulate
axes; and seeing that leaves ordinarily exhibiting definitely-limited
developments, occasionally produce other leaves from their edges; we
are led to suspect the possibility of still greater changes in foliar
organs. When, further, we find that within the limits of one natural
order, petioles usurp the functions and appearances of leaves, at
the same time that in other orders, as in _Ruscus_, lateral axes so
simulate leaves that their axial nature would by most not be suspected,
did they not bear flowers on their mid-ribs or edges; and when, among
Cactuses, we perceive that such metamorphoses and re-metamorphoses
take place with great facility; our suspicion that the morphological
elements of Phænogams admit of profound transformations, is deepened.
And then, on discovering how frequent are the monstrosities which do
not seem satisfactorily explicable without admitting the development
of foliar organs into axial organs; we become ready to entertain
the hypothesis that during the evolution of the phænogamic type, the
distinction between leaves and axes has arisen by degrees.

With our preconceptions loosened by such facts, and carrying with us
the general idea which such facts suggest, let us now consider in what
way the typical structure of a flowering plant may be interpreted.


§ 192. To proceed methodically, we must seek a clue to the structures
of Phanerogams, in the structures of those inferior plants that
approach to them--_Archegoniatæ_. The various divisions of this
class present, along with sundry characters which ally them with
Thallophytes, other characters by which the phænogamic structure is
shadowed forth. While some of the inferior _Hepaticæ_ or Liverworts,
severally consist of little more than a thallus-like frond, among
the higher members of this group, and still more among the Mosses
and Ferns, we find a distinctly marked stem.[8] Some Archegoniates
(or rather Rhizoids) have foliar expansions that are indefinite in
their forms; and some have quite definitely-shaped leaves. Roots are
possessed by all the more-developed genera of the class; but there are
other genera, as _Sphagnum_, which have no roots. Here the fronds are
formed of only a single layer of cells; and there a double layer gives
them a higher character--a difference exhibited between closely-allied
genera of one group, the Mosses. Equally varied are the developments
of the foliar organs in their detailed structures: now being without
mid-ribs or veins; now having mid-ribs but no veins; now having
both mid-ribs and veins. Nor must we omit the similarly-significant
circumstance, that whereas in the lower Archegoniates the reproductive
elements are immersed here and there in the thallus-like frond, they
are, in the higher orders, seated in well-specialized and quite
distinct fructifying organs, having analogies with the flowers of
Phænogams. Thus, many facts imply that if the Phænogamic type is to
be analyzed at all, we must look among the Archegoniates for its
morphological components, and the manner of their integration.

Already we have seen among the lower _Cryptogamia_, how, as they became
integrated and definitely limited, aggregates acquire the habit of
budding out other aggregates, on reaching certain stages of growth.
Cells produce other cells endogenously or exogenously; and fronds
give origin to other fronds from their edges or surfaces. We have
seen, too, that the new aggregates so produced, whether of the first
order or the second order, may either separate or remain connected.
Fissiparously-multiplying cells in some cases part company, while in
other cases they unite into threads or laminæ or masses; and fronds
originating proliferously from other fronds, sometimes when mature
disconnect themselves from their parents, and sometimes continue
attached to them. Whether they do or do not part, is clearly determined
by their nutrition. If the conditions are such that they can severally
thrive better by separating after a certain development is reached,
it will become their habit then to separate; since natural selection
will favour the propagation of those which separate most nearly at
that time. If, conversely, it profits the species for the cells or
fronds to continue longer attached, which it can only do if their
growths and subsequent powers of multiplication are thereby increased,
it must happen, through the continual survival of the fittest, that
longer attachment will become an established characteristic; and,
by persistence in this process, permanent attachment will result
when permanent attachment is advantageous. That disunion is really
a consequence of relative innutrition, and union a consequence of
relative nutrition, is clear _à posteriori_. On the one hand, the
separation of the new individuals, whether in germs or as developed
aggregates, is a dissolving away of the connecting substance; and
this implies that the connecting substance has ceased to perform its
function as a channel of nutriment. On the other hand, where, as
we see among Phænogams, there is about to take place a separation
of new individuals in the shape of germs, at the point where the
nutrition is the lowest, a sudden increase of nutrition will cause the
impending separation to be arrested; and the fructifying elements,
reverting towards the ordinary form, thereupon develop in connexion
with the parent. Turning to the Archegoniates, we find among them
many indications of this transition from discontinuous development
to continuous development. Thus the Liverworts give origin to new
plants by cells which they throw off from their surfaces; as, indeed,
we have seen that much higher plants do. “According to Bischoff,”
says Schleiden, “both the cells of the stem (_Jungermannia_ [now
_Lophocolea_] _bidentata_) and those of the leaves (_J. exsecta_)
separate themselves as propagative cells from the plant, and isolated
cells shoot out and develop while still connected with the parent
plant into small cellular bodies (_Metzgeria furcata_), which separate
from the plant, and grow into new plants, as in _Mnium androgynum_
among the Mosses.” Now in the way above explained, these propagative
cells and proliferous buds, may continue developing in connexion with
the parent to various degrees before separating; or the buds which
are about to become fructifying organs may similarly, under increased
nutrition, develop into young fronds. As Sir W. Hooker says of the male
fructification in _Metzgeria furcata_,--“It has the appearance of being
a young shoot or innovation (for in colour and texture I can perceive
no difference) rolled up into a spherical figure.” On finding in this
same plant, that sometimes the proliferously-produced frond buds out
from itself another frond before separating from the parent, as shown
in Fig. 46, it becomes clear that this long-continued connexion may
readily pass into permanent connexion. And when we see how, even among
Phænogams, buds may either detach themselves as bulbils, or remain
attached and become shoots; we can scarcely doubt that among inferior
plants, less definite in their modes of organization, such transitions
must continually occur.

  [Illustration: Figs. 73–76.]

Let us suppose, then, that Fig. 73 is the frond of some primitive
Archegoniate, similar in general characters to _Pellia epiphylla_,
Fig. 43; bearing, like it, the fructifying buds on its upper surface,
and having a slightly-marked mid-rib and rootlets. And suppose
that, as shown, a secondary frond is proliferously produced from
the mid-rib, and continues attached to it. Evidently the ordinary
discontinuous development, can thus become a continuous development,
only on condition that there is an adequate supply, to the secondary
frond, of such materials as are furnished by the rootlets: the
remaining materials being obtainable by itself from the air. Hence,
that portion of the mid-rib lying between the secondary frond and
the chief rootlets, having its function increased, will increase in
bulk. An additional consequence will be a greater concentration of
the rootlets--there will be extra growth of those which are most
serviceably placed. Observe, next, that the structure so arising
is likely to be maintained. Such a variation implying, as it does,
circumstances especially favourable to the growth of the plant, will
give to the plant extra chances of leaving descendants; since the area
of frond supported by a given area of the soil, being greater than
in other individuals, there may be a greater production of spores.
And then, among the more numerous descendants thus secured by it, the
variation will give advantages to those in which it recurs. Such a
mode of growth having, in this manner, become established, let us ask
what is next likely to result. If it becomes the habit of the primary
frond to bear a secondary frond from its mid-rib, this secondary frond,
composed of physiological units of the same kind, will inherit the
habit; and supposing that the supply of mineral matters obtained by
the rootlets suffices for the full development of the secondary frond,
there is a likelihood that the growth from it of a tertiary frond,
will become an habitual characteristic of the variety. Along with the
establishment of such a tertiary frond, as shown in Fig. 74, there
must arise a further development of mid-rib in the primary frond, as
well as in the secondary frond--a development which must bring with it
a greater integration of the two; while, simultaneously, extra growth
will take place in such of the rootlets as are most directly connected
with this main channel of circulation. Without further explanation it
will be seen, on inspecting Figs. 75 and 76, that there may in this
manner result an integrated series of fronds, placed alternately on
opposite sides of a connecting vascular structure. That this connecting
vascular structure will, as shown in the figures, become more distinct
from the foliar surfaces as these multiply, is no unwarranted
assumption; for we have seen in compound-leaved plants, how, under
analogous conditions, mid-ribs become developed into separate
supporting parts, which acquire some of the characters of axes while
assuming their functions. And now mark how clearly the structure thus
built up by integration of proliferously-growing fronds, corresponds
with the structure of the more-developed _Jungermanniaceæ_. Each of
the fronds successively produced, repeating the characters of its
parent, will bear roots; and will bear them in homologous places, as
shown. Further, the united mid-ribs having but very little rigidity,
will be unable to maintain an erect position. Hence there will result
the recumbent, continuously-rooted stem, which these types exhibit: an
embryo phænogam having the weakness of an embryo.[9]

A natural concomitant of the mode of growth here described, is that
the stem, while it increases longitudinally, increases scarcely at
all transversely: hence the old name Acrogens. Clearly the transverse
development of a stem is the correlative, partly of its function as
a channel of circulation, and partly of its function as a mechanical
support. That an axis may lift its attached leaves into the air,
implies thickness and solidity proportionate to the mass of such
leaves; and an increase of its sap-vessels, also proportionate to the
mass of such leaves, is necessitated when the roots are all at one end
and the leaves at the other. But in the generality of Acrogens, these
conditions, under which arises the necessity for transverse growth
of the axis, are absent wholly or in great part. The stem habitually
creeps below the surface, or lies prone upon the surface; and where
it grows in a vertical or inclined direction, does this by attaching
itself to a vertical or inclined object. Moreover, throwing out
rootlets, as it mostly does, at intervals throughout its length, it
is not called upon in any considerable degree, to transfer nutritive
materials from one of its ends to the other. Hence this peculiarity
which gives their name to the Acrogens, now called Archegoniates, is a
natural accompaniment of the low degree of specialization reached in
them. And that it is an incidental and not a necessary peculiarity, is
demonstrated by two converse facts. On the one hand, in those higher
Acrogens which, like the tree-ferns, lift large masses of foliage into
the air, there is just as decided a transverse expansion of the axis
as in dicotyledonous trees. On the other hand, in those Dicotyledons
which, like the common Dodder, gain support and nutriment from the
surfaces over which they creep, there is no more lateral expansion of
the axis than is habitual among Acrogens or Archegoniates. Concluding,
as we are thus fully justified in doing, that the lateral expansion
accompanying longitudinal extension, which is a general characteristic
of Phanerogams as distinguished from Archegoniates, is nothing more
than a concomitant of their usually-vertical growth;[10] let us now
go on to consider how vertical growth originates, and what are the
structural changes it involves.


§ 193. Plants depend for their prosperity mainly on air and light: they
dwindle where they are smothered, and thrive where they can expand
their leaves into free space and sunshine. Those kinds which assume
prone positions, consequently labour under disadvantages in being
habitually interfered with by one another--they are mutually shaded
and mutually injured. Such of them, however, as happen, by variations
in mode of growth, to rise higher than others, are more likely to
flourish and leave offspring than others. That is to say, natural
selection will favour the more upright-growing forms. Individuals
with structures which lift them above the rest, are the fittest for
the conditions; and by the continual survival of the fittest, such
structures must become established. There are two essentially-different
ways in which the integrated series of fronds above described, may
be modified so as to acquire the stiffness needful for maintaining
perpendicularity. We will consider them separately.

  [Illustration: Figs. 77, 78.]

A thin layer of substance gains greatly in power of resisting a
transverse strain, if it is bent round so as to form a tube: witness
the difference between the pliability of a sheet of paper when
outspread, and the rigidity of the same sheet of paper when rolled up.
Engineers constantly recognize this truth, in devising appliances by
which the greatest strength shall be obtained at the smallest cost of
material; and among organisms, we see that natural selection habitually
establishes structures conforming to the same principle, wherever
lightness and stiffness are to be combined. The cylindrical bones of
mammals and birds, and the hollow shafts of feathers, are examples.
The lower plants, too, furnish cases where the strength needful for
maintaining an upright position, is acquired by this rolling up of a
flat thallus or frond. In Fig. 77 we have an _Alga_ which approaches
towards a tubular distribution of substance; and which has a consequent
rigidity. Sundry common forms of lichen, having the thallus folded
into a branched tube, still more decidedly display the connexion
between this structural arrangement and this mechanical advantage.
And from the particular class of plants we are here dealing with--the
Archegoniates--a type is shown in Fig. 78, _Riella helicophylla_,
similarly characterized by a thin frond that is made stiff enough to
stand, by an incurving which, though it does not produce a hollow
cylinder, produces a kindred form. If, then, as we have seen, natural
selection or survival of the fittest will favour such among these
recumbent Archegoniates as are enabled, by variations in their
structures, to maintain raised postures; it will favour the formation
of fronds that curve round upon themselves, and curve round upon the
fronds growing out of them. What, now, will be the result should such a
modification take place in the group of proliferous fronds represented
in Fig. 76? Clearly, the result will be a structure like that shown
in Fig. 79. And if this inrolling becomes more complete, a form like
_Jungermannia cordifolia_, represented in Fig. 80, will be produced.

  [Illustration: Figs. 79, 80.]

  [Illustration: Figs. 81–89.]

  [Illustration: Figs. 90, 91.]

When the successive fronds are thus folded round so completely that
their opposite edges meet, these opposite edges will be apt to unite:
not that they will grow together after being formed, but that they will
develop in connexion; or, in botanical language, will become “adnate.”
That foliar surfaces which, in their embryonic state, are in close
contact, often join into one, is a familiar fact. It is habitually
so with sepals or divisions of the calyx. In all campanulate flowers
it is so with petals. And in some tribes of plants it is so with
stamens. We are therefore well warranted in inferring that, under the
conditions above described, the successive fronds or leaflets will,
by union of their remote edges, first at their points of origin and
afterwards higher up, form sheaths inserted one within another, and
including the axis. This incurving of the successive fronds, ending
in the formation of sheaths, may be accompanied by different sets of
modifications. Supposing Fig. 81 to be a transverse section of such
type (_a_ being the mid-rib, and _b_ the expansion of an older frond;
while _c_ is a younger frond proliferously developed within it), there
may begin two divergent kinds of changes, leading to two contrasted
structures. If, while frond continues to grow out of frond, the
series of united mid-ribs continues to be the channel of circulation
between the uppermost fronds and the roots--if, as a consequence,
the compound mid-rib, or rudimentary axis, continues to increase in
size laterally; there will arise the series of transitional forms
represented by the transverse sections 82, 83, 84, 85; ending in the
production of a solid axis, everywhere wrapped round by the foliar
surface of the frond, as an outer layer or sheath. But if, on the
other hand, circumstances favour a form of plant which maintains its
uprightness at the smallest cost of substance--if the vascular bundles
of each succeeding mid-rib, instead of remaining concentrated, become
distributed all round the tube formed by the infolded frond; then the
structure eventually reached, through the transitional forms 86, 87,
88, 89, will be a hollow cylinder.[11] And now observe how the two
structures thus produced, correspond with two kinds of Monocotyledons.
Fig. 90 represents a species of _Dendrobium_, in which we see clearly
how each leaf is but a continuation of the external layer of a solid
axis--a sheath such as would result from the infolded edges of a frond
becoming adnate; and on examining how the sheath of each leaf includes
the one above it, and how the successive sheaths include the axis, it
will be manifest that the relations of parts are just such as exist
in the united series of fronds shown in Fig. 79--the successive nodes
answering to the successive points of origin of the fronds. Conversely,
the stem of a grass, Fig. 91, displays just such relations of parts,
as would result from the development of the type shown in Fig. 79,
if instead of the mid-ribs thickening into a solid axis, the matter
composing them became evenly distributed round the foliar surfaces,
at the same time that the incurved edges of the foliar surfaces
united. The arrangements of the tubular axis and its appendages, thus
resulting, are still more instructive than those of the solid axis.
For while, even more clearly than in the _Dendrobium_, we see at the
point _b_, a continuity of structure between the substance of the
axis below the node, and the substance of the sheath above the node:
we see that this sheath, instead of having its edges united as in
_Dendrobium_, has them simply overlapping, so as to form an incomplete
hollow cylinder which may be taken off and unrolled; and we see that
were the overlapping edges of this sheath united all the way from the
node _a_ to the node _b_, it would constitute a tubular axis, like that
which precedes it or like that which it includes. And then, giving an
unexpected conclusiveness to the argument, it turns out that in one
family of grasses, the overlapping edges of the sheaths _do_ unite:
thus furnishing us with a demonstration that tubular structures _are_
produced by the incurving and joining of foliar surfaces; and that so,
hollow axes may be interpreted as above, without making any assumption
unwarranted by fact. One further correspondence between the type thus
ideally constructed, and the monocotyledonous type, must be noted. If,
as already pointed out, the transverse growth of an axis arises when
the axis comes to be a channel of circulation between all the roots at
one of its extremities and all the leaves at the other; and if this
lateral bulging must increase as fast as the quantity of foliage to
be brought in communication with the roots increases--especially if
such foliage has at the same time to be raised high above the earth’s
surface; what must happen to a plant constructed in the manner just
described? The elder fronds or foliar organs, ensheathing the younger
ones, as well as the incipient axis serving as a bond of union, are
at first of such circumference only as suffices to inclose these
undeveloped parts. What, then, will take place when the inclosed
parts grow--when the axis thickens while it elongates? Evidently the
earliest-formed sheaths, not being large enough for the swelling axis,
must burst; and evidently each of the later-formed sheaths must, in
its turn, do the like. There must result a gradual exfoliation of
the successive sheaths, like that indicated as beginning in the above
figure of _Dendrobium_; which, at _a_, shows the bud of the undeveloped
parts just visible above the enwrapping sheaths, while at _b_, and _c_,
it shows the older sheaths in process of being split open. That is to
say, there must result the mode of growth which helped to give the name
Endogens to this class.

  [Illustration: Figs. 92–94.]

  [Illustration: Figs. 95–99.]

The other way in which an integrated series of fronds may acquire
the rigidity needful for maintaining an erect position, has next to
be considered. If the successive fronds do not acquire such habit
of curling as may be taken advantage of by natural selection, so as
to produce the requisite stiffness; then, the only way in which the
requisite stiffness appears producible, is by the thickening and
hardening of the fused series of mid-ribs. The incipient axis will not,
in this case, be inclosed by the rolled-up fronds; but will continue
exposed. Survival of the fittest will favour the genesis of a type, in
which those portions of the successive mid-ribs that enter into the
continuous bond, become more bulky than the disengaged portions of the
mid-ribs: the individuals which thrive and have the best chances of
leaving offspring, being, by the hypothesis, individuals having axes
stiff enough to raise their foliage above that of their fellows. At
the same time, under the same influences, there will tend to result an
elongation of those portions of the mid-ribs, which become parts of the
incipient axis; seeing that it will profit the plant to have its leaves
so far removed from one another, as to prevent mutual interferences.
Hence, from the recumbent type there will evolve, by indirect
equilibration (§ 167), such modifications as are shown in Figs. 92,
93, 94; the first of which is a slight advance on the ideal type
represented in Fig. 76, arising in the way described; and the others
of which are actual plants--_Haplomitrium Hookeri_, and _Plagiochila
decipiens_. Thus the higher Archegoniates show us how, along with
an assumption of the upright attitude, there does go on, as we see
there must go on, a separation of the leaf-producing parts from the
root-producing parts; a greater development of that connecting portion
of the successive fronds, by which they are kept in communication with
the roots, and raised above the ground; and a consequent increased
differentiation of such connecting portion from the parts attached
to it. And this lateral bulging of the axis, directly or indirectly
consequent on its functions as a support and a channel, being here
unrestrained by the early-formed fronds folded round it, goes on
without the bursting of these. Hence arises a leading character of
what is called exogenous growth--a growth which is, however, still
habitually accompanied by exfoliation, in flasks, of the outermost
layers, continually being cracked and split by the accumulation of
layers within them. And now if we examine plants of the exogenous type,
we find among them many displaying the stages of this metamorphosis. In
Fig. 95, is shown a form in which the continuity of the axis with the
mid-rib of the leaf, is manifest--a continuity that is conspicuous in
the common thistle. Here the foliar expansion, running some distance
down the axis, makes the included portion of the axis a part of its
mid-rib; just as in the ideal types above drawn. By the greater growth
of the internodes, which are very variable, not only in different
plants but in the same plant, there results a modification like that
delineated in Fig. 96. And then, in such forms as Fig. 97, there is
shown the arrangement that arises when, by more rapid development of
the proximal end of the mid-rib, the distal part of the foliar surface
is separated from the part which embraces the axis: the wings of the
mid-rib still serving, however, to connect the two portions of the
foliar surface. Such a separation is, as pointed out in § 188, an
habitual occurrence; and in some compound leaves, an actual tearing
of the inter-venous tissue is caused by extra growth of the mid-rib.
Modifications like this, and the further one in Fig. 98, we may expect
to be established by survival of the fittest, among those plants
which produce considerable masses of leaves; since the development of
mid-ribs into foot-stalks, by throwing the leaves further away from
the axes, will diminish the shading of the leaves, one by another. And
then, among plants of bushy growth, in which the assimilating surfaces
become still more liable to intercept one another’s light, natural
selection will continue to give an advantage to those which carry their
assimilating surfaces at the ends of the petioles, and do not develop
assimilating surfaces close to the axis, where they are most shaded.
Whence will result a disappearance of the stipules and the foliar
fringes of the mid-ribs; ending in the production of the ordinary
stalked leaf, Fig. 99, which is characteristic of trees. Meanwhile, the
axis thickens in proportion to the number of leaves it has to carry,
and to put in communication with the roots; and so there comes to be a
more marked contrast between it and the petioles, severally carrying a
leaf each.[12]


§ 194. When, in the course of the process above sketched out, there has
arisen such community of nutrition among the fronds thus integrated
into a series, that the younger ones are aided by materials which the
older ones have elaborated; the younger fronds will begin to show, at
earlier and earlier periods of development, the structures about to
originate from them. Abundant nutrition will abbreviate the intervals
between the successive prolifications; so that eventually, while each
frond is yet imperfectly formed, the rudiment of the next will begin to
show itself. All embryology justifies this inference. The analogies it
furnishes lead us to expect that when this serial arrangement becomes
organic, the growing part of the series will show the general relations
of the forthcoming parts, while they are very small and unspecialized.
What will in such case be the appearances they assume? We shall have no
difficulty in perceiving what it will be, if we take a form like that
shown in Fig. 92, and dwarf its several parts at the same time that we
generalize them. Figs. 100, 101, 102, and 103, will show the result;
and in Fig. 104, which is the bud of a dicotyledon, we see how clear is
the morphological correspondence: _a_ being the rudiment of a foliar
organ beginning to take shape; _b_ being the almost formless rudiment
of the next foliar organ; and _c_ being the quite-undifferentiated
part whence the rudiments of subsequent foliar organs are to arise.

  [Illustration: Figs. 100–104.]

  [Illustration: Figs. 105–106.]

And now we are prepared for entering on a still-remaining question
respecting the structure of Phænogams--what is the origin of axillary
buds? As the synthesis at present stands, it does not account for
these; but on looking a little more closely into the matter, we shall
find that the axillary buds are interpretable in the same manner as the
terminal buds. So to interpret them, however, we must return to that
process of proliferous growth with which we set out, for the purpose
of observing some facts not before named. _Delesseria hypoglossum_,
Fig. 105, represents a seaweed of the same genus as one outlined in
Fig. 40; but of a species in which proliferous growth is carried much
further. Here, not only does the primary frond bud out many secondary
fronds from its mid-rib; but most of the secondary fronds similarly bud
out several tertiary fronds; and even by some of the tertiary fronds,
this prolification is repeated. Besides being shown that the budding
out of several fronds from one frond, may become habitual; we are also
shown that it may become a habit inherited by the fronds so produced,
and also by the fronds they produce: the manifestation of the tendency
being probably limited only by failure of nutrition. That under fit
conditions an analogous mode of growth will occur in fronds of the
acrogenic type, like those we set out with, is shown by the case of
_Metzgeria furcata_, Figs. 45, 46, in which such compound prolification
is partially displayed. Let us suppose, then, that the frond _a_,
Fig. 106, produces not only a single secondary frond _b_, but also
another such secondary frond _b′_. Let us suppose, further, that the
frond _b_ is in like manner doubly proliferous: producing both _c_
and _c′_. Lastly, let us suppose that in the second frond _b′_ which
_a_ produces, as well as in the second frond _c′_ which _b_ produces,
the doubly-proliferous habit is manifested. If, now, this habit grows
organic--if it becomes, as it naturally will become, the characteristic
of a plant of luxuriant growth, the unfolding parts of which can be
fed by the unfolded parts; it will happen with each lateral series,
as with the main series, that its successive components will begin to
show themselves at earlier and earlier stages of development. And in
the same way that, by dwarfing and generalizing the original series, we
arrive at a structure like that of the terminal bud; by dwarfing and
generalizing a lateral series, as shown in Figs. 107–110, we arrive at
a structure answering in nature and position to the axillary bud.

  [Illustration: Figs. 107–110.]

Facts confirming these interpretations are afforded by the structure
and distribution of buds. The phænogamic axis in its primordial form,
being an integrated series of folia; and the development of that part
by which these folia are held together at considerable distances
from one another, taking place afterwards; it is inferable from the
general principles of embryology, that in its rudimentary stages, the
phænogamic shoot will have its foliar parts more clearly marked out
than its axial parts. This we see in every bud. Every bud consists
of the rudiments of leaves packed together without any appreciable
internodal spaces; and the internodal spaces begin to increase with
rapidity, only when the foliar organs have been considerably developed.
Moreover, where nutrition falls short, and arrest of development
takes place--that is, where a flower is formed--the internodes remain
undeveloped: the unfolding ceases before the later-acquired characters
of the phænogamic shoot are assumed. Lastly, as the hypothesis leads us
to expect, axillary buds make their appearances later than the foliar
organs which they accompany; and where, as at the ends of shoots, these
foliar organs show failure of chlorophyll, the axillary buds are not
produced at all. That these are inferable traits of structure, will be
manifest on inspecting Figs. 106–110; and on observing, first, that
the doubly-proliferous tendency of which the axillary bud is a result,
implies abundant nutrition; and on observing, next, that the original
place of secondary prolification, is such that the foliar surface on
which it occurs, must grow to some extent before the bud appears.

On thus looking at the matter--on contemplating afresh the ideal type
shown in Fig. 106, and noting how, by the conditions of the case, the
secondary prolifications must cease before that primary prolification
which produces the main axis; we are enabled to reconcile all the
phenomena of axillary gemmation. We see harmony among the several
facts--first, that the axillary bud becomes a lateral, leaf-bearing
axis if there is abundant material for growth; second, that its
development is arrested, or it becomes a flower-bearing axis, if the
supply of sap is but moderate; third, that it is absent when the
nutrition is failing. We are no longer committed to the gratuitous
assumption that, in the phænogamic type, there must exist an axillary
bud to each foliar organ; but we are led to conclude, _à priori_,
that which we find, _à posteriori_, that axillary buds are as normally
absent in flowers as they are normally present lower down the axis. And
then, to complete the argument, we are prepared for the corollary that
axillary prolification may naturally arise even at the ends of axes,
should the failing nutrition which causes the dwarfing of the foliar
organs to form a flower, be suddenly changed into such high nutrition
as to transform the components of the flower into appendages that are
green, if not otherwise leaf-like--a condition under which only, this
phenomenon is proved to occur.


§ 195. One more question presents itself, when we contrast the early
stages of development in the two classes of Phænogams; and a further
answer, supplied by the hypothesis, gives to the hypothesis a further
probability. It is characteristic of a monocotyledon, to have a single
seed-leaf or cotyledon; and it is characteristic of a dicotyledon, to
have at least two cotyledons, if not more than two. That is to say, the
monocotyledonous mode of germination everywhere co-exists with the
endogenous mode of growth; and along with the exogenous mode of growth, there always goes either a dicotyledonous or
polycotyledonous germination. Why is this? Such correlations cannot be
accidental--cannot be meaningless. A true theory of the phænogamic
types in their origin and divergence, should account for the connexion
of these traits. Let us see whether the foregoing theory does this.

The higher plants, like the higher animals, bequeath to their offspring
more or less of nutriment and structure. Superior organisms of either
kingdom do not, as do all inferior organisms, cast off their progeny
in the shape of minute portions of protoplasm, unorganized and without
stocks of material for them to organize; but they either deposit
along with the germs they cast off, certain quantities of albuminoid
substance to be appropriated by them while they develop themselves,
or else they continue to supply such substance while the germs
partially develop themselves before their detachment. Among plants
this constitutes one distinction between seeds and spores. Every seed
contains a store of food to serve the young plant during the first
stages of its independent life; and usually, too, before the seed is
detached, the young plant is so far advanced in structure, that it
bears to the attached stock of nutriment much the same relation that
the young fish bears to the appended yelk-bag at the time of leaving
the egg. Sometimes, indeed, the development of chlorophyll gives the
seed-leaves a bright green, while the seed is still contained in the
parent-pod. This early organization of the phænogam must be supposed
rudely to indicate the type out of which the phænogamic type arose.
On the foregoing hypothesis, the seed-leaves therefore represent the
primordial fronds; which, indeed, they simulate in their simple,
cellular, unveined structures. And the question here to be asked is--do
the different relations of the parts in young monocotyledons and
dicotyledons correspond with the different relations of the primordial
fronds, implied by the endogenous and the exogenous modes of growth? We
shall find that they do.

  [Illustration: Figs. 111–122.]

Starting, as before, with the proliferous form shown in Fig. 111, it
is clear that if the strength required for maintaining the vertical
attitude, is obtained by the rolling up of the fronds, the primary
frond will more and more conceal the secondary frond within it. At the
same time, the secondary frond must continue to be dependent on the
first for its nutrition; and, being produced within the first, must
be prevented by defective supply of light and air, from ever becoming
synchronous in its development with the first. Hence, this infolding
which leads to the endogenous mode of growth, implies that there must
always continue such pre-eminence of the first-formed frond or its
representative, as to make the germination monocotyledonous. Figs.
111 to 115, show the transitional forms that would result from the
infolding of the fronds. In Fig. 116 (a vertical section of the form
represented in Fig. 115) are exhibited the relations of the successive
fronds to each other. The modified relations that would result, if
the nutrition of the embryo admitted of anticipatory development of
the successive fronds, are shown in Fig. 117. And how readily the
structure may pass into that of the monocotyledonous germ, will be
seen on inspecting Fig. 118; which is a vertical section of an actual
monocotyledon at an early stage--the incomplete lines at the left of
its root, indicating its connexion with the seed.[13] Contrariwise,
where the strength required for maintaining an upright attitude is not
obtained by the rolling up of the fronds, but by the strengthening
of the continuous mid-rib, the second frond, so far from being less
favourably circumstanced than the first, becomes in some respects even
more favourably circumstanced: being above the other, it gets a greater
share of light, and it is less restricted by surrounding obstacles.
There is nothing, therefore, to prevent it from rapidly gaining an
equality with the first. And if we assume, as the truths of embryology
entitle us to do, an increasing tendency towards anticipation in the
development of subsequent fronds--if we assume that here, as in other
cases, structures which were originally produced in succession will,
if the nutrition allows and no mechanical dependence hinders, come to
be produced simultaneously; there is nothing to prevent the passage of
the type represented in Fig. 111, into that represented in Fig. 122.
Or rather, there is everything to facilitate it; seeing that natural
selection will continually favour the production of a form in which the
second frond grows in such way as not to shade the first, and in such
way as allows the axis readily to assume a vertical position.

Thus, then, is interpretable the universal connexion between
monocotyledonous germination and endogenous growth; as well as
the similarly-universal connexion between exogenous growth and
the development of two or more cotyledons. That it explains these
fundamental relations, adds very greatly to the probability of the
hypothesis.


§ 196. While we are in this manner enabled to discern the kinship that
exists between the higher vegetal types themselves, as well as between
them and the lower types; we are at the same time supplied with a
rationale of those truths which vegetal morphologists have established.
Those homologies which Wolff indicated in their chief outlines and
Goethe followed out in detail, have a new meaning given to them when we
regard the phænogamic axis as having been evolved in the way described.
Forming the modified conception which we are here led to do, respecting
the units of which a flowering plant is composed, we are no longer
left without an answer to the question--What is an axis? And we are
helped to understand the naturalness of those correspondences which the
successive members of each shoot display. Let us glance at the facts
from our present standpoint.

  [Illustration: Figs. 123–129.]

The unit of composition of a Phænogam, is such portion of a shoot as
answers to one of the primordial fronds. This portion is neither one
of the foliar appendages nor one of the internodes; but it consists of
a foliar appendage together with the preceding internode, including
the axillary bud where this is developed. The parts intercepted by
the dotted lines in Fig. 123, constitute such a segment; and the true
homology is between this and any other foliar organ with the portion
of the axis below it. And now observe how, when we take this for the
unit of composition, the metamorphoses which the phænogamic axis
displays, are inferable from known laws of development. Embryology
teaches us that arrest of development shows itself first in the absence
of those parts that have arisen latest in the course of evolution;
that if defect of nutrition causes an earlier arrest, parts that are
of more ancient origin abort; and that the part alone produced when
the supply of materials fails near the outset, is the primordial
part. We must infer, therefore, that in each segment of a Phænogam,
the foliar organ, which answers to the primordial frond, will be the
most constant element; and that the internode and the axillary bud,
will be successively less constant. This we find. Along with a smaller
size of foliar surface implying lower nutrition, it is usual to see a
much-diminished internode and a less-pronounced axillary bud, as in
Fig. 124. On approaching the flower, the axillary bud disappears; and
the segment is reduced to a small foliar surface, with an internode
which is in most cases very short if not absent, as in 125 and 126.
In the flower itself, axillary buds and internodes are both wanting:
there remains only a foliar surface (127), which, though often larger
than the immediately-preceding foliar surface, shows failing nutrition
by absence of chlorophyll. And then, in the quite terminal organs of
fructification (129), we have the foliar part itself reduced to a
mere rudiment. Though these progressive degenerations are by no means
regular, being in many cases varied by adaptations to particular
requirements, yet it cannot, I think, be questioned, that the general
relations are as described, and that they are such as the hypothesis
leads us to expect. Nor are we without a kindred explanation of certain
remaining traits of foliar organs in their least-developed forms.
Petals, stamens, pistils, &c., besides reminding us of the primordial
fronds by their diminished sizes, and by the want of those several
supplementary parts which the preceding segments possess, also remind
us of them by their histological characters: they consist of simple
cellular tissue, scarcely at all differentiated. The fructifying cells,
too, which here make their appearance, are borne in ways like those in
which the lower Acrogens bear them--at the edge of the frond, or at the
end of a peduncle, or immersed in the general substance; as in Figs.
128 and 129. Nay, it might even be said that the colours assumed by
these terminal folia, call to mind the plants out of which we conclude
that Phænogams have been evolved; for it is said of the fronds of the
_Jungermanniaceæ_, that, “though under certain circumstances of a pure
green, they are inclined to be shaded with red, purple, chocolate, or
other tints.”

As thus understood, then, the homologies among the parts of the
phænogamic axis are interpretable, not as due to a needless adhesion
to some typical form or fulfilment of a predetermined plan; but as
the inevitable consequences of the mode in which the phænogamic axis
originates.


§ 197. And now it remains only to observe, in confirmation of
the foregoing synthesis, that it at once explains for us various
irregularities. When we see leaves sometimes producing leaflets from
their edges or extremities, we recognize in the anomaly a resumption of
an original mode of growth: fronds frequently do this. When we learn
that a flowering plant, as the _Drosera intermedia_, has been known to
develop a young plant from the surface of one of its leaves, we are
at once reminded of the proliferous growths and fructifying organs in
the Liverworts. The occasional production of bulbils by Phænogams,
ceases to be so surprising when we find it to be habitual among the
inferior Acrogens, and when we see that it is but a repetition,
on a higher stage, of that self-detachment which is common among
proliferously-produced fronds. Nor are we any longer without a solution
of that transformation of foliar organs into axial organs, which not
uncommonly takes place. How this last irregularity of development is to
be accounted for, we will here pause a moment to consider. Let us first
glance at our data.

The form of every organism, we have seen, must depend on the structures
of its physiological [or constitutional] units. Any group of such
units will tend to arrange itself into the complete organism, if
uncontrolled and placed in fit conditions. Hence the development of
fertilized germs; and hence the development of those self-detached
cells which characterize some plants. Conversely, physiological units
which form a small group involved in a larger group, and are subject to
all the forces of the larger group, will become subordinate in their
structural arrangements to the larger group--will be co-ordinated into
a part of the major whole, instead of co-ordinating themselves into a
minor whole. This antithesis will be clearly understood on remembering
how, on the one hand, a small detached part of a hydra soon moulds
itself into the shape of an entire hydra; and how, on the other hand,
the cellular mass that buds out in place of a lobster’s lost claw,
gradually assumes the form of a claw--has its parts so moulded as to
complete the structure of the organism: a result which we cannot but
ascribe to the forces which the rest of the organism exerts upon it.
Consequently, among plants, we may expect that whether any portion of
protoplasm moulds itself into the typical form around an axis of its
own, or is moulded into a part subordinate to another axis, will depend
on the relative mass of its physiological units--the accumulation of
them that has taken place before the assumption of any structural
arrangement. A few illustrations will make clear the validity of this
inference. In the compound leaf, Fig. 65, the several lateral growths
_a_, _b_, _c_, _d_, are manifestly homologous; and on comparing a
number of such leaves together, it will be seen that one of these
lateral growths may assume any degree of complexity, according to the
degree of its nutrition. Every fern-leaf exemplifies the same general
truth still better. Whether each sub-frond remains an undeveloped
wing of the main frond, or whether it organizes itself into a group
of frondlets borne by a secondary rib, or whether, going further, as
it often does, it gives rise to tertiary ribs bearing frondlets, is
determined by the supply of materials for growth; since such higher
developments are most marked at points where the nutrition is greatest;
namely, next the stem. But the clearest evidence is afforded among the
_Algæ_, which, not drawing nutriment from roots, have their parts much
less mutually dependent; and are therefore capable of showing more
clearly, how any part may remain an appendage or may become the parent
of appendages, according to circumstances. In the annexed Fig. 130,
representing a branch of _Ptilota plumosa_, we see how a wing grows
into a wing-bearing branch if its nutrition passes a certain point.
This form, so strikingly like that of the feathery crystallizations of
many inorganic substances, implies that, as in such crystallizations,
the simplicity or complexity of structure at any place depends on the
quantity of matter that has to be arranged at that place in a given
time.[14]

  [Illustration: Fig. 130.]

Hence, then, we are not without an interpretation of those
over-developments which the phænogamic axis occasionally undergoes.
Fig. 104, represents the phænogamic bud in its rudimentary state. The
lateral process _b_, which ordinarily becomes a foliar appendage,
differs very little from the terminal process _c_, which is to become
an axis--differs mainly in having, at this period when its form is
being determined, a smaller bulk. If while thus undifferentiated, its
nutrition remains inferior to that of the terminal process, it becomes
moulded into a part that is subordinate to the general axis. But if,
as sometimes happens, there is supplied to it such an abundance of
the materials needful for growth, that it becomes as large as the
terminal process; then we may naturally expect it to begin moulding
itself round an axis of its own: a foliar organ will be replaced by
an axial organ. And this result will be especially liable to occur,
when the growth of the axis has been previously undergoing that arrest
which leads to the formation of a flower; that is when, from defect
of materials, the terminal process has almost ceased to increase, and
when some concurrence of favourable causes brings a sudden access of
sap which reaches the lateral processes before it reaches the terminal
process.[15]


§198. The general conclusion to which these various lines of evidence
converge, is, then, that the shoot of a flowering plant is an aggregate
of the third degree of composition. Taking as aggregates of the first
order, those small portions of protoplasm which ordinarily assume
the forms under which they are known as cells; and considering as
aggregates of the second order, those assemblages of such cells which,
in the lower cryptogams, compose the various kinds of thallus; then
that structure, common to the higher cryptogams and to phænogams,
in which we find a series of such groups of cells bound up into a
continuous whole, must be regarded as an aggregate of the third
order. The inference drawn from analysis, and verified by a synthesis
which corresponds in a remarkable manner with the facts, is that
those compound parts which, in Monocotyledons and Dicotyledons are
called axes, have really arisen by integration of such simple parts
as in lower plants are called fronds. Here, on a higher level,
appears to have taken place a repetition of the process already
observed on lower levels. The formation of those small groups of
physiological units which compose the lowest protophytes, is itself
a process of integration; and the consolidation of such groups into
definitely-circumscribed and coherent cells or morphological units, is
a completing of the process. In those coalescences by which many such
cells are joined into threads, and discs, and solid or flattened-out
masses, we see these morphological units aggregating into units of a
compound kind: the different phases of the transition being exemplified
by groups of various sizes, various degrees of cohesion, and various
degrees of definiteness. And now we find evidences of a like process
on a larger scale: the compound groups are again compounded. Moreover,
as before, there are not wanting types of organization by which the
stages of this higher integration are shadowed forth. From fronds that
occasionally produce other fronds from their surfaces, we pass to those
that habitually produce them; from those that do so in an indefinite
manner, to those that do so in a definite manner; and from those that
do so singly, to those that do so doubly and triply through successive
generations of fronds. Even within the limits of a sub-class, we find
gradations between fronds irregularly proliferous, and groups of such
fronds united into a regular series.

Nor does the process end here. The flowering plant is rarely
uniaxial--it is nearly always multiaxial. From its primary shoot there
grow out secondary shoots of like kind. Though occasionally among
Phænogams, and frequently among the higher Cryptogams, the germs of
new axes detach themselves under the form of bulbils, and develop
separately instead of in connexion with the parent axis; yet in most
Phænogams the germ of each new axis maintains its connexion with the
parent axis: whence results a group of axes--an aggregate of the fourth
order. Every tree, by the production of branch out of branch, shows
us this integration repeated over and over again; forming an aggregate
having a degree of composition too complex to be any longer defined.

       *       *       *       *       *

[NOTE.--A criticism passed on the general argument set forth in the
foregoing sections, runs as follows:--“I have already pointed out that
the process of evolution by which you believe the Liverworts with a
distinct axis and appendages to have been produced from the thalloid
forms is not founded on sound evidence either in comparative morphology
or development. But even if we admit that such an integration of a
proliferously-produced colony might have given rise to the leafy
_Jungermanniaceæ_, there are even more weighty objections to the
supposition that the same process produced the shoot structures of
the flowering plants. In the first place the flowering plant-body is
_not homologous with the liverwort plant-body_, since they represent
different generations. The liverwort plant-body or _gametophyte_,
_i.e._, the generation bearing sexual organs, is homologous with the
prothallus of ferns and other Pteridophytes, and in the Flowering
Plants with reduced structures contained within the spores (embryo-sac
and pollen-grain) but still giving rise to sexual cells. The liverwort
spore-capsule and its accessory parts (in fact everything produced
from the fertilized egg) is homologous with the sporogonium of the
mosses, and, as most botanists think, with the leafy plant-body
of Pteridophytes and Phanerogams. This generation is called the
_sporophyte_ and from the spores which it produces are developed
the gametophytes of the next generation. These generalizations were
first established by Hofmeister, and all subsequent work has tended
to establish them more firmly. The only doubtful question is (and the
doubt is mainly, I think, peculiar to myself, certainly not being
shared by the majority of botanists) whether the sporophyte of Mosses
and Liverworts is really homologous with that of Pteridophytes and
Phanerogams, whether it may not rather be regarded as a parallel
development along another line of descent from the Green Algæ.

“Hence we must look for the origin of the shoot-structure of flowering
plants in the sporophytes of the Pteridophytes, from which group there
is no reason to doubt that the phanerogams have arisen in descent.
The various groups of Pteridophytes vary much in the organization of
these shoot-systems, as a mental glance at the types exhibited by the
Ferns, Horse-tails, Club-mosses, _Ophioglossaceæ_, and the isolated
Isoetes will convince you at once. It may be that some of these groups
are independent in descent, _i.e._, that the _Pteridophyta_ are
polyphyletic, and the current hypothesis with regard to the phanerogams
is that they have arisen by two, if not three, separate lines of
descent from different groups of Pteridophytes (this is indicated in
the classificatory diagram on p. 377 of vol. I). I should not, however,
care to pin my faith to these or to any such lines of ancestry. Still
I think we must look for the ancestors of the Flowering Plants among
the Pteridophytes, and the latter always have a good distinction
between axis and appendages. The problem of the evolution of these
differentiated sporophytic shoots is undoubtedly the great outstanding
problem of morphology. Various attempts have been made to solve it,
of which probably the most important is the theory of Profs. Bown
and Campbell, who derive the Pteridophytes from some Liverwort like
_Anthoceros_, but the sporophyte of course from the sporophytic portion
of the plant (not much more than a spore-capsule), the prothallus
of the Fern representing the vegetative thallus of Anthoceros. I am
not wholly convinced by these undoubtedly ingenious hypotheses, in
support of which an immense amount of facts have been collected; but
my position would, I know, simply ‘put us to ignorance again’ on this
question.

“I have discussed this at some length in order to bring out clearly
the immense difficulty of constructing a wellgrounded theory of
the origin of the differentiated shoot-system of the higher plant.
I confess I don’t think it can be done at all with the materials at
present at our disposal. Of course it is just possible to suppose that
some ancestral sporophyte had the structure of a proliferous thalloid
liverwort gametophyte, and that from it was evolved the phanerogamic
shoot in the ways you suggest. This gives us absolutely no clue,
however, to any Pteridophytic shoot, which ought to be intermediate
(more or less) between the hypothetical ancestor and the Phanerogam,
and is furthermore, as far as I can see, not supported by an atom of
evidence of any kind. It is true that your theory fits in well with the
phenomena exhibited by phanerogamic shoots themselves, but this fact
you will see must lose much of its significance if the hypothesis lacks
foundation.

“With regard to your method of explaining the fundamental characters
of ‘Exogens’ and ‘Endogens,’ this of course is part of the same
hypothesis; but I may point out that since Von Mohl and Sanio, between
1855 and 1865, showed (1) that the growth at the stem apex of a
monocotyledon was _not_ endogenous, and (2) that the ‘thickening ring’
near the apex of a dicotyledon was not to be confused, as had been
done up till then, with the ring of _secondary meristem_ or _true
cambium_, which arose lower down, and only in woody or practically
woody stem, the terms ‘Exogen’ and ‘Endogen’ have necessarily fallen
into disuse, since they imply a false conception of what happens. Both
monocotyledons and dicotyledons have a ‘thickening ring,’ which gives
rise to the primary vascular cylinder of the stem. When the stem is
of considerable thickness, as in Palms, &c., it grows by the active
cell-division of its outer layers, so that both classes are ‘exogenous’
in this sense; while the addition of a centrifugal zone of secondary
wood is confined to certain Dicotyledons (Trees, shrubs, &c.).

“The distinction between the embryos, moreover, is not absolute. The
single cotyledon is usually terminal in monocotyledons, but not always
(_Dioscoraceæ_ have lateral cotyledons), but the plumule may push
through it (Grasses) or make its exit sideways (Palms), or be formed at
the side (_Alisma_); and Dicotyledons very similarly.

“The occurrence of completely sheathing leaves in grasses is perhaps
correlated with the absence of cambium, but grasses are an aberrant
type among monocotyledons, and secondary thickening is only found in
very few genera of this class, so that the correlation is, so to speak,
negative and indirect.... It is clear that the greater part of the
discussion will have to be re-written.”

For the reasons assigned in the preface I cannot undertake to re-write
the discussion, as suggested. It must stand for what it is worth. All I
can do is here to include along with it the foregoing criticisms.

I may, however, indicate the line of defence I should take were I to
go again into the matter. The objections are based on the structure of
existing Liverworts and Phænogams. But I have already referred to the
probability--or, indeed, the certainty--that in conformity with the
general principle set forth in the note to Chapter I, we must conclude
that the early types of Liverworts out of which the Phænogams are
supposed to have evolved, as well as the early types of Phænogams in
which the stages of evolution were presented, no longer exist. We must
infer that forms simpler than any now known, and more intermediate in
their traits, were the forms concerned; and if so, it may be held that
the incongruities with the hypothesis which are presented by existing
forms, do not negative it. The scepticism my critic himself expresses
respecting the current interpretation is a partial justification of
this view. Moreover, his admission that the theory set forth “fits in
well with the phenomena exhibited by phanerogamic shoots,” must, I
think, be regarded as weighty evidence. On the Evolution hypothesis
we are obliged to suppose that the Monocotyledons and Dicotyledons
respectively arose by integration of fronds; and if to the question
after what manner the integration took place, there is an hypothesis
which renders it comprehensible, and agrees both with the structures of
the two kinds of shoots and the structures of the two kinds of seeds,
as well as with various of the other phenomena the two types present,
it has strong claims for acceptance.

Reconsideration suggests the following remarks.

1. Alternation of generations is a means of furthering multiplication.
To be effective each member of either generation must be a
self-supporting centre of growth or diffusion or both. Hence if, as in
the Liverworts, one of the so-called alternating generations is not
independent, but a permanent growth on the other--a parasite--it is a
misuse of words to call the arrangement Alternation of generations.
(Since this was written I have found that Sir Edward Fry takes the
same view. He approvingly quotes Professor Bower, who says that “the
alternation of generations is not an accurate statement of facts or a
useful analogy.”)

2. The alternating of sexual and non-sexual processes is not
fundamentally distinctive; for, as shown by sundry Archegoniates,
it is an inconstant trait, and as shown by Klebs’ experiments on
_Vaucheria_, the conditions may be varied so as to determine its
occurrence or non-occurrence. Nay, the same individual may reproduce in
either way.

3. Still more significant is the fact that in some of the marine
Thallophytes, there is a process like that which in a moss or a fern
is considered an alternation of generations, whereas in others, as
the Brown Wrack (_Fucus_), each generation is sexual. Thus the
presence or absence of this mode of genesis cannot be a cardinal
distinction.

4. With these facts before us, it is not only a reasonable supposition
but a highly probable supposition, that there have existed plants of
the Liverwort type in which the so-called alternation of generations
did not take place. If so, nearly all the foregoing objections to my
hypothesis fall to the ground.]




                              CHAPTER IV.

               THE MORPHOLOGICAL COMPOSITION OF ANIMALS.


§ 199. What was said in § 180, respecting the ultimate structure of
organisms, holds more manifestly of animals than of plants. That
throughout the vegetal kingdom the cell is the morphological unit, is
a proposition admitting of a better defence, than the proposition that
the cell is the morphological unit throughout the animal kingdom. The
qualifications with which, as we saw, the cell-doctrine must be taken,
are qualifications thrust upon us more especially by the facts which
zoologists have brought to light. It is among the _Protozoa_ that
there occur numerous cases of vital activity displayed by specks of
protoplasm; and from the minute anatomy of all creatures above these,
are drawn the numerous proofs that non-cellular tissues may arise by
direct metamorphosis of mixed colloidal substances.[16]

Our survey of morphological composition throughout the animal kingdom,
must therefore begin with those undifferentiated aggregates of
physiological units [or constitutional units], out of which are formed
what we call, with considerable license, morphological units.


§ 200. In that division of the _Protozoa_ distinguished as _Rhizopoda_,
are presented, under various modifications, these minute portions of
living organic matter, so little differentiated, if not positively
undifferentiated, that animal individuality can scarcely be claimed
for them. Figs. 131, 132, and 133, represent certain nearly-allied
types of these--_Amœba_, _Actinophrys_, and _Lieberkühnia_. The
viscid jelly or sarcode, comparable in its physical properties to
white of egg, out of which one of these creatures is mainly formed,
shows us in various ways, the feebleness with which the component
physiological units are integrated--shows us this by its very slight
cohesion, by the extreme indefiniteness and mutability of its form,
and by the absence of a limiting membrane. It is no longer held even
by unqualified adherents of the cell-doctrine that the _Amœba_ has an
investment. Its outer surface, compared to the film which forms on
the surface of paste, does not prevent the taking of solid particles
into the mass of the body, and does not, in such kindred forms as Fig.
133, prevent the pseudopodia from coalescing when they meet. Hence it
cannot properly have the name of a cell-wall. A considerable portion
of the body, however, in _Difflugia_, Fig. 134, has a denser coating
formed of agglutinated foreign particles; so that the protrusion of
the pseudopodia is limited to one part of it. And in the solitary
_Foraminifera_, like _Gromia_, the sarcode is covered over most of its
surface by a delicate calcareous shell, pierced with minute holes,
through which the slender pseudopodia are thrust. The _Gregarina_
exhibits an advance in integration, and a consequent greater
definiteness. Figs. 135 and 136, exemplifying this type, show the
complete membrane in which the substance of the creature is contained.
Here there has arisen what may be properly called a cell: under its
solitary form this animal is truly unicellular. Its embryology has
considerable significance. After passing through a certain quiescent,
“encysted” state, its interior breaks up into small portions, which,
after their exit, assume forms like that of the _Amœba_; and from
this young condition in which they are undifferentiated, they pass
into that adult condition in which they have limiting membranes. If
this development of the individual _Gregarina_ typifies the mode of
evolution of the species, it yields further support to the belief, that
fragments of sarcode existed earlier than any of the structures which
are called cells. Among aggregates of the first order, there are some
much more highly developed. These are the _Infusoria_, constituting
the most numerous of the _Protozoa_, in species as in individuals.
Figs. 137, 138, and 139, are examples. In them we find, along with
greater definiteness, a considerable heterogeneity. The sarcode of
which the body consists, has an indurated outer layer, bearing cilia
and sometimes spines; there is an opening serving as mouth, a permanent
œsophagus, and a cavity or cavities, temporarily formed in the interior
of the sarcode, to serve as one or more stomachs; and there is a
comparatively specific arrangement of these and various minor parts.

  [Illustration: Figs. 131–139.]

Thus in the animal kingdom, as in the vegetal kingdom, there exists
a class of minute forms having this peculiarity, that no one of them
is separable into a number of visible components homologous with one
another--no one of them can be resolved into minor individualities.
Its proximate units are those physiological units of which we conclude
every organism consists. The aggregate is an aggregate of the first
order.


§ 201. Among plants are found types indicating a transition from
aggregates of the first order to aggregates of the second order; and
among animals we find analogous types. But the stages of progressing
integration are not here so distinct. The reason probably is, that
the simplest animals, having individualities much less marked than
those of the simplest plants, do not afford us the same facilities for
observation. In proportion as the limits of the minor individualities
are indefinite, the formation of major individualities out of them,
naturally leaves less conspicuous traces.

  [Illustration: Figs. 140–145.]

Be this as it may, however, in such types of _Protozoa_ as the compound
_Radiolaria_, we find that though there is reason to regard the
aggregate as an aggregate of the second order, yet its divisibility
into minor individualities like those just described, is less manifest.
Fig. 140 representing _Sphærozoum punctatum_, one of the group,
illustrates this. The sceptically-minded may perhaps doubt whether we
can regard the “cellæform bodies” contained in it, as the morphological
units of the animal. The jelly-like mass in which they are imbedded, is
but indefinitely divisible into portions having each a cell or nucleus
for its centre.[17] Among the _Foraminifera_, we find only indefinite
evidence of the coalescence of aggregates of the first order, into
aggregates of the second order. There are solitary Foraminifers,
allied to the creature represented in Fig. 134. Certain ideal types of
combination among them, are shown in Fig. 141. And setting out from
these, we may ascend in various directions to kinds compounded to
an immense variety of degrees in an immense variety of ways. In all
of them, however, the separability of the major individuality into
minor individualities, is very incomplete. The portion of sarcode
contained in one of these calcareous chambers, gives origin to an
external bud; and this presently becomes covered, like its parent,
with calcareous matter: the position in which each successive chamber
is so produced, determining the form of the compound shell. But the
portions of sarcode thus budded out one from another, do not become
distinctly individualized. Fig. 142, representing the living network
which remains when the shell of an Orbitolite has been dissolved,
shows the continuity that exists among the occupants of its aggregated
chambers.[18] In the compound _Infusoria_, the component units remain
quite distinct. Being, as aggregates of the first order, much more
definitely organized, their union into aggregates of the second
order does not destroy their original individualities. Among the
_Vorticellæ_, of which two kinds are delineated in Figs. 144 and 145,
there are various illustrations of this: the members of the community
being sometimes appended to a single stem; sometimes attached by long
separate stems to a common base; and sometimes massed together.

  [Illustration: Figs. 146–147.]

Thus far, these aggregates of the second order exhibit but indefinite
individualities. The integration is physical; but not physiological.
Though, in the _Polycytharia_, there is a shape that has some symmetry;
and though, in the _Foraminifera_, the formation of successive chambers
proceeds in such methodic ways as to produce quite-regular and
tolerably-specific shells; yet no more in these than in the Sponges or
the compound _Vorticellæ_, do we find such co-ordination as gives the
whole a life predominating over the lives of its parts. We have not
yet reached an aggregate of the second order, so individuated as to be
capable of serving as a unit in still higher combinations. But in the
class _Cœlenterata_, this advance is displayed. The common _Hydra_,
habitually taken as the type of the lowest division of this class,
has specialized parts performing mutually-subservient functions, and
thus exhibiting a total life distinct from the lives of the units.
Fig. 146 represents one of these creatures in its contracted state
and in its expanded state; while Fig. 147 is a diagram showing the
wall of this creature’s sac-like body as seen in section under the
microscope: _a_ and _b_ being the outer and inner cellular layers;
while between them is the “mesoglœa” or “structureless lamella,” the
supporting or skeletal layer. But this lowly-organized tissue of the
Hydra, illustrates a phase of integration in which the lives of the
minor aggregates are only partially-subordinated to the life of the
major aggregate formed by them. For a _Hydra’s_ substance is separable
into _Amœba_-like portions, capable of moving about independently.
If we bear in mind how analogous are the extreme extensibility and
contractility of a _Hydra’s_ body and tentacles, to the properties
displayed by the sarcode among Rhizopods; we may infer that probably
the movements and other actions of a _Hydra_, are due to the
half-independent co-operation of the _Amœba_-like individuals composing
it.


§ 202. A truth which we before saw among plants, we here see repeated
among animals--the truth that as soon as the integration of aggregates
of the first order into aggregates of the second order, produces
compound wholes so specific in their shapes and sizes, and so mutually
dependent in their parts, as to have distinct individualities; there
simultaneously arises the tendency in them to produce, by gemmation,
other such aggregates of the second order. The approach towards
definite limitation in an organism, is, by implication, an approach
towards a state in which growth passing a certain point, results, not
in the increase of the old individual, but in the formation of a new
individual. Thus it happens that the common polype buds out other
polypes, some of which very shortly do the like, as shown in Fig. 148:
a process paralleled by the fronds of sundry _Algæ_, and by those of
the lower _Jungermanniaceæ_. And just as, among these last plants,
the proliferously-produced fronds, after growing to certain sizes
and developing rootlets, detach themselves from their parent fronds;
so among these animals, separation of the young ones from the bodies
of their parents ensues when they have acquired tolerably complete
organizations.

  [Illustration: Figs. 148–150.]

There is reason to think that the parallel holds still further.
Within the limits of the _Jungermanniaceæ_, we found that while some
genera exhibit this discontinuous development, other genera exhibit
a development that is similar to it in all essential respects, save
that it is continuous. And here within the limits of the _Hydrozoa_,
we find, along with this genus in which the gemmiparous individuals
are presently cast off, other genera in which they are not cast off,
but form a permanent aggregate of the third order. Figs. 149 and 150,
exemplify these compound _Hydrozoa_--one of them showing this mode
of growth so carried out as to produce a single axis; and the other
showing how, by repetitions of the process, lateral axes are produced.
Integrations characterizing certain higher genera of the _Hydrozoa_
which swim or float instead of being fixed, are indicated by Figs. 151
and 152: the first of them representing the type of a group in which
the polypes growing from an axis, or cœnosarc, are drawn through the
water by the rhythmical contractions of the organs from which they
hang; and the second of them representing a _Physalia_ the component
polypes of which are united into a cluster, attached to an air-vessel.

  [Illustration: Figs. 151–152.]

A parallel series of illustrations might be drawn from that second
division of the _Cœlenterata_, known as the _Actinozoa_. Here, too,
we have a group of species--the Sea-anemones--the individuals of
which are solitary. Here, too, we have agamogenetic multiplication:
occasionally by gemmation, but more frequently by that modified process
called spontaneous fission. And here, too, we have compound forms
resulting from the arrest of this spontaneous fission before it is
complete. To give examples is needless; since they would but show, in
more varied ways, the truth already made sufficiently clear, that the
compound _Cœlenterata_ are aggregates of the third order, produced by
integration of aggregates of the second order such as we have in the
_Hydra_. As before, it is manifest that on the hypothesis of evolution,
these higher integrations will insensibly arise, if the separation
of the gemmiparous polypes is longer and longer postponed; and that
an increasing postponement will result by survival of the fittest,
if it profits the group of individuals to remain united instead of
dispersing.[19]


§ 203. The like relations exist, and imply that the like processes
have been gone through, among those more highly organized animals
called _Polyzoa_ and _Tunicata_. We have solitary individuals, and we
have variously-integrated groups of individuals: the chief difference
between the evidence here furnished, and that furnished in the last
case, being the absence of a type obviously linking the solitary state
with the aggregated state.

  [Illustration: Figs. 153–155.]

This integration of aggregates of the second order, is carried on
among the _Polyzoa_ in divers ways, and with different degrees of
completeness. The little patches of minute cells, shown as magnified
in Fig. 153, so common on the fronds of sea-weeds and the surfaces of
rocks at low-water mark, display little beyond mechanical combination.
The adjacent individuals, though severally originated by gemmation
from the same germ, have but little physiological dependence. In
kindred kinds, however, as shown in Figs. 154 and 155, one of which
is a magnified portion of the other, the integration is somewhat
greater: the co-operation of the united individuals being shown in the
production of those tubular branches which form their common support,
and establish among them a more decided community of nutrition.

  [Illustration: Figs. 156–159.]

Among the Ascidians this general law of morphological composition is
once more displayed. Each of these creatures subsists on the nutritive
particles contained in the water which it draws in through one orifice
and sends out through another; and it may thus subsist either alone, or
in connexion with others that are in some cases loosely aggregated and
in other cases closely aggregated. Fig. 156, _Phallusia mentula_, is
one of the solitary forms. A type in which the individuals are united
by a stolon that gives origin to them by successive buds, is shown in
_Perophora_, Fig. 157. Among the _Botryllidæ_, of which one kind is
drawn on a small scale in Fig. 159, and a portion of the same on a
larger scale in Fig. 158, there is a combination of the individuals
into annular clusters, which are themselves imbedded in a common
gelatinous matrix. And in this group there are integrations even a
stage higher, in which several such clusters of clusters grow from
a single base. Here the compounding and recompounding appears to be
carried further than anywhere else in the animal kingdom.

Thus far, however, among these aggregates of the third order, we see
what we before saw among the simpler aggregates of the second order--we
see that the component individualities are but to a very small extent
subordinated to the individuality made up of them. In nearly all the
forms indicated, the mutual dependence of the united animals is so
slight, that they are more fitly comparable to societies, of which
the members co-operate in securing certain common benefits. There is
scarcely any specialization of functions among them. Only in the last
type described do we see a number of individuals so completely combined
as to simulate a single individual. And even here, though there appears
to be an intimate community of nutrition, there is no physiological
integration beyond that implied in several mouths and stomachs having a
common vent.[20]


§ 204. We come now to an extremely interesting question. Does there
exist in other sub-kingdoms composition of the third degree, analogous
to that which we have found so prevalent among the _Cœlenterata_ and
the _Polyzoa_ and _Tunicata_? The question is not whether elsewhere
there are tertiary aggregates produced by the branching or clustering
of secondary aggregates, in ways like those above traced; but whether
elsewhere there are aggregates which, though otherwise unlike in the
arrangement of their parts, nevertheless consist of parts so similar to
one another that we may suspect them to be united secondary aggregates.
The various compound types above described, in which the united animals
maintain their individualities so distinctly that the individuality
of the aggregate remains vague, are constructed in such ways that the
united animals carry on their several activities with scarcely any
mutual hindrance. The members of a branched _Hydrozoon_, such as is
shown in Fig. 149 or Fig. 150, are so placed that they can all spread
their tentacles and catch their prey as well as though separately
attached to stones or weeds. Packed side by side on a flat surface or
forming a tree-like assemblage, the associated individuals among the
_Polyzoa_ are not unequally conditioned: or if one has some advantage
over another in a particular case, the mode of growth and the relations
to surrounding objects are so irregular as to prevent this advantage
re-appearing with constancy in successive generations. Similarly
with the Ascidians growing from a stolon or those forming an annular
cluster: each of them is as well placed as every other for drawing in
the currents of sea-water from which it selects its food. In these
cases the mode of aggregation does not expose the united individuals
to multiform circumstances; and therefore is not calculated to produce
among them any structural multiformity. For the same reason no marked
physiological division of labour arises among them; and consequently no
combination close enough to disguise their several individualities. But
under converse conditions we may expect converse results. If there is a
mode of integration which necessarily subjects the united individuals
to unlike sets of incident forces, and does this with complete
uniformity from generation to generation, it is to be inferred that
the united individuals will become unlike. They will severally assume
such different functions as their different positions enable them
respectively to carry on with the greatest advantage to the assemblage.
This heterogeneity of function arising among them, will be followed by
heterogeneity of structure; as also by that closer combination which
the better enables them to utilize one another’s functions. And hence,
while the originally-like individuals are rendered unlike, they will
have their homologies further obscured by their progressing fusion into
an aggregate individual of a higher order.

These converse conditions are in nearly all cases fulfilled where
the successive individuals arising by continuous development are so
budded-off as to form a linear series. I say in nearly all cases,
because there are some types in which the associated individuals,
though joined in single file, are not thereby rendered very unlike
in their relations to the environment; and therefore do not become
differentiated and integrated to any considerable extent. I refer
to such Ascidians as the _Salpidæ_. These creatures float passively
in the sea, attached together in strings. Being placed side by side
and having mouths and vents that open laterally, each of them is as
well circumstanced as its neighbours for absorbing and emitting the
surrounding water; nor have the individuals at the two extremities any
marked advantages over the rest in these respects. Hence in this type,
and in the allied type _Pyrosoma_, which has its component individuals
built into a hollow cylinder, linear aggregation may exist without the
minor individualities becoming obscured and the major individuality
marked: the conditions under which a differentiation and integration
of the component individuals may be expected, are not fulfilled.
But where the chain of individuals produced by gemmation, is either
habitually fixed to some solid body by one of its extremities or moves
actively through the water or over submerged stones and weeds, the
several members of the chain become differently conditioned in the way
above described; and may therefore be expected to become unlike while
they become united. A clear idea of the contrast between these two
linear arrangements and their two diverse results, will be obtained by
considering what happens to a row of soldiers, when changed from the
ordinary position of a single rank to the position of Indian file. So
long as the men stand shoulder to shoulder, they are severally able
to use their weapons in like ways with like efficiency; and could,
if called on, similarly perform various manual processes directly
or indirectly conducive to their welfare. But when, on the word of
command “right face,” they so place themselves that each has one of
his neighbours before him and another behind him, nearly all of them
become incapacitated for fighting and for many other actions. They can
walk or run one after another, so as to produce movement of the file
in the direction of its length; but if the file has to oppose an enemy
or remove an obstacle lying in the line of its march, the front man
is the only one able to use his weapons or hands to much purpose. And
manifestly such an arrangement could become advantageous only if the
front man possessed powers peculiarly adapted to his position, while
those behind him facilitated his actions by carrying supplies, &c. This
simile, grotesque as it seems, serves to convey better perhaps than
any other could do, a clear idea of the relations that must arise in a
chain of individuals arising by gemmation, and continuing permanently
united end to end. Such a chain can arise only on condition that
combination is more advantageous than separation; and for it to be more
advantageous, the anterior members of the series must become adapted to
functions facilitated by their positions, while the posterior members
become adapted to functions which their positions permit. Hence, direct
or indirect equilibration or both, must tend continually to establish
types in which the connected individuals are more and more unlike
one another, at the same time that their several individualities are
more and more disguised by the integration consequent on their mutual
dependence.

Such being the anticipations warranted by the general laws of
evolution, we have now to inquire whether there are any animals which
fulfil them. Very little search suffices; for structures of the kind
to be expected are abundant. In that great division of the animal
kingdom at one time called _Annulosa_, but now grouped into _Annelida_
and _Arthropoda_, we find a variety of types having the looked-for
characters. Let us contemplate some of them.


§ 205. An adult Chætopod is composed of segments which repeat one
another in their details as well as in their general shapes. Dissecting
one of the lower orders, such as is shown in Fig. 160, proves that
the successive segments, besides having like locomotive appendages,
like branchiæ, and sometimes even like pairs of eyes, also have like
internal organs. Each has its enlargement of the alimentary canal;
each its contractile dilatation of the great blood-vessel; each its
portion of the double nervous cord, with ganglia when these exist;
each its branches from the nervous and vascular trunks answering to
those of its neighbours; each its similarly answering set of muscles;
each its pair of openings through the body-wall; and so on throughout,
even to the organs of reproduction. That is to say, every segment is
in great measure a physiological whole--every segment contains most
of the organs essential to individual life and multiplication: such
essential organs as it does not contain, being those which its position
as one in the midst of a chain, prevents it from having or needing. If
we ask what is the meaning of these homologies, no adequate answer is
supplied by any current hypothesis. That this “vegetative repetition”
is carried out to fulfil a predetermined plan, was shown to be quite
an untenable notion (§§ 133, 134). On the one hand, we found nothing
satisfactory in the conception of a Creator who prescribed to himself
a certain unit of composition for all creatures of a particular class,
and then displayed his ingenuity in building up a great variety of
forms without departing from the “archetypal idea.” On the other hand,
examination made it manifest that even were such a conception worthy
of being entertained, it would have to be relinquished; since in each
class there are numerous deviations from the supposed “archetypal
idea.” Still less can these traits of structure be accounted for
teleologically. That certain organs of nutrition and respiration and
locomotion are repeated in each segment of a dorsibranchiate annelid,
may be regarded as functionally advantageous for a creature following
its mode of life. But why should there be a hundred or even two
hundred pairs of ovaries? This is an arrangement at variance with that
physiological division of labour which every organism profits by--is
a less advantageous arrangement than might have been adopted. That
is to say, the hypothesis of a designed adaptation fails to explain
the facts. Contrariwise, these structural traits are just such as
might naturally be looked for, if these annulose forms have arisen by
the integration of simpler forms. Among the various compound animals
already glanced at, it is very general for the united individuals to
repeat one another in all their parts--reproductive organs included.
Hence if, instead of a clustered or branched integration, such as
the _Cœlenterata_, _Polyzoa_ and _Tunicata_ exhibit, there occurs a
longitudinal integration; we may expect that the united individuals
will habitually indicate their original independence by severally
bearing germ-producing or sperm-producing organs.

  [Illustration: Figs. 160–161.]

The reasons for believing one of these creatures to be an aggregate of
the third order, are greatly strengthened when we turn from the adult
structure to the mode of development. Among the _Dorsibranchiata_
and _Tubicolæ_, the embryo leaves the egg in the shape of a ciliated
gemmule, not much more differentiated than that of a polype. As shown
in Fig. 162, it is a nearly globular mass; and its interior consists
of untransformed cells. The first appreciable change is an elongation
and a simultaneous commencement of segmentation. The segments multiply
by a modified gemmation, which takes place from the hinder end of the
penultimate segment. And considerable progress in marking out these
divisions is made before the internal organization begins. Figs. 163,
164, 165, represent some of these early stages. In annelids of other
orders, the embryo assumes the segmented form while still in the
egg. But it does this in just the same manner as before. Indeed, the
essential identity of the two modes of development is shown by the fact
that the segmentation within the egg is only partially carried out: in
all these types the segments continue to increase in number for some
time after hatching. Now this process is as like that by which compound
animals in general are formed, as the different conditions of the case
permit. When new individuals are budded-out laterally, their unfolding
is not hindered--there is nothing to disguise either the process or the
product. But gemmæ produced one from another in the same straight line,
and remaining connected, restrict one another’s developments; and that
the resulting segments are so many gemmiparously-produced individuals,
is necessarily less obvious.

  [Illustration: Figs. 162–165.]


§ 206. Evidence remains which adds very greatly to the weight of
that already assigned. Thus far we have studied only the individual
segmented animal; considering what may be inferred from its mode of
evolution and final organization. We have now to study segmented
animals in general. Comparison of different groups of them and of
kinds within each group, will disclose various phases of progressive
integration of the nature to be anticipated.

  [Illustration: Figs. 166–169.]

Among the simpler _Platyhelminthes_, as in some kinds of _Planaria_,
transverse fission occurs. A portion of a _Planaria_ separated by
spontaneous constriction, becomes an independent individual. Sir J.
G. Dalyell found that in some cases numerous fragments artificially
separated, grew into perfect animals.[21] In these creatures which thus
remind us of the lowest _Hydrozoa_ in their powers of agamogenetic
multiplication, the individuals produced one from another do not
continue connected. As the young ones laterally budded-off by the
_Hydra_ separate when complete, so do the young ones longitudinally
budded-off by the _Planaria_. Fig. 166 indicates this. But there
are allied types which show us a more or less persistent union of
homologous parts, or individuals, similarly arising by longitudinal
gemmation.[22] The cestoid _Entozoa_ furnish illustrations. Without
dwelling on the fact that each segment of a _Tænia_, like each separate
_Planaria_, is an independent hermaphrodite; and without specifying the
sundry common structural traits which add probability to the suspicion
that there is some kinship between the individuals of the one order
and the segments of the other; it will suffice to point out that the
two types are so far allied as to demand their union under the same
sub-class title. And recognizing this kinship, we see significance
in the fact that in the one case the longitudinally-produced gemmæ
separate as complete individuals, and in the other continue united
as segments in smaller or larger numbers and for shorter or longer
periods. In _Tænia echinococcus_, represented in Fig. 167, we have
a species in which the number of segments thus united does not
exceed four. In _Echinobothrium typus_ there are eight or ten; and
in cestoids generally they are numerous.[23] A considerable hiatus
occurs between this phase of integration and the next higher phase
which we meet with; but it is not greater than the hiatus between the
types of the _Platyhelminthes_ and the _Chætopoda_, which present
the two phases. Though it is doubtful whether separation of single
segments occurs among the Annelida,[24] yet very often we find strings
of segments, arising by repeated longitudinal budding, which after
reaching certain lengths undergo spontaneous fission: in some cases
doing this so as to form two or more similar strings of segments
constituting independent individuals; and in other cases doing it so
that the segments spontaneously separated are but a small part of the
string. Thus a _Syllis_, Fig. 168, after reaching a certain length,
begins to transform itself into two individuals: one of the posterior
segments develops into an imperfect head, and simultaneously narrows
its connexion with the preceding segments, from which it eventually
separates. Still more remarkable is the extent to which this process
is carried in certain kindred types; which exhibit to us several
individuals thus being simultaneously formed out of groups of segments.
Fig. 169, copied (omitting the appendages) from one contained in a
memoir by M. Milne-Edwards, represents six worms of different ages
in course of development: the terminal one being the eldest, the
one having the greatest number of segments, and the one that will
first detach itself; and the successively anterior ones, with their
successively smaller numbers of segments, being successively less
advanced towards fitness for separation and independence. Here among
groups of segments we see repeated what in the previous cases occurs
with single segments. And then in other annelids we find that the
string of segments arising by gemmation from a single germ becomes a
permanently united whole: the tendency to any more complete fission
than that which marks out the segments, being lost; or, in other words,
the integration having become relatively complete. Leaving out of sight
the question of alliance among the types above grouped together, that
which it here concerns us to notice is, that longitudinal gemmation
does go on; that it is displayed in that primitive form in which the
gemmæ separate as soon as produced; that we have types in which such
gemmæ hang together in groups of four, or in groups of eight and ten,
from which however the gemmæ successively separate as individuals;
that among higher types we have long strings of similarly-formed
gemmæ which do not become individually independent, but separate into
organized groups; and that from these we advance to forms in which all
the gemmæ remain parts of a single individual. One other significant
fact must be added. There are cases in which annelids multiply by
lateral gemmation.[25] That the longitudinally-produced gemmæ which
compose an annelid, should thus have, one of them or several of them,
the power of laterally budding-off gemmæ, from which other annelids
arise, gives further support to the hypothesis that, primordially, the
segments were independent individuals. And it suggests this belief
the more strongly because, in certain types of _Cœlenterata_, we see
that longitudinal and lateral gemmation _do_ occur together, where the
longitudinally-united gemmæ are demonstrably independent individuals.


§ 207. Though it seems next to impossible that we shall ever be able
to find a type such as that which is here supposed to be the unit of
composition of the annulose type, since we must assume such a type
to have been long since extinct, yet the foregoing evidence goes
far towards showing that an annulose animal is an aggregate of the
third order. This repetition of segments, sometimes numbering several
hundreds, like one another in all their organs even down to those of
reproduction, while it is otherwise unaccountable, is fully accounted
for if these segments are homologous with the separate individuals of
some lower type. The gemmation by which these segments are produced, is
as similar as the conditions allow, to the gemmation by which compound
animals in general are produced. As among plants, and as among
demonstrably-compound animals, we see that the only thing required
for the formation of a permanent chain of gemmiparously-produced
individuals, is that by remaining associated such individuals will
have advantages greater than are to be gained by separation. Further,
comparisons of the annuloid and lower annulose forms, disclose
a number of those transitional phases of integration which the
hypothesis leads us to expect. And, lastly, the differences among these
united individuals or successive segments, are not greater than the
differences in their positions and functions explain--not greater than
such differences are known to produce among other united individuals:
witness sundry compound _Hydrozoa_.

Indirect evidence of much weight has still to be given. Thus far
we have considered only the less developed _Annulosa_. The more
integrated and more differentiated types of the class remain. If in
them we find a carrying further of the processes by which the lower
types are here supposed to have been evolved, we shall have additional
reason for believing them to have been so evolved. If we find that in
these superior orders, the individualities of the united segments are
much less pronounced than in the inferior, we shall have grounds for
suspecting that in the inferior the individualities of the segments are
less pronounced than in those lost forms which initiated the annulose
sub-kingdom.

       *       *       *       *       *

[NOTE.--Partly from the wish to incorporate further evidence, and
partly from the wish to present the evidence, old and new, in a more
effective order, I decide here to recast the foregoing exposition.

Significant traits of development are exhibited in common by two groups
otherwise unallied--certain of the _Platyhelminthes_ and certain of
the lower _Annulosa_. Of the _Platyhelminthes_ the ordinary type is
an unsegmented creature: a Planarian or a Trematode exemplifying it.
Among the free forms, as in some Planarians, there occurs transverse
fission, and prompt separation of the segments; while among some
other free forms, as the _Microstomida_, the two segments first
produced, themselves become segmented while still adherent, and this
process is repeated until a string is formed. Another group of the
_Platyhelminthes_, the Cestoid _Entozoa_, exhibit analogous processes.
There are unsegmented forms, as the _Caryophyllæus_, and there are
forms in which the segments, now few now many, adhere together in
chains; the terminal members of which, however, eventually separate,
and having before separation approached the trematode structure,
become independent individuals which grow, creep about, and continue
the race. In both of these types the condition under which the
gemmiparously-produced members remain connected, is that they shall
be able to feed individually: in the one case by lateral mouths, in
the other case by absorption through the integument. It is further
observable that in both cases separation of the component individuals
occurs at sexual maturity, when advantage in nutrition has ceased
to be the dominant need and dispersion of the species has taken its
place in degree of importance. Among Annelids, higher though they are
in type, we find parallelisms. Usually in its first stage an annelid
is unsegmented, but as fast as it elongates lines of segmentation
indent its surface. This segmentation proceeds in various ways, and
the segments exhibit various degrees of dependence. In some low types,
spontaneous fission goes on to the extent of producing single segments,
each of which has such vitality that it buds out anterior and posterior
parts at its two ends. Thus alike in the simple form which exists
before segmentation and in the form exhibited by a detached segment, we
have a unit analogous to each of the units which are joined together in
certain free _Turbellaria_ and in the Cestoids: the difference being
that in the Annelids the sexually mature units do not individually
disunite. But though there does not take place separation of single
completed segments, there takes place separation of groups of segments,
which are either sexually mature at the time they drop off or presently
become so. And the groups of segments which have become sexually mature
before they drop off, have simultaneously acquired swimming organs
and developed eyes, enabling them to spread and diffuse the species.
Sundry biologists recognize a parallelism between that detachment of
developed segments which goes on in the cestoid _Entozoa_, and that
which goes on in the _Scyphomedusæ_. The successively detached members
of the strobila are sexually matured or maturing individuals which,
as medusæ, are fitted for swimming about, multiplying, and reaching
other habitats; while each detached proglottis of the cestoid is, by
the nature of its medium, limited to creeping about. Clearly this
fissiparous process in such Annelids as the _Syllidæ_, which has
similarly been compared to the strobilization of the _Scyphomedusæ_,
differs simply in the respect that single segments are not adapted for
locomotion, and it therefore profits the species to separate in groups.
All these facts and analogies point to the conclusion that the remote
ancestor of the Annelids was an unsegmented creature homologous with
each of the segments of an existing Annelid.

This conclusion is supported by other kinds of evidence here to
be added. The larvæ of Annelids are very various; but amid their
differences there is a recognizable type. “The Trochophore is the
typical larval form of the Annelid stem”: a trochophore being a curious
spheroidal ciliated structure suggestive of cœlenterate affinities. And
this unsegmented larva, representing the remote ancestor from which the
many Annelid types diverged, is similar to the larvæ of the _Rotifera_
and the _Mollusca_: a trochophore is common to all these great classes.
Moreover since, among the _Rhizota_ (a sub-class of the _Rotiferæ_),
there is a species, _Trochosphæra_, solitary and free-swimming,
resembling in form and structure a trochophore, though it is not a
larva but an adult, we get further evidence that there was a primitive
creature of this general character, of which the trochophores of
_Mollusca_, _Rotifera_, and _Annelida_ are divergent modifications,
and which was unsegmented: the implication being that the segmentation
of the _Annelida_ was superinduced. That this segmentation resulted
from gemmation is implied by what are called polytrochal larvæ. These
“sometimes appear as a stage succeeding other larval types. Thus those
of _Arenicola marina_ arise from larvæ which at first were monotrochal,
later became telotrochal, and finally, by the appearance of new
ciliated rings between those already present, assumed the stage of
polytrochal larvæ.... This condition warrants the assumption that the
segmented forms are to be looked upon as the younger, the unsegmented,
on the other hand, as the phylogenetically older.” (Korschelt and
Heider, i, 278.) And that the above-described rings of cilia mark
off segments is shown by the case of _Ophryotrocha puerilis_, which
“remains, as it were, in a larval condition, since the segments
retain their ciliation throughout life.” (_Ib._, 277.) Yet one more
significant fact must be named. In early stages of development each
segment of an archiannelidan has cœlomic spaces separate from those
of neighbouring segments, but in the adult the septa “generally break
down either partially or completely, so that the peri-visceral cavity
becomes a continuous space from end to end of the animal.” (Sedgwick,
_Text Book_, 449.) While this fact is congruous with the hypothesis
here maintained, it is incongruous with the hypothesis that the annelid
was originally an elongated creature which afterwards became segmented;
since in that case the implication would be that the cœlomic septa, not
arising from recapitulation of an ancestral structure, but originated
by the process of segmentation, were first superfluously formed and
then destroyed.

Various lines of evidence thus converge to the conclusion that an
annulose animal is an aggregate of the third order.

In June, 1865, when No. 14 of my serial containing the foregoing
chapter was issued, I supposed myself to be alone in holding this
belief respecting the annulose type, and long continued to suppose so.
Over thirty years later, however, in M. Edmond Perrier’s work, _La
Philosophie Zoologique avant Darwin_, I found mention of a lecture
delivered by M. Lacaze-Duthiers at the École Normale Supérieure in
Paris, and reported in the _Revue des Cours Scientifiques_ for January
28, 1865, in which he enunciated a like belief. Judging, however, by
the account of this lecture which M. Perrier gives (he was present), it
appears that M. Lacaze-Duthiers simply contended that this view of the
annulose structure as arising by union of once-independent units, is
suggested by certain _à priori_ considerations. There is no indication
that he assigned any of the classes of facts above given, which go to
show that it _has_ thus arisen.

For further facts and arguments concerning the genesis of the annulose
type, see Appendix D 2.]




                              CHAPTER V.

               THE MORPHOLOGICAL COMPOSITION OF ANIMALS,
                             _CONTINUED_.


§ 208. Insects, Arachnids, Crustaceans, and Myriapods, are all members
of that higher division of the _Annulosa_[26] called _Articulata_
or now more generally _Arthropoda_. Though in these creatures the
formation of segments may be interpreted as a disguised gemmation;
and though, in some of them, the number of segments increases by this
modified budding after leaving the egg, as it does among the Annelids;
yet the process is not nearly so dominant: the segments are usually
much less numerous than we find them in the types last considered. In
most cases, too, the segments are in a greater degree differentiated
one from another, at the same time that they are severally more
differentiated within themselves. Nor is there any instance of
spontaneous fission taking place in the series of segments composing
an articulate animal. On the contrary, the integration, always great
enough permanently to unite the segments, is frequently carried so
far as to hide very completely the individualities of some or many of
them; and occasionally, as among the Acari, the consolidation, or the
arrest of segmentation, is so decided as to leave scarcely a trace
of the articulate structure: the type being in these cases indicated
chiefly by the presence of those characteristically-formed limbs, which
give the alternative name _Arthropoda_ to all the higher _Annulosa_.
Omitting the parasitic orders, which, as in other cases, are aberrant
members of their sub-kingdom, comparisons between the different orders
prove that the higher are strongly distinguished from the lower, by
the much greater degree in which the individuality of the tertiary
aggregate dominates over the individualities of those secondary
aggregates called segments or “somites,” of which it is composed. The
successive Figs. 170–176, representing (without their limbs) a Julus,
a Scolopendra, an isopodous Crustacean, and four kinds of decapodous
Crustaceans, ending with a Crab, will convey at a glance an idea of
the way in which that greater size and heterogeneity reached by the
higher types, is accompanied by an integration which, in the extreme
cases, nearly obliterates all traces of composite structure. In the
Crab the posterior segments, usually folded underneath the shell, alone
preserve their primitive distinctness. So completely confluent are the
rest, that it seems absurd to say that a Crab’s carapace is composed of
as many segments as there are pairs of limbs, foot-jaws, and antennæ
attached to it; and were it not that during early stages of the Crab’s
development the segmentation is faintly marked, the assertion might be
considered illegitimate.

  [Illustration: Figs. 170–176.]

That all articulate animals are thus composed from end to end
of homologous segments, is, however, an accepted doctrine among
naturalists. It is a doctrine that rests on careful observation of
three classes of facts--the correspondences of parts in the successive
“somites” of an adult articulate animal; the still more marked
correspondences of such parts as they exist in the embryonic or larval
articulate animal; and the maintenance of such correspondences in some
types, which are absent in types otherwise near akin to them. The
nature of the conclusion which these evidences unite in supporting,
will best be shown by the annexed copies from the lecture-diagrams
of Prof. Huxley; exhibiting the typical structures of a Myriapod, an
Insect, a Spider, and a Crustacean, with their relations to a common
plan, as interpreted by him.

  [Illustration: Figs. 177–186.]

Treating of these homologies, Prof. Huxley says “that a striking
uniformity of composition is to be found in the heads of, at any rate,
the more highly organized members of these four classes; and that,
typically, the head of a Crustacean, an Arachnid, a Myriapod, or an
Insect, is composed of six somites (or segments corresponding with
those of the body) and their appendages, the latter being modified so
as to serve the purpose of sensory and manducatory organs.”[27]

Thus even in the higher _Arthropoda_, the much greater consolidation
and much greater heterogeneity do not obliterate all evidence of the
fact, that the organism is an aggregate of the third order. Comparisons
show that it is divisible into a number of proximate units, each of
which is akin in certain fundamental traits to its neighbours, and each
of which is an aggregate of the second order, in so far as it is an
organized combination of those aggregates of the first order which we
call morphological units or cells. And that these segments or somites,
which make up an annulose animal, were originally aggregates of the
second order having independent individualities, is an hypothesis which
gathers further support from the contrast between the higher and the
lower Arthropods, as well as from the contrast between the Arthropods
in general and the Annelids. For if that masking of the individualities
of the segments which we find distinguishes the higher forms from the
lower, has been going on from the beginning, as we may fairly assume;
it is to be inferred that the individualities of the segments in the
lower forms, were originally more marked than they now are. Reversing
those processes of change by which the most developed _Annulosa_ have
arisen from the least developed; and applying in thought this reversed
process to the least developed, as they were described in the last
Chapter; we are brought to the conception of attached segments that are
all completely alike, and have their individualities in no appreciable
degree subordinated to that of the chain they compose. From which there
is but one step to the conception of gemmiparously-produced individuals
which severally part one from another as soon as they are formed.


§ 209. We must now return to a junction whence we diverged some time
ago. As before explained under the head of Classification, organisms
do not admit of uniserial arrangement, either in general or in detail;
but everywhere form groups within groups. Hence, having traced the
phases of morphological composition up to the highest forms in any
sub-kingdom, we find ourselves at the extremity of a great branch, from
which there is no access to another great branch, except by going back
to some place of bifurcation low down in the tree.

There exist such similarities of shape and structure between the larval
forms of low Molluscs and those of Annelids and Rotifers, as to show
that there was an early type common to them all; and its probable
characters, suggested by comparison, seem to imply that it had arisen
from some cœlenterate type, intermediate between the _Cnidaria_ and
the _Ctenophora_. But there is this noteworthy difference between the
molluscan larva and the allied larvæ, that it gives origin to only one
animal and not to a group of animals, united or disunited. No true
Mollusc multiplies by gemmation, either continuous or discontinuous;
but the product of every fertilized germ is a single individual.

It is a significant fact that here, where for the first time we have
homogenesis holding throughout an entire sub-kingdom, we have also
throughout an entire sub-kingdom no case in which the organism is
divisible into two, three, or more, like parts. There is neither any
such clustering or branching as a cœlenterate or molluscoid animal
usually displays; nor is there any trace of that segmentation which
characterizes the _Annulosa_. Among these animals in which no single
egg produces several individuals, no individual is separable into
several homologous divisions. This connexion will be seen to have
a probable meaning, on remembering that it is the converse of the
connexion which obtains among the _Annulosa_, considered as a group.

A Mollusc, then, is an aggregate of the second order. Not only in the
adult animal is there no sign of a multiplicity of like parts that
have become obscured by integration; but there is no sign of such
multiplicity in the embryo. And this unity is just as conspicuous in
the lowest Lamellibranch as in the highest Cephalopod.

  [Illustration: Figs. 188–190.]

It may be well to note, however, more especially because it illustrates
a danger of misinterpretation presently to be guarded against, that
there are certain Molluscs which simulate the segmented structure.
Externally a _Chiton_, Fig. 188, appears to be made up of divisions
substantially like those of the creature Fig. 189; and one who judged
only by externals, would say that the creature Fig. 190 differs as
much from the creature Fig. 189, as this does from the preceding one.
But the truth is, that while 190 and 189 are closely-allied types, 189
differs from 188 much more widely than a man does from a fish. And the
radical distinction between them is this:--Whereas in the Crustacean
the segmentation is carried transversely through the whole mass of the
body, so as to render the body more or less clearly divisible into
a series of parts which are similarly composed; in the Mollusc the
segmentation is limited to the shell carried on its upper surface,
and leaves its body as completely undivided as is that of a common
slug.[28] Were the body cut through at each of the divisions, the
section of it attached to each portion of the shell would be unlike
all the other sections. Here the segmentation has a purely functional
derivation--is adaptive instead of genetic. The similarly-formed and
similarly-placed parts, are not homologous in the same sense as are the
appendages of a phænogamic axis or the limbs of an insect.


§ 210. In studying the remaining and highest sub-kingdom of animals, it
is important to recognize this radical difference in meaning between
that likeness of parts which is produced by likeness of modifying
forces, and that likeness of parts which is due to primordial identity
of origin. On our recognition of this difference depends the view we
take of certain doctrines that have long been dominant, and have still
a wide currency.

Among the _Vertebrata_, as among the _Mollusca_, homogenesis is
universal. The two sub-kingdoms are like one another and unlike the
remaining sub-kingdoms in this, that in all the types they severally
include, a single fertilized ovum produces only a single individual. It
is true that as the eggs of certain gasteropods occasionally exhibit
spontaneous fission of the vitelline mass, which may or may not result
in the formation of two individuals; so among vertebrate animals we
now and then meet with double monsters, which appear to imply such a
spontaneous fission imperfectly carried out. But these anomalies serve
to render conspicuous the fact, that in both these sub-kingdoms the
normal process is the integration of the whole germ-mass into a single
organism, which at no phase of its development displays any tendency to
separate into two or more parts.

Equally as throughout the _Mollusca_, there holds throughout the
_Vertebrata_ the correlative fact, that not even in its lowest any
more than in its highest types, is the body divisible into homologous
segments. The vertebrate animal, under its simplest as under its most
complex form, is like the molluscous animal in this, that you cannot
cut it into transverse slices, each of which contains a digestive
organ, a respiratory organ, a reproductive organ, &c. The organs of the
least-developed fish as well as those of the most developed mammal,
form but a single physiological whole; and they show not the remotest
trace of having ever been divisible into two or more physiological
wholes. That segmentation which the vertebrate animal usually exhibits
throughout part of its organization, is the same in origin and meaning
as the segmentation of a _Chiton’s_ shell; and no more implies in the
vertebrate animal a composite structure, than do the successive pairs
of branchiæ of the _Doto_, or the transverse rows of branchiæ in the
_Eolis_, imply composite structure in the molluscous animal. To some
this will seem a very questionable proposition; and had we no evidence
beyond that which adult vertebrate animals of developed types supply,
it would be a proposition not easy to substantiate. But abundant
support for it is to be found in the structure of the vertebrate
embryo, and in the comparative morphology of the _Vertebrata_ in
general.

Embryologists teach us that the primordial relations of parts are most
clearly displayed in the early stages of evolution; and that they
generally become partially or completely disguised in its later stages.
Hence, were the vertebrate animal on the same level as the annulose
animal in degree of composition--did it similarly consist of segments
which are homologous in the sense that they are the proximate units
of composition; we ought to find this fundamental fact most strongly
marked at the outset. As in the annelid-embryo the first conspicuous
change is the elongation and division into segments, by constrictions
that encircle the whole body; and as in the arthropod embryo the
blastoderm becomes marked out transversely into pieces which extend
themselves round the yelk before the internal organization has made
any appreciable progress; so in the embryo of every vertebrate animal,
had it an analogous composition, the first decided change should be
a segmentation implicating the entire mass. But it is not so. Sundry
important differentiations occur before any divisions begin to show
themselves. There is the defining of that elongated, elevated area with
its longitudinal groove, which becomes the seat of subsequent changes;
there is the formation of the notochord lying beneath this groove;
there is the growth upwards of the boundaries of the groove into the
dorsal laminæ, which rapidly develop and fold over in the region of the
head. Rathke, as quoted and indorsed by Prof. Huxley, describes the
subsequent changes as follows:--“The gelatinous investing mass, which,
at first, seems only to constitute a band to the right and to the left
of the notochord forms around it, in the further course of development,
a sheath, which ends in a point posteriorly. Anteriorly, it sends
out two processes which underlie the lateral parts of the skull, but
very soon coalesce for a longer or shorter distance. Posteriorly, the
sheath projects but little beyond the notochord; but, anteriorly, for a
considerable distance, as far as the infundibulum. It sends upwards two
plates, which embrace the future central parts of the nervous system
laterally, probably throughout their entire length.” That is to say, in
the _Vertebrata_ the first step is the marking out on the blastoderm
of an integrated structure, within which segments subsequently appear.
When these do appear, they are for some time limited to the middle
region of the spinal axis; and no more then than ever after, do they
implicate the general mass of the body in their transverse divisions.
On the contrary, before vertebral segmentation has made much progress,
the rudiments of the vascular system are laid down in a manner
showing no trace of any primordial correspondence of its parts with
the divisions of the axis. Equally at variance with the belief that
the vertebrate animal is essentially a series of homologous parts,
is the heterogeneity which exists among these parts on their first
appearance. Though in the head of an adult articulate animal there
is little sign of divisibility into segments like those of the body;
yet such segments, with their appropriate ganglia and appendages, are
easily identifiable in the articulate embryo. But in the _Vertebrata_
this antithesis is reversed. At the time when segmentation has become
decided in the dorsal region of the spine, there is no trace of
segments in the parts which are to form the skull--nothing whatever
to suggest that the skull is being formed out of divisions homologous
with vertebræ.[29] And minute observation no more discloses any such
homology than does general appearance. “Remak,” says Prof. Huxley,
“has more fully proved than any other observer, the segmentation into
‘urwirbel,’ or proto-vertebræ, which is characteristic of the vertebral
column, stops at the occipital margin of the skull--the base of which,
before ossification, presents no trace of that segmentation which
occurs throughout the vertebral column.”

  [Illustration: Fig. 191.]

Consider next the evidence supplied by comparative morphology. In
preceding sections (§§ 206, 208) it has been shown that among annulose
animals, the divisibility into homologous parts is most clearly
demonstrable in the lowest types. Though in decapodous Crustaceans,
in Insects, in Arachnids, there is difficulty in identifying some
or many of the component somites; and though, when identified, they
display only partial correspondences; yet on descending to Annelids,
the composition of the entire body out of such somites becomes
conspicuous, and the homology between each somite and its neighbours
is shown by the repetition of one another’s structural details, as
well as by their common gemmiparous origin: indeed, in some cases we
have the homology directly demonstrated by seeing a somite of the body
transformed into a head. If, then, a vertebrate animal had a segmental
composition of kindred nature, we ought to find it most clearly
marked in the lowest _Vertebrata_ and most disguised in the highest
_Vertebrata_. But here, as before, the fact is just the reverse. Among
the _Vertebrata_ of developed type, such segmentation as really exists
remains conspicuous--is but little obscured even in parts of the spinal
column formed out of integrated vertebræ. Whereas in the undeveloped
vertebrate type, segmentation is scarcely at all traceable.[30] The
_Amphioxus_, Fig. 191, is not only without ossified vertebræ; not
only is it without cartilaginous representatives of them; but it is
even without anything like distinct membranous divisions. The spinal
column exists as a continuous notochord: the only signs of incipient
segmentation being given by its membranous sheath, in the upper part
of which “quadrate masses of somewhat denser tissue seem faintly to
represent neural spines.” Moreover, throughout sundry groups of fishes
and amphibians, the segmentation remains very imperfect: only certain
peripheral appendages of the vertebræ becoming defined and solidified,
while in place of the bodies of the vertebræ there still continues the
undivided notochord. Thus, instead of being morphologically composed
of vertebral segments, the vertebrate animal in its primitive form is
entirely without vertebral segments; and vertebral segments begin to
appear only as we advance towards developed forms. Once more, evidence
equally adverse to the current hypothesis meets us on observing that
the differences between the parts supposed to be homologous, are as
great at first as at last. Did the vertebrate animal primordially
consist of homologous segments from snout to tail; then the segments
said to compose the skull ought, in the lowest _Vertebrata_, to show
themselves much more like the remaining segments than they do in the
highest _Vertebrata_. But they do not. Fishes have crania made up of
bones that are no more clearly arrangeable into segments like vertebræ,
than are the cranial bones of the highest mammal. Nay, indeed, the
case is much stronger. The simplest fish possessing a skeleton, has a
cranium composed of cartilage that is not segmented at all!

Besides being inconsistent with the leading truths of Embryology
and Comparative Morphology, the hypothesis of Goethe and Oken is
inconsistent with itself. The facts brought forward to show that there
exists an archetypal vertebra, and that the vertebrate animal is
composed of archetypal vertebræ arranged in a series, and severally
modified to fit their positions--these facts, I say, so far from
proving as much, suffice, when impartially considered, to disprove it.
No assigned, nor any conceivable, attribute of the supposed archetypal
vertebra is uniformly maintained. The parts composing it are constant
neither in their number, nor in their relative positions, nor in their
modes of ossification, nor in the separateness of their several
individualities when present. There is no fixity of any one element,
or connexion, or mode of development, which justifies even a suspicion
that vertebræ are modelled after an ideal pattern. To substantiate
these assertions here would require too much space, and an amount of
technical detail wearisome to the general reader. The warrant for them
will be found in a criticism on the osteological works of Prof. Owen,
originally published in the _British and Foreign Medico-Chirurgical
Review_ for Oct. 1858. This criticism I add in the Appendices, for the
convenience of those who may wish to study the question more fully.
(See Appendix B.)

Everything, then, goes to show that the segmental composition which
characterises the apparatus of external relation in most _Vertebrata_,
is not primordial or genetic, but functionally determined or adaptive.
Our inference must be that the vertebrate animal is an aggregate of
the second order, in which a relatively superficial segmentation has
been produced by mechanical intercourse with the environment. We
shall hereafter see that this conception leads us to a consistent
interpretation of the facts--shows us why there has arisen such unity
in variety as exists in every vertebral column, and why this unity
in variety is displayed under countless modifications in different
skeletons.[31]


§ 211. On glancing back at the facts brought together in these two
chapters, we see it to be probable that there has gone on among animals
a process like that which we saw reason to think has gone on among
plants. Minute aggregates of those physiological units which compose
living protoplasm, exist as _Protozoa_: some of them incoherent,
indefinite, and almost homogeneous, and others of them more coherent,
definite, and heterogeneous. By union of these nucleated particles
of sarcode, are produced various indefinite aggregates of the second
order--Sponges, _Polycytharia_, Foraminifers, &c.; in which the
compound individuality is scarcely enough marked to subordinate the
primitive individualities. But in other types, as in _Hydra_, the lives
of the morphological units are in a considerable degree, though not
wholly, merged in the life of the integrated body they form. As the
primary aggregate, when it passes a certain size, undergoes fission or
gemmation; so does the secondary aggregate. And as on the lower stage
so on the higher, we see cases in which the gemmiparously-produced
individuals part as soon as formed, and other cases in which they
continue united, though in great measure independent. This massing
of secondary aggregates into tertiary aggregates, is variously
carried on among the _Hydrozoa_, the _Actinozoa_, the _Polyzoa_,
and the _Tunicata_. In most of the types so produced, the component
individualities are very little subordinated to the individuality of
the composite mass--there is only physical unity and not physiological
unity; but in certain of the oceanic _Hydrozoa_, the individuals are
so far differentiated and combined as very much to mask them. Forms
showing us clearly the transition to well-developed individuals of
the third order, are not to be found. Nevertheless, in the great
sub-kingdom _Annulosa_, there are traits of structure, development,
and mode of multiplication, which go far to show that its members are
such individuals of the third order; and in the relations to external
conditions involved by the mode of union, we find an adequate cause for
that obscuration of the secondary individualities which we must suppose
has taken place. The two other great subdivisions, _Mollusca_ and
_Vertebrata_, between the lower members of which there are suggestive
points of community, present us only with aggregates of the second
order, that have in many cases become very large and very complex.
We find in them no trace of the union of gemmiparously-produced
individuals. Neither the molluscous nor the vertebrate animal shows
the faintest trace of a segmentation affecting the totality of
its structure; and we see good grounds for concluding that such
segmentation as exceptionally occurs in the one and usually occurs in
the other, is superinduced.

       *       *       *       *       *

[NOTE:--A critic calls in question the statement on p. 121 respecting
the _Amphioxus_. At the outset, however, he admits that in the
_Amphioxus_ “the central nervous system and the notochord are not
segmented.” In the Annelid, however, the central nervous system
is segmented, and there is segmentation of the part which, as a
supporting structure, is analogous to the notochord in respect
of function--the outer part which represents the exo-skeleton in
contrast to the endo-skeleton. He goes on to say that “the gut is
not involved [in the segmentation] and exhibits in _Amphioxus_ just
as it does in worms differentiations entirely independent of the
segmentation of the mesoblast.” Part of this statement is, I think,
not congruous with all the facts. In _Protodrilus_, one of the lowest
of the _Archiannelida_, “the intestine is moniliform, there being a
constriction between each segment” and the next. (Shipley.) Complete
segmentation of the intestine is obviously impossible, since, were
the canal divided into portions by septa, no food could pass. But the
fact that the gut has these successive expansions and constrictions,
corresponding to the successive segments, and giving to each segment
a partially-separate stomach, shows that segmentation has gone as
far as consists with the carrying on of the lives of the segments.
No such partial segmentation exists in the _Amphioxus_. Thus, then,
three fundamental structures--the directive structure, the supporting
structure, and the alimentary structure--are respectively simple in
the lowest vertebrate and segmented, or partially segmented, in the
lowest Annelid. Again, while it is said that the gill-clefts exhibit
segmentation, it is admitted that this has no relevance to any
constitutional segmentation: “they are segmented on a plan of their
own” irrespective of other organs. Another allegation is that the
ovaries of _Amphioxus_ are segmented. Their segmentation, however, like
that of the gills, is isolated, and may be considered as illustrating
those repetitions of like parts seen in supernumerary vertebræ in
various creatures--a repetition which becomes habitual if the resulting
structure is advantageous to the species. On the statement that while
the _Amphioxus_ has no rudiments of a renal system the Elasmobranch
embryo has such rudiments, which are as distinctly segmented as the
nephridia of a worm, two comments may be made. The first is that if
in these Vertebrates the nephridia bear a relation to the general
structure like that which they do in Annelids, then one would expect
to find the segmental arrangement shown in the lowest type, as in
Annelids, rather than in a type considerably advanced in development.
Should it be replied that in the _Amphioxus_ an excretory system had
not yet arisen, though one is required for the higher organization
of an Elasmobranch, then the answer may be that since the segmental
arrangement in the Elasmobranch corresponds with that of the myotomes,
it has no reference to any primordial segmentation, since the myotomes
have been functionally generated. The second comment is that whereas
the nephridia of the Annelid have independent external openings,
the nephridia in the Elasmobranch have not. These discharge their
secretions into certain general tubes of exit common to them all;
showing that each of them, instead of being a member of a partially
independent structure, is united with others in subordination to a
general structure. That is to say, the segmentations are far from being
parallel in their essential natures. The assertion accompanying these
criticisms, that there is “no difference _in principle_ between the
segmentation of _Amphioxus_ and Annelid” is difficult to reconcile
with the visible contrast between the two. Whatever local segmentations
there are in an _Amphioxus_ appear to me quite unlike “in principle”
to those which an Annelid exhibits. Could its portion of gut be duly
supplied with nutriment, the segment of a low Annelid could carry on
its vital functions independently. In the parts of the _Amphioxus_ we
see nothing approaching to this. Cut it into transverse sections and
no one of them contains anything like the assemblage of structures
required for living. The _Amphioxus_ is a physiological whole, and
in that respect differs radically from the Annelid, each segment of
which is in chief measure a physiological whole. No occurrence of
local segmentation in the _Amphioxus_ can obliterate this fundamental
contrast.

An accompanying contrast tells the same story. On ascending from the
lowest to the highest annulose types we see a progressing integration,
morphological and physiological; so that whereas in a low annelid
the successive parts are in large measure independent in their
structures and in their lives, in a high arthropod, as a crab, most
of the parts have lost their individualities and have become merged
in a consolidated organism with a single life. Quite otherwise is it
in the vertebrate series. Its lowest member is at the very outset a
complete morphological and physiological whole, and the formation of
those serial parts which some think analogous to the serial parts of an
Annelid, begins at a later stage and becomes gradually pronounced. That
is to say, the course of transformation is reversed.]




                              CHAPTER VI.

               MORPHOLOGICAL DIFFERENTIATION IN PLANTS.


§ 212. While, in the course of their evolution, plants and animals
have displayed progressive integrations, there have at the same time
gone on progressive differentiations of the resulting aggregates,
both as wholes and in their parts. These differentiations and the
interpretations of them, form the second class of morphological
problems.

We commence as before with plants. We have to consider, first, the
several kinds of modification in shape they have undergone; and,
second, the relations between these kinds of modification and their
factors. Let us glance at the leading questions that have to be
answered.


§ 213. Irrespective of their degrees of composition, plants may, and
do, become changed in their general forms. Are their changes capable of
being formulated? The inquiry which meets us at the outset is--does a
plant’s shape admit of being expressed in any universal terms?--terms
that remain the same for all genera, orders, and classes.

After plants considered as wholes, have to be considered their
proximate components, which vary with their degrees of composition,
and in the highest plants are what we call branches. Is there any law
traceable among the contrasted shapes of different branches in the same
plant? Do the relative developments of parts in the same branch conform
to any law? And are these laws, if they exist, allied with one another
and with that to which the shape of the whole plant conforms?

Descending to the components of these components, which in developed
plants we distinguish as leaves, there meet us kindred questions
respecting their relative sizes, their relative shapes, and their
shapes as compared with those of foliar organs in general. Of their
morphological differentiations, also, it has to be asked whether they
exemplify any truth that is exemplified by the entire plant and by its
larger parts.

Then, a step lower, we come down to those morphological units of
which leaves and fronds consist; and concerning these arise parallel
inquiries touching their divergences from one another and from cells in
general.

The problems thus put together in several groups cannot of course be
rigorously separated. Evolution presupposes transitions which make all
such classings more or less conventional; and adherence to them must be
subordinate to the needs of the occasion.


§ 214. In studying the causes of the morphological differentiations
thus divided out and prospectively generalized, we shall have to bear
in mind several orders of forces which it will be well briefly to
specify.

Growth tends inevitably to initiate changes in the shape of any
aggregate, by altering both the amounts of the incident forces and
the forces which the parts exert on one another. With the mechanical
actions this is obvious. Matter that is sensibly plastic cannot be
increased in mass without undergoing a change in its proportions,
consequent on the diminished ratio of its cohesive force to the force
of gravitation. With the physiological actions it is equally obvious.
Increase of size, other things equal, alters the relations of the parts
to the material and dynamical factors of nutrition; and by so affecting
differently the nutrition of different parts, initiates further changes
of proportions.

In plants of the third order it is thus with the proximate components:
they are subject to mutual influences that are unlike one another and
are continually changing. The earlier-formed units become mechanical
supporters of the later-formed units, and so experience modifying
forces from which the later-formed units are exempt. Further, these
elder units simultaneously begin to serve as channels through which
materials are carried to and from the younger units--another cause
of differentiation that goes on increasing in intensity. Once more,
there arise ever-strengthening contrasts between the amounts of light
which fall upon the youngest or outermost units and the eldest or
innermost units; whence result structural contrasts of yet another
kind. Evidently, then, along with the progressive integration of cells
into fronds, of fronds into axes, and of axes into plants still more
composite, there come into play sundry causes of differentiation which
act on the whole and on each of its parts, whatever their grade. The
forces to be overcome, the forces to be utilized, and the matters to
be appropriated, do not remain the same in their proportions and modes
of action for any two members of the aggregate: be they members of the
first, second, third, or any other order.


§ 215. Nor are these the only kinds and causes of heterogeneity which
we have to consider. Beyond the more general changes produced in the
relative sizes and shapes of plants and their parts by progressive
aggregation, there are the more particular changes determined by the
more particular conditions.

Plants as wholes assume unlike attitudes towards their environments;
they have many ways of articulating their parts with one another; they
have many ways of adjusting their parts towards surrounding agencies.
These are causes of special differentiations additional to those
general differentiations that result from increase of mass and increase
of composition. In each part considered individually, there arises
a characteristic shape consequent on that relative position towards
external and internal forces, which the mode of growth entails. Every
member of the aggregate presents itself in a more or less peculiar way
towards the light, towards the air, and towards its point of support;
and according to the relative homogeneity or heterogeneity in the
incidence of the agencies thus brought to bear on it, will be the
relative homogeneity or heterogeneity of its shape.


§ 216. Before passing from this _à priori_ view of the morphological
differentiations which necessarily accompany morphological
integrations, to an _à posteriori_ view of them, it seems needful to
specify the meanings of certain descriptive terms we shall have to
employ.

Taking for our broadest division among forms, the regular and the
irregular, we may divide the latter into those which are wholly
irregular and those which, being but partially irregular, suggest
some regular form to which they approach. By slightly straining the
difference between them, two current words may be conveniently used
to describe these subdivisions. The entirely irregular forms we may
class as _asymmetrical_--literally as forms without any equalities of
dimensions. The forms which approximate towards regularity without
reaching it, we may distinguish as _unsymmetrical_: a word which,
though it asserts inequality of dimensions, has been associated by
use rather with such slight inequality as constitutes an observable
departure from equality.

Of the regular forms there are several classes, differing in the number
of directions in which equality of dimensions is repeated. Hence
results the need for names by which symmetry of several kinds may be
expressed.

The most regular of figures is the sphere: its dimensions are the same
from centre to surface in all directions; and if cut by any plane
through the centre, the separated parts are equal and similar. This is
a kind of symmetry which stands alone, and will be hereafter spoken of
as _spherical symmetry_.

When a sphere passes into a spheroid, either prolate or oblate, there
remains but one set of planes that will divide it into halves, which
are in all respects alike; namely, the planes in which its axis lies,
or which have its axis for their line of intersection. Prolate and
oblate spheroids may severally pass into various forms without losing
this property. The prolate spheroid may become egg-shaped or pyriform,
and it will still continue capable of being divided into two equal
and similar parts by any plane cutting it down its axis; nor will the
making of constrictions deprive it of this property. Similarly with the
oblate spheroid. The transition from a slight oblateness, like that of
an orange, to an oblateness reducing it nearly to a flat disc, does
not alter its divisibility into like halves by every plane passing
through its axis. And clearly the moulding of any such flattened oblate
spheroid into the shape of a plate, leaves it as before, symmetrically
divisible by all planes at right angles to its surface and passing
through its centre. This species of symmetry is called _radial
symmetry_. It is familiarly exemplified in such flowers as the daisy,
the tulip, and the dahlia.

From spherical symmetry, in which we have an infinite number of axes
through each of which may pass an infinite number of planes severally
dividing the aggregate into equal and similar parts; and from radial
symmetry, in which we have a single axis through which may pass an
infinite number of planes severally dividing the aggregate into equal
and similar parts; we now turn to _bilateral symmetry_, in which the
divisibility into equal and similar parts becomes much restricted.
Noting, for the sake of completeness, that there is a sextuple
bilateralness in the cube and its derivative forms which admit of
division into equal and similar parts by planes passing through the
three diagonal axes and by planes passing through the three axes
that join the centres of the surfaces, let us limit our attention to
the three kinds of bilateralness which here concern us. The first
of these is _triple bilateral symmetry_. This is the symmetry of a
figure having three axes at right angles to one another, through each
of which there passes a single plane that divides the aggregate into
corresponding halves. A common brick will serve as an example; and of
objects not quite so simple, the most familiar is that modern kind of
spectacle-case which is open at both ends. This may be divided into
corresponding halves along its longitudinal axis by cutting it through
in the direction of its thickness, or by cutting it through in the
direction of its breadth; or it may be divided into corresponding
halves by cutting it across the middle. Of objects which illustrate
_double bilateral symmetry_, may be named one of those boats built for
moving with equal facility in either direction, and therefore made
alike at stem and stern. Obviously such a boat is separable into equal
and similar parts by a vertical plane passing through stem and stern;
and it is also separable into equal and similar parts by a vertical
plane cutting it amidships. To exemplify _single bilateral symmetry_
it needs but to turn to the ordinary boat of which the two ends are
unlike. Here there remains but the one plane passing vertically
through stem and stern, on the opposite sides of which the parts are
symmetrically disposed.

These several kinds of symmetry as placed in the foregoing order, imply
increasing heterogeneity. The greatest uniformity in shape is shown
by the divisibility into like parts in an infinite number of infinite
series of ways; and the greatest degree of multiformity consistent
with any regularity, is shown by the divisibility into like parts in
only a single way. Hence, in tracing up organic evolution as displayed
in morphological differentiations, we may expect to pass from the one
extreme of spherical symmetry, to the other extreme of single bilateral
symmetry. This expectation we shall find to be completely fulfilled.




                             CHAPTER VII.

                     THE GENERAL SHAPES OF PLANTS.


§ 217. Among protophytes those exemplified by _Pleurococcus vulgaris_
are by general consent considered the simplest. As shown in Fig.
1, they are globular cells presenting no obvious differentiation
save that between inner and outer parts. Their uniformity of figure
co-exists with a mode of life involving the uniform exposure of all
their sides to incident forces. For though each individual may have
its external parts differently related to environing agencies, yet the
new individuals produced by spontaneous fission, whether they part
company or whether they form clusters and are made polyhedral by mutual
pressure, have no means of maintaining parallel relations of position
among their parts. On the contrary, the indefiniteness of the attitudes
into which successive generations fall, must prevent the rise of any
unlikeness between one portion of the surface and another. Spherical
symmetry continues because, on the average of cases, incident forces
are equal in all directions.

  [Illustration: Figs. 1, 2, 3.]

Other orders of _Protophyta_ have much more special forms, along with
much more special attitudes: their homologous parts maintaining, from
generation to generation, unlike relations to incident forces. The
_Desmidiaceæ_ and _Diatomaceæ_, of which Figs. 2 and 3 show examples,
severally include genera characterized by triple bilateral symmetry.
A _Navicula_ is divisible into corresponding halves by a transverse
plane and by two longitudinal planes--one cutting its valves at right
angles and the other passing between its valves. The like is true
of those numerous transversely-constricted forms of _Desmidiaceæ_,
exemplified by the second of the individuals represented in Fig.
2. If now we ask how a _Navicula_ is related to its environment,
we see that its mode of life exposes it to three different sets of
forces: each set being resolvable into two equal and opposite sets.
A _Navicula_ moves in the direction of its length, with either
end foremost. Hence, on the average, its ends are subject to like
actions from the agencies to which its motions subject it. Further,
either end while moving exposes its right and left sides to amounts
of influence which in the long run must be equal. If, then, the two
ends are not only like one another, but have corresponding right and
left sides, the symmetrical distribution of parts answers to the
symmetrical distribution of forces. Passing to the two edges and the
two flat surfaces, we similarly find a clue to their likenesses and
differences in their respective relations to the things around them.
These locomotive protophytes move through the entangled masses of
fragments and fibres produced by decaying organisms and confervoid
growths. The interstices in such matted accumulations are nearly all
of them much longer in one dimension than in the rest--form crevices
rather than regular meshes. Hence, a small organism will have much
greater facility of insinuating itself through this _débris_, in which
it finds nutriment, if its transverse section is flattened instead of
square or circular. And while we see how, by survival of the fittest, a
flattened form is likely to be acquired by diatoms having this habit;
we also see that likeness will be maintained between the two flat
surfaces and between the two edges. For, on the average, the relations
of the two flat surfaces to the sides of the openings through which
the diatom passes, will be alike; and so, too, on the average, will be
the relations of the two edges. In desmids of the type exemplified by
the second individual in Fig. 2, a kindred equalization of dimensions
is otherwise insured. There is nothing to keep one of the two surfaces
uppermost rather than the other; and hence, in the long succession
of individuals, the two surfaces are sure to be similarly exposed to
light and agencies in general. When to this is added the fact that
spontaneous fission occurs transversely in a constant way, it becomes
manifest that the two ends, while they are maintained in conditions
like one another, are maintained in conditions unlike those of the
two edges. Here then, as before, triple bilateral symmetry in form,
co-exists with a triple bilateral symmetry in the average distribution
of actions.

  [Illustration: Figs. 4, 5, 6.]

Still confining our attention to aggregates of the first order, let us
next note what results when the two ends are permanently subject to
different conditions. The fixed unicellular plants, of which examples
are given in Figs. 4, 5, and 6, severally illustrate the contrast
in shape arising between the part that is applied to the supporting
surface and the part that extends into the surrounding medium. These
two parts which are the most unlike in their relations to incident
forces, are the most unlike in the forms. Observe, next, that the part
which lifts itself into the water or air, is more or less decidedly
radial. Each outward-growing tubule of _Codium adhærens_, Fig. 4, has
its parts disposed with some regularity around its axis; the upper stem
and spore-vessel of _Botrydium_, Fig. 5, display a lateral growth
that is approximately equal in every direction; and the stems of the
_Mucor_, Fig. 6, shoot up with an approach to evenness on all sides.
Plants of this low type are naturally very variable in their modes of
growth: each individual being greatly modified in form by its special
circumstances. But they nevertheless show us a general likeness between
parts exposed to like forces, as well as a general unlikeness between
parts exposed to unlike forces.

Respecting the forms of these aggregates of the first order, it has
only to be added that they are asymmetrical where there is total
irregularity in the incidence of forces. We have an example in the
indefinitely contorted and branched shape of a fungus-cell, growing as
a mycelium among the particles of soil or through the interstices of
organic tissue.


§ 218. Re-illustrations of the general truths which the forms of
these vegetal aggregates of the first order display, are furnished by
vegetal aggregates of the second order. The equalities and inequalities
of growth in different directions, prove to be similarly related to
the equalities and inequalities of environing actions in different
directions.

Of spherical symmetry an instance occurs in _Eudorina elegans_.
The ciliated cells are here so united as to produce a small,
mulberry-shaped, hollow ball which, being similarly conditioned on all
sides, shows no unlikenesses of structure. An allied form, however,
_Volvox globator_, presents a highly instructive, though very trifling,
modification. It is not absolutely homogeneous in its structure and
is not absolutely homogeneous in its motions. The waving cilia of
its component cells have fallen into such slight heterogeneities of
action as to cause rotation in a constant direction; and along with a
fixed axis of rotation there has arisen a fixed axis of progression. A
concomitant fact is that the cells of the colony exhibit an appreciable
differentiation in relation to the fixed axis. There is an incipient
divergence from spherical uniformity along with this slight divergence
from uniformity of conditions.

Vegetal aggregates of the second order are usually fixed: locomotion
is exceptional. Fixity implies that the surface of attachment is
differently circumstanced from the free surface. Hence we may expect
to find, as we do find, that among these rooted aggregates of the
second order, as among those of the first order, the primary contrast
of shape is between the adherent part and the loose part. Sea-weeds
variously exemplify this. In some the fronds are very irregular and
in some tolerably regular; in some the form is pseudo-foliar and in
some pseud-axial; but differing though they do in these respects,
they agree in having the end which is attached to a solid body unlike
the other end. The same truth is seen in such secondary aggregates
as the common Agarics, or rather in their immensely-developed organs
of fructification. A puff-ball, Fig. 192, presents no other obvious
unlikeness of parts than that between its under and upper surfaces. So
too with the stalked kinds that frequent our woods and pastures. In the
types which Figs. 193, 194, 195, delineate, the unlikenesses between
the rooted ends and the expanded ends, as well as between the under
and upper surfaces of the expanded ends, are obviously related to this
fundamental contrast of conditions. Nor is this relation less clearly
displayed in the sessile fungi which grow out from the sides of trees,
as shown at _a_, _b_, Fig. 196. That which is common to this and the
preceding types, is the contrast between the attached end and the free
end.

  [Illustration: Figs. 192–196.]

From what these forms have in common, let us turn to that which they
have not in common, and observe the causes of the want of community.
A puff-ball shows us in the simplest way, the likeness of parts
accompanying likeness of conditions, along with the unlikeness of parts
accompanying unlikeness of conditions. For while, if we cut vertically
through its centre, we find a difference between top and bottom, if
we cut horizontally through its centre, we find no differences among
its several sides. Being, on the average of cases, similarly related
to the environment all round, it remains the same all round. The
radial symmetry of the mushroom and other vertically-growing fungi,
illustrates this connexion of cause and effect still better. But now
mark what happens in the group of _Agaricus noli-tangere_, shown in
Fig. 195. Radially-symmetrical as is the type, and radially symmetrical
as are those centrally-placed individuals which are equally crowded
all round, we see that the peripheral individuals, dissimilarly
circumstanced on their outer sides and on their sides next the group,
have partially changed their radial symmetry into bilateral symmetry.
It is no longer possible to make two corresponding halves by _any_
vertical plane cutting down through the pileus and the stem; but there
is only _one_ vertical plane that will thus produce corresponding
halves--the plane on the opposite sides of which the relations to the
environment are alike. And then mark that the divergence from all-sided
symmetry towards two-sided symmetry, here caused in the individual by
special circumstances, is characteristic of the race where the habits
of the race constantly involve two-sidedness of conditions. Besides
being exemplified by such comparatively undifferentiated types as
certain _Polypori_, Fig. 196, _a_, _b_, this truth is exemplified by
members of the genus just named. In _Agaricus horizontalis_, Fig. 196,
_c_, we have a departure from radial symmetry that is conspicuous only
in the form of the stem. A more decided bilateralness exists in _A.
subpalmatus_, shown in elevation at _d_ and in section at _d′ _. And
_Lentinus flabelliformis_, of which _e_ and _e′_ are different views,
exhibits complete bilateralness--a bilateralness in which there is the
greatest likeness of the parts that are most similarly conditioned,
and the greatest unlikeness of the parts that are most dissimilarly
conditioned.

Among plants of the second order of composition, it will suffice
to note one further class of facts which are the converse of the
foregoing and have the same implications. These are the facts showing
that along with habitual irregularity in the relations to external
forces, there is habitual irregularity in the mode of growth. Besides
finding such facts among Thallophytes, as in the tubers of underground
fungi and in the creeping films of sessile lichens, which severally
show us variations of proportions obviously caused by variations in
the amounts of the influences on their different sides, we also,
among Archegoniates of inferior types, find irregularities of form
along with irregularities in environing actions. The fronds of the
_Marchantiaceæ_ or such _Jungermanniaceæ_ as are shown in
Figs. 41, 42, 43, illustrate the way in which each lowly-organized
aggregate of the second order, not individuated by the mutual
dependence of its parts, has its form determined by the balance of
facilities and resistances which each side of the frond meets with as
it spreads.


§ 219. Among plants displaying integration of the third degree, and
among plants still further compounded, these same truths are equally
manifest. In the forms of such plants we see primary contrasts and
secondary contrasts which, no less clearly than the foregoing, are
related to contrasts of conditions.

That flowering plants from the daisy up to the oak, have in common the
fundamental unlikeness between the upward growing part and the downward
growing part; and that this most marked unlikeness corresponds with the
most marked unlikeness between the two parts of their environment, soil
and air; are facts too conspicuous to be named were they not important
items in the argument. More instructive perhaps, because less familiar,
is the fact that we miss this extreme contrast in flowering plants
which have not their higher and lower portions exposed to conditions
thus extremely contrasted. A parasite like the Dodder, growing in
entangled masses upon other plants, from which it sucks the juices, is
not thus divisible into two strongly-distinguished halves.

Leaving out of consideration the difference between the supporting
part and the supported part in phænogams, and looking at the supported
part only, we observe between its form and the habitual incidence of
forces, a relation like that which we observed in the simpler plants.
Phænogams that are practically if not literally uniaxial, and those
which develop their lateral axes only in the shape of axillary flowers,
when uninterfered with commonly send up vertical stems round which the
leaves and flowers are disposed with a more or less decided radial
symmetry. Gardens and fields supply us with such instances as the Tulip
and the Orchis; and, on a larger scale, the Palms and the Aloes are
fertile in examples. The exceptions, too, are instructive. Besides the
individual divergences arising from special interferences, there are
to be traced general divergences where the habits of the plants expose
them to general interferences in anything approaching to constant ways.
Plants which, like the Foxglove, have spikes of flowers that are borne
on flexible foot-stalks, have their flowers habitually bent round to
one face of the stem: an unlikeness of distribution probably caused
by unlikeness in the relation to the Sun’s rays. The wild Hyacinth,
too, with stem so flexible that its upper part droops, shows us how a
consequent difference in the action of gravity on the flowers, causes
them to deviate from their typically-radial arrangement towards a
bilateral arrangement.

  [Illustration: Figs. 197–199.]

Much more conspicuous are these general and special relations of form
to general and special actions in the environment, among phænogams
that are multiaxial. That when standing alone, and in places where
the winds do not injure them nor adjacent things shade them, shrubs
and trees develop with tolerable evenness on all sides, is an obvious
truth. Equally obvious is the truth that, when growing together in
a wood, and mutually interfered with on all sides, trees still show
obscurely radial distributions of parts; though, under such conditions,
they have tall taper stems with branches directed upwards--a difference
of shape clearly due to the different incidence of forces. And almost
equally obvious is the truth, that a tree of this same kind growing
at the edge of the wood, has its outer branches well developed and
its inner branches comparatively ill-developed. Fig. 197, which
inaccurately represents this difference, will serve to make it manifest
that while one of the peripheral trees can be cut into something like
two similar halves by a vertical plane directed towards the centre of
the wood--a plane on each side of which the conditions are alike--it
cannot be cut into similar halves by any other plane. A like divergence
from an indefinitely-radial symmetry towards an indefinitely-bilateral
symmetry, occurs in trees that have their conditions made bilateral
by growing on inclined surfaces. Two of the common forms observable
in such cases are given in Fig. 198. Here there is divisibility into
parts that are tolerably similar, by a vertical plane running directly
down the hill; but not by any other plane. Then, further, there is the
bilateralness, similar in general meaning though differently caused,
often seen in trees exposed to strong prevailing winds. Almost every
sea-coast has abundant examples of stunted trees which, like the one
shown in Fig. 199, have been made to deviate from their ordinary equal
growth on all sides of a vertical axis, to a growth that is equal only
on the opposite sides of a vertical plane directed towards the wind’s
eye.

From among vegetal aggregates of the third order, we have now only to
add examples of the entirely asymmetrical form which accompanies an
entirely irregular distribution of incident forces. Creeping plants
furnish such examples. They show, both when climbing up vertical or
inclined surfaces and when trailing on the ground, that their branches
grow hither and thither as the balance of forces aids or opposes;
and the general outline is without symmetry of any kind, because the
environing influences have no kind of regularity in their arrangement.


§ 220. Along with some unfamiliar facts, I have here set down facts
which are so familiar as to seem scarcely worth noting. It is because
these facts have become meaningless to perceptions deadened by infinite
repetitions of them, that it is needful here to point out their
meanings. Not alone for its intrinsic importance has the unlikeness
between the attached ends and the free ends been traced among plants
of all degrees of integration. Nor is it simply because of the
significance they have in themselves, that instances have been given
of those varieties of symmetry and asymmetry which the free ends of
plants equally display: be they plants of the first, second, third,
or any higher order. Neither has the only other purpose been that of
showing how, in the radial symmetry of some vegetal aggregates and
the single bilateral symmetry of others, there are traceable the same
ultimate principles as in the spherical symmetry and triple bilateral
symmetry of certain minute plants first described. But the main object
has been to present, under their simplest aspects, those general laws
of morphological differentiation which are fulfilled by the component
parts of each plant.

If organic form is determined by the distribution of forces, and the
approach in every case towards an equilibrium of inner actions with
outer actions; then this relation between forms and forces must hold
alike in the organism as a whole in its proximate units, and in its
units of lower orders. Formulas which express the shapes of entire
plants in terms of surrounding conditions, must be formulas which also
express the shapes of their several parts in terms of surrounding
conditions. If, therefore, we find that a plant as a whole is radially
symmetrical or bilaterally symmetrical or asymmetrical, according as
the incident forces affect it equally on all sides of an axis, or
affect it equally only on the opposite sides of one plane, or affect
it equally in no two directions; then, we may expect that, in like
manner, each member of a plant will display radial symmetry where
environing influences are alike along many radii, bilateral symmetry
where there is bilateralness of environing influences, and unsymmetry
or asymmetry where there is partial or entire departure from a balance
of surrounding actions.

To show that this expectation is borne out by the facts, will be the
object of the following four chapters. Let us begin with the largest
parts into which plants are divisible; and proceed to the successively
smaller parts.




                             CHAPTER VIII.

                        THE SHAPES OF BRANCHES.


§ 221. Aggregates of the first order supply a few examples of forms
ramified in an approximately-regular manner, under conditions
which subject their parts to approximately-regular distributions
of forces. Some unicellular _Algæ_, becoming elaborately branched,
assume very much the aspects of small trees; and show us in their
branches analogous relations of forms to forces. _Bryopsis plumosa_
may be instanced. Fig. 200 represents the end of one of its lateral
ramifications, above and beneath which come others of like characters.
Here it will be seen that the attached and free ends differ; that the
two sides are much alike; and that they are unlike the upper and under
surfaces, which resemble one another. The more highly developed members
of the same group of _Algæ_, the _Siphoneæ_, show a marked radial
symmetry co-existing with very elaborate branching, _e.g._, _Neomeris_,
_Cymopolia_, and others.

  [Illustration: Fig. 200.]


§ 222. Fig. 201 shows us how, in an aggregate of the second order,
each proximate component is modified by its relations to the rest;
just as we before saw a whole fungus of the same type modified by
its relations to environing objects. If a branch of the fungus here
figured, be compared with one of the fungi clustered together in Fig.
195, or, still better, with one of the laterally-growing fungi shown
in Fig. 196, there will be perceived a kindred transition from radial
to bilateral symmetry, occurring under kindred conditions. The portion
of the pileus next to the side of attachment is undeveloped in this
branched form as in the simpler form; and in the one case as in the
other, the stem is modified towards the side of attachment. A division
into similar halves, which, as shown in Fig. 196 _e_, might be made of
the whole fungus by a vertical plane passing through the centre of the
pileus and the axis of the supporting body, might here be made of the
branch, by a vertical plane passing through the centre of its pileus
and the axis of the main stem. Among aggregates of this order, the
_Algæ_ furnish cases of kindred nature. In the branches of _Lessonia_,
Fig. 37, may be observed a substantially-similar relationship. As their
inner parts are less developed than their outer parts, while their two
sides are developed in approximately equal degrees, they are rendered
bilateral.

  [Illustration: Fig. 201.]


§ 223. These few cases introduce us to the more familiar but more
complex cases which plants of the third degree of aggregation present.
At _a_, _b_, _c_, Fig. 202, are sketched three homologous parts of the
same tree: _a_ being the leading shoot; _b_ a lateral branch near
the top, and _c_ a lateral branch lower down. There is here a double
exemplification. While the branch _a_, as a whole, has its branchlets
arranged with tolerable regularity all round, in correspondence with
its equal exposure on all sides, each branchlet shows by its curve
as much bilateral symmetry as its simple form permits. The branch
_b_, dissimilarly circumstanced on the side next the main stem and
on the side away from it, has an approximate bilateralness as a
whole, while the bilateralness of its branchlets varies with their
respective positions. And in the branch _c_, having its parts still
more differently conditioned, these traits of structure are still more
marked. Extremely strong contrasts of this kind occur in trees having
very regular modes of growth. The uppermost branches of a Spruce-fir
have radially-arranged branchlets: each of them, if growing vigorously,
repeats the type of the leading shoot, as shown in Fig. 203, _a_,
_b_. But if we examine branches lower and lower down the tree, we
find the vertically-growing branchlets bear a less and less ratio to
the horizontally-growing ones; until, towards the bottom, the radial
arrangement has wholly merged into the bilateral. Shaded and confined
by the branches above them, these eldest branches develop their
offshoots in those directions where there is most space and light:
becoming finally quite flattened and fan-shaped, as shown at Fig.
203, _c_. And on remembering that each of these eldest branches, when
first it diverged from the main stem, was radial, we see not only that
between the upper and lower branches does this contrast in structure
hold, but also that each branch is transformed from the radial to
the bilateral by the progressive change in its environment. Other
forces besides those which aid or hinder growth, conspire to produce
this two-sided character in lateral branches. The annexed Fig. 204,
sketched from an example of the _Pinus Coulterii_ at Kew, shows very
clearly how, by mere gravitation, the once radially-arranged branchlets
may be so bent as to produce in the branch as a whole a decided
bilateralness. A full-grown _Araucaria_, too, exhibits in its lower
branches modifications similarly caused; and in each of such branches
there may be remarked the further fact, that its upward-bending
termination has a partially-modified radialness, at the same time that
its drooping lateral branchlets give to the part nearer the trunk a
completely bilateral character.

  [Illustration: Figs. 202–204.]

Now in these few instances, typical of countless instances which might
be given, we see, as we saw in the case of the fungi, that the same
thing is true of the parts in their relations to the whole and to one
another, which is true of the whole in its relations to the environment
at large. Entire trees become bilateral instead of radial, when exposed
to forces that are equal only on opposite sides of one plane; and in
their branches, parallel changes of form occur under parallel changes
of conditions.


§ 224. There remains to be said something respecting the distribution
of leaves. How a branch carries its leaves constitutes one of
its characters as a branch, and is to be considered apart from
the characters of the leaves themselves. The principles hitherto
illustrated we shall here find illustrated still further.

The leading shoot and all the upper twigs of a fir-tree, have their
pin-shaped leaves evenly distributed all round, or placed radially;[32]
but as we descend we find them beginning to assume a bilateral
distribution; and on the lower, horizontally-growing branches, their
distribution is quite bilateral.[33] Between the Irish and English
kinds of Yew, there is a contrast of like significance. The branches
of the one, shooting up as they do almost vertically, are clothed with
leaves all round; while those of the other, which spread laterally,
bear their leaves on the two sides. In trees with better-developed
leaves, the same principle is more or less manifest in proportion as
the leaves are more or less enabled by their structures to maintain
fixed positions. Where the foot-stalks are long and slender, and where,
consequently, each leaf, according to its weight, the flexibility and
twist of its foot-stalk, and the direction of the branch it grows from,
falls into some indefinite attitude, the relations are obscured. But
where the foot-stalks are stiff, as in the Laurel, it will be found, as
before, that from the topmost and upward growing branches the leaves
diverge on all sides; while the undermost branches, growing out from
the shade of those above, have their leaves so turned as to bring them
into rows horizontally spread out on the two sides of each branch.

  [Illustration: Fig. 205.]

A kindred truth, having like implications, comes into view when we
observe the relative sizes of leaves on the same branch, where their
sizes differ. Fig. 205 represents a branch of a Horse-chestnut, taken
from the lowermost fringe of the tree, where the light has been to
a great extent intercepted from all but the most protruded parts.
Beyond the fact that the leaves become by appropriate growths of their
foot-stalks bilaterally distributed on this drooping branch, instead of
being distributed symmetrically all round, as on one of the ascending
shoots, we have here to note the fact that there is unequal development
on the upper and lower sides. Each of the compound leaves acquires a
foot-stalk and leaflets that are large in proportion to the supply
of light; and hence, as we descend towards the bottom of the tree,
the clusters of leaves display increasing contrasts. How marked these
contrasts become will be seen on comparing _a_ and _b_, which form one
pair of leaves that are normally equal, or _c_ and _d_, which form
another pair normally equal.

Let us not omit to note, while we have this case before us, the proof
it affords that these differences of development are in a considerable
degree determined by the different conditions of the parts after they
have been unfolded. Though those inequalities of dimensions whence
the differentiations of form result, may be in many cases largely due
to the inequalities in the circumstances of the parts while in the
bud (which are, however, representative of inequalities in ancestral
circumstances); yet these are clearly not the sole causes of the
unlikenesses which eventually arise. This bilateralness resulting from
the unequal sizes of the leaves, must be considered as due to the
differential actions that come into play after the leaves have assumed
their typical structures.


§ 225. How, in the arrangement of their twigs and leaves, branches tend
to lapse from forms that are approximately symmetrical to forms that
are quite asymmetrical, need not be demonstrated: it is sufficiently
conspicuous. But it may be well to point out how the tendency to
do this further enforces our argument. The comparatively regular
budding out of secondary axes and tertiary axes, does not usually
produce an aggregate which maintains its regularity, for the simple
reason that many of the axes abort. Terminal buds are some of them
destroyed by birds; others are burrowed into by insects; others are
nipped by frost; others are broken off or injured during gales of wind.
The environment of each branch and its branchlets is thus ever being
varied on all sides: here, space being left vacant by the death of some
shoot that would ordinarily have occupied it; and there, space being
trenched on by the lateral growth of some adjacent branch that has had
its main axis broken. Hence the asymmetry, or heterogeneity of form,
assumed by the branch, is caused by the asymmetrical distribution of
incident forces--a result and a cause which go on ever complicating.


§ 226. One conspicuous trait in the shapes of branches has still to be
named. Their proximal or attached ends differ from their distal or free
ends, in the same way that the lower ends of trees differ from their
upper ends. This fact, like the fact to which it is here paralleled,
has had its significance obscured by its extreme familiarity. But
it shows in a striking way how the most differently conditioned
parts become the most strongly contrasted in their structures. A
phænogamic axis is made up of homologous segments, marked off from
one another by the nodes; and a compound branch consists of groups of
such segments. The earliest-formed segments, alike of the tree and of
each branch, serve as mechanical supports and channels for sap to the
successive generations of segments that grow out of them; and become
more and more shaded by their progeny as these increase. Hence the
progressively-increasing contrasts which, while mainly due to the
unlikenesses of bulk accompanying differences of age, are in part due
to the unlikenesses of structure which differences of relation to the
environment have caused.


§ 227. Thus, then, it is with the proximate parts of plants as it is
with plants as wholes. The radial symmetry, the bilateral symmetry, and
the asymmetry, which branches display in different trees, in different
parts of the same tree, and at different stages of their own growths,
prove to be all consequent on the ways in which they stand towards the
entire plexus of surrounding actions. The principle that the growths
are unequal in proportion as the relations of parts to the environment
are unequal, serves to explain all the leading traits of structure.




                              CHAPTER IX.

                         THE SHAPES OF LEAVES.


§ 228. Next in the descending order of composition come compound
leaves. The relative sizes and distributions of their leaflets, as
affecting their forms as wholes, have to be considered in their
relations to conditions. Figs. 206, 207, represent leaves of the
common _Oxalis_ and of the _Marsilea_, in which radial symmetry is
as completely displayed as the small number of leaflets permits.
This equal development of the leaflets on all sides, occurs where
the foot-stalks, growing up vertically from creeping or underground
stems, are so long that the leaves either do not interfere with one
another or do it in an inconstant way: the leaflets are not differently
conditioned on different sides, as they are where the foot-stalks grow
out in the ordinary manner. How unlikeness of position influences the
leaflets is clearly shown in a Clover-leaf, Fig. 208, which deviates
from the Oxalis-leaf but slightly towards bilateralness, as it deviates
from it but slightly in the attitude of its petiole; which is a little
inclined away from the others borne by the same procumbent axis.
A familiar example of an almost radial symmetry along with almost
equal relations to surrounding conditions, occurs in the root-leaves
of the Lupin, Fig. 209 _b_. Here though we have lateral divergence
from a vertical axis, yet the long foot-stalks preserve nearly erect
positions, and carry their leaves to such distances from the axis,
that the development of the leaflets on the side next it is not much
hindered. Still the interference of the leaves with one another is, on
the average, somewhat greater on the proximal side than on the distal
side; and hence the interior leaflets are rather less than the exterior
leaflets. In further proof of which influence, let it be added that, as
shown in the figure, at _a_, the leaves growing out of the flowering
stem deviate towards the two-sided form more decidedly. Two-sidedness
is much greater where there is a greater relative proximity of the
inner leaflets to the axis, or where the foot-stalk approaches towards
a horizontal position. The Horse-chestnut, Fig. 205, already instanced
as showing how the arrangements and sizes of leaflets are determined
by the incidence of forces, serves also to show how the incidence of
forces determines the relative sizes and arrangements of leaflets.
Fig. 210, which shows a leaf of the _Bombax_, further illustrates this
relation of structure to conditions.

  [Illustration: Figs. 206–210.]

  [Illustration: Figs. 211, 212.]

Compound leaves that are completely bilateral, present us with
modifications of form exemplifying the same general truth in another
way. In them the proximal and distal parts have none of that
resemblance which we see in those intermediate forms just described.
The portion next the axis and the portion furthest from the axis
are entirely different; and the only likeness is between the wings
or leaflets on opposite sides of the main foot-stalk or mid-rib. On
turning back to Fig. 65, it will be seen that the compound leaf there
drawn to exemplify another truth, serves also to exemplify this truth:
the homologous parts _a_, _b_, _c_, _d_, while they are unlike one
another, are, in their main proportions, severally like the parts with
which they are paired. And here let us not overlook a characteristic
which is less conspicuous but not less significant. Each of the lateral
wings has winglets that are larger on the one side than on the other;
and in each case the two sides are dissimilarly conditioned. Even in
the several components of each wing may be traced a like divergence
from symmetry, along with a like inequality in the relations to the
rest: the proximal half of each leaflet is habitually larger than the
distal half. In the leaves of the Bramble, previously figured, kindred
facts are presented. How far such differences of development are due to
the positions of the parts in the bud; how far the respective spaces
available for the parts when unfolded affect them; and how far the
parts are rendered unlike by unlikenesses in their relations to light;
it is difficult to say. Probably these several factors operate in all
varieties of proportion. That the habitual shading of some parts by
others largely aids in causing these divergences from symmetry, is very
instructively shown by the compound leaves of the Cow-parsnip. Fig.
211 represents one of these. While the leaf as a whole is bilaterally
symmetrical, each of the wings has an unsymmetrical bilateralness: the
side next the axis being larger than the remoter side. How does this
happen? Fig. 212, which is a diagrammatic section down the mid-rib
of the leaf, showing its inclined attitude and the positions of the
wings _a_, _b_, _c_, will make the cause clear. As the wings overlap,
like the bars of a Venetian blind, each intercepts some light from the
one below it; and the one below it thus suffers more on its distal
side than on its proximal side. Hence the smaller development of the
distal side. That this is the cause is further shown by the proportion
that is maintained between the degree of obscuration and the degree
of non-development; for this unlikeness is greater between the two
sides _a_ and _a′_, than between _b_ and _b′_ or _c_ and _c′_, at the
same time that the interference is greater in the lower wings than in
the upper. Of course in this case and in the kindred cases hereafter
similarly interpreted, it is not meant that this differentiation
is consequent solely, or even chiefly, on the differential actions
experienced by the individual plant. Though there is good reason to
believe that the rate of growth in each part of each leaf is affected
by the incidence of light, yet contrasts so marked and so systematic as
these are not explicable without taking into account the inheritance
of modifications either functionally caused or caused by spontaneous
variation. Clearly, the tendency will be towards the preservation of a
plant which distributes its chlorophyll in the most advantageous way;
and hence there will always be a gravitation towards a form in which
shaded parts of leaves are undeveloped.


§ 229. From compound leaves to simple ones, we find transitions in
leaves of which the divisions are partial instead of total; and in
these we see, with equal clearness, the relations between forms and
positions that have been traced thus far. Fig. 213 is the leaf of a
Winter-aconite in which, round a vertical petiole, there is a radial
distribution of half-separated leaflets. The _Cecropia_-leaf, Fig. 214,
shows us a two-sided development of the parts beginning to modify, but
not obliterating, the all-sided arrangement; and this mixed symmetry
occurs under conditions that are intermediate. A more marked degree of
the same relation is presented in the leaf of the Lady’s Mantle, Fig.
215. And then in the Sycamore and the Vine, we have a cleft type of
leaf in which a decided bilateralness of form co-exists with a decided
bilateralness of conditions.

  [Illustration: Figs. 213–215.]

  [Illustration: Figs. 216–218.]

The quite simple leaves to which we now descend, exhibit, very
distinctly, a parallel series of facts. Where they grow up on long and
completely-independent foot-stalks, without definite subordination
to some central vertical axis, the leaves of water-plants are
symmetrically peltate. Of this the sacred Indian-bean, Fig. 216,
furnishes an example. Here there is only a trace of bilateralness in
the venation of the leaf, corresponding to the very small difference
of the conditions on the proximal and distal sides. In the _Victoria
regia_, Fig. 217, the foot-stalks, though radiating almost
horizontally from a centre, are so long as to keep the leaves quite
remote from one another; and in it each leaf is almost symmetrically
peltate, with a bilateralness indicated only by a seam over the
line of the foot-stalk. The leaves of the _Nymphæa_, Fig. 218,
more closely clustered, and having less room transversely than
longitudinally, exhibit a marked advance to the two-sided form; not
only in the excess of the length over the breadth, but in the existence
of a cleft, where in the _Victoria regia_ there is merely a seam.
Among land-plants similar forms are found under analogous conditions.
The common _Hydrocotyle_, Fig. 219, which sends up direct from its
roots a few almost upright leaf-stalks, has these surmounted by peltate
leaves; which leaves, however, diverge slightly from radial symmetry in
correspondence with the slight contrast of circumstances which their
grouping involves. Another case is supplied by the Nasturtium, Fig.
220, which combines the characters--a creeping stem, long leaf-stalks
growing up at right angles to it, and unsymmetrically peltate leaves,
of which the least dimension is, on the average, towards the stem.
But perhaps the most striking illustration is that furnished by the
_Cotyledon umbilicus_, Fig. 221, in which different kinds of
symmetry occur in the leaves of the same plant, along with differences
in their relations to conditions. The root-leaves, _a_, growing up
on vertical petioles before the flower-stalk makes its appearance, are
symmetrically peltate; while the leaves which subsequently grow out of
the flower-stalk, _b_, are at the bottom transitionally bilateral,
and higher up completely bilateral.

  [Illustration: Figs. 219–221.]

That the bilateral form of leaf is the ordinary form, corresponds with
the fact that, ordinarily, the circumstances of the leaf are different
in the direction of the plant’s axis from what they are in the opposite
direction, while transversely the circumstances are alike. It is
needless to give diagrams to illustrate this extremely familiar truth.
Whether they are broad or long, oval or heart-shaped, pointed or
obtuse, the leaves of most trees and plants will be remembered by all
as having the ends by which they are attached unlike the free ends,
while the two sides are alike. And it will also be remembered that
these equalities and inequalities of development correspond with the
equalities and inequalities in the incidence of forces.


§ 230. A confirmation that is interesting and important, is furnished
by the cases in which leaves present unsymmetrical forms in positions
where their parts are unsymmetrically related to the environment. A
considerable deviation from bilateral symmetry may be seen in a leaf
which habitually so carries itself, that the half on the one side of
the mid-rib is more shaded than the other half. The drooping branches
of the Lime, delineated in Fig. 222, show us leaves so arranged and
so modified. On examining their attitudes and their relations one to
another, it will be found that each leaf is so inclined that the half
of it next to the shoot grows over the shoot and gets plenty of light;
while the other half so hangs down that it comes a good deal into the
shade of the preceding leaf. The result is that having leaves which
fall into these positions, the species profits by a large development
of the exposed halves; and by survival of the fittest, acting along
with the direct effect of extra exposure, this modification becomes
established. How unquestionable is the connexion between the relative
positions of the halves and their relative developments, will be
admitted on observing a converse case. Fig. 223 represents a shoot
of _Strobilanthes glomeratus_. Here the leaves are so set on the stem
that the inner half of each leaf is shaded by the subsequently-formed
leaf, while its outer half is not thus shaded; and here we find the
inner half less developed than the outer half. But the most conclusive
evidence of this relation between unsymmetrical form and unsymmetrical
distribution of surrounding forces, is supplied by the genus _Begonia_;
for in it we have a manifest proportion between the degree of the
alleged effect and the degree of the alleged cause. These plants
produce their leaves in pairs, in such ways that the connate leaves
interfere with one another, much or little according as the foot-stalks
are short or long; and the result is a correlative divergence from
symmetry. In _Begonia nelumbiifolia_, which has petioles so long that
the connate leaves are not kept close together, there is but little
deviation from a bilaterally-peltate form; whereas, accompanying the
comparatively marked and constant proximity in _B. pruinata_, Fig. 224,
we see a more decidedly unsymmetrical shape; and in _B. mahringii_,
Fig. 225, the modification thus caused is pushed so far as to destroy
the peltate structure.[34]

  [Illustration: Figs. 222–225.]


§ 231. Again, then, we are taught the same truth. Here, as before,
we see that homologous units of any order become differentiated in
proportion as their relations to incident forces become different. And
here, as before, we see that in each unit, considered by itself, the
differences of dimension are greatest in those directions in which the
parts are most differently conditioned; while there are no differences
between the dimensions of the parts that are not differently
conditioned.[35]




                              CHAPTER X.

                        THE SHAPES OF FLOWERS.


§ 232. Following an order like that of preceding chapters, let us first
note a few typical facts respecting the forms of clusters of flowers,
apart from the forms of the flowers themselves. Two kindred kinds of
_Leguminosæ_ serve to show how the members of clusters are distributed
in an all-sided manner or in a two-sided manner, according as the
circumstances are alike on all sides or alike on only two sides. In
_Hippocrepis_, represented in Fig. 226, the flowers growing at the
end of a vertical stem, are arranged round it in radial symmetry.
Contrariwise in _Melilotus_, Fig. 227, where the axillary stem bearing
the flowers is so placed in relation to the main stem, that its outer
and inner faces are differently conditioned, the flowers are all on the
outer face: the cluster is bilaterally symmetrical, since it may be cut
into approximately equal and similar groups by a vertical plane passing
through the main axis.

  [Illustration: Figs. 226, 227.]

Plants of this same tribe furnish clusters of intermediate characters
having intermediate conditions. Among these, as among the clusters
which other types present, may be found some in which conformity
to the general law is not obvious. The discussion of these apparent
anomalies would carry us too much out of our course. A clue to the
explanation of them will, I believe, be found in the explanation
presently to be given of certain kindred anomalies in the forms of
individual flowers.


§ 233. The radially-symmetrical form is common to all individual
flowers that have vertical axes. In plants which are practically if
not literally uniaxial, and bear their flowers at the ends of upright
stalks, so that the faces open horizontally, the petals are disposed in
an all-sided way. Crocuses, Tulips, and Poppies are familiar examples
of this structure occurring under these conditions. A Ranunculus
flower, Fig. 228, will serve as a typical one. Similarly, flowers which
have peduncles flexible enough to let them hang directly downwards, and
are not laterally incommoded, are also radial; as in the _Fuchsia_,
Fig. 229, as in _Cyclamen_, _Hyacinth_, &c. These relations of form
to position are, I believe, uniform. Though some flowers carried at
the ends of upright or downright stems have oblique shapes, it is only
when they have inclined axes or are not equally conditioned all round.
No solitary flower having an axis habitually vertical, presents a
bilateral form. This is as we should expect; since flowers which open
out their faces horizontally, whether facing upwards or downwards, are,
on the average, similarly affected on all sides.

  [Illustration: Figs. 228, 229.]

  [Illustration: Figs. 230, 231.]

At first it seems that flowers thus placed should alone be radial;
but further consideration discloses conditions under which this
type of symmetry may exist in flowers otherwise placed. Remembering
that the radial form is the primitive form--that, morphologically
speaking, it results from the contraction into a whorl, of parts that
are originally arranged in the same spiral succession as the leaves;
we must expect it to continue wherever there are no forces tending
to change it. What now must be the forces tending to change it? They
must be forces which do not simply affect differently the different
parts of an individual flower. They must be forces which affect in
like contrasted ways the homologous parts of other individual flowers,
both on the same plant and on surrounding plants of the same species.
A permanent modification can be expected only in cases where, by
inheritance, the effects of the modifying causes accumulate. That
they may accumulate the flowers must keep themselves so related to
the environment, that the homologous parts may, generation after
generation, be subjected to like differentiating forces. Hence, among
a plant’s flowers which maintain no uniformity in the relations of
their parts to surrounding influences, the radial form will continue.
Let us glance at the several causes which entail this variability.
When flowers are borne on many branches, which have all inclinations
from the vertical to the horizontal--as are the flowers of the Apple,
the Plum, the Hawthorn--they are placed in countless different
attitudes. Consequently, any spontaneous variation in shape which
might be advantageous were the attitude constant, is not likely to
be advantageous; and any functionally-produced modification in one
flower, is likely to be neutralized in offspring by some opposite
functionally-produced modification in another flower. It is quite
comprehensible, therefore, that irregularly-branched plants should
thus preserve their laterally-borne flowers from undergoing permanent
deviations from their primitive radial symmetry. Fig. 230, representing
a blossoming twig of the Blackthorn, illustrates this. Again, upright
panicles, such as those of the Saxifrage exemplified in Fig. 231,
and irregular terminal groups of flowers otherwise named, furnish
conditions under which there is similarly an absence of determinate
relations between the parts of the flowers and the incident forces;
and hence an absence of bilateralness. This inconstancy of relative
position is produced in various other ways--by extreme flexibility of
the stems, as in the Blue-bell; by the tendency of the peduncles to
curl to a greater or less extent in diverse directions, as in _Pyrola_;
by special twistings of the peduncles, differing in degree in different
individuals, as in _Convolvulus_; by unusual laxity of the petals,
as in _Lythrum_. Elsewhere the like general result arises from a
progressive change of attitude, as in _Myosotis_, the stem of which as
it unfolds causes each flower to undergo a transition from an upward
position of the mouth to a lateral position; or as in most _Cruciferæ_,
where the like effect follows from an altered direction of the peduncle.

There are, however, certain seemingly-anomalous cases where radial
symmetry is maintained by laterally-placed flowers, which keep
their parts in relative positions that are tolerably constant. The
explanation of these exceptions is not manifest. It is only when
we take into account certain incident actions liable to be left
unremembered, that we find a probable solution. It will be most
convenient to postpone the consideration of these cases until we have
reached the general rule to which they are exceptions.


§ 234. Transitions varying in degree from the radial towards the
bilateral, are common in flowers that are borne at the ends of branches
or axes which are inclined in tolerably constant ways. We may see this
in sundry garden flowers such as _Petunia_, or such as _Isoloma_ and
_Achimenes_, shown in Figs. 232 and 233. If these plants be examined,
it will be perceived that the mode of growth makes the flower unfold
in a partially one-sided position; that its parts of attachment have
rigidity sufficient to prevent this attitude from being very much
interfered with; and that though the individual flowers vary somewhat
in their attitudes, they do not vary to the extent of neutralizing the
differentiating conditions--there remains an average divergence from a
horizontal unfolding of the flower, to account for its divergence from
radial symmetry.

  [Illustration: Figs. 232, 233.]

We pass insensibly from forms like these, to forms having bilateral
symmetry strongly pronounced. Some such forms occur among flowers that
grow at the ends of upright stems; as in _Pinguicula_, and in the
Violet tribe. But this happens only where, in successive generations,
the flower unfolds its parts sideways in constant relative positions.
And in the immense majority of flowers having well-marked two-sided
forms, the habitual exposure of the different parts to different
sets of forces, is effectually secured by the mode of placing. As
illustrations, I may name the genera--_Orchis_, _Utricularia_,
_Salvia_, _Salix_, _Delphinium_, _Mentha_, _Teucrium_, _Ajuga_,
_Ballota_, _Galeopsis_, _Lamium_, _Stachys_, _Nepeta_, _Marrubium_,
_Calamintha_, _Melittis_, _Prunella_, _Scutellaria_, _Bartsia_,
_Euphrasia_, _Rhinanthus_, _Melampyrum_, _Pedicularis_, _Linaria_,
_Digitalis_, _Orobanche_, _Fumaria_, _&c._; to which may be added all
the Grasses and all the _Papilionaceæ_. In most of these cases the
flowers, being sessile on the sides of upright stems, are kept in quite
fixed attitudes; and in the other cases the peduncles are very short,
or else stiff enough to secure general uniformity in the positions. A
few of the more marked types are shown in Figs. 234 to 241.

  [Illustration: Figs. 234–241.]

  [Illustration: Figs. 242–246.]

  [Illustration: Figs. 247, 248.]

Very instructive evidences here meet us. Sometimes within the limits
of one genus we find radial flowers, bilateral flowers, and flowers of
intermediate characters. The genus

_Begonia_ may be instanced. In _B. rigida_ the flowers, various in
their attitudes, are in their more conspicuous characters radial:
though there is a certain bilateralness in the calyx, the five petals
are symmetrically disposed all round. _B. Wageneriana_ furnishes
two forms of flowers. On the same individual plant may be found
radial flowers like Fig. 242, and others, like Fig. 243, which are
merging into the bilateral. More decided is the bilateralness in _B.
albo-coccinea_, Fig. 244; and still more in _B. nitida_, Fig. 245.
While in _B. heracleifolia_, Fig. 246, the change reaches its extreme
by the disappearance of the lateral petals. On examining the modes
of growth in these several species, they will be seen to explain
these changes in the manner alleged. Even more conclusive are the
nearly-allied transformations occurring in artificially-produced
varieties of the same species. _Gloxinia_ may be named in illustration.
In Fig. 247 is represented one of the ordinary forms, which shows us
bilateralness of shape along with a mode of growth that renders the
conditions alike on the two sides while different above and below.
But in _G. erecta_, Fig. 248, we have the flower assuming an upright
attitude, and at the same time assuming the radial type. This is not
to be interpreted as a production of radial symmetry out of bilateral
symmetry, under the action of the appropriate conditions. It is rather
to be taken as a case of what is termed “peloria”--a reversion to the
primitive radial type, from which the bilateral modification had been
derived. The significant inference to be drawn from it is, that this
primitive radial type had an upright attitude; and that the derivation
of a bilateral type from it, occurred along with the assumption of an
inclined attitude.

We come now to a group of cases above referred to, in which radial
symmetry continues to co-exist with that constant lateral attitude
ordinarily accompanied by the two-sided form. Two examples will
suffice: one a very large flower, the Hollyhock, and the other a very
small flower, the Agrimony. Why does the radial form here remain
unchanged? and how does its continuance consist with the alleged
general law?

Until quite recently I have been unable to find any probable answers
to these questions. When the difficulty first presented itself, I
could think of no other possible cause for the anomaly, than that the
parts of the Hollyhock-flower, unfolding spirally as they do, might
have different degrees of spiral twist in different flowers, and might
thus not be unfolded in sufficiently-constant positions. But this
seemed a questionable interpretation; and one which did not obviously
apply to the case of the Agrimony. It was only on inquiring what are
the special causes of modifications in the forms of flowers, that a
more feasible explanation suggested itself; and this would probably
never have suggested itself, had not Mr. Darwin’s investigations into
the fertilization of Orchids led me to take into account an unnoticed
agency.

  [Illustration: Fig. 249.]

The actions which affect the forms of leaves, affect much less
decidedly the forms of flowers; and the forms of flowers are influenced
by actions which do not influence the forms of leaves. Partly through
the direct action of incident forces and partly through the indirect
action of natural selection, leaves get their parts distributed in
ways that most facilitate their assimilative functions, under the
circumstances in which they are placed; and their several types
of symmetry are thus explicable. But in flowers, the petals and
fructifying organs of which do not contain chlorophyll, the tendency to
grow most where the supply of light is greatest, is less decided, if
not absent; and a shape otherwise determined is hence less liable to
alter in consequence of altered relations to sun and air. Gravity, too,
must be comparatively ineffective in causing modifications: the smaller
sizes of the parts, as well as their modes of attachment, giving them
greater relative rigidity. Not, indeed, that these incident forces of
the inorganic world are here quite inoperative. Fig. 249, representing
a species of _Campanula_, shows that the developments of individual
flowers are somewhat modified by the relations of their parts to
general conditions. But the fact to be observed is, that the extreme
transformations which flowers undergo are not likely to be thus caused:
some further cause must be sought. And if we bear in mind the functions
of flowers, we shall find in their adaptations to these functions,
under conditions that are extremely varied, an adequate cause for the
different types of symmetry, as well as for the exceptions to them.
Flowers are parts in which fertilization is effected; and the active
agents of this fertilization are insects--bees, moths, butterflies,
&c. Mr. Darwin has shown in many cases, that the forms and positions
of the essential organs of fructification, are such as to facilitate
the actions of insects in transferring pollen from the anthers of one
flower to the pistil of another--an arrangement produced by natural
selection. And here we shall find reason for concluding, that the forms
and positions of those subsidiary parts which give their shapes to
flowers, similarly arise by the survival of individuals which have the
subsidiary parts so adjusted as to aid this fertilizing process--the
deviations from radial symmetry being among such adjustments. The
reasoning is as follows. So long as the axis of a flower is vertical
and the conditions are similar all round, a bee or butterfly alighting
on it, will be as likely to come from one side as from another; and
hence, hindrance rather than facilitation would result if the several
sides of the flower did not afford it equally free access. In like
manner, flowers which are distributed over a plant in such ways that
their discs open out on planes of all directions and inclinations, will
have no tendency to lose their radial symmetry; since, on the average,
no part of the periphery is differently related to insect-agency
from any other part. But flowers so fixed as to open out sideways in
tolerably-constant attitudes, have their petals differently related
to insect-agency. A bee or butterfly coming to a laterally-growing
flower, does not settle on it in one way as readily as in another; but
almost of necessity settles with the axis of its body inclined upwards
towards the stem of the plant. Hence the side-petals of a flower so
fixed, habitually stand to the alighting insect in relations different
from those in which the upper and lower petals stand; and the upper
and lower petals differ from one another in their relations to it. If,
then, there so arises an habitual attitude of the insect towards the
petals, there is likely to be some arrangement of the petals that will
be most convenient to the insect--will most facilitate its entrance
into the flower. Thus we see in many cases, that a long undermost
petal or lip, by enabling the insect to settle in such way as to bring
its head opposite to the opening of the tube, aids its fertilizing
agency. But whatever be the special modifications of the corolla which
facilitate the actions of the particular insects concerned, all of them
will conduce to bilateral symmetry; since they will be alike for the
two sides but unlike for the top and bottom. And now we are prepared
for understanding the exceptions. Flowers growing sideways can become
thus adapted by survival of the fittest, only if they are of such sizes
and structures that insect-agency can affect them in the way described.
But in the plants named above, this condition is not fulfilled. A
Hollyhock-flower is so open, as well as so large, that its petals
are not in any appreciable degree differently related to the insects
which visit it. On the other hand, the flower of the Agrimony is so
small, that unless visited by insects of a corresponding size which
settle as bees and butterflies settle, its parts will not be affected
in the alleged manner. That all anomalies of this kind can at once be
satisfactorily explained, is scarcely to be expected: the circumstances
of each case have to be studied. But it seems not improbable that they
are due to causes of the kind indicated.[36]


§ 235. We have already glanced at clusters of flowers for the purpose
of considering their shapes as clusters. We must now return to them to
observe the modifications undergone by their component flowers. Among
these occur illustrations of great significance.

An example of transition from the radial to the bilateral form in
clustered flowers of the same species, is furnished by the cultivated
_Geraniums_, called by florists _Pelargoniums_. Some of these, bearing
somewhat small terminal clusters of flowers, which are closely packed
together with their faces almost upwards, have radially-symmetrical
flowers. But among other varieties having terminal clusters of which
the members are mutually thrust on one side by crowding, the flowers
depart very considerably from the radial shape towards the bilateral
shape. A like result occurs under like conditions in Rhododendrons
and Azaleas. The _Verbena_, too, furnishes an illustration of radial
flowers rendered slightly two-sided by the slight two-sidedness
of their relations to other flowers in the cluster. And among the
_Cruciferæ_ a kindred case occurs in the cultivated Candytuft.

Evidence of a somewhat different kind is offered us by clustered
flowers in which the peripheral members of the clusters differ from
the central members; and this evidence is especially significant
where we find allied species that do not exhibit the deviation, at
the same time that they do not fulfil the conditions under which it
may be expected. Thus, in _Scabiosa succisa_, Fig. 250, which bears
its numerous small flowers in a hemispherical knob, the component
flowers, similarly circumstanced, are all equal and all radial; but in
_Scabiosa arvensis_, Fig. 251, in which the numerous small flowers form
a flattened disk only the confined central ones are radial: round the
edge the flowers are much larger and conspicuously bilateral.

  [Illustration: Figs. 250, 251.]

  [Illustration: Fig. 252.]

But the most remarkable and most conclusive proofs of these relations
between forms and positions, are those given by the clustered flowers
called _Umbelliferæ_. In some cases, as where the component flowers
have all plenty of room, or where the surface of the umbel is more or
less globular, the modifications are not conspicuous; but where, as
in _Viburnum_, _Chærophyllum_, _Anthriscus_, _Torilis_, _Caucalis_,
_Daucus_, _Tordylium_, &c., we have flowers clustered in such ways as
to be differently conditioned, we find a number of modifications that
are marked and varied in proportion as the differences of conditions
are marked and varied. In _Chærophyllum_, where the flowers of each
umbellule are closely placed so as to form a flat surface, but
where the umbellules are wide apart and form a dispersed umbel, the
umbellules do not differ from one another; though among the flowers
of each umbellule there are decided differences: the central flowers
being small and radial, while the peripheral ones are large and
bilateral. But in other genera, where not only the flowers of each
umbellule but also the umbellules themselves, are closely clustered
into a flat surface, the umbellules themselves become contrasted;
and many remarkable secondary modifications arise. In an umbel of
_Heracleum_, for instance, there are to be noted the facts;--first,
that the external umbellules are larger than the internal ones; second,
that in each umbellule the central flowers are less developed than the
peripheral ones; third, that this greater development of the peripheral
flowers is most marked in the outer umbellules; fourth, that it is
most marked on the outer sides of the outer umbellules; fifth, that
while the interior flowers of each umbellule are radial, the exterior
ones are bilateral; sixth, that this bilateralness is most marked in
the peripheral flowers of the peripheral umbellules; seventh, that the
flowers on the outer sides of these peripheral umbellules are those in
which the bilateralness reaches a maximum; and eighth, that where the
outer umbellules touch one another, the flowers, being unsymmetrically
placed, are unsymmetrically bilateral.[37] The like modifications are
displayed, though not in so clearly-traceable a way, in an umbel of
_Tordylium_, Fig. 252. Considering how obviously these various forms
are related to the various conditions, we should be scarcely able, even
in the absence of all other facts, to resist the conclusion that the
differences in the conditions are the causes of the differences in the
forms.

  [Illustration: Fig. 253.]

Composite flowers furnish evidence so nearly allied to that which
clustered flowers furnish, that we may fitly glance at them under
the same head. Such a common type of this order as the Sun-flower,
exemplifies the extremely marked difference which arises in many
of these plants between the closely-packed internal florets, each
similarly circumstanced on all sides, and the external florets, not
similarly circumstanced on all sides. In Fig. 253, representing the
inner and outer florets of a Daisy, the contrast is marked between the
small radial corolla of the one and the larger bilateral corolla of the
other. In many cases, however, this contrast is less marked: the inner
florets also having their outward-growing prolongations--a difference
possibly related to some difference in the habits of the insects that
fertilize them. Nevertheless, these composite flowers which have
inner florets with strap-shaped corollas outwardly directed, equally
conform to the general principle; both in the radial arrangement of
the assemblage of florets, and in the bilateral shape of each floret;
which has its parts alike on the two sides of a line passing from the
centre of the assemblage to the circumference. Certain other members
of this order fulfil the law somewhat differently. In _Centaurea_,
for instance, the inner florets are small and vertical in direction,
while the outer florets are large and lateral in direction. And here
may be remarked, in passing, a clear indication of the effect which
great flexibility of the petals has in preventing a flower from
losing its original radiate form; for while in _C. cyanus_, the large
outward-growing florets, having short, stiff divisions of the corolla,
are decidedly bilateral, in _C. scabiosa_, where the divisions of the
corolla are long and flexible, the radial form is scarcely at all
modified. On bearing in mind the probable relations of the forms to
insect-agency, the meaning of this difference will not be difficult to
understand.[38]


§ 236. In extremely-varied ways there are thus re-illustrated among
flowers, the general laws of form which leaves and branches and
entire plants disclose to us. Composed as each cluster of flowers
is of individuals that are originally similar; and composed as each
flower is of homologous foliar organs; we see both that the like
flowers become unlike and the like parts of each flower become unlike,
where the positions involve unlike incidence of forces. The symmetry
remains radial where the conditions are equal all round; shows
deviation towards two-sidedness where there is slight two-sidedness
of conditions; becomes decidedly bilateral where the conditions are
decidedly bilateral; and passes into an unsymmetrical form where the
relations to the environment are unsymmetrical.




                              CHAPTER XI.

                     THE SHAPES OF VEGETAL CELLS.


§ 237. We come now to aggregates of the lowest order. Already something
has been said (§ 217) concerning the forms of those morphological units
which exist as independent plants. But it is here requisite briefly to
note the modifications undergone by them where they become components
of larger plants.

  [Illustration: Fig. 254.]

Of the numerous cell-forms which are found in the tissues of the higher
plants, it will suffice to give, in Fig. 254, representing a section
of a leaf, a single example. In this it will be seen that the cells
forming the upper and lower surfaces, _a_ and _b_, have differences of
shape related to differences in the incidence of forces: they are more
or less flattened in relation to the environment. The underneath cells
at _c_, form a class which, similarly exposed to light at their outer
ends, and, as we may assume, largely developed in adjustment to their
active assimilative functions, are, by mutual pressure, made to grow
more in the direction of their lengths than in the direction of their
breadths. Then on the other side we see that the cells _d_, next above
the outer layer, while approximately similar, become more and more
dissimilar as they diverge from the surface, and are quite irregular
in the interior _e_, where there is no definiteness in the conditions
to which they are exposed. Thus the divergences of these cells from
primordial sphericity are such as correspond with unlikenesses in their
circumstances. And throughout the more complex modifications which the
cells of other tissues exhibit, the like correspondences hold.

  [Illustration: Figs. 32–35.]

Among plants of a lower order of aggregation, we have already seen
how cells become metamorphosed as they become integrated into masses
having definite organizations. The higher _Algæ_, exemplified in Figs.
32, 34, 35, show this very clearly. Here the departure from the simple
cell-form to the form of an elongated prism, is manifestly subordinated
to the contrasts in the relations of the parts. And it is interesting
to observe how, in one of the branches of Fig. 32, we pass from the
small, almost-spherical cells which terminate the branchlets, to the
large, much-modified cells which join the main stem, through gradations
obviously related in their changed forms to the altered actions their
positions expose them to.

  [Illustration: Figs. 19–23.]

More simply, but quite as conclusively, do the inferior _Algæ_, of
which Figs. 19–23 are examples, show us how cells pass from their
original spherical symmetry into radial symmetry, as they pass from a
state in which they are similarly-conditioned on all sides, to a state
in which two of their opposite sides or ends are conditioned in ways
that are like one another, but unlike the ways in which all other sides
are conditioned.

Still more instructive are the morphological differentiations of
those protophytes in which the first steps towards a higher degree of
integration are shown. In Fig. 10, representing one of the transitional
forms of _Desmidiaceæ_, it is to be noted that besides the difference
between the transverse and longitudinal dimensions, which the component
units display in common, the two end-units differ from the rest:
they have appendages which the rest have not. Once more, where the
integration is carried on in such ways as to produce not strings but
clusters, there arise contrasts and correspondences just such as might
be looked for. All the four members of the group shown in Fig. 12,
are similarly conditioned; and each of them has a bilateral shape
answering to its bilateral relations. In Fig. 14 we have a number of
similarly-bilateral individuals on the circumference, including a
central individual differing from the rest by having the bilateral
character nearly obliterated. And then, in Fig. 15, we have two central
components of the group, deviating more decidedly from those that
surround them.[39]

  [Illustration: Figs. 7–17.]




                             CHAPTER XII.

                  CHANGES OF SHAPE OTHERWISE CAUSED.


§ 238. Besides the more special causes of modification in the shapes of
plants and of their parts, certain more general causes must be briefly
noticed. These may be described as consequences of variations in the
total quantities of the matters and forces furnished to plants by their
environments. Some of the changes of form so produced are displayed by
plants as wholes, and others only by their parts. We will glance at
them in this order.


§ 239. It is a familiar fact that luxuriant shoots have relatively-long
internodes; and, conversely, that a shoot dwarfed from lack of sap,
has its nodes closely clustered: a concomitant result being that the
lateral axes, where these are developed, become in the one case far
apart and in the other case near together. Fig. 255 represents a branch
to the parts of which the longer and shorter internodes so resulting
give differential characters. A whole tree being in many cases
simultaneously thus affected by states of the earth or the air, all
parts of it may have such variations impressed on them; and, indeed,
such variations, following more or less regularly the changes of the
seasons, give to many trees manifest traits of structure. In Fig. 256,
a shoot of _Phyllocactus crenatus_, we have an interesting example
of a variation essentially of the same nature, little as it appears
to be so. For each of the lateral indentations is here the seat of
an axillary bud; and these we see are separated by internodes which,
becoming broader as they become longer, and narrower as they become
shorter, produce changes of form that correspond with changes in the
luxuriance of growth.

  [Illustration: Figs. 255, 256.]

To complete the statement it must be added that these variations of
nutrition often determine the development or non-development of lateral
axes; and by so doing cause still more marked structural differences.
The Foxglove may be named as a plant which illustrates this truth.[40]


§ 240. From the morphological differentiations caused by unlikenesses
of nutrition felt by the whole plant, we pass now to those which are
thus caused in some of its parts and not in others. Among such are
the contrasts between flowering axes, and the axes that bear leaves
only. It has already been shown in § 78, that the belief expressed by
Wolff in a direct connexion between fructification and innutrition,
is justified inductively by many facts of many kinds. Deductively
too, in § 79, we saw reason to conclude that such a relation would be
established by survival of the fittest; seeing that it would profit
a species for its members to begin sending off migrating germs from
the ends of those axes which innutrition prevented from further
agamogenetic multiplication. Once more, when considering the nature of
the phænogamic axis, we found support for this belief in the fact that
the components of a flower exhibit a reversion to that type from which
the phænogamic type has probably arisen--a reversion which the laws of
embryology would lead us to look for where innutrition had arrested
development.

Hence, then, we may properly count those deviations of structure
which constitute inflorescence, as among the morphological
differentiations produced by local innutrition. I do not mean that the
detailed modifications which the essential and subservient organs of
fructification display, are thus accounted for: we have seen reason
to think them otherwise caused. But I mean that the morphological
characters which distinguish gamogenetic axes in general from
agamogenetic axes, such as non-development of the internodes and
dwarfing of the foliar organs, are primarily results of failure in the
supply of some material required for further growth.[41]


§ 241. Another trait which has to be noticed under this head, is the
spiral, or rather the helical, arrangement of parts. The successive
nodes of a phænogam habitually bear their appendages in ways implying
more or less twist in the substance of the axis; and in climbing plants
the twist is such as to produce a corkscrew shape. This structure is
ascribable to differences of interstitial nutrition. Take a shoot which
is growing vertically. It is clear that if the molecules are added
with perfect equality on all sides, there will be no tendency towards
any kind of lateral deviation; and the successively-produced parts
will be perpendicularly over one another. But any inequality in the
rate of growth on the different sides of the shoot, will destroy this
straightness in the lines of growth. If the greatest and least rates
of molecular increase happen to be on opposite sides, the shoot must
assume a curve of single curvature; but in every other case of unequal
molecular increase, a curve of double curvature must result. Now it is
a corollary from the instability of the homogeneous, that the rates of
growth on all sides of a shoot can never be exactly alike; and it is
also to be inferred from the same general law, that the greatest and
least rates of growth will not occur on exactly opposite sides of the
shoot, at the same time that equal rates of growth are preserved by the
two other sides. Hence, there must almost inevitably arise more or less
of twist; and the appendages of the internodes will so be prevented
from occurring perpendicularly one over another.

A deviation of this kind, necessarily initiated by physical causes
in conformity with the general laws of evolution, is likely to be
made regular and decided by natural selection. For under ordinary
circumstances, a plant profits by having its axis so twisted as to
bring the appended leaves into positions which prevent them from
shading one another. And, manifestly, modifications in the forms,
sizes, and insertions of the leaves, may, under the same agency, lead
to adapted modifications of the twist. We must therefore ascribe this
common characteristic of phænogams, primarily to local differences of
nutrition, and secondarily to survival of the fittest.

It is proper to add that there are some Monocotyledons, as _Ravenala
madagascariensis_, in which this character does not occur. What
conditions of existence they are that here hold this natural tendency
in check, it is not easy to see.[42]




                             CHAPTER XIII.

               MORPHOLOGICAL DIFFERENTIATION IN ANIMALS.


§ 242. The general considerations which preluded our inquiry into the
shapes of plants and their parts, equally serve, so far as they go, to
prelude an inquiry into the shapes of animals and their parts. Among
animals, as among plants, the formation of aggregates greater in bulk
or higher in degree of composition, or both, is accompanied by changes
of form in the aggregates as wholes as well as by changes of form in
their parts; and the processes of morphological differentiation conform
to the same general laws in the one kingdom as in the other.

It is needless to recapitulate the several kinds of modification to be
explained, and the several factors that co-operate in working them.
In so far as these are common to plants and animals, the preceding
chapters have sufficiently familiarized them. Nor is it needful to
specify afresh the several types of symmetry and their descriptive
names; for what is true of them in the one case is true of them in the
other. There is, however, one new and all-important factor which we
shall have now to take into account; and about this a few preliminary
remarks are requisite.


§ 243. This new factor is motion--motion of the organism in relation
to surrounding objects, or of the parts of the organism in relation
to one another, or both. Though there are plants, especially of the
simpler kinds, which move, and though a few of the simpler animals do
not move; yet movements are so exceptional and unobtrusive in the one
kingdom, while they are so general and conspicuous in the other, that
the broad distinction commonly made is well warranted. What, among
plants, is an inappreciable cause of morphological differentiation,
becomes, among animals, the chief cause of morphological
differentiation.

Rooted animals or animals otherwise fixed, of course present traits
of structure nearest akin to those we have lately been studying. The
motions of parts in relation to one another and to the environment,
being governed by the mode of aggregation and mode of fixing, we are
presented with morphological differentiations similar in their general
characters to those of plants, and showing us parallel kinds of
symmetry under parallel conditions. But animals which move from place
to place are subject to an additional class of actions and reactions.
These actions and reactions affect them in various ways according to
their various modes of movement. Let us glance at the several leading
relations between shape and motion which we may expect to find.

If an organism advances through a homogeneous medium with one end
always foremost, that end, being exposed to forces unlike those to
which the other end is exposed, may be expected to become unlike it;
and supposing this to be the only constant contrast of conditions,
we may expect an equal distribution of the parts round the axis
of movement--a radial symmetry. If, in addition to this habitual
attitude of the ends, one surface of the body is always uppermost
and another always lowermost, there arise between the top and bottom
dissimilarities of conditions, while the two sides remain similarly
conditioned. Hence it is inferable that such an organism will be
divisible into similar halves by a vertical plane passing through
its axis of motion--will have a bilateral symmetry. We may presume
that this symmetry will deviate but little from double bilateralness
where the upper and under parts are not exposed to strongly-contrasted
influences; while we may rationally look for single bilateral symmetry
of a decided kind, in creatures having dorsal and ventral parts
conversant with very unlike regions of the environment: as in all cases
where the movement is over a solid surface. If the movement, though
over a solid surface, is not constant in direction, but takes place as
often on one side as on another, radial symmetry may be again looked
for; and if the motions are still more variously directed--if they are
not limited to approximately-plane surfaces, but extend to surfaces
that are distributed all around with a regular irregularity--an
approach of the radial towards the spherical symmetry is to be
anticipated. Where the habits are such that the intercourse between the
organism and its environment, does not involve an average equality of
actions and reactions on any two or more sides, there may be expected
either total irregularity or some divergence from regularity.

The like general relations between forms and incident forces are
inferable in the component parts of animals, as well as in the animals
as wholes. It is needless, however, to occupy space by descriptions of
these. Let us now pass to the facts, and see how they confirm, _à
posteriori_, the conclusions here reached _à priori_.




                             CHAPTER XIV.

                    THE GENERAL SHAPES OF ANIMALS.


§ 244. Certain of the _Protozoa_ are quite indefinite in their shapes,
and quite inconstant in those indefinite shapes which they have--the
relations of their parts are indeterminate both in space and time.
In one of the simpler Rhizopods, at least during the active stage
of its existence, no permanent distinction of inside and outside is
established; and hence there can arise no established correspondence
between the shape of the outside and the distribution of environing
actions. But when the relation of inner and outer becomes fixed,
either over part of the mass or over the whole of it, we have kinds
of symmetry that correspond with the habitual incidence of forces. An
_Amœba_ in becoming encysted, passes from an indefinite, ever-changing
form into a spherical form; and the order of symmetry which it thus
assumes, is in harmony with the average equality of the actions on all
its sides. In _Difflugia_, Fig. 134, and still better in _Arcella_, we
have an indefinitely-radial symmetry occurring where the conditions are
different above and below but alike all around. Among the _Gregarinida_
the spherical symmetry and symmetry passing from that into the radial,
are such as appear to be congruous with the simple circumstances
of these creatures in the intestines of insects. But the relations
of these lowest types to their environments are comparatively so
indeterminate, and our knowledge of their actions so scanty, that
little beyond negative evidence can be expected from the study of them.

  [Illustration: Figs. 131–139.]

The like may be said of the _Infusoria_. These are more or less
irregular. In some cases, where the line of movement through the
water is tolerably definite and constant, we have a form that is
approximately radial--externally at least. But usually, as shown
in Figs. 137, 138, 139, there is either an unsymmetrical or an
asymmetrical shape. And when one of these creatures is watched under
the microscope, the congruity of this shape with the incidence of
forces is manifest. For the movements are conspicuously varied and
indeterminate--movements which do not expose any two or more sides of
the mass to approximately equal sets of actions.[43]


§ 245. Among aggregates of the second order, as among aggregates of the
first order, we find that of those possessing any definite shapes the
lowest are spherical or spheroidal. Such are some of the _Radiolaria_,
as _Collozoum inerme_. These bodies which float passively in the sea,
and present in turn all their sides to the same influences, have their
parts disposed with approximate regularity round a centre--approximate,
because in the absence of locomotion a slight irregularity of
growth, almost certain to take place, may cause a fixed attitude and
a resulting deviation from spherical symmetry. The best cases in
illustration of the truth here named, are furnished by rotating and
locomotive organisms respecting which there is a dispute whether they
are animal or vegetal--the _Volvocineæ_. These, already instanced
under the one head in § 218, may here be instanced afresh under the
other. Further, among these secondary aggregates in which the units,
only physically integrated, have not had their individualities merged
into an individuality of a higher order, must be named the compound
_Infusoria_. The cluster of _Vorticellæ_ in Fig. 144, will sufficiently
exemplify them; and the striking resemblance borne by its individuals
to those of a radially-arranged cluster of flowers, will show how,
under analogous conditions, the general principles of morphological
differentiation are similarly illustrated in the two kingdoms.


§ 246. Radial symmetry is usual in low aggregates of the second order
which have their parts sufficiently differentiated and integrated
to give individualities to them as wholes. The _Cœlenterata_
offer numerous examples of this. Solitary polypes--hydroid or
helianthoid--mostly stationary, and when they move, moving with any
side foremost, do not by locomotion subject their bodies to habitual
contrasts of conditions. Seated with their mouths upwards or downwards,
or else at all degrees of inclination, the individuals of a species
taken together, are subject to no mechanical actions affecting some
parts of their discs more than other parts. And this indeterminateness
of attitude similarly prevents their relations to prey from being such
as subject some of their prehensile organs to forces unlike those to
which the rest are subject. The fixed end is differently conditioned
from the free end, and the two are therefore different; but around the
axis running from the fixed to the free end the conditions are alike
in all directions, and the form therefore is radial. Again, among many
of the simple free-swimming _Hydrozoa_, the same general truth is
exemplified under other circumstances. In a common _Medusa_, advancing
through the water by the rhythmical contractions of its disc, the
mechanical reactions are the same on all sides; and as, from accidental
causes, every part of the edge of the disc comes uppermost in its
turn, no part is permanently affected in a different way from the rest.
Hence the radial form continues.

  [Illustration: Figs. 257, 258.]

In others of this same group, however, there occur forms which show
us an incipient bilateralness; and help us to see how a more decided
bilateralness may arise. Sundry of the _Medusidæ_ are proliferous,
giving origin to gemmæ from the body of the central polypite or from
certain points on the edge of the disc; and this budding, unless it
occurs equally on all sides, which it does not and is unlikely to do,
must tend to destroy the balance of the disc, and to make its attitude
less changeable. In other cases the growth of a large process [a
much-developed tentacle] from the edge of the disc on one side, as in
_Steenstrupia_, Fig. 257, constitutes a similar modification, and a
cause of further modification. The animal is no longer divisible into
any two quite similar halves, except those formed by a plane passing
through the process; and unless the process is of the same specific
gravity as the disc, it must tend towards either the lowest or the
highest point, and must so serve to increase the bilateralness, by
keeping the two sides of the disc similarly conditioned while the
top and bottom are differently conditioned. Fig. 258 represents the
underside of another _Medusa_, in which a more decided bilateralness
is produced by the presence of two such processes. Among the simple
free-swimming _Actinozoa_, occur like deviations from radial symmetry,
along with like motions through the water in bilateral attitudes.
Of this a _Cydippe_ is a familiar example. Though radial in some of
its characters, as in the distribution of its meridional bands of
locomotive paddles with their accompanying canals, this creature
has a two-sided distribution of tentacles and various other parts,
corresponding with its two-sided attitude in moving through the water.
And in other genera of this group, as in _Cestum_, _Eurhamphæa_,
and _Callianira_, that almost equal distribution of parts which
characterizes the _Beroe_ is quite lost.

Here seems a fit place to meet the objection which some may feel to
this and other such illustrations, that they amount very much to
physical truisms. If the parts of a _Medusa_ are disposed in radial
symmetry round the axis of motion through the water, there will of
course be no means of maintaining one part of its edge uppermost
more than another; and the equality of conditions may be ascribed
to the radiateness, as much as the radiateness to the equality of
conditions. Conversely, when the parts are not radially arranged
around the axis of motion, they must gravitate towards some one
attitude, implying a balance on the two sides of a vertical plane--a
bilateralness; and the two-sided conditions so necessitated, may be
as much ascribed to the bilateralness as the bilateralness to the
two-sided conditions. Doubtless the form and the conditions are, in
the way alleged, necessary correlates; and in so far as it asserts
this, the objection harmonizes with the argument. To the difficulty
which it at the same time raises by the implied question--Why make
the form the result of the conditions, rather than the conditions
the result of the form? the reply is this:--The radial type, both as
being the least differentiated type and as being the most obviously
related to lower types, must be taken as antecedent to the bilateral
type. The individual variations which incidental circumstances produce
in the radial type, will not cause divergence of a species from the
radial type, unless such variations give advantages to the individuals
displaying them; which there is no reason to suppose they will always
do. Those occasional deviations from the radial type, which the law of
the instability of the homogeneous warrants us in expecting to take
place, will, however, in some cases be beneficial; and will then be
likely to establish themselves. Such deviations must tend to destroy
the original indefiniteness and variability of attitude--must cause
gravitation towards an habitual attitude. And gravitation towards an
habitual attitude having once commenced, will continually increase,
where increase of it is not negatived by adverse agencies: each further
degree of bilateralness rendering more decided the actions that conduce
to bilateralness. If this reply be thought insufficient, it may be
enforced by the further one, that as, among plants, the incident forces
are the antecedents and the forms the consequents (changes of forces
being in many cases visibly followed by changes of forms) we are
warranted in concluding that the like order of cause and effect holds
among animals.[44]


§ 247. Keeping to the same type but passing to a higher degree of
composition, we meet more complex and varied illustrations of the same
general laws. In the compound _Cœlenterata_, presenting clusters
of individuals which are severally homologous with the solitary
individuals last dealt with, we have to note both the shapes of the
individuals thus united, and the shapes of the aggregates made up of
them.

  [Illustration: Figs. 149, 150.]

Such of the fixed _Hydrozoa_ and _Actinozoa_ as form branched
societies, continue radial; both because their varied attitudes do not
expose them to appreciable differences in their relations to those
surrounding actions which chiefly concern them (the actions of prey),
and because such differences, even if they were appreciable, would
be so averaged in their effects on the dissimilarly-placed members
of each group as to be neutralized in the race. Among the tree-like
coral-polypedoms, as well as in such ramified assemblages of simpler
polypes as are shown in Figs. 149, 150, we have, indeed, cases in many
respects parallel to the cases of scattered flowers (§ 233), which
though placed laterally remain radial, because no differentiating
agency can act uniformly on all of them. Meanwhile, in the groups which
these united individuals compose, we see the shapes of plants further
simulated under a further parallelism of conditions. The attached ends
differ from the free ends as they do in plants; and the regular or
irregular branches obviously stand to environing actions in relations
analogous to those in which the branches of plants stand.

The members of those compound _Cœlenterata_ which move through the
water by their own actions, in attitudes that are approximately
constant, show us a more or less distinct two-sidedness. _Diphyes_,
Fig. 259, furnishes an example. Each of the largely-developed and
modified polypites forming its swimming sacs is bilateral, in
correspondence with the bilateralness of its conditions; and in each of
the appended polypites the insertion of the solitary tentacle produces
a kindred divergence from the primitive radial type. The aggregate,
too, which here very much subordinates its members, exhibits the same
conformity of structure to circumstances. It admits of symmetrical
bisection by a plane passing through its two contractile sacs, or
nectocalyces, but not by any other plane; and the plane which thus
symmetrically bisects it, is the vertical plane on the two sides of
which its parts are similarly conditioned as it propels itself through
the water.

  [Illustration: Fig. 259.]

Another group of the oceanic _Hydrozoa_, the _Physophoridæ_, furnishes
interesting evidence--not so much in respect of the forms of the united
individuals, which we may pass over, as in respect of the forms of the
aggregates. Some of these are without swimming organs, and have their
parts suspended from air-vessels which habitually float on the surface
of the water. Hence the distribution of their parts is asymmetrical.
The _Physalia_, Fig. 152, is an example. Here the relations of the
integrated group of individuals to the environment are indefinite; and
there is thus no agency tending to change that comparatively irregular
mode of growth which is probably derived from a primordial type of the
branched _Hydrozoa_.

  [Illustration: Fig. 152.]

So various are the modes of union among the compound _Cœlenterata_,
that it is out of the question to deal with them all. Even did
space permit, it would be impracticable for any one but a professed
naturalist, to trace throughout this group the relations between
shapes and conditions of existence. The above must be taken simply as a
few of the most significant and easily-interpretable cases.


§ 248. In the sub-kingdoms _Polyzoa_ and _Tunicata_ we meet with
examples not wholly unlike the foregoing. Among the types assembled
under these names there are simple individuals or aggregates of the
second order, and societies or tertiary aggregates produced by their
union. The relations of forms to forces have to be traced in both.

Solitary Ascidians, fixed or floating, carry on an inactive and
indefinite converse with the actions in the environment. Without
power to move about vivaciously, and unable to catch any prey but
that contained in the currents of water they absorb and expel, these
creatures are not exposed to sets of forces which are equal on two
or more sides; and their shapes consequently remain vague. Though
internally their parts have a partially-symmetrical arrangement, due
to their derivation, yet they are substantially unsymmetrical in that
part of the body which is concerned with the environment. Fig. 156 is
an example.[45] Among the composite Ascidians, floating and fixed,
the shape of the aggregate, partly determined by the habitual mode of
gemmation and partly by the surrounding conditions in each case, is in
great measure indefinite. We can say no more about it than that it is
not obviously at variance with the laws alleged.

Evidence of a more positive kind occurs among those compound
_Molluscoida_ which are most like the compound _Cœlenterata_ in
their modes of union--the _Polyzoa_. Many of these form groups that
are more or less irregular--spreading as films over solid surfaces,
combining into seaweed-like fronds, budding out from creeping stolons,
or growing up into tree-shaped societies; and besides aggregating
irregularly they are irregularly placed on surfaces inclined in all
directions. Merely noting that this asymmetrical distribution of the
united individuals is explained by the absence of definiteness in the
relations of the aggregate to incident forces, it concerns us chiefly
to observe that the united individuals severally exemplify the same
truth as do similarly-united individuals among the _Cœlenterata_.
Averaging the members of each society, the ciliated tentacles they
protrude are similarly related to prey on all sides; and therefore
remain the same on all sides. This distribution of tentacles is not,
however, without exception. Among the fresh-water _Polyzoa_ there
are some genera, as _Plumatella_ and _Crystatella_, in which the
arrangement of these parts is very decidedly bilateral. Some species of
them show us such relations of the individuals to one another and to
their surface of attachment, as give a clue to the modification; but in
other species the meaning of this deviation from the radial type is not
obvious.


§ 249. In the _Platyhelminthes_ good examples of the connexions between
forms and forces occur. The _Planaria_ exemplifies the single bilateral
symmetry which, even in very inferior forms, accompanies the habit of
moving in one direction over a solid surface. Humbly organized as are
these creatures and their allies the _Nemertidæ_, we see in them, just
as clearly as in the highest animals, that where the movements subject
the body to different forces at its two ends, different forces on its
under and upper surfaces, and like forces along its two sides, there
arises a corresponding form, unlike at its extremities, unlike above
and below, but having its two sides alike.

The _Echinodermata_ furnish us with instructive
illustrations--instructive because among types that are nearly allied,
we meet with wide deviations of form answering to marked contrasts in
the relations to the environment. The facts fall into four groups.
The _Crinoidea_, once so abundant and now so rare, present a radial
symmetry answering to an incidence of forces that are equal on all
sides. In the general attitudes of their parts towards surrounding
actions, they are like uniaxial plants or like polypes; and show, as
those do, marked differences between the attached ends and the free
ends, along with even distributions of parts all round their axes.
In the _Ophiuridea_, and in the Star-fishes, we have radial symmetry
co-existing with very different habits; but habits which nevertheless
account for the maintenance of the form. Holding on to rocks and weeds
by its simple or branched arms, or by the suckers borne on the under
surface of its rays, one of these creatures moves about not always
with one side foremost, but with any side foremost. Consequently,
averaging its movements, its arms or rays are equally affected, and
therefore remain the same on all sides. On watching the ways of the
common Sea-urchin, we are similarly furnished with an explanation of
its spherical, or rather its spheroidal, figure. Here the habit is
not to move over any one approximately-flat surface; but the habit is
to hold on by several surfaces on different sides at the same time.
Frequenting crevices and the interstices among stones and weeds, the
Sea-urchin protrudes the suckers arranged in meridional bands over
its shell, laying hold of objects now on this side and now on that,
now above and now below: the result being that it does not move in
all directions over one plane but in all directions through space.
Hence the approach in general form towards spherical symmetry--an
approach which is, however, restrained by the relations of the parts
to the mouth and vent: the conditions not being exactly the same at
the two poles as at other parts of the surface. Still more significant
is that deviation from this shape which occurs among such of the
_Echinidea_ as have habitats of a different kind, and consequently,
different habits. The genera _Echinocyamus_, _Spatangus_, _Brissus_,
and _Amphidotus_, diverge markedly towards a bilateral structure. These
creatures are found not on rocky shores but on flat sea-bottoms, and
some of them only on bottoms of sand or mud. Here, there is none of
that distribution of surfaces on all sides which makes the spheroidal
form congruous with the conditions. Having to move about over an
approximately-horizontal plane, any deviation of structure arising
accidentally which leads to one side being kept always foremost, will
be an advantage: greater fitness to function becoming possible in
proportion as function becomes fixed. Survival of the fittest will
therefore tend to establish, under such conditions, a form that keeps
the same part in advance--a form in which, consequently, the original
radial symmetry diverges more and more towards bilateral symmetry.


§ 250. Very definite and comparatively uniform, are the relations
between shapes and circumstances among the _Annulosa_: including under
that title the _Annelida_ and the _Arthropoda_. The agreements and the
disagreements are equally instructive.

At one time or other of its life, if not throughout its life, every
annulose animal is locomotive; and its temporary or permanent
locomotion, being carried on with one end habitually foremost and one
surface habitually uppermost, it fulfils those conditions under which
bilateral symmetry arises. Accordingly, bilateral symmetry is traceable
throughout the whole of this sub-kingdom. Traceable, we must say,
because, though it is extremely conspicuous in the immense majority
of annulose types, it is to a considerable extent obscured where
obscuration is to be expected. The embryos of the _Tubicolæ_, after
swimming about a while, settle down and build themselves tubes, from
which they protrude their heads; and in them, or in some of them, the
bilateral symmetry is disguised by the development of head-appendages
in an all-sided manner. The tentacles of _Terebella_ are distributed
much in the same way as those of a polype. The breathing organs in
_Sabella unispira_, Fig. 260, do not correspond on opposite sides of
a median plane. Even here, however, the body retains its primitive
bilateralness; and it is further to be remarked that this loss of
bilateralness in the external appendages, does not occur where the
relations to external conditions continue bilateral: witness the
_Serpula_, Fig. 261, which has its respiratory tufts arranged in a
two-sided way, under the two-sided conditions involved by the habitual
position of its tube.

  [Illustration: Figs. 260–261.]

The community of symmetry among the higher _Annulosa_, has an
unobserved significance. That Flies, Beetles, Lobsters, Centipedes,
Spiders, Mites, have in common the characters, that the end which moves
in advance differs from the hinder end, that the upper surface differs
from the under surface, and that the two sides are alike, is a truth
received as a matter of course. After all that has been said above,
however, it will be seen to have a meaning not to be overlooked; since
it supplies a million-fold illustration of the laws which have been set
forth. It is needless to give diagrams. Every reader can call to mind
the unity indicated.

  [Illustration: Figs. 263–270.]

While, however, annulose animals repeat so uniformly these traits of
structure, there are certain other traits in which they are variously
contrasted; and their contrasts have to be here noted, as serving
further to build up the general argument. In them we see the stages
through which bilateral symmetry becomes gradually more marked, as
the conditions it responds to become more decided. A common Earth-worm
may be instanced as a member of this sub-kingdom that is among the
least-conspicuously bilateral. Though internally its parts have a
two-sided arrangement; and though the positions of its orifices give
it an external two-sidedness, at the same time that they establish
a difference between the two ends; yet its two-sidedness is not
strongly-marked. The form deviates but little from what we have
distinguished as triple bilateral symmetry: if the creature is cut
across the middle, the head and tail ends are very much alike; if
cut in two along its axis by a horizontal plane, the under and upper
halves are very much alike, externally if not internally; and if cut
in two along its axis by a vertical plane, the two sides are quite
alike. Figs. 263 and 264 will make this clear. Such creatures as
the _Julus_ and the Centipede, may be taken as showing a transition
to double bilateral symmetry. Besides being divisible into exactly
similar halves by a vertical plane passing through its axis, one of
these animals may be bisected transversely into parts that differ only
slightly; but if cut in two by a horizontal plane passing through its
axis, the under and upper halves are decidedly unlike. Figs. 265, 266,
exhibit these traits. Among the isopodous crustaceans, the departure
from these low types of symmetry is more marked. As shown in Figs. 267
and 268, the contrast between the upper and under parts is greater,
and the head and tail ends differ more obviously. In all the higher
_Arthropoda_, the unlikeness between the front half and the hind half
has become conspicuous. There is in them single bilateral symmetry of
so pronounced a kind, that no other resemblance is suggested than that
between the two sides. By Figs. 269 and 270, representing a decapodous
crustacean divided longitudinally and transversely, this truth is made
manifest. On calling to mind the habits of the creatures here drawn and
described, it will be seen that they explain these forms. The incidence
of forces is the same all around the Earth-worm as it burrows through
the compact ground. The Centipede, creeping amid loose soil or _débris_
or beneath stones, insinuates itself between solid surfaces--the
interstices being mostly greater in one dimension than in others.
And all the higher _Annulosa_, moving about as they do over exposed
objects, have their dorsal and ventral parts as dissimilarly acted upon
as are their two ends.

  [Illustration: Fig. 271.]

One other fact only respecting annulose animals needs to be noticed
under this head--the fact, namely, that they become unsymmetrical
where their parts are unsymmetrically related to the environment.
The common Hermit-crab serves as an instance. Here, in addition
to the unlikeness of the two sides implied by that curvature of
the body which fits the creature to the shell it inhabits, there
is an unlikeness due to the greater development of the limbs, and
especially the claws, on the outer side. As in the embryo of the
Hermit-crab the two sides are alike; and as both the embryo and
the ancestor lived in such a way, being free, that the conditions
were alike on the two sides; and as the embryo may be taken to
represent the type from which the Hermit-crab has been derived; we
have in this case evidence that a symmetrically-bilateral form has
been moulded into an unsymmetrically-bilateral form, by the action
of unsymmetrically-bilateral conditions. A further illustration
is supplied by _Bopyrus_, Fig. 271: a parasite which lives in the
branchial chamber of prawns, and whose habits similarly account for its
distorted shape.


§ 251. Among the _Mollusca_ we find more varied relations between
shapes and circumstances. Some of these relations are highly
instructive.

Mollusks of one order, the _Pteropoda_, swim in the sea much in
the same way that butterflies fly in the air, and have shapes not
altogether unlike those of butterflies. Fig. 272 represents one of
these creatures. That its bilaterally-symmetrical shape harmonizes with
its bilaterally-symmetrical conditions is sufficiently obvious.

  [Illustration: Fig. 272.]

Among the _Lamellibranchiata_, we have diverse forms accompanying
diverse modes of life. Such of them as frequently move about, like
the fresh-water Mussel, have their two valves and the contained parts
alike on the opposite sides of a vertical plane: they are bilaterally
symmetrical in conformity with their mode of movement. The marine
Mussel, too, though habitually fixed, and though not usually so fixed
that its two valves are similarly conditioned, still retains that
bilateral symmetry which is characteristic of the order; and it does
this because in the species considered as a whole, the two valves
are not dissimilarly conditioned. If the positions of the various
individuals are averaged, it will be seen that the differentiating
actions neutralize one another. In certain other fixed Lamellibranchs,
however, there is a considerable deviation from bilateral symmetry;
and it is a deviation of the kind to be anticipated under the
circumstances. Where one valve is always downwards, or next to the
surface of attachment, while the other valve is always upwards, or next
to the environing water, we may expect to find the two valves become
unlike. This we do find: witness the Oyster. In the Oyster, too, we see
a further irregularity. There is a great indefiniteness of outline,
both in the shell and in the animal--an indefiniteness made manifest by
comparing different individuals. We have but to remember that growing
clustered together, as Oysters do, they must interfere with one another
in various ways and degrees, to see how the indeterminateness of form
and the variety of form are accounted for.

Among the Gasteropods modifications of a more definite kind occur. “In
all Mollusks,” says Professor Huxley, “the axis of the body is at first
straight, and its parts are arranged symmetrically with regard to a
longitudinal vertical plane, just as in a vertebrate or an articulate
embryo.” In some Gasteropods, as the _Chiton_, this bilateral
symmetry is retained--the relations of the body to surrounding actions
not being such as to disturb it. But in those more numerous types
which have spiral shells, there is a marked deviation from bilateral
symmetry, as might be expected. “This asymmetrical over-development
never affects the head or foot of the mollusk”: only those parts
which, by inclosure in a shell, are protected from environing actions,
lose their bilateralness; while the external parts, subjected by the
movements of the creatures to bilateral conditions, remain bilateral.
Here, however, a difficulty meets us. Why is it that the naked
Gasteropods, such as our common slugs, deviate from bilateral symmetry,
though their modes of movement are those along with which complete
bilateral symmetry usually occurs? The reply is that their deviations
from bilateral symmetry are probably inherited, and that they are
maintained in such parts of their organization as are not exposed to
bilaterally-symmetrical conditions. There is reason to believe that the
naked Gasteropods are descended from Gasteropods which had shells: the
evidence being that the naked Gasteropods have shells during the early
stages of their development, and that some of them retain rudimentary
shells throughout life. Now the shelled Gasteropods deviate from
bilateral symmetry in the disposition of both the alimentary system
and the reproductive system. The naked Gasteropods, in losing their
shells, have lost that immense one-sided development of the alimentary
system which fitted them to their shells, and have acquired that
bilateral symmetry of external figure which fits them for their habits
of locomotion; but the reproductive system remains one-sided, because,
in respect to it, the relations to external conditions remain one-sided.

The Cephalopods show us bilaterally-symmetrical external forms along
with habits of movement through the water in two-sided attitudes. At
the same time, in the radial distribution of the arms, enabling one
of these creatures to take an all-sided grasp of its prey, we see how
readily upon one kind of symmetry there may be partially developed
another kind of symmetry, where the relations to conditions favour it.


§ 252. The _Vertebrata_ illustrate afresh the truths which we have
already traced among the _Annulosa_. Flying through the air, swimming
through the water, and running over the earth as vertebrate animals
do, in common with annulose animals, they are, in common with annulose
animals, different at their anterior and posterior ends, different
at their dorsal and ventral surfaces, but alike along their two
sides. This single bilateral symmetry remains constant under the
extremest modifications of form. Among fish we see it alike in the
horizontally-flattened Skate, in the vertically-flattened Bream, in the
almost-spherical _Diodon_, and in the greatly-elongated _Syngnathus_.
Among reptiles the Turtle, the Snake, and the Crocodile all display it.
And under the countless modifications of structure displayed by birds
and mammals, it remains conspicuous.

  [Illustration: Figs. 273–280.]

A less obvious fact which it concerns us to note among the
_Vertebrata_, parallel to one which we noted among the _Annulosa_, is
that whereas the lower vertebrate forms deviate but little from triple
bilateral symmetry, the deviation becomes great as we ascend. Figs.
273 and 274 show how, besides being divisible into similar halves by
a vertical plane passing through its axis, a Fish is divisible into
halves that are not very dissimilar by a horizontal plane passing
through its axis, and also into other not very dissimilar halves by
a plane cutting it transversely. If, as shown in Figs. 275 and 276,
analogous sections be made of a superior Reptile, the divided parts
differ more decidedly. When a Mammal and a Bird are treated in the
same way, as shown in Figs. 277, 278, and Figs. 279, 280, the parts
marked off by the dividing planes are unlike in far greater degrees. On
considering the mechanical converse between organisms of these several
types and their environments--on remembering that the fish habitually
moves through a homogeneous medium of nearly the same specific gravity
as itself, that the terrestrial reptile either crawls on the surface or
raises itself very incompletely above it, that the more active mammal,
having its supporting parts more fully developed, thereby has the under
half of its body made more different from the upper half, and that the
bird is subject by its mode of life to yet another set of actions and
reactions; we shall see that these facts are quite congruous with the
general doctrine, and furnish further support to it.

One other significant piece of evidence must be named. Among the
_Annulosa_ we found unsymmetrical bilateralness in creatures having
habits exposing them to unlike conditions on their two sides; and
among the _Vertebrata_ we find parallel cases. They are presented
by the _Pleuronectidæ_--the order of distorted flat fishes to which
the Sole and the Flounder belong. On the hypothesis of evolution, we
must conclude that fishes of this order have arisen from an ordinary
bilaterally-symmetrical type of fish, which, feeding at the bottom of
the sea, gained some advantage by placing itself with one of its sides
downwards, instead of maintaining the vertical attitude. Besides the
general reason there are special reasons for concluding this. In the
first place, the young Sole or Flounder is bilaterally symmetrical--has
its eyes on opposite sides of its head and swims in the usual way. In
the second place, the metamorphosis which produces the unsymmetrical
structure sometimes does not take place--there are abnormal Flounders
that swim vertically, like other fishes. In the third place, the
transition from the symmetrical structure to the unsymmetrical
structure may be traced. Almost incredible though it seems, one of the
eyes is transferred from the underside of the head to the upper side:
the transfer being effected by a distorted development of the cranial
bones--atrophy of some and hypertrophy of others, along with a general
twist. This metamorphosis furnishes several remarkable illustrations
of the way in which forms become moulded into harmony with incident
forces. For besides the divergence from bilateral symmetry involved
by presence of both eyes upon the upper side, there is a further
divergence from bilateral symmetry involved by differentiation of the
two sides in respect to the contours of their surfaces and the sizes of
their fins. And then, what is still more significant, there is a near
approach to likeness between the halves that were originally unlike,
but are, under the new circumstances, exposed to like conditions. The
body is divisible into similarly-shaped parts by a plane cutting it
along the side from head to tail: “the dorsal and ventral instead of
the lateral halves become symmetrical in outline and are equipoised.”


§ 253. Thus, little as there seems in common between the shapes of
plants and the shapes of animals, we yet find, on analysis, that the
same general truths are displayed by both. The one ultimate principle
that in any organism equal amounts of growth take place in those
directions in which the incident forces are equal, serves as a key to
the phenomena of morphological differentiation. By it we are furnished
with interpretations of those likenesses and unlikenesses of parts,
which are exhibited in the several kinds of symmetry; and when we take
into account inherited effects, wrought under ancestral conditions
contrasted in various ways with present conditions, we are enabled to
comprehend, in a general way, the actions by which animals have been
moulded into the shapes they possess.

To fill up the outline of the argument, so as to make it correspond
throughout with the argument respecting vegetal forms, it would be
proper here to devote a chapter to the differentiations of those
homologous segments out of which animals of certain types are composed.
Though, among most animals of the third degree of composition, such
as the rooted _Hydrozoa_, the _Polyzoa_, and the _Ascidioida_, the
united individuals are not reduced to the condition of segments of a
composite individual, and do not display any marked differentiations;
yet there are some animals in which such subordinations, and consequent
heterogeneities, occur. The oceanic _Hydrozoa_ form one group of them;
and we have seen reason to conclude that the _Annulosa_ form another
group. It is not worth while, however, to occupy space in detailing
these unlikenesses of homologous segments, and seeking specific
explanations of them. Among the oceanic _Hydrozoa_ they are extremely
varied; and the habits and derivations of these creatures are so little
known, that there are no adequate data for interpreting the forms of
the parts in terms of their relations to the environment. Conversely,
among the _Annulosa_ those differentiations of the homologous segments
which accompany their progressing integration, have so much in common,
and have general causes which are so obvious, that it is needless to
deal with them at any length. They are all explicable as due to the
exposure of different parts of the chain of segments to different sets
of actions and reactions: the most general contrast being that between
the anterior segments and the posterior segments, answering to the most
general contrast of conditions to which annulose animals subject their
segments; and the more special contrasts answering to the contrasts of
conditions entailed by their more special habits.

Were an exhaustive treatment of the subject practicable, there should
here, also, come a chapter devoted to the internal structures of
animals--meaning, more especially, the shapes and arrangements of the
viscera. The relations between forms and forces among these inclosed
parts are, however, mostly too obscure to allow of interpretation.
Protected as the viscera are in great measure from the incidence of
external forces, we are not likely to find much correspondence between
their distribution and the distribution of external forces. In this
case the influences, partly mechanical, partly physiological, which
the organs exercise on one another, become the chief causes of their
changes of figure and arrangement; and these influences are complex and
indefinite. One general fact may, indeed, be noted--the fact, namely,
that the divergence towards asymmetry which generally characterizes
the viscera, is marked among those of them which are most removed from
mechanical converse with the environment, but not so marked among
those of them which are less removed from such converse. Thus while,
throughout the _Vertebrata_, the alimentary system, with the exception
of its two extremities, is asymmetrically arranged, the respiratory
system, which occupies one end of the body, generally deviates but
little from bilateral symmetry, and the reproductive system, partly
occupying the other end of the body, is in the main bilaterally
symmetrical: such deviation from bilateral symmetry as occurs, being
found in its most interiorly-placed parts, the ovaries. Just indicating
these facts as having a certain significance, it will be best to leave
this part of the subject as too involved for detailed treatment.

Internal structures of one class, however, not included among
the viscera, admit of general interpretation--structures which,
though internal, are brought into tolerably-direct relations with
environing forces, and are therefore subordinate in their forms to
the distribution of those forces. These internal structures it will
be desirable to deal with at some length; both because they furnish
important illustrations enforcing the general argument, and because an
interpretation of them which we have seen reason to reject, cannot be
rejected without raising the demand for some other interpretation.




                              CHAPTER XV.

                  THE SHAPES OF VERTEBRATE SKELETONS.


§ 254. When an elongated mass of any substance is transversely
strained, different parts of the mass are exposed to forces of opposite
kinds. If, for example, a bar of metal or wood is supported at its two
ends, as shown in Fig. 281, and has to bear a weight on its centre, its
lower part is thrown into a state of tension, while its upper part is
thrown into a state of compression. As will be manifest to any one who
observes what happens on breaking a stick across his knee, the greatest
degree of tension falls on the fibres forming the convex surface, while
the fibres forming the concave surface are subject to the greatest
degree of compression. Between these extremes the fibres at different
depths are subject to different forces. Progressing upwards from the
under surface of the bar shown in Fig. 281, the tension of the fibres
becomes less; and progressing downwards from the upper surface, the
compression of the fibres becomes less; until, at a certain distance
between the two surfaces, there is a place at which the fibres are
neither extended nor compressed. This, shown by the dotted line in the
figure, is called in mechanical language the “neutral axis.” It varies
in position with the nature of the substance strained: being, in common
pine-wood, at a distance of about five-eighths of the depth from the
upper surface, or three-eighths from the under surface. Clearly, if
such a piece of wood, instead of being subject to a downward force, is
secured at its ends and subject to an upward force, the distribution
of the compressions and tensions will be reversed, and the neutral
axis will be nearest to the upper surface. Fig. 282 represents these
opposite attitudes of the bar and the changed position of its neutral
axis: the arrow indicating the direction of the force producing the
upward bend, and the faint dotted line _a_, showing the previous
position of the neutral axis. Between the two neutral axes will be seen
a central space; and it is obvious that when the bar has its strain
from time to time reversed, the repeated changes of its molecular
condition must affect the central space in a way different from that
in which they affect the two outer spaces. Fig. 283 is a diagram
conveying some idea of these contrasts in molecular condition. If A B C
D be the middle part of a bar thus treated, while G H and K L are the
alternating neutral axes; then the forces to which the bar is in each
case subject, may be readily shown. Supposing the deflecting force to
be acting in the direction of the arrow E, then the tensions to which
the fibres between G and F are exposed, will be represented by a series
of lines increasing in length as the distance from G increases; so that
the triangle G F M, will express the amount and distribution of all
the molecular tensions. But the molecular compressions throughout the
space from G to E, must balance the molecular tensions; and hence, if
the triangle G E N be made equal to the triangle G F M, the parallel
lines of which it is composed (here dotted for the sake of distinction)
will express the amount and distribution of the compressions between
E and G. Similarly, when the deflecting force is in the direction of
the arrow F, the compressions and tensions will be quantitatively
symbolized by the triangles K F O, and K E P. And thus the several
spaces occupied by full lines and by dotted lines and by the two
together, will represent the different actions to which different
parts of the transverse section are subject by alternating transverse
strains. Here, then, it is made manifest to the eye that the central
space between G and K, is differently conditioned from the spaces above
and below it; and that the difference of condition is sharply marked
off. The fibres forming the outer surface C D, are subject to violent
tensions and violent compressions. Progressing inwards the tensions and
compressions decrease--the tensions the more rapidly. As we approach
the point G, the tensions to which the fibres are alternately subject,
bear smaller and smaller ratios to the compressions, and disappear
at the point G. Thence to the centre occur compressions only, of
alternating intensities, becoming at the centre small and equal; and
from the centre we advance, through a reverse series of changes, to the
other side.

  [Illustration: Fig. 281.]

  [Illustration: Fig. 282.]

  [Illustration: Fig. 283.]

Thus it is demonstrable that any substance in which the power of
resisting compression is unequal to the power of resisting tension,
cannot be subject to alternating transverse strains, without having
a central portion differentiated in its conditions from the outer
portions, and consequently differentiated in its structure. This
conclusion may easily be verified by experiment. If something having
a certain toughness but not difficult to break, as a thick piece of
sheet lead, be bent from side to side till it is broken, the surface of
fracture will exhibit an unlikeness of texture between the inner and
outer parts.


§ 255. And now for the application of this seemingly-irrelevant
truth. Though it has no obvious connection with the interpretation of
vertebral structure, we shall soon see that it fundamentally concerns
us.

  [Illustration: Fig. 284.]

The simplest type of vertebrate animal, the fish, has a mode of
locomotion which involves alternating transverse strains. It is not,
indeed, subjected to alternating transverse strains by some outer
agency, as in the case we have been investigating: it subjects itself
to them. But though the strains are here internally produced instead
of externally produced, the case is not therefore removed into a
wholly different category. For supposing Fig. 284 to represent the
outline of a fish when bent on one side (the dotted lines representing
its outline when the bend is reversed), it is clear that part of the
substance forming the convex half must be in a state of tension. This
state of tension implies the existence in the other half of some
counter-balancing compression. And between the two there must be a
neutral axis. The way in which this conclusion is reconcilable with the
fact that there is tension somewhere in the concave side of a fish,
since the curve is caused by muscular contractions on the concave
side, will be made clear by the rude illustration which a bow supplies.
A bow may be bent by a thrust against its middle (the two ends being
held back), or it may be bent by contracting a string that unites its
ends; but the distributions of mechanical forces within the wood of the
bow, though not quite alike in the two cases, will be very similar.
Now while the muscular action on the concave side of a fish differs
from that represented by the tightened string of a bow, the difference
is not such as to destroy the applicability of the illustration: the
parallel holds so far as this, that within that portion of the fish’s
body which is passively bent by the contracting muscles, there must be,
as in a strung bow, a part in compression, a part in tension, and an
intermediate part which is neutral.

After thus seeing that even in the developed fish with its complex
locomotive apparatus, this law of the transverse strain holds in a
qualified way, we shall understand how much more it must hold in any
form that may be supposed to initiate the vertebrate type--a form
devoid of that segmentation by which the vertebrate type is more or
less characterized. We shall see that assuming a rudimentary animal,
still simpler than the _Amphioxus_, to have a feeble power of moving
itself through the water by the undulations of its body, or some part
of its body, there will necessarily come into play certain reactions
which must affect the median portion of the undulating mass in a way
unlike that in which they affect its lateral portions. And if there
exists in this median portion a tissue which keeps its place with any
constancy, we may expect that the differential conditions produced in
it by the transverse strain, will initiate a differentiation. It is
true that the distribution of the viscera in the _Amphioxus_, Fig. 191,
and in the type from which we may suppose it to have arisen, is such as
to interfere with this process. It is also true that the actions and
reactions described would not of themselves give to the median portion
a cylindrical shape, like that of the cartilaginous rod running along
the back of the _Amphioxus_. But what we have here to note in the
first place is, that these habitual alternate flexions have a tendency
to mark off from the outer parts an unlike inner part, which may be
seized hold of, maintained, and further modified, by natural selection,
should any advantage thereby result. And we have to note in the second
place, that an advantage _is_ likely to result. The contractions
cannot be effective in producing undulations, unless the general shape
of the body is maintained. External muscular fibres unopposed by an
internal resistant mass, would cause collapse of the body. To meet
the requirements there must be a means of maintaining longitudinal
rigidity without preventing bends from side to side; and such a means
is presented by a structure initiated as described. In brief, whether
we have or have not the actual cause, we have here at any rate “a true
cause.” Though there are difficulties in tracing out the process in a
definite way, it may at least be said that the mechanical genesis of
this rudimentary vertebrate axis is quite conceivable. And even the
difficulties may, I think, be more fully met than at first sight seems
possible.

  [Illustration: Fig. 191.]

What is to be said of the other leading trait which the simplest
vertebrate animal has in common with all higher vertebrate animals--the
segmentation of its lateral muscular masses? Is this, too, explicable
on the mechanical hypothesis? Have we, in the alternating transverse
strains, a cause for the fact that while the rudimentary vertebrate
axis is without any divisions, there are definite divisions of the
substance forming the animal’s sides? I think we have. A glance at the
distribution of forces under the transverse strain, as represented in
the foregoing diagrams, will show how much more severe is the strain
on the outer parts than on the inner parts; and how, consequently,
any modifications of structure eventually necessitated, will arise
peripherally before they arise centrally. The perception of this may be
enforced by a simple experiment. Take a stick of sealing-wax and warm
it slowly and moderately before the fire, so as to give it a little
flexibility. Then bend it gently until it is curved into a semi-circle.
On the convex surface small cracks will be seen, and on the concave
surface wrinkles; while between the two the substance remains
undistorted. If the bend be reversed and re-reversed, time after time,
these cracks and wrinkles will become fissures which gradually deepen.
But now, if changes of this class, entailed by alternating transverse
strains, commence superficially, as they manifestly must; there arise
the further questions--What will be the special modifications produced
under these special conditions? and through what stages will these
modifications progress? Every one has literally at hand an example
of the way in which a flexible external layer that is now extended
and now compressed, by the bending of the mass it covers, becomes
creased; and a glance at the palms and the fingers will show that the
creases are near one another where the skin is thin, and far apart
where the skin is thick. Between this familiar case and the case of
the rhinoceros-hide, in which there are but a few large folds, various
gradations may be traced. Now the like must happen with the increasing
layers of contractile fibres forming the sides of the muscular tunic
in such a type as that supposed. The bendings will produce in them
small wrinkles while they are thin, but more decided and comparatively
distant fissures as they become thick. Fig. 289, which is a horizontal
longitudinal section, shows how these thickening layers will adjust
themselves on the convex and the concave surfaces, supposing the fibres
of which they are composed to be oblique, as their function requires;
and it is not difficult to see that when once definite divisions have
been established, they will advance inwards as the layers develop;
and will so produce a series of muscular bundles. Here then we have
something like the _myocommata_ [or myotomes as now called] which are
traceable in the _Amphioxus_, and are conspicuous in all superior
fishes.

  [Illustration: Fig. 289.]


§ 256. These are highly speculative conceptions. I have ventured to
present them with the view of implying that the hypothesis of the
mechanical genesis of vertebrate structure is not wholly at fault when
applied to the most rudimentary vertebrate animal. Lest it should be
alleged that the question is begged if we set out with a type which,
like the _Amphioxus_, already displays segmentation throughout
its muscular system, it seemed needful to indicate conceivable modes
in which there may have been mechanically produced those leading
traits that distinguish the _Amphioxus_. All I intend to suggest
is that mechanical actions have been at work, and that probably they
have operated in the manner alleged: so preparing the way for natural
selection.

But now let us return to the region of established fact, and consider
whether such actions and reactions as we actually witness, are adequate
causes of those observed differentiations and integrations which
distinguish the more-developed vertebrate animals. Let us see whether
the theory of mechanical genesis affords us a deductive interpretation
of the inductive generalizations.

Before proceeding, we must note a process of functional adaptation
which here co-operates with natural selection. I refer to the usual
formation of denser tissues at those parts of an organism which are
exposed to the greatest strains--either compressions or tensions.
Instances of hardening under compression are made familiar to us by
the skin. We have the general contrast between the soft skin covering
the body at large, and the indurated skin covering the inner surfaces
of the hands and the soles of the feet. We have the fact that even
within these areas the parts on which the pressure is habitually
greatest have the skin always thickest; and that in each person special
points exposed to special pressures become specially dense--often as
dense as horn. Further, we have the converse fact that the skin of
little-used hands becomes abnormally thin--even losing, in places, that
ribbed structure which distinguishes skin subject to rough usage. Of
increased density directly following increased tension, the skeletons,
whether of men or animals, furnish abundant evidence. Anatomists easily
discriminate between the bones of a strong man and those of a weak
man, by the greater development of those ridges and crests to which
the muscles are attached; and naturalists, on comparing the remains of
domesticated animals with those of wild animals of the same species,
find kindred differences. The first of these facts shows unmistakably
the immediate effect of function on structure, and by obvious alliance
with it the second may be held to do the same: both implying that the
deposit of dense substance capable of great resistance, constantly
takes place at points where the tension is excessive.

Taking into account, then, this adaptive process, continually aided by
the survival of individuals in which it has taken place most rapidly,
we may expect, on tracing up the evolution of the vertebrate axis, to
find that as the muscular power becomes greater there arise larger and
harder masses of tissue, serving the muscles as _points d’appui_; and
that these arise first in those places where the strains are greatest.
Now this is just what we _do_ find. The _myocommata_ are so placed
that their actions are likely to affect first that upper coat of the
notochord, where there are found “quadrate masses of somewhat denser
tissue,” which “seem faintly to represent neural spines,” even in the
_Amphioxus_. It is by the development of the neural spines, and after
them of the hæmal spines, that the segments of the vertebral column are
first marked out; and under the increasing strains of more-developed
_myocommata_, it is just these peripheral appendages of the vertebral
segments that must be most subject to the forces which cause the
formation of denser tissue. It follows from the mechanical hypothesis
that as the muscular segmentation must begin externally and progress
inwards, so, too, must the vertebral segmentation. Besides thus
finding reason for the fact that in fishes with wholly cartilaginous
skeletons, the vertebral segments are indicated by these processes,
while yet the notochord is unsegmented; we find a like reason for the
fact that the transition from the less-dense cartilaginous skeleton
to the more-dense osseous skeleton, pursues a parallel course. In the
existing _Lepidosiren_, which by uniting certain piscine and amphibian
characters betrays its close alliance with primitive types, the axial
part of the vertebral column is unossified, while there is ossification
of the peripheral parts. Similarly with numerous genera of fishes
classed as palæozoic. The fossil remains of them show that while the
neural and hæmal spines consisted of bone, the central parts of the
vertebræ were not bony. It may in some cases be noted, too, both in
extant and in fossil forms, that while the ossification is complete at
the outer extremities of the spines it is incomplete at their inner
extremities--thus similarly implying centripetal development.


§ 257. After these explanations the process of eventual segmentation
in the spinal axis itself, will be readily understood. The original
cartilaginous rod has to maintain longitudinal rigidity while
permitting lateral flexion. As fast as it becomes definitely marked
out, it will begin to concentrate within itself a great part of those
pressures and tensions caused by transverse strains. As already said,
it must be acted upon much in the same manner as a bow, though it is
bent by forces acting in a more indirect way; and like a bow, it must,
at each bend, have the substance of its convex side extended and the
substance of its concave side compressed. So long as the vertebrate
animal is small or inert, such a cartilaginous rod may have sufficient
strength to withstand the muscular strains; but, other things equal,
the evolution of an animal that is large, or active, or both, implies
muscular strains which must tend to cause modification in such a
cartilaginous rod. The results of greater bulk and of greater vivacity
may be best dealt with separately. As the animal increases in size,
the rod will grow both longer and thicker. On looking back at the
diagrams of forces caused by transverse strains, it will be seen that
as the rod grows thicker, its outer parts must be exposed to more
severe tensions and pressures if the degree of bend is the same. It is
doubtless true that when the fish, advancing by lateral undulations,
becomes longer, the curvature assumed by the body at each movement
becomes less; and that from this cause the outer parts of the notochord
are, other things equal, less strained--the two changes thus partially
neutralizing one another. But other things are _not_ equal. For while,
supposing the shape of the body to remain constant, the force exerted
in moving the body increases as the cubes of its dimensions, the
sectional area of the notochord, on which fall the reactions of this
exerted force, increases only as the squares of the dimensions: whence
results a greater stress upon its substance. This, however, will not
be very decided where there is no considerable activity. It is clear
that augmenting bulk, taken alone, involves but a moderate residuary
increase of strain on each portion of the notochord; and this is
probably the reason why it is possible for a large _sluggish_ fish like
the Sturgeon, to retain the notochordal structure. But now, passing to
the effects of greater activity, a like dynamical inquiry at once shows
us how rapidly the violence of the actions and reactions rises as the
movements become more vivacious. In the first place, the resistance
of a medium such as water increases as the square of the velocity of
the body moving through it; so that to _maintain_ double the speed, a
fish has to expend four times the energy. But the fish has to do more
than this--it has to _initiate_ this speed, or to impress on its mass
the force implied by this speed. Now the _vis viva_ of a moving body
varies as the square of the velocity; whence it follows that the energy
required to generate that _vis viva_ is measured by the square of the
velocity it produces. Consequently, did the fish put itself in motion
_instantaneously_, the expenditure of energy in generating its own
_vis viva_ and simultaneously overcoming the resistance of the water,
would vary as the fourth power of the velocity. But the fish cannot
put itself in motion instantaneously--it must do it by increments; and
thus it results that the amounts of the forces expended to give itself
different velocities must be represented by some series of numbers
falling between the squares and the fourth powers of those velocities.
Were the increments slowly accumulated, the ratios of increasing effort
would but little exceed the ratios of the squares; but whoever observes
the sudden, convulsive action with which an alarmed fish darts out
of a shallow into deep water, will see that the velocity is rapidly
generated, and that therefore the ratios of increasing effort probably
exceed the ratios of the squares very considerably. At any rate it
will be clear that the efforts made by fishes in rushing upon prey or
escaping enemies (and it is these extreme efforts which here concern
us) must, as fishes become more active, rapidly exalt the strains to
be borne by their motor organs; and that of these strains, those which
fall upon the notochord must be exalted in proportion to the rest. Thus
the development of locomotive power, which survival of the fittest must
tend in most cases to favour, involves such increase of stress on the
primitive cartilaginous rod as will tend, other things equal, to cause
its modification.

  [Illustration: Figs. 291–293.]

What must its modification be? Considering the complication of the
influences at work, conspiring, as above indicated, in various ways and
degrees, we cannot expect to do more than form an idea of its average
character. The nature of the changes which the notochord is likely
to undergo, where greater bulk is accompanied by higher activity, is
rudely indicated by Figs. 291, 292, and 293. The successively thicker
lines represent the successively greater strains to which the outer
layers of tissue are exposed; and the widening interspaces represent
the greater extensions which they have to bear when they become convex,
or else the greater gaps that must be formed in them. Had these outer
layers to undergo extension only, as on the convex side, continued
natural selection might result in the formation of a tissue elastic
enough to admit of the requisite stretching. But at each alternate
bend these outer layers, becoming concave, are subject to increased
compression--a compression which they cannot withstand if they have
become simply more extensible. To withstand this greater compression
they must become harder as well as more extensible. How are these two
requirements to be reconciled? If, as facts warrant us in supposing, a
formation of denser substance occurs at those parts of the notochord
where the strain is greatest; it is clear that this formation cannot
so go on as to produce a continuous mass: the perpetual flexions must
prevent this. If matter that will not yield at each bend, is deposited
while the bendings are continually taking place, the bendings will
maintain certain places of discontinuity in the deposit--places at
which the whole of the stretching consequent on each bend will be
concentrated. And thus the tendency will be to form segments of hard
tissue capable of great resistance to compression, with intervals
filled by elastic tissue capable of great resistance to extension--a
vertebral column.

And now observe how the progress of ossification is just such as
conforms to this view. That centripetal development of segments which
holds of the vertebrate animal as a whole, as, if caused by transverse
strains, it ought to do, and which holds of the vertebral column as
a whole, as it ought to do, holds also of the central axis. On the
mechanical hypothesis, the outer surface of the notochord should be
the first part to undergo induration, and that division into segments
which must accompany induration. And accordingly, in a vertebral column
of which the axis is beginning to ossify, the centrums consist of bony
rings inclosing a still-continuous rod of cartilage.


§ 258. Sundry other general facts disclosed by the comparative
morphology of the _Vertebrata_, supply further confirmation. Let
us take first the structure of the skull.

On considering the arrangement of the muscular flakes, or _myocommata_,
in any ordinary fish which comes to table--an arrangement already
sketched out in the _Amphioxus_--it is not difficult to see that that
portion of the body out of which the head of the vertebrate animal
becomes developed, is a portion which cannot subject itself to bendings
in the same degree as the rest of the body. The muscles developed
there must be comparatively short, and much interfered with by the
pre-existing orifices. Hence the cephalic part will not partake in any
considerable degree of the lateral undulations; and there will not tend
to arise in it any such distinct segmentation as arises elsewhere. We
have here, then, an explanation of the fact, that from the beginning
the development of the head follows a course unlike that of the spinal
column; and of the fact that the segmentation, so far as it can be
traced in the head, is most readily to be traced in the occipital
region and becomes lost in the region of the face. For if, as we have
seen, the segmentation consequent on mechanical actions and reactions
must progress from without inwards, affecting last of all the axis; and
if, as we have seen, the region of the head is so circumstanced that
the causes of segmentation act but feebly even on its periphery; then
that terminal portion of the primitive notochord which is included in
the head, having to undergo no lateral bendings, may ossify without
division into segments.

Of other incidental evidences supplied by comparative morphology, let
me next refer to the supernumerary bones, which the theory of Goethe
and Oken as elaborated by Prof. Owen, has to get rid of by gratuitous
suppositions. In many fishes, for example, there are what have been
called interneural spines and interhæmal spines. These cannot by any
ingenuity be affiliated upon the archetypal vertebra, and they are
therefore arbitrarily rejected as bones belonging to the exo-skeleton;
though in shape and texture they are similar to the spines between
which they are placed. On the hypothesis of evolution, however, these
additional bones are accounted for as arising under actions like those
that gave origin to the bones adjacent to them. And similarly with such
bones as those called sesamoid; together with others too numerous to
name.


§ 259. Of course the foregoing synthesis is to be taken simply as an
adumbration of the process by which the vertebrate structure may have
arisen through the continued actions of known agencies. The motive
for attempting it has been two-fold. Having, as before said, given
reasons for concluding that the segments of a vertebrate animal are not
homologous in the same sense as are those of an annulose animal, it
seemed needful to do something towards showing how they are otherwise
to be accounted for; and having here, for our general subject, the
likenesses and differences among the parts of organisms, as determined
by incident forces, it seemed out of the question to pass by the
problem presented by the vertebrate skeleton.

Leaving out all that is hypothetical, the general argument may be
briefly presented thus:--The evolution from the simplest known
vertebrate animal of a powerful and active vertebrate animal, implies
the development of a stronger internal fulcrum. The internal fulcrum
cannot be made stronger without becoming more dense. And it cannot
become more dense while retaining its lateral flexibility, without
becoming divided into segments. Further, in conformity with the general
principles thus far traced, these segments must be alike in proportion
as the forces to which they are exposed are alike, and unlike in
proportion as these forces are unlike; and so there necessarily results
that unity in variety by which the vertebral column is from the
beginning characterized. Once more, we see that the explanation extends
to those innumerable and more marked divergences from homogeneity,
which vertebræ undergo in the various higher animals. Thus, the
production of vertebræ, the production of likenesses among vertebræ,
the production of unlikenesses among vertebræ, and the production of
unlikenesses among vertebral columns, are interpretable as parts of one
general process, and as harmonizing with one general principle.

Whether sufficient or insufficient, the explanation here given assigns
causes of known kinds producing effects such as they are known to
produce. It does not, as a solution of one mystery, offer another
mystery of which no solution is to be asked. It does not allege a
Platonic ἰδέα, or fictitious entity, which explains the vertebrate
skeleton by absorbing into itself all the inexplicability. On the
contrary, it assumes nothing beyond agencies by which structures in
general are moulded--agencies by which these particular structures
are, indeed, notoriously modifiable. An ascertained cause of certain
traits in vertebræ and other bones, it extends to all other traits of
vertebræ; and at the same time assimilates the morphological phenomena
they present to much wider classes of morphological phenomena.

       *       *       *       *       *

[NOTE.--The theory set forth in the foregoing chapter, is an
elaboration of one suggested at the close of a criticism of Prof.
Owen’s _Archetype and Homologies of the Vertebrate Skeleton_,
already referred to in § 210 as having been published in the
_Medico-Chirurgical Review_ for October, 1858. It is now reproduced in
Appendix B. Since the issue of this elaborated exposition, in No. 15 of
my serial in December, 1865, verifications of it have from time to time
been published. In his work _The Primary Factors of Organic Evolution_,
Prof. Cope of Philadelphia writes:--

“Mr. Herbert Spencer has endeavoured to account for the origin of the
segmentation of muscles into myotomes, and the division of the sheath
of the notochord into vertebræ, by supposing it to be due to the
lateral swimming movements of the fishes, which first exhibit these
structures. With this view various later authors have agreed, and I
have offered some additional evidence of the soundness of this position
with respect to the vertebral axis of Batrachia, and the origin of
limb articulations. It is true that the origin of segmentation in the
vertebral column of the true fishes and the Batrachia turns out to have
been less simple in its process than was suggested by Mr. Spencer, but
his general principle holds good, now that paleontology has cleared up
the subject” (pp. 367–8).

An allusion in the foregoing extract is made by Prof. Cope to certain
observations set forth in his work entitled _The Origin of the
Fittest_. On pp. 305–6 of it will be found the following sentences:--

“Now, all the Permian land-animals, reptiles and batrachians, retain
this notochord with the elements of osseous vertebræ, in a greater or
less degree of completeness. There are some in South Africa, I believe,
in which the ossification has come clear through the notochord; but
they are few.... There is something to be said as to the condition
of the column from a mechanical standpoint, and it is this: that the
chorda exists, with its osseous elements disposed about it; and in the
Permian batrachians, equally related to salamanders and frogs, these
osseous elements are arranged in the sheath or skin of the chorda;
and they are in the form of regular concave segments, very much like
such segments as you can take from the skin of an orange--but parts
of a cylinder, and having greater or less dimensions according to the
group or species. Now, the point of divergence of these segments is
on the side of the column. The contacts are placed on the side of the
column where the segments separate--the upper segments rising and the
lower segments coming downward. To the upper segments are attached the
arches and their articulations, and the lower segments are like the
segments of a cylinder. If you take a flexible cylinder, and cover it
with a more or less inflexible skin or sheath, and bend that cylinder
sidewise, you of course will find that the wrinkles or fractures of
that part of the surface will take place along the line of the shortest
curve, which is on the side; and, as a matter of fact, you have breaks
of very much the character of the segments of the Permian Batrachia....
In the cylinder bending both ways, of course the shortest line of
curve is right at the centre of the side of that cylinder, and the
longest curve is of course at the summit and base, and the shortest
curve will be the point of fracture. And that is exactly what I presume
has happened in the case of the construction of the segments of the
sheath of the vertebral column, by the lateral motion of the animal in
swimming, and which has been the actual cause of the disposition of the
osseous material in its form.... That is the state of the vertebral
column of many of the Vertebrata of the Permian period.”

In his essay on “The Mechanical Causes of the Development of the
Hard Parts of the Mammalia,” published in the American _Journal of
Morphology_ (Vol. III), Prof. Cope has carried the interpretation
further, by showing that in kindred ways the genesis of articulations
and limb-bones may be explained. On p. 163 he enunciates the general
principle of his interpretation as follows:--

“It cannot have been otherwise than that, since the motions of animals
continued during the evolution of their hard parts, these hard parts
grew in exact adaptation to these movements. Thus at the points of
greatest flexure joints would be formed, and between these joints the
deposit would be continuous.”

Evidently if osseous structures are produced by deposits of calcareous
matters in pre-existing cartilaginous structures, or other structures
of flexible materials, the deposits must be so carried on that while
dense resistant masses are produced these must admit of such free
movements as the creature’s life necessitates, and must so form adapted
joints.

Let it be understood, however, that the hypothesis set forth in
the foregoing chapter and extended by Prof. Cope, which serves to
interpret a large part of the phenomena of osseous structures in the
_Vertebrata_, does not serve to interpret them all. While the formation
of hard parts has been in large measure initiated and regulated by
tensions and pressures, there are hard parts the formation of which
cannot be thus explained. The bones of the skull are the most obvious
instances. These are apparently referable to no other cause than the
survival of the fittest--the survival of individual animals in which
greater density of the brain-covering yielded better protection against
external injuries. Without enumerating other instances which might be
given, it will suffice to recognize the truth that natural selection
of favourable variations and the inheritance of functionally-produced
changes have all along co-operated: each of them in some cases acting
alone, but in other cases both acting together.]




                             CHAPTER XVI.

                      THE SHAPES OF ANIMAL CELLS.


§ 260. Among animals as among plants, the laws of morphological
differentiation must be conformed to by the morphological units, as
well as by the larger parts and by the wholes formed of them. It
remains here to point out that the conformity is traceable where the
conditions are simple.

  [Illustration: Fig. 294.]

In the shapes assumed by those rapidly-multiplying cells out of which
each animal is developed, there is a conspicuous subordination to
the surrounding actions. Fig. 294 represents the cellular embryonic
mass that arises by repeated spontaneous fissions. In it we see how
the cells, originally spherical, are changed by pressure against one
another and against the limiting membrane; and how their likenesses
and unlikenesses are determined by the likenesses and unlikenesses
of the forces to which they are exposed. This fact may be thought
scarcely worth pointing out. But it is worth pointing out, because what
is here so obvious a consequence of mechanical actions, is in other
cases a consequence of actions composite in their kinds and involved
in their distribution. Just as the equalities and inequalities of
dimensions among aggregated cells, are here caused by the equalities
and inequalities among their mutual pressures in different directions;
so, though less manifestly, the equalities and inequalities of
dimensions among other aggregated cells, are caused by the equalities
and inequalities of the osmotic, chemical, thermal, and other forces
besides the mechanical, to which their different positions subject them.


§ 261. This we shall readily see on observing the ordinary structures
of limiting membranes, internal and external. In Fig. 295, is shown
a much-magnified section of a papilla from the gum. The cells of
which it is composed originate in its deeper part; and are at first
approximately spherical. Those of them which, as they develop, are
thrust outwards by the new cells that continually take their places,
have their shapes gradually changed. As they grow and successively
advance to replace the superficial cells, when these exfoliate, they
become exposed to forces which are more and more different in the
direction of the surface from what they are in lateral directions; and
their dimensions gradually assume corresponding differences.

  [Illustration: Fig. 295.]

  [Illustration: Fig. 296.]

Another species of limiting membrane, called cylinder-epithelium,
is represented in Fig. 296. Though its mode of development is
such as to render the shapes of its cells quite unlike those of
pavement-epithelium, as the above-described kind is sometimes called,
its cells equally exemplify the same general truth. For the chief
contrast which each of them presents, is the contrast between its
dimension at right angles to the surface of the membrane, and its
dimension parallel to that surface.

It is needless for our present purpose to examine further the evidence
furnished by Histology; nor, indeed, would further examination of this
evidence be likely to yield definite results. In the cases given above
we have marked differences among the incident forces; and therefore
have a chance of finding, as we do find, relations between these and
differences of form. But the cells composing masses of tissue are
severally subject to forces which are indeterminate; and therefore
the interpretation of their shapes is impracticable. It must suffice
to observe that so far as the facts go they are congruous with the
hypothesis.




                             CHAPTER XVII.

                 SUMMARY OF MORPHOLOGICAL DEVELOPMENT.


§ 262. That any formula should be capable of expressing a common
character in the shapes of things so unlike as a tree and a cow, a
flower and a centipede, is a remarkable fact; and is a fact which
affords strong _primâ facie_ evidence of truth. For in proportion to
the diversity and multiplicity of the cases to which any statement
applies, is the probability that it sets forth the essential relations.
Those connexions which remain constant under all varieties of
manifestation, are most likely to be the causal connexions.

Still higher will appear the likelihood of an alleged law of organic
form possessing so great a comprehensiveness, when we remember that on
the hypothesis of Evolution, there must exist between all organisms
and their environments, certain congruities expressible in terms of
their actions and reactions. The forces being, on this hypothesis, the
causes of the forms, it is inferable, _à priori_, that the forms
must admit of generalization in terms of the forces; and hence, such
a generalization arrived at _à posteriori_, gains the further
probability due to fulfilment of anticipation.

Nearer yet to certainty seems the conclusion thus reached, on finding
that it does but assert in their special manifestations, the laws of
Evolution in general--the laws of that universal re-distribution of
matter and motion which hold throughout the totality of things, as
well as in each of its parts.

It will be useful to glance back over the various minor inferences
arrived at, and contemplate them in their _ensemble_ from these
higher points of view.


§ 263. That process of integration which every plant displays during
its life, we found reason to think has gone on during the life of the
vegetal kingdom as a whole. Protoplasm into cells, cells into folia,
folia into axes, axes into branched combinations--such, in brief, are
the stages passed through by every shrub; and such appear to have been
the stages through which plants of successively-higher kinds have been
evolved from lower kinds. Even among certain groups of plants now
existing, we find aggregates of the first order passing through various
gradations into aggregates of the second order--here forming small,
incoherent, indefinite assemblages, and there forming large, definite,
coherent fronds. Similar transitions are traceable through which these
integrated aggregates of the second order pass into aggregates of
the third order: in one species the unions of parent-fronds with the
fronds that bud out from them, being temporary, and in another species
such unions being longer continued; until, in species still higher,
by a gemmation which is habitual and regular, there is produced a
definitely-integrated aggregate of the third order--an axis bearing
fronds or leaves. And even between this type and a type further
compounded, a link occurs in the plants which cast off, in the shape of
bulbils, some of the young axes they produce. As among plants, so among
animals. A like spontaneous fission of cells ends here in separation,
there in partial aggregation, while elsewhere, by closer combination
of the multiplying units, there arises a coherent and tolerably
definite individual of the second order. By the budding of individuals
of the second order, there are in some cases produced other separate
individuals like them; in some cases temporary aggregates of such like
individuals; and in other cases permanent aggregates of them: certain
of which become so definitely integrated that the individualities of
their component members are almost lost in a tertiary individuality.

Along with this progressive integration there has gone on a progressive
differentiation. Vegetal units of whatever order, originally
homogeneous, have become heterogeneous while they have become united.
Spherical cells aggregating into threads, into laminæ, into masses, and
into special tissues, lose their sphericity; and instead of remaining
all alike assume innumerable unlikenesses--from uniformity pass
into multiformity. Fronds combining to form axes, severally acquire
definite differences between their attached ends and their free ends;
while they also diverge from one another in their shapes at different
parts of the axes they compose. And axes, uniting into aggregates of
a still higher order, become contrasted in their sizes, curvatures,
and the arrangements of their appendages. Similarly among animals.
Those components of them which, with a certain license, we class as
morphological units, while losing their minor individualities in the
major individualities formed of them, grow definitely unlike as they
grow definitely combined. And where the aggregates so produced become,
by coalescence, segments of aggregates of a still higher order, they,
too, diverge from one another in their shapes.

The morphological differentiation which thus goes hand in hand with
morphological integration, is clearly what the perpetually-complicating
conditions would lead us to anticipate. Every addition of a new unit
to an aggregate of such units, must affect the circumstances of the
other units in all varieties of ways and degrees, according to their
relative positions--must alter the distribution of mechanical strains
throughout the mass, must modify the process of nutrition, must affect
the relations of neighbouring parts to surrounding diffused actions;
that is, must initiate a changed incidence of forces tending ever to
produce changed structural arrangements.


§ 264. This broad statement of the correspondence between the general
facts of Morphological Development and the principles of Evolution
at large, may be reduced to statements of a much more specific kind.
The phenomena of symmetry and unsymmetry and asymmetry, which we have
traced out among organic forms, are demonstrably in harmony with those
laws of the re-distribution of matter and motion to which Evolution
conforms. Besides the myriad-fold illustrations of the instability of
the homogeneous, afforded by these aggregates of units of each order,
which, at first alike, lapse gradually into unlikeness; and besides
the myriad-fold illustrations of the multiplication of effects, which
these ever-complicating differentiations exhibit to us; we have also
myriad-fold illustrations of the definite equalities and inequalities
of structures, produced by definite equalities and inequalities of
forces.

The proposition arrived at when dealing with the causes of Evolution,
“that in the actions and reactions of force and matter, an unlikeness
in either of the factors necessitates an unlikeness in the effects; and
that in the absence of unlikeness in either of the factors the effects
must be alike” (_First Principles_, § 169), is a proposition which
implies all these particular likenesses and unlikenesses of parts
which we have been tracing. For have we not everywhere seen that the
strongest contrasts are between the parts that are most contrasted in
their conditions; while the most similar parts are those most similarly
conditioned? In every plant the leading difference is between the
attached end and the free end; in every branch it is the same; in every
leaf it is the same. And in every plant the leading likenesses are
those between the two sides of the branch, the two sides of the leaf,
and the two sides of the flower, where these parts are two-sided in
their conditions; or between all sides of the branch, all sides of
the leaf, and all sides of the flower, where these parts are similarly
conditioned on all sides. So, too, is it with animals which move about.
The most marked contrasts they present are those between the part in
advance and the part behind, and between the upper part and the under
part; while there is complete correspondence between the two sides.
Externally the likenesses and differences among limbs, and internally
the likenesses and differences among vertebræ, are expressible in terms
of this same law.

And here, indeed, we may see clearly that these truths are corollaries
from that ultimate truth to which all phenomena of Evolution are
referable. It is an inevitable deduction from the persistence of force,
that organic forms which have been progressively evolved, must present
just those fundamental traits of form which we find them present. It
cannot but be that during the intercourse between an organism and its
environment, equal forces acting under equal conditions must produce
equal effects; for to say otherwise is, by implication, to say that
some force can produce more or less than its equivalent effect, which
is to deny the persistence of force. Hence those parts of an organism
which are, by its habits of life, exposed to like amounts and like
combinations of actions and reactions, must develop alike; while
unlikenesses of development must as unavoidably follow unlikenesses
among these agencies. And this being so, all the specialities of
symmetry and unsymmetry and asymmetry which we have traced, are
necessary consequences.




                                PART V.

                      PHYSIOLOGICAL DEVELOPMENT.




                              CHAPTER I.

                      THE PROBLEMS OF PHYSIOLOGY.


§ 265. The questions to be treated under the above title are widely
different from those which it ordinarily expresses. We have no
alternative, however, but to use Physiology in a sense co-extensive
with that in which we have used Morphology. We must here consider
the facts of function in a manner parallel to that in which we
have, in the foregoing Part, considered the facts of structure.
As, hitherto, we have concerned ourselves with those most general
phenomena of organic form which, holding irrespective of class and
order and sub-kingdom, illustrate the processes of integration and
differentiation characterizing Evolution at large; so, now, we have
to concern ourselves with the evidences of those differentiations and
integrations of organic functions which have simultaneously arisen,
and which similarly transcend the limits of zoological and botanical
divisions. How heterogeneities of action have progressed along with
heterogeneities of structure--that is the inquiry before us; and
obviously, in pursuing it, all the specialities with which Physiology
usually deals can serve us only as materials.

Before entering on the study of Morphological Development, it
was pointed out that while facts of structure may be empirically
generalized apart from facts of function, they cannot be rationally
interpreted apart; and throughout the foregoing pages this truth has
been made abundantly manifest. Here we are obliged to recognize the
interdependence still more distinctly; for the phenomena of function
cannot even be conceived without direct and perpetual consciousness of
the phenomena of structure. Though the subject-matter of Physiology
is as broadly distinguished from the subject-matter of Morphology as
motion is from matter; yet, just as the laws of motion cannot be known
apart from some matter moved, so there can be no knowledge of function
without a knowledge of some structure as performing function.

Much more than this is obvious. The study of functions, considered from
our present point of view as arising by Evolution, must be carried on
_mainly_ by the study of the correlative structures. Doubtless, by
experimenting on the organisms which are growing and moving around
us, we may ascertain the connexions existing among certain of their
actions, while we have little or no knowledge of the special parts
concerned in those actions. In a living animal that can be conveniently
kept under observation, we may learn the way in which conspicuous
functions vary together--how the rate of a man’s pulse increases with
the amount of muscular exertion he is undergoing; or how a horse’s
rapidity of breathing is in part dependent on his speed. But though
observations of this order are indispensable--though by accumulation
and comparison of such observations we learn which parts perform which
functions--though such observations, prosecuted so as to disclose
the actions of all parts under all circumstances, constitute, when
properly generalized and co-ordinated, what is commonly understood as
Physiology; yet such observations help us but a little way towards
learning how functions came to be established and specialized.
We have next to no power of tracing up the genesis of a function
considered purely as a function--no opportunity of observing the
progressively-increasing quantities of a given action that have arisen
in any order of organisms. In nearly all cases we are able only to
show the greater growth of the part which we have found performs the
action, and to infer that greater action of the part has accompanied
greater growth of it. The tracing out of Physiological Development,
then, becomes substantially a tracing out of the development of
the organs by which the functions are known to be discharged--the
differentiation and integration of the functions being presumed to have
progressed hand in hand with the differentiation and integration of the
organs. Between the inquiry pursued in Part IV, and the inquiry to be
pursued in this Part, the contrast is that, in the first place, facts
of structure are now to be used to interpret facts of function, instead
of conversely; and, in the second place, the facts of structure to be
so used are not those of conspicuous shape so much as those of minute
texture and chemical composition.


§ 266. The problems of Physiology, in the wide sense above
described, are, like the problems of Morphology, to be considered
as problems to which answers must be given in terms of incident
forces. On the hypothesis of Evolution these specializations of
tissues and accompanying concentrations of functions, must, like the
specializations of shape in an organism and its component divisions,
be due to the actions and reactions which its intercourse with the
environment involves; and the task before us is to explain how they are
wrought--how they are to be comprehended as results of such actions and
reactions.

Or, to define these problems still more specifically:--Those extremely
unstable substances composing the protoplasm of which organisms are
mainly built, have to be traced through the various modifications in
their properties and powers, that are entailed on them by changes of
relation to agencies of all kinds. Those organic colloids which pass
from liquid to solid and from soluble to insoluble on the slightest
molecular disturbance--those albuminoid matters which, as we see in
clotted blood or the coagulable lymph poured out on abraded surfaces
and causing adhesion between inflamed membranes, assume new forms
with the greatest readiness--are to have their metamorphoses studied
in connexion with the influences at work. Those compounds which, as
we see in the quickly-acquired brownness of a bitten apple or in the
dark stains produced by the milky juice of a Dandelion, immediately
begin to alter when the surrounding actions alter, are to be everywhere
considered as undergoing modifications by modified conditions. Organic
bodies, consisting of substances that, as I here purposely remind the
reader, are prone beyond all others to change when the incident forces
are changed, we must contemplate as in all their parts differently
changed in response to the different changes of the incident forces.
And then we have to regard the concomitant differentiations of their
reactions as being concomitant differentiations of their functions.

Here, as before, we must take into account two classes of factors.
We have to bear in mind the inherited results of actions to which
antecedent organisms were exposed, and to join with these the results
of present actions. Each organism is to be considered as presenting
a moving equilibrium of functions, and a correlative arrangement
of structures, produced by the aggregate of actions and reactions
that have taken place between all ancestral organisms and their
environments. The tendency in each organism to repeat this adjusted
arrangement of functions and structures, must be regarded as from time
to time interfered with by actions to which its inherited equilibrium
is not adjusted--actions to which, therefore, its equilibrium has to be
re-adjusted. And in studying physiological development we have in all
cases to contemplate the progressing compromise between the old and the
new, ending in a restored balance or adaptation.

Manifestly our data are so scanty that nothing more than very general
and approximate interpretations of this kind are possible. If the
hypothesis of Evolution furnishes us with a rude conception of the
way in which the more conspicuous and important differentiations of
functions have arisen, it is as much as can be expected.


§ 267. It will be best, for brevity and clearness, to deal with these
physiological problems as we dealt with the morphological ones--to
carry on the inductive statement and the deductive interpretation
hand in hand: so disposing of each general truth before passing
to the next. Treating separately vegetal organisms and animal
organisms, we will in each kingdom consider:--first, the physiological
differentiations and accompanying changes of structure which arise
between outer tissues and inner tissues; next, those which arise
between different parts of the outer tissues; and, finally, those which
arise between different parts of the inner tissues. What little has
to be said concerning physiological integration must come last. For
though, in tracing up Morphological Evolution, we have to study those
processes of integration by which organic aggregates are formed, before
studying the differentiations that arise among their parts; we must,
contrariwise, in tracing up Physiological Evolution, study the genesis
of the different functions before we study the interdependence that
eventually arises among them and constitutes physiological unity.




                              CHAPTER II.

    DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF PLANTS.


§ 268. The simplest plant presents a contrast between its peripheral
substance and its central substance. In each protophyte, be it a
spherical cell or a branched tube, or such a more-specialized form as a
Desmid, a marked unlikeness exists between the limiting layer and that
which it limits. These vegetal aggregates of the first order may differ
widely from one another in the natures of their outer coats and in the
natures of their contents. As in the Palmella-form of one of the lower
_Algæ_, there may exist a clothing of jelly; or, as in Diatom, the
walls may take the form of silicious valves variously sculptured. The
contained matter may be partly or wholly here green, there red, and in
other cases brown. But amid all these diversities there is this one
uniformity--a strong distinction between the parts in contact with the
environment and the parts not in contact with the environment.

When we remember that this trait is one which these simple living
bodies have in common with bodies that are not living--when we
remember that each inorganic mass eventually has its outer part more
or less differentiated from its inner part, here by oxidation, there
by drying, and elsewhere by the actions of light, of moisture, of
frost; we can scarcely resist the conclusion that, in the one case
as in the other, the contrast is due to the unlike actions to which
the parts are subject. Given an originally-homogeneous portion of
protoplasm, and it follows from the general laws of Evolution (First
Principles, §§ 149–155), first, that it must lose its homogeneity, and,
second, that the leading dissimilarities must arise between the parts
most-dissimilarly conditioned--that is, between the outside and the
inside. The exterior must bear amounts and kinds of force unlike the
amounts and kinds which the interior bears; and from the persistence
of force it follows inevitably that unlike effects must be wrought on
them--they must be differentiated.

What is the limit towards which the differentiation tends? We
have seen that the re-distribution of matter and motion whence,
under certain conditions, evolution results, can never cease until
equilibrium is reached--proximately a moving equilibrium, and finally
a complete equilibrium (_First Principles_, §§ 170–175). Hence, the
differentiation must go on until it establishes such differences in the
parts as shall balance the differences in the forces acting on them.
When dealing with equilibration in general, we saw that this process
is what is called adaptation (_First Principles_, § 173); and, in this
work, we saw that by it the totality of functions of an organism is
brought into correspondence with the totality of actions affecting it
(§§ 159–163). Manifestly in this case, as in all others, either death
or adjustment must eventually result. A force falling on one of these
minute aggregates of protoplasm, must expend itself in working its
equivalent of change. If this force is such that in expending itself
it disturbs beyond rectification the balance of the organic processes,
then the aggregate is disintegrated or decomposed. But if it does
not overthrow that moving equilibrium constituting the life of the
aggregate, then the aggregate continues in that modified form produced
by the expenditure of the force. Thus, by direct equilibration,
continually furthered by indirect equilibration, there must arise this
distinction between the outer part adapted to meet outer forces, and
the inner part adapted to meet inner forces. And their respective
actions, as thus meeting outer and inner forces, must be what we call
their respective functions.


§ 269. Aggregates of the second order exhibit parallel traits,
admitting of parallel interpretations. Integrated masses of cells or
units homologous with protophytes, habitually show us contrasts between
the characters of the superficial tissues and the central tissues. Such
among these aggregates of the second order as have their component
units arranged into threads or laminæ, single or double, cannot, of
course, furnish contrasts of this kind; for all their units are as much
external as internal. We must turn to the more or less massive forms.

Of these, among _Fungi_, the common Puff-ball is a good example--good
because it presents this fundamental differentiation but little
complicated by others. In it we have a cortical layer of interwoven
hyphæ obviously unlike the mass of spores which it incloses. So far as
the unlikeness between external and internal parts is concerned, we see
here a relation analogous to that existing in the simple cell; and we
see in it a similar meaning: there is a physiological differentiation
corresponding to the difference in the incidence of forces.

Under various forms the _Algæ_ show just the same relation. Where,
as in _Codium Bursa_, we have the ramified tubular branches of the
thallus aggregated into a hollow globular mass, the outer and inner
surfaces are contrasted both in colour and structure, though the
tubules composing the two surfaces are continuous with one another. In
_Rivularia_, again, we see the like, both in the radial arrangement
of the imbedded threads and in the difference of colour between the
exterior of the imbedding jelly and its interior. The more-developed
_Algæ_ of all kinds repeat the antithesis. In branched stems, when
they consist of more than single rows of cells, the outer cells become
unlike the inner, as shown in Fig. 35. Such types as _Chrysymenia
rosea_ show us this unlikeness very conspicuously. And it holds even
with ribbon-shaped fronds. Wherever one of these is composed of three,
four, or more layers, as in _Laminaria_ and _Punctaria_, the cells
of the external layers are strongly distinguished from those of the
internal layers, both by their comparative smallness and by their deep
colour.


§ 270. The higher plants variously display the like fundamental
distinction between outer and inner tissues. Each leaf, thin as it is,
exemplifies this differentiation of the parts immediately in contact
with the environment from the parts not in immediate contact with
the environment. Its epidermal cells, forming a protecting envelope,
diverge physically and chemically from the mesophyll cells, which carry
on the more active functions. And the contrast may be observed to
establish itself in the course of development. At first the component
cells of the leaf are all alike; and this unlikeness between the cells
of the outer and inner layers, arises simultaneously with the rise of
differences in their conditions--differences that have acted on all
ancestral leaves as they act on the individual leaf.

An unlikeness more marked in kind but similar in meaning, exists
between the bark of every branch and the tissues it clothes. The
phænogamic axis, especially when it undergoes what is known as
secondary thickening, is commonly characterized by an outer zone of
cells (the cork layer) differing from the inner layers in character and
function, as it differs from them in position. Subject as this outer
layer is to the unmitigated actions of forces around--to abrasions, to
extremes of heat and cold, to evaporation and soaking with water--its
units have to be brought into equilibrium with these more violent
actions, and have acquired molecular constitutions more stable
than those of the interior cells. That is to say, the forces which
differentiate the cortical part from the rest are the forces which it
has to resist, and from which it passively protects the parts within.
How clearly this heterogeneity of structure and function is consequent
upon intercourse with the environment, every tree and shrub shows. The
young shoots, alike of annuals and perennials, are quite green and soft
at their extremities. Among plants of short lives, there is usually
but a slight development of bark or none at all: such traces of it as
the surface of the axis acquires being seen only at its lowermost or
oldest portion. In long-lived plants, however, this formation of a
tough opaque coating takes place more rapidly; and shows us distinctly
the connexion between the degree of differentiation and the length of
exposure. For, in a growing twig, we see that the bark, invisible at
the bud, thickens by insensible gradations as we go downwards to the
junction of the twig with the branch; and we come to still thicker
parts of it as we descend along the branch towards the main stem.
Moreover, on examining main stems we find that while in some trees the
bark, cracked by expansion of the wood, drops off in flakes, leaving
exposed patches of the inner tissue which presently become green and
finally develop new bark; in other trees the exfoliated flakes continue
adherent, and in the course of years form a rugged fissured coat: so
producing a still more marked contrast between outside and inside. Of
course the establishment of this heterogeneity is furthered by natural
selection, which, where a protective covering is needed, gives an
advantage to those individuals in which it has become strongest. But
that this divergence of structure commences as a direct adaptation, is
clearly shown by other facts than the foregoing. There is the fact that
many of the plants which in our gardens develop bark with considerable
rapidity, do not develop it with the same rapidity in a greenhouse. And
there is the fact that plants which, in some climates, have their stems
covered only by thin semi-transparent layers, acquire thick opaque
layers when taken to other climates.

Just noting, for the sake of completeness, that in the roots of the
higher plants there arises a contrast between outer and inner parts,
parallel to the one we have traced in their branches, let me draw
attention to another differentiation of the same ultimate nature, which
the higher plants exhibit to us--a differentiation which, familiar
though it is, gains a new meaning by association with those named
above, and makes their meaning still more manifest. Each great plant
shows it. When, by the budding of axes out of axes, there is produced
one of those highly-compounded Phænogams which we call a tree, the
central part of the aggregate becomes functionally and structurally
unlike the peripheral part. On looking into a large tree, or even
a small one which has thick foliage, like the Laurel, we see that
the internal branches are almost or quite bare of leaves, while the
leaf-clad branches form an external stratum; and all our experience
unites in proving that this contrast arises by degrees, as fast as
the growth of the tree entails a contrast between the conditions
to which inner and outer branches are exposed. Now when, in these
most-composite aggregates, we see a differentiation between peripheral
and central parts demonstrably caused by a difference in the relations
of these parts to environing forces, we get support for the conclusion
otherwise reached, that there is a parallel cause for the parallel
differentiations exhibited by all aggregates of lower orders--branches,
leaves, cells.


§ 271. Before leaving this most general physiological differentiation,
it may be well to say something respecting certain secondary
unlikenesses which usually arise between interior and exterior. For the
contrast is not, as might be supposed from the foregoing descriptions,
a simple contrast: it is a compound contrast. The outer structure
itself is usually divisible into concentric structures. This is equally
true of a protophyte and of a phænogamic axis. Between the centre of
an independent vegetal cell and its surface, there are at least two
layers; and the bark coating the substance of a shoot, besides being
itself compound, includes another tissue lying between it and the
wood. What is the physical interpretation of these facts?

When a mass of something we distinguish as inert matter is exposed
to external agencies capable of working changes in it--when it is
chemically acted upon, or when, being dry, it is allowed to soak, or
when, being wet, it is allowed to dry--the changes set up progress in
an equable way from the surface towards the centre. At any time during
the process (supposing no other action supervenes) the modification
wrought, first completed at the outside, either gradually diminishes
as we approach the centre, or ceases suddenly at a certain distance
from the centre. But now suppose that the mass, instead of being
inert, is the seat of active changes--suppose that it is a portion of
complex colloidal substance, permeable by light and by fluids capable
of affecting its unstable molecules--suppose that its interior is
a source of forces continually liberated and diffusing themselves
outwards. Is it not likely that while at the centre the action of the
internally-liberated forces will dominate, and while at the surface
the action of the environing forces will dominate, there will be
between the two a certain place at which their actions balance? May
we not expect that this will be the place where the most unstable
matter exists--the place outside of which the matter becomes relatively
stable in the face of external forces, and inside of which the matter
becomes relatively stable in the face of internal forces? And must we
not conclude that though part of the adjustment is due to indirect
equilibration, the initiation of it is due to direct equilibration?

But we are here chiefly concerned with the more general interpretation,
which is independent of any such speculation as the foregoing. These
contrasted tissues and the contrasted functions they severally perform
are, beyond question, subordinated to the relations of outside and
inside. And the evidence makes it tolerably clear that the unlike
actions or forces involved by the relations of outside and inside,
determine these contrasts--partly directly and partly indirectly.




                             CHAPTER III.

          DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS.


§ 272. The motionless protococcoid forms of lower _Algæ_, which do
not permanently expose any parts of their surfaces to actions unlike
those which other parts are exposed to, have no parts of their
surfaces unlike the rest in function and composition. This is what
the hypothesis prepares us for. If physiological differentiations
are determined by differences in the incidence of forces, then there
will be no such differentiations where there are no such differences.
Contrariwise, it is to be expected that the most conspicuous
unlikeness of function and minute structure will arise between the
most-dissimilarly circumstanced parts of the surface. We find that
they do. The upper end and the lower end, or, more strictly speaking,
the free end and the attached end, habitually present the strongest
physiological contrasts.

Even aggregates of the first order illustrate this truth. Such
so-called unicellular plants as those delineated in Figs. 4, 5, and 6,
show us, on comparing the contents of their fixed ends and their loose
ends, that different processes are going on in them, and that different
functions are being performed by their limiting membranes. _Caulerpa
prolifera_, which “consists of a little creeping stem with roots below
and leaves above,” originating “in the growth of a body which may be
regarded as an individual cell,” supplies a still-better example. Among
aggregates of the second order a like connexion is displayed in more
various modes but with equal consistency. As before, the Puff-ball
served to exemplify the primary physiological differentiation of outer
parts from inner parts; so, here, it supplies a simple illustration of
the way in which the differentiated outer part is re-differentiated,
in correspondence with the chief contrast in its relations to the
environment. The only marked unlikeness which the cortical layer of
the Puff-ball presents, is that between the portion next the ground
and the opposite portion. The better-developed _Fungi_ exhibit a
more decided heterogeneity of parallel kind. Such incrusting _Algæ_
as _Ralfsia verrucosa_ furnish a kindred contrast; and in the higher
_Algæ_ it is uniformly repeated. Phænogams display this physiological
differentiation very conspicuously. That earth and air are unlike
portions of the environment, subjecting roots and leaves to unlike
physical forces, which entail on them unlike reactions, and that the
unlike functions and structures of their respective surfaces are fitted
to these unlike physical forces, are familiar facts which it would be
needless here to name, were it not that they must be counted as coming
within a wider group of facts.

Is this unlikeness between the outer tissues of the attached ends
and those of the free ends in plants, determined by their converse
with the unlike parts of the environment? That they result from an
equilibration partly arising in the individual and partly arising by
the survival of individuals in which it has been carried furthest, is
inferable _à priori_; and this _à priori_ argument may be adequately
enforced by arguments of the inductive order. A few typical ones must
here suffice. The gemmules of the _Marchantia_ are little disc-shaped
masses of cells composed of two or more layers. Their sides being
alike, there is nothing to determine which side falls lowermost when
one of them is detached. Whichever side falls lowermost, however,
presently begins to send out rootlets, while the uppermost side
begins to assume those characters which distinguish the face of the
frond. When this differentiation has commenced, the tendency to its
complete establishment becomes more and more decided; as is proved by
the fact that if the positions of the surfaces be altered, the gemmule
bends itself so as to re-adjust them: the change towards equilibrium
with environing forces having been once set up, there is acquired,
as it were, an increasing momentum which resists any counter-change.
But the evidence shows that at the outset, the relations to earth
and air alone determine the differentiation of the under surface
from the upper. The experiences of the gardener, multiplying his
plants by cuttings and layers, constitute another class of evidences
not to be omitted: they are commonplace but instructive examples of
physiological differentiation. While circumstanced as it usually is,
the meristematic tissue of each branch in a Phænogam continues to
perform its ordinary function--regularly producing on its outer side
the cortical substances, and on its inner side the vascular and woody
tissues. But change the conditions to those which the underground part
of the plant is exposed to, and there begins another differentiation
resulting in underground structures. Contact with water often suffices
alone to produce this result, as in the branches of some trees when
they droop into a pool, or as occasionally with a cutting placed in a
bottle of water; and when the light is excluded by imbedding the end
of the cutting, or the middle of the still-attached branch, in the
earth, this production of tissues adapted to the function of absorbing
moisture and mineral constituents proceeds still more readily.
With such cases may be grouped those in which this development of
underground organs by an above-ground tissue, is not exceptional but
habitual. Creeping plants furnish good illustrations. From the shoots
of the Ground-Ivy, rootlets are put out into the soil in a manner
differing but little from that in which they are put out by an imbedded
layer; save that the process follows naturally-induced conditions
instead of following artificially-induced conditions. But in the common
Ivy which, instead of running along the surface of the earth, runs
up inclined or vertical surfaces, we see the process interestingly
modified without being essentially changed. The rootlets, here
differentiated by their conditions into organs of attachment much more
than organs of absorption, still develop on that side of the shoot next
the supporting surface, and do not develop where the shoot, growing
away from the tree or wall, is surrounded equally on all sides by light
and air: thus showing, undeniably, that the production of the rootlets
is determined by the differential incidence of forces. Though survival
of the fittest doubtless furthered this transition yet it clearly
did not initiate it. That greenness which may be observed in these
Ivy-branch rootlets while they are quite young, soft, and unshaded,
introduces us to facts which are the converse of the foregoing facts;
and proves that the parts ordinarily imbedded in the soil and adapted
to its actions, acquire, often in very marked degrees, the superficial
structures of the aërial parts, when they are exposed to light and air.
This may be witnessed in Maize, which, when luxuriant, sends out from
its nodes near the ground, clusters of roots that are thick, succulent,
and of the same colour as the leaves. Examples more familiar to us
in England occur in every field of turnips. On noting how green is
the uncovered part of a turnip-root, and how manifestly the area over
which the greenness extends varies with the area exposed to light, as
well as with the degree of the exposure, it will be seen that beyond
question, root-tissue assumes to a considerable extent the appearances
and function of leaf tissue, when subject to the same agencies. Let
us not forget, too, that where exposed roots do not approach in
superficial character towards leaves, they approach in superficial
character towards stems: becoming clothed with a thick, fissured bark,
like that of the trunk and branches. But the most conclusive evidence
is furnished by the actual substitutions of surface-structures and
functions, that occur in aërial organs which have taken to growing
permanently under ground, and in underground organs which have taken to
growing permanently in the air. On the one hand, there is the rhizome
exemplified by Ginger--a stem which, instead of shooting up vertically,
runs horizontally below the surface of the soil, and assumes the
character of a root, alike in colour, texture, and production of
rootlets; and there is that kind of swollen underground axis, bearing
axillary buds, which the Potato exemplifies--a structure which, though
homologically an axis, simulates a tuberous root in surface-character,
and when exposed to the air, manifests no greater readiness to develop
chlorophyll than a tuberous root does. On the other hand, there are the
aërial roots of certain Orchids which, habitually green at their tips,
continue green throughout their whole lengths when kept moist; which
have become leaf-like not only by this development of chlorophyll, but
also by the acquirement of stomata; and which do not bury themselves in
the soil when they have the opportunity.[46] Thus we have aërial organs
so completely changed to fit underground actions, that they will not
resume aërial functions; and underground organs so completely changed
to fit aërial actions, that they will not resume underground functions.

That the physiological differentiation between the part of a plant’s
surface which is exposed to light and air and the part which is exposed
to darkness and moisture and solid matter, is primarily due to the
unlike actions of these unlike parts of the environment, is, then,
clearly implied by observed facts--more clearly, indeed, than was to
be expected. Considering how strong must be the inherited tendency
of a plant to assume those special characters, physiological as well
as morphological, which have resulted from an enormous accumulation
of antecedent actions, it may be even thought surprising that this
tendency can be counteracted to so great an extent by changed
conditions. Such a degree of modifiability becomes comprehensible only
when we remember how little a plant’s functions are integrated, and how
much, therefore, the functions going on in each part may be altered
without having to overcome the momentum of the functions throughout
the whole plant. But this modifiability being as great as it is, we
can have no difficulty in understanding how, by the cumulative aid
of natural selection, this primary differentiation of the surface in
plants has become what we see it.


§ 273. We will leave now these contrasts between the free surfaces of
plants and their attached or imbedded surfaces, and turn our attention
to the secondary contrasts existing between different parts of their
free surfaces. Were a full statement of the evidence practicable,
it would be proper here to dwell on that which is furnished by the
inferior classes. It might be pointed out in detail that where, as
among the _Algæ_, the free surfaces are not dissimilarly conditioned,
there is no systematic differentiation of them--that the frond of
an _Ulva_, the ribbon-shaped divisions of a _Laminaria_, and the
dichotomous expansions of the _Fuci_ which clothe the rocks between
tide-marks, are alike on both sides; because, swayed about in all
directions as they are by the waves and tides, their sides are equally
affected. Conversely, from the _Fungi_ might be drawn abundant proof
that even among Thallophytes, unlikenesses arise between different
parts of the free surfaces when their circumstances are unlike.
In such laterally-growing kinds as are shown in Fig. 196_b_, the
honey-combed under surface and the smooth leathery upper surface,
have their contrasts related to contrasted conditions; and in the
adjacently-figured Agarics, and other stalked genera, the pileus
exhibits a parallel difference, explicable in a parallel way. But
passing over Cryptogams it must suffice if we examine more at length
these traits as they are displayed by Phænogams. Let us first note the
dissimilarities between the outer tissues of stems and leaves.

That these dissimilarities arose by degrees, as fast as the units
of which the phænogamic axis is composed became integrated, is a
conclusion in harmony with the truth that in every shoot of every
plant, they are at first slight and become gradually marked. Already,
in briefly tracing the contrasts between the outer and inner tissues
of plants, some facts have been named showing, by implication, how
the cessation of the leaf-function in axes is due to that change of
conditions entailed by the discharge of other functions. Here we have
to consider more closely facts of this class, together with others
immediately to the point. On pulling off from a stem of grass the
successive sheaths of its leaves, the more-inclosed parts of which are
of a fainter green than the outer parts, it will be found that the
tubular axis eventually reached is of a still fainter green; but when
the axis eventually shoots up into a flowering stem, its exposed part
acquires as bright a green as the leaves. In other Monocotyledons, the
leaf-sheaths of which are successively burst and exfoliated by the
swelling axis, it may be observed that where the dead sheaths do not
much obstruct the light and air, the surface of the axis underneath
is full of chlorophyll. _Dendrobium_ is an example. But when the dead
sheaths accumulate into an opaque envelope, the chlorophyll is absent,
and also, we may infer, the function which its presence habitually
implies. Carrying with us this evidence, we shall recognize a like
relation in Dicotyledons. While its outer layer remains tolerably
transparent, an exogenous stem or branch continues to show, by the
formation of chlorophyll, that it shares in the duties of the leaves;
but in proportion as a bark which the light cannot penetrate is
produced by the adherent flakes of dead skin, or by the actual deposit
of a protective substance, the differentiation of duties becomes
more decided. Cactuses and Euphorbias supply us with converse facts
having the same implication. The succulent axes so strangely combined
in these plants, maintain for a long time the translucency of their
outer layers and their greenness; and they so efficiently perform the
offices of leaves that leaves are not produced. In some cases, axes
that are not succulent participate largely in the leaf-function, or
entirely usurp it--still, however, by fulfilling the same essential
conditions. Occasionally, as in _Statice brassicæfolia_, stems become
fringed; and the fringes they bear assume, along with the thinness of
leaves, their darker green and general aspect. In the genus _Ruscus_,
the flattened axis simulates so closely the leaf-structure, that were
it not for the flower borne on its mid-rib, or edge, its axial nature
would hardly be suspected. And let us not omit to note that where axes
usurp the characters of leaves, in their attitudes as well as in their
shapes and thicknesses, there are contrasts between their under and
upper surfaces, answering to the contrasts between the relations of
these surfaces to the light. Of this _Ruscus androgynus_ furnishes a
striking example. In it the difference which the unaided eye perceives
is much less conspicuous than that disclosed by the microscope; for
I find that while the face of the pseudo-leaf has no stomata, the
back is abundantly supplied with them. One more illustration must
be added. Equally for the morphological and physiological truths
which it enforces, the _Mühlenbeckia platyclada_ is one of the most
instructive of plants. In it the simulation of forms and usurpation
of functions, are carried out in a much more marvellous way than
among the _Cactaceæ_. Imagine a growth resembling in outline a very
long willow-leaf, but without a mid-rib, and having its two surfaces
alike. Imagine that across this thin, green, semi-transparent
structure, there are from ten to thirty divisions, which prove to be
the successive nodes of an axis. Imagine that along the edges of this
leaf-shaped aggregate of internodes, there arise axillary buds, some
of which unfold into flowers, and others of which shoot up vertically
into growths like the one which bears them. Imagine a whole plant thus
seemingly composed of jointed willow-leaves growing from one another’s
edges, and some conception will be formed of the _Mühlenbeckia_.
The two facts which have meaning for us here are--first, that the
performance of leaf-functions by these axes goes along with the
assumption of a leaf-like translucency; and, second, that these
flattened axes, retaining their upright attitudes, and therefore
keeping their two faces similarly conditioned, have these two faces
alike in colour and texture.

That physiological differentiation of the surface which arises in
Phænogams between axial organs and foliar organs, is thus traceable
with tolerable clearness to those differences between their conditions
which integration has entailed--partly in the way above described and
partly in other ways still to be named. By its relative position, as
being shaded by the leaves, the axis is less-favourably circumstanced
for performing those assimilative actions effected by the aid of light.
Further, that relatively-small ratio of surface to mass in the axis,
which is necessitated by its functions as a support and a channel
for circulation, prevents it from taking in, with the same facility
as the leaves, those surrounding gases from which matter is to be
assimilated. Both these special causes, however, in common with that
previously assigned, fall within the general cause. And in the fact
that where the differential conditions do not exist, the physiological
differentiation does not arise, or is obliterated, we have clear proof
that it is determined by unlikenesses in the relations of the parts to
the environment.


§ 274. From this most general contrast between aërial
surface-tissues--those of axes and those of folia--we pass now to the
more special contrasts of like kind existing in folia themselves.
Leaves present us with superficial differentiations of structure and
function; and we have to consider the relations between these and the
environing forces.

Over the whole surface of every phænogamic leaf, as over the fronds
of the _Pteridophyta_, there extends a simple or compound epidermal
layer, formed of cells that are closely united at their edges and
devoid (in the Flowering Plants) of that granular colouring matter
(chlorophyll) contained in the layers of cells they inclose: the result
being that the membrane formed of them is comparatively transparent. On
the submerged leaves of aquatic Phænogams, this outer layer is thin,
delicate, and permeable by water; but on leaves exposed to the air, and
especially on their upper surfaces, is comparatively strong, dense,
often smooth and impermeable by water: being thus fitted to prevent the
rapid escape of the contained juices by evaporation. Similarly, while
the leaves of terrestrial plants which live in temperate climates,
usually have comparatively thin coats thus composed, in climates that
are both hot and dry, leaves are commonly clothed with a very thick
cuticle. Nor is this all. The outside of an aërial leaf differs from
that of a submerged leaf by containing a deposit of waxy substance.
Whether this be exuded by the exposed surfaces of the cells, as some
contend, or whether it is deposited within the cells, as thought by
others, matters not in so far as the general result is concerned. In
either case a waterproof coating is formed at the outermost sides of
these outermost cells; and in many cases produces that polish by which
the upper surface of the leaf is more or less distinguished from the
under surface. This external pellicle presents us with another contrast
of allied meaning. On the upper surfaces of leaves subject to the
direct action of the sun’s rays, there are either few or none of those
minute openings, or stomata, through which gases can enter or escape;
but on the under surfaces these stomata are abundant: a distribution
which, while permitting free absorption of the needful carbonic acid,
puts a check on the exit of watery vapour. Two general exceptions to
this arrangement may be noted. Leaves that float on the water have all
their stomata on their upper sides, and leaves that are submerged have
no stomata--modifications obviously appropriate to the conditions.
What is to be said respecting the genesis of these differentiations?
For the last there seems no direct cause: its cause must be indirect.
The unlike actions to which the upper and under surfaces of leaves are
subject, have no apparent tendency to produce unlikeness in the number
of their breathing holes. Here the natural selection of spontaneous
variations furnishes the only feasible explanation. For the first,
however, there is a possible cause in the immediate actions of incident
forces, which survival of the fittest continually furthers.

The fluid exhaling through the walls of the cells next the air, will be
likely to leave behind suspended substances on their outer surfaces.
On remembering the pellicle which is apt to form on thick solutions or
emulsions as they dry, and how this pellicle as it grows retards the
further drying, it will be perceived that the deposit of waxy matter
next to the outer surfaces of the cuticular cells in leaves, is not
improbably initiated by the evaporation which it eventually checks.
Should it be so, there results a very simple case of equilibration.
Where the loss of water is too great, this waxy pellicle left behind by
the escaping water will protect most those individuals of the species
in which it is thickest or densest; and by inheritance and continual
survival of the fittest, there will be established in the species that
thickness of the layer which brings the evaporation to a balance with
the supply of water.

Another superficial differentiation, still more familiar, has to be
noted. Every child soon learns to distinguish by its colour the upper
side of a leaf from its under side, if the leaf is one that has grown
in such way as to establish the relations of upper and under. The upper
surfaces of leaves are habitually of a deeper green than the under.
Microscopic examination shows that this deeper green results from the
closer clustering of those parenchyma-cells full of chlorophyll that
are in some way concerned with the assimilative actions; while beneath
them are more numerous intercellular passages communicating with those
openings or stomata through which is absorbed the needful air. Now when
it is remembered that the formation of chlorophyll is clearly traceable
to the action of light--when it is remembered that leaves are pale
where they are much shaded and colourless when developed in the dark,
as in the heart of a Cabbage--when it is remembered that succulent axes
and petioles, like those of Sea-kale and Celery, remain white while
the light is kept from them and become green when exposed; it cannot
be questioned that this greater production of chlorophyll next to the
upper surface of a leaf, is directly consequent on the greater amount
of light received. Here, as in so many other cases, we must regard
the differentiation as in part due to direct equilibration and in
part to indirect equilibration. Familiar facts compel us to conclude
that from the beginning, each individual foliar organ has undergone
a certain immediate adaptation of its surfaces to the incidence of
light; that when there arose a mode of growth which exposed the leaves
of successive generations in similar ways, this immediately-produced
adaptation, ever tending to be transmitted, was furthered by the
survival of individuals inheriting it in the greatest degree; and that
so there was gradually established that difference between the two
surfaces which each leaf displays before it unfolds to the light, but
which becomes more marked when it has unfolded.[47]

From the ordinary cases let us now pass to the exceptional cases. We
will look first at those in which the two faces of the leaves differ
but little, or not at all--their circumstances being similar or equal.
Leaves that grow in approximately-upright attitudes, and attitudes
which do not maintain the relative positions of the two surfaces with
constancy, may be expected to display an unusual likeness between the
two surfaces; and among them we see it. The Grasses may be named as a
group exemplifying this relation; and if, instead of comparing them as
a group with other groups, we compare those dwarf kinds of them which
spread out their leaves horizontally, with the large aspiring kinds,
such as _Arundo_, we trace a like antithesis: in the one the contrast
of upper and under is very obvious, while in the other it is scarcely
to be detected. Leaves of various other Monocotyledons that grow in a
similar way, similarly show us a near approach to uniformity of the
two surfaces; as instance the genus _Clivia_ and the thinner-leaved
kinds of _Yucca_. Where the contrast of upper and under is greatly
diminished by the assumption of a rounded or cylindrical form, instead
of a flattened form, the same thing happens. The genus _Kleinia_
furnishes illustrations. It may be remarked, too, that even within the
limits of this genus there are instructive variations; for while in
_Kleinia ficoides_ the leaves, shaped like pea-pods, are broadest in a
vertical direction, and have their lateral surfaces alike in conditions
and structure, in other species the leaves, broader horizontally than
vertically, exhibit unlikeness between the upper and under sides.
Equally to the point is the evidence furnished by vertically-growing
leaves that are cylindrical, as those of _Sanseviera cylindrica_, or as
those of the Rush-tribe: the similarly-placed surface has all around
a similar character. Of kindred meaning, and still more conclusive,
are the cases in which the under side of the leaf, being more exposed
to light than the upper side, usurps the character and function of
the upper side. If a common Flag be pulled to pieces, it will be seen
that what answers to the face in other leaves, forms merely the inside
of the sheath including the younger leaves, and is obliterated higher
up. The two surfaces of the blade answer to the two under halves of a
leaf that has been, as it were, folded together lengthways, with the
two halves of its upper surface in contact. And here, in default of an
upper surface, the under surface acquires its character and discharges
its function. A like substitution occurs in _Aristea corymbosa_; and
there are some of the Orchids, as _Lockhartia_, which display it in a
very obvious way.

When joined with the foregoing evidence, the evidence which another
kind of substitution supplies is of great weight. I refer to that which
occurs in the Australian Acacias, already instanced as throwing light
on morphological changes. In these plants the leaves properly so-called
are undeveloped, and the foot-stalks, flattened out into foliaceous
shapes, acquire veins and mid-ribs, and so far simulate leaves as
ordinarily to be taken for them: a fact in itself of much physiological
significance. But that which it concerns us especially to note, is
the absence of distinction between the two faces of these phyllodes,
as they are named, and the cause of its absence. These transformed
petioles do not flatten themselves out horizontally, so as to acquire
under and upper sides, as most true leaves do; but they flatten
themselves out vertically: the result being that their two sides are
similarly circumstanced with respect to light and other agencies; and
there is consequently nothing to cause their differentiation. And then
we find an analogous case where differential conditions arise, and
where some differentiation results. In _Oxalis bupleurifolia_, Fig. 66,
there is a similar flattening out of the petiole into a pseudo-leaf;
but in it the flattening takes place in the same plane as the leaf, so
as to produce an under and an upper surface; and here the two surfaces
of the pseudo-leaf are slightly unlike--in contour if in nothing else.


§ 275. We now come to such physiological differentiations among the
outer tissues of plants, as are displayed in the contrasts between
foliar organs on the same axis, or on different axes--contrasts between
the seed-leaves and the leaves subsequently formed, between submerged
and aërial leaves in certain aquatic plants, between leaves and bracts,
and between bracts and sepals. To deal even briefly with these implies
information which even a professed botanist would have to increase by
special inquiries, before attempting interpretations. Here it must
suffice to say something respecting those marked unlikenesses existing
between the tissues of the more characteristic parts of flowers, and
the tissues of the homologous foliar organs.

It was pointed out in § 196, that the terminal folia of a phænogamic
axis have sundry characters in common with such fronds as those
out of which we concluded that the phænogamic axis has arisen by
integration--common characters of a kind to be expected. In their
simple cellular composition, comparative want of chlorophyll, and
deficiency of vascular structures, the undeveloped ends of leaf-shoots
and the developed ends of flower-shoots, approach to the fronds of the
simpler Archegoniates. We also noted between them another resemblance.
It is said of the _Jungermanniaceæ_, that “though under certain
circumstances of a pure green, they are inclined to be shaded with
red, purple, chocolate, or other tints;” and answering to this we have
the facts that such colours commonly occur in the terminal folia of a
phænogamic axis, when arrest of its development leads to the formation
of a flower, and that very frequently they are visible at the ends
of leaf-axes. In the unfolding parts of shoots, more or less of red,
or copper-colour, or chocolate-colour, may generally be seen: often,
indeed, it characterizes the leaves for some time after they are
unfolded. Occasionally the traces of it are permanent; and, as in the
scarlet terminal leaves of _Poinsettia pulcherrima_, we see that it may
become, and continue, extremely conspicuous. The question, then, now
to be asked is--has this colouring by which the immature part of the
phænogamic axis is characterized, anything to do with the colouring of
flowers? Has this difference between undeveloped folia and folia that
are further developed, been increased by natural selection where an
advantage accrued from it, until it has ended in the strong contrast we
now see? I think we may not irrationally infer that this has happened.

Facts, very numerous and varied, united to warrant us in concluding
that gamogenesis commences where the forces which conduce to growth are
nearly equilibrated by the forces which resist growth (§ 78); and the
induction that in plants, fertilized germs are produced at places where
there is an approach towards this balance, we found to be in harmony
with the deduction that an advantage to the species must be gained by
sending off migrating progeny from points where nutrition is failing.
Other things equal, failure of nutrition may be expected in parts which
have the most remote or most indirect access to the materials furnished
by the roots--materials which have to be carried great distances by
a very imperfect apparatus. The ends of lateral axes are therefore
the probable points of fructification, in aggregates of the third
order that have taken to growing vertically. But if these points at
which nutrition is failing, are also the points at which the colours
inherited from lower types are likely to recur in more marked degrees
than elsewhere; then we may infer that the organs of fructification
will not unfrequently co-exist with such colours at the ends of such
axes. How may the resulting contrast between the older fronds and the
fronds next the germ-producing organs be increased? If uninterfered
with it would be likely to diminish. These traits inherited from remote
ancestry might be expected slowly to fade away. How, then, is the
intensification of them to be explained?

If a contrast of the kind described favours the propagation of a race
in which it exists, it will be maintained and increased; and if we
take into account an agency of which Mr. Darwin has shown the great
importance--the agency of insects--we shall have little difficulty
in understanding how such a contrast may facilitate propagation. We
cannot, of course, here assume the agency of insects so specialized
in their habits as Bees and Butterflies; for their specialized habits
imply the pre-existence of the contrast to be explained. But there is
an insect-agency of a more general kind which may be fairly counted
upon as coming into action. Various small Flies and Beetles wander
over the surfaces of plants in search of food. It is a legitimate
assumption that they will frequent most those parts in which they find
most food, or food most to their liking--especially if at the same time
they gain the advantage of concealment. Now the ends of axes, formed
of young, soft, and closely-packed folia, are the parts which more
than any others offer these several advantages. They afford shelter
from enemies; they frequently contain exuded juices; and when they do
not, their tissues are so tender as to be easily pierced in search
of the sap. If, then, from the first, as at present, these ends of
axes have been favourite haunts of small insects; and if, where the
closely-clustered folia contained the generative organs, the insects
frequenting them occasionally carried adherent fructifying cells from
one plant to another, and so aided fertilization; it would follow
that anything which made such terminal clusters more attractive to
such insects, or more conspicuous to them, or both, would further the
multiplication of the race, and would so be continually increased by
the extra multiplication of individuals in which it was greatest.
Here we find the clue. This contrast of colour between the folia next
to the fructifying parts and all other folia, must constantly have
facilitated insect-agency; supposing the insects to have had the power
of distinguishing between colours. That Bees and Butterflies have this
power is manifest. They may be watched flying from flower to flower,
disregarding all other parts of the plants. And if the less-specialized
insects possessed some degree of such discrimination, then the initial
contrasts of colour above described would be maintained and increased.
Let such a connexion be once established, and it must tend to become
more decided. Insects most able to discern the parts of plants which
afford what they seek, will be those most likely to survive and leave
offspring. Plants presenting most of the desired food, and showing most
clearly where it lies, will have their fertilization and multiplication
furthered in the greatest degree. And so the mutual adaptation will
become ever closer; while it is rendered at the same time more varied
by the special requirements of the insects and of the plants in each
locality, under each change of conditions. Of course, the genesis
of the sweet secretions and the odours of flowers, has a parallel
interpretation. The simultaneous production of honey, or some kindred
substance, is implied above; since, unless a bait co-existed with
the colour, the colour would not attract insects, and would not be
maintained and intensified by natural selection. Gums, and resins, and
balsams, are familiar products of plants; apparently, in many cases,
excreted as useless matters from various parts of their surfaces. These
substances, admitting of wide variations in quality, as they do, afford
opportunities for the action of natural selection wherever any of
them, attractive to insects, happen to be produced near the organs of
fructification. And this action of natural selection once set up, may
lead to the establishment of a local excretion, to the production of an
excretion more and more attractive, and to the disposal of the organ
containing it in such a way as most to facilitate the carrying away of
pollen. Similarly and simultaneously with odours. Odours, like colours,
draw insects to flowers. After observing how Bees come swarming into a
house where honey is largely exposed, or how Wasps find their way into
a shop containing much ripe fruit, it cannot be questioned that insects
are to a considerable extent guided by scent. Being thus sensitive
to the aromatic substances which flowers exhale, they may, when the
flowers are in large masses, be attracted by them from distances
at which the flowers themselves are invisible. And manifestly, the
flowers which so attract them from the greatest distances, increasing
thereby their chances of efficient fertilization, will be most likely
to perpetuate themselves. That is to say, survival of the fittest
must tend to produce perfumes that are both more powerful and more
attractive.

These physiological differentiations, then, which mark off the
foliar organs constituting flowers from other foliar organs, are
the consequences of indirect equilibration. They are not due to the
immediate actions of unlike incident forces on the parts of the
individual plant; but they are due to the actions of such unlike
incident forces on the aggregate of individuals, generation after
generation.[48]


§ 276. The unity of interpretation which we here find for phenomena
of such various orders, could hardly be found were the phenomena
otherwise caused. That the stronger and the feebler contrasts among the
different parts of the outer tissues in plants, should so constantly
occur along with stronger and feebler contrasts among the incident
forces, is in itself weighty evidence that unlike outer actions have
caused unlike inner actions, and correspondingly-unlike structures;
either by changing the functional equilibrium in the individual, or by
changing it in the race, or by both.

Even in the absence of more direct proof, there would be great
significance in the marked differences that habitually exist between
the exposed and imbedded parts of plants, between the stems and the
leaves, and between the upper and under surfaces of the leaves. The
significance of these differences is increased when we discover that
they vary in degree as the differences in the conditions vary in
degree. Still greater becomes the force of the evidence on finding
that these strongly-contrasted parts may, when placed in one another’s
conditions, and kept in them from generation to generation, permanently
assume one another’s functions, and, in a great degree, one another’s
structures. Even more conclusive yet is the argument rendered, by the
discovery that, where these substitutions of function and structure
take place, the superinduced modifications differ in different
circumstances; just as the original modifications do. The fact that
a flattened stem simulating a vertically-growing leaf has its two
surfaces alike, while when it simulates a horizontally-growing leaf its
upper and under surfaces differ, is a fact which, standing alone, might
prove little, but proves much when joined with all the other evidence.
And its profound meaning becomes the more obvious on discovering that
the same thing happens with petioles when they usurp leaf-functions.

Finally, when we remember how rapidly analogous modifications of
function and structure arise in the superficial tissues of individual
plants, the general inference can scarcely be resisted. When we meet
with so striking a case as that of the _Begonia_-leaf, a fragment
of which stuck in the ground produces roots from its under surface
and leaves from its upper surface--when we see that though, in this
case, the typical structure of the plant presently begins to control
the organizing process, yet the initial differentiations are set
up by the differential actions of the environment; the presumption
becomes extremely strong that the heterogeneities of surface which
we have considered, result, as alleged, directly or indirectly from
heterogeneities in the incident forces.




                              CHAPTER IV.

        DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS.[49]


§ 277. In passing from plants formed of threads or thin laminæ, to
plants having some massiveness, we find that after the external and
internal parts have become distinguished from one another, there arise
distinctions among the internal parts themselves, as well as among the
external parts themselves: the primarily-differentiated parts are both
re-differentiated.

From types of very low organisation illustrations of this may be
drawn. In the thinner kinds of _Laminaria_ there exists but the
single contrast between the outer layer of cells and an inner layer;
but in larger species of the same genus, as _L. digitata_, there
are three unlike layers on each side of a central layer differing
from them--augmentation of bulk is accompanied by multiplication of
concentric internal structures, having their unlikenesses obviously
related to unlikenesses in their conditions. In _Furcellaria_ and
various _Algæ_ of similarly swollen forms, the like relation may be
traced.

Just indicating the generality of this contrast, but not attempting to
seek in these lower types for any more specific interpretation of it,
let us pass to the higher types. The argument will be amply enforced by
the evidence obtained from them. We will look first at the conditions
which they have to fulfil; and then at the ways in which the functions
and structures adapting them to these conditions arise.


§ 278. A terrestrial plant that grows vertically needs no marked
modification of its internal tissues, so long as the height it reaches
is very small. As we before saw, the spiral or cylindrical rolling
up of a simple cellular frond, or the more bulky growth of a simple
cellular axis, may give the requisite strength; and the requisite
circulation may be carried on through the unchanged cellular tissue.
But in proportion as the height to be attained and the mass to be
supported increase, the supporting part must acquire greater bulk or
greater density, or both; and some modification that shall facilitate
the transfer of nutritive liquids must take place. Hence, in the
inner tissues of plants we may expect to find that structural changes
answering to these requirements become marked, as the growth of the
aërial part becomes great. Facts correspond with these expectations.

Among the humbler Cormophytes, which creep over or raise themselves
but little above, the surfaces they flourish upon, there is scarcely
any internal differentiation: the vascular and woody structures, if
not in all cases absolutely unrepresented, are rarely and very feebly
indicated. But among the higher types--the Ferns and Lycopodiums--which
raise their fronds to considerable heights, there are vascular bundles
and hard tissues like wood; and by the Tree-Ferns massive axes are
developed. That the relation which thus shows itself among Cryptogams
is habitual among Phænogams, scarcely needs saying.

Phænogams, however, are not universally thus characterized in a decided
way. Besides the comparative want of woody tissue in flowering plants
of humble growth, and besides the paucity of vessels in ordinary
water-plants, there are cases of much more marked divergence from
this typical internal structure. These exceptional cases occur under
exceptional conditions, and are highly instructive. They are of two
kinds. One group of them is furnished by certain plants which are
parasitic on the exposed roots of trees--parasitic not partially,
as the Mistletoe, but to the extent of subsisting wholly on the sap
they absorb. Fungus-like in colour and texture, and having scales for
leaves, these _Balanophoræ_ and _Rafflesiaceæ_ are recognizable as
Phænogams by scarcely any other traits than their fructifications.
Along with their aborted leaves and absence of chlorophyll, there is
a great degradation of those internal tissues by which Phænogams are
commonly distinguished. Though Dr. [now Sir J.] Hooker has shown that
they are not, as some botanists thought, devoid of spiral vessels; yet,
as shown by the mistake previously made in classifying them, their
appliances for circulation are rudimentary. And this trait goes along
with a greatly-simplified distribution of nutriment. In the absence
of leaves there can be but little down-current of sap, such as leaves
usually supply to roots: there cannot be much beyond an upward current
of the absorbed juices. The other cases occur where circulation is
arrested or checked in a different way; namely, in plants that are
wholly submerged. These are the _Podostemaceæ_. Clothing as they do
the submerged rocks, their roots play the part of rhizomes, being
attached to the substratum by hairs and other processes, and having
the leaf-bearing and flower-bearing shoots on their surfaces. The
latter spread out more or less horizontally and are also fixed to the
substratum in the same manner as the roots. Observe then the connexion
of facts. One of these _Podostemaceæ_ needs no internal stiffening
substance, for it exists in a medium of its own specific gravity; and
being in a position to absorb water over its entire surface, it has no
need for a circulation of crude sap--nor, indeed, in the absence of
evaporation from any part of its surface, could any active circulation
take place. Here, accordingly, the tracheal and mechanical elements are
undeveloped. Though spiral vessels are not entirely absent, yet they
are so rare as to do no more than verify the inference of phænogamic
relationship drawn from the flowers.

The method of agreement, the method of difference, and the method of
concomitant variations, thus unite in proving a direct relation between
the demand for support and circulation, and the existence of these
vascular woody bundles which the higher plants habitually possess.
The question which we have to consider is--Under what influences are
these structures, answering to these requirements, developed? How are
these internal differentiations caused? The inquiry may be conveniently
divided. Though the supporting tissues and the tissues concerned in the
circulation of liquids are closely connected, and indeed entangled,
with one another, we may fitly deal with them apart. Let us take first
the supporting tissue.


§ 279. Many commonplace facts indicate that the mechanical strains to
which upright-growing plants are exposed, themselves cause increase
of the dense deposits by which such plants are enabled to resist
such strains. There is the fact that the massiveness of a tree-trunk
varies according to the stress habitually put upon it. If the contrast
between the slender stem of a tree growing in a wood and the bulky stem
of a kindred tree growing in the fields, be ascribed to difference
of nutrition rather than difference of exposure to winds; there is
still the fact that a tree trained against a wall has a less bulky
stem than a tree of the same kind growing unsupported; and that
between the long weak branches of the one and the stiff ones of the
other there are decided contrasts. If it be objected that a tree so
trained and branches so borne have relatively less foliage, and that
therefore these unlikenesses also are due to unlikenesses of general
nutrition, which may in part be true; there are still such cases as
those of garden plants, which when held up by tying them to sticks
have weaker stems than when they are unpropped, and sink down if their
props are taken away. Again, there is the evidence supplied by roots.
Though the contrast between the feeble roots of a sheltered tree
and the strong roots of an exposed tree, may, like the contrast of
their stems, be mainly due to difference of nutrition, and therefore
supplies but doubtful evidence, we get tolerably clear evidence where
trees growing on inclined rocky surfaces, send into crevices that
afford little moisture or nutriment, roots which nevertheless become
thick where they are so directed as to bear great strains. Suspicion
thus raised is strengthened into conviction by special evidences
occurring in the places where they are to be expected. The Cactuses,
with their succulent growths that pass into woody growths slowly
and irregularly, give us the opportunity of tracing the conditions
under which the wood is formed. Good examples occur in the genus
_Cereus_, and especially in forms like _C. crenulatus_. Here,
from a massive vertically-growing rod of fleshy tissue, two inches
or more in diameter, there grow at intervals lateral rods similarly
bulky, which, quickly curving themselves, take vertical directions.
One of these heavy branches puts great strains on its own substance
and that of the stem at their point of junction; and here both of them
become brown and hard, while they continue green and succulent all
around. Such differentiations may be traced internally before they
are visible on the surface. If a joint of an _Opuntia_ be sliced
through longitudinally, the greater resistance to the knife all around
the narrow neck, indicates there a larger deposit of lignin than
elsewhere; and a section of the tissue placed under the microscope,
exhibits at the narrowest part a concentration of the woody and
vascular bundles. Clear evidence of another kind has been noted by
Mr. Darwin, in the organs of attachment of climbing plants. Speaking
of _Solanum jasminoides_ he says:--“When the flexible petiole
of a half-or a quarter-grown leaf has clasped any object, in three
or four days it increases much in thickness, and after several weeks
becomes wonderfully hard and rigid; so that I could hardly remove one
from its support. On comparing a thin transverse slice of this petiole
with one from the next or older leaf beneath, which had not clasped
anything, its diameter was found to be fully doubled, and its structure
greatly changed.... This clasped petiole had actually become thicker
than the stem close beneath; and this was chiefly due to the greater
thickness of the ring of wood, which presented, both in transverse
and longitudinal sections, a closely similar structure in the petiole
and axis. The assumption by a petiole of this structure is a singular
morphological fact; but it is a still more singular physiological fact
that so great a change should have been induced by the mere act of
clasping a support.”

If there is a direct relation between mechanical stress and the
formation of wood, it ought to explain for us the internal distribution
of the wood. Let us see whether it does this.

When seeking in mechanical actions and reactions the cause of that
indurated structure which forms the vertebrate axis (§§ 254–7), it
was pointed out that in a transversely-strained mass, the greatest
pressures and tensions are thrown on the molecules of the concave and
convex surfaces. Hence, supposing the transversely-strained mass to
be a cylinder, bent backwards and forwards not in one plane but now
in this plane and now in that, its peripheral layers will be those on
which the greatest stress falls. An ordinary dicotyledonous axis is
such a cylinder so strained. The maintenance of its attitude either as
a lateral shoot or a vertical shoot, implies subjection to the bendings
caused by its own weight and by the ever-varying wind. These bendings
imply tensions and pressures falling most severely first on one side of
its outer layers and then on another. And if the dense substance able
to resist these tensions and pressures is deposited most where they are
greatest, we ought to find it taking the shape of a cylindrical casing.
This is just what we do find. On cutting across a shoot in course of
formation, we see its central space either unoccupied or occupied only
by soft tissue. That the layer of hard tissue surrounding this is not
the outermost layer, is true: there lies beyond it the cambium layer,
from which it is formed, the phloëm, and the cortex. But outside of the
soft phloëm there is frequently another layer of dense tissue now known
as the pericyclic fibres, having frequently a tenacity greater even
than that of the wood--a layer which, while it protects the cambium
and offers additional resistance to the transverse strain, admits of
being fissured as fast as the cylinder of wood thickens. That is to
say, the deposit of resisting substance is as completely peripheral as
the exogenous mode of growth permits. So, too, in general arrangement
is it with the ordinary monocotyledonous stem. Different as is here
the internal structure, there yet holds the same general distribution
of tissues, answering to the same mechanical conditions. The vascular
woody bundles, more abundant towards the outside of the stem than
near the centre, produce a harder casing surrounding a softer core.
In the supporting structures of leaves we find significant deviations
from this arrangement. While axes are on the average exposed to equal
strains on all sides, most leaves, spreading out their surfaces
horizontally, have their petioles subject to strains that are not alike
in all directions; and in them the hard tissue is differently arranged.
Its transverse section is not ring-shaped but crescent-shaped: the two
horns being directed towards the upper surface of the petiole. That
this arrangement is one which answers to the mechanical conditions,
is not easy to demonstrate: we must satisfy ourselves by noting that
here, where the distribution of forces is different, the distribution
of resisting tissue is different. And then, showing conclusively the
connexion between these differences, we have the fact that in petioles
growing vertically and supporting peltate leaves--petioles which are
therefore subject to equal transverse strains on all sides--the
vascular bundles are arranged cylindrically, as in axes.

Such, then, are some of the reasons for concluding that the development
of the supporting tissue in plants, is caused by the incident forces
which this tissue has to resist. The individuals in which this direct
balancing of inner and outer actions progresses most favourably, are
those which, other things equal, are most likely to prosper; and, by
habitual survival of the fittest, there is established a systematic and
constant distribution of a deposit adapted to the circumstances of each
type.


§ 280. The function of circulation may now be dealt with. We have to
consider here by what structures this is discharged; and what connexion
exists between the demand for them and the genesis of them.

The contrast between the rates at which a dye passes through simple
cellular tissue and cellular tissue of which the units have been
elongated, indicates one of the structural changes required to
facilitate circulation. If placed with its cut surface in a coloured
liquid, the parenchyma of a potato or the medullary mass of a
cabbage-stalk, will absorb the liquid with extreme slowness; but if
the stalk of a fungus be similarly placed, the liquid runs up it, and
especially up its loose central substance, very quickly. On comparing
the tissues which thus behave so differently, we find that whereas in
the one case the component cells, packed close together, have deviated
from their primitive sphericity only as much as mutual pressure
necessitates, in the other case they are drawn out into long tubules
with narrow spaces among them--the greatest dimensions of the tubules
and the spaces being in the direction which the dye takes so rapidly.
That which we should infer, then, from the laws of capillary action,
is experimentally shown: liquid moving through tissues follows the
lines in which the elements of the tissues are most elongated. It does
this for two reasons. That narrowing of the cells and intercellular
spaces which accompanies their elongation, facilitates capillarity;
and at the same time fewer of the septa formed by the joined ends of
the cells have to be passed through in a given distance. Hence the
general fact that the establishment of a rudimentary vascular system,
is the formation of bundles of cells lengthened in the direction
which the liquid is to take. This we see very obviously among the
lower Cormophytes. In one of the lichen-like Liverworts, the veins
which, branching through its frond, serve as communications with its
scattered rootlets, are formed of cells longer than those composing the
general tissue of the frond: the lengths of these cells corresponding
in their directions with the lengths of the veins. So, too, is it
with the mid-ribs of such fronds as assume more definite shapes; and
so, too, is it with the creeping stems which unite many such fronds.
That is to say, the current which sets towards the growing part from
the part which supplies certain materials for growth, sets through
a portion of the tissues composed of units that are longer in the
line of the current than at right angles to that line. The like is
true of Phænogams. Omitting all other characteristics of those parts
of them through which chiefly the currents of sap flow, we find the
uniform fact to be that they consist of cells and intercellular spaces
distinguished from others by their lengths. It is thus with veins,
and mid-ribs, and petioles; and if we wish proof that it is thus with
stems, we have but to observe the course taken by a coloured solution
into which a stem is inserted.

What is the original cause of this differentiation? Is it possible
that this modification of cell-structure which favours the transfer of
liquid towards each place of demand, is itself caused by the current
which the demand sets up? Does the stream make its own channel? There
are various reasons for thinking that it does. In the first place,
the simplest and earliest channels, such as we see in the Liverworts,
do not develop in any systematic way, but branch out irregularly,
following everywhere the irregular lobes of the fronds as these spread;
and on examining under a magnifier the places at which the veins are
lost in the cellular tissue, it will be seen that the cells are there
slightly longer than those around: suggesting that the lengthening of
them which produces an extension of the veins, takes place as fast
as the growth of the tissue beyond causes a current to pass through
them. In the second place, a disappearance of the granular contents of
these cells accompanies their union into a vein--a result which the
transmission of a current may not improbably bring about. But be the
special causes of this differentiation what they may, the evidence
favours very much the conclusion that the general cause is the setting
up of a current towards a place where the sap is being consumed. In the
histological development of the higher plants we find confirmation.
The more finished distributing canals in Phænogams are formed of cells
previously lengthened. At parts of which the typical structure is
fixed, and the development direct, this fact is not easy to trace;
the cells rapidly take their elongated structures in anticipation of
their predetermined functions. But in places where new vessels are
required in adaptation to a modifying growth, we may clearly trace
this succession. The swelling root of a turnip, continually having its
vascular system further developed, and the component vessels lengthened
as well as multiplied, gives us an opportunity of watching the process.
In it we see that the reticulated cells which unite to form ducts,
arise in the midst of bundles of cells that have previously become
elongated, and that they arise by transformation of such elongated
cells; and we also see that these bundles of elongated cells have an
arrangement suggestive of their formation by passing currents.

Are there grounds for thinking that these further transformations by
which strings of elongated cells pass into vessels lined with spiral,
annular, reticulated, or other frameworks, are also in any way
determined by the currents of sap carried? There are some such grounds.

As just indicated, the only places where we may look for evidence
with any rational hope of finding it, are places where some local
requirement for vessels has arisen in consequence of some local
development which the type does not involve. In these cases we
find such evidence. Good illustrations occur in those genera of
the _Cactaceæ_, which simulate leaves, like _Epiphyllum_ and
_Phyllocactus_. A branch of one of these is outlined in Fig. 256. As
before explained this is a flattened axis; and the notches along its
edges are the seats of the axillary buds. Most of these axillary buds
are arrested; but occasionally one of them grows. Now if, taking an
_Epiphyllum_-shoot which bears a lateral shoot, we compare the parts
of it that are near the aborted axillary buds with the part that is
near the developed axillary bud, we find a conspicuous difference. In
the neighbourhood of an aborted axillary bud there is no external sign
of any internal differentiation; and on holding up the branch against
the light, the uniform translucency shows that there is no greater
amount of dense tissue near it than in other parts of the succulent
mass. But where an axillary bud has developed, a prominent rounded
ridge joins the mid-rib of the lateral branch with the mid-rib of the
parent branch. In the midst of this rounded ridge an opaque core may be
seen. And on cutting through it, this opaque core proves to be full of
vascular bundles imbedded in woody deposits. Clearly, these clusters
of vessels imply transformations of the tissues, caused by the passage
of increased currents of sap. The vessels were not there when the
axillary bud was formed; they would not have developed had the axillary
bud proved abortive; but they arise as fast as growth of the axillary
bud draws the sap along the lines in which they lie. Verification
is obtained by examining the internal structures. If longitudinal
sections be made through a growing bud of _Opuntia_ or _Cereus_, it
will be found that the vessels in course of formation converge towards
the point of growth, as they would do if the sap-currents determined
their formation; that they are most developed near their place of
convergence, which they would be if so produced; and that their
terminations in the tissue of the parent shoot are partially-formed
lines of irregular elongated cells, like those out of which the vessels
of a leaf or bud are developed.

Concluding, then, that sap-vessels arise along the lines of least
resistance, through which currents are drawn or forced, the question
to be asked is--What physical process produces them? Their component
cells, united end to end more or less irregularly in ways determined
by their original positions, form a channel much more permeable, both
longitudinally and laterally, than the tissue around. How is this
greater permeability caused? The idea, first propounded I believe by
Wolff, that the adjoined ends of the cells are perforated or destroyed
by the passing current, is one for which much is to be said. Whether
these septa are dissolved by the liquids they transmit, or whether they
are burst by those sudden gushes which, as we shall hereafter see, must
frequently take place along these canals, need not be discussed: it is
sufficient for us that the septa do, in many cases, disappear, leaving
internal ridges showing their positions; and, in other cases, become
extremely porous. Though it is manifest that this is not the process
of vascular development in tissues that unfold after pr-determined
types, since, in these, the dehiscences or perforations of septa occur
before such direct actions can have come into play; yet it is still
possible that the disappearances of septa which now arise by repetition
of the type were established in the type by such direct actions. Be
this as it may, however, a simultaneous change undergone by these
longitudinally-united cells must be otherwise caused. Frame-works are
formed in them--frameworks which, closely fitting their inner surfaces,
may consist either of successive rings, or continuous spiral threads,
or networks, or structures between spirals and networks, or networks
with openings so far diminished that the cells containing them are
distinguished as fenestrated. Their differences omitted, however, these
structures have the common character that, while supporting the coats
of the vessels, they also give special facilities for the passage of
liquids, both through the sides of the transformed cells and through
their united ends, where these are not destroyed.

To attempt any physical interpretation of this change is scarcely
safe: the conditions are so complex. There are reasons for suspecting,
however, that it arises from a vacuolation of the substance deposited
on the cell-wall. If rapidly deposited, as it is likely to be along
lines where sap is freely supplied, this may, in passing from the
state of a soluble colloid to that of an insoluble colloid, so
contract as to leave uncovered spaces on the cell-membrane; and this
change, originally consequent on a physico-chemical action, may be
so maintained and utilized by natural selection, as to result in
structures of definite kinds, regularly formed in growing parts in
anticipation of functions to be afterwards discharged. But, without
alleging any special cause for this metamorphosis, we may reasonably
conclude that it is in some way consequent upon the carrying of sap.
If we examine tissues such as that in the interior of a growing
turnip that has not yet become stringy, we may, in the first place,
find bundles of elongated cells not having yet developed in them
those fenestrated or reticulated structures by which the ducts are
eventually characterized. Along the centres of adjacent bundles we may
find incomplete lines of such cells--some that are partially or wholly
transformed, with some between them that are not transformed. In other
bundles, completed chains of such transformed cells are visible. And
then, in still older bundles, there are several complete chains running
side by side. All which facts imply a metamorphosis of the elongated
cells, indirectly caused by the continued action of the currents
carried.


§ 281. Here, however, presents itself a further problem. Taking
it as manifest that there is a typical distribution of supporting
tissue adapted to meet the mechanical strains a plant is exposed to
by its typical mode of growth, and also that there goes on special
adaptation of the supporting tissue to the special strains the
individual plant has to bear; and taking it as tolerably evident that
the sap-channels are originally determined by the passage of currents
along lines of least resistance; there still remains the ultimate
question--Through what physical actions are established these general
and special adjustments of supporting tissue to the strains borne,
and these distributions of nutritive liquid required to make possible
such adjustments? Clearly, if the external actions produce internal
reactions; and if this play of actions and reactions results in a
balancing of the strains by the resistances; we may rationally suspect
that the incident forces are directly conducive to the structural
changes by which they are met. Let us consider how they must work.

When any part of a plant is bent by the wind, the tissues on its convex
surface are subject to longitudinal tension, and these extended outer
layers compress the layers beneath them. Such of the vessels or canals
in these subjacent layers as contain sap, must have some of this sap
expelled. Part of it will be squeezed through the more or less porous
walls of the canals into the surrounding tissue, thus supplying it with
assimilable materials; while part of it, and probably the larger part,
will be thrust along the canals longitudinally upwards and downwards.
When the branch or twig or leaf-stalk recoils, these vessels, relieved
from pressure, expand to their original diameters. As they expand, the
sap rushes back into them from above and below. In whichever of these
directions least has been expelled by the compression, from that
direction most must return during the dilation; seeing that the force
which more efficiently resisted the thrusting back of the sap is the
same force which urges it into the expanded vessels again, when they
are relieved from pressure. At the next bend of the part a further
portion of sap will be squeezed out, and a further portion thrust
forwards along the vessels. This rude pumping process thus serves for
propelling the sap to heights which it could not reach by capillary
action, at the same time that it incidentally serves to feed the parts
in which it takes place. It strengthens them, too, just in proportion
to the stress to be borne; since the more severe and the more repeated
the strains, the greater must be the exudation of sap from the vessels
or ducts into the surrounding tissue, and the greater the thickening
of this tissue by secondary deposits. By this same action the movement
of the sap is determined either upwards or downwards, according to the
conditions. While the leaves are active and evaporation is going on
from them, these oscillations of the branches and petioles urge forward
the sap into them; because so long as the vessels of the leaves are
being emptied, the sap in the compressed vessels of the oscillating
parts will meet with less resistance in the direction of the leaves
than in the opposite direction. But when evaporation ceases at night,
this will no longer be the case. The sap drawn to the oscillating
parts, to supply the place of the exuded sap, must come from the
directions of least resistance. A slight breeze will bring it back from
the leaves into the gently-swaying twigs, a stronger breeze into the
bending branches, a gale into the strained stem and roots--roots in
which longitudinal tension produces, in another way, the same effects
that transverse tension does in the branches.

Two possible misinterpretations must be guarded against. It is not to
be supposed that this force-pump action causes movement of the sap
towards one point rather than another: it is simply an aid to its
movement. From the stock of sap distributed through the plant, more
or less is everywhere being abstracted--here by evaporation, here by
the unfolding of the parts into their typical shapes, here by both. The
result is a tension on the contained liquid columns, which is greatest
now in this direction and now in that. This tension it is which must be
regarded as the force that determines the current upwards or downwards;
and all which the mechanical actions do is to facilitate the transfer
to the places of greatest demand. Hence it happens that in a plant
prevented from oscillating, but having a typical tendency to assume
a certain height and bulk, the demands set up by its unfolding parts
will still cause currents; and there will still be alternate ascents
and descents, according as the varying conditions change the direction
of greatest demand--the only difference being that, in the absence of
oscillations, the growth will be less vigorous. Similarly, it must not
be supposed that mechanical actions are here alleged to be the sole
causes of wood formation in the individual plant. The tendency of the
individual plant to form wood at places where wood has been habitually
formed by ancestral plants, is manifestly a cause, and, indeed, the
chief cause. In this, as in all other cases, inherited structures
repeat themselves irrespective of the circumstances of the individual:
absence of the appropriate conditions resulting simply in imperfect
repetition of the structures. Hence the fact that in trained trees and
hothouse shrubs, dense substance is still largely deposited; though
not so largely as where the normal mechanical strains have acted.
Hence, too, the fact, that in such plants as the Elephant’s-foot or the
_Welwitschia mirabilis_, which for untold generations can have
undergone no oscillations, there is an extensive formation of wood
(though not to any considerable height above the ground), in repetition
of an ancestral type: natural selection having here maintained the
habit as securing some other advantage than that of support.

Still, it must be borne in mind that though intermittent mechanical
strains cannot be assigned as the direct causes of these internal
differentiations in plants that are artificially sheltered or
supported, they are assignable as the indirect causes; since the
inherited structures, repeated apart from such strains, are themselves
interpretable as accumulated results of such strains acting on
successive generations of ancestral plants. This will become clear on
combining the several threads of the argument and bringing it to a
close, which we may now do.


§ 282. To put the co-operative actions in their actual order,
would require us to consider them as working on individuals small
modifications that become conspicuous and definite only by inheritance
and gradual increase; but it will aid our comprehension without leading
us into error, if we suppose the whole process resumed in a single
continuously-existing plant.

As the plant erects the integrated series of fronds whose united
parts form its rudimentary axis, the increasing area of frond surface
exposed to the sun’s rays entails an increasing draught upon the
liquids contained in the rudimentary axis. The currents of sap so
produced, once established along certain lines of cells that offer
least resistance, render them by their continuous passage more and
more permeable. This establishment of channels is aided by the wind.
Each bend produced by it while yet the tissue is undifferentiated,
squeezes towards the place of growth and evaporation the liquids
that are passing by osmosis from cell to cell; and when the lines of
movement become defined, each bend helps, by forcing the liquid along
these lines, to remove obstructions and make continuous canals. As fast
as this transfer of sap is facilitated, so fast is the plant enabled
further to raise itself, and add to its assimilating surfaces; and so
fast do the transverse strains, becoming greater, give more efficient
aid. The canals thus formed can be neither in the centre of the
rudimentary axis nor at its surface: for at neither of these places
can the transverse strains produce any considerable compressions.
They must arise along a tract between the outside of the axis and its
core--a tract along which there occur the severest squeezes between the
stretched outer layers and the internal mass. Just that distribution
which we find, is the distribution which these mechanical actions tend
to establish.

As the plant gains in height, and as the mass of its foliage
accumulates, the strains thrown upon its axis, and especially the
lower part of its axis, rapidly increase. Supposing the forms to
remain similar, the strains must increase in the ratio of the cubes of
the dimensions; or even in a somewhat higher ratio. One consequence
must be that the compressions to which the vessels at the lower
part of the incipient stem are subject, become greater as fast as
the height to which the sap has to be raised becomes greater; and
another consequence must be that the local exudation of sap produced
by the pressure is proportionately augmented. Hence the materials for
interstitial nutrition being there supplied more abundantly, we may
expect thickening of the surrounding tissues to show itself there
first: in other words, wood will be formed round the vessels of the
lower part of the incipient stem. The resulting greater ability of this
lower part of the stem to bear strains, renders possible an increase of
height; and while after an increase of height the lowest part becomes
still further strained, and still further thickens, the part above it,
exposed to like actions, undergoes a like thickening. This induration,
while it spreads upwards, also spreads outwards. As fast as the rude
cylinder of dense matter formed in this way, begins to inclose the
original vessels, it begins to play the part of a resistant mass, which
more and more prevents the contained vessels from being squeezed;
while between it and the outer layers the greatest compression occurs
at each bend. Thus at the same time that the original vessels become
useless, the peripheral cells of the developing wood become those which
have their liquid contents squeezed out longitudinally and laterally
with increasing force; and, consequently, amid them are formed new
sap-channels, from which there is the most active local exudation,
producing the greatest deposit of dense matter.

Thus fusing together, as it were, the individualities of successive
generations of plants, and recognizing as all-important that
facilitation of the process which natural selection has all along
given, we are enabled to interpret the chief internal differentiations
of plants as consequent on an equilibration between inner and outer
forces. Here, indeed, we see illustrated in a way more than usually
easy to follow, the eventual balancing of outer actions by inner
reactions. The relation between the demand for liquid and the formation
of channels that supply liquid, as well as that between the incidence
of strains and the deposit of substance which resists strains, are
among the clearest special examples of the general truth that the
moving equilibrium of an organism, if not overthrown by an incident
force, must eventually be adjusted to it.

The processes here traced out are, of course, not to be taken as the
only differentiating processes to which the inner tissues of plants
have been subject. Besides the chief changes we have considered,
various less conspicuous changes have taken place. These must be
passed over as arising in ways too involved to admit of specific
interpretations; even supposing them to have been produced by causes of
the kind assigned. But the probability, or rather indeed the certainty,
is that some of them have not been so produced. Here, as in nearly
all other cases, indirect equilibration has worked in aid of direct
equilibration; and in many cases indirect equilibration has been the
sole agency. Besides ascribing to natural selection the rise of various
internal modifications of other classes than those above treated, we
must ascribe some even of these to natural selection. It is so with the
dense deposits which form thorns and the shells of nuts: these cannot
have resulted from any inner reactions immediately called forth by
outer actions; but must have resulted immediately through the effects
of such outer actions on the species. Let it be understood, therefore,
that the differentiations to which the foregoing interpretation
applies, are only those most conspicuous ones which are directly
related to the most conspicuous incident forces. They must be taken as
instances on the strength of which we may conclude that other internal
differentiations have had a natural genesis, though in ways that we
cannot trace.




                              CHAPTER V.

                 PHYSIOLOGICAL INTEGRATION IN PLANTS.


§ 283. A good deal has been implied on this topic in the preceding
chapters. Here, however, we must for a brief space turn our attention
immediately to it.

Plants do not display integration in such distinct and multiplied
ways as do animals. But its advance may be traced both directly and
indirectly--directly in the increasing co-ordination of actions, and
indirectly in the effect of this upon the powers and habits.

Let us group the facts under these heads: ascending in both cases from
the lower to the higher types.


§ 284. The inferior _Algæ_, along with little unlikeness of parts,
show us little mutual dependence of parts. Having surfaces similarly
circumstanced everywhere, much physiological division of labour
cannot arise; and therefore there cannot be much physiological unity.
Among the superior _Algæ_, however, the differentiation between the
attached part and the free part is accompanied by some integration.
There is evidently a certain transfer of materials, which is doubtless
facilitated by the elongated forms of the cells in the stem, and
probably leads to the formation of dense tissue at the places of
greatest strain, in a way akin to that recently explained in other
cases. And where there is this co-ordination of actions, the parts are
so far mutually dependent that each dies if detached from the other.
That though the organization is so low neither part can reproduce the
other and survive by so doing, is probably due to the circumstance
that neither part contains any considerable stock of untransformed
protoplasm, out of which new tissues may be produced.

Fungi and Lichens present no very significant advances of integration.
We will therefore pass at once to the Archegoniates. In those of them
which, either as single fronds or strings of fronds, spread over
surfaces, and which, rooting themselves as they spread, do not need
that each part should receive aid from remote parts, there is no
developed vascular system serving to facilitate transfer of nutriment:
the parts being little differentiated there is but little integration.
But along with assumption of the upright attitude and the accompanying
specializations, producing vessels for distributing sap and hard tissue
for giving mechanical support, there arises a decided physiological
division of labour; rendering the aërial part dependent on the embedded
part and the embedded part dependent on the aërial part. Here, indeed,
as elsewhere, these concomitant changes are but two aspects of the
same change. Always the gain of power to discharge a special function
involves a loss of power to perform other functions; and always,
therefore, increased mutual dependence constituting physiological
integration, must keep pace with that increased fitting of particular
parts to particular duties which constitutes physiological
differentiation.

Making a great advance among the Archegoniates, this physiological
integration reaches its climax among Phænogams. In them we see
interdependence throughout masses that are immense. Along with
specialized appliances for support and transfer, we find an exchange of
aid at great distances. We see roots giving the vast aërial growth a
hold tenacious enough to withstand violent winds, and supplying water
enough even during periods of drought; we see a stem and branches of
corresponding strength for upholding the assimilating organs under
ordinary and extraordinary strains; and in these assimilating organs
we see elaborate appliances for yielding to the stem and roots the
materials enabling them to fulfil their offices. As a consequence of
which greater integration accompanying the greater differentiation,
there is ability to maintain life over an immense period under marked
vicissitudes.

Even more conspicuously exemplified in Phænogams, is that physiological
integration which holds together the functions not of the individual
only but of the species as a whole. The organs of reproduction, both
in their relations to other parts of the individual bearing them and
in their relations to corresponding parts of other individuals, show
us a kind of integration conducing to the better preservation of the
race; as those already specified conduce to the better preservation
of the individual. In the first place, this greater co-ordination of
functions just described, itself enables Phænogams to bequeath to
the germs they cast off, stores of nutriment, protective envelopes,
and more or less of organization: so giving them greater chances of
rooting themselves. In the second place, certain differentiations
among the parts of fructification, the meaning of which Mr. Darwin
has so admirably explained, give to the individuals of the species a
kind of integration that makes possible a mutual aid in the production
of vigorous offspring. And it is interesting to observe how, in
that dimorphism by which in some cases this mutual aid is made more
efficient, the greater degree of integration is dependent on the
greater degree of differentiation--not simply differentiation of
the fructifying organs from other parts of the plant bearing them,
but differentiation of these fructifying organs from the homologous
organs of neighbouring individuals of the same race. Another form of
this co-ordination of functions which conduces to the maintenance of
the species, may be here named--partly for its intrinsic interest.
I refer to the strange processes of multiplication occurring in the
genus _Bryophyllum_. It is well known that the succulent leaves of
_B. calycinum_, borne on foot-stalks so brittle that they are easily
snapped by the wind, send forth from their edges when they fall to the
ground, buds which root themselves and grow into independent plants.
The correlation here obviously furthering the preservation of the race,
is more definitely established in another species of the genus--_B.
proliferum_. This plant, shooting up to a considerable height, and
having a stem containing but little woody fibre, habitually breaks
near the bottom while still in flower; and is thus generally prevented
from ripening its seeds. The multiplication is, however, secured in
another way. Before the stem is broken young plants have budded out
from the pedicels of the flowers, and have grown to considerable
lengths; and on the fall of the parent they forthwith commence their
separate lives. Here natural selection has established a remarkable
kind of co-ordination between a special habit of growth and decay, and
a special habit of proliferation.


§ 285. The advance of physiological integration among plants as we
ascend to the higher types, is implied by their greater constancy of
structure, as well as by the stricter limitations of their habitats and
modes of life. “Complexity of structure is generally accompanied with a
greater tendency to permanence in form,” says Dr. [now Sir J.] Hooker;
or, conversely, “the least complex are also the most variable.” This is
the second aspect under which we have to contemplate the facts.

The differences between the simpler _Algæ_ and _Fungi_ are so feebly
marked that botanists have had great difficulty in framing definitions
of these classes. This structural indefiniteness is accompanied by
functional indefiniteness. _Algæ_, which are mostly aquatic, include
many small forms that frequent the damp places preferred by _Fungi_.
Among _Fungi_, there are kinds which lead submerged lives like the
_Algæ_. Besides this indistinctness of the classes, there is great
variability in the shapes and modes of life of their species--a
variability so great that what were at first taken to be different
species, or different genera, or even different orders, have proved
to be merely varieties of one species. So inconstant in structure are
the _Algæ_ that Schleiden quotes with approval the opinion of Kutzing,
that “there are no species but merely forms of _Algæ_:” an opinion
which though now rejected sufficiently implies extreme indefiniteness.
In all which facts we see that these lowest types of plants, little
differentiated, are also but little integrated.

Archegoniates present a like relation between the small specialization
of functions which constitutes physiological differentiation, and
the small combination of functions which constitutes physiological
integration. “Mosses,” says Mr. Berkeley, “are no less variable than
other cryptogams, and are therefore frequently very difficult to
distinguish. Not only will the same species exhibit great diversity
in the size, mode of branching, form and nervation of the leaves, but
the characters of even the peristome itself are not constant.” And
concerning the classification of the remaining group, _Filicales_,
he says:--“Not only is there great difficulty in arranging ferns
satisfactorily, but it is even more difficult to determine the limits
of species.”

After this vagueness of separation as well as inconstancy of structure
and habit among the lower plants, the stability of structure and
habit and divisibility of groups among the higher plants, appear
relatively marked. Though Phænogams are much more variable than most
botanists have until lately allowed, yet the definitions of species and
genera may be made with far greater precision, and the forms are far
less capable of change, than among Cryptogams. And this comparative
fixity of type, implying, as it does, a closer combination of the
component functions, we see to be the accompaniment of the greater
differentiation of those functions and of the structures performing
them. That these characters are correlatives is further shown by the
fact that the higher plants are more restricted in their habitats
than the lower plants, both in space and time. “The much narrower
delimitation in area of animals than plants,” says Sir J. Hooker, “and
greater restriction of Faunas than Floras, should lead us to anticipate
that plant-types are, geologically speaking, more ancient and permanent
than the higher animal types are, and so I believe them to be, and I
would extend the doctrine even to plants of highly complex structure.”
“Those classes and orders which are the least complex in organization
are the most widely distributed.”


§ 286. Thus that which the general doctrine of evolution leads us
to anticipate, we find implied by the facts. The physiological
division of labour among parts, can go on only in proportion to the
mutual dependence of parts; and the mutual dependence of parts can
progress only as fast as there arise structures by which the parts are
efficiently combined, and the mutual utilization of their actions made
easy.

To say definitely by what process is brought about this co-ordination
of functions which accompanies their specialization, is hardly
practicable. Direct and indirect equilibration doubtless co-operate
in establishing it. We may see, for example, that every increase of
fitness for function produced in the aërial part of a plant by light,
as well as every increase of fitness for function produced in its
imbedded part by the direct action of the moist earth, must conduce
to an increased current of the liquid evaporated from the one and
supplied by the other--must serve, therefore, to aid the formation
of sap-channels in the ways already described; that is--must serve
to develop the structures through which mutual aid of the parts is
given: the additional differentiation tends immediately to bring
about the additional integration. Contrariwise, it is obvious that
an interdependence such as we see between the secretion of honey and
the fertilization of germs, or between the deposit of albumen in the
cotyledons of an embryo-plant and its subsequent striking root, is
a kind of integration in the actions of the individual or of the
species, which no differentiation has a direct tendency to initiate.
Hence we must regard the total results as due to a plexus of influences
acting simultaneously on the individual and on the species: some
chiefly affecting the one and some chiefly affecting the other.

       *       *       *       *       *

[NOTE.--In _Nature_ for June 11, 1896, Dr. Maxwell Masters, in an
essay on “Plant Breeding,” names an instructive fact concerning the
production of varieties by selection of slightly divergent forms. He
says:--

“To the untrained eye, the primordial differences noted are often very
slight; even the botanist, unless his attention be specially directed
to the matter, fails to see minute differences which are perceptible
enough to the raiser or his workmen. Nor must it be thought that these
variations, difficult as they are to recognise in the beginning, are
unimportant. On the contrary, they are interesting, physiologically,
as the potential origin of new species, and very often they are
commercially valuable also. These apparently trifling morphological
differences are often associated with physiological variations which
render some varieties, say of wheat, much better enabled to resist
mildew and disease generally than others. Some, again, prove to be
better adapted for certain soils or for some climates than others; some
are less liable to injury from predatory birds than others, and so on.”

Thus we are shown that, to a much greater degree than might be
supposed, minute changes of forms and functions in one part of a plant
are correlated with changes of forms and functions throughout it. The
interdependence--that is to say, the physiological integration--is very
close at the same time that it is very complex.

Here while naming these facts in illustration of physiological
integration in plants I name them because they illustrate an important
truth bearing upon the general question of heredity which I have dealt
with in Appendix G, and to which I now especially draw attention.]




                              CHAPTER VI.

   DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF ANIMALS.


§ 287. What was said respecting the primary physiological
differentiation in plants, applies with little beyond change of terms
to animals. Among _Protozoa_, as among _Protophyta_, the first definite
contrast of parts is that between outside and inside. The speck of
jelly or sarcode which appears to constitute the simplest animal,
proves, on closer examination, to be a mass of substance containing a
nucleus--a periplast in the midst of which there is a minute endoplast,
consisting of a spherical membrane and its contents.

This parallel, only just traceable among these Rhizopods, which are
perpetually changing the distribution of their outer substance, becomes
at once marked in those higher _Protozoa_ which have fixed shapes,
and maintain constant relations between their surfaces and their
environments. Indeed the Rhizopods themselves, on passing into a state
of quiescence in which the relations of outer and inner parts are
fixed, become encysted: there is formed a hardened outer coat different
from the matter which it contains. And what is here a temporary
character answering to a temporary definiteness of conditions, is
in the _Infusoria_ a constant character, answering to definite
conditions that are constant. Each of these minute creatures, though
not coated by a distinct membrane, has an outer layer of excreted
substance forming a delicate cuticle.


§ 288. The early establishment of this primary contrast of tissues
answering to this primary contrast of conditions, is no less
conspicuous in aggregates of the second order. The feebly-integrated
units of a Sponge, with individualities so little merged in that of
the whole they form that most of them still retain their separate
activities, nevertheless show us, in the unlikeness that arises between
the outermost layer and the contained mass, the effect of converse with
unlike conditions. This outermost layer is composed of units somewhat
flattened and united into a continuous membrane--a kind of rudimentary
skin.

Secondary aggregates in which the lives of the units are more
subordinate to the life of the whole, carry this distinction further.
The leading physiological trait of every cœlenterate animal is the
divisibility of its substance into endoderm and ectoderm--the part
next the food and the part next the environment. Fig. 147 (§ 201),
representing a portion of the body-wall of a _Hydra_ seen in section,
gives some idea of this fundamental differentiation. The creature
consists of a simple sac, the cavity of which is in communication with
the surrounding water; and hence the unlikeness between the outer and
inner layers has not become great. The essential contrast is that
between the differentiated parts of what was originally the same
part--a uniform membrane composed of juxtaposed cells.

For here, indeed, we are shown unmistakably how the primary contrast
of structures follows upon the primary contrast of conditions. The
ordinary form from which low types of the _Metazoa_ set out, is a
hollow sphere formed of cells packed side by side--a blastula, as it
is called: all these cells being similarly exposed to the environment.
The blastula presently changes into what is called a gastrula--a form
resulting from the introversion of one of the sides of the blastula.
If there be taken a small ball of vulcanized india-rubber, say an
inch or more in diameter, and having a hole in it through which the
air may escape, and if one side of it be thrust inwards so as to
produce a cup, and if the wide opening of the cup be supposed to
contract, thus becoming a narrow opening, there will result something
like the gastrula form. Manifestly that part of the original layer
which has become internal is differently conditioned from the rest
which remains external: the one continuing to hold converse with the
forces of the environment, while the other begins to hold converse
with the nutritive matters taken into the sac-formed chamber--the
archenteron or primitive stomach. Interesting evidence of the primitive
externality of the digestive cavity is yielded by the fact that whereas
the blastula consisted of ciliated cells, and whereas the ciliation
persists throughout life on the outer layer, or parts of it, in sundry
low types--even in some Chætopods--it persists also on the alimentary
tract of sundry low types: not only in the _Hydra_ but commonly in
Nemertines, in some _Platyhelminthes_, and even in some leeches.

Besides being enabled thus to understand how an aggregate of
_Amœba_-form units, originally consisting of a single layer, may pass
into an aggregate consisting of a double layer; we may also understand
under what influences the transition takes place. If the habit which
some of the primary aggregates have, of wrapping themselves round
masses of nutriment, is followed by a secondary aggregate, there will
naturally arise just that re-differentiation which the _Hydra_ shows us.


§ 289. This account of the primary differentiation carries us only
half-way towards a true conception of the distinction between outer
and inner tissues. Though, using words in their current senses, this
introverted part of the primitive layer has become internal in contrast
with the remainder, which continues external, yet this introverted
part has not become internal in the strict physiological sense. For
it remains subject to the actions of those environing matters which
are taken in as food: such environing matters, when they happen to be
moving prey, acting upon it much as they might act upon the exterior.
So that this introverted part has a quasi-externality. It has not
the same absolute internality as have those parts which never come
in contact with products of the outer world. Here we must briefly
recognize the distinction between these parts and the parts thus far
considered.

Reverting to our symbol, the india-rubber ball, it will be seen that
the introversion may be so complete that the cavity is obliterated,
with the result that the internal surfaces of the outer and inner
layers come in contact. This is the state reached in the simplest
cœlenterate animal, the _Hydra_: there being in it nothing more
than a thin structureless lamella between the ectoderm and endoderm,
as shown in Fig. 147. This lamella represents all that there is of
strictly internal tissues. But the introversion, instead of bringing
the inner surfaces of the ball into contact, may be so far incomplete
as to leave a space, and in various creatures and embryos of others,
symbolized by this arrangement, this space becomes occupied by a tissue
formed from one or other or both of the two primary tissues--the
mesoblast or mesoderm. This intermediate layer, sometimes, as in the
_Medusa_, growing into a mass of jelly serving as a fulcrum for
the creature’s contractions, or, as in the Sponge, giving a passive
basis to the active tissues, becomes in higher animals the layer out
of which the structures that support the body and move it about, as
well as those that distribute prepared nutriment, are developed. From
it arise the bones, the muscles, and the vascular system--the masses
of differentiated tissue which are truly internal and occupy what is
called the body-cavity or peri-visceral space.

In the higher types of animals this space comes to be partially
occupied by a structure that may be described as a cavity within
a cavity--the cœlom. Most zoologists regard this as arising by a
re-introversion of the _archenteron_ or primary alimentary sac. It
is easily to be perceived that after the introversion which produces
this digestive cavity, the wall of the cavity may be again introverted
in such way as to intrude into the peri-visceral space. The cœlom thus
formed is subsequently shut off. Becoming included among the more
truly internal structures, and in part giving origin to certain lining
membranes, it has for its chief function the formation of organs for
the excretion and emission of nitrogenous waste and of the generative
products: some portions of it retaining, as a consequence, indirect
connexions with the environment and characters usually accompanying
such connexions.

Here we are not concerned with further details: the aim being simply to
indicate the way in which out of the original layer, wholly external,
there arise, by primary and secondary introversions, and the formation
of intermediate membranes and spaces, the chief contrasts between
outer and inner tissues, and how there simultaneously go on the
differentiations accompanying different conditions.


§ 289_a_. Another all-important differentiation between outer tissues
and inner tissues has now to be set forth--that by which the nervous
system becomes established and distinguished. Strangely enough, like
the one above described, it is sequent upon an introversion: the
nervous system is primarily a skin-structure and develops by the
infolding of this skin-structure.

In creatures possessing the earliest rudiments of nerves these exist
in certain superficial cells. Each has a small tubular orifice from
which projects a minute hair, and each has on its under side processes
running into the tissue below, and serving, as it seems, to conduct
impressions from the projecting hair when it is disturbed by contacts
with foreign bodies. A plexus of fibres bringing the inner processes
of such cells into communication arises, and forms something like
a nervous layer capable of propagating impulses in all directions.
At a subsequent stage some of the superficial cells, ceasing to be
themselves the recipients of external stimuli, sink inwards and become
ganglion-cells connected with the nervous plexus--agents, as we must
suppose, for the reception, multiplication, and diffusion of the
impulses received from the outer cells.

As thus far developed, the nervous structure is one fitted only
for a vague stimulation of dispersed contractile fibres, causing
movements of an undirected kind. A concentration of these superficial
nervous structures is a probable preliminary to the next change--an
all-important change. For a part of the surface begins to sink inwards,
forming, in the _Vertebrata_, a groove; and from the lining cells
of this groove, which presently closes over, the central parts of
the nervous system arise: definite nerves having meantime, as we may
suppose, been developed out of the indefinite nervous plexus.

Neglecting what there is in this of a speculative nature, it is
sufficient for the present purpose to recognize the undoubted fact that
the nervous system is developed from the ectoderm, and that, originally
external, it is made internal by a process of sinking in or by a
process of definite introversion.


§ 290. Whether direct equilibration or indirect equilibration has had
the greater share in producing these fundamental contrasts between
the inner and outer tissues of animals, must be left undecided. The
two causes have all along co-operated--modification of the individual
accumulated by inheritance predominating in some cases, and in other
cases modification of the race by survival of the incidentally fittest.
On the other hand, the action of the medium on the organism cannot fail
to change its surface more than its centre, and so differentiate the
two; while, on the other hand, the surfaces of organisms inhabiting
the same medium display extreme unlikenesses which cannot be due to
the immediate actions of their medium. Let us dwell a moment on the
antithesis.

We have abundant evidence that animal protoplasm is rapidly modified by
light, heat, air, water, and the salts contained in water--coagulated,
turned from soluble into insoluble, partially changed into isomeric
compounds, or otherwise chemically altered. Immediate metamorphoses
of this kind are often obviously produced in ova by changes of their
media. At the outset, therefore, before yet there existed any such
differentiation as that which now usually arises by inheritance,
these environing agencies must have tended to originate a protective
envelope. For a modification produced by them on the superficial part
of the protoplasm, must either have been a decomposition or else the
formation of a compound which remained stable under their subsequent
action. There would be generated an outer layer of substance that was
so molecularly immobile as to be incapable of further metamorphoses,
while it would shield the contained protoplasm from that too-great
action of external forces which, by rapidly changing the unstable
equilibrium of its molecules into a relatively stable equilibrium,
would arrest development. Evidently organic evolution, whether
individual or general, must always and everywhere have been subordinate
to these physical necessities. Though natural selection, beginning with
minute portions of protoplasm, must all along have tended to establish
a molecular composition apt to undergo this differentiation of surface
from centre to the most favourable extent, yet it must all along have
done so while controlled by this process of direct equilibration.

Contrariwise, the many and great unlikenesses among the dermal
structures of creatures inhabiting the same element, cannot be ascribed
to any such cause. The contrasts between naked and shelled Gastropods,
between marine Worms and Crustaceans, between soft-skinned Fishes
and Fishes in armour like the _Pterichthys_, must have been produced
entirely by natural selection. Environing forces are, as before, the
ultimate causes; but the forces are now not so much those exercised by
the medium as those exercised by the other inhabitants of the medium;
and they do not act by modifying the surface of the individual, but
by killing off individuals whose surfaces are least fitted to the
requirements: thus slowly affecting the species. Still the dermal
skeleton bristling with spines, which protects the _Diodon_ or the
_Cyclichthys_ from enemies it could not escape, comes within the
general formula of an outer tissue differentiated from inner tissues by
the outer actions to which the creature is exposed: the differentiation
having gone on until there is equilibrium between the destructive
forces to be met and the protective forces which meet them.

If we venture to apportion the respective shares which mediate and
immediate actions have had in differentiating outer from inner tissues,
we shall probably not be far wrong in ascribing that part of the result
which is alike in all animals, mainly to the direct actions of their
media, while we ascribe the multitudinous unlikenesses of the results
in various animals, partly to the indirect actions of the media, and
partly to the indirect actions of other animals by which the media are
inhabited. That is to say, while assigning the specialities of the
differentiations to the specialities of converse with the agencies in
the environment, most of them organic, we may assign to the constant
and universal converse with its inorganic agencies, the universal
characteristic of tegumentary structures--their growth outwards from a
layer lying below the surface which continually produces new substance
to replace the substance worn away or cast off.

Here let me add a piece of evidence which strengthens the general
argument, at the same time that it justifies this apportionment. When
ulceration has gone deep enough to destroy the tegumentary structures,
these are never reproduced. The puckered surface formed where an ulcer
heals, or where a serious burn has destroyed the skin, consists of
modified connective tissue, which, as the healing goes on, spreads
inwards from the edges of the ulcer: some of it, perhaps, growing from
the portions of connective tissue that dip down between the muscular
bundles. This connective tissue is normally covered by the epidermis
and thus sheltered from environing actions. What has happened to it?
It has now become the outermost layer. And how does it comport itself
under its new conditions? It produces a superficial substance which
plays the part of the epidermis and grows outwardly. For since the
surface, subject to friction and exfoliation, has to be continually
renewed, there must be a continual reproduction of an outermost layer
from a layer beneath. That is to say, the contact of this deep-seated
tissue with outer agencies, produces in it some approach towards
that character which we find universally characterizes outer tissue.
But while we see under this exposure to the conditions common to
all integument, a tendency to assume the structure common to all
integument, we see no tendency to assume any of the specialities of
tegumentary structure: no rudiments of glands or hair sacs make their
appearance.

Analogous conclusions may be drawn respecting the processes of
differentiation by which from the outer layer nervous tissue and
finally a nervous system are evolved. Here, also, both direct and
indirect equilibration appear to have operated. Two reasons may be
assigned for the belief that the transformation of certain superficial
cells into sensitive cells was initiated by exposure to external
stimuli. The first is that, extremely unstable as protoplasm is,
disturbances received by the outer side of a specially-exposed cell
could scarcely fail to cause changes passing through it towards the
interior mass of the body, and that perpetual repetition of such
changes would tend to generate channels of easy transmission through
the protoplasm. The second reason is that, if we do not assume this
process of initiation but assume that survival of the fittest was
the sole agency, then no reason can be assigned why the nervous
system should not have been at the outset formed internally instead
of being initiated externally and then transferred to the interior:
the roundabout process would be inexplicable. At the same time the
production of a central nervous system by introversion of superficial
sensitive cells cannot be ascribed to the differentiating effects
of external stimuli, but must be ascribed to natural selection. No
perpetual repetition of outer disturbances would cause the sinking
inwards, and covering up, of the specially-sensitive area and
the plexus below it. But it is manifest that since these nervous
structures, at once all-important and easily injured, would be safer
if removed from the surface, survival of the fittest, continually
preserving those in which they were more deeply seated, would tend to
produce an arrangement in which all parts but the actual receivers of
external stimuli became internal.

Hence, contemplating generally these two fundamental differentiations
of inner from outer tissues, we may conclude that though their first
stages resulted from direct equilibration, their subsequent and higher
stages resulted from indirect equilibration.




                             CHAPTER VII.

         DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS.


§ 291. The outer tissues of animals, originally homogeneous over their
whole surfaces, pass into a heterogeneity which fits their respective
parts to their respective conditions. So numerous and varied are the
implied differentiations, that it is impracticable here to deal with
them all even in outline. To trace them up through classes of animals
of increasing degrees of aggregation, would carry us into undue detail.

Did space permit, it would be possible to point out among the
_Protozoa_, various cases analogous to that of the _Arcella_; which may
be described as like a microscopic Limpet, having a sarcode body of
which the upper surface has become horny, while the lower surface with
its protruding pseudopodia, retains the primitive jelly-like character.
That differentiations of this kind have been gradually established
among these minute creatures through the unlike relations of their
parts to the environment, is an inference supported by a form which,
while the rest of the body has a scarcely distinguishable coating,
“agrees with _Arcella_ and _Difflugia_ in having the pseudopodia
protrusible from one extremity only of the body.”

Many parallel specializations of surface among aggregates of the second
order might be instanced from the _Cœlenterata_. In the _Hydra_, the
ectoderm presents over its whole area no conspicuous unlikenesses;
but there usually exist in the hydroid polypes of superior types,
decided contrasts between the higher and lower parts. While the
higher parts retain their original characters, the lower parts excrete
hard outer layers yielding support and protection. Various stages
of the differentiation might be followed. “In _Hydractinia_,” says
Prof. Green, this horny layer “becomes elevated at intervals to form
numerous rough processes or spines, while over the general surface
of the ectoderm its presence is almost imperceptible.” In other
types, as in _Cordylophora_, it spreads part way up the animal’s
sides, ending indefinitely. In _Bimeria_ it “extends itself so as to
enclose the entire body of each polypite, leaving bare only the mouth
and tips of the tentacles.” While in _Campanularia_ it has become a
partially-detached outer cell, into which the creature can retract its
exposed parts.

But it is as needless as it would be wearisome to trace through the
several sub-kingdoms the rise of these multiform contrasts, with the
view of seeking interpretations of them. It will suffice if we take a
few groups of the illustrations furnished by the higher animals.


§ 292. We may begin with those modifications of surface which subserve
respiration. Though we ordinarily think of respiration as the quite
special function of a quite special organ, yet originally it is not
so. Little-developed animals part with their carbonic acid and absorb
oxygen, through the general surface of the body. Even in the lower
types of the higher classes, the general surface of the body aids
largely in aërating the blood; and the parts which discharge the
greater part of this function are substantially nothing more than
slightly altered and extended portions of the skin.

Such differentiations, marked in various degrees, are to be seen among
_Mollusca_. In the _Pteropoda_ the only modification which appears to
facilitate respiration, is the minute vascularity of one part of the
skin. Higher types possess special skin-developments. The _Doris_ has
appendages developed into elaborately-branched forms--small trees of
blood-vessels covered by slightly-changed dermal tissues. And these
arborescent branchiæ are gathered together into a single cluster. Thus
there is evidence that large external respiratory organs have arisen
by degrees from simple skin: as, indeed, they do arise during the
development of each individual having them. Just as gradually as in
the embryo a simple bud on the integument, with its contained vascular
loop, passes by secondary buddings into a tree-like growth penetrated
everywhere by dividing and subdividing blood-vessels; so gradually has
there probably proceeded the differentiation which has turned part
of the outer surface into an organ for excreting carbonic acid and
absorbing oxygen.

Certain inferior vertebrate animals present us with a like
metamorphosis of tissues. These are the _Amphibia_. The branchiæ
here developed from the skin, are covered with cellular epidermis, not
much thinner than that covering the rest of the body. Like it they
have their surfaces speckled with pigment-cells; and are not even
conspicuous by their extra vascularity--where they are temporary at
least. They facilitate the exchange of gases in scarcely any other
way than by affording a larger area of contact with the water, and
interposing a rather thinner layer of tissue between the water and the
blood-vessels. Those very simple branchiæ of the larval _Amphibia_
that have them but for a short time, graduate into the more complex
ones of those that have them for a long time or permanently; showing,
as before, the small stages by which this heterogeneity of surface
accompanying heterogeneity of function may arise.

In what way are such differentiations established? Mainly, no doubt, by
natural selection; but also to some degree, I think, by the inheritance
of direct adaptations. That a portion of the integument at which
aëration is favoured by local conditions, should thereby be led to grow
into a larger surface of aëration, appears improbable. Survival of
those individuals which happen to have this portion of the integument
somewhat more-developed, seems here the only likely cause.


§ 293. Among the conspicuous modifications by which the
originally-uniform outer layer is rendered multiform, are the
protective structures. Let us look first at the few cases in which the
formation of these is ascribable mainly to direct equilibration.

Already reference has been more than once made to those thickenings
that occur where the skin is exposed to unusual pressure and friction.
Are these adaptations inheritable? and may they, by accumulation
through many generations, produce permanent dermal structures fitted
to permanent or frequently-recurring stress? Take, for instance, the
callosities on the knuckles of the _Gorilla_, which are adapted to its
habit of partially supporting itself on its closed hands when moving
along the ground. Shall we suppose that these defensive thickenings are
produced afresh in each individual by the direct actions; or that they
are inherited modifications caused by such direct actions; or that they
are wholly due to the natural selection of spontaneous variations? The
last supposition does not seem a probable one. Such thickenings, if
spontaneous, would be no more likely to occur on the knuckles than on
any other of the hundred equal areas forming the skin-surface at large;
and the chances against their simultaneous occurrence on all eight
knuckles would be incalculable. Moreover, the implication would be that
those slight extra thicknesses of skin on the knuckles, with which we
must suppose the selection to have commenced, were so advantageous
as to cause survivals of the individuals having them, in presence of
other superiorities possessed by other individuals. Then that survivals
so caused, if they ever occurred at all, should have occurred with
the frequency requisite to establish and increase the variation, is
hardly supposable. And if we reject, as also unlikely, the reproduction
of these callosities _de novo_ in each individual (for this would
imply that after a thousand generations each young gorilla began with
knuckles having skin no thicker than elsewhere), there remains only the
inference that they have arisen by the transmission and accumulation of
functional adaptations. Another case which seems interpretable only in
an analogous way, is that of the spurs that are developed on the wings
of certain birds--on those of the Chaja screamer for example. These are
weapons of offence and defence. It is a familiar fact that some birds
strike with their wings, often giving severe blows; and in the birds
named, the blows are made more formidable by the horny, dagger-shaped
growths standing out from those points on the wings which deliver
them. Are these spurs directly or indirectly adaptive? To conclude
that natural selection of spontaneous variations has caused them, is
to conclude that, without any local stimulus, thickenings of the skin
occurred symmetrically on the two wings at the places required; that
such thickenings, so localized, happened to arise in birds given to
using their wings in fight; and that on their first appearance the
thickenings were decided enough to give appreciable advantages to the
individuals distinguished by them--advantages in bearing the reactions
of the blows if not in inflicting the blows. But to conclude this
is, I think, to conclude against probability. Contrariwise, if we
assume that the thickening of the epidermis produced by habitual rough
usage is inheritable, the development of these structures presents no
difficulty. The points of impact would become indurated in wings used
for striking with unusual frequency. The callosities of surface thus
generated, rendering the parts less sensitive, would enable the bird in
which they arose to give, without injury to itself, more violent blows
and a greater number of them: so, in some cases, helping it to conquer
and multiply. Among its descendants, inheriting the modification and
the accompanying habit, the thickening would be further increased in
the same way: survival of the fittest tending ever to accelerate the
process. Presently the horny nodes so formed, hitherto defensive only
in their effects, would, by their prominence, become offensive--would
make the blows given more hurtful. And now natural selection, aiding
more actively, would mould the nodes into spurs: the individuals
in which the nodes were most pointed would be apt to survive and
propagate; and the pointedness generation after generation thus
increased, would end in the well-adapted shape we see.

But if in these cases the differentiations which fit particular parts
of the outer tissues to bear rough usage are caused mainly by the
direct balancing of external actions by internal reactions, then we
may suspect that the like is true of other modifications that occur
where special strains and abrasions have to be met. Possibly it is
true of sundry parts that are formed of hardened epidermis, such as
the nails, claws, hoofs, and hollow horns of Mammals; “all of which,”
says Prof. Huxley, “are constructed on essentially the same plan, being
diverticula of the whole integument, the outer layer of whose ecderon
has undergone horny metamorphosis.” Leaving open, however, the question
what tegumentary structures are due to direct equilibration, furthered
and controlled by indirect equilibration, it is tolerably clear that
direct equilibration has been one of the factors.


§ 294. Dermal structures of another class are developed mainly, if not
wholly, by the actions of external causes on species rather than on
individuals. These are the various kinds of clothing--hairs, feathers,
quills, scales, scutes. Though it is no longer thought as at one time
that all these various tegumentary structures are homologous with one
another, yet it is unquestionable that sundry of the more conspicuous
ones are. Those which are extremely unlike may be seen linked together
by a long series of graduated forms. A retrograde metamorphosis from
feathers to appendages that are almost scale-like, is well seen in
the coat of the Penguin. There is manifest a transition from the
bird-like covering to the fish-like covering--a transition so gradual
that no place can be found where an appreciable break occurs; and if
the scale-like appendages are not truly scales yet they exemplify
an extreme metamorphosis. Less striking, perhaps, but scarcely less
significant, are the modifications through which we pass from feathers
to hairs, on the surfaces of the Ostrich and the Cassowary. The skin
of the Porcupine shows us hairs and quills united by a series of
intermediate structures, differing from one another inappreciably. Even
more remarkable are certain other alliances of dermal structures. “It
may be taken as certain, I think,” says Prof. Huxley, “that the scales,
plates, and spines of all fishes are homologous organs; nor as less so
that the tegumentary spines of the Plagiostomes are homologous with
their teeth, and thence with the teeth of all vertebrata.”

Further details concerning these tegumentary structures are not needful
for present purposes, and are indeed but indirectly relevant to the
subject of physiological development. Here they are of interest to us
only by involving the general question--What physical influences have
brought them into existence? Still with a view to definite presentation
of the problem, it will be well to contemplate the mode of development
common to the most familiar of them.

Suppose a small pit to be formed on the previously flat skin; and
suppose that the growth and casting off of horny cells which goes on
over the skin in general, continues to go on at the usual rate over the
depressed surface of this pit. Clearly the quantity of horny matter
produced within this hollow, will be greater than that produced on a
level portion of the skin subtending an equal area of the animal’s
outside. Suppose such a pit to be deepened until it becomes a small
sac. If the exfoliation goes on as before, the result will be that the
horny matter, expelled, as it must be, through the mouth of the sac,
which now bears a small proportion to the internal surface of the sac,
will be large in quantity compared with that exfoliated from a portion
of the skin equal in area to the mouth of the sac: there will be a
conspicuous thrusting forth of horny matter. Suppose once more that
the sac, instead of remaining simple, has its bottom pushed up into
its interior, like the bottom of a wine-bottle--the introversion being
carried so far that the introverted part reaches nearly to the external
opening, and leaves scarcely any space between the introverted part and
the walls of the sac. It is easy to see that the exfoliation continuing
from the surface of the introverted part, as well as from the inside
of the sac generally, the horny matter cast off will form a double
layer; and will come out of the sac in the shape of a tube having
within its lower end the introverted part, as the core on which it is
moulded, and from the apex of which is cast off the substance filling,
less densely, its interior. The structure resulting will be what we
know as a hair. Manifestly by progressive enlargement of the sac, and
further complication of that introverted part on which the excreted
substance is moulded, the protruding growth may be rendered larger and
more involved, as we see it in quills and feathers. So that insensible
steps, thus indicated in principle, carry us from the exfoliation of
epidermis by a flat surface, to the exfoliation of it by a hollow
simple sac, an introverted sac, and a sac further complicated; each of
which produces its modified kind of tegumentary appendage.

But now, after contemplating this typical illustration, we return to
the general question. What are the agencies which have been operative
in developing these skin-structures? Indirect equilibration must have
worked almost alone in producing them. No direct incidence of forces
can have developed the enamelled armour of the _Lepidosteus_ or the
tesselated plates of the _Glyptodon_ and its modern allies. Survival of
the fittest must here and in multitudinous other cases be regarded as
the sole cause.


§ 295. Among many other differentiations of the outer tissues, the
most worthy to be noticed in the space that remains, are those by which
organs of sense are formed. We will begin with the simplest and most
closely-allied to the foregoing.

Every hair that is not too long or flexible to convey to its rooted end
a strain put upon its free end, is a rudimentary tactual organ; as may
be readily proved by touching one of those growing on the back of the
hand. If, then, a creature has certain hairs so placed that they are
habitually touched by the objects with which it deals, or amid which
it moves, an advantage is likely to accrue if these hairs are modified
in a way that enables them the better to transmit the impressions
derived. Such modified hairs we have in the _vibrissæ_, or, as they are
commonly called, the “whiskers” possessed by Cats and feline animals
generally, as well as by Seals and many Rodents. These hairs are long
enough to reach objects at considerable distances; they are so stiff
that forces applied to their free ends, cause movements of their
imbedded ends; and the sacs containing their imbedded ends being well
covered with nerve-fibres, these developed hairs serve as instruments
of exploration. By constant use of them the animal learns to judge of
the relative positions of objects past which, or towards which, it
is moving. When stealthily approaching prey or stealthily escaping
enemies, such aids to perception are obviously important: indeed their
importance has been proved by the diminished power of self-guidance in
the dark, that results from cutting them off. These, then, are dermal
appendages originally serving the purpose of clothing, but afterwards
differentiated into sense-organs.

That eyes are essentially dermal structures seems scarcely conceivable.
Yet an examination of their rudimentary types, and of their genesis
in creatures that have them well developed, shows us that they really
arise by successive modifications of the double layer composing the
integument. They make their first appearance among the simpler animals
as specks of pigment, covered by portions of epidermis slightly
convex and a little more transparent than that around it. Here their
fundamental community of structure with the skin is easy to trace; and
the formation of them by differentiation of it presents no difficulty.
Not so far in advance of these as much to obscure the relationship,
are the eyes which the Crustaceans possess. In every fishmonger’s shop
we may see that the eyes of a Lobster are carried on pedicles; and
when the Lobster casts its shell, the outer coat of each eye, being
continuous with the epidermis of its pedicle, is thrown off along with
the rest of the exo-skeleton. Beneath the transparent epidermic layer,
there exists a group of eyes of the kind which we see in an insect;
and these, according to a high authority, are inclosed in the dermal
system. Describing the arrangement of the parts, M. Milne Edwards
writes:--“But the most remarkable circumstance is, that the large
cavity within which the whole of these parallel columns, every one of
which is itself a perfect eye, are contained, is closed posteriorly by
a membrane, which appears to be neither more nor less than the middle
tegumentary membrane, pierced for the passage of the optic nerve; so
that the ocular chamber at large results from the separation at a point
of the two external layers of the general envelope.” Thus too is it,
in the main, even with the highly developed eyes of the _Vertebrata_.
“The three pairs of sensory organs appertaining to the higher senses,”
says Prof. Huxley--“the nasal sacs, the eyes, and the ears--arise as
simple cœcal involutions of the external integument of the head of
the embryo. That such is the case, so far as the olfactory sacs are
concerned, is obvious, and it is not difficult to observe that the
lens and the anterior chamber of the eye are produced in a perfectly
similar manner. It is not so easy to see that the labyrinth of the ear
arises in this way, as the sac resulting from the involution of the
integument is small, and remains open but a very short time. But I have
so frequently verified Huschke’s and Remak’s statement that it does so
arise, that I entertain no doubt whatever of the fact. The outer ends
of the olfactory sacs remain open, but those of the ocular and auditory
sacs rapidly close up, and shut off their contents from all direct
communication with the exterior.” That is to say, the eye considered as
an optical apparatus is produced by metamorphoses of the skin: the only
parts of it not thus produced, being the membranes lying between the
sclerotic and the vitreous humour, including those retinal structures
formed in them. All is tegumentary save that which has to appreciate
the impressions which the modified integument concentrates upon it.

Thus, as Prof. Huxley has somewhere pointed out, there is a
substantial parallelism between all the sensory organs in their modes
of development; as there is, too, between their modes of action.
A _vibrissa_ may be taken as their common type. Increased
impressibility by an external stimulus, requires an increased
peripheral expansion of the nervous system on which the stimulus may
fall; and this is secured by an introversion of the integument, forming
a sac on the walls of which a nerve may ramify. That the more extended
sensory area thus constituted may be acted upon, there requires some
apparatus conveying to it from without the appropriate stimulus; and
in the case of the _vibrissa_, this apparatus is the epidermic
growth which, under the form of a hair, protrudes from the sac. And
that the greatest sensitiveness may be obtained, the external action
must be exaggerated or multiplied by the apparatus which conveys it
to the recipient nerve; as, in the case of the _vibrissa_, it is
by the development of a hair into an elastic lever, that transforms
the slight force acting through considerable space on its exposed end,
into a greater force acting through a smaller space at its rooted end.
Similarly with the organs of the higher senses. In a rudimentary eye,
the slightly modified sense cell has but a rudimentary nerve to take
cognizance of the impression; and to concentrate the impression upon
it, there is nothing beyond a thickening of the epidermis into a
lens-shape. But the developed eye shows us a termination of the nerve
greatly expanded and divided to receive the external stimulus. It shows
us an introverted portion of the integument containing the apparatus
by which the external stimulus is conveyed to the recipient nerve. The
structure developed in this sac not only conveys the stimulus, but
also, like its homologue, concentrates it; and in the one case as in
the other, the structure which does this is an epidermic growth from
the bottom of the sac. Even with the ear it is the same. Again we have
an introverted portion of the integument, on the walls of which the
nerve is distributed in the primitive ear. The otolithes contained
in the sac thus formed, are bodies which are set in motion by the
vibrations of the surrounding water, and convey these vibrations in an
exaggerated form to the nerves. And though it is not alleged that these
otolithes are developed from the epidermic lining of the chamber, yet
as, if not so developed, they are concretions from the contents of an
epidermic sac, they must still be regarded as epidermic products.

Whether these differentiations are due wholly to indirect
equilibration, or whether direct equilibration has had a share in
working them, are questions that must be left open. Possibly a short
hair so placed on a mammal’s face as to be very often touched, may,
by conveying excitations to the nerves and vessels at its root, cause
extra growth of the bulb and its appendages, and so the development of
a _vibrissa_ may be furthered. Possibly, too, the light itself, to
which the tissues of some inferior animals are everywhere sensitive,
may aid in setting up certain of the modifications by which the nervous
parts of visual organs are formed: producing, as it must, the most
powerful effects at those points on the surface which the movements of
the animal expose to the greatest and most frequent contrasts of light
and shade; and propagating from those points currents of molecular
change through the organism. But it seems clear that the complexities
of the sensory organs are not thus explicable. They must have arisen by
the natural selection of favourable variations.


§ 296. A group of facts, serving to elucidate those put together in
the several foregoing sections, has to be added. I have reserved
this group to the last, partly because it is transitional--links the
differentiations of the literally outer tissues with those of the truly
inner tissues. Though physically internal, the mucous coat of the
alimentary canal has a _quasi_-externality from a physiological
point of view. As was pointed out in the last chapter, the skin and
the assimilating surface have this in common, that they come in direct
contact with matters not belonging to the organism; and we saw that
along with this community of relation to alien substances, there is a
certain community of structure and development. The like holds with the
linings of all internal cavities and canals that have external openings.

The transition from the literally outer tissues to those tissues
which are intermediate between them and the truly inner tissues,
is visible at all the orifices of the body; where skin and mucous
membrane are continuous, and the one passes insensibly into the
other. This visible continuity is associated not simply with a great
degree of morphological continuity, but also with a great degree of
physiological continuity. That is to say, these literally outer and
_quasi_-outer layers are capable of rapidly assuming one another’s
structures and functions when subject to one another’s conditions.
Mucous surfaces, normally kept covered, become skin-like if exposed
to the air; but resume more or less fully their normal characters
when restored to their normal positions. These are truths familiar to
pathologists. They continually meet with proofs that permanent eversion
of the mucous membrane, even where it is by prolapse of a part deeply
seated within the body, is followed by an adaptation eventually almost
complete: originally moist, tender to the touch, and irritated by the
air, the surface gradually becomes covered with a thick, dry cuticle;
and is then scarcely more sensitive than ordinary integument.

Whether this equilibration between new outer forces and reactive inner
forces, which is thus directly produced in individuals, is similarly
produced in races, must remain as a question not to be answered in a
positive way. On the one hand, we have the fact that among the higher
animals there are cases of _quasi_-outer tissues which are in
one species habitually ensheathed, while in another species they are
not ensheathed; and that these two tissues, though unquestionably
homologous, differ as much as skin and mucous membrane differ. On the
other hand, there are certain analogous changes of surface, as on the
abdomen of the Hermit-Crab, which give warrant to the supposition
that survival of the fittest is the chief agent in establishing such
differentiations; since the abdomen of a Hermit-Crab, bathed by water
within the shell it occupies, is not exposed to physical conditions
that directly tend to differentiate its surface from the surface of
the thorax. But though in cases like this last, we must assign the
result to the natural selection of variations arising incidentally; we
may, I think, legitimately assign the result to the immediate action
of changed conditions where, as in cases like the first, we see these
producing in the individual, effects of the kinds observed in the race.

However this may be, the force of the general argument remains the
same. In these exchanges of structure and function between the outer
and _quasi_-outer tissues, we get undeniable proof that they are
easily differentiable. And seeing this, we are enabled the more clearly
to see how there have, in course of time, arisen those extreme and
multitudinous differentiations of the outer tissues which have been
glanced at.




                             CHAPTER VIII.

         DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS.


§ 297. The change from the outside of the lips to their inside,
introduces us to a new series of interesting and instructive facts,
joining on to those with which the last chapter closed. They concern
the differentiations of those coats of the alimentary canal which, as
we have seen, are physiologically outer, though physically inner.

These coats are greatly modified at different parts; and their
modifications vary greatly in different animals. In the lower types,
where they compose a simple tube running from end to end of the body,
they are almost uniform in their histological characters; but on
ascending from these types, we find them presenting an increasing
variety of minute structures between their two ends. The argument
will be adequately enforced if we limit ourselves to the leading
modifications they display in some of the higher animals.

The successive parts of the alimentary canal are so placed with respect
to its contents, that the physical and chemical changes undergone by
its contents while passing from one end to the other, inevitably tend
to transform its originally homogeneous surface into a heterogeneous
surface. Clearly, the effect produced on the food at any part of
the canal by trituration, by adding a secretion, or by absorbing
its nutritive matters, implies the delivery of the food into the
next part of the canal in a state more or less unlike its previous
states--implies that the surface with which it now comes in contact
is differently affected by it from the preceding surfaces--implies,
that is, a differentiating action. To use concrete language;--food
that is broken down in the mouth acts on the œsophagus and stomach
in a way unlike that which it would have done had it been swallowed
whole; the masticated food, to which certain solvents or ferments
are added, becomes to the intestine a different substance from that
which it must have otherwise been; and the altered food, resolved
by these additions into its proximate principles, cannot have those
proximate principles absorbed in the next part of the intestine,
without the remoter parts being affected as they would not have been
in the absence of absorption. It is true that in developed alimentary
canals, such as the reasoning here tacitly assumes, these marked
successive differentiations of the food are themselves the results
of pre-established differentiations in the successive parts of the
canal. But it is also true that actions and reactions like those
here so definitely marked, must go on indefinitely in an undeveloped
alimentary canal. If the food is changed at all in the course of its
transit, which it must be if the creature is to live by it, then it
cannot but act dissimilarly on the successive tracts of the alimentary
canal, and cannot but be dissimilarly reacted on by them. Inevitably,
therefore, the uniformity of the surface must lapse into greater or
less multiformity: the differentiation of each part tending ever to
initiate differentiations of other parts.

Not, indeed, that the implied process of direct equilibration can
be regarded as the sole process. Indirect equilibration aids; and,
doubtless, there are some of the modifications which only indirect
equilibration can accomplish. But we have here one unquestionable
cause--a cause that is known to work in individuals, changes of the
kind alleged. Where, for instance, cancerous disease of the œsophagus
so narrows the passage into the stomach as to prevent easy descent of
the food, the œsophagus above the obstruction becomes enlarged into a
kind of pouch; and the inner surface of this pouch begins to secrete
juices that produce in the food a kind of rude digestion. Again,
stricture of the intestine, when it arises gradually, is followed by
hypertrophy of the muscular coat of the intestine above the constricted
part: the ordinary peristaltic movements being insufficient to force
the food forwards, and the lodged food serving as a constant stimulus
to contraction, the muscular fibres, habitually more exercised,
become more bulky. The deduction from general principles being thus
inductively enforced, we cannot, I think, resist the conclusion that
the direct actions and reactions between the food and the alimentary
canal have been largely instrumental in establishing the contrasts
among its parts. And we shall hold this view with the more confidence
on observing how satisfactorily, in pursuance of it, we are enabled to
explain one of the most striking of these differentiations, which we
will take as a type of the class.

The gizzard of a bird is an expanded portion of the alimentary canal,
specially fitted to give the food that trituration which the toothless
mouth of a bird cannot give. Besides having a greatly-developed
muscular coat, this grinding-chamber is lined with a thick, hard
cuticle, capable of bearing the friction of the pebbles swallowed to
serve as grindstones. This differentiation of the mucous coat into a
ridged and tubercled layer of horny matter--a differentiation which,
in the analogous organs of certain _Mollusca_, is carried to the
extent of producing from this membrane cartilaginous plates, and even
teeth--varies in birds of different kinds, according to their food.
It is moderate in birds that feed on flesh and fish, and extreme in
granivorous birds and others that live on hard substances. How does
this immense modification of the alimentary canal originate? In the
stomach of a mammal, the macerating and solvent actions are united
with that triturating action which finishes what the teeth have mainly
done; but in the bird, unable to masticate, these internal functions
are specialized, and while the crop is the macerating chamber, the
gizzard becomes a chamber adapted to triturate more effectually. This
adaptation requires simply an exaggeration of certain structures
and actions which characterize stomachs in general, and, in a less
degree, alimentary canals throughout their whole lengths. The massive
muscles of the gizzard are simply extreme developments of the muscular
tunic, which is already considerably developed over the stomach, and
incloses also the œsophagus and the intestine. The indurated lining of
the gizzard, thickened into horny buttons at the places of severest
pressure, is nothing more than a greatly strengthened and modified
epithelium. And the grinding action of the gizzard is but a specialized
form of that rhythmical contraction by which an ordinary stomach kneads
the contained food, and which in the œsophagus effects the act of
swallowing, while in the intestine it becomes the peristaltic motion.
Allied as the gizzard thus clearly is in structure and action to the
stomach and alimentary canal in general; and capable of being gradually
differentiated from a stomach where a growing habit of swallowing
food unmasticated entails more trituration to be performed before
the food passes the pylorus; the question is--Does this change of
structure arise by direct adaptation? There is warrant for the belief
that it does. Besides such collateral evidence as that mucous membrane
becomes horny on the toothless gums of old people, when subject to
continual rough usage, and that the muscular coat of the intestine
thickens where unusual activity is demanded of it, we have the direct
evidence of experiment. Hunter habituated a sea-gull to feed on grain,
and found that the lining of its gizzard became hardened, while the
gizzard-muscles doubled in thickness. A like change in the diet of a
kite was followed by like results. Clearly, if differentiations so
produced in the individuals of a race under changed habits, are in any
degree inheritable, a structure like a gizzard will originate through
the direct actions and reactions between the food and the alimentary
canal.

Another case--a very interesting one, somewhat allied to this--is
presented by the ruminating animals. Here several dilatations of
the alimentary canal precede the true stomach; and in them large
quantities of unmasticated food are stored, to be afterwards returned
to the mouth and masticated at leisure. What conditions have made
this specialization advantageous? and by what process has it been
established? To both these questions the facts indicate answers
which are not unsatisfactory. [Creatures that obtain their food very
irregularly--now having more than they can consume, and now being for
long periods without any--must, in the first place, be apt, when very
hungry, to eat to the extreme limits of their capacities; and must,
in the second place, profit by peculiarities which enable them to
compensate themselves for long fasts, past and future. A perch which,
when its stomach is full of young frogs, goes on filling its œsophagus
also; or a trout which, rising to the fisherman’s fly, proves when
taken off the hook to be full of worms and insect-larvæ up to the very
mouth, gains by its ability to take in such unusual supplies of food
when it meets with them--obviously thrives better than it would do
could it never eat more than a stomachful. That this ability to feed
greatly in excess of immediate requirement, is one that varies in
individuals of the same race, we see in the marked contrast between
our own powers in this respect, and the powers of uncivilized men;
whose fasting and gorging are to us so astonishing. Carrying with us
these considerations, we shall not be surprised at finding dilatations
of the œsophagus in vultures and eagles, which get their prey at long
intervals in large masses; and we may naturally look for them, too, in
birds like pigeons, which, coming in flocks upon occasional supplies
of grain, individually profit by devouring the greatest quantity in
a given time. Now where the trituration of the food is, as in these
cases, carried on in a lower part of the alimentary canal, nothing
further is required than the storing-chamber; but for a mammal,
having its grinding apparatus in its mouth, to gain by the habit of
hurriedly swallowing unmasticated food, it must also have the habit of
regurgitating the food for subsequent mastication. This correlation of
habits with their answering structures, may, as we shall see, arise in
a very simple way. The starting point of the explanation is a familiar
fact--the fact that indigestion, often resulting from excess of food,
is apt to cause that reversed peristaltic action known as vomiting.
From this we pass to the fact, also within the experience of most
persons, that during slight indigestion the stomach sometimes quietly
regurgitates a small part of its contents as far as the back of the
mouth--giving an unpleasant acquaintance with the taste of the gastric
juices. Exceptional facts of the same class help the argument a step
further. “There are certain individuals who are capable of returning,
at will, a greater or smaller portion of the contents of the digesting
stomach into the cavity of the mouth.... In some of these cases, the
expulsion of the food has required a violent effort. In the majority
it has been easily evoked or suppressed. While in others, it has been
almost uncontrollable; or its non-occurrence at the habitual time
has been followed by a painful feeling of fulness, or by the act of
vomiting.” Here we have a certain physiological action, occasionally
happening in most persons and in some developed into a habit more or
less pronounced: indigestion being the habitual antecedent. Suppose,
then, that gregarious animals, living on innutritive food such as
grass, are subject to a like physiological action, and are capable of
like variations in the degree of it. What will naturally happen? They
wander in herds, now over places where food is scarce and now coming
to places where it is abundant. Some masticate their food completely
before swallowing it, while some masticate it incompletely. If an
oasis, presently bared by their grazing, has not supplied to the whole
herd a full meal, then the individuals which masticate completely
will have had less than those which masticate incompletely--will
not have had enough. Those which masticate incompletely and distend
their stomachs with food difficult to digest, will be liable to these
regurgitations; but if they re-masticate what is thus returned to
the mouth (and we know that animals often eat again what they have
vomited), then the extra quantity of food taken, eventually made
digestible, will yield them more nourishment than is obtained by those
which masticate completely at first. The habit initiated in this
natural way, and aiding survival when food is scarce, will be apt to
cause modifications of the alimentary canal. We know that dilatations
of canals readily arise under habitual distensions. We know that canals
habitually distended become gradually more tolerant of the contained
masses that at first irritated them. And we know that there commonly
take place adaptive modifications of their surfaces. Hence if a habit
of this kind and the structural changes resulting from it, are in
any degree inheritable, it is clear that, increasing in successive
generations, both immediately by the cumulative effect of repetitions
and mediately by survival of the individuals in which they are most
decided, they may go on until they end in the peculiarities which
Ruminants display.


§ 298. There are structures belonging to the same group which cannot,
however, be accounted for in this way. They are the organs that
secrete special products facilitating digestion--the liver, pancreas,
and various smaller glands. All these appendages of the alimentary
canal, large and independent as some of them seem, really arise by
differentiations from its coats. The primordial liver consists of
nothing more than bile-cells scattered along a tract of the intestinal
surface. Accumulation of these bile-cells is accompanied by increased
growth of the surface which bears them--a growth which at first takes
the form of a _cul-de-sac_, having an outside that projects from the
intestine into the peri-visceral cavity. As the mass of bile-cells
becomes greater, there arise secondary lateral cavities opening into
the primary one, and through it into the intestine; until, eventually,
these cavities with their coatings of bile-cells, become ramifying
ducts distributed through the solid mass we know as a liver. How is
this differentiation caused?

Before attempting any answer to this question, it is requisite to
inquire the nature of bile. Is that which the liver throws into
the intestines a waste product of the organic actions? or is it a
secretion aiding digestion? or is it a mixture of these? Modern
investigations imply that it is most likely the last. The liver
is found to have a compound function. Bernard has proved to the
satisfaction of physiologists, that there goes on in it a formation of
glycogen--a substance which is transformed into sugar before it leaves
the liver and is afterwards carried away by the blood to eventually
disappear in the active organs, chiefly the muscles. It is also shown,
experimentally, that there are generated in the liver certain biliary
acids; and by the aid either of these or of some other compounds, it is
clear that bile renders certain materials more absorbable. Its effect
on fat is demonstrable out of the body; and the greatly diminished
absorption of fat from the food when the discharge of bile into the
intestine is prevented, is probably one of the causes of that pining
away which results. But while recognizing the fact that the bile
consists in part of a solvent, or solvents, aiding digestion, there
is abundant evidence that one element of it is an effete product;
and probably this is the primary element. The yellow-green substance
called biliverdine in herbivora and bilirubin in man and carnivora,
which gives its colour to bile, is a product the greater part of which
is normally cast out from the system continually, as is shown by the
contrast between the normal and abnormal colours of fæcal matters, and
as is still more strikingly shown by the effects on the system when
there is a stoppage of the excretion, and an attack of jaundice. Hence
we are warranted in classing biliverdine as a waste product, and we
may fairly infer that the excretion of it is the original function of
the liver.

One further preliminary is requisite. We must for a moment return to
those physico-chemical data set down in the first chapter of this work
(§§ 7–8). We there saw that the complex and large-atomed colloids which
mainly compose living organic matter, have extremely little molecular
mobility; and, consequently, extremely little power of diffusing
themselves. Whereas we saw not only that those absorbed matters,
gaseous and liquid, which further the decomposition of living organic
matter, have very high diffusibilities, but also that the products
of the decomposition are much more diffusible than the components of
living organic matter. And we saw that, as a consequence of this, the
tissues give ready entrance to the substances which decompose them,
and ready exit to the substances into which they are decomposed. Hence
it follows that, under its initial form, uncomplicated by nervous and
other agencies, the escape of effete matters from the organism, is a
physical action parallel to that which goes on among mixed colloids
and crystalloids that are dead or even inorganic. Excretion is a
specialized form of this spontaneous action; and we have to inquire how
the specialization arises.

Two causes conspire to establish it. The first is that these products
of decomposition are diffusible in widely different degrees. While
the carbonic acid and water permeate the tissues with ease in all
directions, and escape more or less from the exposed surfaces, urea,
and other waste substances incapable of being vaporized, cannot escape
thus readily. The second is that the different parts of the body, being
subject to different physical conditions, are from the outset sure
severally to favour the exit of these various products of decomposition
in various degrees. How these causes must have co-operated in
localizing the excretions, we shall see on remembering how they now
co-operate in localizing the separation of morbid materials. The
characteristic substances of gout and rheumatism have their habitual
places of deposit. Tuberculous matter, though it may be present in
various organs, gravitates towards some much more than towards others.
Certain products of disease are habitually got rid of by the skin,
instead of collecting internally. Mostly, these have special parts of
the skin which they affect rather than the rest; and there are those
which, by breaking out symmetrically on the two sides of the body,
show how definitely the places of their excretion are determined by
certain favouring conditions, which corresponding parts may be presumed
to furnish in equal degrees. Further, it is to be observed of these
morbid substances circulating in the blood, that having once commenced
segregating at particular places, they tend to continue segregating
at those places. Assuming, then, as we may fairly do, that this
localization of excretion, which we see continually commencing afresh
with morbid matters, has always gone on with the matters produced by
the waste of the tissues, let us take a further step, and ask how
localizations become fixed. Other things equal, that which from its
physical conditions is a place of least resistance to the exit of
an effete product, will tend to become established as the place of
excretion; since the rapid exit of an effete product will profit the
organism. Other things equal, a place at which the excreted matter
produces least detrimental effect will become the established place. If
at any point the excreted matter produces a beneficial effect, then,
other things equal, survival of the fittest will determine it to this
point. And if facility of escape anywhere goes along with utilization
of the escaping substance, then, other things equal, the excretion will
be there localized still more decisively by survival of the fittest.

Such being the conditions of the problem, let us ask what will happen
with the lining membrane of the alimentary canal. This, physiologically
considered, is an external surface; and matters thrown off from it
make their way out of the body. It is also a surface along which is
moving the food to be digested. Now, among the various waste products
continually escaping from the living tissues, some of the more complex
ones, not very stable in composition, are likely, if added to the food,
to set up changes in it. Such changes may either aid or hinder the
preparation of the food for absorption. If an effete matter, making its
exit through the wall of the intestine, hinders the digestive process,
the enfeeblement and disappearance of individuals in which this
happens, will prevent the intestine from becoming the established place
for its exit. While if it aids the digestive process, the intestine
will, for converse reasons, become more and more the place to which
its exit is limited. Equally manifest is it that if there is one part
of this alimentary canal at which, more than at any other part, the
favourable effect results, this will become the place of excretion.

Thus, then, reverting to the case in question, we may understand how a
product to be cast out, such as biliverdine, if it either directly or
indirectly serves a useful purpose, when poured into a particular part
of the intestine, may lead to the formation of a patch of excreting
cells on its wall; and once this place of excretion having been
established, the development of a liver is simply a question of time
and natural selection.


§ 299. A differentiation of another order occurring in the alimentary
canal, is that by which a part of it is developed into a lateral
chamber or chambers, through which carbonic acid exhales and oxygen
is absorbed. Comparative anatomy and embryology unite in showing
that a lung is formed, just as a liver or other appendage of the
alimentary canal is formed, by the growth of a hollow bud into the
peri-visceral cavity, or space between the alimentary canal and the
wall of the body. The interior of this bud is simply a _cul-de-sac_ of
the alimentary canal, with the mucous lining of which its own mucous
lining is continuous. And the development of this _cul-de-sac_ into
an air-chamber, simple or compound, is merely a great extension of
area in the internal surface of the _cul-de-sac_, along with that
specialization which fits it for excreting and absorbing substances
different from those which other parts of the mucous surface excrete
and absorb. These lateral air-chambers, universal among the higher
_Vertebrata_ and very general among the lower, and everywhere attached
to the alimentary canal between the mouth and the stomach, have not
in all cases the respiratory function. In most fishes that have them
they are what we know as swim-bladders. In some fishes the cavities of
these swim-bladders are completely shut off from the alimentary canal:
nevertheless showing, by the communications which they have with it
during the embryonic stages, that they are originally _diverticula_
from it. In other fishes there is a permanent _ductus pneumaticus_,
uniting the cavity of the swim-bladder with that of the gullet: the
function, however, being still not respiratory in an appreciable
degree, if at all. But in certain still extant representatives of the
sauroid fishes, as the _Lepidosteus_, the air-bladder is “divided into
two sacs that possess a cellular structure,” and “the trachea which
proceeds from it opens high up in the throat, and is surrounded with
a glottis.” In the _Amphibia_ the corresponding organs are chambers
over the surfaces of which there are saccular depressions, indicating a
transition towards the air-cells characterizing lungs; and accompanying
this advance we see, as in the common _Triton_, the habit of coming up
to the surface and taking down a fresh supply of air in place of that
discharged.

How are the internal air-chambers, respiratory or nonrespiratory,
developed? Upwards from the amphibian stage, in which they are
partially refilled at long intervals, there is no difficulty in
understanding how, by infinitesimal steps, they pass into complex
and ever-moving lungs. But how is the differentiation that produces
them initiated? How comes a portion of the internal surface to be
specialized for converse with a medium to which it is not naturally
exposed? The problem appears a difficult one; but there is a not
unsatisfactory solution of it.

When many gold-fish are kept in a small aquarium, as with thoughtless
cruelty they frequently are, they swim close to the surface, so as to
breathe that water which is from instant to instant absorbing fresh
oxygen. In doing this they often put their mouths partly above the
surface, so that in closing them they take in bubbles of air; and
sometimes they may be seen to continue doing this--the relief due to
the slight extra aëration of blood so secured, being the stimulus
to continue. Air thus taken in may be detained. If a fish that has
taken in a bubble turns its head downwards, the bubble will ascend
to the back of its mouth, and there lodge; and coming within reach
of the contractions of the œsophagus, it may be swallowed. If, then,
among fish thus naturally led upon occasion to take in air-bubbles,
there are any having slight differences in the alimentary canal that
facilitate lodgment of the air, or slight nervous differences such as
in human beings cause an accidental action to become “a trick,” it
must happen that if an advantage accrues from the habitual detention
of air-bubbles, those individuals most apt to detain them will,
other things equal, be more likely than the rest to survive; and by
the survival of descendants inheriting their peculiarities in the
greatest degrees, and increasing them, an established structure and an
established habit may arise. And that they do in some way arise we have
proof. The common Loach swallows air, which it afterwards discharges
loaded with carbonic acid.

From air thus swallowed the advantages that may be derived are of two
kinds. In the first place, the fish is made specifically lighter, and
the muscular effort needed to keep it from sinking is diminished--or,
indeed, if the bubble is of the right size, is altogether saved.
The contrast between the movements of a Goby, which, after swimming
up towards the surface, falls rapidly to the bottom on ceasing its
exertions, and the movements of a Trout, which remains suspended just
balancing itself by slight undulations of its fins, shows how great
an economy results from an internal float, to fishes which seek their
food in mid-water or at the surface. Hence the habit of swallowing
air having been initiated in the way described, we see why natural
selection will, in certain fishes, aid modifications of the alimentary
canal favouring its lodgment--modifications constituting air-sacs.
In the second place, while from air thus lodged in air-sacs thus
developed, the advantage will be that of flotation only if the air
is infrequently changed or never changed, the advantage will be that
of supplementary respiration if the air-sacs are from time to time
partially emptied and refilled. The requirements of the animal will
determine which of the two functions predominates. Let us glance at the
different sets of conditions under which these divergent modifications
may be expected to arise.

The respiratory development is not likely to take place in fishes
that inhabit seas or rivers in which the supply of aërated water
never fails: there is no obvious reason why the established branchial
respiration should be replaced by a pulmonic respiration. Indeed, if
a fish’s branchial respiration is adequate to its needs, a loss would
result from the effort of coming to the surface for air; especially
during those first stages of pulmonic development when the extra
aëration achieved was but small. Hence in fishes so circumstanced,
the air-chambers arising in the way described would naturally become
specialized mainly or wholly into floats. Their contained air being
infrequently changed, no advantage would arise from the development
of vascular plexuses over their surfaces; nothing would be gained
by keeping open the communication between them and the alimentary
canal; and there might thus eventually result closed chambers the
gaseous contents of which, instead of being obtained from without,
were secreted from their walls, as gases often are from mucous
membranes. Contrariwise, aquatic vertebrates in which the swallowing of
air-bubbles, becoming habitual, had led to the formation of sacs that
lodged the bubbles; and which continued to inhabit waters not always
supplying them with sufficient oxygen, might be expected to have the
sacs further developed, and the practice of changing the contained
air made regular, if either of two advantages resulted--either the
advantage of being able to live in old habitats that had become
untenable without this modification, or the advantage of being able to
occupy new habitats. Now it is just where these advantages are gained
that we see the pulmonic respiration coming in aid of the branchial
respiration, and in various degrees replacing it. Shallow waters are
liable to three changes which conspire to make this supplementary
respiration beneficial. The summer’s sun heats them, and raising the
temperatures of the animals they contain, accelerates the circulation
in these animals, exalts their functional activities, increases the
production of carbonic acid, and thus makes aëration of the blood more
needful than usual. Meanwhile the heated water, instead of yielding
to the highly carbonized blood brought to the branchiæ the usual
quantity of oxygen, yields less than usual; for as the heat of the
water increases, the quantity of air it contains diminishes. And this
greater demand for oxygen joined with smaller supply, pushed to an
extreme where the water is nearly all evaporated, is at last still
more intensely felt in consequence of the excess of carbonic acid
discharged by the numerous creatures congregated in the muddy puddles
that remain. Here, then, it is, that the habit of taking in air-bubbles
is likely to become established, and the organs for utilizing them
developed; and here it is, accordingly, that we find all stages of the
transition to aërial respiration. The Loach before-mentioned, which
swallows air, frequents small waters liable to be considerably warmed.
The _Amphipnous Cuchia_, an anomalous eel-shaped fish, which has
vascular air-sacs opening out at the back of the mouth, “is generally
found lurking in holes and crevices, on the muddy banks of marshes or
slow-moving rivers”; and though its air-sacs are not morphological
equivalents of those above described, yet they equally well illustrate
the relation between such organs and the environing condition. Still
more significant is the fact that the _Lepidosiren_, or “mudfish” as it
is called from its habits, though it is a true fish nevertheless has
lungs. But it is among the _Amphibia_ that we see most conspicuously
this relation between the development of air-breathing organs, and
the peculiarities of the habitats. Pools, more or less dissipated
annually, and so rendered uninhabitable by most fishes, are very
generally peopled by these transitional types. Just as we see, too,
that in various climates and in various kinds of shallow waters, the
supplementary aërial respiration is needful in different degrees; so
do we find among the _Amphibia_ many stages in the substitution of the
one respiration for the other. The facts, then, are such as give to the
hypothesis a _vraisemblance_ greater than could have been expected.

The relative effects of direct and indirect equilibration in
establishing this further heterogeneity, must, as in many other
cases, remain undecided. The habit of taking in bubbles is scarcely
interpretable as a result of spontaneous variation: we must regard it
as arising accidentally during the effort to obtain the most aërated
water; as being persevered in because of the relief obtained; and as
growing by repetition into a tendency bequeathed to offspring, and
by them, or some of them, increased and transmitted. The formation
of the first slight modifications of the alimentary canal favouring
the lodgment of bubbles, is not to be thus explained. Some favourable
variation in the shape of the passage must here have been the initial
step. But the gradual increase of this structural modification by the
survival of individuals in which it is carried furthest, will, I think,
be all along aided by immediate adaptation. The part of the alimentary
canal previously kept from the air, but now habitually in contact with
the air, must be in some degree modified by the action of the air;
and the directly-produced modification, increasing in the individual
and in successive individuals, cannot cease until there is a complete
balance between the actions of the changed agency and the changed
tissue.


§ 300. We come now to differentiations among the truly inner
tissues--the tissues which have direct converse neither with the
environment nor with the foreign substances taken into the organism
from the environment. These, speaking broadly, are the tissues
which lie between the double layer forming the integument with its
appendages, and the double layer forming the alimentary canal with
its _diverticula_. We will take first the differentiation which
produces the vascular system.

Certain forces producing and aiding distribution of liquids in animals,
come into play before any vascular system exists; and continue to
further circulation after the development of a vascular system. The
first of these is osmotic exchange, acting locally and having an
indirect general action; the second is local variation of pressure,
which movement of the body throws on the tissues and their contained
liquids. A few words are needed in elucidation of each. If in any
creature, however simple, different changes are going on in parts
that are differently conditioned--if, as in a _Hydra_, one surface
is exposed to the surrounding medium while the other surface is
exposed to dissolved food; then between the unlike liquids which the
dissimilarly-placed parts contain, osmotic currents must arise; and
a movement of liquid through the intermediate tissue must go on as
long as an unlikeness between the liquids is kept up. This primary
cause of re-distribution remains one of the causes of re-distribution
in every more-developed organism: the passage of matters into and
out of the capillaries is everywhere thus set up. And obviously in
producing these local currents, osmose must also indirectly produce
general currents, or aid them if otherwise produced. In the absence
of a pumping organ, this force is probably an important aid to that
movement of the nutritive liquids which the functions set up. How
the second cause--the changes of internal pressure which an animal’s
movements produce--furthers circulation, will be sufficiently manifest.
That parts which are bent or strained necessarily have their contained
vessels squeezed, has been shown (§ 281); and whether the bend or
strain is caused, as in a plant, by an external force, or, as usually
in an animal, by an internal force, there must be a thrusting of
liquids towards places of least resistance--commonly places of greatest
consumption. This which in animals without hearts is a main agent of
circulation, continues to further it very considerably even among the
highest animals. In these the effect becomes as it were systematized.
The valves in the veins necessitate perpetual propulsions towards the
heart.

Even in such simple types as the _Hydrozoa_, cavities in the tissues
faintly indicate a structure which facilitates the transfer of
nutritive matters. These cavities become reservoirs filled with the
plasma that slowly oozes through the substance of the body; and every
movement of the animal, accompanied as it must be by changed pressures
and tensions on these reservoirs, tends here to fill them and there to
squeeze out their contents in that or the other direction--possibly
aiding to produce, by union of several cavities, those lacunæ or
irregular canals which the body in some cases presents.

Irregular canals of this kind, not lined with any membranes but being
simply cavities running through the flesh, mainly constitute the
vascular system in _Polyzoa_ and _Brachiopoda_ and some _Mollusca_.
Though the central parts of a vascular system are rudely developed, yet
its peripheral parts consist of sinuses permeating the tissues. The
higher orders of _Mollusca_ have a more-developed system of vessels or
arteries, which run into the substance of the body and end in lacunæ or
simple fissures. This ending in lacunæ takes place at various distances
from the vascular centre. In some genera the arterial structure is
carried to the periphery of the blood-system, while in others it stops
short midway. Throughout most orders of the _Mollusca_ the back
current of blood continues to be carried by channels of the original
kind: there are no true veins, but the blood having been delivered
into the tissues, finds its way back to the peri-visceral cavity
through inosculating sinuses. Among the Cephalopods, however, the
afferent blood-canals, as well as the efferent ones, acquire distinct
walls. On putting together these facts, we may conceive pretty clearly
the stages of vascular development. From the original reservoir of
nutritive liquid between the alimentary canal and the wall of the body,
a portion partially shut off becomes a contractile vessel; and by its
actions there is produced a more rapid transfer of the nutritive liquid
than was originally produced by the motions of the animal. Clearly,
the extension of this contractile tube and the development from it
of branches running hither and thither into the tissues, must, by
defining the channels of blood throughout a part of its course, render
its distribution more regular and active. As fast as this centrifugal
growth advances, so fast are the efferent currents of blood, prevented
from escaping laterally, obliged to move from the centre towards
the circumference; and so fast also does the less developed set of
channels become, of necessity, occupied by afferent currents. When, by
a parallel increase of definiteness, the lacunæ and irregular sinuses
through which the afferent currents pass, become transformed into
veins, the accompanying disappearance of all stagnant or slow-moving
collections of blood, implies a further improvement in the circulation.

By what agency is effected this differentiation of a definite vascular
system? No sufficient reply is obvious. The genesis of the primordial
heart is not comprehensible as a result of direct equilibration, and we
cannot readily see our way to it as a result of indirect equilibration;
for it is difficult to imagine what favourable variation natural
selection could have seized hold of to produce such a structure. A
contractile tube that aided the distribution of nutritive liquid,
having been once established, survival of the fittest would suffice
for its gradual extension and its successive modifications. But what
were the early stages of the contractile tube, while it was yet not
sufficiently formed to help circulation, and while it must nevertheless
have had some advantage without which no selective process could go
on? The question seems insoluble. To another part of the question,
however, an answer may be ventured. If we ask the origin of these
ramifying channels which, first appearing as simple lacunæ, eventually
become vessels having definite walls, a reply admitting of considerable
justification, is, that the currents of nutritive liquid forced and
drawn hither and thither through the tissues, themselves initiate these
channels. We know that streams running over and through solid and
quasi-solid inorganic matter, tend to excavate definite courses. We saw
reason for concluding that the development of sap-channels in plants
conforms to this general principle. May we not then suspect that the
nutritive liquid contained in the tissue of a simple animal, made to
ooze now in this direction and now in that by the changes of pressure
which the animal’s movements cause, comes to have certain lines along
which it is thrust backwards and forwards more than along other lines;
and must by repeated passings make these more and more permeable until
they become lacunæ? Such actions will inevitably go on; and such
actions appear competent to produce some, at least, of the observed
effects. The leading facts which indicate that this is a part-cause of
vascular development are these.

Growths normally recurring in certain places at certain intervals,
are accompanied by local formations of blood-vessels. The periodic
maturation of ova among the _Mammalia_ supplies an instance. Through
the stroma of an ovarium are distributed innumerable minute vesicles,
which, in their early stages, are microscopic. Of these, severally
contained in their minute ovi-sacs, any one may develop: the
determining cause being probably some slight excess of nutrition. When
the development is becoming rapid, the capillaries of the neighbouring
stroma increase and form a plexus on the walls of the ovi-sac. Now
since there is no typical distribution of the developing ova; and since
the increase of an ovum to a certain size precedes the increase of
vascularity round it; we can scarcely help concluding that the setting
up of currents towards the point of growth determines the formation of
the blood-vessels. It may be that having once commenced, this local
vascular structure completes itself in a typical manner; but it seems
clear that this greater development of blood-vessels around the growing
ovum is initiated by the draught towards it. Abnormal growths show
still better this relation of cause and effect. The false membranes
sometimes found in the bronchial tubes in inflammatory diseases, may
perhaps fairly be held abnormal in but a partial sense: it may be said
that their vascular systems are formed after the type of the membranes
to which they are akin. But this can scarcely be said of the morbid
growths classed as malignant. The blood-vessels in an encephaloid
cancer, are led to enlarge and ramify, often to an immense extent, by
the unfolding of the morbid mass to which they carry blood. Alien as
is the structure as a whole to the type of the organism; and alien in
great measure as is its tissue to the tissue on which it is seated;
it nevertheless happens that the growth of the alien tissue and
accompanying abstraction of materials from the blood-vessels, determine
a corresponding growth of these blood-vessels. Unless, then, we say
that there is a providentially-created type of vascular structure for
each kind of morbid growth (and even this would not much help us, since
the vascular structure has no constancy within the limits of each
kind), we are compelled to admit that in some way or other the currents
of blood are here directly instrumental in forming their own channels.
One more piece of evidence, before cited as exemplifying adaptation (§
67), may be called to mind. When any main channel for blood, leading
to or from a certain part of the body, has been rendered impervious,
others among the channels leading to or from this same part, enlarge to
the extent requisite for fulfilling the extra function that falls upon
them: the enlargement being caused, as we must infer, by the increase
of the currents carried.

Here, then, are facts warranting inductively the deduction above drawn.
It is true that we are left in the dark respecting the complexities
of the process. How the channels for blood come to have limiting
membranes, and many of them muscular coats, the hypothesis does not
help us to say. But the evidence assigned goes far to warrant the
belief that vascular development is initiated by direct equilibration;
though indirect equilibration may have had the larger share in
establishing the structures which distinguish finished vascular systems.


§ 301. Of the inner tissues which remain let us next take bone. In what
manner is differentiated this dense substance serving in most cases for
internal support?

When considering the vertebrate skeleton under its morphological aspect
(§ 256), it was pointed out that the formation of dense tissues,
internal as well as external, is, in some cases at least, brought
about by the mechanical forces to be resisted. Through what process
it is brought about we could not then stay to inquire: this question
being not morphological but physiological. Answers to some kindred
questions have since been attempted. Certain actions to which the
internal dense tissues of plants may be ascribed, have been indicated;
and more recently, analogous actions have been assigned as causes of
some external dense tissues of animals. We have now to ask whether
actions of the same nature have produced these internal dense tissues
of animals.

The problem is an involved one. Bones have more than one stage. They
are membranous or cartilaginous before they become osseous; and their
successive component substances so far differ that the effects
of mechanical actions upon them differ. And having to deal with
transitional states in which bone is formed of mixed tissues, having
unlike physical properties and unlike minute structures, the effects of
strains become too complicated to follow with precision. Anything in
the way of interpretation must therefore be regarded as tentative. If
analysis and comparison show that the phenomena are not inconsistent
with the hypothesis of mechanical genesis, it is as much as can be
expected. Let us first observe more nearly the mechanical conditions to
which bones are subject.

The endo-skeleton of a mammal with the muscles and ligaments holding
it together, may be rudely compared to a structure built up of struts
and ties; of which, speaking generally, the struts bear the pressures
and the ties bear the tensions. The framework of an ordinary iron
roof will give an idea of the functions of these two elements, and of
the mechanical characters required by them. Such a framework consists
partly of pieces which have each to bear a thrust in the direction of
its length, and partly of pieces which have each to bear a pull in the
direction of its length; and these struts and ties are differently
formed to adapt them to these different strains. Further, it should
be remarked that though the rigidity of the framework depends on the
ties which are flexible, as much as on the struts which are stiff, yet
the ties help to give the rigidity simply by so holding the struts
in position that they cannot escape from the thrusts which fall on
them. Now the like relation holds with a difference among the bones
and muscles: the difference being that here the ties admit of being
lengthened or shortened and the struts of being moved about upon their
joints. The mechanical relations are not altered by this, however.
The actions are of essentially the same kind in an animal that is
standing, or keeping itself in a strained attitude, as in one that is
changing its attitude--the same in so far that we have in each a set
of flexible parts that are pulling and a set of rigid parts that are
resisting. It needs but to remember the sudden collapse and fall which
take place when the muscles are paralyzed, or to remember the inability
of a bare skeleton to support itself, to see that the struts without
the ties cannot suffice. And we have but to think of the formless mass
into which a man would sink when deprived of his bones, to see that
the ties without the struts cannot suffice. To trace the way in which
a particular bone has its particular thrust thrown upon it, may not
always be practicable. Though it is easy to perceive how a flexor or
extensor of the arm causes by its tension a reactive pressure along the
line of the humerus, and is enabled to produce its effect only by the
rigidity of the humerus; yet it is not so easy to perceive how such
bones as those of a horse’s pelvis are similarly acted upon. Still, as
the weight of the hind quarters has to be transferred from the back to
the feet, and must be so transferred through the bones, it is manifest
that though these bones form a very crooked line, the weight must
produce a pressure along the axis of each: the muscles and ligaments
concerned serving here, as in other cases, so to hold the bones that
they bear the pressure instead of being displaced by it. Not forgetting
that many processes of the bones have to bear tensions, we may then
say that generally, though by no means universally, bones are internal
dense masses that have to bear pressures--pressures which in the
cylindrical bones become longitudinal thrusts. Leaving out exceptional
cases, let us consider bones as masses thus circumstanced.

When giving reasons for the belief that the vertebrate skeleton is
mechanically originated, one of the facts put in evidence was, that
in the vertebrate series the transition from the cartilaginous to the
osseous spine begins peripherally (§ 257): each vertebra being at first
a ring of bone surrounding a mass of cartilage. And it was pointed
out that this peripheral ossification is ossification at the region
of greatest pressures. Now it is not vertebræ only that follow this
course of development. In a cylindrical bone, though it is differently
circumstanced, the places of commencing ossification are still the
places on which the severest stress falls. Let us consider how such a
bone that has to bear a longitudinal pressure is mechanically affected.
If the end of a walking-cane be thrust with force against the ground,
the cane bends; and partially resuming its straightness when relieved,
again bends, usually towards the same side, when the thrust is renewed.
A bend so caused acts on the fibres of the cane in nearly the same
way as does a bend caused by supporting the cane horizontally at its
two ends and suspending a weight from its middle. In either case the
fibres on the convex side are extended and the fibres on the concave
side compressed. Kindred actions occur in a rod that is so thick
as not to yield visibly under the force applied. In the absence of
complete homogeneity of its substance, complete symmetry in its form,
and an application of a force exactly along its axis, there must be
some lateral deflection; and therefore some distribution of tensions
and pressures of the kind indicated. And then, as the fact which here
specially concerns us, we have to note that the strongest tensions
and pressures are borne by the outer layers of fibres. Now the shaft
of a long bone, subject to mechanical actions of this kind, similarly
has its outer layer most strained. In this layer, therefore, on the
mechanical hypothesis, ossification should commence, and here it does
commence--commences, too, midway between the ends, where the bends
produce on the superficial parts their most intense effects. But we
have not in this place simply to observe that ossification commences at
the places of greatest stress, but to ask what causes it to do this.
Can we trace the physical actions which set up this deposit of dense
tissue? It is, I think, possible to indicate a “true cause” that is at
work; though whether it is a sufficient cause may be questioned. We
concluded that in certain other cases, the formation of dense tissue
indirectly results from the alternate squeezing and relaxation of the
vessels running through the part; and the inquiry now to be made is,
whether, in developing bone, the same actions go on in such ways as to
produce the observed effects. At the outset we are met by what seems a
fatal difficulty--cartilage is a non-vascular tissue: this substance
of which unossified bones consist is not permeated by minute canals
carrying nutritive liquid, and cannot, therefore, be a seat of actions
such as those assigned. This apparent difficulty, however, furnishes a
confirmation. For cartilage that is wholly without permeating canals
does not ossify: ossification takes place only at those parts of it
into which the canals penetrate. Hence, we get additional reason for
suspecting that bone-formation is due to the alleged cause; since it
occurs where mechanical strains can produce the actions described, but
does not occur where mechanical strains cannot produce them. Let us
consider more closely what the several factors are. It will suffice
for the argument if we commence with the external vascular layer as
already existing, and consider what will take place in it. Cartilage
is elastic--is somewhat extensible, and spreads out laterally under
pressure, but resumes its form when relieved. How, then, will the
minute channels traversing it in all directions be affected at the
places where it is strained by a bend? Those on the convex side will
be laterally squeezed, in the same way that we saw the sap-vessels on
the convex side of a bent branch are squeezed; and as exudation of the
sap into the adjacent prosenchyma will be caused in the one case, so,
in the other, there will be caused exudation of serum into the adjacent
cartilage: extra nutrition and increase of strength resulting in both
cases. The parallel ceases here, however. In the shoot of a plant, bent
in various directions by the wind, the side which was lately compressed
is now extended; and hence that squeezing of the sap-vessels which
results from extension, suffices to feed and harden the tissue on all
sides of the shoot. But it is not so with a bone. Having yielded on
one side under longitudinal pressure, and resumed as nearly as may be
its previous shape when the pressure is taken off, the bone yields
again towards the same side when again longitudinally pressed. Hence
the substance of its concave side, never rendered convex by a bend
in the opposite direction, would not receive any extra nutrition did
no other action come into play. But if we consider how intermittent
pressures must act on cartilage, we shall see that there will result
extra nutrition of the concave side also. Squeeze between two pieces of
glass a thin bit of caoutchouc which has a hole through it. While the
caoutchouc spreads out away from the centre, it also spreads inwards,
so as partially to close the hole. Everywhere its molecules move away
in directions of least resistance; and for those near the hole, the
direction of least resistance is towards the hole. Let this hole stand
for the transverse section of one of the minute canals or channels
passing through cartilage, and it will be manifest that on the side of
the unossified bone made concave in the way described, the compressed
cartilage will squeeze the canals traversing it; and, in the absence
of perfect homogeneity in the cartilage, the squeeze will cause extra
exudation from the canals into the cartilage. Thus every additional
strain will give to the cartilage it falls upon, an additional supply
of the materials for growth. So that presently the side which, by
yielding more than any other, proves itself to be the weakest, will
cease to be the weakest. What further will happen? Some other side
will yield a little--the bends will take place in some other plane;
and the portions of cartilage on which repeated tensions and pressures
now fall will be strengthened. Thus the rate of nutrition, greatest at
the place where the bending is greatest, and changing as the incidence
of forces changes, will bring about at every point a balance between
the resistances and the strains. Thus, too, there will be determined
that peripheral induration which we see in bones so circumstanced.
As in a shoot we saw that the woody deposit takes place towards the
outside of the cylinder, where, according to the hypothesis, it ought
to take place; so, here, we see that the excess of exudation and
hardening, occurring where the strains are most intense, will form a
cylinder having a dense outside and a porous or hollow inside. These
processes will be essentially the same in bones subject to more complex
mechanical actions, such as sundry of the flat bones and others that
serve as internal fulcra. Be the strains transverse or longitudinal,
be they torsion strains or mixed strains, the outer parts of the
bone will be more affected by them than its inner parts. They will
therefore tend everywhere to produce resisting masses having outer
parts more dense than their inner parts. And by causing most growth
where they are most intense, they will call out reactive forces
adequate to balance them. There are doubtless obstacles in the way
of this interpretation. It may be said that the forces acting on the
outer layers in the manner described, would compress the canals too
little to produce the alleged effects; and if evenly distributed along
the whole lengths of the layers, they would probably do so. But it
needs only to bend a flexible mass and observe the tendency to form
creases on the concave surface, to feel assured that along the surface
of an ossifying bone, the yielding of the tissue when bent will not
be uniform. In the absence of complete homogeneity, the interstitial
yielding will take place at some points more than others, and at one
point above all others. When, at the weakest point--the centre of
commencing ossification--an extra amount of deposit has been caused,
it will cease to be the weakest; and adjacent points, now the weakest,
will become the places of yielding and induration. It may be further
objected that the hypothesis is incompatible with the persistence
of cartilage for so long a time between the epiphysis of bones and
the bony masses which they terminate. But there is the reply that
the places occupied by this cartilage being places at which the bone
lengthens, the non-ossification is in part apparent only--it is rather
that new cartilage is formed as fast as the pre-existing cartilage
ossifies; and there is the further reply that the slowness of the
ultimate ossification of this part, is due to its non-vascularity, and
to mechanical conditions which are unfavourable to its acquirement of
vascularity. Once more, there is the demurrer that in the epiphyses
ossification does not begin at the surface but within the mass of
the cartilage. Explanation of this implies ability to follow out the
mechanical actions in a resilient substance which, like india-rubber,
admits of being distorted in all ways by pressure and recovering its
form, and it seems impossible to say how the more superficial and more
deep-seated canals traversing it will be respectively affected.

Of course it is not meant that this osseous development by direct
equilibration takes place in the individual. Though it is a corollary
from the argument that in each individual the process must be furthered
and modified by the particular actions to which the particular bones
are exposed; yet the leading traits of structure assumed by the bones
are assumed in conformity with the inherited type. This, however, is
no difficulty. The type itself is to be regarded as the accumulated
result of such modifications, transmitted and increased from generation
to generation. The actions above described as taking place in the
bone of an individual, must be understood as producing their total
effect little by little in the corresponding bones of a long series of
individuals. Even if but a small modification can be so wrought in the
individual, yet if such modification, or a part of it, is inheritable,
we may readily understand how, in the course of geologic epochs, the
observed structures may arise in the assigned way.

Here may fitly be added a strong confirmation. If we find cases
where individual bones, subject in exceptional degrees to the
actions described, present in exceptional amounts the modifications
attributed to them, we are greatly helped in understanding how there
may be produced in the race that aggregate of modifications which
the hypothesis implies. Such cases occur in ricketty children. I
am indebted to Mr. Busk for pointing out these abnormal formations
of dense tissue, that are not apparently explicable as results of
mechanical actions and reactions. It was only on tracing out the
processes here at work, that there suggested itself the specific
interpretation of the normal process, as above set forth. When, from
constitutional defect, bones do not ossify with due rapidity, and are
meanwhile subject to the ordinary strains, they become distorted.
Remembering how a mass which has been made to yield in any direction
by a force it cannot withstand, is some little time before it recovers
completely its previous form, and usually, indeed, undergoes what
is called a “permanent set;” it is inferable that when a bone is
repeatedly bent at the same time that the liquid contained in its
canals is poor in the materials for forming dense tissue, there
will not take place a proportionate strengthening of the parts most
strained; and these parts will give way. This happens in rickets. But
this having happened, there goes on what, in teleological language,
we call a remedial process. Supposing the bone to be one commonly
affected--a femur; and supposing a permanent bend to have been caused
in it by the weight of the body; the subsequent result is an unusual
deposition of cartilaginous and osseous matter on the concave side of
the bone. If the bone is represented by a strung bow, then the deposit
occurs at the part represented by the space between the bow and the
string. And thus occurring where its resistance is most effective,
it increases until the approximately-straight piece of bone formed
within the arc, has become strong enough to bear the pressure without
appreciably yielding. Now this direct adaptation, seeming so like a
special provision, and furnishing so remarkable an instance of what,
in medical but unscientific language, is called the _vis medicatrix
naturæ_, is simply a result of the above-described mechanical actions
and reactions, going on under the exceptional conditions. Each time
such a bent bone is subject to a force which again bends it, the
severest compression falls on the substance of its concave side. Each
time, then, the canals running through this part of its substance are
violently squeezed--far more squeezed than they or any other of the
canals would have been, had the bone remained straight. Hence, on every
repetition of the strain, these canals near the concave surface have
their contents forced out in more than normal abundance. The materials
for the formation of tissue are supplied in quantity greater than can
be assimilated by the tissue already formed; and from the excess of
exuded plasma, new tissue arises.[50] A layer of organizable material
accumulates between the concave surface and the periosteum; in this,
according to the ordinary course of tissue-growth, new vessels appear;
and the added layer presently assumes the histological character of the
layer from which it has grown. What next happens? This added layer,
further from the neutral axis than that which has thrown it out, is now
the most severely compressed, and its vessels are the most severely
squeezed. The place of greatest exudation and most rapid deposit
of matter, is therefore transferred to this new layer; and at the
same time that active nutrition increases its density, the excess of
organizable material forms another layer external to it: the successive
layers so added, encroaching on the space between the concave surface
of the bone and the chord of its arc. What limits the encroachment on
this space?--what stops the process of filling it up? The answer to
this question will be manifest when observing that there comes into
play a cause which gradually diminishes the forces falling on each new
layer. For the transverse sectional area is step by step increased; and
an increase of the area over which the weight borne is distributed,
implies a relatively smaller pressure upon each part of it. Further, as
the transverse dimensions of the bone increase, the materials composing
its convex and concave layers, becoming further from the neutral axis,
become better placed for resisting the strains to be borne. So that
both by the increased quantity of dense matter and by its mechanically
more-advantageous position, the bendings of the bone are progressively
decreased. But as they are decreased, each new layer formed on the
concave surface has its substance and its vessels less compressed; and
the resulting growth and induration are rendered less rapid. Evidently,
then, the additions, slowly diminishing, will eventually cease; and
this will happen when the bone no longer bends. That is to say, the
thickening of the bone will reach its limit when there is equilibrium
between the incident forces and the forces which resist them. Here,
indeed, we may trace with great clearness the process of direct
equilibration--may see how an unusual force, falling on the moving
equilibrium of an organism and not overthrowing it, goes on working
modifications until the reaction balances the action.

That, however, which now chiefly concerns us, is to note how this
marked adaptation supports the general argument. Unquestionably bone
is in this case formed under the influence of mechanical stress, and
formed just where it most effectually meets the stress. This result,
not otherwise explained, is explained by the hypothesis above set
forth. And when we see that this special deposit of bone is accounted
for by actions like those to which bone-formation in general is
ascribed, the probability that these are the actions at work becomes
very great.[51]

Of course it is not alleged that osseous structures arise in this
way alone. The bones of the skull and various dermal bones cannot be
thus interpreted. Here the natural selection of favourable variations
appears the only assignable cause--the equilibration is indirect. We
know that ossific deposits now and then occur in tissues where they
are not usually found; and such deposits, originally abnormal, if they
occurred in places where advantages arose from them, might readily be
established and increased by survival of the fittest. Especially might
we expect this to happen when a constitutional tendency to form bone
had been established by actions of the kind described; for it is a
familiar fact that differentiated types of tissue, having once become
elements of an organism, are apt occasionally to arise in unusual
places, and there to repeat all their peculiar histological characters.
And this may possibly be the reason why the bones of the skull, though
not exposed to forces such as those which produce, in other bones,
dense outer layers including less dense interiors, nevertheless repeat
this general trait of bony structure. While, however, it is beyond
doubt that some bones are not due to the direct influence of mechanical
stress, we may, I think, conclude that mechanical stress initiates
bone-formation.


§ 302. What is the origin of nerve? In what way do its properties stand
related to the properties of that protoplasm whence the tissues in
general arise? and in what way is it differentiated from protoplasm
simultaneously with the other tissues? These are profoundly interesting
questions; but questions to which positive answers cannot be expected.
All that can be done is to indicate answers which seem feasible.

That the property specially displayed by nerve, is a property which
protoplasm possesses in a lower degree, is manifest. The sarcode
of a Rhizopod and the substance of an unimpregnated ovum, exhibit
movements that imply a propagation of stimulus from one part of
the mass to another. We have not far to seek for a probable origin
of this phenomenon. There is good reason for ascribing it to the
extreme instability of the organic colloids of which protoplasm
consists. These, in common with colloids in general, assume
different isomeric forms with great facility; and they display not
simply isomerism but polymerism. Further, this readiness to undergo
molecular re-arrangement, habitually shows itself in colloids by
the rapid propagation of the re-arrangement from part to part. As
Prof. Graham has shown, matter in this state often “pectizes” almost
instantaneously--a touch will transform an entire mass. That is to say,
the change of molecular state once set up at one end, spreads to the
other end--there is a progress of a stimulus to change; and this is
what we see in a nerve. So much being understood, let us re-state the
case more completely.

Molecular change, implying as it does motion of molecules, communicates
motion to adjacent molecules; be they of the same kind or of a
different kind. If the adjacent molecules, either of the same kind or
of a different kind, be stable in composition, a temporary increase
of oscillation in them as wholes, or in their parts, may be the only
result; but if they are unstable there are apt to arise changes of
arrangement among them, or among their parts, of more or less permanent
kinds. Especially is this so with the complex molecules which form
colloidal matter, and with the organic colloids above all. Hence it is
to be inferred that a molecular disturbance in any part of a living
animal, set up by either an external or internal agency, will almost
certainly disturb and change some of the surrounding colloids not
originally implicated--will diffuse a wave of change towards other
parts of the organism: a wave which will, in the absence of perfect
homogeneity, travel further in some directions than in others. Let us
ask next what will determine the differences of distance travelled in
different directions. Obviously any molecular agitation spreading from
a centre, will go furthest along routes that offer least resistance.
What routes will these be? Those along which there lie most molecules
that are easily changed by the diffused molecular motion, and which
yet do not take up much molecular motion in assuming their new states.
Molecules which are tolerably stable will not readily propagate the
agitation; for they will absorb it in the increase of their own
oscillations, instead of passing it on. Molecules which are unstable
but which, in assuming isomeric forms, absorb motion, will not readily
propagate it; since it will disappear in working the changes in them.
But unstable molecules which, in being isomerically transformed, do not
absorb motion, and still more those which, in being so transformed,
give out motion, will readily propagate any molecular agitation; since
they will pass on the impulse either undiminished, or increased, to
adjacent molecules. If then we assume, as we are not only warranted
in doing but are obliged to do, that protoplasm contains two or more
colloids, either mingled or feebly combined (since it cannot consist of
simple albumen or fibrin or casein, or any allied proximate principle);
it may be concluded that any molecular agitation set up by what we call
a stimulus, will diffuse itself further along some lines than along
others, if the components of the protoplasm are not quite homogeneously
dispersed, and if some of them are isomerically transformed more
easily, or with less expenditure of motion, than others; and it will
especially travel along spaces occupied chiefly by those molecules
which give out molecular motion during their metamorphoses, if there
should be any such. But now let us ask what structural effects
will be wrought along a tract traversed by this wave of molecular
disturbance. As is shown by those transformations which so rapidly
propagate themselves through colloids, molecules that have undergone a
certain change of form, are apt to communicate a like change of form
to adjacent molecules of the same kind--the impact of each overthrow
is passed on and produces another overthrow. Probably the proneness
towards isochronism of molecular movements necessitates this. If any
molecule has had its components re-arranged, and their oscillations
consequently altered, there result movements not concordant with the
movements in adjacent untransformed molecules, but which, impressing
themselves on the parts of such untransformed molecules, tend to
generate in them concordant movements--tend, that is, to produce
the re-arrangements involved by these concordant movements. Is this
action limited to strictly isomeric substances? or may it extend to
substances that are closely-allied? If along with the molecules of
a compound colloid there are mingled those of some kindred colloid;
or if with the molecules of this compound colloid there are mingled
the components out of which other such molecules may be formed; then
there arises the question--does the same influence which tends to
propagate the isomeric transformations, tend also to form new molecules
of the same kind out of the adjacent components? There is reason to
suspect that it does. Already when treating of the nutrition of parts
(§ 64), it was pointed out that we are obliged to recognize a power
possessed by each tissue to build up, out of the materials brought
to it, molecules of the same type as those of which it is formed.
This building up of like molecules seems explicable as caused by the
tendency of the new components which the blood supplies, to acquire
movements isochronous with those of the like components in the tissue;
which they can do only by uniting into like compound molecules.
Necessarily they must gravitate towards a state of equilibrium; such
state of equilibrium--moving equilibrium of course--must be one in
which they oscillate in the same times with neighbouring molecules;
and so to oscillate they must fall into groups identical with the
groups around them. If this be a general principle of tissue-growth
and repair, we may conclude that it will apply in the case before
us. A wave of molecular disturbance passing along a tract of mingled
colloids closely-allied in composition, and isomerically transforming
the molecules of one of them, will be apt at the same time to form
some new molecules of the same type, at any place where there exist
the proximate components, either uncombined or feebly combined in some
not very different way. And this will be most likely to occur where
the molecules of the colloid that are undergoing the isomeric change,
predominate, but have scattered through them the other molecules out of
which they may be formed, either by composition or modification. That
is to say, a wave of molecular disturbance diffused from a centre, and
travelling furthest along a line where lie most molecules that can be
isomerically transformed with facility, will be likely at the same time
to further differentiate this line, and make it more characterized than
before by the easy-transformability of its molecules. One additional
step, and the interpretation is reached. Analogy shows it to be not
improbable that these organic colloids, isomerically transformed by
slight molecular impact or increase of molecular motion, will some of
them resume their previous molecular structures after the disturbance
has passed. We know that what are stable molecular arrangements under
one degree of molecular agitation, are not stable under another degree;
and there is evidence that re-arrangements of an inconspicuous kind
are occasionally brought about by very slight changes of molecular
agitation. Water supplies a clear case. Prof. Graham infers that water
undergoes a molecular re-arrangement at about 32°--that ice has a
colloid form as well as a crystalloid form, dependent on temperature.
Send through it an extra wave of the molecular agitation we call heat,
and its molecules aggregate in one way. Let the wave die away, and its
molecules resume their previous mode of aggregation. And obviously
such transformations may be repeated backwards and forwards within
narrow limits of temperature. Now among the extremely unstable organic
colloids, such a phenomenon is far more likely to happen. Suppose,
then, that the nerve-colloid is one of which the molecules are changed
in form by a passing wave of extra agitation, but resume their previous
form when the wave has passed: the previous form being the most stable
under the conditions which then recur. What follows? It follows that
these molecules will be ready again to undergo isomeric transformation
when there again occurs the stimulus; will, as before, propagate the
transformation most along the tract where such molecules are most
abundant; will, as before, tend to form new molecules of their own
type; will, as before, make the line along which they lie one of easier
transfer for the molecular agitation. Every repetition will help to
increase, to integrate, to define more completely, the course of the
escaping molecular motion--extending its remoter part while it makes
its nearer part more permeable--will help, that is, to form a line of
discharge, a line for conducting impressions, a nerve.

Such seems to me a not unfair series of deductions from the known
habitudes of colloids in general and the organic colloids in
particular. And I think that the implied nature and properties of nerve
correspond better with the observed phenomena than do the nature and
properties implied by other hypotheses. Of course the speculation as
it here stands is but tentative, and leaves much unexplained. It gives
no obvious reply to the questions--what causes the formation of nerves
in directions adapted to the needs? what determines their appropriate
connexions?--questions, however, to which, when we come to deal with
physiological integration, we may find not unsatisfactory answers.
Moreover it says nothing about the genesis of ganglia. A ganglion,
it is clear, must consist of a colloidal matter equally unstable, or
still more unstable, which, when disturbed, falls into some different
molecular arrangement, perhaps chemically simpler, and gives out in
so doing a large amount of molecular motion--serves as a reservoir of
molecular motion which may be suddenly discharged along an efferent
nerve or nerves, when excitement of an afferent nerve has disengaged
it. How such a structure as this results, the hypothesis does not show.
But admitting these shortcomings it may still be held that we are, in
the way pointed out, enabled to form some idea of the actions by which
nervous tissue is differentiated.


§ 303. A speculation akin to, and continuous with, the last,
is suggested by an inquiry into the origin of muscular tissue.
Contractility as well as irritability is a property of protoplasm
or sarcode; and, as before suggested (§ 22), is not improbably
due to isomeric change in one or more of its component colloids.
It is a feasible supposition that of the several isomeric changes
simultaneously set up among these component colloids, some may be
accompanied by change of bulk and some not. Clearly the isomeric
change undergone by the colloid which we suppose to form nerve, must
be one not accompanied by appreciable change of bulk; since change of
bulk implies “internal work,” as physicists term it, and therefore
expenditure of force. Conversely, the colloid out of which muscle
originates, may be one that readily passes into an isomeric state
in which it occupies less space: the molecular disturbance causing
this contraction being communicated to it from adjacent portions of
nerve-substance that are molecularly disturbed; or being otherwise
communicated to it by direct mechanical or chemical stimuli: as happens
where nerves do not exist, or where their influence has been cut off.
This interpretation seems, indeed, to be directly at variance with
the fact that muscle does not diminish in bulk during contraction
but merely changes its shape. That which we see take place with the
muscle as a whole, is said also to take place with each fibre--while
it shortens it also broadens. There is, however, a possible solution
of this difficulty. A contracting colloid yields up its water; and
the contracted colloid _plus_ the free water, may have the same bulk
as before though the colloid has less. If it be replied that in this
case the water should become visible between the substance of the
fibre and its sarcolemma or sheath, it may be rejoined that this
is not necessary--it may be deposited interstitially. Possibly the
striated structure is one that facilitates its exudation and subsequent
re-absorption; and to this may be due the superiority of striated
muscle in rapidity of contraction. Granting the speculative character
of this interpretation, let us see how far it agrees with the facts.
If the actions are as here supposed, the contracted or more integrated
state of the muscular colloid will be that which it tends continually
to assume--that into which it has an increasing aptitude to pass when
artificial paralysis has been produced, as shown by Dr. Norris--that
into which it lapses completely in _rigor mortis_. The sensible motion
generated by the contraction can arise only from the transformation
of insensible motion. This insensible motion suddenly yielded up by
a contracting mass, implies the fall of its component molecules into
more stable arrangements. And there can be no such fall unless the
previous arrangement is unstable. From this point of view, too, it is
possible to see how the hydro-carbons and carbo-hydrates consumed in
muscular action, may produce their effects. For these non-nitrogenous
elements of food, when consumed in the tissues, give out large amounts
of molecular motion. They do this in presence of the muscular colloids
which have lost molecular motion during their fall in the stable or
contracted state. From the molecular motion they give out, may be
restored the molecular motion lost by the contracted colloids; and
these contracted colloids may thus have their molecules raised to that
unstable state from which, again falling, they can again generate
mechanical motion.

This conception of the nature and mode of action of muscle, while it
is suggested by known properties of colloidal matter and conforms to
the recent conclusions of organic chemistry and molecular physics,
establishes a comprehensible relation between the vital actions of
the lower and the higher animals. If we contemplate the movements
of cilia, of a Rhizopod’s pseudopodia, of a Polype’s body, or of the
long pendant tentacles of a _Medusa_, we shall see great congruity
between them and this hypothesis. Bearing in mind that the contractile
substance of developed muscle is affected not by nervous influence
only, but, where nervous influence is destroyed, is made to contract
by mechanical disturbance and chemical action, we may infer that
it does not differ intrinsically from the primordial contractile
substance which, in the lowest animals, changes its bulk under other
stimuli than the nervous. We shall see significance in the fact
ascertained by Dr. Ransom, that various agents which excite and arrest
nervo-muscular movements in developed animals, excite and arrest the
protoplasmic movements in ova. We shall understand how tissues not yet
differentiated into muscle and nerve, have this joint irritability and
contractility; how muscle and nerve may arise by the segregation of
their mingled colloids, the one of which, not appreciably altering its
bulk during isomeric change, readily propagates molecular disturbance,
while the other, contracting when isomerically changed, less readily
passes on the molecular disturbance; and how, by this differentiation
and integration of the conducting and the contracting colloids, the one
ramifying through the other, it becomes possible for a whole mass to
contract suddenly, instead of contracting gradually, as it does when
undifferentiated.

The question remaining to be asked is--What causes the specialization
of contractile substance?--What causes the growth of colloid masses
which monopolize this contractility, and leave kindred colloids to
monopolize other properties? Has natural selection gradually localized
and increased the primordial muscular substance? or has the frequent
recurrence of irritations and consequent contractions at particular
parts done it? We have, I think, reason to conclude that direct
equilibration rather than indirect equilibration has been chiefly
operative. The reasoning that was used in the case of nerve applies
equally in the case of muscle. A portion of undifferentiated tissue
containing a predominance of the colloid that contracts in changing,
will, during each change, tend to form new molecules of its own type
from the other colloids diffused through it: the tendency of these
entangled colloids to fall into unity with those around them, will
be aided by every shock of isomeric transformation. Hence, repeated
contractions will further the growth of the contracting mass, and
advance its differentiation and integration. If, too, we remember that
the muscular colloid is made to contract by mechanical disturbance, and
that among mechanical disturbances one which will most readily affect
it simultaneously throughout its mass is caused by stretching, we
shall be considerably helped towards understanding how the contractile
tissues are developed. If extension of a muscular colloid previously
at rest, produces in it that molecular disturbance which leads to
isomeric change and decrease of bulk, then there is no difficulty in
explaining the movements of cilia; the formation of a contractile
layer in the vascular system becomes comprehensible; each dilatation
of a blood-vessel caused by a gush of blood, will be followed by a
constriction; the heart will pulsate violently in proportion as it is
violently distended; arteries will develop in power as the stress upon
them becomes greater; and we shall similarly have an explanation of the
increased muscularity of the alimentary canal which is brought about by
increased distension of it.

That the production of contractile tissue in certain localities,
is due to the more frequent excitement in those localities of the
contractility possessed by undifferentiated tissue in general, is a
view harmonizing with traits which the differentiated contractile
tissue exhibits. These are the relations between muscular exercise,
muscular power, and muscular structure; and it is the more needful
for us here to notice them because of certain anomalies they present,
which, at first sight, seem inconsistent with the belief that the
functionally-determined modifications of muscle are inheritable.

Muscles disagree greatly in their tints: all gradations between white
and deep red being observable. Contrasts are visible between the
muscles of different animals, between the muscles of the same animal
at different ages, and between different muscles of the same animal
at the same age. We will glance at the facts under these heads:
noting under each of them the connexion which here chiefly concerns
us--that between the activity of muscle and its depth of colour. The
cold-blooded _Vertebrata_ are, taken as a group, distinguished
from the warmblooded by the whiteness of their flesh; and they are
also distinguished by their comparative inertness. Though a fish
or a reptile can exert considerable force for a short time, it is
not capable of prolonged exertion. Birds and mammals show greater
endurance along with the darker-coloured muscles. If among birds
themselves or mammals themselves we make comparisons, we meet with
kindred contrasts--especially between wild and domestic creatures of
allied kinds. Barn-door fowls are lighter-fleshed than most untamed
gallinaceous birds; and among these last the pheasant, moving about but
little, is lighter-fleshed than the partridge and the grouse which are
more nomadic. The muscles of the sheep are not on the average so dark
as those of the deer; and it is said that the flesh of the wild-boar is
darker than that of the pig. Perhaps, however, the contrast between the
hare and the rabbit affords, among familiar animals, the best example
of the alleged relation: the dark-fleshed hare having no retreat and
making wide excursions, while the white-fleshed rabbit, passing a
great part of its time in its burrow, rarely wanders far from home.
The parallel contrast between young and old animals has a parallel
meaning. Veal is much whiter than beef, and lamb is of lighter colour
than mutton. Though at first sight these facts may not seem to furnish
confirmatory evidence, since lambs in their play appear to expend
more muscular force than their sedate dams; yet the meaning of the
contrast is really as alleged. For in consequence of the law that the
strains which animals have to overcome, increase as the cubes of the
dimensions, while their powers of overcoming them increase only as
the squares (§ 46), the movements of an adult animal cost much more
in muscular effort than do those of a young animal: the result being
that the sheep and the cow exercise their muscles more vigorously in
their quiet movements, than the lamb and the calf in their lively
movements. It may be added as significant, that the domestic animal
in which no very marked darkening of the flesh takes place along with
increasing age, namely the pig, is one which, ordinarily kept in a sty,
leads so quiescent a life that the assigned cause of darkening does
not come into action. But perhaps the most conclusive evidences are
the contrasts which exist between the active and inactive muscles of
the same animal. Between the leg-muscles of fowls and their pectoral
muscles, the difference of colour is familiar; and we know that fowls
exercise their leg-muscles much more than the muscles which move their
wings. Similarly in the turkey, in the guinea fowl, in the pheasant.
And then, adding much to the force of this evidence, we see that in
partridges and grouse, which belong to the same order as our domestic
fowls but use their wings as constantly as their legs, little or
no difference is visible between the colour of these two groups of
muscles. Special contrasts like these do not, however, exhaust the
proofs; for there is a still more significant general contrast. The
muscle of the heart, which is the most active of all muscles, is the
darkest of all muscles.

The connexion of phenomena thus shown in so many ways, implies that
the bulk of a muscle is by no means the sole measure of the quantity
of force it can evolve. It would seem that, other things equal, the
depth of colour varies with the constancy of action; while, other
things equal, the bulk varies with the amount of force that has to be
put forth upon occasion. These of course are approximate relations.
More correctly we may say that the actions of pale muscles are either
relatively feeble though frequent (as in the massive flanks of a fish),
or relatively infrequent though strong (as in the pectoral muscles of
a common fowl); while the actions of dark muscles are both frequent
and strong. Some such differentiation may be anticipated by inference
from the respective physiological requirements. A muscle which has
upon occasion to evolve considerable force, but which has thereafter a
long period of rest during which repair may restore it to efficiency,
requires neither a large reserve of the contractile substance that is
in some way deteriorated by action, nor highly developed appliances for
bringing it nutritive materials and removing effete products. Where,
contrariwise, an exerted muscle which has undergone much molecular
change in evolving much mechanical force, has soon again to evolve
much mechanical force, and so on continually; it is clear that either
the quantity of contractile substance present must be great, or the
apparatus for nutrition and depuration must be very efficient, or both.
Hence we may look for marked unlikenesses of minute structure between
muscles which are markedly contrasted in activity. And we may suspect
that these conspicuous contrasts of colour between active and inactive
muscles, are due to these implied differences of minute structure:
partly differences between the numbers of blood-vessels and partly
differences between the quantities or qualities of sarcous matter.

Here, then, we have a key to the apparent anomaly above hinted at--the
maintenance of bulk by certain muscles which have been rendered
comparatively inactive by changed habits of life. That the pectoral
muscles of those domestic birds which fly but little, have not dwindled
to any great extent, has been thought a fact at variance with the
conclusion that functionally-produced adaptations are inheritable.
It has been argued that if parts which are exercised increase, not
only in the individual but in the race, while parts which become
less active decrease; then a notable difference of size should exist
between the muscles used for flight in birds that fly much, and those
in birds of an allied kind that fly little. But, as we here see, this
is not the true implication. The change in such cases must be chiefly
in vascularity and abundance of contractile substance; and cannot be,
to any great extent, in bulk. For a bird to fly at all, its pectoral
muscles, bones of attachment, and all accompanying appliances, must be
kept up to a certain level of power. If the parts dwindle much, the
creature will be unable to lift itself from the ground. Bearing in mind
that the force which a bird expends to sustain itself in the air during
each successive instant of a short flight is, other things equal,
the same as it expends in each successive instant of a long flight,
we shall see that the muscles employed in the two cases must have
something like equal intensities of contractile power; and that the
structural differences between them must have relation mainly to the
lengths of time during which they can continue to repeat contractions
of like intensity. That is to say, while the power of flight is
retained at all, the muscles and bones cannot greatly dwindle; but the
dwindling, in birds whose flights are short or infrequent or both,
will be in the reserve stock of the substance that is incapacitated by
action, or in the appliances that keep the apparatus in repair, or in
both. Only where, as in the struthious birds, the habit of flight is
lost, can we expect atrophy of all the parts concerned in flight; and
here we find it.

Are such differentiations among the muscles functionally-produced? or
are they produced by the natural selection of variations distinguished
as spontaneous? We have, I think, good grounds for concluding that
they are functionally-produced. We know that in individual men and
animals, the power of sustained action in muscles is rapidly adaptable
to the amount of sustained action required. We know that being “out of
condition,” is usually less shown by the inability to put out a violent
effort than by the inability to continue making violent efforts; and
we know that the result of training for prize-fights and races, is more
shown in the prolongation of energy than in the intensification of
energy. At the same time, experience has taught us that the structural
change which accompanies this functional change, is not so much a
change in the bulk of the muscles as a change in their internal state:
instead of being soft and flabby they become hard. We have inductive
proof, then, that exercise of a muscle causes some interstitial
growth along with the power of more sustained action; and there can
be no doubt that the one is a condition to the other. What is this
interstitial growth? There is reason to suspect that it is in part an
increased deposit of the sarcous substance and in part a development of
blood-vessels. Microscopic observation tends to confirm the conclusions
before drawn, that repetition of contractions furthers the formation
of the matter which contracts, and that greater draughts of blood
determine greater vascularity. And if the contrasts of molecular
structure and the contrasts of vascularity, directly caused in muscles
by contrasts in their activities, are to any degree inheritable;
there results an explanation of those constitutional differences in
the colours and textures of muscles, which accompany constitutional
differences in their degrees of activity.

It may be added that if we are warranted in so ascribing the
differentiations of muscles from one another to direct equilibration,
then we have the more reason for thinking that the differentiation
of muscles in general from other structures is also due to direct
equilibration. That unlikenesses between parts of the contractile
tissues having unlike functions, are caused by the unlikenesses of
their functions, renders it the more probable that the unlikenesses
between contractile tissue and other tissues, have been caused by
analogous unlikenesses.


§ 304. These interpretations, which have already occupied too
large a space, must here be closed. Of course out of phenomena so
multitudinous and varied, it has been impracticable to deal with any
but the most important; and it has been practicable to deal with these
only in a general way. Much, however, as remains to be explained, I
think the possibility of tracing, in so many cases, the actions to
which these internal differentiations may rationally be ascribed,
makes it likely that the remaining internal differentiations are
due to kindred actions. We find evidence that, in more cases than
seemed probable, these actions produce their effects directly on the
individual; and that the unlikenesses are produced by accumulation
of such effects from generation to generation. While for all the
other unlikenesses, we have, as an adequate cause, the indirect
effects wrought by the survival, generation after generation, of the
individuals in which favourable variations have occurred--variations
such as those of which human anatomy furnishes endless instances.
Thus accounting for so much, we may not unreasonably presume that
these co-operative processes of direct and indirect equilibration will
account for what remains.

       *       *       *       *       *

[NOTE.--After having dismissed this revised chapter as done
with, and sent it to the printer, further thought concerning those
differentiations which produce bone, has reminded me of a fact of
extreme and varied significance named in the first volume. I refer
to the formation of adaptive structures round the ends of dislocated
bones, and to the formation of “false joints.”

These are ontogenetic changes of which phylogeny yields no explanation.
They do not repeat the traits of ancestral organisms, and they cannot
be ascribed to either of the recognized evolutionary factors. If a
humerus be broken across and, failing to set, presently comes to have
its two loose ends so modified as in a measure to simulate the parts of
a normal joint--the ends becoming smooth, covered with periosteum and
supplied with fibrous tissue, and attached by ligaments in such ways
as to allow of restrained movements--it is impossible to think that
natural selection has had anything to do with the power of adjustment
thus shown. No survival of individuals in which adaptations of this
kind, now in one place and now in another, were better and better
effected, could account for acquirement of the ability. Nor can it be
supposed that the ability might result from a functionally-produced
habit; since it is scarcely conceivable that the number of cases in
which individuals profited by it (at first a little and gradually more)
could be such (even did they survive) as to affect the constitution of
the species. Both of the alleged causes of structural modifications
are out of court. It is manifest, too, that the foregoing hypothesis
respecting bone-formation yields us not the slightest help.

But on carefully considering the facts, certain phenomena of profound
meaning may strike us. Here, in a part of the body where no such
tissues ordinarily exist and to which no such structures are ordinarily
appropriate, there arise tissues and structures adapted to the physical
circumstances imposed on that part. Out of what do these abnormal
but appropriate tissues arise? The substances around--osseous,
cartilaginous, membranous--consist of differentiated elements too
far specialized to allow of transformation. These new tissues, then,
must originate from the undifferentiated protoplasm pervading the
part. The units of this protoplasm, subject to the actions proper
to an articulation, begin to assume the appropriate histological
traits--are determined by local stimuli to form tissues ordinarily
associated with such stimuli. What is the inevitable implication? These
units--physiological or constitutional, as we may call them--must
have possessed latent potentialities of falling into these special
arrangements under stress of such conditions. At one point there
arises periosteum and at another ligamentous tissue, while for the
shaping of the ends of the bones--here into a rude hinged form and
there into a rude ball-and-socket form, according to the habitual
movements--there goes on some appropriate deposit of bone. Hence we
must conclude that in the units of protoplasm which have not yet been
organized into special tissues, there resides the ability to take on
one or other type of histological structure according to circumstances;
and, further, that there resides in each of them the still more
marvellous ability to co-operate with kindred units dispersed around
in developing that arrangement of the parts required to constitute a
“false joint.” So that while these units have a general proclivity
towards the structure of the organism as a whole, they have also
proclivities towards structures proper to the local conditions into
which they fall. There is latent in each unit the constitution of the
entire organism and by implication the constitution of every organ; and
each unit while co-operating with the aggregate is ready to take part
in that particular arrangement proper to the position it has fallen
into. If the reader will refer back to §§ 97_d_, 97_e_, in which it is
shown that each member of a human society possesses a combination of
potentialities like these, he will be the better enabled to believe
that this thing _may be_ so while he is unable to conceive how it _is_
so.

And here, indeed, let it be pointed out how completely irrelevant is
the test of conceivableness as applied to these ultimate physiological
actions. For as here, from the un-united ends of the broken bone, there
presently arises a rude joint with fit membranes, ligaments, and even
synovial fluid, though we are absolutely unable to imagine the process
by which the adjacent tissues produce this structure; so there may
be from an organ enlarged by function, such reactive effect upon the
system at large as eventually to influence the reproductive cells,
though we may be absolutely unable to imagine how this can be done.]




                              CHAPTER IX.

                 PHYSIOLOGICAL INTEGRATION IN ANIMALS.


§ 305. Physiological differentiation and physiological integration,
are correlatives that vary together. We have but to recollect the
familiar parallel between the division of labour in a society and the
physiological division of labour, to see that as fast as the kinds
of work performed by the component parts of an organism become more
numerous, and as fast as each part becomes more restricted to its
own work, so fast must the parts have their actions combined in such
ways that no one can go on without the rest and the rest cannot go on
without each one.

Here our inquiry must be, how the relationship of these two processes
is established--what causes the integration to advance _pari passu_
with the differentiation. Though it is manifest, _à priori_, that
the mutual dependence of functions must be proportionate to the
specialization of functions; yet it remains to find the mode in which
the increasing co-ordination is determined.

Already, among the Inductions of Biology, this relation between
differentiation and integration has been specified and illustrated
(§ 59). Before dealing with it deductively, a few further examples,
grouped so as to exhibit its several aspects, will be advantageous.


§ 306. If the lowly-organized _Planaria_ has its body broken up and
its gullet detached, this will, for a while, continue to perform
its function when called upon, just as though it were in its place:
a fragment of the creature’s own body placed in the gullet, will
be propelled through it, or swallowed by it. But, as the seeming
strangeness of this fact implies, we find no such independent actions
of analogous parts in the higher animals. Again, a piece cut out of the
disc of a _Medusa_ continues with great persistence repeating those
rhythmical contractions which we see in the disc as a whole; and thus
proves to us that the contractile function in each portion of the disc,
is in great measure independent. But it is not so with the locomotive
organs of more differentiated types. When separated from the rest these
lose their powers of movement. The only member of a vertebrate animal
which continues to act after detachment, is the heart; and the heart
has motor powers complete within itself.

Where there is this small dependence of each part upon the whole,
there is but small dependence of the whole upon each part. The longer
time which it takes for the arrest of a function to produce death in
a less-differentiated animal than in a more-differentiated animal,
may be illustrated by the case of respiration. Suffocation in a man
speedily causes resistance to the passage of the blood through the
capillaries, followed by congestion and stoppage of the heart: great
disturbance throughout the system results in a few seconds, and in
a minute or two all the functions cease. But in a frog, with its
undeveloped respiratory organ, and a skin through which a considerable
aëration of the blood is carried on, breathing may be suspended for
a long time without injury. Doubtless this difference is proximately
due to the greater functional activity in the one case than in the
other, and the more pressing need for discharging the produced carbon
dioxide; but the greater functional activity being itself made possible
by the higher specialization of functions, this remains the primary
cause of the greater dependence of the other functions on respiration,
where the respiratory apparatus has become highly specialized. Here
indeed, we see the relation under another aspect. This more rapid
rhythm of the functions which increased heterogeneity of structure
makes possible, is itself a means of integrating the functions. Watch,
when it is running down, a complicated machine of which the parts are
not accurately adjusted, or are so worn as to be somewhat loose. There
will be observed certain irregularities of movement just before it
comes to rest--certain of the parts which stop first, are again made to
move a little by the continued movement of the rest, and then become
themselves, in turn, the causes of renewed motion in other parts which
have ceased to move. That is to say, while the connected rhythmical
changes of the machine are quick, their actions and reactions on one
another are regular--all the motions are well integrated; but as the
velocity diminishes irregularities arise--the motions become somewhat
disintegrated. Similarly with organic functions: increase of their
rapidity involves increase of a joint momentum which controls each
and co-ordinates all. Thus if we compare a snake with a mammal, we
see that its functions are not tied together so closely. The mammal,
and especially the superior mammal, requires food with considerable
regularity; keeps up a respiration which varies within but moderate
limits; and has periods of activity and rest that alternate evenly and
frequently. But the snake, taking food at long intervals, may have
these intervals greatly extended without fatal results; its dormant and
its active states recur less uniformly; and its rate of respiration
varies within much wider limits--now being scarcely perceptible and
now, as you may prove by exciting it, becoming conspicuous. So that
here, where the rhythms are very slow, they are individually less
regular, and are united into a less regular compound rhythm--are less
integrated.

Perhaps the clearest general idea of the co-ordination of functions
that accompanies their specialization, is obtained by observing the
slowness with which a little-differentiated animal responds to a
stimulus applied to one of its parts, and the rapidity with which
such a local stimulus is responded to by a more-differentiated animal.
A sea-anemone and a fly will serve for the comparison. A tentacle of
a sea-anemone, when touched, slowly contracts; and if the touch has
been rude, the contraction presently extends to the other tentacles
and eventually to the entire body: the stimulus to movement is
gradually diffused throughout the organism. But if you touch a fly,
or rather if you come near enough to threaten a touch, the entire
apparatus of flight is instantly brought into combined action. Whence
arises this contrast? The one creature has but faintly specialized
contractile organs, and fibres for conveying impressions. The other
has definite muscles and nerves and a co-ordinating centre. The parts
of the little-differentiated sea-anemone have their functions so
feebly co-ordinated, that one may be strongly affected for some time
before any effect is felt by another at a distance from it; but in the
much-differentiated fly, various remote parts instantly have changes
propagated to them from the affected part, and by their united actions
thus set up, the whole organism adjusts itself so as to avoid the
danger.

These few added illustrations will make the nature of this general
relation sufficiently clear. Let us now pass to the interpretation of
it.


§ 307. If a _Hydra_ is cut in two, the nutritive liquids diffused
through its substance cannot escape rapidly, since there are no
open channels for them; and hence the conditions of the parts at
a distance from the cut is but little affected. But where, as in
the more-differentiated animals, the nutritive liquid is contained
in vessels which have continuous communications, cutting the body
in two, or cutting off any considerable portion of it, is followed
by escape of the liquid from these vessels to a large extent; and
this affects the nutrition and efficiency of organs remote from the
place of injury. Then where, as in further-developed creatures,
there exists an apparatus for propelling the blood through these
ramifying channels, injury of a single one will cause a loss of blood
that quickly prostrates the entire organism. Hence the rise of a
completely-differentiated vascular system, is the rise of a system
which integrates all members of the body, by making each dependent on
the integrity of the vascular system, and therefore on the integrity
of each member through which it ramifies. In another mode, too, the
establishment of a distributing apparatus produces a physiological
union that is great in proportion as this distributing apparatus is
efficient. As fast as it assumes a function unlike the rest, each part
of an animal modifies the blood in a way more or less unlike the rest,
both by the materials it abstracts and by the products it adds; and
hence the more differentiated the vascular system becomes, the more
does it integrate all parts by making each of them feel the qualitative
modification of the blood which every other has produced. This is
simply and conspicuously exemplified by the lungs. In the absence of a
vascular system, or in the absence of one that is well marked off from
the imbedding tissues, the nutritive plasma or the crude blood, gets
what small aëration it can, only by coming near the creature’s outer
surface, or those inner surfaces which are bathed by water. But where
there have been formed definite channels branching throughout the body,
and particularly where there exist specialized organs for pumping the
blood through these channels, it manifestly becomes possible for the
aëration to be carried on in one part peculiarly modified to further
it, while all other parts have the aërated blood brought to them. And
how greatly the differentiation of the vascular system thus becomes a
means of integrating the various organs, is shown by the fatal result
that follows when the current of aërated blood is interrupted.

Here, indeed, it becomes obvious both that certain physiological
differentiations make possible certain physiological integrations;
and that, conversely, these integrations make possible other
differentiations. Besides the waste products which escape through the
lungs, there are waste products which escape through the skin, the
kidneys, the liver. The blood has separated from it in each of these
structures, the particular product which this structure has become
adapted to separate; leaving the other products to be separated by
the other adapted structures. How have these special adaptations been
made possible? By union of the organs as recipients of one circulating
mass of blood. While there is no efficient apparatus for transfer of
materials through the body, the waste products of each part have to
make their escape locally; and the local channels of escape must be
competent to take off indifferently all the waste products. But it
becomes practicable and advantageous for the differently-localized
excreting structures to become fitted to separate different waste
products, as soon as the common circulation through them grows so
efficient that the product left unexcreted by one is quickly carried to
another better fitted to excrete it. So that the integration of them
through a common vascular system, is the condition under which only
they can become differentiated. Perhaps the clearest idea of the way in
which differentiation leads to integration, and how, again, increased
integration makes possible still further differentiation, will be
obtained by contemplating the analogous dependence in the social
organism. While it has no roads, a country cannot have its industries
much specialized: each locality must produce, as best it can, the
various commodities it consumes, so long as it has no facilities for
barter with other localities. But the localities being unlike in their
natural fitnesses for the various industries, there tends ever to
arise some exchange of the commodities they can respectively produce
with least labour. This exchange leads to the formation of channels
of communication. The currents of commodities once set up, make
their foot-paths and horse-tracks more permeable; and as fast as the
resistance to exchange becomes less, the currents of commodities become
greater. Each locality takes more of the products of adjacent ones,
and each locality devotes itself more to the particular industry for
which it is naturally best fitted: the functional integration makes
possible a further functional differentiation. This further functional
differentiation reacts. The greater demand for the special product of
each locality, excites improvements in production--leads to the use of
methods which both cheapen and perfect the commodity. Hence results a
still more active exchange; a still clearer opening of the channels of
communication; a still closer mutual dependence. Yet another influence
comes into play. As fast as the intercourse, at first only between
neighbouring localities, makes for itself better roads--as fast as
rivers are bridged and marshes made easily passable, the resistance to
distribution becomes so far diminished, that the things grown or made
in each district can be profitably carried to a greater distance; and
as the economical integration is thus extended over a wider area, the
economical differentiation is again increased; since each district,
having a larger market for its commodity, is led to devote itself more
exclusively to producing this commodity. These actions and reactions
continue until the various localities, becoming greatly developed
and highly specialized in their industries, are at the same time
functionally integrated by a network of roads, and finally railways,
along which rapidly circulate the currents severally sent out and
received by the localities. And it will be manifest that in individual
organisms a like correlative progress must have been caused in an
analogous way.


§ 308. Another and higher form of physiological integration in animals,
is that which the nervous system effects. Each part as it becomes
specialized, begins to act upon the rest not only indirectly through
the matters it takes from and adds to the blood, but also directly
through the molecular disturbances it sets up and diffuses. Whether
nerves themselves are differentiated by the molecular disturbances
thus propagated in certain directions, or whether they are otherwise
differentiated, it must equally happen that as fast as they become
channels along which molecular disturbances travel, the parts they
connect become physiologically integrated, in so far that a change
in one initiates a change in the other. We may dimly perceive that
if portions of what was originally a uniform mass having a common
function, undertake subdivisions of the function, the molecular changes
going on in them will be in some way complementary to one another:
that peculiar form of molecular motion which the one has lost in
becoming specialized, the other has gained in becoming specialized.
And if the molecular motion that was common to the two portions while
they were undifferentiated, becomes divided into two complementary
kinds of molecular motion; then between these portions there will be
a contrast of molecular motions such that whatever is _plus_ in the
one will be _minus_ in the other; and hence there will be a special
tendency towards a restoration of the molecular equilibrium between
the two: the molecular motion continually propagated away from either
will have its line of least resistance in the direction of the
other. If, as argued in the last chapter, repeated restorations of
molecular equilibrium, always following the line of least resistance,
tend ever to make it a line of diminished resistance; then, in
proportion as any parts become more physiologically integrated by
the establishment of this channel for the easy transmission of
molecular motion between them, they may become more physiologically
differentiated. The contrast between their molecular motions leads to
the line of discharge; the line of discharge, once formed, permits a
greater contrast of their molecular motions to arise; thereupon the
quantities of molecular motion transferred to restore equilibrium,
being increased, the channel of transfer is made more permeable; and
its further permeability, so caused, renders possible a still more
marked unlikeness of action between the parts. Thus the differentiation
and the integration progress hand in hand as before. How the same
principle holds throughout the higher stages of nervous development,
can be seen only still more vaguely. Nevertheless, it is comprehensible
that as functions become further divided, there will arise the
need for sub-connexions along which there may take place secondary
equilibrations subordinate to the main ones. It is manifest, too, that
whereas the differentiation of functions proceeds, not necessarily by
division into two, but often by division into several, and usually in
such ways as not to leave any two functions that are just complementary
to one another, the restorations of equilibrium cannot be so simple
as above supposed. And especially when we bear in mind that many
differentiated functions, as those of the senses, cannot be held
complementary to any other functions in particular; it becomes manifest
that the equilibrations that have to be made in an organism of much
heterogeneity, are extremely complex, and do not take place between
each organ and some other, but between each organ and all the others.
The peculiarity of the molecular motion propagated from each organ,
has to be neutralized by some counter-peculiarity in the average of
the molecular motions with which it is brought into relation. All the
variously-modified molecular motions from the various parts, must have
their pluses and minuses mutually cancelled: if not locally, then at
some centre to which each unbalanced motion travels until it meets with
some opposite unbalanced motion to destroy it. Still, involved as these
actions must become, it is possible to see how the general principle
illustrated by the simple case above supposed, will continue to hold.
For always the molecular motion proceeding from any one differentiated
part, will travel most readily towards that place where a molecular
motion most complementary to it in kind exists--no matter whether this
complementary molecular motion be that proceeding from any one other
organ, or the _resultant_ of the molecular motions proceeding from
many other organs. So that the tendency will be for each channel of
communication or nerve, to unite itself with some centre or ganglion,
where it comes into relation with other nerves. And if there be any
parts of its peculiar molecular motion uncancelled by the molecular
motions it meets at this centre; or if, as will probably happen, the
average molecular motion which it there unites to produce, differs from
the average molecular motion elsewhere; then, as before, there will
arise a discharge along another channel or nerve to another centre
or ganglion, where the residuary difference may be cancelled by the
differences it meets; or whence it may be still further propagated till
it is so cancelled. Thus there will be a tendency to a general nervous
integration keeping pace with the differentiation.

Of course this must be taken as nothing more than the indication of
initial tendencies--not as an hypothesis sufficient to account for all
the facts. It leaves out of sight the origin and functions of ganglia,
considered as something more than nerve-junctions. Were there only
these lines of easy transmission of molecular disturbance, a change
set up in one organ could never do more than produce its equivalent of
change in some other or others; and there could be none of that large
amount of motion initiated by a small sensation, which we habitually
see. The facts show, unmistakably, that the slight disturbance
communicated to a ganglion, causes an overthrow of that highly-unstable
nervous matter contained in it, and a discharge from it of the
greatly-increased quantity of molecular motion so generated. This,
however, is beyond our immediate topic. All we have here to note is the
interdependence and unification of functions that naturally follow the
differentiation of them.


§ 309. Something might be added concerning the further class of
integrations by which organisms are constituted mechanically-coherent
wholes. Carrying further certain of the arguments contained in the
last chapter, it might be not unreasonably inferred that the binding
together of parts by bones, muscles, and ligaments, is a secondary
result of those same actions by which bones, muscles, and ligaments
are specialized. But adequate treatment of this division of the subject
is at present scarcely possible.

What little of fact and inference has been above set down, will,
however, serve to make comprehensible the general truths respecting
which, in their main outlines, there can be no question. Beginning with
the feebly-differentiated sponge, of which the integration is also so
feeble that cutting off a piece interferes in no appreciable degree
with the activity and growth of the rest, it is undeniable that the
advance is through stages in which the multiplication of unlike parts
having unlike actions, is accompanied by an increasing interdependence
of the parts and their actions; until we come to structures like our
own, in which a slight change initiated in one part will instantly and
powerfully affect all other parts--will convulse an immense number of
muscles, send a wave of contraction through all the blood-vessels,
awaken a crowd of ideas with an accompanying gush of emotions, affect
the action of the lungs, of the stomach, and of all the secreting
organs. And while it is a manifest necessity that along with this
subdivision of functions which the higher organisms show us, there
must be this close co-ordination of them, the foregoing paragraphs
suggest how this necessary correlation is brought about. For a great
part of the physiological union that accompanies the physiological
specialization, there appears to be a sufficient cause in the process
of direct equilibration; and indirect equilibration may be fairly
presumed a sufficient cause for that which remains.




                              CHAPTER X.

                 SUMMARY OF PHYSIOLOGICAL DEVELOPMENT.


§ 310. Intercourse between each part and the particular conditions
to which it is exposed, either habitually in the individual
or occasionally in the race, thus appears to be the origin of
physiological development; as we found it to be the origin of
morphological development. The unlikenesses of form that arise among
members of an aggregate that were originally alike, we traced to
unlikenesses in the incident forces. And in the foregoing chapters we
have traced to unlikenesses in the incident forces, those unlikenesses
of minute structure and chemical composition that simultaneously arise
among the parts.

In summing up the special truths illustrative of this general truth,
it will be proper here to contemplate more especially their dependence
on first principles. Dealing with biological phenomena as phenomena of
evolution, we have to interpret not only the increasing morphological
heterogeneity of organisms, but also their increasing physiological
heterogeneity, in terms of the re-distribution of matter and motion.
While we make our rapid re-survey of the facts, let us then more
particularly observe how they are subordinate to the universal course
of this re-distribution.


§ 311. The instability of the homogeneous, or, strictly speaking, the
inevitable lapse of the more homogeneous into the less homogeneous,
which we before saw endlessly exemplified by the morphological
differentiations of the parts of organisms, we have here seen
afresh exemplified in ways also countless, by the physiological
differentiations of their parts. And in the one case as in the other,
this change from uniformity to multiformity in organic aggregates, is
caused, as it is in all inorganic aggregates, by the necessary exposure
of their component parts to actions unlike in kind or quantity or
both. General proof of this is furnished by the order in which the
differences appear. If parts are rendered physiologically heterogeneous
by the heterogeneity of the incident forces, then the earliest
contrasts should be between parts that are the most strongly contrasted
in their relations to incident forces; the next earliest contrasts
should occur where there are the next strongest contrasts in these
relations; and so on. It turns out that they do so.

Everywhere the differentiation of outside from inside comes first.
In the simplest plants the unlikeness of the cell-wall to the
cell-contents is the conspicuous trait of structure. The contrasts
seen in the simplest animals are of the same kind: the film that
covers a Rhizopod and the more indurated coat of an Infusorian, are
more unlike the contained sarcode than the other parts of this are
from one another; and the tendency during the life of the animal is
for the unlikeness to become greater. What is true of _Protophyta_ and
_Protozoa_, is true of the germs of all organisms up to the highest:
the differentiation of outer from inner is the first step. When the
protoplasm of an _Alga_-cell has broken up into the clusters of
granules which are eventually to become spores, each of these quickly
acquires a membranous coating, constituting an unlikeness between
surface and centre. Similarly with the ovule of every higher plant:
the mass of cells forming it, early exhibits an outside layer of cells
distinguished from the cells within. With animal-germs it is the
same. Be it in a ciliated gemmule, be it in the unfertilized ova of
_Aphides_ and of the _Cecidomyia_, or be it in true ova, the primary
differentiation conforms to the relations of exterior and interior.
If we turn to adult organisms, vegetal or animal, we see that whether
they do or do not display other contrasts of parts, they always display
this contrast. Though otherwise almost homogeneous, such _Fungi_ as the
puff-ball, or, among _Algæ_, all which have a thallus of any thickness,
present marked differences between those of their cells which are in
immediate contact with the environment and those which are not. Such
differences they present in common with every higher plant; which,
here in the shape of bark and there in the shape of cuticle, has an
envelope inclosing it even up to its petals and stamens. In like manner
among animals, there is always either a true skin or an outer coat
analogous to one. Wherever aggregates of the first order have united
into aggregates of the second and third orders--wherever they have
become the morphological units of such higher aggregates--the outermost
of them have grown unlike those lying within. Even the Sponge is not
without a layer that may by analogy be called dermal.

This lapse of the relatively homogeneous into the relatively
heterogeneous, first showing itself, as on the hypothesis of evolution
it must do, by the rise of an unlikeness between outside and inside,
goes on next to show itself, as we infer that it must do, by the
establishment of secondary contrasts among the outer parts answering to
secondary contrasts among the forces falling on them. So long as the
whole surface of a plant remains similarly related to the environment,
as in a _Protococcus_, it remains uniform; but when there come to be
an attached surface and a free surface, these, being subject to unlike
actions, are rendered unlike. This is visible even in a unicellular
_Alga_ when it becomes fixed; it is shown in the distinction between
the under and upper parts of ordinary _Fungi_; and we see it in the
universal difference between the imbedded ends and the exposed ends of
the higher plants. And then among the less marked contrasts of surface
answering to the less marked contrasts in the incident forces, come
those between the upper and under sides of leaves; which, as we have
seen, vary in degree as the contrasts of forces vary in degree, and
disappear where these contrasts disappear. Equally clear proof is
furnished by animals, that the original uniformity of surface lapses
into multiformity, in proportion as the actions of the environment
upon the surface become multiform. In a Worm, burrowing through damp
soil which acts equally on all its sides, or in a _Tænia_, uniformly
bathed by the contents of the intestine it inhabits, the parts of the
integument do not appreciably differ from one another; but in creatures
not surrounded by the same agencies, as those that crawl and those
that have their bodies partially inclosed, there are unlikenesses of
integument corresponding to unlikenesses of the conditions. A snail’s
foot has an under surface not uniform with the exposed surface of
its body, and this again is not uniform with the protected surface.
Among articulate animals there is usually a distinction between the
ventral and the dorsal aspects; and in those of the _Arthropoda_ which
subject their anterior and posterior ends to different environing
agencies, as do the ant-lion and the hermit-crab, these become
superficially differentiated. Analogous general contrasts occur among
the _Vertebrata_. Fishes, though their outsides are uniformly bathed
by water, have their backs more exposed to light than their bellies,
and the two are commonly distinct in colour. When it is not the back
and belly which are thus dissimilarly conditioned, but the sides, as in
the _Pleuronectidæ_, then it is the sides which become contrasted; and
there may be significance in the fact that those abnormal individuals
of this order which revert to the ancestral undistorted type, and
swim vertically, have the two sides alike. In such higher vertebrates
as reptiles, we see repeated this differentiation of the upper and
under surfaces: especially in those of them which, like snakes, expose
these surfaces to the most diverse actions. Even in birds and mammals
which usually, by raising the under surface considerably above the
ground, greatly diminish the contrast between its conditions and the
conditions to which the upper surface is subject, there still remains
some unlikeness of clothing answering to the remaining unlikeness
between the conditions. Thus, without by any means saying that all such
differentiations are directly caused by differences in the actions of
incident forces, which, as before shown (§ 294), they cannot be, it is
clear that many of them are so caused. It is clear that parts of the
surface exposed to very unlike environing agencies, become very unlike;
and this is all that needs to be shown.

Complex as are the transformations of the inner parts of organisms
from the relatively homogeneous into the relatively heterogeneous, we
still see among them a conformity to the same general order. In both
plants and animals the earlier internal differentiations answer to the
stronger contrasts of conditions. Plants, absorbing all their nutriment
through their outer surfaces, are internally modified mainly by the
transfer of materials and by mechanical stress. Such of them as do not
raise their fronds above the surface, have their inner tissues subject
to no marked contrasts save those caused by currents of sap; and the
lines of lengthened and otherwise changed cells which are formed where
these currents run, and are most conspicuous where these currents must
obviously be the strongest, are the only decided differentiations of
the interior. But where, as in the higher Cryptogams and in Phænogams,
the leaves are upheld, and the supporting stem is transversely bent by
the wind, the inner tissues, subject to different amounts of mechanical
strain, differentiate accordingly: the deposit of dense substance
commences in that region where the sap-containing cells and canals
suffer the greatest intermittent compressions. Animals, or at least
such of them as take food into their interiors, are subject to forces
of another class tending to destroy their original homogeneity. Food
is a foreign substance which acts on the interior as an environing
object which touches it acts on the exterior--is literally a portion
of the environment which, when swallowed, becomes a cause of internal
differentiations as the rest of the environment continues a cause
of external differentiations. How essentially parallel are the two
sets of actions and reactions, we have seen implied by the primordial
identity of the endoderm and ectoderm in simple animals, and of the
skin and mucous membrane in complex animals (§§ 288, 289). Here
we have further to observe that as food is the original source of
internal differentiations, these may be expected to show themselves
first where the influence of the food is greatest; and to appear
later in proportion as the parts are more removed from the influence
of the food. They do this. In animals of low type, the coats of the
alimentary cavity or canal are more differentiated than the tissue
which lies between the alimentary canal and the wall of the body. This
tissue in the higher _Cœlenterata_, is a feebly-organized parenchyma
traversed by canals lined with simple ciliated cells; and in the lower
_Mollusca_ the structures bounding the peri-visceral cavity and its
ramifying sinuses, are similarly imperfect. Further, it is observable
that the differentiation of this peri-visceral sac and its sinuses into
a vascular system, proceeds centrifugally from the region where the
absorbed nutriment enters the mass of circulating liquid, and where
this liquid is qualitatively more unlike the tissues than it is at the
remoter parts of the body.

Physiological development, then, is initiated by that instability
of the homogeneous which we have seen to be everywhere a cause of
evolution (_First Principles_, §§ 149–155). That the passage
from comparative uniformity of composition and minute structure to
comparative multiformity, is set up in organic aggregates, as in all
other aggregates, by the necessary unlikenesses of the actions to which
the parts are subject, is shown by the universal rise of the primary
differentiation into the parts that are universally most contrasted in
their circumstances, and by the rise of secondary differentiations
obviously related in their order to secondary contrasts of conditions.


§ 312. How physiological development has all along been aided by the
multiplication of effects--how each differentiation has ever tended to
become the parent of new differentiations, we have had, incidentally,
various illustrations. Let us here review the working of this cause.

Among plants we see it in the production of progressively-multiplying
heterogeneities of tissue by progressive increase of bulk. The
integration of fronds into axes and of axes into groups of axes,
sets up unlikenesses of action among the integrated units, followed
by unlikenesses of minute structure. Each gust transversely strains
the various parts of the stem in various degrees, and longitudinally
strains in various degrees the roots; and while there is inequality of
stress at every place in stem and branch, so, at every place in stem
and branch, the outer layers and the successively inner layers are
severally extended and compressed to unequal amounts, and have unequal
modifications wrought in them. Let the tree add to its periphery
another generation of the units composing it, and immediately the
mechanical strains on the supporting parts are all changed in different
degrees, initiating new differences internally. Externally, too, new
differences are initiated. Shaded by the leaf-bearing outer stratum
of shoots, the inner structures cease to bear leaves, or to put out
shoots which bear leaves; and instead of that green covering which
they originally had, become covered with bark of increasing thickness.
Manifestly, then, the larger integration of units that are originally
simple and uniform, entails physiological changes of various orders,
varying in their degrees at all parts of the aggregate. Each branch
which, favourably circumstanced, flourishes more than its neighbours,
becomes a cause of physiological differentiations, not only in its
neighbours from which it abstracts sap and presently turns from
leaf-bearers into fruit-bearers, but also in the remoter parts.

That among animals physiological development is furthered by the
multiplication of effects, we have lately seen proved by the many
changes in other organs, which the growth or modification of each
excreting and secreting organ initiates. By the abstracted as well as
by the added materials, it alters the quality of the blood passing
through all members of the body; or by the liquid it pours into the
alimentary canal, it acts on the food, and through it on the blood,
and through it on the system as a whole: an additional differentiation
in one part thus setting up additional differentiations in many other
parts; from each of which, again, secondary differentiating forces
reverberate through the organism. Or, to take an influence of another
order, we have seen how the modified mechanical action of any member
not only modifies that member, but becomes, by its reactions, a cause
of secondary modifications--how, for example, the burrowing habits
of the common mole, leading to an almost exclusive use of the fore
limbs, have entailed a dwindling of the hind limbs, and a concomitant
dwindling of the pelvis, which, becoming too small for the passage of
the young, has initiated still more anomalous modifications.

So that throughout physiological development, as in evolution at large,
the multiplication of effects has been a factor constantly at work, and
working more actively as the development has advanced. The secondary
changes wrought by each primary change, have necessarily become more
numerous in proportion as organisms have become more complex. And
every increased multiplication of effects, further differentiating the
organism and, by consequence, further integrating it, has prepared the
way for still higher differentiations and integrations similarly caused.


§ 313. The general truth next to be resumed, is that these processes
have for their limit a state of equilibrium--proximately a moving
equilibrium and ultimately a complete equilibrium. The changes we have
contemplated are but the concomitants of a progressing equilibration.
In every aggregate which we call living, as well as in all other
aggregates, the instability of the homogeneous is but another name
for the absence of balance between the incident forces and the forces
which the aggregate opposes to them; and the passage into heterogeneity
is the passage towards a state of balance. And to say that in every
aggregate, organic or other, there goes on a multiplication of effects,
is but to say that one part which has a fresh force impressed on it,
must go on changing and communicating secondary changes, until the
whole of the impressed force has been used up in generating equivalent
reactive forces.

The principle that whatever new action an organism is subject to, must
either overthrow the moving equilibrium of its functions and cause the
sudden equilibration called death, or else must progressively alter the
organic rhythms until, by the establishment of a new reaction balancing
the new action a new moving equilibrium is produced, applies as much
to each member of an organism as to the organism in its totality. Any
force falling on any part not adapted to bear it, must either cause
local destruction of tissue, or must, without destroying the tissue,
continue to change it until it can change it no further; that is--until
the modified reaction of the part has become equal to the modified
action. Whatever the nature of the force this must happen. If it is a
mechanical force, then the immediate effect is some distortion of the
part--a distortion having for its limit that attitude in which the
resistance of the structures to further change of position, balances
the force tending to produce the further change; and the ultimate
effect, supposing the force to be continuous or recurrent, is such a
permanent alteration of form, or alteration of structure, or both, as
establishes a permanent balance. If the force is physico-chemical, or
chemical, the general result is still the same: the component molecules
of the tissue must have their molecular arrangements changed, and the
change in their molecular arrangements must go on until their molecular
motions are so re-adjusted as to equilibrate the molecular motions of
the new physico-chemical or chemical agent. In other words, the organic
matter composing the part, if it continues to be organic matter at all,
must assume that molecular composition which enables it to bear, or as
we say adapts it to, the incident forces.

Nor is it less certain that throughout the organism as a whole,
equilibration is alike the proximate limit of the changes wrought by
each action, as well as the ultimate limit of the changes wrought by
any recurrent actions or continuous action. The movements every instant
going on, are movements towards a new state of equilibrium. Raising
a limb causes a simultaneous shifting of the centre of gravity, and
such altered tensions and pressures throughout the body as re-adjust
the disturbed balance. Passage of liquid into or out of a tissue,
implies some excess of force in one direction there at work; and ceases
only when the force so diminishes or the counter-forces so increase
that the excess disappears. A nervous discharge is reflected and
re-reflected from part to part, until it has all been used up in the
re-arrangements produced--equilibrated by the reactions called out. And
what is thus obviously true of every normal change, is equally true of
every abnormal change--every disturbance of the established rhythm of
the functions. If such disturbance is a single one, the perturbations
set up by it, reverberating throughout the system, leave its moving
equilibrium slightly altered. If the disturbance is repeated or
persistent, its successive effects accumulate until they have produced
a new moving equilibrium adjusted to the new force.

Each re-balancing of actions, having for its necessary concomitant a
modification of tissues, it is an obvious corollary that organisms
subjected to successive changes of conditions, must undergo successive
differentiations and re-differentiations. Direct equilibration in
organisms, with all its accompanying structural alterations, is as
certain as is that universal progress towards equilibrium of which
it forms part. And just as certain is that indirect equilibration
in organisms to which the remaining large class of differentiations
is due. The development of favourable variations by the killing of
individuals in which they do not occur or are least marked, is, as
before, a balancing between certain local structures and the forces
they are exposed to; and is no less inevitable than the other.


§ 314. In all which universal laws, we find ourselves again brought
down to the persistence of force, as the deepest knowable cause of
those modifications which constitute physiological development; as
it is the deepest knowable cause of all other evolution. Here, as
elsewhere, the perpetual lapse from less to greater heterogeneity,
the perpetual begetting of secondary modifications by each primary
modification, and the perpetual approach to a temporary balance on the
way towards a final balance, are necessary implications of the ultimate
fact that force cannot disappear but can only change its form.

It is an unquestionable deduction from the persistence of force, that
in every individual organism each new incident force must work its
equivalent of change; and that where it is a constant or recurrent
force, the limit of the change it works must be an adaptation of
structure such as opposes to the new outer force an equal inner force.
The only thing open to question is, whether such re-adjustment is
inheritable; and further consideration will, I think, show, that to say
it is not inheritable is indirectly to say that force does not persist.
If all parts of an organism have their functions co-ordinated into a
moving equilibrium, such that every part perpetually influences all
other parts, and cannot be changed without initiating changes in all
other parts--if the limit of change is the establishment of a complete
harmony among the movements, molecular and other, of all parts; then
among other parts that are modified, molecularly or otherwise, must
be those which cast off the germs of new organisms. The molecules of
their produced germs must tend ever to conform the motions of their
components, and therefore the arrangements of their components, to the
molecular forces of the organism as a whole; and if this aggregate of
molecular forces be modified in its distribution by a local change
of structure, the molecules of the germs must be gradually changed
in the motions and arrangements of their components, until they are
re-adjusted to the aggregate of molecular forces.




                             CHAPTER X^A.

                 THE INTEGRATION OF THE ORGANIC WORLD.


§ 314_a_. That from the beginning of life there has been an
ever-increasing heterogeneity in the Earth’s Flora and Fauna, is
a truth recognized by all biologists who accept the doctrine of
evolution. In discussing the origin of species Mr. Darwin and others
have been mainly occupied in explaining the genesis of now this and
now that form of organism, considered as a member of one or other
series, and regarded as becoming differentiated from its allies. But by
implication, if not avowedly, there has been simultaneously accepted
the belief that the forms continually produced by divergences and
re-divergences, have constituted an assemblage increasingly multiform
in its included kinds. And this, which we are shown by the process of
organic evolution as followed out in its details, is a corollary from
the doctrine of evolution at large, as was pointed out in § 159 of
_First Principles_.

Meanwhile there has been little if any recognition of an accompanying
change, no less fundamental. In the general transformation which
constitutes Evolution, differentiation and integration advance hand
in hand; so that along with the production of unlike parts there
progresses the union of these unlike parts into a whole. Examples of
various kinds before given will recur to the reader, and an addition
to them has just been set forth in the chapter on “Physiological
Integration.” One more example, world-wide in its reach, has still to
be named.

For here it remains to point out that along with the increasing
multiplication of types of organisms covering the Earth’s surface,
there has been ever going on an increasing mutual dependence of
them--an increasing integration of the entire aggregate of living
things.

Many facts which are obvious and many which are quite familiar will
be named as evidence. But I must be excused for reminding the reader
of things that he knows and things that he may easily observe, since,
unless the evidence, trite as it may be, is gathered together and
properly marshalled, the generalization enunciated will not be thought
valid.


§ 314_b_. Respecting the physiological characters of the earliest
forms there is an assumption from which no escape seems possible--the
assumption that they united animal and vegetal characters. Even among
existing microscopic types of the lowest classes, there is such
community of plant-traits and animal-traits that doubts respecting
their proper places in one or the other kingdom are continually
raised--doubts, too, whether, if regarded as vegetal, they are to be
grouped as algoid or fungoid.

Here, however, without entering on moot questions, we may draw the
_à priori_ conclusion that these earliest living things were
double-natured, in so far that they must have had the ability to
assimilate from the inorganic world all the materials of which
protoplasm consists--must therefore, along with the power of
appropriating carbon from its gaseous compound, also have had the power
of appropriating nitrogen, either from one of its combined oxides
or directly from the air with which water is more or less charged.
For before organic substances existed there could have been none but
inorganic sources from which nitrogen could be obtained.

This conclusion concerns us only because it implies homogeneity of
nature in these primordial forms of life. There could not at first
have existed among these minutest of _Protozoa_ even such vague
distinctions as are now presented in a shadowy way by their modern
representatives. And the implication is that during the period
throughout which these smallest, lowest, and simplest living things
alone existed, there could have been, in the absence of kinds, no
mutual dependence.

Since, among various of the lowest types now known to us, the same
individual exhibits a life which is now predominantly vegetal and now
predominantly animal, we cannot err in assuming that there eventually
took place differentiations of this original plant-animal type into
types permanently unlike: some in which the traits were more markedly
vegetal and others in which they were more markedly animal. As fast as
this differentiation arose, there came the beginnings of co-operation
between the predominantly vegetal types which by the aid of light
formed organic matter from the inorganic world, and the predominantly
animal types which, in chief measure, utilized the matter so formed.
Evidently with the rise of such a differentiation came an incipient
mutual dependence. If to the implied algoid type and the animal type
there be added the fungoid type, somewhat intermediate in character,
which in a large proportion of cases lives on the decaying remnants of
the other two, we are furnished with a rude conception of the primary
differentiations and the accompanying vague mutual dependences.

Speculation aside, it suffices to say that early in the history of
life there must have arisen the distinction between _Protozoa_ and
_Protophyta_, and that this distinction foreshadowed that widest
contrast which the higher organic world presents--the contrast
between plants and animals. It is needless to do more than name the
mutual dependence between these two great divisions. That, as being
respectively decomposers of carbon dioxide and exhalers of carbon
dioxide, they act reciprocally, as also in some measure by interchange
of nitrogenous matters; and that the implied general co-operation
serves in an indirect way to unite their lives, and in that sense to
integrate the two kingdoms; needs not to be insisted upon. Further
complications of the mutual dependence will be mentioned by and by. For
the present it suffices to recognize this division of organic functions
as the first which arose and as continuing to be that fundamental one
which more than all others binds organisms at large together.


§ 314_c_. It will be thought by many readers that in speaking of the
contrasted vital activities of plants and animals as constituting a
“division of organic functions,” I am straining words beyond their
meanings; since the conception of organic functions postulates an
organized whole in which they exist, and plants and animals constitute
no such organized whole. But there is at hand an unexpected defence for
this conception--a defence not forthcoming a generation ago, but which
now all biologists will recognize as relevant. I refer to the phenomena
of symbiosis. These present various cases in which the plant-function
and the animal-function are carried on in the same body,--cases in
which the co-operation is not between separate vegetal organisms which
accumulate nutritive matters and separate animal organisms which
consume them, but is a co-operation between vegetal elements and animal
elements forming parts of the same organism.

As introductory to examples of these must first, however, be named an
example of such co-operation between the two great classes of vegetal
organisms--the fungoid and the algoid. Incredible as the statement once
seemed, it is a statement now accepted, that what we know as lichens,
and used to consider as plants forming a certain low class, are now
found to be not plants in the ordinary sense at all, but compound
growths formed of minute algæ and minute fungi, carrying on their lives
together: the algæ furnishing to the fungi certain constituents they
need but cannot directly obtain, and the fungi profiting by certain
materials they obtain from the algæ, either while living or while
individually decaying. Whence it would seem that after the microscopic
vegetal type had become in a large degree differentiated into two main
types, in adaptation to different conditions of life, and had acquired
appropriate specialities of nature, there grew up this communistic
arrangement between certain of them, enabling each to benefit by the
powers which the other had acquired: evidently an exchange of services,
a physiological division of labour, a mutual dependence of functions
analogous to that which exists between functions in an ordinary plant
or animal.

Not differing in principle but only in application, is that symbiosis
above referred to as existing between _Protophyta_ and many _Protozoa_,
as well as between such _Protophyta_ and the lowest kinds of _Metazoa_.
A recent statement that certain amœbæ, made green by contained
chlorophyll, continue to grow and multiply after they have consumed
what nutritive matter may be at hand, is in harmony with various facts
alleged of other _Protozoa_--various other kinds of _Rhizopods_,
various _Heliozoa_, numerous ciliated and flagellated _Infusoria_.
Among _Metazoa_ the like association occurs in one of the sponges,
in the _Hydra viridis_, in various turbellarians, in a rotifer, and
even in two molluscs. In these cases the partnership between the
vegetal cells and the animal cells (existing either as units or as an
organized group such as a polype), is a partnership which, as before,
profits each of the partners--an inference supported by the fact that
_Metazoa_ containing these algoid cells usually place themselves where
the light falls upon them, and can therefore further the production of
the carbo-hydrates which eventually become useful to the animal cells,
while these in some way reciprocate the benefit.

Here, then, we have exchange of services between associated
plant-elements and animal-elements--a performance by them of different
organic functions for the benefit of the aggregate which they unite
to form. Hence, when these vegetal elements and animal elements
are separately embodied in plants and animals, which profit by
one another, we may still properly regard their respective lives
as mutually-dependent organic functions, as said in the preceding
section. We are enabled the better to see how the Earth’s Flora and
Fauna, which are respectively accumulators of motion and expenders of
motion, form mutually-dependent parts of a whole, and are in that sense
integrated. And we shall be prepared to see how all other relations
between organisms which make them subservient one to another, similarly
constitute elements in a general integration of the organic world.


§ 314_d_. Another form of mutual dependence and consequently of
integration is conspicuous--that which accompanied the progressive
increase of size in organisms of the higher classes. We have but to
contemplate the possibilities to see that life must necessarily have
commenced with minute forms, and that the progress to larger ones must
have been by small steps.

For had creatures of appreciable sizes been the first to exist they
would inevitably have disappeared from lack of food. Having no resource
but to devour one another, they would quickly have brought life to an
end. There must have been smaller types serving as prey for larger
ones before these could continue to exist and to multiply: microbes
affording food to infusoria, infusoria affording food to such sized
creatures as the _Entomostraca_, these again supplying food to
small fishes, such as loch-trout, and these last yielding to larger
fishes masses sufficiently great for their needs: each higher grade
requiring lower grades of appropriate bulk. It needs but to ask what
would become of tigers if there were no mammals larger than mice,
to see that the animal world is a linked assemblage, of which the
connected members stand within certain ratios of mass; and that during
the evolution of higher and larger types the linking of grades has
become closer.

That among plants considered as an aggregate relations of like kind,
though far less distinct ones, have all along been growing may be
reasonably concluded. In a world peopled only by microscopic types
there could not have existed the conditions needful for large trees.
Gradual disintegration of rock-surfaces, partly effected by physical
agencies and partly by low forms of plants, had to prepare the way for
superior plants. The production of sufficient soil by mineralogical
decay as well as by the decay of organisms, plant and animal, may be
regarded as having been a preliminary to larger plant-growth; and
though at present the dependence is far less close than that among
animals, yet the benefits yielded to metaphytes by the decomposing
actions carried on by protophytes, as well as those carried on by
microbes permeating the soil, imply a continued general interdependence
throughout the aggregate of plant-forms, apart from more special
interdependences. And then along with this indebtedness of the greater
plants to the smaller during the process of evolution, there must be
named that indebtedness of plant-life to animal-life which Mr. Darwin
has shown in his book on the agency of worms as producers of mould.


§ 314_e_. Services of one to another, and consequent unions, of
more special kinds are infinitely varied, alike within each kingdom
and between the two kingdoms. I refer to those seen in parasitism,
commensalism, and other forms of association. While they do not
conduce to unions of the kind thus far considered, these nevertheless
constitute innumerable links whereby the lives of organisms, plant and
animal, are tied together; sometimes for the advantage of both but in
most cases for the benefit of one to the injury of the other.

Among plants the degrees of dependence are various. Unable to raise
themselves into the air and light, some climb, like the ivy, by
modified rootlets, or spirally coil themselves, or hang by tendrils.
Others there are which gradually strangle the trees they embrace, or
which, like lichens in damp climates, festooning the smaller trees,
by and by cause their decay. Of higher types of epiphytes which
use trees only to gain elevation, the orchids may be instanced.
And then we have plants which, like the mistletoe, fix themselves
on the bark of their hosts, utilizing them partly for purposes of
elevation and partly by appropriation of their juices. After these
may be named those extreme cases in which the parasitic plants,
ceasing to have any chlorophyll-bearing leaves, live wholly on the
juices of the invaded plants. At home the common dodder, and in the
tropics the _Rafflesiaceæ_, belong to this group. There must be
added the numerous forms of minute fungi which in like manner thrive
at the expense of the plants they infest. In all these cases the
interdependence is one-sided, though, as we shall presently see, while
detrimental to one of the two concerned, it is not always detrimental
to the organic world as a whole.

That utilization of one by another among animals which causes immediate
death, is familiar enough in the relations between carnivores and
herbivores. Almost as familiar are those seen in parasitism. Less
familiar are those seen in commensalism; and the least familiar are
those which show us exchange of services. Among these last--the
mutually-beneficial relations--that between the crocodile and the
bird which picks parasites out of its teeth is a striking one; and no
less so is that of the pique-gouffe, an African bird which pierces
the tumour on a buffalo’s back that incloses a parasite. Then of
another kind we have the connexion between aphides and ants: the
one profiting by being carried to better pastures and the other by
increased saccharine excretion. Next comes the class of messmates, the
connexions between some of which are relatively innocent, as witness
the Sea-anemone which settles itself on the shell occupied by a
Hermit-crab, or as witness the _Remora_ fixed on a shark’s skin. Less
innocent is the relation under which one of the two seizes a share
of the food obtained by the other, like the annelid which insinuates
itself between the Hermit-crab and the whelk-shell it inhabits, or like
the small fishes inhabiting certain _Medusæ_, or those which nestle
in the branchial sac of the _Lophius_. After these may be named the
less injurious forms of parasites proper--those which, distinguished
as _Epizoa_, fix themselves on the skins of their hosts, permanently
or temporarily, such as, of the one kind, the _Lernæa_ on fishes, and
of the other kind the Tick on mammals and birds. Then there come the
other class of parasites, most of them highly injurious, distinguished
as _Entozoa_, living within the bodies of their hosts, now in parts of
their alimentary canals, now on other of their mucous surfaces, and now
in various of their organs: these last two groups being so numerous in
their kinds that there are commonly more species than one proper to
each larger animal. One stage further in the complication meets us in
the parasites upon parasites.

But now the general fact, to which these brief indications are
introductory, is that the use made of one organism by another has been
ever widening and becoming more involved. Among plants utilization of
the larger by the smaller--of trees by epiphytes and parasites--must
have arisen since the times when the larger came into existence--times
relatively late in the course of organic evolution. Moreover most
of the plants which utilize others, either by climbing up them or
settling themselves high up on their stems or sucking their juices,
are phænogams, and the plants they utilize are also phænogams; so that
these innumerable interdependences must have been established since
the phænogamic type has become so predominant in respect of both size
and kind. Similarly among animals. Though there are many parasites
belonging, like the Trematodes, to very low classes, there are many
which belong to the _Arthropoda_, and, being degraded forms of that
class, must have come into existence after Arthropods of considerable
structure had been evolved. Again, a large part of the animals infested
by _Epizoa_ and _Entozoa_ are vertebrates--many of the highest types;
and as these are relatively modern all this parasitism must be of
late date. So, too, of much commensalism and many mutually-beneficial
associations. The reciprocal services of ants and aphides must have
originated since the _Hymenoptera_ and _Hemiptera_ became established
types, and since the days when certain insects of the ant-type had
become social, and since the days when aphides had become degraded
members of their order: both dates being relatively recent. And still
more recent must have been the commensalism between the ants and the
many species of other insects which inhabit their nests.

Leaving out relations of the kinds just named, it seems that down from
those between carnivores and their prey to those between lice and their
hosts, such relations profit one of the two species concerned and
injure the other, and that there the matter ends. But it does not end
there; for that multiplication of effects to which people are usually
blind, brings about changes which, as hinted above, though injurious
to the individual are beneficial to the species, and which, when not
beneficial to the species, are often beneficial to the aggregate of
species.

Even where animals of one class live by devouring animals of another
class, we see, on looking beyond the immediate results, certain remote
results that are advantageous. In the first place the process is
one by which inferior individuals--the least agile, swift, strong,
or sagacious--are picked out and prevented from leaving posterity
and lowering the average quality of their kind. At the same time
individuals made feeble by injury or old age, are among those to be
killed and saved from suffering prolonged pains: the evils of death by
disease and starvation being thus limited to the predatory animals,
relatively small in their numbers. Meanwhile a check is put on undue
multiplication. Where a tract of country has been overrun by rabbits,
weasels, thriving on the abundant supply of food, presently become
numerous enough to bring the population of rabbits within moderate
limits; and by doing this benefit not only all those kinds of plants
which are being eaten down, and all those other animals which live on
such plants, but also the rabbits themselves; since, increasing beyond
the means of subsistence, a large part of them would, if not killed,
die of hunger. Between aphides and lady-birds we see a connexion
of like nature: great increase of the first yielding abundant food
to larvæ of the second, ending after a season or so in swarms of
lady-birds, and consequently of their larvæ, whereby the aphides,
immensely diminished, cease so greatly to injure various plants and
the animals dependent on them. Even minute parasites, by the evils
they inflict on one species, profit others: instance the enormous
destruction of flies which a microscopic fungus caused a few years
ago--a destruction which relieved not only man but all the animals
which flies irritate: often so much as to hinder them from feeding.
Such instances remind us how numerous are the bonds by which the lives
of organisms are tied together.


§ 314_f_. I have reserved to the last the clearest and most striking
illustration of this progressing integration throughout the organic
world. I refer to the mutually-beneficial relations established between
plants and animals through the agency of flowers and insects.

Everyone nowadays has been made familiar with the process of
plant-fertilization, and knows that (leaving out of consideration
plants fertilized by wind-borne pollen) the ability to bear seed
depends largely on the aid given by bees, butterflies, and moths. The
exchange of services has been growing ever more various and complicated
during long past periods. We have the acquirement by flowers of bright
colours serving to guide these insects to places where honey is to be
found; and we have their perfumes, also serving for guidance. Then we
have the many different arrangements, often complicated, by which the
visiting insects are obliged to carry away pollen and dust with it the
stigmas of flowers on which they subsequently settle: thus effecting
crossfertilization. _Pari passu_ have gone on insect-developments
made possible by these arrangements and furthering them. Especially
must be named the modification of certain _Hymenoptera_ into
honey-storing bees: the implication being that the entire economy
established by these social insects has been sequent on the growth
of this system of reciprocal benefits. And then, just instancing the
dependence between a particular flower having a long tubular corolla,
and a particular moth having an appropriately long proboscis, it
suffices to say that innumerable specialities of this general relation
everywhere multiply the links by which the vegetal world and the animal
world are here connected. That the effects of the connections tell
largely on the prosperity of both, is suggested by some instances Mr.
Darwin gives, and by a statement recently made in the United States,
by Dr. L. O. Howard, that the greater fostering of bees would much
increase certain of the crops.

But now observe the broad fact to which these few details concerning
plant-fertilization are introductory. All these general and special
relations between plants and animals have arisen since the phænogamic
type came into existence--have, indeed, arisen since the higher members
of that type, the Angiosperms, have appeared; for the Gymnosperms do
not play any part in this intercommunion. But so far as we can judge
of present results of geologic explorations, there were no Angiosperms
during the Eozoic and Paleozoic periods. So that this class of
connexions between animals and vegetals must have been established
since carboniferous times--a period long, indeed, but far shorter than
that which organic evolution at large has occupied.


§ 314_g_. I have but just touched on some salient parts of a subject,
immense in extent and extremely involved, which it would take a volume
to set forth adequately. Enough has been said, however, to indicate the
truth which it is the purpose of the chapter to bring into view and
emphasize--the truth that both of the two great laws of evolution are
exemplified in the organic world as a whole, as they are exemplified in
every organism, and in all other things.

The reader has long since become familiar with the generalization that
while Evolution is a change from the homogeneous to the heterogeneous,
it is also a change from the incoherent to the coherent; and this
change from the incoherent to the coherent has been above exhibited
as going on even throughout that vast assemblage of organisms, plant
and animal, which cover the Earth’s surface. In what we are obliged
to conceive as the earliest stage, when the most minute types of life
alone existed, the aggregate of living things was at once homogeneous
and incoherent. In the course of epochs immeasurable in duration, this
uniform aggregate of beings has been becoming more multiform. And now
we see that instead of forms of life everywhere without the slightest
union caused by mutual dependence, there have slowly arisen forms of
life among which mutual dependences have entailed vital connexions
correspondingly marked. Along with progressing differentiation there
has ever been progressing integration. So that we may recognize
something like a growing life of the entire aggregate of organisms in
addition to the lives of individual organisms--an exchange of services
among parts enhancing the life of the whole.

In this final generalization the law of Evolution is manifested under
its most transcendental form.




                               PART VI.

                        LAWS OF MULTIPLICATION.




                              CHAPTER I.

                           THE FACTORS.[52]


§ 315. If organisms have been evolved, their respective powers of
multiplication must have been determined by natural causes. Grant that
the countless specialities of structure and function in plants and
animals, have arisen from the actions and reactions between them and
their environments, continued from generation to generation; and it
follows that from these actions and reactions have also arisen those
countless degrees of fertility which we see among them. As in all other
respects an adaptation of each species to its conditions of existence
is directly or indirectly brought about; so must there be directly or
indirectly brought about an adaptation of its reproductive activity to
its conditions of existence.

We may expect to find, too, that permanent and temporary differences
of fertility have the same general interpretation. If the small
variations of structure and function that arise within the limits
of each species, are due to actions like those which, by their
long-accumulating effects, have produced the immense contrasts between
the various types; we may conclude that, similarly, the actions to
which changes in the rate of multiplication of each species are due,
also produce, in great periods of time, the enormous differences
between the rates of multiplication of different species.

Before inquiring in what ways the rapidities of increase are adjusted
to the requirements, both temporary and permanent, it will be needful
to look at the factors. Let us set down first those which belong to the
environment, and then those which belong to the organism.


§ 316. Every living aggregate being one of which the inner actions are
adjusted to balance outer actions, it follows that the maintenance of
its moving equilibrium depends on its exposure to the right amounts of
these actions. Its moving equilibrium may be overturned if one of these
actions is either too great or too small in amount; and it may be so
overturned either by excess or defect of some inorganic agency in its
environment, or by excess or defect of some organic agency.

Thus a plant, constitutionally fitted to a certain warmth and
humidity, is killed by extremes of temperature, as well as by extremes
of drought and moisture. It may dwindle away from want of soil, or
die from the presence of too great or too small a quantity of some
mineral substance which the soil supplies to it. In like manner, every
animal can maintain the balance of its functions so long only as the
environment adds to or deducts from its heat at rates not exceeding
definite limits. Water, too, must be accessible in amount sufficient to
compensate loss. If the parched air is rapidly abstracting its liquid
which there is no pool or river to restore, its functions cease; and
if it is an aquatic creature, drought may kill it either by drying up
its medium or by giving it a medium inadequately aërated. Thus each
organism, adjusted to a certain average in the actions of its inorganic
environment, or rather, we should say, adjusted to certain moderate
deviations from this average, is destroyed by extreme deviations. So,
too, is it with the environing organic agencies. Among plants, only
the parasitic kinds and those united by symbiosis (as well as a few
innocent “lodgers”) depend for their individual preservation on the
presence of certain other organisms (though the presence of certain
other organisms is needful to most plants for the preservation of the
race by aiding fertilization). Here, for the continuance of individual
life, particular organisms must be absent or not very numerous--beasts
that browse, caterpillars that devour leaves, aphides that suck juices.
Among animals, however, the maintenance of the functional balance is
both positively and negatively dependent on the amounts of surrounding
organic agents. There must be an accessible sufficiency of the plants
or animals serving for food; and of organisms that are predatory or
parasitic or otherwise detrimental, the number must not pass a certain
limit.

This dependence of the moving equilibrium in every individual organism
on an adjustment of its forces to the forces of the environment, and
the overthrow of this equilibrium by failure of the adjustment, is
comprehensive of all cases. At first sight it does not seem to include
what we call natural death; but only death by violence, or starvation,
or cold, or drought. But in reality natural death, no less than every
other kind of death, is caused by the failure to meet some outer action
by a proportionate inner action. The apparent difference is due to
the fact that in old age, when the quantity of force evolved in the
organism gradually diminishes, the momentum of the functions becomes
step by step less, and the variations of the external forces relatively
greater; until there finally comes an occasion when some quite moderate
deviation from that average to which the feeble moving equilibrium is
adjusted, produces in it a fatal perturbation.


§ 317. The individuals of every species being thus dependent on certain
environing actions; and severally having their moving equilibria sooner
or later overthrown by one or other of these environing actions; we
have next to consider in what ways the environing actions are so met
as to prevent extinction of the species. There are two essentially
different ways. There may be in each individual a small or great
ability to adjust itself to variations of the agencies around it and to
a small or great number of such varying agencies--there may be little
or much power of preserving the balance of the functions. And there may
be much or little power of producing new individuals to replace those
whose moving equilibria have been overthrown. A few facts must be set
down to enforce these abstract statements.

There are both active and passive adaptations by which organisms
are enabled to survive adverse influences. Plants show us but few
active adaptations: that of the Pitcher-plant and those of the
reproductive parts of some flowers (which do not, however, conduce to
self-preservation) are exceptional instances. But plants have various
passive adaptations; as thorns, stinging hairs, poisonous and acrid
juices, repugnant odours, and the woolliness or toughness that makes
their leaves uneatable. Animals exhibit far more numerous adjustments,
both passive and active. In some cases they survive desiccation, they
hybernate, they acquire thicker clothing, and so are fitted to bear
unfavourable inorganic actions; and they are in many cases fitted
passively to meet the adverse actions of other organisms, by bearing
spines or armour or shells, by simulating neighbouring objects in
colour or form or both, by emitting disagreeable odours, or by having
disgusting tastes. In still more numerous ways they actively contend
with unfavourable conditions. Against the seasons they guard by storing
up food, by secreting themselves in crevices, or by forming burrows
and nests. They save themselves from enemies by developed powers of
locomotion, taking the shape of swiftness or agility or aptitude for
changing their media; by their strength either alone or aided by
weapons; lastly by their intelligence, without which, indeed, their
other superiorities would avail them little. And then these various
active powers serving for defence, become, in other cases, the powers
that enable animals to aggress, and to preserve their lives by the
success of their aggressions.

The second process by which extinction is prevented--the formation of
new individuals to replace the individuals destroyed--is carried on,
as described in the chapter on “Genesis,” by two methods, the sexual
and the asexual. Plants multiply by spontaneous fission, by gemmation,
by proliferation, and by the evolution of young ones from detached
cells and scales and leaves; and they also multiply by the casting
off of spores and sporangia and seeds. In like manner among animals,
there are varied kinds of agamogenesis, from spontaneous fission up to
parthenogenesis, all of them conducing to rapid increase of numbers;
and we have the more familiar process of gamogenesis, also carried
on in a great variety of ways. This formation of new individuals to
replace the old, is, however, inadequately conceived if we contemplate
only the number born or detached on each occasion. There are four
factors, all variable, on which the rate of multiplication depends.
The first is the age at which reproduction commences; the second is
the frequency with which broods are produced; the third is the number
contained in each brood; and the fourth is the length of time during
which the bringing forth of broods continues. There must be taken into
account a further element--the amount of aid given by the parent to
each germ in the shape of stored-up nutriment, continuous feeding,
warmth, protection, &c.: on which amount of aid, varying between
immensely wide limits, depends the number of the new individuals
that survive long enough to replace the old, and perform the same
reproductive process.

Thus, regarding every living organism as having a moving equilibrium
dependent on environing forces, but ever liable to be overthrown by
irregularities in those forces, and always so overthrown sooner or
later; we see that each species of organism can be maintained only
by the generation of new individuals with a certain rapidity, and by
helping them more or less fully to establish their moving equilibria.


§ 318. Such are the factors with which we are here concerned. I have
presented them in abstract shapes for the purpose of showing how they
are expressible in general terms of force--how they stand related to
the ultimate laws of re-distribution of matter and motion.

For the purposes of the argument now to follow, we may, however,
conveniently deal with these factors under a more familiar guise.
Ignoring their other aspects, we may class the factors which affect
each race of organisms as forming two conflicting sets. On the one
hand, by what we call natural death, by enemies, by lack of food, by
atmospheric changes, &c., the race is constantly being destroyed. On
the other hand, partly by the endurance, the strength, the swiftness,
and the sagacity of its members, and partly by their fertility, it
is constantly being maintained. These conflicting sets of factors
may be generalized as--the forces destructive of race and the forces
preservative of race. So generalizing them, let us ask what are the
necessary implications.




                              CHAPTER II.

                         _À PRIORI_ PRINCIPLE.


§ 319. The number of a species must at any time be either decreasing
or stationary or increasing. If, generation after generation, its
members die faster than others are born, the species must dwindle
and finally disappear. If its rate of multiplication is equal to its
rate of mortality, there can be no numerical change in it. And if the
deductions by death are fewer than the additions by birth, the species
must become more abundant. These we may safely set down as necessities.
The forces destructive of race must be either greater than the forces
preservative of race, or equal to them, or less than them; and there
cannot but result these effects on number.

We are here concerned only with races that continue to exist; and may
therefore leave out of consideration those in which the destructive
forces, remaining permanently in excess of the preservative forces,
cause extinction. Practically, too, we may exclude the stationary
condition; for the chances are infinity to one against the maintenance
of a permanent equality between the births and the deaths. Hence, our
inquiry resolves itself into this:--In races that continue to exist,
what laws of numerical variation result from these variable conflicting
forces, which are respectively destructive of race and preservative of
race?


§ 320. Clearly if the forces destructive of race, when once in excess,
had nothing to prevent them from remaining in excess, the race would
disappear; and clearly if the forces preservative of race, when once in
excess, had nothing to prevent them from remaining in excess, the race
would go on increasing to infinity. In the absence of any compensating
actions, the only possible avoidance of these opposite extremes would
be an unstable equilibrium between the conflicting forces, resulting
in a perfectly constant number of the species: a state which we know
does not exist, and against the existence of which the probabilities
are, as already said, infinite. It follows, then, that as in every
continuously-existing species, neither of the two conflicting sets
of forces remains permanently in excess; there must be some way of
stopping that excess of the one or the other which is ever occurring.

How is this done? Should any one allege, in conformity with the old
method of interpretation, that there is in each case a providential
interposition to rectify the disturbed balance, he commits himself to
the supposition that of the millions of species inhabiting the Earth,
each one is yearly regulated in its degree of fertility by a miracle;
since in no two years do the forces which foster, or the forces which
check, each species, remain the same; and therefore, in no two years
is there required the same fertility to balance the mortality. Few if
any will say that God continually alters the reproductive activity
of every parasitic fungus and every Tape-worm or _Trichina_, so as
to prevent its extinction or undue multiplication; which they must
say if they adopt the hypothesis of supernatural adjustment. And in
the absence of this hypothesis there remains only one other. The
alternative possibility is, that the balance of the preservative and
destructive forces is self-sustaining--is of the kind distinguished as
a stable equilibrium: an equilibrium such that any excess of one of
the forces at work, itself generates, by the deviation it produces,
certain counter-forces which eventually out-balance it, and initiate an
opposite deviation. Let us consider how, in the case before us, such a
stable equilibrium must be constituted.


§ 321. When a season favourable to it, or a diminution of creatures
detrimental to it, causes any species to become more numerous than
usual, an immediate increase of certain destructive influences takes
place. If it be a plant, the supposed greater abundance itself implies
fuller occupation of the places available for growth--an occupation
which, leaving fewer such places as the multiplication goes on,
becomes a check on further multiplication--itself causes a greater
mortality of seeds that fail to root themselves. And afterwards, in
addition to this passive resistance to continued increase, there comes
an active resistance: the creatures which thrive at the expense of
the species--the larvae, the birds, the herbivores--increase too. If
it be an animal that has grown more numerous, then, unless by some
exceptional coincidence a simultaneous and proportionate addition to
the animals or plants serving for food has occurred, there must result
a relative scarcity of food. Enemies, too, be they beasts of prey or
be they parasites, must quickly begin to multiply. Hence, each kind
of organism, previously existing in something like its normal number,
cannot have its number raised without a rise of the destructive forces,
negative and positive, quickly commencing. Both negative and positive
destructive forces must augment until this increase of the species is
arrested. The competition for places on which to grow, if the species
be vegetal, or for food if it be animal, must become more intense as
the over-peopling of the habitat progresses; until there is reached
the limit at which the mortality equals the reproduction. And as, at
the same time, enemies will multiply with a rapidity which soon brings
them abreast of the augmented supply of prey, the positive restraint
they exert will help to bring about an earlier arrest of the expansion
than pressure of population alone would cause. One more inference may
be drawn. Had the species to meet no repressing influence save that
negative one of relatively-diminished space or relatively-diminished
food-supply, the cause leading to its increase might carry it up
to the limit set by this, and there leave it: its enlarged number
might be permanent. But the positive repressing influence that has
been called into existence, will prevent this. For the increase of
enemies, commencing, as it must, after the increase of the species,
and advancing in geometrical progression until it is itself checked
in like manner, will end in an excess of enemies. Whereupon must
result a mortality of the species greater than its multiplication--a
decrease which will continue until its habitat is under-peopled, its
unduly-numerous enemies decimated by starvation, and the destroying
agencies reduced to a minimum. Whence will follow another increase.

Thus, as before indicated (_First Prin._ §§ 85, 173), there is
here, as wherever antagonistic forces are in action, an alternate
predominance of each, causing a rhythmical movement--a rhythmical
movement which constitutes a moving equilibrium in those cases where
the forces are not dissipated with appreciable rapidity, or are
re-supplied as fast as they are dissipated. While, therefore, on the
one hand, we see that the continued existence of a species necessarily
implies some action by which the destructive and preservative forces
are self-adjusted; we see, on the other hand, that such an action is an
inevitable consequence of the universal process of equilibration.


§322. Is this the sole equilibration which must exist? Clearly not.
The temporary compensating adjustments of multiplication to mortality
in each species, are but introductory to the permanent compensating
adjustments of multiplication to mortality among species in general.
The above reasoning would hold just as it now does, were all species
equally prolific and all equally short-lived. It yields no answer
to the inquiries--why do their fertilities differ so enormously,
or why do their mortalities differ so enormously? and how is the
general fertility adapted to the general mortality in each? The
balancing process we have contemplated can go on only within moderate
limits--must fail entirely in the absence of a due proportion
between the ordinary birth-rate and the ordinary death-rate. If the
reproduction of mice proceeded as slowly as the reproduction of men,
mice would be extinct before a new generation could arise: even did
their natural lives extend to fifteen or sixteen years, it would still
be extremely improbable that any would for so long survive all the
dangers they are exposed to. Conversely, did oxen propagate as fast
as infusoria, the race would die of starvation in a week. Hence, the
minor adjustment of varying multiplication to varying mortality in each
species, implies some major adjustment of average multiplication to
average mortality. What must this adjustment be?

We have already seen that the forces preservative of race are
two--ability in each member of the race to preserve itself, and ability
to produce other members--power to maintain individual life, and power
to generate the species. These must vary inversely. When, from lowness
of organization, the ability to contend with external dangers is
small, there must be great fertility to compensate for the consequent
mortality; otherwise the race must die out. When, on the contrary, high
endowments give much capacity of self-preservation, a correspondingly
low degree of fertility is requisite. Given the dangers to be met as a
constant quantity; then as the ability to meet them must be a constant
quantity too; and as this is made up of the two factors, power to
maintain individual life and power to multiply, these cannot do other
than vary inversely: one must decrease as the other increases.

It needs but to conceive the results of nonconformity to this law,
to see that every species must either conform to it or cease to
exist. Suppose, first, a species whose individuals, having but small
self-preservative powers, are rapidly destroyed, to be at the same
time without reproductive powers proportionately great. The defect
of fertility, if extreme, will result in the death of one generation
before another has grown up. If less extreme, it will entail a
scarcity such that in the next generation sexual congress will be too
infrequent to maintain even the small number which remains; and the
race will dwindle with increasing rapidity. If still less extreme, the
consequent degree of sparseness, while not so great as to prevent an
adequate number of procreative unions, will be so great as to render
special food abundant and special enemies few--will thus diminish the
destructive forces so much that the self-preservative forces will
become _relatively_ great: so great, relatively, that when combined
with the small ability to propagate the species, they will suffice to
balance the small destructive forces. Suppose, next, a species whose
individuals have high powers of self-preservation, while they have
powers of multiplication much beyond what is needful. The excess of
fertility, if extreme, will cause sudden extinction of the species by
starvation. If less extreme, it must produce a permanent increase in
the number of the species; and this, followed by intenser competition
for food and augmented number of enemies, will involve such an increase
of the dangers to individual life, that the great self-preserving
powers of the individuals will not be more than sufficient to cope with
them. That is to say, if the fertility is relatively too great, then
the ability to maintain individual life inevitably becomes smaller,
_relatively_ to the requirements; and the inverse proportion is thus
established.

So that when, from comparing the different states of the same species,
we go on to compare the states of different species, we see that there
is an analogous adjustment--analogous in the sense that great mortality
is associated with great multiplication, and small mortality with small
multiplication. And we see that the unlikeness of the cases consists
merely in this, that what is a temporary relation in the one is a
permanent relation in the other.


§ 323. For the moment it does not concern us to inquire what is the
origin of this permanent relation. That which we have now to note, is
simply that in some way or other there must be established an inverse
proportion between the power to sustain individual life and the power
to produce new individuals. Whether or not this permanent relation is
self-adjusting in long periods of time, as the temporary relation is
self-adjusting in short periods of time, is a separate question. The
purpose of this chapter is fulfilled by showing that such a permanent
relation must exist.

But having recognized the _à priori_ principle that in races which
continuously survive, the forces destructive of race must be
equilibrated by the forces preservative of race; and that, supposing
these are constant, there must be an inverse proportion between
self-preservation and race-preservation; we may go on to inquire how
this relation, necessary in theory, arises in fact. Leaving out the
untenable hypothesis of a supernatural pre-adjustment, we have to ask
in what way an adjustment comes about as a result of Evolution. Is it
due to the survival of varieties in which the proportion of fertility
to mortality happens to be the best? Or is the fertility adapted to the
mortality in a more direct way? To these questions let us now address
ourselves.




                             CHAPTER III.

                     OBVERSE _À PRIORI_ PRINCIPLE.


§ 324. When dealing with its phenomena inductively, we saw that however
it may be carried on, Genesis “is a process of negative or positive
disintegration; and is thus essentially opposed to that process of
integration which is the primary process in individual evolution.”
(§ 76.) Each new individual, whether separated as a germ or in some
more-developed form, is a deduction from the mass of a pre-existing
individual or of two pre-existing individuals. Whatever nutritive
matter is stored-up along with the germ, if it be deposited in the
shape of an egg, is so much nutritive matter lost to the parent.
No drop of blood can be absorbed by the fœtus, nor any draught of
milk sucked by the young when born, without taking from the mother
tissue-forming and force-evolving materials to an equivalent amount.
And all subsequent supplies given to progeny, if they are nurtured,
involve, to a parent or parents, so much waste in exertion which does
not bring its return in assimilated food.

Conversely, the continued aggregation of materials into one organism,
renders impossible the formation of other organisms out of those
materials. As much assimilated food as is united into a single whole,
is so much assimilated food withheld from a plurality of wholes which
might else have been produced. Given the absorbed nutriment as a
constant quantity, and the longer the building of it up into a concrete
shape goes on, the longer must be postponed any building of it up into
discrete shapes. And, similarly, the larger the proportion of matter
consumed in the functional actions of parents, the smaller must be the
proportion of matter which can remain to establish and support the
functional actions of offspring.

Though the necessity of these universal relations is tolerably obvious
as thus stated generally, it will be useful to dwell for a brief space
on their leading aspects.


§ 325. That disintegration which constitutes genesis, may be such as
to disperse entirely the aggregate which integration has previously
produced--the parent may dissolve wholly into progeny. This dissolution
of each aggregate into two or many aggregates, may occur at very short
intervals, in which case the bulk attained can be but extremely small;
or it may occur at longer intervals, in which case a larger bulk may be
attained.

Instead of quickly losing its own individuality in the individualities
of its offspring, each member of the race may, after growing for a
time, have portions of its substance begin to develop into the parental
shape and presently detach themselves; and the parent, maintaining its
own identity, may continue indefinitely so to produce young ones. But
clearly, the earlier it commences doing this, and the more rapidly it
does it, the sooner must the increase of its own bulk be stopped.

Or again, growth and development continuing for a long period without
any deduction of materials, an individual of considerable size and
organization may result; and then the abstraction of substance for
the formation of new individuals, or rather the eggs of them, may
be so great that as soon as the eggs are laid the parent dies of
exhaustion--dies, that is, from an excessive loss of the nutritive
matters needed for its own activities.[53]

Once more, the deduction of materials for the propagation of the
species may be postponed long enough to allow of great bulk and complex
structure being attained. The procreative subtraction then setting
in, while it checks and presently stops growth, may be so moderate
as to leave vital capital sufficient to carry on the activities of
the parent; may go on as long as parental vigour suffices to furnish,
without fatal result, the materials needed to produce young ones; and
may cease when such a surplus cannot be supplied, leaving the parental
life to continue.


§ 326. The opposite side of this antagonism has also several aspects.
Progress of organic evolution may be shown in increased bulk, in
increased structure, in increased amount or variety of action, or
in combinations of these; and under any of its forms this carrying
higher of each individuality, implies a correlative retardation in the
establishment of new individualities.

Other things equal, every normal addition to the bulk of an organism
is an augmentation of its life.[54] Besides being an advance in
integration, it implies a greater total of activities gone through in
the assimilation of materials; and it implies, thereafter, a greater
total of the vital changes taking place from moment to moment in all
parts of the enlarged mass. Moreover, while increased size is thus,
in so far, the expression of increased life, it is also, where the
organism is active, the expression of increased ability to maintain
life--increased strength. Aggregation of substance is almost the only
mode in which self-preserving power is shown among the lowest types;
and even among the highest, sustaining the body in its integrity
is that in which self-preservation fundamentally consists--is the
end which the widest intelligence is indirectly made to subserve.
While, on the one hand, the increase of tissue constituting growth is
conservative both in essence and in result; on the other hand, decrease
of tissue, either from injury, disease, or old age, is in both essence
and result the reverse. And if so, every addition to individual life
thus implied, necessarily delays or diminishes the casting off of
matter to form new individuals.

Other things equal, too, a greater degree of organization involves
a smaller degree of that disorganization shown by the separation
of reproductive gemmæ and germs. Detachment of a living portion or
portions from what was previously a living whole, is a ceasing of
co-ordination; and is therefore essentially at variance with that
establishment of greater co-ordination which is achieved by structural
development. In the extreme cases where a living mass is continually
dividing and subdividing, it is manifest that there cannot arise
much physiological division of labour; since progress towards mutual
dependence of parts is prevented by the parts becoming independent.
Contrariwise, it is equally clear that in proportion as the
physiological division of labour is carried far, the separative process
must be localized in some comparatively small portion of the organism,
where it may go on without affecting the general structure--must
become relatively subordinate. The advance that is shown by greater
heterogeneity, must be a hindrance to multiplication in another way.
For organization entails cost. That transfer and transformation
of materials implied by differentiation, can be effected only by
expenditure of force; and this supposes consumption of digested and
absorbed food, which might otherwise have gone to make new organisms,
or the germs of them. Hence, that individual evolution which consists
in progressive differentiation, as well as that which consists in
progressive integration, necessarily diminishes that species of
dissolution, general or local, which propagation of the race exhibits.

In active organisms we have yet a further opposition between
self-maintenance and maintenance of the race. All motion, sensible and
insensible, generated by an animal for the preservation of its life,
is motion liberated from decomposed nutriment--nutriment which, if
not thus decomposed, would have been available for reproduction; or
rather--might have been replaced by nutriment fitted for reproductive
purposes, absorbed from other kinds of food. Hence, in proportion as
the activities increase--in proportion as, by its more varied, complex,
rapid, and vigorous actions, an animal gains power to support itself
and to cope with surrounding dangers, it must lose power to propagate.


§ 327. How may this antagonism be best expressed in a brief way? If
self-preservation displayed itself in the highest organisms, as it
does in the lowest, in little else but continuous growth; and if
race-preservation consisted always, as it does often, of nothing beyond
detachment of portions from the parental mass; then the antagonism
would be, throughout, the obviously-necessary one of integration and
disintegration. Maintenance of the individual and propagation of the
species, being respectively aggregative and separative, it would be
as self-evident that they vary inversely, as it is self-evident that
addition and subtraction undo one another. But though the simplest
types show us the opposition of self-maintenance and race-maintenance
almost wholly under this form; and though higher types, up to the
most complex, exhibit it to a great extent under this form; yet, as
we have just seen, this is not its only form. The total material
monopolized by the individual and withheld from the race, must be
stated as the quantity united to form its fabric, _plus_ the quantity
expended in differentiating its fabric, _plus_ the quantity expended
in its self-conserving actions. Similarly, the total material devoted
to the race at the expense of the individual, includes that which is
directly subtracted from the parent in the shape of egg or fœtus,
_plus_ that which is directly subtracted in the shape of milk, _plus_
that which is indirectly subtracted in the shape of matter consumed
in exertions for fostering the young. Hence this inverse variation
is not expressible in simple terms of aggregation and separation. As
we advance to more highly-evolved organisms, the total cost of an
individual becomes very much greater than is implied by the amount of
tissue composing it. So, too, the total cost of producing each new
individual becomes very much greater than that of its mere substance.
And it is between these two total costs that the antagonism exists.

We may, indeed, reduce the antagonism to a form comprehensive of all
cases, if we consider it as existing between the sums of the forces,
latent and active, used for the two purposes. The molecules which make
up a plant or animal, have been formed by the absorption of forces
directly or indirectly derived from the Sun; and hence the quantity
of matter raised to the form called organic, which a plant or animal
presents, is equivalent to a certain amount of force. Another amount of
force is expressed by the totality of its differentiations. A further
amount of force is that dissipated in its actions. And in these three
amounts added together, we have the whole expense of the individual
life. So, too, the whole expense of establishing each new individual
includes--first the forces latent in the substance composing it when
born or hatched; second the forces latent in the prepared nutriment
afterwards supplied; and third the forces expended in feeding and
protecting it. These two sets of forces being taken from a common fund,
it is manifest that either set can increase only by decrease of the
other. If, of the force which the parent obtains from the environment,
much is consumed in its own life, little remains to be consumed in
producing other lives; and, conversely, if there is a great consumption
in producing other lives, it can only be where comparatively little is
reserved for parental life.

Hence, then, Individuation and Genesis are necessarily antagonistic.
Grouping under the word Individuation all processes by which
individual life is completed and maintained; and enlarging the
meaning of the word Genesis so as to include all processes aiding
the formation and perfecting of new individuals; we see that the
two are fundamentally opposed. Assuming other things to remain the
same--assuming that environing conditions as to climate, food,
enemies, &c., continue constant; then, inevitably, every higher
degree of individual evolution is followed by a lower degree of
race-multiplication, and _vice versâ_. Progress in bulk, complexity, or
activity, involves retrogress in fertility; and progress in fertility
involves retrogress in bulk, complexity, or activity.

This statement needs a slight qualification. For reasons to be
hereafter assigned, the relation described is never completely
maintained; and in the small departure from it, we shall find a
remarkable self-acting tendency to further the supremacy of the most
developed types. Here, however, this hint must suffice: explanation
would carry us too far out of our line of argument. For the present
it will not lead us astray if we regard this inverse variation of
Individuation and Genesis as exact.


§ 328. Thus, then, the condition which each race must fulfil if it is
to survive, is a condition which, in the nature of things, it ever
tends to fulfil. In the last chapter we saw that a species cannot be
maintained unless the power to preserve individual life and the power
to propagate other individuals vary inversely. And here we have seen
that, irrespective of an end to be subserved, these powers cannot do
other than vary inversely. On the one hand, given a certain totality
of destroying forces with which the species has to contend; and in
proportion as its members have severally but small ability to resist
these forces, it is requisite that they should have great ability to
form new individuals, and _vice versâ_. On the other hand, given the
quantity of force, absorbed as food or otherwise, which the species
can use to counterbalance these destroying forces; and in proportion
as much of it is expended in preserving the individual, little of it
can be reserved for producing new individuals, and _vice versâ_. There
is thus complete accordance between the requirements considered under
each aspect. The two necessities correspond.

We might rest on these deductions and their several corollaries.
Without going further we might with safety assert the general truths
that, other things equal, advancing evolution must be accompanied
by declining fertility; and that, in the highest types, fertility
must still further decrease if evolution still further increases.
We might be sure that if, other things equal, the relations between
an organism and its environment become so changed as permanently
to diminish the difficulties of self-preservation, there will be a
permanent increase in the rate of multiplication; and, conversely, that
a decrease of fertility will result where altered circumstances make
self-preservation more laborious.

But we need not content ourselves with these _à priori_ inferences. If
they are true, there must be an agreement between them and the observed
facts. Let us see how far such an agreement is traceable.




                              CHAPTER IV.

                DIFFICULTIES OF INDUCTIVE VERIFICATION.


§ 329. Were all species subject to the same kinds and amounts of
destructive forces, it would be easy, by comparing different species,
to test the inverse variation of Individuation and Genesis. Or if
either the power of self-preservation or the power of multiplication
were constant, there would be little difficulty in seeing how the
other changed as the destroying forces changed. But comparisons are
nearly always partially vitiated by some want of parity. Each factor,
besides being variable as a whole, is compounded of factors that are
severally variable. Not simply is the sum of the forces destructive of
race different in every case; and not simply are both sets of forces
preservative of race unlike in their totalities in every case; but
each is made up of actions that bear such changing proportions to one
another as to prevent any positive estimation of its amount.

Before dealing with the facts as well as we can, it will be best to
glance at the chief difficulties; so that we may see the kind of
verification which is alone possible.


§ 330. Either absolutely, or relatively to any species, every
environment differs more or less from every other.

There are the unlikenesses of media--air, water, earth, organic matter;
severally involving special resistances to movement, and special losses
of heat. There are the contrasts of climate: here great expenditure
for the maintenance of temperature is needed, and there very little;
in one zone an organism is supplied with abundant light all the year
round, and in another only for a few months; this region yields an
almost unfailing supply of water, while that entails the exertion of
travelling many miles every night for a draught.

Permanent differences in the natures and distributions of aliment
greatly interfere with the comparisons. The Swallow goes through more
exertion than the Sparrow in securing a given weight of food; but then
their foods are dissimilar in nutritive qualities. There is a want of
parallelism between the circumstances of those herbivores which live
where the plains are annually covered for a time with rich herbage, but
afterwards become parched up, and of those inhabiting more temperate
regions. Insects whose larvæ feed on an abundant plant, as do several
of the genus _Vanessa_ on the Nettle, have practically an environment
very unlike that of insects such as _Deilephila Euphorbiæ_, whose larvæ
feed on a comparatively rare plant--the Sea-Spurge.

Again, comparisons between creatures otherwise akin in their
constitutions and circumstances, are hindered by inequalities in their
relations to enemies. Two animals, of which one is predatory and has
no foes but parasites while the other is much pursued, cannot properly
be contrasted with a view to determining the influence of size or
complexity.

Without multiplying instances, it will be clear enough then that the
aggregate of destructive actions, positive and negative, which each
species has to contend with, is so undefinable in the amounts and kinds
of its components, that nothing beyond a vague idea of its relative
total can be formed.


§ 331. Besides these immense variations in the outer actions to
be counter-balanced, there are immense variations in the inner
actions required to counterbalance them. Even were species similarly
conditioned, self-preservation would require of them extremely unlike
expenditures of force.

The cost of locomotion increases in a greater ratio than the size. In
virtue of the law that the weights of animals increase as the cubes
of their dimensions, while their powers of bearing strains increase
only as the squares of their dimensions (§ 46), preservation of its
various attitudes requires a large animal to consume more substance in
proportion to its weight, than it requires a small animal to consume;
and there results, other things equal, a difficulty of self-maintenance
which augments in a more rapid ratio than the bulk. Nor must we
overlook the further complication, that among aquatic creatures the
variation of resistance of the medium tends to produce an opposite
effect.

Again, the heat-consumption is a changing element in the total expense
of self-preservation. Creatures which have temperatures scarcely
above that of the air or water, may, other things equal, accumulate
more surplus nutriment than creatures which have to keep their bodies
warm spite of the continual loss by radiation and conduction. This
difference of cost is modified by the presence or absence of natural
clothing; and it is also modified by unlikenesses of size. Here the
bulky animals have the advantage: small masses cooling more rapidly
than large ones.

Dissimilarities of attack and defence are also causes of variation
in the outlay for self-maintenance. A creature that has to hunt, as
compared with another that gets a sufficiency of prey by lying in wait,
or a creature that escapes by speed as compared with another that
escapes by concealment, obviously leads a life that is physiologically
more costly. Animals which protect themselves passively, as the
Hedge-hog by its spines or as the Skunk and the Musk-rat by their
intolerable odours, are relatively economical; and have the more vital
capital for other purposes.

Amplification is needless. These instances will show that anything
beyond very general conceptions of the individual expenditures in
different cases, cannot be reached.


§ 332. Still more entangled are we among qualifying considerations
when we contrast species in their powers of multiplication. The total
cost of Genesis admits of even less definite estimation than does the
total cost of Individuation. I do not refer merely to the truth that
the degree of fertility depends on four factors--the age of commencing
reproduction, the number in each brood, the frequency of the broods,
and the time during which broods continue to be repeated. There are
many further obstacles in the way of comparisons.

Were all multiplication carried on sexually, the problem would be
less involved; but there are many kinds of asexual multiplication
alternating with the sexual. This asexual multiplication is in some
cases perpetual instead of occasional; and often has more forms than
one in the same species. The result is that we have to compare what
is here a periodic process with what is elsewhere a cyclical process
partly continuous and partly periodic: the calculation of fertility in
this last case being next to impossible.

We have to avoid being misled by the assumption that the cost of
Genesis is measured by the number of young produced, instead of being
measured, as it is, by the weight of nutriment abstracted to form
the young, _plus_ the weight consumed in caring for them. This total
weight may be very diversely apportioned. In contrast to the Cod with
its millions of small ova spawned without protection, we may put the
_Hippocampus_, or the Pipe-fish, with its few relatively-large ova
carried about by the male in a caudal pouch, or seated in hemispherical
pits in its skin; or we may put the still more remarkable genus
_Arius_, and especially _Arius Boakeii_--a fish some six or seven
inches long, which produces ten or a dozen eggs 5–10 mm. in diameter,
that are carried by the male in his mouth till they are hatched.
Here though the degrees of fertility, if measured by the numbers of
fertilized germs deposited, are extremely unlike, they are less unlike
if measured by the numbers of young which are hatched and survive long
enough to take care of themselves; nor will the tax on the parent-Cod
seem so immensely different from that on the parent-_Arius_, if the
masses of the ova, instead of their numbers, are compared. Again, while
sometimes the parental loss is little else but the matter deducted to
form eggs, &c., at other times it takes the shape of a small direct
deduction joined with a large indirect outlay. The Mason-wasp furnishes
a typical instance. In journeyings hither and thither to fetch bit
by bit the materials for building a cell; in putting together these
materials, as well as in secreting glutinous matter to act as cement;
and then, afterwards, in the labour of seeking for, and carrying, the
small caterpillars with which it fills up the cell to serve its larva
with food when it emerges from the egg; the Mason-wasp expends more
substance than is contained in the egg itself. And this supplementary
expenditure is manifestly so great that but few eggs can be housed and
provisioned.

Estimates of the cost of Genesis are further complicated by variations
in the ratio borne by the two sexes. Among Fishes the mass of milt
approaches in size the mass of spawn; but among higher _Vertebrata_
the substance lost by the one sex in the shape of sperm-cells is small
compared with that lost by the other sex in the shape of albumen
stored-up in the eggs, or blood supplied to the fœtus, or milk given
to the young. Then there come the differences of indirect tax on males
and females. While, frequently, the fostering of the young devolves
entirely on the female, occasionally the male undertakes it wholly or
in part. After building a nest, the male Stickleback guards the eggs
till they are hatched; as does also the great _Silurus glanis_ for some
forty days, during which he takes no food. And then, among most birds,
we have the male occupied in feeding the female during incubation,
and the young afterwards. Evidently all these differences affect the
proportion between the total cost of reproduction and the total cost of
individuation.

Whether the species is monogamous or polygamous, and whether there are
marked differences of size or of structure between males and females,
are also questions not to be overlooked. If there are many females to
one male, the total quantity of assimilated matter devoted by each
generation to the production of a new generation, is greater than if
there is a male to each female. Similarly, where the requirements
are such that small males will suffice, the larger quantity of food
left for the females makes possible a greater surplus available for
reproduction. Another cause has a like effect. Where the habits of the
race render it needless that both sexes should have developed powers of
locomotion--where, as in the Glow-worm and sundry _Lepidoptera_, the
female is wingless while the male has wings--the cost of Individuation
not being so great for the species as a whole, there arises a greater
reserve for Genesis: the matter which would otherwise have gone to the
production of wings and the using of them, may go to the production of
ova.

Other complications, as those which we see in Bees and Ants, might be
dwelt on; but the foregoing will amply serve the intended purpose.


§ 333. To ascertain by comparison of cases whether Individuation
and Genesis vary inversely, is thus an undertaking so beset with
difficulties, that we might despair of any satisfactory results,
were not the relation too marked a one to be hidden even by all
these complexities. Species are so extremely contrasted in their
degrees of evolution, and so extremely contrasted in their rates of
multiplication, that the law of relation between these traits becomes
unmistakable when the evidence is looked at in its _ensemble_. This we
shall soon find on ranging in order a number of typical cases.

In doing this it will be convenient to neglect, temporarily,
all unlikenesses among the circumstances in which organisms are
placed. At the outset, we will turn our attention wholly to the
antagonism displayed between the integrative process which results in
individual evolution and the disintegrative process which results in
multiplication of individuals; and this we will consider first as we
see it under the several forms of agamogenesis, and then as we see
it under the several forms of gamogenesis. We will next look at the
antagonism between propagation and that evolution which is shown by
increased complexity. And then we will consider the remaining phase of
the antagonism, as it exists between the degree of fertility and the
degree of evolution expressed by activity.

Afterwards, passing to the varying relations between organisms and
their environments, we will note how relative increase in the supply
of food, or relative decrease in the quantity of force expended by the
individual, entails relative increase in the quantity of force devoted
to multiplication, and _vice versâ_.

Certain minor qualifications, together with sundry important
corollaries, may then be entered upon.




                              CHAPTER V.

            ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS.


§ 334. When illustrating, in Part IV, the morphological composition of
plants and animals, there were set down in groups, numerous facts which
we have here to look at from another point of view. Then we saw how,
by union of small simple aggregates, there are produced large compound
aggregates. Now we have to observe the reactive effect of this process
on the relative numbers of the aggregates. Our present subject is the
antagonism of Individuation and Genesis as seen under its simplest
form, in the self-evident truth that the same quantity of matter may
be divided into many small wholes or few large wholes; but that number
negatives largeness and largeness negatives number.

In setting down some examples we may conveniently adopt the same
arrangement as before. We will look at the facts as they are presented
by vegetal aggregates of the first order, of the second order, and of
the third order; and then as they are presented by animal aggregates of
the same three orders.


§ 335. The ordinary unicellular plants are at once microscopic and
enormously prolific. The often cited _Sphærella nivalis_, which shows
its immense powers of multiplication by reddening wide tracts of snow
in a single night, does this by developing in its cavity a brood of
young cells, which, being presently set free by the bursting of the
parent-cell, severally grow and quickly repeat the process. The like
occurs among sundry of those kindred forms of minute _Algæ_ which,
by their enormous numbers, sometimes suddenly change pools to an
opaque green. So, too, the _Desmidiaceæ_ often multiply so greatly as
to colour the water; and among the _Diatomaceæ_ the rate of genesis
by self-division, “is something really extraordinary. So soon as a
frustule is divided into two, each of the latter at once proceeds
with the act of self-division; so that, to use Professor Smith’s
approximative calculation of the possible rapidity of multiplication,
supposing the process to occupy, in any single instance, twenty-four
hours, ‘we should have, as the progeny of a single frustule, the
amazing number of one thousand millions in a single month.’” In these
cases the multiplication is so carried on that the parent is lost in
the offspring--the old individuality disappears either in the swarms of
zoospores it dissolves into, or in the two or four new individualities
simultaneously produced by fission. Vegetal aggregates of the first
order, have, however, a form of agamogenesis in which the parent
individuality is not lost: the young cells arise from the old cells
by external gemmation. This process, too, repeated as it is at short
intervals, results in immense fertility. The Yeast-fungus, which in a
few hours thus propagates itself throughout a large vat of wort, offers
a familiar example.

In certain compound forms that must be classed as plants of the second
order of aggregation, though very minute ones, self-division similarly
increases the numbers at high rates. The _Sarcina ventriculi_, a
parasitic plant which infests the stomach and swarms afresh as fast as
previous swarms are vomited, shows us a spontaneous fission of clusters
of cells. An allied mode of increase occurs in _Gonium pectorale:_
each cell of the cluster resolving itself into a secondary cluster,
and the secondary clusters then separating. “Supposing, which is very
probable, that a young _Gonium_ after twenty-four hours is capable of
development by fission, it follows that under favourable conditions
a single colony may on the second day develop 16, on the third 256,
on the fourth 4,096, and at the end of a week 268,435,456 other
organisms like itself.” In the _Volvocineæ_ this continual dissolution
of a primary compound individual into secondary compound individuals,
is carried on endogenously, and on a modified system: some only of
the component cells giving origin to young colonies, and the parent
bursting to liberate them. The numbers arising by this method also,
are sometimes so great as to tint large bodies of water. More fully
established and organized aggregates of the second order, such as the
higher Thallophytes and the lower Archegoniates, do not sacrifice
their individualities by fission; but nevertheless, by the kindred
process of gemmation, are continually hindered in the increase of their
individualities. The gemmæ called tetraspores are cast off in great
numbers by the marine _Algæ_. Among those simple _Jungermanniaceæ_
which consist of single fronds, the young ones that bud out grow for a
time in connexion with their parents, send rootlets from their under
sides into the soil, and presently separate themselves--a habit which
augments the number of individuals in proportion as it checks their
growths.

Plants of the third order of composition, arising by arrest of this
separation, exhibit a further corresponding decrease in the abundance
of the aggregates formed. Archegoniates of inferior types, in which
the axes produced by integration of fronds are but small and feeble,
are characterized by the habit of throwing off bulbils--bud-shaped
axes which, falling and taking root, add to the number of distinct
individuals. This agamic multiplication, very general among the
Mosses and their kindred, and not uncommon under a modified form
in such higher types as the Ferns, many of which produce young
ones from the surfaces of their fronds, becomes very unusual among
Phænogams. The detachment of bulbils, though not unknown among them,
is exceptional. And while it is true that some flowering plants, as
the Strawberry, multiply by a process allied to gemmation, yet this
is not characteristic of the class. A leading trait of these highest
groups, to which the largest members of the vegetal kingdom belong, is
that agamogenesis has so far ceased that it does not usually originate
independent plants. Though the axes which, budding one out of another,
compose a tree, are the equivalents of asexually-produced individuals;
yet the asexual production of them stops short of separation. These
vast integrations arise where spontaneous disintegration, and the
multiplication effected by it, have come to an end.

Thus, not forgetting that certain Phænogams, as _Begonia
phyllomaniaca_, revert to quite primitive modes of increase, we may
hold it as beyond question that while among the most minute plants
asexual multiplication is universal, and produces enormous numbers in
short periods, it becomes step by step more restricted in range and
frequency as we advance to large and compound plants; and disappears so
generally from the highest and largest, that its occurrence is regarded
as anomalous.


§ 336. Parallel examples furnished by animals make clear the purely
quantitative nature of this relation under its original form. Among
the _Protozoa_, as among the _Protophyta_, there occurs that process
by which the individuality of the parent is wholly lost in producing
offspring--the breaking up of the parental mass into a number of germs.
Some of the _Infusoria_, as for instance those of the genus _Kolpoda_
and several allied genera, become encysted and subsequently break
up into young ones. The more familiar mode of increase among these
animal-aggregates of the first order, by fission, though it sacrifices
the parent individuality by merging it in the individualities of the
two produced, sacrifices it less completely than does the dissolution
into a great number of germs. Occurring, however, as this fission does,
very frequently, and being completed, in some cases that have been
observed, in the course of half-an-hour, it results in immensely-rapid
multiplication. If all its offspring survive, and continue dividing
themselves, a single _Paramœcium_ is said to be capable of thus
originating 268 millions in the course of a month.[55] Nor is this
the greatest known rate of increase. Another animalcule, visible only
under a high magnifying power, “is calculated to generate 170 billions
in four days.”[56] And these enormous powers of propagation are
accompanied by a minuteness so extreme, that of some species one drop
of water would contain as many individuals as there are human beings on
the Earth! Even if we allow a large margin for exaggeration in these
estimates, it is beyond question that among these smallest of animals
the rate of asexual multiplication is immensely the greatest; and this
suffices for the purposes of argument.

Of animal aggregates belonging to the second order, that multiply
asexually with rapidity, the familiar Polypes furnish conspicuous
examples. By gemmation in most cases, in other cases by fission, and
in some cases by both, the agamogenesis is carried on among these
tribes. As shown in Fig. 148, the budding of young ones from the parent
_Hydra_ is carried on so actively, that before the oldest of them is
cast off half-a-dozen or more others have reached various stages of
growth; and even while still attached, the first-formed of the group
have commenced budding out from their sides a second generation of
young ones. In the _Hydra tuba_ this gemmiparous multiplication is
from time to time interrupted by a transverse splitting-up of the body
into segments, which successively separate and swim away: the result
of the two processes being that, in the course of a season, there are
produced from a single germ great numbers of young _Medusæ_, which
are the adult or sexual forms of the species. Respecting cœlenterate
animals of this degree of composition, it may be added that when we
ascend to the larger kinds we find asexual genesis far less active.
Though comparisons are interfered with by differences of structure and
mode of life, yet the contrasts are too striking to have their meanings
much obscured. If, for instance, we take a solitary _Actinozoon_ and
a solitary _Hydrozoon_, we see that the relatively-great bulk of the
first, goes along with a relatively-slow agamogenesis. The common
Sea-anemones are but occasionally observed to undergo self-division:
multiplication by budding being in some cases largely followed, but
their numbers are not rapidly increased by either process. A higher
class of secondary aggregates exemplifies the same general truth with
a difference. In the smaller members the agamogenesis is incomplete,
and in the larger it disappears. The gemmation of the minute _Polyzoa_,
though it does not end in the separation of the young individuals,
habitually goes to the extent of producing families of partially
independent individuals; but their near ally, the _Phoronis_, which
immensely exceeds them in size, is solitary and not gemmiparous. So,
too, is it with the _Ascidioida_. And then among the true _Mollusca_,
which are relatively large, no such thing is known as fission or
gemmation.

Take next the _Annulosa_, including under this title the _Annelida_
and _Arthropoda_. When treating of morphological composition, reasons
were given for the belief that the annulose animal is an aggregate of
the third order, the segments of which, produced one from another by
gemmation, originally became separate; but by progressive integration,
or arrested disintegration, there resulted a type in which many such
segments were permanently united (§§ 205–7 and note to § 207). Part
of the evidence there assigned, is evidence to be here repeated in
illustration of the direct antagonism of Growth and Asexual Genesis.
We saw how, among the lower Annelids, the string of segments produced
by gemmation presently divides transversely into two strings; and how,
in some cases, this resolution of the elongating string of segments
into groups that are to form separate individuals, goes on so actively
that as many as six groups are found in different stages of progress
to ultimate independence--a fact implying a high rate of fissiparous
multiplication.[57] Then we saw that, in the superior annulose types,
distinguished in the mass by including the larger species, fission
does not occur. The higher Annelids do not propagate in this way;
there is no known case of new individuals being so formed among the
_Myriapoda_; nor do the Crustaceans afford us a single instance of this
primordial mode of increase. It is, indeed, true that while articulate
animals never multiply asexually after this simplest method, and while
they are characterized in the mass by the cessation of agamogenesis
of every kind, there nevertheless occur in a few of their small
species, those higher forms of agamogenesis known as parthenogenesis
and pseudo-parthenogenesis; and that by these some of them multiply
very rapidly. Hereafter we shall find, in the interpretation of these
anomalies, further support for the general doctrine.

To the above evidence has to be added that which the _Vertebrata_
present. This may be very briefly summed up. On the one hand this
class, whether looked at in the aggregate or in its particular species,
immensely exceeds all other classes in the sizes of its individuals;
and on the other hand, agamogenesis under any form is absolutely
unknown in it. If it be said that budding occurs among the _Tunicata_
which, under the common title of _Chordata_, are included in the same
phylum with the _Vertebrata_, then it may firstly be replied that those
types which have no vertebræ cannot properly be called _Vertebrata_,
and secondly that if, as being _Chordata_, they must be recognized,
then the exception which they present further illustrates the truth
that agamogenetic multiplication occurs only in creatures small in
size, or low in structure, or both.


§ 337. Such are a few leading facts serving to show how deduction
is inductively verified, in so far as the antagonism between Growth
and Asexual Genesis is concerned. In whatever way we explain this
opposition of the integrative and disintegrative processes, the facts
and their implications remain the same. Indeed we need not commit
ourselves to any hypothesis respecting the physical causation. It
suffices to recognize the results under their most general aspects. We
cannot help admitting there are at work these two antagonist tendencies
to aggregation and separation; and we cannot help admitting that the
proportion between the aggregative and separative tendencies, must
in each case determine the relation between increase in bulk of the
individual and increase of the race in number.

The antithesis is as manifest _à posteriori_ as it is necessary _à
priori_. While the minutest organisms multiply asexually in their
billions; while the _Infusoria_ thus multiply in their millions;
while the small compound types next above them thus multiply in their
thousands; while larger and more compound types thus multiply in their
hundreds and their tens; the largest types do not thus multiply at all.
Conversely, those which do not multiply asexually at all, are a billion
or a million times the size of those which thus multiply with greatest
rapidity; and are a thousand times, or a hundred times, or ten times
the size of those which thus multiply with less and less rapidity.
Without saying that this inverse proportion is regular, which, as
we shall hereafter see, it cannot be, we may unhesitatingly assert
its average truth. That the smallest organisms habitually reproduce
asexually with immense rapidity; that the largest organisms never
reproduce at all in this manner; and that between these extremes there
is a general decrease of asexual reproduction along with an increase of
bulk; are propositions which admit of no dispute.




                              CHAPTER VI.

             ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS.


§ 338. In so far as it is a process of separation, sexual genesis is
like asexual genesis; and is therefore, equally with asexual genesis,
opposed to that aggregation which results in growth. Whether deduction
is made from one parent or from two, whether it is made from any part
of the body indifferently or from a specialized part, or whether it is
made directly or indirectly, it remains in any case a deduction; and in
proportion as it is great, or frequent, or both, it must restrain the
increase of the individual.

Here we have to group together the leading illustrations of this truth.
We will take them in the same order as before.


§ 339. The lowest vegetal forms, or rather, we may say, those forms
which we cannot class as either distinctly vegetal or distinctly
animal, show us a process of sexual multiplication that differs much
less from the asexual process than in the higher forms. The common
character which distinguishes sexual from asexual genesis, is that
the mass of protoplasm whence a new generation is to arise, has been
produced by the union of two portions of matter which were before more
widely separated. I use this general expression because, among the
simplest _Algæ_, this is not invariably matter supplied by different
individuals: certain _Diatomaceæ_ exhibit within a single cell, the
formation of a sporangium by a drawing together of the opposite halves
of the endochrome into a ball. Mostly, however, sporangia are products
of conjugation. The protoplasmic contents of two cells unite to form
the germ-mass or zygote; and these conjugating cells may be either
entirely independent, as in many _Desmidiaceæ_ and in the gametes of
many _Confervoideæ_; or they may be two of the adjacent cells forming
a thread, as in some _Conjugateæ_ and the gametes of _Confervoideæ_;
or they may be cells belonging to adjacent threads, as in other
_Conjugateæ_. But whether it is originated by a single parent-cell, or
by two parent-cells, the zygote, after remaining quiescent until there
recur the fit conditions for growth, either breaks up into a multitude
of spores, each of which produces an individual that usually multiplies
asexually, or germinates directly to produce one new individual;
and the fact here to be noted is, that as the entire contents of
the parent-cells unite to form the zygote, their individualities
are lost in the germs of a new generation. In these minute simple
types, sexual propagation just as completely sacrifices the life of
the parent or parents, as does that form of asexual propagation in
which the protoplasm resolves itself directly into zoospores. And in
the one case as in the other, this sacrifice is the concomitant of a
prodigious fertility. Slightly in advance of this, but still showing
us an almost equal loss of parental life in the lives of offspring,
is the process seen in such unicellular _Algæ_ as _Botrydium_, and
in minute _Fungi_ of the same degree of composition. These exhibit a
relatively-enormous development of the spore-producing part, and an
almost entire absorption of the parental substance into it. As evidence
of the resulting powers of multiplication, we have but to remember that
the spread of mould over stale food, the rapid destruction of crops
by mildew, and other kindred occurrences, are made possible by the
incalculably numerous spores thus generated and universally dispersed.

Plants a degree higher in composition supply a parallel series of
illustrations. We have among the larger _Fungi_, in which the
reproductive apparatus is relatively so enormous as to constitute
the ostensible plant, a similar subordination of the individual to
the race, and a similarly-immense fertility. Thus, as quoted by Dr.
Carpenter, Fries says--“in a single individual of _Reticularia maxima_,
I have counted (calculated?) 10,000,000 sporules.” It needs but to
note the clouds of particles, so minute as to look like smoke, which
ripe puffballs give off when they are burst, and then to remember
that each particle is a potential fungus, to be impressed with the
almost inconceivable powers of propagation which these plants possess.
The Lichens, too, furnish examples. Though they are nothing like so
prolific as the _Fungi_ (the difference yielding, as we shall hereafter
see, further support to the general argument), yet there is a great
production of germs, and a proportionate sacrifice of the parental
individuality. Considerable areas of the thallus develop into the
fruit-bodies characteristic of the various fungi which, combined with
algæ, form the different lichens (various members of the _Ascomycetes_
and the _Basidiomycetes_). From these are produced great numbers of
ascospores or basidiospores, as the case may be. Very many lichens also
reproduce themselves by means of _Soredia_, _i.e._, little masses of
algal cells closely wrapped in a weft of fungal hyphæ. Some contrasts
presented by the higher _Algæ_ may also be named as exemplifying the
inverse proportion between the size of the individual and the extent of
the generative structures. While in the smaller kinds relatively large
portions of the fronds are transformed into reproductive elements, in
the larger kinds these portions are relatively small: instance the
_Macrocystis pyrifera_, a gigantic seaweed which sometimes attains a
length of 1,500 feet, of which Dr. Carpenter remarks--“This development
of the nutritive surface takes place at the expense of the fructifying
apparatus, which is here quite subordinate.”

When we turn to vegetal aggregates of the third order of composition,
facts having the same meaning are conspicuous. On the average
these higher plants are far larger than plants of a lower degree of
composition; and on the average their rates of sexual reproduction are
far less. Similarly if, among Archegoniates and Phænogams, we compare
the smaller types with the larger, we find them proportionately more
prolific. This is not manifest if we simply calculate the number of
seeds ripened by an individual in a single season; but it becomes
manifest if we take into account the further factor which here
complicates the result--the age at which sexual genesis commences. The
smaller Phænogams are mostly either annuals, or perennials that die
down annually; and seeding as they do annually before their deaths, or
the deaths of their reproductive parts, it results that in the course
of a year each gives origin to a multitude of potential plants, of
which every one may the next year, if preserved, give origin to an
equal multitude. Supposing but a hundred offspring to be produced the
first year, ten thousand may be produced in the second year, a million
in the third, a hundred millions in the fourth. Meanwhile, what has
been the possible multiplication of a large Phænogam? While its small
congener has been seeding and dying, and leaving multitudinous progeny
to seed and die, it has simply been growing; and may so continue to
grow for ten or a dozen years without bearing fruit. Before a Cocoa-nut
tree has ripened its first cluster of nuts, the descendants of a wheat
plant, supposing them all to survive and multiply, will have become
numerous enough to occupy the whole surface of the Earth. So that
though, when it begins to bear, a tree may annually shed as many seeds
as an herb, yet in consequence of this delay in bearing, its fertility
is incomparably less; and its relatively-small fertility becomes still
further reduced where, as in _Lodoicea callipyge_, the seeds take two
years from the date of fertilization to the date of germination.


§ 340. Some observers state that in certain _Protozoa_ there occurs a
process of conjugation akin to that which the _Protophyta_ exhibit--a
coalescence of the substance of two individuals to form a germ-mass.
This has been alleged more especially of _Actinophrys_. If this
statement should be proved true,[58] then of the minute forms that
appear to be more animal than vegetal in their characters, some have
a mode of sexual multiplication by which the parents are sacrificed
bodily in the production of a new generation.

Among small animal aggregates of the second order, the first to be
considered are of course the _Cœlenterata_. A _Hydra_ occasionally
devotes a large part of its substance to sexual genesis. In the walls
of its body groups of ova, or spermatozoa, or both, take their rise;
and develop into masses greatly distorting the creature’s form, and
leaving it much diminished when they escape. Here, however, gamogenesis
is obviously supplementary to agamogenesis--the immensely rapid
multiplication by budding continues as long as food is abundant and
warmth sufficient, and is replaced by gamogenesis only at the close
of the season. A better example of the relation between small size
and active gamogenesis among low types of the _Metazoa_ is supplied
by the _Rotifera_. Microscopic as these are, they have a great rate
of sexual increase. According to Ehrenberg, _Hydatina senta_ “is
capable of a four-fold propagation every twenty-four or thirty hours,
bringing forth in this time four ova, which grow from the embryo to
maturity, and exclude their fertile ova in the same period. The same
individual, producing in ten days forty eggs, developed with the
rapidity above cited, this rate, raised to the tenth power, gives
one million of individuals from one parent, on the eleventh day
four millions, and on the twelfth day sixteen millions, and so on.”
Ehrenberg, however, characterized by Huxley as “the greatest looker
and the worst observer,” is not a safe authority, and it is better to
state the estimate of Ludwig Plate, who says that _Hydatina_ lays fifty
eggs in two to three weeks--a number which, multiplying in the manner
described, will yield in the time named a much smaller total though
still an enormous total.

The _Annulosa_, including among them the inferior types, have habits
and conditions of life so various that only the broadest contrasts can
be instanced in support of the proposition before us. The differences
of organization and activity greatly complicate the inverse variation
of fertility and bulk. Bearing in mind, however, that the rate of
multiplication depends much less on the number of each brood than on
the quickness with which maturity is reached and a new generation
commenced, it will be obvious that though Annelids, relatively
enormous in size, produce great numbers of ova, yet as they do this at
comparatively long intervals, their rates of increase fall immensely
below that just instanced in the Rotifers. And when at the other
extreme we come to the large articulate animals, such as the Crab
and the Lobster, the further diminution of fertility is seen in the
still longer delay which occurs before each new generation begins to
reproduce.

Perhaps the best examples are supplied by vertebrate animals, and
especially those that are most familiar to us. Comparisons between
Fishes are unsatisfactory, because of our ignorance of their histories.
In some cases Fishes equal in bulk produce widely different numbers of
eggs; as the Cod which spawns millions at once, and the Salmon by which
nothing like so great a number is spawned. But then the eggs are very
unlike in size; and if the ovaria of the two fishes be compared, the
difference between their masses is comparatively moderate. There are,
indeed, contrasts which seem at variance with the alleged relation; as
that between the Cod and the Stickleback which, though so much smaller,
produces fewer ova. The Stickleback’s ova, however, are relatively
large; and their total bulk bears as great a ratio to the bulk of the
Stickleback as does the bulk of the Cod’s ova to that of the Cod.
Moreover if, as is not improbable, the reproductive age is arrived
at earlier by the Stickleback than by the Cod, the fertility of the
species may be greater notwithstanding the smaller number produced
by each individual. Evidence which admits of being tolerably well
disentangled is furnished by Birds. They differ but little in their
grades of organization; and the habits of life throughout extensive
groups of them are so similar, that comparisons may be fairly made. It
is true that, as hereafter to be shown, the differences of expenditure
which differences of bulk entail, have doubtless much to do with the
differences of fertility. But we may set down under the present head
some of those cases in which the activity, being relatively slight,
does not greatly interfere with the relation we are considering; and
may note that among such birds having similarly slight activities, the
small produce more eggs than the large, and eggs that bear in their
total mass a greater ratio to the mass of the parent. Consider, for
example, the gallinaceous birds; which are like one another and unlike
birds of most other groups in flying comparatively little. Taking
first the wild members of this order, which rarely breed more than
once in a season, we find that the Pheasant has from 10 to 14 eggs,
the Black-cock from 6 to 10, the Grouse 8 to 14, the Partridge 12 to
20, the Quail still more, sometimes reaching two broods of 7 to 12 in
each. Here the only exception to the relation between decreasing bulk
and increasing number of eggs, occurs in the cases of the Pheasant
and the Black-cock; and it is to be remembered, in explanation, that
the Pheasant is constitutionally adapted to a warmer region, is
better fed--often artificially--and leads a less active life. If we
pass to domesticated genera of the same order, we meet with parallel
differences. From the numbers of eggs laid, little can be inferred;
for under the favourable conditions artificially maintained, the
laying is carried on indefinitely. But though in the sizes of their
broods the Turkey and the Fowl do not greatly differ, the Fowl begins
breeding at a much earlier age than the Turkey, and produces broods
more frequently: a considerably higher rate of multiplication being
the result. Now these contrasts among domestic creatures which are
similarly conditioned, and closely-allied by constitution, may be held
to show, more clearly than most other contrasts, the inverse variation
between bulk and sexual genesis; since here the cost of activity is
diminished to a comparatively small amount. There is little expenditure
in flight--sometimes almost none; and the expenditure in walking about
is not great: there is more of standing than of actual movement. It
is true that young Turkeys commence their existence as larger masses
than chickens; but it is tolerably manifest that the total weight of
the eggs laid by a Turkey during each season, bears a less ratio to
the Turkey’s weight, than the total weight of the eggs which a Hen
lays during each season, bears to the Hen’s weight; and this is the
fairest way of making the comparison. The comparison so made shows
a greater difference than appears likely to be due to the different
costs of locomotion; considering the inertness of the creatures.
Remembering that the assimilating surface increases only as the
squares of the dimensions, while the mass of the fabric to be built up
by the absorbed nutriment increases as the cubes of the dimensions,
it will be seen that the expense of growth becomes relatively greater
with each increment of size; and that hence, of two similar creatures
commencing life with different sizes, the larger one in reaching its
superior adult bulk, will do this at a more than proportionate expense;
and so will either be delayed in commencing its reproduction, or will
have a diminished reserve for reproduction, or both. Other orders
of Birds, active in their habits, show more markedly the connexion
between augmenting mass and declining fertility. But in them the
increasing cost of locomotion becomes an important, and probably the
most important, factor. The evidence they furnish will therefore
come better under another head. Contrasts among Mammals, like those
which Birds present, have their meanings obscured by inequalities of
the expenditures for motion. The smaller fertility which habitually
accompanies greater bulk, must in all cases be partly ascribed to this.
Still, it may be well if we briefly note, for as much as they are
worth, the broader contrasts. While a large Mammal bears but a single
young one at a time, is several years before it commences doing this,
and then repeats the reproduction at long intervals; we find, as we
descend to the smaller members of the class, a very early commencement
of breeding, an increasing number at a birth, reaching in small Rodents
to 10 or even more, and a much more frequent recurrence of broods: the
combined result being a relatively prodigious fertility. If a specific
comparison be desired between Mammals that are similar in constitution,
in food, in conditions of life, and all other things but size, the
Deer-tribe supplies it. While the large Red-deer has but one at a
birth, the small Roe-deer has frequently two at a birth.[59]


§ 341. The antagonism between growth and sexual genesis, visible in
these general contrasts, may also be traced in the history of each
plant and animal. So familiar is the fact that sexual genesis does not
occur early in life, and in all organisms which expend much begins only
when the limit of size is nearly reached, that we do not sufficiently
note its significance. It is a general physiological truth, however,
that while the building-up of the individual is going on rapidly, the
reproductive organs remain imperfectly developed and inactive; and that
the commencement of reproduction at once indicates a declining rate
of growth, and becomes a cause of arresting growth. As was shown in §
78, the exceptions to this rule are found where the limit of growth is
indefinite; either because the organism expends little or nothing in
action, or expends in action so moderate an amount that the supply of
nutriment is never equilibrated by its expenditure.

We will pass over the inferior plants and, limiting ourselves to
Phænogams, will not dwell on the less conspicuous evidence with the
smaller types present. A few cases such as gardens supply will serve.
All know that a Pear-tree increases in size for years before it begins
to bear; and that, producing but few pears at first, it is long before
it fruits abundantly. A young Mulberry-tree, branching out luxuriantly
season after season, but covered with nothing but leaves, at length
blossoms sparingly and sets some small and imperfect berries, which
it drops while they are green; and it makes these futile attempts
time after time before it succeeds in ripening any seeds. But these
multiaxial plants, or aggregates of individuals some of which continue
to grow while others become arrested and transformed into seed-bearers,
show us the relation less definitely than certain plants that are
substantially, if not literally, uniaxial. Of these the Cocoa-nut may
be instanced. For some years it goes on shooting up without making
any sign of becoming fertile. About the sixth year it flowers; but
the flowers wither without result. In the seventh year it flowers and
produces a few nuts; but these prove abortive and drop. In the eighth
year it ripens a moderate number of nuts; and afterwards increases
the number until, in the tenth year, it comes into full bearing.
Meanwhile, from the time of its first flowering its growth begins
to diminish, and goes on diminishing till the tenth year, when it
ceases. Here we see the antagonism between growth and sexual genesis
under both its aspects--see a struggle between self-evolution and
race-evolution, in which the first for a time overcomes the last, and
the last ultimately overcomes the first. The continued aggrandizement
of the parent-individual makes abortive for two seasons the tendency to
produce new individuals; and the tendency to produce new individuals,
becoming more decided, stops any further aggrandizement of the parent
individual.

Parallel illustrations occur in the animal kingdom. The eggs laid by a
pullet are relatively small and few. Similarly, it is alleged that, as
a general rule, “a bitch has fewer puppies at first, than afterwards.”
According to Burdach, as quoted by Dr. Duncan, “the elk, the bear, &c.,
have at first only a single young one, then they come to have most
frequently two, and at last again only one. The young hamster produces
only from three to six young ones, while that of a more advanced age
produces from eight to sixteen. The same is true of the pig.” It is
remarked by Buffon that when a sow of less than a year old has young,
the number of the litter is small, and its members are feeble and even
imperfect. Here we have evidence that in animals growth checks sexual
genesis. And then, on the other hand, we have evidence that sexual
genesis checks growth. It is well known to breeders that if a filly
is allowed to bear a foal, she is thereby prevented from reaching
her proper size. And a like loss of perfection as an individual,
is suffered by a cow which breeds too early. It may be added, as a
converse fact, that castrated animals, as capons and notably cats,
often become larger than their unmutilated associates.


§ 342. Notwithstanding the way in which the inverse variation of
growth and sexual genesis is complicated with other relations, its
existence is, I think, sufficiently manifest. Individually, many of
the foregoing instances are open to criticism, and have to be taken
with qualifications; but when looked at in the mass their meaning is
beyond doubt. Comparisons between the largest with the smallest types,
whether vegetal or animal, yield results which are unmistakable. On
the one hand, remembering the fact that during its centuries of life
an Oak does not produce as many acorns as a Fungus does spores in a
single night, we see that the Fungus has a fertility exceeding that
of the Oak in a degree literally beyond our powers of calculation or
imagination. On the other hand when, taking a microscopic protophyte
which has billions of descendants in a few days, we ask how many such
would be required to build up the forest tree which is years before
it drops a seed, we are met by a parallel difficulty in conceiving
the number, if not in setting it down. Similarly, if from the minute
and prodigiously-fertile Rotifer we turn to the Elephant, which
approaches thirty years before it bears a solitary young one, we find
the connexions between small size and great fertility and between great
size and small fertility, too intensely marked to be much disguised by
the perturbing relations that have been indicated. Finally, as this
induction, reached by a survey of organisms in general, is verified by
observations on the relation between decreasing growth and commencing
reproduction in individual organisms, we may, I think, consider the
alleged antagonism as proved.[60]




                             CHAPTER VII.

  THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS, ASEXUAL AND SEXUAL.


§ 343. By Development, as here to be dealt with apart from Growth, is
meant increase of structure as distinguished from increase of mass. As
was pointed out in § 50, this is the biological definition of the word.
In the following sections, then, we have to note how complexity of
organization is hindered by reproductive activity, and conversely.

This relation partially coincides with that which we have just
contemplated; for, as was shown in § 44, degree of growth is to a
considerable extent dependent on degree of organization. But while the
antagonism to be illustrated in this chapter is much entangled with
that illustrated in the last chapter, it may be so far separated as to
be identified as an additional antagonism.

Besides the direct opposition between that continual disintegration
which rapid genesis implies, and the fulfilment of that pre-requisite
to extensive organization--the formation of an extensive aggregate,
there is an indirect opposition which we may recognize under several
aspects. The change from homogeneity to heterogeneity takes time;
and time taken in transforming a relatively-structureless mass into
a developed individual, delays the period of reproduction. Usually
this time is merged in that taken for growth; but certain cases of
metamorphosis show us the one separate from the other. An insect,
passing from its lowly-organized caterpillar-stage into that of
chrysalis, is afterwards a week, a fortnight, or a longer period in
completing its structure: the re-commencement of genesis being by so
much postponed, and the rate of multiplication therefore diminished.
Further, that re-arrangement of substance which development implies,
entails expenditure. The chrysalis loses weight in the course of its
transformation; and that its loss is not loss of water only, may
be inferred from the fact that it respires, and that respiration
indicates consumption. Clearly the matter consumed is, other things
equal, a deduction from the surplus which may go to reproduction.
Yet again, the more widely and completely an organic mass becomes
differentiated, the smaller is the portion of it which retains the
relatively-undifferentiated state that admits of being moulded into
new individuals, or the germs of them. Protoplasm which has become
specialized tissue cannot be generalized afresh, and afterwards
transformed into something else; and hence the progress of structure
in an organism, by diminishing the unstructured part, diminishes the
amount available for making offspring.

It is true that higher structure, like greater growth, may insure to
a species advantages which eventually further its multiplication--may
give it access to larger supplies of food, or enable it to obtain food
more economically; and we shall hereafter see how the inverse variation
we are considering is thus qualified. But here we are concerned
only with the necessary and direct effects; not with those that are
contingent and remote. These necessary and direct effects we will now
look at as exemplified.


§ 344. Speaking generally, the simpler plants propagate both
sexually and asexually; and, speaking comparatively, the complex
plants propagate only sexually: their asexual propagation is usually
incomplete--produces a united aggregate of individuals instead of
numerous distinct individuals. The Protophytes that perpetually
subdivide, the merely cellular _Algæ_ that shed their tetraspores, the
Archegoniates that spontaneously separate their fronds or drop their
gemmæ, show us an extra mode of multiplication which, among flowering
plants, is exceptional. This extra mode of multiplication among these
simpler plants, is made easy by their low development. Tetraspores
arise only where the frond consists of untransformed cells; gemmæ bud
out and drop off only where the tissue is comparatively homogeneous.

Should it be said that this is but another aspect of the antagonism
already set forth, since these undeveloped forms are also the smaller
forms; the reply is that though in part true this is not wholly true.
Various marine _Algæ_ which propagate asexually, are larger than some
Phænogams which do not thus propagate. The objection that difference of
medium vitiates this comparison, is met by the fact that it is the same
among land-plants themselves. Sundry of the lowly-organized Liverworts
which are habitually gemmiparous, exceed in size many flowering plants.
And the Ferns show us agamic multiplication occurring in plants which,
while they are inferior in complexity of structure, are superior in
bulk to numbers of annual Monocotyledons and Dicotyledons.


§ 345. In the ability of the lowly-organized substance of a Sponge to
transform itself into multitudes of gemmules, we have an instance of
this same direct relation in the animal kingdom. Moreover, the instance
yields very distinct proof of an antagonism between development and
genesis, independent of the antagonism between growth and genesis; for
the Sponge which thus multiplies itself asexually, as well as sexually,
is far larger than hosts of more complex animals which do not multiply
asexually.

Once again may be cited the creature so often brought in evidence, the
_Hydra_, as showing us how rapidity of agamic propagation is associated
with inferiority of structure. Its power to produce young ones from
nearly all parts of its body, is due to the comparative homogeneity
of its body. In kindred but more-organized types, the gemmiparity is
greatly restricted, or disappears. Among the free-swimming _Hydrozoa_,
multiplication by budding, when it occurs at all, occurs only at
special places. That increase of structure apart from increase of size,
is here a cause of declining agamogenesis, we may see in the contrast
between the simple _Hydra_ and the compound Hydroids. These last, along
with more-differentiated tissues, show us a gemmation which does not
go on all over the body of each polype, and much of it does not end in
separation.

It is, however, among the _Annulosa_ that progressing organization
is most conspicuously operative in diminishing agamogenesis. The
segments or “somites” composing an animal belonging to this class, are
primordially alike; and, as before argued (§§ 205–7), are probably
the homologues of what were originally independent individuals. The
progress from the lower to the higher types of the class, is at once
a progress towards types in which the strings of segments cease to
undergo subdivision, and towards types in which the segments, no longer
alike in their structures and functions, have become physiologically
integrated or mutually dependent. Already this group of cases has
been named as illustrating the antagonism between growth and asexual
genesis; but it is proper also to name it here, since, on the one hand,
the greater size due to the ceasing of fission, is made possible only
by the specialization of parts and the development of a co-ordinating
apparatus to combine their actions, and since, on the other hand,
specialization and co-ordination can advance only in proportion as
fission ceases.


§ 346. The inverse variation of development and sexual genesis is by no
means easy to follow. One or two facts indicative of it may, however,
be named.

Phænogams that have but little supporting tissue may fairly be classed
as structurally inferior to those having stems with a bulky and
complex woody system; for these imply additional differentiations,
and constitute wider departures from the primitive type of vegetal
tissue. That the concomitant of this higher organization is a slower
gamogenesis, scarcely needs pointing out. While the herbaceous annual
is blossoming and ripening seed, the young tree is transforming its
originally-succulent axis into dense fibrous substance; and year
by year the young tree expends in doing the like, nutriment which
successive generations of the annual expend in fruit. Here the inverse
relation is between sexual reproduction and complexity, and not between
sexual reproduction and bulk, seeing that besides seeding, the annual
often grows to a size greater than that reached by the young infertile
tree in several years.

Proof of the antagonism between complexity and gamogenesis in animals,
is still more difficult to disentangle. Perhaps the evidence most to
the point is furnished by the contrast between Man and certain other
Mammals approaching him in mass. To compare him with the domestic Sheep
which, though not very unlike in size, is relatively prolific, is
objectionable because of the relative inactivity of Sheep; and this,
too, may be alleged as a reason why the Ox, though far more bulky, is
also far more fertile, than Man. Further, against a comparison with
the Horse which, while both larger and more prolific, is tolerably
active, it may be urged that in his case, and the cases of herbivorous
creatures generally, the small exertion required to procure food,
joined with the great ratio borne by the alimentary organs to the
organs they have to build up and repair, vitiates the result. We may,
however, fairly draw a parallel between Man and a large carnivore. The
Lion, superior in size, and perhaps equal in activity, has a digestive
system not proportionately greater; and yet has a higher rate of
multiplication than Man. Here the only decided want of parity, besides
that of organization, is that of food. Possibly a carnivore gains an
advantage in having a surplus nutriment consisting almost wholly of
those nitrogenous materials from which the bodies of young ones are
mainly formed. But, allowing for all other differences, it appears not
improbable that the smallness of human fertility compared with the
fertility of large feline animals, is due to the greater complexity of
the human organization--more especially the organization of the nervous
system. Taking degree of nervous organization as the chief correlative
of mental capacity; and remembering the physiological cost of that
slow evolution whereby high mental capacity is reached; we may suspect
that nervous organization is very expensive: the inference being that
bringing it up to the level it reaches in Man, whose digestive system,
by no means large, has at the same time to supply materials for general
growth and daily waste, involves a great retardation of maturity and
sexual genesis.




                             CHAPTER VIII.

              ANTAGONISM BETWEEN EXPENDITURE AND GENESIS.


§ 347. Under this head we have to set down no evidence derived from the
vegetal kingdom. Plants are not expenders of force in such degrees as
to affect the general relations with which we are dealing. They have
not to maintain a heat above that of their environment, nor have they
to generate motion; and hence consumption for these two purposes does
not diminish the stock of material which serves on the one hand for
growth and on the other hand for propagation.

It will be well, too, if we pass over the lower animals: especially
those aquatic ones which, being nearly of the same temperature as
the water, and nearly of the same specific gravity, lose but little
in evolving motion, sensible and insensible. A further reason for
excluding from consideration these inferior types, is that we do not
know enough of their rates of genesis to permit of our making, with any
satisfaction, those involved comparisons here to be entered upon.

The facts on which we must mainly depend are those to be gathered
from terrestrial animals, and chiefly from those higher classes of
them which are at the same time great expenders and have rates of
multiplication about which our knowledge is tolerably definite. We
will restrict ourselves, then, to the evidence which Birds and Mammals
supply.


§ 348. Satisfactory proof that loss of substance in the maintenance of
heat diminishes the rapidity of propagation, is difficult to obtain.
It is, indeed, obvious that the warmblooded _Vertebrata_ are less
prolific than the cold-blooded; but then they are at the same time
more vivacious. Similarly, between Mammals and Birds (which are the
warmer-blooded of the two) there is, other things equal, a parallel,
though much smaller, difference; but here, too, the unlikenesses of
muscular action complicate the evidence. Again, the annual return of
generative activity has an average correspondence with the annual
return of a warmer season, which, did it stand alone, might be taken
as evidence that a diminished cost of heat-maintenance leads to such a
surplus as makes reproduction possible. But then, this periodic rise of
temperature is habitually accompanied by an increase in the quantity
of food--a factor of equal or greater importance. We must be content,
therefore, with such few special facts as admit of being disentangled.

Certain of these we are introduced to by the general relation last
named--the habitual recurrence of genesis with the recurrence of
spring. For in some cases a domesticated creature has its supplies
of food almost equalized; and hence the effect of varying nutrition
may be in great part eliminated from the comparison. The common
Fowl yields an illustration. It is fed through the cold months, but
nevertheless, in mid-winter, it either wholly leaves off laying or
lays very sparingly. And then we have the further evidence that if it
lays sparingly, it does so only on condition that the heat, as well as
the food, is artificially maintained. Hens lay in cold weather only
when they are kept warm. To which fact may be added the kindred one
that “when pigeons receive artificial heat, they not only continue to
hatch longer in autumn, but will recommence in spring sooner than they
would otherwise do.” An analogous piece of evidence is that, in winter,
inadequately sheltered Cows either cease to give milk or give it in
diminished quantity. For though giving milk is not the same thing as
bearing a young one, yet, as milk is part of the material from which
a young one is built up, it is part of the outlay for reproductive
purposes, and diminution of it is a loss of reproductive power. Indeed
the case aptly illustrates, under another aspect, the struggle between
self-preservation and race-preservation. Maintenance of the cow’s life
depends on maintenance of its heat; and maintenance of its heat may
entail such reduction in the supply of milk as to cause the death of
the calf.

Evidence derived from the habits of the same or allied genera in
different climates, may naturally be looked for; but it is difficult
to get, and it can scarcely be expected that the remaining conditions
of existence will be so far similar as to allow of a fair comparison
being made. The only illustrative facts I have met with which seem
noteworthy, are some named by Mr. Gould in his work on _The Birds
of Australia_. He says:--“I must not omit to mention, too, the
extraordinary fecundity which prevails in Australia, many of its
smaller birds breeding three or four times in a season; but laying
fewer eggs in the early spring when insect life is less developed, and
a greater number later in the season, when the supply of insect food
has become more abundant. I have also some reason to believe that the
young of many species breed during the first season, for among others,
I frequently found one section of the Honey-eaters (the _Melithrepti_)
sitting upon eggs while still clothed in the brown dress of immaturity;
and we know that such is the case with the introduced _Gallinaceæ_ (or
poultry) three or four generations of which have been often produced in
the course of a year.” Though here Mr. Gould refers only to variation
in the quantity of food as a cause of variation in the rate of
multiplication, may we not suspect that warmth is a part-cause of the
high rate which he describes as general?


§ 349. Of the inverse variation between activity and genesis, we get
clear proof. Let us begin with that which Birds furnish.

First we have the average contrast, already hinted, between the
fertility of Birds and the fertility of Mammals. Comparing the large
with the large and the small with the small, we see that creatures
which continually go through the muscular exertion of sustaining
themselves in the air and propelling themselves rapidly through it,
are less prolific than creatures of equal weights which go through the
smaller exertion of moving about over solid surfaces. Predatory Birds
have fewer young ones than predatory Mammals of approximately the same
sizes. If we compare Rooks with Rats, or Finches with Mice, we find
like differences. And these differences are greater than at first
appears. For whereas among Mammals a mother is able, unaided, to bear
and suckle and rear half-way to maturity, a brood that probably weighs
more in proportion than does the brood of a Bird; a Bird, or at least
a Bird that flies much, is unable to do this. Both parents have to
help; and this indicates that the margin for reproduction in each adult
individual is smaller.

Among Birds themselves occur contrasts which may be next considered.
In the Raptorial class, various species of which, differing in their
sizes, are similarly active in their habits, we see that the small
are more prolific than the large. The Golden Eagle has usually 2
eggs: sometimes 3, sometimes only 1. As we descend to the Kites and
Falcons, the number is 2 or 3, and 3 or 4. And when we come to the
Sparrow-Hawk, 3 to 5 is the specified number. Similarly among the
Owls: while the Great Eagle-Owl has 2 or 3 eggs, the comparatively
small Common Owl has 4 or 5. As before hinted, it is impossible to say
what proportions of these differences are due to unlikenesses of bulk
merely, and what proportions are due to unlikenesses in the costs of
locomotion. But we may fairly assume that the unlikenesses in the costs
of locomotion are here the more important factors. Weights varying as
the cubes of the dimensions, while the surfaces of digestive systems
vary as the squares, the expense of flight increases more rapidly than
does the ability to take in nutriment; and as motion through the
air requires more effort than motion on the ground, this geometrical
progression tells more rapidly on Birds than on Mammals. Be this as
it may, however, these contrasts support the argument; as do various
others which may be set down. The Finch-family, for example, have
broods averaging about 5 in number, and have commonly 2 broods in
the season; while in the Crow-family the number of the brood is on
the average less, and there is but one brood in the season. And then
on descending to such small birds as the Wrens and the Tits, we have
8, 10, 12 to 15 eggs, and sometimes two broods in the year. One of
the best illustrations is furnished by the Swallow-tribe, throughout
which there is little or no difference in mode of life or in food. The
Sand-Martin, much the least of them, has 4 to 6 eggs and two broods;
the Swallow, somewhat larger, has 4 or 5; and the Swift (similar in
habits though unrelated), larger still, has but 2. Here we see a lower
fertility associated in part with greater size, but associated still
more conspicuously with greater expenditure. For the difference of
fertility is more than proportionate to the difference of bulk, as
shown in other cases; and for this greater difference there is the
reason, that the Swift has to support not only the cost of propelling
its larger mass through the air, but also the cost of propelling it at
a higher velocity.

Omitting much evidence of like nature, let us note that disclosed by
comparisons of certain groups of birds with other groups. “Skulkers”
is the descriptive title applied to the Water-Rail, the Corn-Crake,
and their allies, which evade enemies by concealment--consequently
expending but little in locomotion. These birds have relatively large
broods--6 to 11, 8 to 12, &c. Not less instructive are the contrasts
between the Gallinaceous Birds and other Birds of like sizes but more
active habits. The Partridge and the Wood-Pigeon are about equal in
bulk and have much the same food. Yet while the one has from 12 to 20
young ones, the other has but 2 young ones twice a-year: its annual
reproduction is less than one-third. It may be said that the ability
of the Partridge to bring up so large a brood, is due to that habit of
its tribe which one of its names, “Scrapers,” describes; and to the
accompanying habit of the young, which begin to get their own living as
soon as they are hatched: so saving the parents’ labour. Conversely,
it may be said that the inability of Pigeons to rear more than 2 at a
time, is caused by the necessity of fetching everything they eat. But
the alleged relation holds nevertheless. On the one hand, a great part
of the food which the Partridge chicks pick up, is food which, in their
absence, the mother would have picked up. Though each chick costs her
far less than a young Pigeon costs its parents, yet the whole of her
chicks cost her a great deal in the shape of abstinence--an abstinence
she can bear because she has to fly but little. On the other hand,
the Pigeon’s habit of laying and hatching but two eggs, must not be
referred to any foreseen necessity of going through so much labour in
supporting the young, but to a constitutional tendency established by
such labour. This is proved by the curious fact that when domesticated,
and saved from such labour by artificial feeding, Pigeons, says
Macgillivray (quoting Aitkin), “are frequently seen sitting on eggs
long before the former brood is able to leave the nest, so that the
parent bird has at the same time young birds and eggs to take care of.”


§ 350. Made to illustrate the effect of activity on fertility, most
comparisons among Mammals are objectionable: other circumstances are
not equal. A few, however, escape this criticism.

One is that between the Hare and the Rabbit. These are closely-allied
species of the same genus, similar in their diet but unlike in their
expenditures for locomotion. The relatively-inert Rabbit has 6 young
ones in a litter, and four litters a-year; while the relatively-active
Hare has but 2 to 5 in a litter. This is not all. The Rabbit begins to
breed at six months old; but a year elapses before the Hare begins
to breed. These two factors compounded, result in a difference of
fertility far greater than can be ascribed to unlikeness of the two
creatures in size.

Perhaps the most striking piece of evidence which Mammals furnish,
is the extreme infertility of our common Bat. The _Cheiroptera_ and
the _Rodentia_ are not very dissimilar in their internal structures.
Diversity of constitution, therefore, cannot vitiate the comparison
between Bats and Mice, which are about the same in size. Though their
diets differ, the difference is in favour of the Bat: its food being
exclusively animal while that of the Mouse is mainly vegetal. What now
are their respective rates of genesis? The Mouse has several litters in
a year of 5 to 7 in each; while the Bat produces only one at a time.
Whether the Bat repeats its one more frequently than the Mouse repeats
its 7 is not stated; but it is quite certain that even if it does so
(an absurd supposition), the more frequent repetition cannot be such
as to raise its fertility to anything like that of the Mouse. And this
relatively-low rate of multiplication we may fairly ascribe to its
relatively-high rate of expenditure.

Here let us note, in passing, an interesting example of the
way in which a species which has no specially-great power of
self-preservation, while its power of multiplication is extremely
small, nevertheless avoids extinction because it has to meet an
unusually-small total of race-destroying forces. Leaving out parasites,
the only enemy of the Bat is the Owl; and the Owl is sparingly
distributed.


§ 351. These general evidences may be enforced by some special
evidences. We have few opportunities of observing how, within the
same species, variations of expenditure are related to variations of
fertility. But a fact or two showing the connexion may be named.

Doctor Duncan quotes a statement to the point respecting the breeding
of dogs. Already in §341 I have extracted a part of this statement,
to the effect that before her growth is complete, a bitch bears at a
birth fewer puppies than when she becomes full-grown. An accompanying
allegation is, that her declining vigour is shown by a decrease in the
number of puppies contained in a litter, “ending in one or two.” And
then it is further alleged that, “as regards the amount of work a dog
has to perform, so will the decline be rapid or gradual; and hence, if
a bitch is worked hard year after year, she will fail rapidly, and the
diminution of her puppies will be accordingly; but if worked moderately
and well kept, she will fail gradually, and the diminution will be less
rapid.”

In this place, more fitly than elsewhere, may be added a fact of like
implication, though of a different order. Of course whether excessive
expenditure be in the continual repairs of nervo-muscular tissues or in
replacing other tissues, the reactive effects, if not quite the same,
will be similar--there will be a decrease of the surplus available for
genesis. If, then, in any animals there from time to time occur unusual
outlays for self-maintenance, we may expect the periods of such outlays
to be periods of diminished or arrested reproduction. That they are so
the moulting of birds shows us. When hens begin to moult they cease to
lay. While they are expending so much in producing new clothing, they
have nothing to expend for producing eggs.




                              CHAPTER IX.

            COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS.


§ 352. Under this head may be grouped various facts which, in another
way, tell the same tale as those contained in the last chapter. The
evidence there put together went to show that increased cost of
self-maintenance entailed decreased power of propagation. The evidence
to be set down here, will go to show that power of propagation is
augmented by making self-maintenance unusually easy. For into this may
be translated the effect of abundant food.

To put the proposition more specifically--we have seen that after
individual growth, development, and daily consumption, have
been provided for, the surplus nutriment measures the rate of
multiplication. This surplus may be raised in amount by such changes in
the environment as bring a larger supply of the materials or forces on
which both parental life and the lives of offspring depend. Be there,
or be there not, any expenditure, a higher nutrition will make possible
a greater propagation. We may expect this to hold both of agamogenesis
and of gamogenesis; and we shall find that it does so.


§ 353. On multiaxial plants, the primary effect of surplus nutriment
is a production of large and numerous leaf-shoots. How this asexual
multiplication results from excessive nutrition, is well shown when the
leading axis, or a chief branch, is broken off towards its extremity.
The axillary buds below the breakage quickly swell and burst into
lateral shoots, which often put forth secondary shoots: two generations
of agamic individuals arise where there probably would have been none
but for the local abundance of sap, no longer drawn off. In like manner
the abnormal agamogenesis which we have in proliferous flowers, is
habitually accompanied by a general luxuriance, implying an unusual
plethora.

No less conclusive is the evidence furnished by agamogenesis in
animals. Sir John Dalyell, speaking of _Hydra tuba_, and of the period
before strobilization commences, says--“It is singular how much
propagation is promoted by abundant sustenance.” This Polype goes on
budding out young polypes from its sides, with a rapidity proportionate
to the supply of materials. So, too, is it with the agamic reproduction
of the _Aphis_. As cited by Professor Huxley, Kyber “states that he
raised viviparous broods of both this species (_Aphis Dianthi_) and
_A. Rosæ_ for four consecutive years, without any intervention of
males or oviparous females, and that the energy of the power of agamic
reproduction was at the end of that period undiminished. The rapidity
of the agamic proliferation throughout the whole period was directly
proportional to the amount of warmth and food supplied.”

In these cases the relation is not appreciably complicated by
expenditure. The parent having reached its limit of growth, the
absorbed food goes to asexual multiplication: scarcely any being
deducted for the maintenance of parental life.


§ 354. The sexual multiplication of organisms under changed conditions,
undergoes variations conforming to a parallel law. Cultivated plants
and domesticated animals yield us proof of this.

Facts showing that in cultivated plants sexual genesis increases with
nutrition, are obscured by facts showing that a less rapid asexual
genesis, and an incipient sexual genesis, accompany the fall from a
high to a moderate nutrition. The confounding of these two relations
has led to mistaken inferences. When treating of Genesis inductively,
we reached the generalization that “the products of a fertilized germ
go on accumulating by simple growth, so long as the forces whence
growth results are greatly in excess of the antagonist forces; but
that when diminution of the one set of forces, or increase of the
other, causes a considerable decline in this excess, and an approach
towards equilibrium, fertilized germs are again produced.” (§ 78.)
It was pointed out that this holds of organisms which multiply by
heterogenesis, as well as those which multiply by homogenesis.
And plants were referred to as illustrating, both generally and
locally, the decline of agamic multiplication and commencement of
gamic multiplication, along with a lessening rate of nutrition.
Now the many cases which are given of fruitfulness caused in trees
by depletion, are really cases of this change from agamogenesis
to gamogenesis; and simply go to prove that what would naturally
arise when decreased peripheral growth had followed increased size,
may be brought about artificially by diminishing the supply of
materials for growth. Cramping its roots in a pot, or cutting them,
or ringing its branches, will make a tree bear very early: bringing
about a premature establishment of that relative innutrition which
would have spontaneously arisen in course of time. Such facts by
no means show that in plants sexual genesis increases as nutrition
diminishes. When it has once set in, sexual genesis is scanty or
imperfect unless nutrition is good. Though the starved plant may
blossom, yet many of its blossoms will fail; and such seeds as it
produces will be ill-furnished with those enveloping structures and
that store of albumen, &c., needed to give good chances of successful
germination--the number of surviving offspring will be diminished. Were
it otherwise, the manuring of fields which are to bear seed-crops,
would be not simply useless but injurious. Were it otherwise, dunging
the roots of a fruit-tree would in all cases be impolitic; instead
of being impolitic only where the growth of sexless axes is still
luxuriant. Were it otherwise, a tree which has borne a heavy crop
should, by the consequent depletion, be led to bear a still heavier
crop next year; whereas it is apt to be wholly or partially barren next
year--has to recover a state of tolerably-high nutrition before its
sexual genesis again becomes large.

But the best illustrations are yielded by animals--those animals at
least in which we have, besides an increased supply of nutriment, a
diminished expenditure. Two classes of comparisons, alike in their
implications, may be made--comparisons between tame and wild animals of
the same species or genus, and comparisons between tame animals of the
same species differently treated.

To begin with Birds, let us first contrast the farm-yard _Gallinaceæ_
with their kindred of the fields and woods. Notwithstanding their
greater size, which, other things equal, should be accompanied by
smaller fertility, the domesticated kinds have more numerous offspring
than the wild kinds. A Turkey has a dozen in a brood, while a Pheasant
has from 6 to 10. Twice or thrice in a season, a Hen rears as many
chickens as a Partridge rears once in a season. Anserine birds show us
parallel differences. The Tame Goose sits on 13 to 18 eggs and often
sits a second time; but the Wild Goose sits on 5, 6, or 7, and these
are noted as considerably smaller. It is the same with Ducks. The
domesticated variety lays and hatches twice as many eggs as the wild
variety. And the like holds of Pigeons. After remarking of the _Columba
livia_ that “in spring when they have plenty of corn to pick from
the newly-sown fields, they begin to get fat and pair; and again in
harvest, when the corn is cut down,” Macgillivray goes on to say that
“the same pair when tamed generally breed four times” in the year. That
between different poultry-yards inequalities of fertility are caused by
inequalities in the supplies of food, is a familiar truth. High feeding
shows its effects not only in the continuous laying, but also in the
sizes of the eggs. Among directions given for obtaining eggs from
pullets late in the year, it is especially insisted on that they shall
have a generous diet. Respecting Pigeons Macgillivray writes:--“that
their breeding depends much on their having plenty of food to fatten
them, seems, I think, evident from the circumstance that, when tamed,
which they easily are, they are observed to breed in every month of
the year. I do not mean that the same pair will breed every month; but
some in the flock, if well fed, will breed at any season.” There may
be added a fact of like meaning which partially-domesticated birds
yield. The Sparrow is one of the Finch tribe that has taken to the
neighbourhood of houses; and by its boldness secures food not available
to its congeners. The result is that it has several broods in a season,
while its field-haunting kindred have none of them more than two
broods, and some have only one.

Equally clear proof that abundant nutriment raises the rate of
multiplication, occurs among Mammals. Compare the litters of the Dog
with the litters of the Wolf and the Fox. Whereas those of the one
range in number from 6 to 14, those of the others contain respectively
5 or 6 or occasionally 7, and 4 or 5 or rarely 6. Again, the Wild Cat
has 4 or 5 kittens; but the tame Cat has 5 or 6 kittens 2 or 3 times
a-year. So, too, is it with the Weasel tribe. The Stoat has 5 young
ones once a-year. The Ferret has 2 litters yearly, each containing
from 6 to 9; and this notwithstanding that it is the larger of the
two. Perhaps the most striking contrast is that between the wild and
tame varieties of the Pig. While the one produces, according to its
age, from 4 to 8 or 10 young ones once a year, the other produces
sometimes as many as 17 in a litter; or, in other cases, will bring
up 5 litters of 10 each in two years--a rate of reproduction which is
unparalleled in animals of as large a size.[61] And let us not omit to
note that this excessive fertility occurs where there is the greatest
inactivity--where there is plenty to eat and nothing to do. There is no
less distinct evidence that among domesticated Mammals themselves, the
well-fed individuals are more prolific than the ill-fed individuals. On
the high and comparatively-infertile Cotswolds, it is unusual for ewes
to have twins; but they very commonly have twins in the adjacent rich
valley of the Severn. Similarly, among the barren hills of the west of
Scotland, two lambs will be borne by about one ewe in twenty; whereas
in England, something like one ewe in three will bear two lambs. Nay,
in rich pastures, twins are more frequent than single births; and it
occasionally happens that, after a genial autumn and consequent good
grazing, a flock of ewes will next spring yield double their number of
lambs--the triplets balancing the uniparæ. So direct is this relation,
that I have heard a farmer assert his ability to foretell, from the
high, medium, or low, condition of an ewe in the autumn, whether she
will next spring bear two, or one, or none.


§ 355. An objection must here be met. Many facts may be brought to
prove that fatness is not accompanied by fertility but by barrenness;
and the inference drawn is that high feeding is unfavourable to
genesis. The premiss may be admitted while the conclusion is denied.

There is a distinction between what may be called normal plethora,
and an abnormal plethora, liable to be confounded with it. The one
is a mark of constitutional wealth; but the other is a mark of
constitutional poverty. Normal plethora is a superfluity of materials
both for the building up of tissue and the evolution of force; and
this is the plethora which we have found to be associated with unusual
fecundity. Abnormal plethora which, as truly alleged, is accompanied by
infecundity, is a superfluity of force-evolving materials joined with
either a positive or a relative deficiency of tissue-forming materials:
the increased bulk indicating this state, being really the bulk of so
much inert or dead matter. Note, first, a few of the facts which show
us that obesity implies physiological impoverishment.

Neither in brutes nor men does it ordinarily occur either in youth or
in that early maturity during which the vigour is the greatest and the
digestion the best: it does not habitually accompany the highest power
of taking up nutritive materials. When fatness arises in the prime of
life, whether from peculiarity of food or other circumstance, it is
not the sign of an increased total vitality. On the contrary, if great
muscular action has to be gone through, the fat must be got rid of;
either, as in a man, by training, or as in a horse that has grown bulky
while out at grass, by putting him on such more nutritive diet as oats.
The frequency of senile fatness, both in domesticated creatures and in
ourselves, has a similar implication. Whether we consider the smaller
ability of those who display it to withstand large demands on their
powers, or whether we consider the comparatively-inferior digestion
common among them, we see that the increased size indicates, not an
abundance of materials which the organism requires, but an abundance of
materials which it does not require. Of like meaning is the fact that
women who have had several children, and animals after they have gone
on bearing young for some time, frequently become fat; and lose their
fecundity as they do this. In such cases the fatness is not to be taken
as the cause of the infecundity; but the constitutional exhaustion
which the previous production of offspring has left, shows itself at
once in the failing fecundity and the commencing fatness. There is yet
another kind of evidence. Obesity not uncommonly sets in after the
system has been subject to debilitating influences. Often a serious
illness is followed by a corpulence to which there was previously no
tendency. And the prolonged administration of mercury, constitutionally
injurious as it is, sometimes produces a like effect.

Closer inquiry verifies the conclusion to which these facts point.
The microscope shows that along with the increase of bulk common in
advanced life, there goes on what is called “fatty degeneration:”
oil-globules are deposited where there should be particles of flesh--or
rather, we may say, the hydrocarbonaceous molecules locally produced
by decomposition of the nitrogenous molecules, have not been replaced
by other nitrogenous molecules, as they should have been. This fatty
degeneration is, indeed, a kind of local death. For so regarding it
we have not simply the reason that an active substance has its place
occupied by an inert substance; but we have the further reason that the
flesh of dead bodies, under certain conditions, is transformed into a
fatty matter called adipocere.

The infertility that accompanies fatness in domestic animals has,
however, other causes than that declining constitutional vigour which
the fatness commonly indicates. Being artificially fed, these animals
cannot always obtain what their systems need. That which is given to
them is given expressly because of its fattening quality. And since the
capacity of the digestive apparatus remains the same, the absorption
of fat-producing materials in excess, implies defect in the absorption
of materials from which the tissues are formed, and out of which young
ones are built up. Moreover, this special feeding with a view to rapid
and early fattening, continued as it is through generations, and
accompanied as it is by a selection of individuals and varieties which
fatten most readily, tends to establish a modified constitution, more
fitted for producing fat and correspondingly-less fitted for producing
flesh--a constitution which, from this relatively-deficient absorption
of nitrogenous matters, is likely to become infertile; as, indeed,
these varieties often do become. Hence, no conclusions respecting the
effects of high nutrition, properly so-called, can be drawn from cases
of this kind. The cases are, in truth, of a kind which could not exist
but for human agency. Under natural conditions no animal would diet
itself in the way required to produce such results. And if it did its
race would quickly disappear.[62]

There is yet another mode in which accumulation of fat diminishes
fertility. Even supposing it unaccompanied by a smaller absorption of
nitrogenous materials, it is still a cause of lessening the surplus
of nitrogenous materials. For the repair of the motor tissues becomes
more costly. Fat stored-up is weight to be carried. A creature loaded
with inert matter must, other things equal, consume a greater amount
of tissue-forming substances for keeping its locomotive apparatus
in order; and thus expending more for self-maintenance can expend
less for race-maintenance. Abnormal plethora is thus antagonistic
to reproduction in a double way. It ordinarily implies a smaller
absorption of tissue-forming matters, and an increased demand on
the diminished supply. Hence fertility decreases in a geometrical
progression.

The counter-conclusion drawn from facts of this class is, then, due
to a misconception of their nature--a misconception arising partly
from the circumstance that the increase of bulk produced by fat is
somewhat like the increase of bulk which growth of tissues causes,
and partly from the circumstance that abundance of good food normally
produces a certain quantity of fat, which, within narrow limits, is
a valuable store of force-evolving material. When, however, we limit
the phrase high nutrition to its proper meaning--an abundance of, and
due proportion among, all the substances which the organism needs--we
find that, other things equal, fertility always increases as nutrition
increases. And we see that these apparently-exceptional cases, are
cases which really show us the same thing; since they are cases of
relative innutrition.

       *       *       *       *       *

[NOTE.--By a strange oversight when writing this chapter
in the first edition--an oversight I was on the eve of repeating in
this present edition--I omitted to bring forward the familiar and
all-important evidence furnished by the variations of genesis which
ordinarily accompany the alternations of the seasons. These variations,
in multitudinous creatures of all types, show unmistakably that
reproduction begins at those times of the year when greater warmth
and larger supplies of food render maintenance of individual life
relatively easy, and when there is therefore a surplus available for
producing new individuals. Conversely, along with the decrease of
heat and the relative deficiency of food which make it comparatively
difficult in winter to maintain individual life, there ceases to be
the power of producing other lives: the reproductive organs become
quiescent and often dwindle. With this general fact is associated
a special fact. Though among wild animals--birds, mammals, and
others--breeding ceases when Nature no longer supplies abundant food
and warmth; in domesticated mammals and birds, artificially supplied
with food and warmth, the breeding season is greatly extended and often
made continuous, as, under the same conditions, it is in Man himself.

Evidence yielded by the vegetal world is less conspicuous, for the
reason that the cold which arrests reproductive activity also arrests
individual activity: growth of the individual and multiplication of
the race vary simultaneously with variations in the seasons. Still
there are some familiar facts showing that the external conditions
which favour nutrition also bring about reproduction. Early in the
year we are supplied with flowers from regions warmer than our own,
and by and by there come to our markets fruits and vegetables from the
south of France, the Channel Islands, and even from the Scilly Isles,
which are much in advance of those furnished by the gardens of our own
colder regions: reproduction commences earlier where the light and
heat furthering nutrition are greater. And then there is a kindred
meaning in the not unfrequent occurrence of a second flowering and even
of a second fruiting in warm, bright and prolonged autumns. Here the
abnormal re-commencement of reproduction is determined by an abnormal
increase of nutrition.]




                              CHAPTER X.

                   SPECIALITIES OF THESE RELATIONS.


§ 356. Tests of the general doctrines set forth in preceding chapters,
are afforded by organisms having modes of life which diverge widely
from ordinary modes. Here, as elsewhere, aberrant cases yield crucial
proofs.

If certain organisms are so circumstanced that highly-nutritive matter
is supplied to them without stint, and they have nothing to do but
absorb it, we may infer that their powers of propagation will be
enormous.

If there are classes of creatures which expend very little for
self-support in comparison with allied creatures, a relatively-extreme
prolificness may be expected of them.

Or if, again, we find species presenting the peculiarity that while
some of their individuals have much to do and little to eat, others of
their individuals have much to eat and little to do, we may look for
great fertility in these last and comparative infertility or barrenness
in the first.

These several anticipations we shall find completely verified.


§ 357. Plants which, like the _Rafflesiaceæ_, carry their parasitism
to the extent of living on the juices they absorb from other plants,
exhibit one of these relations in the vegetal kingdom. In them the
organs for self-support being needless, are rudimentary; and the parts
directly or indirectly concerned in the production and distribution of
germs, constitute the mass of the organism. That small ratio which the
race-preserving structures bear to the self-preserving structures in
ordinary Phænogams, is, in these Phænogams, inverted. A like relation
occurs in the common Dodder.

There may be added a kindred piece of evidence which the _Fungi_
present. Those of them which grow on living plants, repeat the above
connection completely; and those of them which, though not parasitic,
nevertheless subsist on organized materials previously elaborated by
other plants, substantially repeat it. The spore-producing part is
relatively enormous; and the fertility is far greater than that of
Cryptogams of like sizes, which have to form for themselves the organic
compounds of which they and their germs consist.


§ 358. The same lesson is taught us by animal-parasites. Along with the
decreased cost of Individuation, they similarly show us an increased
expenditure for Genesis; and they show us this in the most striking
manner where the deviation from ordinary conditions of life is the
greatest.

Take, among the _Epizoa_, such an instance as _Chondracanthus
gibbosus_. Belonging to the _Entomostraca_, both males and females
of this species are, in their early days, similar to their allies;
and the males, practically parasitic, though they become greatly
degraded, continue throughout life to show by their segmentation and
other external traits their original nature. The female, however,
having fixed herself where she can suck the juices of her host, the
_Lophius_, grows to twelve times the length of the male and probably
a thousand times its bulk, and becomes utterly transformed by loss of
the organs of animal life and enormous development of the organs of
reproduction. “No heart is discoverable, and the nervous system and
organs of sense (if any) are equally undistinguishable. The interspace
between the alimentary canal and the walls of the body is almost wholly
occupied by the ovarium.”[63] And then beyond this there are appended
ovi-sacs twice the length of the body. So that the germ-producing
organs and their contents, eventually acquire a total bulk many times
that of all the other organs put together. Numerous species of this
type and habit, repeat this relation between a life of inaction with
high feeding, and an enormous rate of genesis. Parasites belonging to
another great division of the animal kingdom, the _Platyhelminthes_,
supply an example of an _epizoon_ in which the rate of multiplication
is made great not so much by immense development of the egg-producing
organs as by the rapidity with which generations succeed one another--a
rapidity such that each generation partially develops the next before
it is itself anything like ready for independent life. This is the
_Gyrodactylus elegans_, of which it is said that “its most remarkable
feature is that it is viviparous, and its embryos before they leave the
body of their mother have already developed their embryos inside them;
and the latter may contain their embryos, so that four generations may
be included under the cuticle of the sexually mature animal.”[64]

_Entozoa_ yield us many examples of this causal relation, raised to
a still higher degree. The _Gordius_, or Hair-worm, is a creature
which, finding its way when young into the body of an insect which is
afterwards swallowed by a fish, there grows rapidly, and then emerging
to breed, lays as many as 8,000,000 eggs in less than a day. Similarly
with those larger types infesting the higher animals. It has been
calculated by Dr. Eschricht, as quoted by Professor Owen, that there
are “64,000,000 of ova in the mature female _Ascaris lumbricoides_.”
Very many of the _Entozoa_ belong to the _Platyhelminthes_, and among
them occur examples of fertility caused not only by great numbers of
ova, but by rapid succession of partially-developed individuals and
also examples of fertility caused by production of ova almost exceeding
numeration. Among the first the Liver-fluke may be named. Of the
half-million eggs it produces each yields a free-swimming ciliated
embryo, and any one of these, which finds its way into a water-snail,
becomes a sporocyst--a bag, presently occupied exclusively by masses
of cells: each mass by and by becoming a _Redia_, which makes its way
out. Like all its fellows which develop in succession, this, with
the exception of a small space occupied by the stomach, devotes the
whole of its interior partly to the formation of other _Rediæ_ (which
presently escape and become similarly transformed), and partly to the
development of _Cercariæ_, into which the internal substance of all
the _Rediæ_ is eventually transformed: _Cercariæ_ which, escaping
from the host, become agents for infecting other creatures. So that
each ovum thus gives rise to a number of forms which severally
subserve multiplication in different ways. Of the other division of
_Platyhelminthes_ referred to as carrying on its multiplication by
production of ova only, the commonest of the _Cestoidea_ furnishes the
best example. Immersed as a Tape-worm is in nutritive liquid, which it
absorbs through its integument, it requires no digestive apparatus. The
room which one would occupy, and the materials it would use up, are
therefore available for germ-producing organs, which nearly fill each
segment: each segment, sexually complete in itself, is little else than
an enormous reproductive system, with just enough of other structures
to bind it together. Remembering that the Tape-worm, retaining its
hold, continues to bud out such segments as fast as the fully-developed
ones are cast off, and goes on doing this as long as the infested
individual lives; we see that here, where there is no expenditure,
where the cost of individuation is reduced to the greatest extent while
the nutrition is the highest possible, the degree of fertility reaches
its extreme. These _Entozoa_ yield us further interesting evidence. Of
their various species, most if not all undergo passive migration from
animal to animal before they become mature. Usually, the form assumed
in the body of the first host is devoid of all that part in which the
reproductive structures take their rise; and this part grows and
develops reproductive structures, only in some predatory animal to
which its first host falls a sacrifice. Occasionally, however, the egg
gives origin to the sexual form in the animal that originally swallowed
it, but the development remains incomplete--there is no sexual genesis,
no formation of eggs in the rudimentary segments. That these may
become fertile it is needful, as before, for the containing animal to
be devoured; so that the imperfect Tape-worm may find its way into
the intestine of a higher animal. Thus the _Bothriocephalus solidus_,
found in the abdominal cavity of the Stickleback, is barren while it
remains there; but if the Stickleback be eaten by a Water-fowl, the
reproductive system of the transferred _Bothriocephalus_ (then known
as _B. nodosus_) becomes developed and active. So, too, a kind of
Tape-worm which remains infertile while in the intestine of a Mouse,
becomes fertile in the intestine of a Cat that devours the mouse. May
we not regard these facts as again showing the dependence of fertility
on nutrition? Barrenness here accompanies conditions unfavourable
to the absorption of nutriment; and it gives way to fecundity where
nutriment is large in quantity and superior in quality.


§ 359. Extremely significant are those cases of partial reversion to
primitive forms of genesis, which occur under special conditions in
some of the higher _Annulosa_. I refer to the pseudo-parthenogenesis
and metagenesis in Insects.

Under what conditions do the _Aphides_ exhibit this strange deviation
from the habits of their order? Why among them should imperfect
females produce, agamically, others like themselves, generation after
generation, with great rapidity? There is the obvious explanation that
they get plenty of easily-assimilated food without exertion. Piercing
the tender coats of young shoots, they sit and suck--appropriating
the nitrogenous elements of the sap and ejecting its saccharine
matter as “honey dew.” Along with a sluggishness strongly contrasted
with the activity of most insects--along with a very low rate of
consumption and a correlative degradation of structure; we have here
a retrogression to asexual genesis, and a greatly-increased rate of
multiplication.

The recently discovered instance of internal metagenesis in the
maggots of certain Flies has a like meaning. Incredible as it at first
seemed to naturalists, it is now proved that the _Cecydomia_-larva
develops in its interior a brood of larvæ of like structure with
itself. In this case, as in the last, abundant food is combined
with low expenditure. These larvæ are found in such habitats as the
refuse of beet-root-sugar factories--masses of nitrogenous _débris_
remaining after the extraction of the saccharine matter. Each larva
has a practically-unlimited supply of sustenance imbedding it on all
sides.[65]

It is true that some other maggots, as those of the Flesh-fly,
are similarly, or still better, circumstanced; and, it may be
said, ought therefore to have the same habit. But this does not
necessarily follow. Survival of the fittest will determine whether
such specially-favourable conditions result in aggrandizement of the
individual or in multiplication of the race. And in the case of the
Flesh-fly there is a reason why greater individuation rather than
more rapid genesis will occur. For a decomposing animal body lasts
so short a time, that were Flesh-fly larvae to multiply agamically,
the second generation would die from the disappearance of their food.
Hence individuals in which the excessive nutrition led to internal
metagenesis, would leave no posterity, and natural selection would
establish the variety in which greater growth resulted. All which the
argument requires is that when such reversion to agamogenesis _does_
take place, it shall be where the food is unusually abundant and the
expenditure unusually small; and this the cases instanced go to show.


§ 360. The physiological lesson taught us by Bees and Ants, not quite
harmonizing with the moral lesson they are supposed to teach, is that
highly-fed idleness is favourable to fertility, and that excessive
industry has barrenness for its concomitant.

The egg of a Bee develops into a small barren female or into a large
fertile female, according to the supply of food given to the larva
hatched from it. We here see that the germ-producing action is an
overflow of the surplus remaining after completion of the individual;
and that the lower feeding which the larva of a working Bee has,
results in a dwarfing of the adult and an arrested development of
the generative organs. Further, we have the fact that the condition
under which the perfect female, or mother-Bee, goes on, unlike insects
in general, laying eggs continuously, is that she has plenty of
food brought to her, is kept warm, and goes through no considerable
exertion. While, contrariwise, it is to be noted that the infertility
of the workers is associated with the ceaseless labour of bringing
materials for the combs and building them, as well as the labour of
feeding the queen, the larvæ, and themselves.

Ants also show us these relations, and they are shown in a greatly
exaggerated form by what are called white ants--insects belonging to
a quite different order. The contrast in bulk between the fecund and
infecund females is here immensely greater. The mother-Ant has the
reproductive system so enormously developed, that the remainder of her
body is relatively insignificant. Entirely incapable of locomotion,
she is unable to deposit her eggs in the places where they are to
be hatched; so that they have to be carried away by the workers as
fast as they are extruded. Her life is thus reduced substantially to
that of a parasite--an absorption of abundant food supplied gratis,
a total absence of expenditure, and a consequent excessive rate of
genesis. “The queen-ant of the African _Termites_ lays 80,000 eggs in
twenty-four hours.”


§ 361. It may be needful to say that these exceptional relations cannot
be ascribed to the assigned causes acting alone. The extreme fertility
which, among parasites and social insects, accompanies extremely high
feeding and an expenditure reduced nearly to zero, presupposes typical
structures and tendencies of suitable kinds; and these are not directly
accounted for. On creatures otherwise organized, unlimited supplies
of food and total inactivity are not followed by such results. There
of course requires a constitution fitted to the special conditions,
and the evolution of this cannot be due simply to plethora joined with
rest. These cases are given as illustrating the conditions under which
extreme exaltations of fertility become possible. Their meanings, thus
limited, are clear, and completely to the point. We see in them that
the devotion of nutriment to race-preservation, is carried furthest
where the cost of self-preservation is reduced to a minimum; and,
conversely, that nothing is devoted directly to race-preservation
by individuals on which falls an excessive expenditure for
self-preservation and preservation of other’s offspring.

       *       *       *       *       *

[NOTE.--Among specialities of these relations may be fitly added here
a very strange one, for a description of which I am indebted to M.
Charles Julin, Professor of Comparative Anatomy in the University of
Liège. In the _Revue Générale des Sciences_ for 30th August, 1894, in
an account of certain investigations of M. Giard, he describes what
he calls “la castration parasitaire”--a castration not of a literal
kind but one effected by the arrest of development which follows from
the depletion caused by a parasite. The _Sacculina_ is an amazingly
transformed type belonging to the _Cirrhipedia_--a type without
segments or appendages and without mouth and alimentary canal. Fixing
itself, during its early locomotive stage, under the abdomen of a
decapodous crustacean, and leaving behind its exo-skeleton, it makes
its way into the interior, and there becoming a mere bag containing
the reproductive organs, obtains the needful nutriment by developing
what are practically roots and rootlets which run everywhere among the
viscera and absorb nutriment from the surrounding tissues. Here we
are concerned merely with the effect produced upon the host by this
physiological robbery. This effect is to arrest the development not
only of the primary sexual organs devoted to the production of germs,
but also of those secondary sexual organs which characterize the male.
M. Julin writes:--

   “Il convient cependant de dire, pour être plus exact, que,
   dans les cas des Crabes infesté par des Sacculines, il n’y a
   pas, en réalité, apparition de caractères femelles chez le
   sexe mâle, mais plutôt absence de développement des caractères
   mâles. En fait, l’animal reste à un stade jeune, non différencié
   sexuellement, tout en prenant une taille plus considérable. Cela
   nous porte à attribuer les modifications dont nous avons parlé à
   un simple arrêt de développement, qui est plus sensible chez le
   mâle, parce que chez lui les caractères sexuels secondaires sont
   à l’état normal plus développés que chez la femelle.

   D’une manière générale, nous croyons, avec M. Giard, qu’il faut
   assimiler les modifications dues à la castration parasitaire à
   celles qui sont le résultat de la progenèse ou qui engendrent le
   dimorphisme saisonnier.

   Il y a _progenèse_ lorsque, chez un animal, la reproduction
   sexuée s’opère d’une façon plus ou moins précoce, c’est-à-dire
   lorsque les produits sexuels (œufs ou spermatozoïdes) se
   forment et mûrissent avant que l’être n’ait atteint son complet
   développement. On peut citer comme exemples les Axolotls et
   les larves de Tritons qui, les uns normalement, les autres
   accidentellement, pondent en ayant encore leurs branchies.

   Très souvent la progenèse n’affecte qu’un seul sexe. Tantôt,
   c’est le sexe femelle qui mûrit à l’état larvaire comme chez
   les pucerons, les _Stylops_, etc.... Tantôt c’est le sexe mâle,
   comme chez la Bonellie, les mâles complémentaires de Cirripèdes,
   les mâles pygmées des Rotifères, le mâle de l’Anguille, etc.
   D’autres fois, enfin, l’animal présente successivement les deux
   sexes avec progenèse pour l’un d’entre eux. C’est ainsi qu’il
   y a _progenèse protandrique_ chez les Crustacés cymothoadiens,
   et, parmi les Vertébrés, chez les Myxines, qui, mâles dans le
   jeune âge, deviennent femelles en vieillissant et en achevant
   de prendre leur développement. Le cas des vieilles femelles de
   Gallinacés à plumage et à instincts masculins semble être, au
   contraire, un exemple imparfait de _progenèse protogynique_,
   puisque ces femelles ont pondu lorsqu’elles avaient encore
   la livrée des jeunes et qu’elles ont continué plus tard leur
   développement, et présentent le caractère des mâles sans que,
   cependant, l’on ait constaté la production de spermatozoïdes.

   Dans les cas extrêmes de progenèse femelle, la reproduction se
   fait même sans le concours de l’élément mâle, revenant ainsi
   à la forme agamique primordiale. Ces cas sont connus depuis
   longtemps sous le nom de _pédogenèse_. On les a observé chez les
   larves de _Miastor_, de _Chironomus_ et chez certains pucerons.

   Chaque fois qu’il y a progenèse dans un type déterminé, on
   constate soit momentanément, soit d’une façon définitive, un
   arrêt de croissance et de développement: l’animal progénétique
   a, par suite, l’aspect d’une larve sexuée, lorsqu’on le compare
   soit à l’autre sexe, soit aux formes voisines, qui ne présentent
   pas le phénomène de la progenèse.

   Cela est en parfaite harmonie avec le principe, si bien mis en
   lumière par Herbert Spencer, de _l’antagonisme entre la genèse
   et la croissance et entre la genèse et le développement_.
   Cet antagonisme s’explique facilement si l’on songe que les
   matériaux employés pour la reproduction ne peuvent servir à
   l’accroissement de l’individu. S’il est avantageux pour un
   organisme de se reproduire sans acquérir des organes inutiles,
   la sélection naturelle déterminera bientôt une progenèse de plus
   en plus complète. Les animaux parasites, outre qu’ils tirent de
   leur hôte une nourriture abondante, n’ont guère besoin d’une
   foule d’organes qui servent à leurs congénères libres dans la
   vie de relation. Aussi voyons-nous qu’un très grand nombre
   d’animaux parasites sont progénétiques. Les mâles progénétiques
   de la Bonellie et des Cirripèdes vivent en parasites dans leurs
   femelles. Chez certains types, les pucerons, la progenèse cesse
   dès que, la nourriture devenant moins abondante, un déplacement
   pourra être nécessaire.

   En résumé, l’arrêt de développement dû à la progenèse résulte
   d’une dérivation des principes nourriciers au détriment
   de l’animal progénétique. Dans les exemples de castration
   parasitaire que nous avons examinés, le parasite joue, par
   rapport à son hôte, absolument le même rôle que la glande
   génitale d’un type progénétique. Il détourne, pour sa propre
   subsistance, une partie des principes qui auraient servi au
   développement de l’animal. Aussi les effets produits sont-ils
   tout à fait de même ordre.”

A phenomenon so anomalous as this, explicable upon the hypothesis set
forth but not otherwise explicable, furnishes striking verification.]




                              CHAPTER XI.

                   INTERPRETATION AND QUALIFICATION.


§ 362. Considering the difficulties of inductive verification, we
have, I think, as clear a correspondence between the _à priori_ and
_à posteriori_ conclusions, as can be expected. The many factors
co-operating to bring about the result in every case, are so
variable in their absolute and relative amounts, that we can rarely
disentangle the effect of each one, and have usually to be content
with qualified inferences. Though in the mass organisms show us an
unmistakable relation between great size and small fertility, yet
special comparisons among them are nearly always partially vitiated
by differences of structure, differences of nutrition, differences
of expenditure. Though it is beyond question that the more complex
organisms are the less prolific, yet as complexity has a certain
general connexion with bulk, and in animals with expenditure, we cannot
often identify its results as independent of these. And, similarly,
though the creatures which waste much matter in producing motion,
sensible and insensible, have lower rates of multiplication than those
which waste less, yet, as the creatures which waste much are generally
larger and more complex, we are again met by an obstacle which limits
our comparisons, and compels us to accept conclusions less definite
than are desirable.

Such difficulties arise, however, only when we endeavour, as in
foregoing chapters, to prove the inverse variation between Genesis
and each separate element of Individuation--growth, development,
activity. We are scarcely at all hampered by qualifications when,
from contemplating these special relations, we return to the general
relation. The antagonism between Individuation and Genesis is shown
by all the facts which have been grouped under each head. We have
seen that in ascending from the lowest to the highest types, there is
a decrease of fertility so great as to be absolutely inconceivable,
and even inexpressible by figures; and whether the superiority of
type consists in relative largeness, in greater complexity, in
higher activity, or in some or all of these combined, matters not
to the ultimate inference. The broad fact, enough for us here, is
that organisms in which the integration and differentiation of
matter and motion have been carried furthest, are those in which the
rate of multiplication has fallen lowest. How much of the decline
of reproductive power is due to the greater integration of matter,
how much to its greater differentiation, how much to the larger
amounts of integrated and differentiated motions generated, it may
be impossible to say; and it is not needful to say. These are all
elements of a higher degree of life, an augmented ability to maintain
the organic equilibrium amid environing actions, an increased power
of self-preservation; and we find their invariable accompaniment
to be, a diminished expenditure of matter, or motion, or both, in
race-preservation.

In brief, then, examination of the evidence shows that there _does_
exist that relation which we inferred _must_ exist. Arguing from
general data, we saw that for the maintenance of a species, the ability
to produce offspring must be great, in proportion as the ability
of the individuals to contend with destroying forces is small; and
conversely. Arguing from other general data, we saw that, derived as
the self-sustaining and race-sustaining forces are from a common stock
of force, it necessarily happens that, other things equal, increase
of one involves decrease of the other. And then, turning to special
facts, we have found that this inverse variation is clearly traceable
throughout both the animal and vegetal kingdoms. We may therefore set
it down as a law, that every higher degree of organic evolution, has
for its concomitant a lower degree of that peculiar organic dissolution
which is seen in the production of new organisms.


§ 363. Something remains to be said in reply to the inquiry--how is the
ratio between Individuation and Genesis established in each case? This
inquiry has been but partially answered in the course of the foregoing
argument.

Many specialities of the reproductive process are manifestly due to
the natural selection of favourable variations. Whether a creature
lays a few large eggs or many small ones equal in weight to the few
large, is not determined by any physiological necessity: here the only
assignable cause is the survival of varieties in which the matter
devoted to reproduction happens to be divided into portions of such
size and number as most to favour multiplication. Whether in any case
there are frequent small broods or larger broods at longer intervals,
depends wholly on the constitutional peculiarity that has arisen from
the dying out of families in which the sizes and intervals of the
broods were least suited to the conditions of life. Whether a species
of animal produces many offspring of which it takes no care or a few
of which it takes much care--that is, whether its reproductive surplus
is laid out wholly in germs or partly in germs and partly in labour on
their behalf--must have been decided by that moulding of constitution
to conditions slowly effected through the more frequent preservation
of descendants from those whose reproductive habits were best adapted
to the circumstances of the species. Given a certain surplus available
for race-preservation, and it is clear that by indirect equilibration
only, can there be established the more or less peculiar distribution
of this surplus which we see in each case. Obviously, too, survival
of the fittest has a share in determining the proportion between the
amount of matter that goes to Individuation and the amount that goes
to Genesis. Whether the interests of the species are most subserved
by a higher evolution of the individual joined with a diminished
fertility, or by a lower evolution of the individual joined with an
increased fertility, are questions ever being experimentally answered.
If the more-developed and less-prolific variety has a greater number of
survivors, it becomes established and predominant. If, contrariwise,
the conditions of life being simple, the larger or more-organized
individuals gain nothing by their greater size or better organization;
then the greater fertility of the less evolved ones, will insure to
their descendants an increasing predominance.

But direct equilibration all along maintains the limits within which
indirect equilibration thus works. The necessary antagonism we have
traced, rigidly restricts the changes that natural selection can
produce, under given conditions, in either direction. A greater demand
for Individuation, be it a demand caused by some spontaneous variation
or by an adaptive increase of structure and function, inevitably
diminishes the supply for Genesis; and natural selection cannot, other
things remaining the same, restore the rate of Genesis while the higher
Individuation is maintained. Conversely, survival of the fittest,
acting on a species that has, by spontaneous variation or otherwise,
become more prolific, cannot again raise its lowered Individuation, so
long as everything else continues constant.


§ 364. Here, however, a qualification must be made. It was
parenthetically remarked in § 327, that the inverse variation between
Individuation and Genesis is not exact; and it was hinted that a slight
modification of statement would be requisite at a more advanced stage
of the argument. We have now reached the proper place for specifying
this modification.

Each increment of evolution entails a decrement of reproduction which
is not accurately proportionate, but somewhat less than proportionate.
The gain in the one direction is not wholly cancelled by a loss in
the other direction, but only partially cancelled: leaving a margin
of profit to the species. Though augmented power of self-maintenance
habitually necessitates diminished power of race-propagation, yet the
product of the two factors is greater than before; so that the forces
preservative of race become, thereafter, in excess of the forces
destructive of race, and the race spreads. We shall soon see why this
happens.

Every advance in evolution implies an economy. That any increase in
bulk, or structure, or activity, may become established, the life of
the organism must be to some extent facilitated by the change--the
cost of self-support must be, on the average, reduced. If the greater
complexity, or the larger size, or the more agile movement, entails
on the individual an outlay that is not repaid in food more-easily
obtained, or danger more-easily escaped; then the individual will
be at a relative disadvantage, and its diminished posterity will
disappear. If the extra outlay is but just made good by the extra
advantage, the modified individual will not survive longer, or leave
more descendants, than the unmodified individuals. Consequently, it
is only when the expense of greater individuation is out-balanced by
a subsequent saving, that it can tend to subserve the preservation of
the individual, and, by implication, the preservation of the race.
The vital capital invested in the alteration must bring a more than
equivalent return. A few instances will show that, whether the change
results from direct equilibration or from indirect equilibration, this
must happen. Suppose a creature takes to performing some act in an
unusual way--leaps where ordinarily its kindred crawl, eludes pursuit
by diving instead of, like others of its kind, by swimming along the
surface, escapes by doubling instead of by speed. Clearly, perseverance
in the modified habit will, other things equal, imply that it takes
less effort. The creature’s sensations will ever prompt desistance from
the more laborious course; and hence a congenital habit is not likely
to be diverged from unless an economy of force is achieved by the
divergence. Assuming, then, that the new method has no advantage over
the old in directly diminishing the chances of death, the establishment
of it, and of the structural complications involved, nevertheless
implies a physiological gain. Suppose, again, that an animal takes
to some abundant food previously refused by its kind. It is likely
to persist only if the comparative ease in obtaining this food, more
than compensates for any want of adaptation to its digestive organs;
so that superposed modifications of the digestive organs are likely
to arise only when an average economy results. What now must be the
influence on the creature’s system as a whole? Diminished expenditure
in any direction, or increased nutrition however effected, will leave
a greater surplus of materials. The animal will be physiological
richer. Part of its augmented wealth will go towards its own greater
individuation--its size, or its strength, or both, will increase; while
another part will go towards more active genesis. Just as a state of
plethora directly produced enhances fertility; so will such a state
indirectly produced.

In another way, the same thing must result from those additions
to bulk or complexity or activity that are due to survival of the
fittest. Any change which prolongs individual life will, other things
remaining the same, further the production of offspring. Even when it
is not, like the foregoing, a means of economizing the forces of the
individual, still, if it increases the chances of escaping destruction,
it increases the chances of leaving posterity. Any further degree of
evolution, therefore, will be established only where the cost of it
is more than repaid: part of the gain being shown in the lengthened
life of the individual, and part in the greater production of other
individuals.

We have here the solution of various minor anomalies by which the
inverse variation of Individuation and Genesis is obscured. Take as an
instance the fertility of the Blackbird as compared with that of the
Linnet. Both birds lay five eggs, and both usually have two broods. Yet
the Blackbird is far the larger of the two, and ought, according to the
general law, to be much less prolific. What causes this nonconformity?
We shall find an answer in their respective foods and habits. Except
during the time that it is rearing its young, the Linnet collects only
vegetal food--lives during the winter on the seeds it finds in the
fields, or, when hard pressed, picks up around farms; and to obtain
this spare diet is continually flying about. The result, if it survives
the frost and snow, is a considerable depletion; and it recovers its
condition only after some length of spring weather. The Blackbird, on
the other hand, is omnivorous. While it eats grain and fruit when they
come in its way, it depends largely on animal food. It cuts to pieces
and devours the dew-worms which, morning and evening, it finds on the
surface of a lawn, and, even discovering where they are, unearths
them; it swallows slugs, and breaking snail-shells, either with its
beak or by hammering them against stones, tears out their tenants; and
it eats beetles and larvæ. Thus the strength of the Blackbird opens
to it a store of good food, much of which is inaccessible to so small
and weak a bird as a Linnet--a store especially helpful to it during
the cold months, when the hybernating snails in hedge-bottoms yield
it abundant provision. The result is that the Blackbird is ready to
breed very early in spring, and is able during the summer to rear
a second, and sometimes even a third, brood. Here, then, a higher
degree of Individuation secures advantages so great, as to much more
than compensate its cost. It is not that the decline of Genesis is
less than proportionate to the increase of Individuation, but there
is no decline at all. Comparison of the Rat with the Mouse yields a
parallel result. Though they differ greatly in size, yet the one is as
prolific as the other. This absence of difference cannot be ascribed
to their unlike degrees of activity. We must seek its cause in some
facility of living secured to the Rat by its greater intelligence,
greater power and courage, greater ability to utilize what it finds.
The Rat is notoriously cunning; and its cunning gives success to its
foraging expeditions. It is not, like the Mouse, limited mainly to
vegetal food; but while it eats grain and beans like the Mouse, it also
eats flesh and carrion, devours young poultry and eggs. The result is
that, without a proportionate increase of expenditure, it gets a far
larger supply of nourishment than the Mouse; and relative excess of
nourishment makes possible a larger size without a smaller rate of
multiplication. How clearly this is the cause, we see in the contrast
between the common Rat and the Water-Rat. While the common Rat has
ordinarily several broods a-year of from 10 to 12 each, the Water-Rat,
though somewhat smaller, has but 5 or 6 in a brood, and but one brood,
or sometimes two broods, a-year. But the Water-Rat lives on vegetal
food, and it lacks all that its bold, sagacious, omnivorous congener
gains from the warmth as well as the abundance which men’s habitations
yield.

The inverse variation of Individuation and Genesis is, therefore, but
approximate. Recognizing the truth that every increment of evolution
which is appropriate to the circumstances of an organism, brings an
advantage somewhat in excess of its cost; we see the general law, as
more strictly stated, to be that Genesis decreases not quite so fast
as Individuation increases. Whether the greater Individuation takes
the form of a larger bulk and accompanying access of strength; whether
it be shown in higher speed or agility; whether it consists in a
modification of structure which facilitates some habitual movement,
or in a visceral change that helps to utilize better the absorbed
aliment; the ultimate effect is identical. There is either a more
economical performance of the same actions, internal or external, or
there is a securing of greater advantages by modified actions, which
cost no more, or have an increased cost less than the increased gain.
In any case the result is a greater surplus of vital capital, part
of which goes to the aggrandizement of the individual, and part to
the formation of new individuals. While the higher tide of nutritive
matters, everywhere filling the parent-organism, adds to its power of
self-maintenance, it also causes a reproductive overflow larger than
before.

Hence every type which is best adapted to its conditions, (and this
on the average means every higher type), has a rate of multiplication
that insures a tendency to predominate. Survival of the fittest, acting
alone, is ever replacing inferior species by superior species. But
beyond the longer survival, and therefore greater chance of leaving
offspring, which superiority gives, we see here another way in which
the spread of the superior is insured. Though the more-evolved organism
is the less fertile absolutely, it is the more fertile relatively.




                             CHAPTER XII.

                   MULTIPLICATION OF THE HUMAN RACE.


§ 365. The relative fertility of Man considered as a species, and
those changes in Man’s fertility which occur under changed conditions,
must conform to the laws which we have traced thus far. As a matter
of course, the inverse variation between Individuation and Genesis
holds of him as of all other organized beings. His extremely low rate
of multiplication--far below that of all terrestrial Mammals except
the Elephant, (which though otherwise less evolved is, in extent
of integration, more evolved)--we shall recognize as the necessary
concomitant of his much higher evolution. And the causes of increase or
decrease in his fertility, special or general, temporary or permanent,
we shall expect to find in those changes of bulk, of structure, or of
expenditure, which we have in all other cases seen associated with such
effects.

In the absence of detailed proof that these parallelisms exist, it
might suffice to contemplate the several communities between the
reproductive function in human beings and other beings. I do not refer
simply to the fact that genesis proceeds in a similar manner; but I
refer to the similarity of the relation between the generative function
and the functions which have for their joint end the preservation
of the individual. In Man, as in other creatures that expend much,
genesis commences only when growth and development are declining in
rapidity and approaching their termination. Among the higher organisms
in general, the reproductive activity, continuing during the prime
of life, ceases when the vigour declines, leaving a closing period of
infertility; and in like manner among ourselves, barrenness supervenes
when middle age brings the surplus vitality to an end. So, too, it is
found that in Man, as in beings of lower orders, there is a period
at which fecundity culminates. In § 341, facts were cited showing
that at the commencement of the reproductive period, animals bear
fewer offspring than afterwards; and that towards the close of the
reproductive period, there is a decrease in the number produced. In
like manner it is shown by the tables of Dr. Duncan’s recent work,
that the fecundity of women increases up to the age of about 25
years, and continuing high with but slight diminution till after 30,
then gradually wanes. It is the same with the sizes and weights of
offspring. Infants born of women from 25 to 29 years of age, are both
longer and heavier than infants born of younger or older women; and
this difference has the same implication as the greater total weight
of the offspring produced at a birth, during the most fecund age of
a pluriparous animal. Once more, there is the fact that a too-early
bearing of young produces on a woman the same injurious effects as
on an inferior creature--an arrest of growth and an enfeeblement of
constitution.

Considering these general and special parallelisms, we might safely
infer that variations of human fertility conform to the same laws
as do variations of fertility in general. But it is not needful to
content ourselves with an implication. Evidence is assignable that
what causes increase or decrease of genesis in other creatures, causes
increase or decrease of genesis in Man. It is true that, even more
than hitherto, our reasonings are beset by difficulties. So numerous
are the inequalities in the conditions, that but few unobjectionable
comparisons can be made. The human races differ considerably in their
sizes, and notably in their degrees of cerebral development. The
countries they inhabit entail on them widely different consumptions
of matter for maintenance of temperature. Both in their qualities
and quantities the foods they live on are unlike; and the supply is
here regular and there very irregular. Their expenditures in bodily
action are extremely unequal; and even still more unequal are their
expenditures in mental action. Hence the factors, varying so much in
their amounts and combinations, can scarcely ever have their respective
effects identified. Nevertheless there are a few comparisons the
results of which may withstand criticism.


§ 366. The increase of fertility caused by a nutrition that is greatly
in excess of the expenditure, is to be detected by contrasting
populations of the same race, or allied races, one of which obtains
good and abundant sustenance much more easily than the other. Three
cases may here be set down.

The traveller Barrow, describing the Cape-Boers, says:--“Unwilling
to work and unable to think,” ... “indulging to excess in the
gratification of every sensual appetite, the African peasant grows to
an unwieldy size;” and respecting the other sex, he adds--“the women
of the African peasantry lead a life of the most listless inactivity,”
Then, after illustrating these statements, he goes on to note “the
prolific tendency of all the African peasantry. Six or seven children
in a family are considered as very few; from a dozen to twenty are
not uncommon.” The native races of this region yield evidence to the
same effect. Speaking of the cruelly-used Hottentots (he is writing a
century ago), who, while they are poor and ill-fed, have to do all the
work for the idle Boers, Barrow says that they “seldom have more than
two or three children; and many of the women are barren.” This unusual
infertility stands in remarkable contrast with the unusual fertility of
the Kaffirs, of whom he afterwards gives an account. Rich in cattle,
leading easy lives, and living almost exclusively on animal food
(chiefly milk with occasional flesh), these people were then reputed
to have a very high rate of multiplication. Barrow writes:--“They are
said to be exceedingly prolific; that twins are almost as frequent as
single births, and that it is no uncommon thing for a woman to have
three at a time.” Probably both these statements are in excess of the
truth; but there is room for large discounts without destroying the
extreme difference. A third instance is that of the French-Canadians.
“_Nous sommes terribles pour les enfants!_” observed one of them to
Prof. Johnston, who tells us that the man who said this “was one of
fourteen children--was himself the father of fourteen, and assured
me that from eight to sixteen was the usual number of the farmers’
families. He even named one or two women who had brought their husbands
five-and-twenty, and threatened ‘_le vingt-sixième pour le prêtre_.’”
From these large families, joined with the early marriages and low
rate of mortality, it results that, by natural increase, “there are
added to the French-Canadian population of Lower Canada four persons
for every one that is added to the population of England.” Now these
French-Canadians are described by Prof. Johnston as home-loving,
contented, unenterprising; and as living in a region where “land and
subsistence are easily obtained.” Very moderate industry brings to
them liberal supplies of necessaries; and they pass a considerable
portion of the year in idleness. Hence the cost of Individuation
being much reduced, the rate of Genesis is much increased. That this
uncommon fertility is not due to any direct influence of the locality,
is implied by the fact that along with the “restless, discontented,
striving, burning energy of their Saxon neighbours,” no such rate of
multiplication is observed; while further south, where the physical
circumstances are more favourable if anything, the Anglo-Saxons,
leading lives of excessive activity, have a fertility below the
average. And that the peculiarity is not a direct effect of race, is
proved by the fact that in Europe, the rural French are certainly not
more prolific than the rural English.

To every reader there will probably occur the seemingly-adverse
evidence furnished by the Irish; who, though not well fed, multiply
fast. Part of this more rapid increase is due to the earlier marriages
common among them, and consequent quicker succession of generations--a
factor which, as we have seen, has a larger effect than any other on
the rate of multiplication. Part of it is due to the greater generality
of marriage--to the comparative smallness of the number who die without
having had the opportunity of producing offspring. The effects of
these causes having been deducted, we may doubt whether the Irish,
individually considered, would be found more prolific than the English.
Perhaps, however, it will be said that, considering their diet, they
ought to be less prolific. This is by no means obvious. It is not
simply a question of nutriment absorbed. It is a question of how much
remains after the expenditure in self-maintenance. Now a notorious
peculiarity in the life of the Irish peasant is, that he obtains a
return of food which is large in proportion to his outlay in labour.
The cultivation of his potatoe-ground occupies each cottager but a
small part of the year; and the domestic economy of his wife is not of
a kind to entail on her much daily exertion. Consequently the crop,
tolerably abundant in quantity though innutritive in quality, possibly
suffices to meet the comparatively-low expenditure, and to leave a good
surplus for genesis--perhaps a greater surplus than remains to the
males and females of the English peasantry, who, though fed on better
food, are harder worked.

We conclude, then, that in the human race, as in all other races, such
absolute or relative abundance of nutriment as leaves a large excess
after defraying the cost of carrying on parental life, is accompanied
by a high rate of genesis.[66]


§ 367. Evidence of the converse truth, that relative increase of
expenditure, leaving a diminished surplus, reduces the degree of
fertility, is not wanting. Some of it has been set down for the sake of
antithesis in the foregoing section. Here may be grouped a few facts of
a more special kind having the same implication.

To prove that much bodily labour renders women less prolific, requires
more evidence than has at present been collected. Nevertheless it may
be noted that De Boismont in France and Dr. Szukits in Austria, have
shown by extensive statistical comparisons, that the reproductive
age is reached a year later by women of the labouring class than by
middle-class women; and while ascribing this delay in part to inferior
nutrition, we may suspect that it is in part due to greater muscular
expenditure. A kindred fact, admitting of a kindred interpretation,
may be added. Though the comparatively-low rate of increase in France
is attributed to other causes, yet, very possibly, one of its causes
is the greater proportion of hard work entailed on French women, by
the excessive abstraction of men for non-productive occupations,
military and civil. The higher rate of multiplication in England than
in continental countries generally, is not improbably furthered by the
easier lives which English women lead.

That absolute or relative infertility is commonly produced in women
by mental labour carried to excess, is more clearly shown. Though the
regimen of upper-class girls is not what it should be, yet, considering
that their feeding is better than that of girls belonging to the poorer
classes, while, in most other respects, their physical treatment is
not worse, the deficiency of reproductive power among them may be
reasonably attributed to the overtaxing of their brains--an overtaxing
which produces a serious reaction on the physique. This diminution
of reproductive power is not shown only by the greater frequency of
absolute sterility; nor is it shown only in the earlier cessation of
child-bearing; but it is also shown in the very frequent inability of
such women to suckle their infants. In its full sense, the reproductive
power means the power to bear a well-developed infant and to supply
that infant with the natural food for the natural period. Most of the
flat-chested girls who survive their high-pressure education, are
incompetent to do this. Were their fertility measured by the number
of children they could rear without artificial aid, they would prove
relatively very infertile.

The cost of reproduction to males being so much less than it is to
females, the antagonism between Genesis and Individuation is not often
shown in men by suppression of generative power consequent on unusual
expenditure in bodily action. Nevertheless, there are indications that
this results in extreme cases. We read that the ancient _athletæ_
rarely had children; and among such of their modern representatives as
acrobats, an allied relation of cause and effect is alleged. Indirectly
this truth, or rather its converse, appears to have been ascertained
by those who train men for feats of strength--they find it needful to
insist on continence.

Special proofs that in men great cerebral expenditure diminishes or
destroys generative power, are difficult to obtain. It is, indeed,
asserted that intense application to mathematics, requiring as it does
extreme concentration of thought, is apt to have this result; and it is
asserted, too, that this result is produced by the excessive emotional
excitement of gambling. Then, again, it is a matter of common remark
how frequently men of unusual mental activity leave no offspring. But
facts of this kind admit of another interpretation. The reaction of
the brain on the body is so violent--the overtaxing of the nervous
system is so apt to prostrate the heart and derange the digestion; that
the incapacities caused in these cases, are probably often due more to
constitutional disturbance than to the direct deduction which excessive
action entails. Such instances harmonize with the hypothesis; but how
far they yield it positive support we cannot say.


§ 368. An objection must here be guarded against. It is likely to
be urged that since the civilized races are, on the average, larger
than many of the uncivilized races; and since they are also somewhat
more complex as well as more active; they ought, in conformity with
the alleged general law, to be less prolific. There is, however, no
evidence to prove that they are so: on the whole, they seem rather the
reverse.

The reply is that were all other things equal, these superior varieties
of men should have inferior rates of increase. But other things are not
equal; and it is to the inequality of other things that this apparent
anomaly is attributable. Already we have seen how much more fertile
domesticated animals are than their wild kindred; and the causes of
this greater fertility are also the causes of the greater fertility,
relative or absolute, which civilized men exhibit when compared with
savages.

There is the difference in amount of food. Australians, Fuegians, and
sundry races that might be named as having low rates of multiplication,
are obviously underfed. The sketches of natives contained in the
volumes of Livingstone, Baker, and others, yield clear proofs of the
extreme depletion common among the uncivilized. In quality as well
as in quantity, their feeding is bad. Wild fruits, insects, larvæ,
vermin, &c., which we refuse with disgust, often enter largely into
their dietary. Much of this inferior food they eat uncooked; and they
have not our elaborate appliances for mechanically-preparing it, and
rejecting its useless parts. So that they live on matters of less
nutritive value, which cost more both to masticate and to digest.
Further, to uncivilized men supplies of food come very irregularly.
Long periods of scarcity are divided by short periods of abundance.
And though by gorging when opportunity occurs, something is done
towards compensating for previous fasting, yet the effects of prolonged
starvation cannot be neutralized by occasional enormous meals. Bearing
in mind, too, that improvident as they are, savages often bestir
themselves only under pressure of hunger, we may fairly consider them
as habitually ill-nourished--may see that even the poorer classes of
civilized men, making regular meals on food separated from innutritive
matters, easy to masticate and digest, tolerably good in quality and
adequate if not abundant in quantity, are much better nourished.

Then, again, though a greater consumption in muscular action appears
to be undergone by civilized men than by savages; and though it is
probably true that among our labouring people the daily repairs cost
more; yet in many cases there does not exist so much difference as we
are apt to suppose. The chase is very laborious; and great amounts of
exertion are gone through by the lowest races in seeking and securing
the odds and ends of wild food on which they largely depend. We
naturally assume that because barbarians are averse to regular labour,
their muscular action is less than our own. But this is not necessarily
true. The monotonous toil is what they cannot tolerate; and they may
be ready to go through as much or more exertion when it is joined with
excitement. If we remember that the sportsman who gladly scrambles
up and down rough hill-sides all day after grouse or deer, would
think himself hardly used had he to spend as much effort and time in
digging; we shall see that a savage who is the reverse of industrious,
may nevertheless be subject to a muscular waste not very different in
amount from that undergone by the industrious. When it is added that a
larger physiological expenditure is entailed on the uncivilized than
on the civilized by the absence of good appliances for shelter and
protection--that in some cases they have to make good a greater loss
of heat, and in other cases suffer much wear from irritating swarms of
insects; we shall see that the total cost of self-maintenance among
them is probably in many cases little less, and in some cases more,
than it is among ourselves.

So that though, on the average, the civilized are probably larger than
the savage; and though they are, in their nervous systems at least,
somewhat more complex; and though, other things equal, they ought to
be the less prolific; yet other things are so unequal as to make it
quite conformable to the general law that they should be more prolific.
In § 365 we observed how, among inferior animals, higher evolution
sometimes makes self-preservation far easier, by opening the way to
resources previously unavailable: so involving an undiminished, or
even an increased, rate of genesis. And similarly we may expect that
among races of men, those whose slight further developments have been
followed by habits and arts which immensely facilitate life, will not
exhibit a lower degree of fertility, and may even exhibit a higher.


§ 369. One more objection has to be met--a kindred objection to which
there is a kindred reply. Cases may be named of men conspicuous
for activity, bodily and mental, who were also noted, not for less
generative power than usual, but for more. As their superiorities
indicate higher degrees of evolution, it may be urged that such men
should, according to the theory, have lower degrees of reproductive
activity. The fact that here, along with increased powers of
self-preservation, there go increased powers of race-propagation,
seems irreconcilable with the general doctrine. Reconciliation is not
difficult however.

The cases are analogous to some before named, in which more abundant
food simultaneously aggrandizes the individual and adds to the
production of new individuals: the difference between the cases
being, that instead of a better external supply of materials there
is a better internal utilization of materials. Creatures of the same
species notoriously differ in goodness of constitution. Here there
is some visceral defect, showing itself in feebleness of all the
functions; while here some peculiarity of organic balance, some high
quality of tissue, some abundance or potency of the digestive juices,
gives to the system a perpetual high tide of rich blood, which serves
at once to enhance the vital activities and to raise the power of
propagation. Such variations, however, are independent of changes in
the _proportion_ between Individuation and Genesis. This remains the
same, while both are increased or decreased by the increase or decrease
of the common stock of materials.

An illustration will best clear up any perplexity. Let us say that
the fuel burnt in the furnace of a locomotive steam-engine, answers
to the food which a man consumes. Let us say that the produced steam
expended in working the engine, corresponds to that portion of absorbed
nutriment which carries on the man’s functions and activities. And
let us say that the steam blowing off at the safety-valve, answers to
that portion of the absorbed nutriment which goes to the propagation
of the race. Such being the conditions of the case, several kinds of
variations are possible. All other circumstances remaining the same,
there may be changes of proportion between the steam used for working
the engine and the steam that escapes by the safety-valve. There may
be a structural or organic change of proportion. By enlarging the
safety-valve or weakening its spring, while the cylinders are reduced
in size, there may be established a constitutionally-small power of
locomotion and a constitutionally-large amount of escape-steam; and
inverse variations so produced, will answer to the inverse variations
between Individuation and Genesis which different types of organisms
show us. Again, there may be a functional change of proportion. If the
engine has to draw a considerable load, the abstraction of steam by
the cylinders greatly reduces the discharge by the safety-valve; and
if a high velocity is kept up, the discharge from the safety-valve
entirely ceases. Conversely, if the velocity is low, the escape-steam
bears a large ratio to the steam consumed by the motor apparatus;
and if the engine becomes stationary the whole of the steam escapes
by the safety-valve. This inverse variation answers to that which
we have traced between Expenditure and Genesis, as displayed in the
contrasts between species of the same type but unlike activities,
and in the contrasts between active and inactive individuals of the
same species. But now beyond these inverse variations between the
quantities of consumed steam and escape-steam, which are structurally
and functionally caused, there are coincident variations, producible
in both by changes in the quantity of steam supplied--changes which
may be caused in several ways. In the first place, the fuel thrown
into the furnace may be increased or made better. Other things equal,
there will result a more active locomotion as well as a greater escape;
and this will answer to that simultaneous addition to its individual
vigour and its reproductive activity, caused in an animal by a larger
quantity, or a superior quality, of food. In the second place, the
steam generated may be economized. Loss by radiation from the boiler
may be lessened by a covering of non-conducting substances; and part
of the steam thus prevented from condensing, will go to increase the
working power of the engine, while part will be added to the quantity
blowing off. This variation corresponds to that simultaneous addition
to bodily vigour and propagative power, which results in animals
that have to expend less in keeping up their temperatures. In the
third place, by improvement of the steam-generating apparatus, more
steam may be obtained from a given weight of fuel. A better-formed
evaporating surface, or boiler tubes which conduct more rapidly, or
an increased number of them may cause a larger absorption of heat
from the burning mass or the hot gases it gives off; and the extra
steam generated by this extra heat will, as before, augment both the
motive force and the emission through the safety-valve. And this last
case of coincident variation, is parallel to the case with which we
are here concerned--the augmentation of individual expenditure and of
reproductive energy, that may be caused by a superiority of some organ
on which the utilizing or economizing of materials depends.

Manifestly, therefore, an increased expenditure for Genesis, or an
increased expenditure for Individuation, may arise in one of two quite
different ways--either by diminution of the antagonistic expenditure,
or by addition to the store which supplies both expenditures; and
confusion results from not distinguishing between these. Given the
ratio 4 to 20, as expressive of the relative costs of Genesis and
Individuation; then the expenditure for Genesis may be raised to 5
while the expenditure for Individuation is raised to 25, without any
alteration of type, merely by favourable circumstances or superiority
of constitution. On the other hand, circumstances remaining the same,
the expenditure for Genesis may be raised from 4 to 5, by lowering the
expenditure for Individuation from 20 to 19: which change of ratio may
be either functional and temporary, or structural and permanent. And
only when it is the last does it illustrate that inverse variation
between degree of evolution and degree of procreative dissolution,
which we have everywhere seen.


§ 370. There is no reason to suppose, then, that the laws of
multiplication which hold of other beings, do not hold of the human
being. On the contrary, there are special facts which unite with
general implications to show that these laws do hold of the human
being. The absence of direct evidence in some cases where it might be
looked for, we find fully explained when all the factors are taken into
account. And certain seemingly-adverse facts prove, on examination, to
be facts belonging to a different category from that in which they are
placed, and harmonize with the rest when rightly interpreted.

The conformity of human fertility to the laws of multiplication in
general, being granted, it remains to inquire what effects must be
caused by permanent changes in men’s natures and circumstances. Thus
far we have observed how, by their exceptionally-high evolution and
exceptionally-low fertility, mankind display the inverse variation
between Individuation and Genesis, in one of its extremes. And we have
also observed how mankind, like other kinds, are functionally changed
in their rates of multiplication by changes of conditions. But we have
not observed how alteration of structure in Man entails alteration
of fertility. The influence of this factor is so entangled with the
influences of other factors which are for the present more potent, that
we cannot recognize it. Here, if we proceed at all, we must proceed
deductively.

       *       *       *       *       *

[NOTE.--From among the publications of the American Academy of
Political and Social Science, there was sent to me some years ago an
essay entitled “The Significance of a Decreasing Birth Rate” by (Miss)
J. L. Brownell, Fellow in Political Science, Bryn Mawr College. This
essay contains a number of elaborate comparisons drawn from the vital
statistics of the tenth United States Census. The results of these
comparisons are thus summed up:--

   “1. Whether or not it be true that the means spoken of by Dr.
   Billings, M. Dumont, M. Levasseur, and Dr. Edson has become an
   important factor in the diminishing birth-rate of civilized
   countries, it is evident that it is not the only factor, and
   that, quite apart from voluntary prevention, there is a distinct
   problem to be investigated. This is shown by the fact that the
   white and the colored birth-rate vary together.

   “2. Mr. Spencer’s generalization that the birth-rate diminishes
   as the rate of individual evolution increases is confirmed by a
   comparison of the birth-rates with the death-rates from nervous
   diseases, and also with the density of population, the values
   of agricultural and manufactured products, and the mortgage
   indebtedness.”

Of course multitudinous differences of race, class, mode of living,
occupation, locality, make it difficult to draw positive inferences
from the data; but the inferences above drawn are held to remain
outstanding after allowing for all the qualifying conditions.]




                             CHAPTER XIII.

                    HUMAN POPULATION IN THE FUTURE.


§ 371. Any further evolution in the most highly-evolved of terrestrial
beings, Man, must be of the same nature as evolution in general.
Structurally considered, it may consist in greater integration,
or greater differentiation, or both--augmented bulk, or increased
heterogeneity and definiteness, or a combination of the two.
Functionally considered, it may consist in a larger sum of actions,
or more multiplied varieties of actions, or both--a larger amount of
sensible and insensible motion generated, or motions more numerous in
their kinds and more intricate and exact in their co-ordinations, or
motions that are greater alike in quantity, complexity, and precision.

Expressing the change in terms of that more special evolution displayed
by organisms; we may say that it must be one which further adapts the
moving equilibrium of organic actions. As was pointed out in _First
Principles_, § 173, “the maintenance of such a moving equilibrium,
requires the habitual genesis of internal forces corresponding in
number, directions, and amounts to the external incident forces--as
many inner functions, single or combined, as there are single or
combined outer actions to be met.” And it was also pointed out that
“the structural complexity accompanying functional equilibration, is
definable as one in which there are as many specialized parts as are
capable, separately and jointly, of counteracting the separate and
joint forces amid which the organism exists.” Clearly, then, since
all incompletenesses in Man as now constituted, are failures to meet
certain of the outer actions (mostly involved, remote, irregular), to
which he is exposed; every advance implies additional co-ordinations of
actions and accompanying complexities of organization.

Or, to specialize still further this conception of future progress, we
may consider it as an advance towards completion of that continuous
adjustment of internal to external relations, which Life shows us.
In Part I. of this work, where it was shown that the correspondence
between inner and outer actions which under its phenomenal aspect,
we call Life, is a particular kind of what, in terms of Evolution,
we called a moving equilibrium; it was shown that the degree of life
varies as the degree of correspondence. Greater evolution or higher
life implies, then, such modifications of human nature as shall make
more exact the existing correspondences, or shall establish additional
correspondences, or both. Connexions of phenomena of a rare, distant,
unobtrusive, or intricate kind, which we either suffer from or do not
take advantage of, have to be responded to by new connexions of ideas,
and acts properly combined and proportioned: there must be increase
of knowledge, or skill, or power, or of all these. And to effect this
more extensive, more varied, and more accurate, co-ordination of
actions, there must be organization of still greater heterogeneity and
definiteness.


§ 372. Let us, before proceeding, consider in what particular ways this
further evolution, this higher life, this greater co-ordination of
actions, may be expected to show itself.

Will it be in strength? Probably not to any considerable degree.
Mechanical appliances are fast supplanting brute force, and doubtless
will continue doing this. Though at present civilized nations largely
depend for self-preservation on vigour of limb, and are likely to do
so while wars continue; yet that progressive adaptation to the social
state which must at last bring wars to an end, will leave the amount
of muscular power to adjust itself to the requirements of a peaceful
_regime_. Though, taking all things into account, the muscular power
then required may not be less than now, there seems no reason why more
should be required.

Will it be swiftness or agility? Probably not. In savages these are
important elements of the ability to maintain life; but in civilized
men they aid self-preservation in quite minor degrees, and there
seems no circumstance likely to necessitate an increase of them.
By games and gymnastic competitions, such attributes may indeed be
artificially increased; but no artificial increase which does not bring
a proportionate advantage can be permanent; since, other things equal,
individuals and societies that devote the same amounts of energy in
ways which subserve life more effectually, must by and by predominate.

Will it be in mechanical skill, that is, in the better-co-ordination
of complex movements? Most likely in some degree. Awkwardness is
continually entailing injuries and deaths. Moreover the complicated
tools which civilization brings into use, are constantly requiring
greater delicacy of manipulation. All the arts, industrial and
æsthetic, as they develop, imply a corresponding development of
perceptive and executive faculties in men: the two act and react.

Will it be in intelligence? Largely, no doubt. There is ample room
for advance in this direction, and ample demand for it. Our lives are
universally shortened by our ignorance. In attaining complete knowledge
of our own natures and of the natures of surrounding things--in
ascertaining the conditions of existence to which we must conform,
and in discovering means of conforming to them under all variations
of seasons and circumstances; we have abundant scope for intellectual
progress.

Will it be in morality, that is, in greater power of self-regulation?
Largely also: perhaps most largely. Right conduct is usually come short
of more from defect of will than defect of knowledge. For the right
co-ordination of those complex actions which constitute human life in
its civilized form, there goes not only the pre-requisite--recognition
of the proper course; but the further pre-requisite--a due impulse to
pursue that course. On calling to mind our daily failures to fulfil
often-repeated resolutions, we shall perceive that lack of the needful
desire, rather than lack of the needful insight, is the chief cause of
faulty action. A further endowment of those feelings which civilization
is developing in us--sentiments responding to the requirements of the
social state--emotive faculties that find their gratifications in the
duties devolving on us--must be acquired before the crimes, excesses,
diseases, improvidences, dishonesties, and cruelties, that now so
greatly diminish the duration of life, can cease.

Thus, looking at the several possibilities, and asking what direction
this further evolution, this more complete moving equilibrium, this
better adjustment of inner to outer relations, this more perfect
co-ordination of actions, is likely to take; we conclude that it must
take mainly the direction of a higher intellectual and emotional
development.


§ 373. This conclusion we shall find equally forced on us if we
inquire for the causes which are to bring about such results. No
more in the case of Man than in the case of any other being, can
we presume that evolution has taken place, or will hereafter take
place, spontaneously. In the past, at present, and in the future, all
modifications, functional and organic, have been, are, and must be,
immediately or remotely consequent on surrounding conditions. What,
then, are those changes in the environment to which, by direct or
indirect equilibration, the human organism has been adjusting itself,
is adjusting itself now, and will continue to adjust itself? And how
do they necessitate a higher evolution of the organism?

Civilization, everywhere having for its antecedent the increase
of population, and everywhere having for one of its consequences
a decrease of certain race-destroying forces, has for a further
consequence an increase of certain other race-destroying forces. Danger
of death from predatory animals lessens as men grow more numerous.
Though, as they spread over the Earth and divide into tribes, men
become wild beasts to one another, yet the danger of death from this
cause also diminishes as tribes coalesce into nations. But the danger
of death which does not diminish, is that produced by augmentation of
numbers itself--the danger from deficiency of food. Supposing human
nature to remain unchanged, the mortality hence resulting would, on
the average, rise as human beings multiplied. If mortality, under
such conditions, does not rise, it must be because the supply of food
also augments; and this implies some change in human habits wrought
by stress of human needs. Here, then, is the permanent cause of
modification to which civilized men are exposed. Though the intensity
of its action is ever being mitigated in one direction by greater
production of food, it is, in the other direction, ever being added
to by the greater production of individuals. Manifestly, the wants of
their redundant numbers constitute the only stimulus mankind have to
obtain more necessaries of life. Were not the demand beyond the supply,
there would be no motive to increase the supply. And manifestly, this
excess of demand over supply is perennial: this pressure of population,
of which it is the index, cannot be eluded. Though by the emigration
that takes place when the pressure arrives at a certain intensity,
temporary relief is from time to time obtained; yet as, by this
process, all habitable countries must become peopled, it follows that
in the end the pressure, whatever it may then be, must be borne in full.

This constant increase of people beyond the means of subsistence
causes, then, a never-ceasing requirement for skill, intelligence, and
self-control--involves, therefore, a constant exercise of these and
gradual growth of them. Every industrial improvement is at once the
product of a higher form of humanity, and demands that higher form of
humanity to carry it into practice. The application of science to the
arts, is the bringing to bear greater intelligence for satisfying our
wants, and implies continued progress of that intelligence. To get more
produce from the acre, the farmer must study chemistry, must adopt new
mechanical appliances, and must, by the multiplication of processes,
cultivate both his own powers and the powers of his labourers. To
meet the requirements of the market, the manufacturer is perpetually
improving his old machines and inventing new ones; and by the
premium of high wages incites artizans to acquire greater skill. The
daily-widening ramifications of commerce entail on the merchant a need
for more knowledge and more complex calculations; while the lessening
profits of the ship-owner force him to build more scientifically, to
get captains of higher intelligence and better crews. In all cases
pressure of population is the original cause. Were it not for the
competition this entails, more thought and energy would not daily be
spent on the business of life; and growth of mental power would not
take place. Difficulty in getting a living is alike the incentive to a
higher education of children, and to a more intense and long-continued
application in adults. In the mother it prompts foresight, economy,
and skilful house-keeping; in the father, laborious days and constant
self-denial. Nothing but necessity could make men submit to this
discipline; and nothing but this discipline could produce a continued
progression.

In this case, as in many others, Nature secures each step in advance by
a succession of trials; which are perpetually repeated, and cannot fail
to be repeated, until success is achieved. All mankind in turn subject
themselves more or less to the discipline described; they either
may or may not advance under it; but, in the nature of things, only
those who _do_ advance under it eventually survive. For, necessarily,
families and races whom this increasing difficulty of getting a living
which excess of fertility entails, does not stimulate to improvements
in production--that is, to greater mental activity--are on the high
road to extinction; and must ultimately be supplanted by those whom
the pressure does so stimulate. This truth we have recently seen
exemplified in Ireland. And here, indeed, without further illustration,
it will be seen that premature death, under all its forms and from all
its causes, cannot fail to work in the same direction. For as those
prematurely carried off must, in the average of cases, be those in whom
the power of self-preservation is the least, it unavoidably follows
that those left behind to continue the race, must be those in whom the
power of self-preservation is the greatest--must be the select of their
generation. So that, whether the dangers to existence be of the kind
produced by excess of fertility, or of any other kind, it is clear that
by the ceaseless exercise of the faculties needed to contend with them,
and by the death of all men who fail to contend with them successfully,
there is ensured a constant progress towards a higher degree of skill,
intelligence, and self-regulation--a better co-ordination of actions--a
more complete life.[67]


§ 374. The proposition at which we have thus arrived is, then, that
excess of fertility, through the changes it is ever working in Man’s
environment, is itself the cause of Man’s further evolution; and the
obvious corollary here to be drawn is, that Man’s further evolution so
brought about, itself necessitates a decline in his fertility.

All future progress in civilization which the never-ceasing pressure
of population must produce, will be accompanied by an enhanced cost of
Individuation, both in structure and function; and more especially in
nervous structure and function. The peaceful struggle for existence in
societies ever growing more crowded and more complicated, must have
for its concomitant an increase of the great nervous centres in mass,
in complexity, in activity. That larger body of emotion needed as a
fountain of energy for men who have to hold their places and rear their
families under the intensifying competition of social life, is, other
things equal, the correlative of larger brain. Those higher feelings
presupposed by the better self-regulation which, in a better society,
can alone enable the individual to leave a persistent posterity, are,
other things equal, the correlatives of a more complex brain; as are
also those more numerous, more varied, more general, and more abstract
ideas, which must also become increasingly requisite for successful
life as society advances. And the genesis of this larger quantity of
feeling and thought, in a brain thus augmented in size and developed
in structure, is, other things equal, the correlative of a greater
wear of nervous tissue and greater consumption of materials to repair
it. So that both in original cost of construction and in subsequent
cost of working, the nervous system must become a heavier tax on the
organism. Already the brain of the civilized man is larger by nearly
thirty per cent. than the brain of the savage. Already, too, it
presents an increased heterogeneity--especially in the distribution
of its convolutions. And further changes like these which have taken
place under the discipline of civilized life, we infer will continue
to take place. But everywhere and always, evolution is antagonistic
to procreative dissolution. Whether it be in greater growth of the
organs which subserve self-maintenance, whether it be in their added
complexity of structure, or whether it be in their higher activity,
the abstraction of the required materials implies a diminished reserve
of materials for race-maintenance. And we have seen reason to believe
that this antagonism between Individuation and Genesis, becomes
unusually marked where the nervous system is concerned, because of the
costliness of nervous structure and function. In § 346 was pointed out
the apparent connexion between high cerebral development and prolonged
delay of sexual maturity; and in §§ 366, 367, the evidence went to show
that where exceptional fertility exists there is sluggishness of mind,
and that where there has been during education excessive expenditure
in mental action, there frequently follows a complete or partial
infertility. Hence the particular kind of further evolution which Man
is hereafter to undergo, is one which, more than any other, may be
expected to cause a decline in his power of reproduction.

The higher nervous development and greater expenditure in nervous
action, here described as indirectly brought about by increase of
numbers, and as thereafter becoming a check on the increase of numbers,
must not be taken to imply an intenser strain--a mentally-laborious
life. The greater emotional and intellectual power and activity above
contemplated, must be understood as becoming, by small increments,
organic, spontaneous, and pleasurable. As, even when relieved from
the pressure of necessity, large-brained Europeans voluntarily enter
on enterprises and activities which the savage could not keep up even
to satisfy urgent wants; so, their still larger-brained descendants
will, in a still higher degree, find their gratifications in careers
entailing still greater mental expenditures. This enhanced demand for
materials to establish and carry on the psychical functions, will be a
constitutional demand. We must conceive the type gradually so modified,
that the more-developed nervous system irresistibly draws off, for its
normal and unforced activities, a larger proportion of the common stock
of nutriment; and while so increasing the intensity, completeness, and
length of the individual life, necessarily diminishing the reserve
applicable to the setting up of new lives--no longer required to be so
numerous.

Though the working of this process will doubtless be interfered
with and modified in the future, as it has been in the past, by the
facilitations of living which civilization brings; yet nothing beyond
temporary interruptions can so be caused. However much the industrial
arts may be improved, there must be a limit to the improvement; while,
with a rate of multiplication in excess of the rate of mortality,
population must continually tread on the heels of production. So that
though, during the earlier stages of civilization, an increased amount
of food may accrue from a given amount of labour, there must come a
time when this relation will be reversed, and when every additional
increment of food will be obtained by a more than proportionate labour:
the disproportion growing ever higher, and the diminution of the
reproductive power becoming greater.


§ 375. There now remains but to inquire towards what limit this
progress tends. So long as the fertility of the race is more than
sufficient to balance the diminution by deaths, population must
continue to increase. So long as population continues to increase,
there must be pressure on the means of subsistence. And so long
as there is pressure on the means of subsistence, further mental
development must go on, and further diminution of fertility must
result; provided that the actions and reactions which have been
described are not artificially interfered with. I append this
qualifying clause advisedly, and especially emphasize it, because
these actions and reactions have been hitherto, and are now,
greatly interfered with by governments, and the continuance of the
interferences may retard, if not stop, that further evolution which
would else go on.

I refer to those hindrances to the survival of the fittest which
in earlier times resulted from the undiscriminating charities of
monasteries and in later times from the operation of Poor Laws. Of
course if the competition which increasing pressure of population
entails, is prevented from acting on a considerable part of the
community, such part, saved from the needed intellectual and moral
stress, will not undergo any further mental development; and must
ever tend to leave a posterity, and an increasing posterity, in which
none of that higher individuation which checks genesis takes place.
Such State-meddlings with the natural play of actions and reactions
produce a further evil equally great or greater. For those who are
not self-maintained, or but partially self-maintained, are supplied
with the means they lack by the better members of the community; and
these better members have thus not only to support themselves and
their offspring, but also to support or aid the inferior members and
their offspring. The under-working of one part is accompanied by the
over-working of the other part--by a working which at each stage of
progress exceeds that which the normal conditions necessitate, and
results sometimes in illness, premature age, or death, or in lessened
number of children, or in imperfect rearing of children: the bad are
fostered and the good are repressed.

It does not follow that the struggle for life and the survival of the
fittest must be left to work out their effects without mitigation. It
is contended only that there shall not be a forcible burdening of the
superior for the support of the inferior. Such aid to the inferior as
the superior voluntarily yield, kept as it will be within moderate
limits, may be given with benefit to both--relief to the one, moral
culture to the other. And aid willingly given (little to the least
worthy and more to the most worthy) will usually be so given as not
to further the increase of the unworthy. For in proportion as the
emotional nature becomes more evolved, and there grows up a higher
sense of parental responsibility, the begetting of children that cannot
be properly reared will be universally held intolerable. If, as we
see, public opinion in many places and times becomes coercive enough
to force men to fight duels, we can scarcely doubt that at a higher
stage of evolution it may become so coercive as to prevent men from
marrying improvidently. If the frowns of their fellows can make men
commit immoral acts, surely they may make men refrain from immoral
acts--especially when the actors themselves feel that the threatened
frowns would be justified. Hence with a higher moral nature will come a
restriction on the multiplication of the inferior.

In brief, the sole requirement is that there shall be no extensive
suspension of that natural relation between merit and benefit which
constitutes justice. Holding, then, that this all-essential condition
will itself come to be recognized and enforced by a more evolved
humanity, let us consider what is the goal towards which the restraint
on genesis by individuation progresses.


§ 375_a_. Supposing the Sun’s light and heat, on which all terrestrial
life depends, to continue abundant for a period long enough to allow
the entire evolution we are contemplating; there are still certain
changes which must prevent such complete adjustment of human nature
to surrounding conditions, as would permit the rate of multiplication
to become equal to the rate of mortality. As before pointed out (§
148), during an epoch of 21,000 years each hemisphere goes through
a cycle of temperate seasons and seasons extreme in their heat and
cold--variations which are themselves alternately exaggerated and
mitigated in the course of far longer cycles; and we saw that these
cause perpetual ebbings and flowings of species over different parts
of the Earth’s surface. Further, by slow but inevitable geologic
changes, especially those of elevation and subsidence, the climate and
physical characters of every habitat are modified; while old habitats
are destroyed and new are formed. This, too, we noted as a constant
cause of migrations and of resulting alterations of environment. Now
though the human race differs from other races in having a power of
artificially counteracting external changes, yet there are limits to
this power; and, even were there no limits, the changes could not
fail to work their effects indirectly, if not directly. If, as is
thought probable, these astronomic cycles entail recurrent glacial
periods in each hemisphere, then parts of the Earth which are at one
time thickly peopled, will at another time be almost deserted, and
_vice versâ_. The geologically-caused alterations of climate and
surface, must produce further slow re-distributions of population;
and other currents of people, to and from different regions, will be
necessitated by the rise of successive centres of higher civilization.
Consequently, mankind cannot but continue to undergo changes of
environment, physical and moral, analogous to those which they have
thus far been undergoing. Such changes may eventually become slower
and less marked; but they can never cease. And if they can never cease
there can never arise a perfect adaptation of human nature to its
conditions of existence. To establish that complete correspondence
between inner and outer actions which constitutes the highest life
and greatest power of self-preservation, there must be a prolonged
converse between the organism and circumstances which remain the
same. If the external relations are being altered while the internal
relations are being adjusted to them, the adjustment can never become
exact. And in the absence of exact adjustment, there cannot exist that
theoretically-highest power of self-preservation with which there would
co-exist the theoretically-lowest power of race-production.

Hence though the number of premature deaths may ultimately become very
small, it can never become so small as to allow the average number of
offspring from each pair to fall so low as two. Some average number
between two and three may be inferred as the limit--a number, however,
which is not likely to be quite constant, but may be expected at
one time to increase somewhat and afterwards to decrease somewhat,
according as variations in physical and social conditions lower or
raise the cost of self-preservation.

To this qualification must be added a further qualification. The
foregoing argument tacitly assumes that the causes described will
continuously operate on all mankind; whereas a survey of the facts
makes it clear that some parts only of the Earth’s surface are capable
of bearing high types of civilization, and consequently high types
of Man. There must remain hereafter, as there are now, considerable
parts of its surface which can support only groups of nomads, or other
groups obliged by their habitats to lead simple and inferior kinds of
life. Only by subjection to the discipline we have been contemplating
can there be produced the fully-developed Man; and evidently in many
parts of the world this discipline will continue to be eluded. Not
only must we conclude that the varieties of our race now living in
desert regions and arctic climates will continue hereafter to do so,
but we may conclude that always, as now, a certain proportion of men
who are born in civilized societies, impatient of the stress which
pressure of population puts on them, will escape into unoccupied or
sparsely-peopled regions, where they can lead unrestrained lives
though lives of hardship. Recognizing as we must the probability
that in common with all other things, humanity will continue to
differentiate and produce a more heterogeneous assemblage of types,
we must infer that only in some of the highest of these will the
antagonism of individuation and genesis have the anticipated effects.

Restricting ourselves to these, then, we may conclude that in the
end, pressure of population and its accompanying evils will almost
disappear; and will leave a state of things requiring from each
individual little more than a normal and pleasurable activity.
Cessation in the decrease of fertility implies cessation in the
development of the nervous system; and this implies a nervous system
which has become equal to all that is demanded of it--has not to do
more than is natural to it. But that exercise of faculties which does
not exceed what is natural, constitutes gratification.

The necessary antagonism of Individuation and Genesis, not only,
then, fulfils the _à priori_ law of maintenance of race, from the
monad up to Man, but ensures final attainment of the highest form of
this maintenance--a form in which the amount of life shall be the
greatest possible and the births and deaths the fewest possible. From
the beginning pressure of population has been the proximate cause of
progress. It produced the original diffusion of the race. It compelled
men to abandon predatory habits and take to agriculture. It led to
the clearing of the Earth’s surface. It forced men into the social
state; made social organization inevitable; and has developed the
social sentiments. It has stimulated to progressive improvements
in production, and to increased skill and intelligence. It is
daily thrusting us into closer contact and more mutually-dependent
relationships. And after having caused, as it ultimately must, the due
peopling of the globe, and the raising of its habitable parts into the
highest state of culture--after having perfected all processes for the
satisfaction of human wants--after having, at the same time, developed
the intellect into competence for its work, and the feelings into
fitness for social life--after having done all this, the pressure of
population must gradually approach to an end--an end, however, which
for the reasons given it cannot absolutely reach.


§ 377. In closing the argument let us not overlook the
self-sufficingness of those universal processes by which the results
reached thus far have been wrought out, and which may be expected to
work out these future results.

Evolution under all its aspects, general and special, is an advance
towards equilibrium. We have seen that the theoretical limit towards
which the integration and differentiation of every aggregate advances,
is a state of balance between all the forces to which its parts are
subject, and the forces which its parts oppose to them (_First Prin._ §
170). And we have seen that organic evolution is a progress towards a
moving equilibrium completely adjusted to environing actions.

It has been also pointed out that, in civilized Man, there is going
on a new class of equilibrations--those between his actions and the
actions of the societies he forms (_First Prin._ § 175). Social
restraints and requirements are ever altering his activities and by
consequence his nature; and as fast as his nature is altered, social
restraints and requirements undergo more or less re-adjustment. Here
the organism and the conditions are both modifiable; and by successive
conciliations of the two, there is effected a progress towards
equilibrium.

More recently we have seen that in every species, there establishes
itself an equilibrium of an involved kind between the total
race-destroying forces and the total race-preserving forces--an
equilibrium which implies that where the ability to maintain individual
life is small, the ability ta propagate must be great, and _vice
versâ_. Whence it follows that the evolution of a race more in
equilibrium with the environment, is also the evolution of a race in
which there is a correlative approach towards equilibrium between the
number of new individuals produced and the number which survive and
propagate.

The final result to be observed is that in Man, all these
equilibrations between constitution and conditions, between the
structure of society and the nature of its members, between fertility
and mortality, advance simultaneously towards a common climax. In
approaching an equilibrium between his nature and the ever-varying
circumstances of his inorganic environment, and in approaching an
equilibrium between his nature and all the requirements of the social
state, Man is at the same time approaching that lowest limit of
fertility at which the equilibrium of population is maintained by the
addition of as many infants as there are subtractions by death in old
age. But in a universe of which all parts are in motion and every
part is consequently subject to change of conditions, neither this
equilibrium nor any other equilibrium can become complete.


                               THE END.




                              APPENDICES.




                              APPENDIX A.

          SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS.


I append here the evidences referred to in § 190. The most numerous and
striking I have met with among the _Umbelliferæ_. Monstrosities having
the alleged implication, are frequent in the common Cow-Parsnep--so
frequent that they must be familiar to botanists; and wild Angelica
supplies many over-developments of like meaning. Omitting numerous
cases of more or less significance, I will limit myself to two.

  [Illustration: Fig. 69.]

One of them is that of a terminal umbel, in which nine of the outer
umbellules are variously transformed--here a single flower being
made monstrous by the development of some of its members into buds;
there several such malformed flowers being associated with rays that
bear imperfect umbellules; and elsewhere, flowers being replaced by
umbellules: some of which are perfect, and others imperfect only in
the shortness of the flower-stalks. The annexed Fig. 69, representing
in a somewhat conventionalized way, a part of the dried specimen,
will give an idea of this Angelica. At _a_ is shown a single flower
partially changed; in the umbellule marked _b_, one of the rays bears a
secondary umbellule; and there may be seen at _c_ and _d_, several such
over-developments.

But the most conclusive instance is that of a Cow-Parsnep, in which a
single terminal umbel, besides the transformations already mentioned,
exhibits higher degrees of such transformations.[68] The components
of this complex growth are;--three central umbellules, abnormal only
in minor points; one umbellule, external to these, which is partially
changed into an umbel; one rather more out of the centre, which is
so far metamorphosed as to be more an umbel than an umbellule: nine
peripheral clusters formed by the development of umbellules into
umbels, some of which are partially compounded still further. Examined
in detail, these structures present the following facts:--1. The
innermost umbellule is normal, save in having a peripheral flower of
which one member (apparently a petal) is transformed into a flower-bud.
2. The next umbellule, not quite so central, has one of its peripheral
flowers made monstrous by the growth of a bud from the base of the
calyx. 3. The third of the central umbellules has two abnormal outer
flowers. One of them carries a flower-bud on its edge, in place of
a foliar member. The other is half flower and half umbellule: being
composed of three petals, three stamens, and five flower-buds growing
where the other petals and stamens should grow. 4. Outside of these
umbellules comes one of the mixed clusters. Its five central flowers
are normal. Surrounding these are several flowers transformed in
different degrees: one having a stamen partially changed into a flower
bud. And then, at the periphery of this mixed cluster, come three
complete umbellules and an incomplete one in which some petals and
stamens of the original flower remain. 5. A mixed cluster, in which
the umbel-structure predominates, stands next. Its three central
flowers are normal. Surrounding them are five flowers over-developed in
various ways, like those already described. And on its periphery are
seven complete umbellules in place of flowers; besides an incomplete
umbellule that contains traces of the original flower, one of them
being a petal imperfectly twisted up into a bud. 6. Of the nine
external clusters, in which the development of simple into compound
umbels is most decided, nearly all present anomalies. Three of them
have each a central flower untransformed; and in others, the central
umbellule is composed of two, three, or four flowers. 7. But the most
remarkable fact is, that in sundry of these peripheral clusters,
resulting from the metamorphosis of simple umbels into compound umbels,
the like metamorphosis is carried a stage higher. Some of the component
rays, are themselves the bearers of compound umbels instead of simple
umbels. In Fig. 70, a portion of the dried specimen is represented.
Two of the central umbellules are marked _a_ and _b_; those marked
_c_ and _d_ are mixed clusters; at _e_ and _f_ are compound umbels
replacing simple ones; and _g_ shows one of the rays on which the
over-development goes still further.

  [Illustration: Fig. 70.]

Does not this evidence, enforced as it is by much more of like kind,
go far to prove that foliar organs may be developed into axial organs?
Even were not the transitional forms traceable, there would still,
I think, be no other legitimate interpretation of the facts last
detailed. The only way of eluding the conclusion here drawn, is by
assuming that where a cluster of flowers replaces a single flower, it
is because the axillary buds which hypothetically belong to the several
foliar organs of the flower, become developed into axes; and assuming
this, is basing an hypothesis on another hypothesis that is directly at
variance with facts. The foliar organs of flowers do _not_ bear buds
in their axils; and it would never have been supposed that such buds
are typically present, had it not been for that mistaken conception of
“type” which has led to many other errors in Biology. Goethe writes:
“Now as we cannot realize the idea of a leaf apart from the node out of
which it springs, or of a node without a bud, we may venture to infer,”
&c. See here an example of a method of philosophizing not uncommon
among the Germans. The method is this--Survey a portion of the
facts, and draw from them a general conception; project this general
conception back into the objective world, as a mould in which Nature
casts her products; expect to find it everywhere fulfilled; and allege
potential fulfilment where no actual fulfilment is visible.

If instead of imposing our ideal forms on Nature, we are content to
generalize the facts as Nature presents them, we shall find no warrant
for the morphological doctrine above enunciated. The only conception
of type justified by the logic of science, is--that correlation
of parts which remains constant under all modifications of the
structure to be defined. To ascertain this, we must compare all these
modifications, and note what traits are common to them. On doing so
with the successive segments of a phænogamic axis, we are brought to
a conclusion widely different from that of Goethe. Axillary buds are
almost universally absent from the cotyledons; they are habitually
present in the axils of fully-developed leaves higher up the axis; they
are often absent from leaves that are close to the flower; they are
nearly always absent from the bracts; absent from the sepals; absent
from the petals; absent from the stamens; absent from the carpels.
Thus, out of eight leading forms which folia assume, one has the
axillary bud and seven are without it. With these facts before us, it
seems to me not difficult to “realize the idea” “of a node without a
bud.” If we are not possessed by a foregone conclusion, the evidence
will lead us to infer, that each node bears a foliar appendage and
_may_ bear an axillary bud.

Even, however, were it granted that the typical segment of a Phænogam
includes an axillary bud, which must be regarded as always potentially
present, no legitimate counter-interpretation of the monstrosities
above described could thence be drawn. If when an umbellule is
developed in place of a flower, the explanation is, that its component
rays are axillary to the foliar organs of the flower superseded; we may
fairly require that these foliar organs to which they are axillary,
shall be shown. But there are none. In the last specimen figured,
the inner rays of each such umbellule are without them; most of the
outer rays are also without them; and in one cluster, only a single
ray has a bract at its point of origin. There is a rejoinder ready,
however: the foliar organs are said to be suppressed. Though Goethe
could not “realize the idea” “of a node without a bud,” those who
accept his typical form appear to find no difficulty in realizing the
idea of an axillary bud without anything to which it is axillary. But
letting this pass, suppose we ask what is the warrant for this assumed
suppression. Axillary buds normally occur where the nutrition is high
enough to produce fully-developed leaves; and when axillary buds are
demonstrably present in flowers, they accompany foliar organs that
are more leaf-like than usual--always greener if not always larger.
That is to say, the normal and the abnormal axillary buds, are
alike the concomitants of foliar organs coloured by that chlorophyll
which habitually favours foliar development. How, then, can it be
supposed that when, out of a flower there is developed a cluster
of flower-bearing rays, the implied excess of nutrition causes the
foliar organs to abort? It is true that very generally in a branched
inflorescence, the bracts of the several flower-branches are very
small (their smallness being probably due to that defective supply of
certain chlorophyll-forming matters, which is the proximate cause of
flowering); and it is true that, under these conditions, a flowering
axis of considerable size, for the development of which chlorophyll is
less needful, grows from the axil of a dwarfed leaf. But the inference
that the foliar organ may therefore be entirely suppressed, seems
to me irreconcilable with the fact, that the foliar organ is always
developed to some extent _before_ the axillary bud appears. Until it
has been shown that in some cases a lateral bud first appears, and a
foliar organ _afterwards_ grows out beneath it, to form its axil, the
conception of an axillary bud of which the foliar organ is suppressed,
will remain at variance with the established truths of development.

       *       *       *       *       *

The above originally formed a portion of § 190. I have transferred it
to the Appendix, partly because it contains too much detail to render
it fit for the general argument, and partly because the interpretations
being open to some question, it seemed undesirable to risk compromising
that argument by including them. The criticisms passed upon these
interpretations have not, however, sufficed to convince me of their
incorrectness. Unfortunately, I have since had no opportunity of
verifying the above statements by microscopic examinations, as I had
intended.

Though unable to enforce the inference drawn by further facts more
minutely looked into, I may add some arguments based on facts that are
well known. One of these is the fact that the so-called axillary bud
is not universally axillary--is not universally seated in the angle
made by the axis and an appended foliar organ. In certain plants the
axillary bud is placed far above the node, half-way between it and the
succeeding node. So that not only may a segment of a phæenogamic axis
be without the axillary bud, but the axillary bud, when present, may be
removed from that place in which, according to Goethe, it necessarily
exists. Another fact not congruous with the current doctrine, is the
common occurrence of “adventitious” buds--the buds that are put out
from roots and from old stems or branches bare of leaves. The name
under which they are thus classed, is meant to imply that they may be
left out of consideration. Those, however, who have not got a theory to
save by putting anomalies out of sight, may be inclined to think that
the occurrence of buds where they are avowedly unconnected with nodes,
and are axillary to nothing, tells very much against the assumption
that every bud implies a node and a corresponding foliar organ. And
they may also see that the development of these adventitious buds at
places where there is excess of nutritive materials, favours the view
above set forth. For if a bud thus arises at a place where it is not
morphologically accounted for, simply because there happens to be at
that place an abundance of unorganized protoplasm; then, clearly, it is
likely that if the mass of protoplasm from which a leaf would usually
arise, is greatly increased in mass by excess of nutrition, it may
develop into an axis instead of a leaf.

       *       *       *       *       *

Many years after this work was published, I discovered among my papers
a memorandum which unfortunately I had overlooked, containing further
evidence in support of the foregoing conclusion. With the omission of
an error concerning the species of plant, I reproduce this memorandum
just as it stood:--

“I found at Dieppe, July 1, 1860, in a garden near the sea a sample of
cultivated wild flower (I thought it was grown as an ornamental flower)
in which some of the single flowers of the umbel were developed into
groups of flowers thus:--

  [Illustration]

“In the case where the transformation was fully effected the umbellule
had _six_ flowers, answering to the _six petals_ of the original
flowers. In other cases the transformation was incomplete. There were
instances where but _two_ of the petals were developed into flowers;
and the other petals remained unchanged. Others in which _three_ were
developed; and others where four were developed. In some cases, too,
the development of a petal into a flower was imperfect, in the absence
of the flower-stalk--the flowers were sessile in the place where the
petals would have been. In one case there was an _imperfect_ flower
sessile; another _imperfect_ flower on a short stalk; and three perfect
flowers on long stalks.

“I was in some doubt whether the petals or the stamens were developed.
In cases of imperfect transformation the petals at the base of the
umbellule seemed to stand in the position of calyx or involucrum,
giving the idea that the stamens were developed into flowers. But in
the case where there were _six_ flowers developed there were no petals
at the base.

“That it was a matter of extra nutrition was shown by this:--

“1. That they were cultivated as garden flowers.

“2. That where there was one perfectly developed umbellule, it was the
only one in the umbel.

“3. That where there were three umbellules they were all imperfect.

“4. That in this imperfect umbellule the perfect flowers were on long
stalks and the imperfect ones sessile.

“5. That the umbellules were on stalks both longer and thicker than
those of single flowers.”

       *       *       *       *       *

[Concerning the foregoing argument at large an expert writes:--“The
abnormalities you describe certainly show that an axis may arise
abnormally in the place of a normal leaf-structure, and every modern
botanist would be in agreement with you in your criticism of the
older form of the doctrine of axillary buds. I think we are largely
emancipated from the dextrous juggling with the arrangements and
relations of organs which used to pass current as morphology.

“You have quoted sufficient evidence in the text (§ 190) to establish
the conclusion that no sharp line can be drawn between axes and
leaf-structure; and a very great deal more could be added in the same
sense. Petioles for instance, exist which the most highly trained
histological observer could not distinguish from stems.

“But I must demur to the suggestion that the replacement of one by the
other is primarily a question of nutrition. We are as ignorant as ever
of the proximate cause of the production of a leaf or a shoot at a
certain spot in meristematic tissue.”

To this last remark I had at first made only the reply that the plants
exhibiting the abnormalities were in all cases excessively luxuriant in
their growths; but to this I am now able to add a more definite reply.
The expert from whom I have just quoted, had read this appendix before
there had been made to it the above addition describing the flower
from Dieppe; and I was not myself aware, until I came to read over
this addition, what clear evidence it contains that extra nutrition
was the cause of these transformations of foliar structures into axial
structures; but the above paragraphs 1, 2, 3, 4, 5, contain different
evidences conspiring to prove this.]




                              APPENDIX B.

    A CRITICISM ON PROF. OWEN’S THEORY OF THE VERTEBRATE SKELETON.

[_From the_ BRITISH & FOREIGN MEDICO-CHIRURGICAL REVIEW FOR OCT., 1858.]


   I. _On the Archetype and Homologies of the Vertebrate Skeleton._
   _By_ RICHARD OWEN, _F.R.S._--_London_, 1848. _pp._ 172.

   II. _Principes d’Ostéologie Comparée, ou Recherches sur
   l’Archétype et les Homologies du Squelette Vertébré._ _Par_
   RICHARD OWEN.--_Paris._

   _Principles of Comparative Osteology; or, Researches on the
   Archetype and the Homologies of the Vertebrate Skeleton._ _By_
   RICHARD OWEN.

   III. _On the Nature of Limbs. A Discourse delivered on Friday,
   February 9, at an Evening Meeting of the Royal Institution of
   Great Britain._ _By_ RICHARD OWEN, _F.R.S._--_London_, 1849.
   _pp._ 119.

Judging whether another proves his position is a widely different thing
from proving your own. To establish a general law requires an extensive
knowledge of the phenomena to be generalized; but to decide whether an
alleged general law is established by the evidence assigned, requires
merely an adequate reasoning faculty. Especially is such a decision
easy where the premises do _not_ warrant the conclusion. It may be
dangerous for one who has but little previous acquaintance with the
facts, to say that a generalization is demonstrated; seeing that the
argument may be one-sided: there may be many facts unknown to him which
disprove it. But it is not dangerous to give a negative verdict when
the alleged demonstration is manifestly insufficient. If the data put
before him do not bear out the inference, it is competent for every
logical reader to say so.

From this standpoint, then, we venture to criticize some of Professor
Owen’s osteological theories. For his knowledge of comparative
osteology we have the highest respect. We believe that no living man
has so wide and detailed an acquaintance with the bony structure of
the _Vertebrata_. Indeed, there probably has never been any one whose
information on the subject was so nearly exhaustive. Moreover, we
confess that nearly all we know of this department of biology has been
learnt from his lectures and writings. We pretend to no independent
investigations, but merely to such knowledge of the phenomena as he has
furnished us with. Our position, then, is such that, had Professor Owen
simply enunciated his generalizations, we should have accepted them on
his authority. But he has brought forward evidence to prove them. By
so doing he has tacitly appealed to the judgments of his readers and
hearers--has practically said, “Here are the facts; do they not warrant
these conclusions?” And all we propose to do, is to consider whether
the conclusions _are_ warranted by the facts brought forward.

Let us first limit the scope of our criticisms. On that division
of comparative osteology which deals with what Professor Owen
distinguishes as “special homologies,” we do not propose to enter. That
the wing of a bird is framed upon bones essentially parallel to those
of a mammal’s fore-limb; that the cannon-bone of a horse’s leg answers
to the middle metacarpal of the human hand; that various bones in the
skull of a fish are homologous with bones in the skull of a man--these
and countless similar facts, we take to be well established. It may
be, indeed, that the doctrine of special homologies is at present
carried too far. It may be that, just as the sweeping generalization
at one time favoured, that the embryonic phases of the higher animals
represent the adult forms of lower ones, has been found untrue in a
literal sense, and is acceptable only in a qualified sense; so the
sweeping generalization that the skeletons of all vertebrate animals
consist of homologous parts, will have to undergo some modification.
But that this generalization is substantially true, all comparative
anatomists agree.

The doctrine which we are here to consider, is quite a separate
one--that of “general homologies.” The truth or falsity of this may
be decided on quite apart from that of the other. Whether certain
bones in one vertebrate animal’s skeleton correspond with certain
bones in another’s, or in every other’s, is one question; and whether
the skeleton of every vertebrate animal is divisible into a series of
segments, each of which is modelled after the same type, is another
question. While the first is answered in the affirmative, the last
may be answered in the negative; and we propose to give reasons why it
should be answered in the negative.

       *       *       *       *       *

In so far as his theory of the skeleton is concerned, Professor Owen
is an avowed disciple of Plato. At the conclusion of his _Archetype
and Homologies of the Vertebrate Skeleton_, he quotes approvingly
the Platonic hypothesis of ἰδέαι, “a sort of models, or moulds in
which matter is cast, and which regularly produce the same number and
diversity of species.” The vertebrate form in general (see diagram of
the _Archetypus_), or else the form of each kind of vertebrate animal
(see p. 172, where this seems implied), Professor Owen conceives to
exist as an “idea”--an “archetypal exemplar on which it has pleased the
Creator to frame certain of his living creatures.” Whether Professor
Owen holds that the typical vertebra also exists as an “idea,” is not
so certain. From the title given to his figure of the “ideal typical
vertebra,” it would seem that he does; and at p. 40 of his _Nature of
Limbs_, and indeed throughout his general argument, this supposition is
implied. But on the last two pages of the _Archetype and Homologies_,
it is distinctly alleged that “the repetition of similar segments in
a vertebral column, and of similar elements in a vertebral segment,
is analogous to the repetition of similar crystals as the result of
polarizing force in the growth of an inorganic body;” it is pointed
out that, “as we descend the scale of animal life, the forms of the
repeated parts of the skeleton approach more and more to geometrical
figures;” and it is inferred that “the Platonic ἰδέα or specific
organizing principle or force, would seem to be in antagonism with the
general polarizing force, and to subdue and mould it in subserviency
to the exigencies of the resulting specific form.” If Professor Owen’s
doctrine is to be understood as expressed in these closing paragraphs
of his _Archetype and Homologies_--if he considers that “the ἰδέα”
“which produces the diversity of form belonging to living bodies of the
same materials,” is met by the “counter-operation” of “the polarizing
force pervading all space,” which produces “the similarity of forms,
the repetition of parts, the signs of unity of organization,” and which
is “_subdued_” as we ascend “in the scale of being;” then we may pass
on with the remark that the hypothesis is too cumbrous and involved to
have much _vraisemblance_. If, on the other hand, Professor Owen holds,
as every reader would suppose from the general tenor of his reasonings,
that not only does there exist an archetypal or ideal vertebrate
skeleton, but that there also exists an archetypal or ideal vertebra;
then he carries the Platonic hypothesis much further than Plato does.
Plato’s argument, that before any species of object was created it
must have existed as an idea of the Creative Intelligence, and that
hence all objects of such species must be copies of this original
idea, is tenable enough from the anthropomorphic point of view. But
while those who, with Plato, think fit to base their theory of creation
upon the analogy of a carpenter designing and making a table, must
yield assent to Plato’s inference, they are by no means committed
to Professor Owen’s expansion of it. To say that before creating a
vertebrate animal, God must have had the conception of one, does not
involve saying that God gratuitously bound himself to make a vertebrate
animal out of segments all moulded after one pattern. As there is
no conceivable advantage in this alleged adhesion to a fundamental
pattern--as, for the fulfilment of the intended ends, it is not only
needless, but often, as Professor Owen argues, less appropriate than
some other construction would be (see _Nature of Limbs_, pp. 39, 40),
to suppose the creative processes thus regulated, is not a little
startling. Even those whose conceptions are so anthropomorphic as to
think they honour the Creator by calling him “the Great Artificer,”
will scarcely ascribe to him a proceeding which, in a human artificer,
they would consider a not very worthy exercise of ingenuity.

But whichever of these alternatives Professor Owen contends
for--whether the typical vertebra is that more or less crystalline
figure which osseous matter ever tends to assume in spite of “the ἰδέα
or organizing principle,” or whether the typical vertebra is itself
an “ἰδέα or organizing principle”--there is alike implied the belief
that the typical vertebra has an abstract existence apart from actual
vertebræ. It is a form which, in every endo-skeleton, strives to embody
itself in matter--a form which is potentially present in each vertebra;
which is manifested in each vertebra with more or less clearness; but
which, in consequence of antagonizing forces, is nowhere completely
realized. Apart from the philosophy of this hypothesis, let us here
examine the evidence which is thought to justify it.

       *       *       *       *       *

And first as to the essential constituents of the “ideal typical
vertebra.” Exclusive of “_diverging appendages_” which it “may also
support,” “it consists in its typical completeness of the following
elements and parts”:--A _centrum_ round which the rest are arranged in
a somewhat radiate manner; above it two _neurapophyses_--converging
as they ascend, and forming with the centrum a trianguloid space
containing the neural axis; a _neural spine_, surmounting the two
neurapophyses, and with them completing the neural arch; below the
centrum two _hæmapophyses_ and a _hæmal spine_, forming a hæmal arch
similar to the neural arch above, and enclosing the hæmal axis;
two _pleurapophyses_ radiating horizontally from the sides of the
centrum; and two _parapophyses_ diverging from the centrum below the
pleurapophyses. “These,” says Professor Owen, “being usually developed
from distinct and independent centres, I have termed ‘autogenous
elements.’” The remaining elements, which he classes as “exogenous,”
because they “shoot out as continuations from some of the preceding
elements,” are the _diapophyses_ diverging from the upper part of the
centrum as the parapophyses do below, and the _zygapophyses_ which grow
out of the distal ends of the neurapophyses and hæmapophyses.

If, now, these are the constituents of the vertebrate segment “in its
typical completeness;” and if the vertebrate skeleton consists of a
succession of such segments; we ought to have in these constituents,
representatives of all the elements of the vertebrate skeleton--at
any rate, all its essential elements. Are we then to conclude
that the “diverging appendages,” which Professor Owen regards as
rudimental limbs, and from certain of which he considers actual
limbs to be developed, are typically less important than some of the
above-specified exogenous parts--say the zygapophyses?

That the meaning of this question may be understood, it will be needful
briefly to state Professor Owen’s theory of _The Nature of Limbs_;
and such criticisms as we have to make on it must be included in the
parenthesis. In the first place, he aims to show that the scapular
and pelvic arches, giving insertion to the fore and hind limbs
respectively, are displaced and modified hæmal arches, originally
belonging in the one case to the occipital vertebra, and in the
other case to some trunk-vertebra not specified. In support of this
assumption of displacement, carried in some cases to the extent of
_twenty-seven_ vertebræ, Professor Owen cites certain acknowledged
displacements which occur in the human skeleton to the extent of half
a vertebra--a somewhat slender justification. But for proof that such
a displacement _has_ taken place in the scapular arch, he chiefly
relies on the fact that in fishes, the pectoral fins, which are the
homologues of the fore-limbs, are directly articulated to certain bones
at the back of the head, which he alleges are parts of the occipital
vertebra. This appeal to the class of fishes is avowedly made on the
principle that these lowest of the _Vertebrata_ approach closest to
archetypal regularity, and may therefore be expected to show the
original relations of the bones more nearly. Simply noting the facts
that Professor Owen does not give us any transitional forms between
the alleged normal position of the scapular arch in fishes, and its
extraordinary displacement in the higher _Vertebrata_; and that he
makes no reference to the embryonic phases of the higher _Vertebrata_,
which might be expected to exhibit the progressive displacement; we
go on to remark that, in the case of the pelvic arch, he abandons
his principle of appealing to the lowest vertebrate forms for proof
of the typical structure. In fishes, the rudimentary pelvis, widely
removed from the spinal column, shows no signs of having belonged to
any vertebra; and here Professor Owen instances the perennibranchiate
_Batrachia_ as exhibiting the typical structure: remarking that
“mammals, birds, and reptiles show the rule of connexion, and fishes
the exception.” Thus in the case of the scapular arch, the evidence
afforded by fishes is held of great weight, _because_ of their
archetypal regularity; while in the case of the pelvic arch, their
evidence is rejected as exceptional. But now, having, as he considers,
shown that these bony frames to which the limbs are articulated are
modified hæmal arches, Professor Owen points out that the hæmal
arches habitually bear certain “diverging appendages;” and he aims
to show that the “diverging appendages” of the scapular and pelvic
arches respectively, are developed into the fore and hind limbs.
There are several indirect ways in which we may test the probability
of this conclusion. If these diverging appendages are “rudimental
limbs”--“future possible or potential arms, legs, wings, or feet,” we
may fairly expect them always to bear to the hæmal arches a relation
such as the limbs do. But they by no means do this. “As the vertebræ
approach the tail, these appendages are often transferred gradually
from the pleurapophysis to the parapophysis, or even to the centrum
and neural arch.” (_Arch. and Hom._, p. 93.) Again, it might naturally
be assumed that in the lowest vertebrate forms, where the limbs are
but little developed, they would most clearly display their alliance
with the appendages, or “rudimental limbs,” by the similarity of their
attachments. Instead of this, however, Professor Owen’s drawings
show that whereas the appendages are habitually attached to the
pleurapophyses, the limbs, in their earliest and lowest phase, alike in
fishes and in the _Lepidosiren_, are articulated to the hæmapophyses.
Most anomalous of all, however, is the process of development. When
we speak of one thing as being developed out of another, we imply
that the parts next to the germ are the first to appear, and the most
constant. In the evolution of a tree out of a seed, there come at the
outset the stem and the radicle; afterwards the branches and divergent
roots; and still later the branchlets and rootlets; the remotest parts
being the latest and most inconstant. If, then, a limb is developed out
of a “diverging appendage” of the hæmal arch, the earliest and most
constant bones should be the humerus and femur; next in order of time
and constancy should come the coupled bones based on these; while the
terminal groups of bones should be the last to make their appearance,
and the most liable to be absent. Yet, as Professor Owen himself shows,
the actual mode of development is the very reverse of this. At p. 16 of
the _Archetype and Homologies_, he says:--

   “The earlier stages in the development of all locomotive
   extremities are permanently retained or represented in the
   paired fins of fishes. First the essential part of the member,
   the hand or foot, appears: then the fore-arm or leg, both much
   shortened, flattened, and expanded, as in all fins and all
   embryonic rudiments of limbs: finally come the humeral and
   femoral segments; but this stage I have not found attained in
   any fish.”

That is to say, alike in ascending through the _Vertebrata_, generally,
and in tracing up the successive phases of a mammalian embryo, the
last-developed and least constant division of the limb, is that basic
one by which it articulates with the hæmal arch. It seems to us that,
so far from proving his hypothesis, Professor Owen’s own facts tend
to show that limbs do not belong to the vertebræ at all: that they
make their first appearance peripherally; that their development is
centripetal; and that they become fixed to such parts of the vertebrate
axis as the requirements of the case determine.

But now, ending here this digressive exposition and criticism,
and granting the position that limbs “are developments of costal
appendages,” let us return to the question above put--Why are not
these appendages included as elements of the “ideal typical vertebra?”
It cannot be because of their comparative inconstancy; for judging
from the illustrative figures, they seem to be as constant as the
hæmal spine, which is one of the so-called autogenous elements: in
the diagram of the _Archetypus_, the appendage is represented as
attached to every vertebrate segment of the head and trunk, which the
hæmal spine is not. It cannot be from their comparative unimportance;
seeing that as potential limbs they are essential parts of nearly
all the _Vertebrata_--much more obviously so than the diapophyses
are. If, as Professor Owen argues, “the divine mind which planned the
archetype also foreknew all its modifications;” and if, among these
modifications, the development of limbs out of diverging appendages was
one intended to characterize all the higher _Vertebrata_; then, surely,
these diverging appendages must have been parts of the “ideal typical
vertebra.” Or, if the “ideal typical vertebra” is to be understood as
a crystalline form in antagonism with the organizing principle; then
why should not the appendages be included among its various offshoots?
We do not ask this question because of its intrinsic importance. We
ask it for the purpose of ascertaining Professor Owen’s method of
determining what are true vertebral constituents. He presents us with a
diagram of the typical vertebra, in which are included certain bones,
and from which are excluded certain others. If relative constancy
is the criterion, then there arises the question--What degree of
constancy entitles a bone to be included? If relative importance is the
criterion, there comes not only the question--What degree of importance
suffices? but the further question--How is importance to be measured?
If neither of these is the criterion, then what is it? And if there is
no criterion, does it not follow that the selection is arbitrary?

       *       *       *       *       *

This question serves to introduce a much wider one:--Has the “ideal
typical vertebra” any essential constituents at all? It might
naturally be supposed that though some bones are so rarely developed
as not to seem worth including, and though some that are included are
very apt to be absent, yet that certain others are invariable: forming,
as it were, the basis of the ideal type. Let us see whether the facts
bear out this supposition. In his “summary of modifications of corporal
vertebræ” (p. 96), Professor Owen says--“The _hæmal spine_ is much
less constant as to its existence, and is subject to a much greater
range of variety, when present, than its vertical homotype above,
which completes the neural arch.” Again he says--“The _hæmapophyses_,
as osseous elements of a vertebra, are less constant than the
pleurapophyses.” And again--“The _pleurapophyses_ are less constant
elements than the neurapophyses.” And again--“Amongst air-breathing
vertebrates the _pleurapophyses_ of the trunk segments are present
only in those species in which the septum of the heart’s ventricle is
complete and imperforate, and here they are exogenous and confined
to the cervical and anterior thoracic vertebræ.” And once more, both
the _neurapophyses_ and the _neural spine_ “are absent under both
histological conditions, at the end of the tail in most air-breathing
vertebrates, where the segments are reduced to their central elements.”
That is to say, of all the peripheral elements of the “ideal typical
vertebra,” there is not one which is always present. It will be
expected, however, that at any rate the _centrum_ is constant: the bone
which “forms the axis of the vertebral column, and commonly the central
bond of union of the peripheral elements of the vertebrate (p. 97), is
of course an invariable element. No: not even this is essential.

   “The centrums do not pass beyond the primitive stage of the
   notochord (undivided column) in the existing lepidosiren, and
   they retained the like rudimental state in every fish whose
   remains have been found in strata earlier than the permian
   æra in Geology, though the number of vertebræ is frequently
   indicated in Devonian and Silurian ichthyolites by the
   fossilized neur-and hæmapophyses and their spines” (p. 96).

Indeed, Professor Owen himself remarks that “the neurapophyses are
more constant as osseous or cartilaginous elements of the vertebræ
than the centrums” (p. 97). Thus, then, it appears that the several
elements included in the “ideal typical vertebra” have various degrees
of constancy, and that no one of them is essential. There is no one
part of a vertebra which invariably answers to its exemplar in the
pattern-group. How does this fact consist with the hypothesis? If
the Creator saw fit to make the vertebrate skeleton out of a series
of segments, all formed on essentially the same model--if, for the
maintenance of the type, one of these bony segments is in many cases
formed out of a coalesced group of pieces, where, as Professor Owen
argues, a single piece would have served as well or better; then we
ought to find this typical repetition of parts uniformly manifested.
Without any change of shape, it would obviously have been quite
possible for every actual vertebra to have contained all the parts
of the ideal one--rudimentally where they were not wanted. Even one
of the terminal bones of a mammal’s tail might have been formed
out of the nine autogenous pieces, united by suture but admitting
of identification. As, however, there is no such uniform typical
repetition of parts, it seems to us that to account for the typical
repetition which _does_ occur, by supposing the Creator to have fixed
on a pattern-vertebra, is to ascribe to him the inconsistency of
forming a plan and then abandoning it.

If, on the other hand, Professor Owen means that the “ideal typical
vertebra” is a crystalline form in antagonism with “the idea or
organizing principle;” then we might fairly expect to find it most
clearly displaying its crystalline character, and its full complement
of parts, in those places where the organizing principle may be
presumed to have “subdued” it to the smallest extent. Yet in the
_Vertebrata_ generally, and even in Professor Owen’s _Archetypus_,
the vertebræ of the tail, which must be considered as, if anything,
less under the influence of the organizing principle than those
of the trunk, do not manifest the ideal form more completely. On
the contrary, as we approach the end of the tail, the successive
segments not only lose their remaining typical elements, but become as
uncrystalline-looking as can be conceived.

       *       *       *       *       *

Supposing, however, that the assumption of suppressed or undeveloped
elements be granted--supposing it to be consistent with the hypothesis
of an “ideal typical vertebra,” that the constituent parts may
severally be absent in greater or less number, sometimes leaving only
a single bone to represent them all; may it not be that such parts
as _are_ present, show their respective typical natures by some
constant character: say their mode of ossification?

To this question some parts of the _Archetype and Homologies_ seem
to reply, “Yes;” while others clearly answer, “No.” Criticising the
opinions of Geoffrey St. Hilaire and Cuvier, who agreed in thinking
that ossification from a separate centre was the test of a separate
bone, and that thus there were as many elementary bones in the skeleton
as there were centres of ossification, Professor Owen points out that,
according to this test, the human femur, which is ossified from four
centres, must be regarded as four bones; while the femur in birds and
reptiles, which is ossified from a single centre, must be regarded as
a single bone. Yet, on the other hand, he attaches weight to the fact
that the skull of the human fœtus presents “the same ossific centres”
as do those of the embryo kangaroo and the young bird. (_Nature of
Limbs_, p. 40.) And at p. 104 of the _Homologies_, after giving
a number of instances, he says--

   “These and the like correspondences between the points of
   ossification of the human fœtal skeleton, and the separate
   bones of the adult skeletons of inferior animals, are pregnant
   with interest, and rank among the most striking illustrations of
   unity of plan in the vertebrate organization.”

It is true that on the following page he seeks to explain this seeming
contradiction by distinguishing

   “between those centres of ossification that have homological
   relations, and those that have teleological ones--_i.e._,
   between the separate points of ossification of a human bone
   which typify vertebral elements, often permanently distinct
   bones in the lower animals; and the separate points which,
   without such signification, facilitate the progress of
   osteogeny, and have for their obvious final cause the well-being
   of the growing animal.”

But if there are thus centres of ossification which have homological
meanings, and others which have not, there arises the question--How are
they always to be distinguished? Evidently independent ossification
ceases to be a homological test, if there are independent ossifications
that have nothing to do with the homologies. And this becomes the
more evident when we learn that there are cases where neither a
homological nor a teleological meaning can be given. Among various
modes of ossification of the centrum, Professor Owen points out that
“the body of the human atlas is sometimes ossified from two, rarely
from three, distinct centres placed side by side” (p. 89); while at
p. 87 he says:--“In osseous fishes I find that the centrum is usually
ossified from six points.” It is clear that this mode of ossification
has here no homological signification; and it would be difficult to
give any teleological reason why the small centrum of a fish should
have more centres of ossification than the large centrum of a mammal.
The truth is, that as a criterion of the identity or individuality of
a bone, mode of ossification is quite untrustworthy. Though, in his
“ideal typical vertebra,” Professor Owen delineates and classifies
as separate “autogenous” elements, those parts which are “usually
developed from distinct and independent centres;” and though by
doing so he erects this characteristic into some sort of criterion;
yet his own facts show it to be no criterion. The parapophyses are
classed among the autogenous elements; yet they are autogenous in
fishes alone, and in these only in the trunk vertebræ, while in all
air-breathing vertebrates they are, when present at all, exogenous.
The neurapophyses, again, “lose their primitive individuality by
various kinds and degrees of confluence:” in the tails of the higher
_Vertebrata_ they, in common with the neural spine, become exogenous.
Nay, even the centrum may lose its autogenous character. Describing
how, in some batrachians, “the ossification of the centrum is completed
by an extension of bone from the bases of the neurapophyses, which
effects also the coalescence of these with the centrum,” Professor Owen
adds:--“In _Pelobates fuscus_ and _Pelobates cultripes_, Müller found
the entire centrum ossified from this source, without any independent
points of ossification” (p. 88). That is to say, the centrum is in
these cases an exogenous process of the neurapophyses. We see, then,
that these so-called typical elements of vertebræ have no constant
developmental character by which they can be identified. Not only
are they undistinguishable by any specific test from other bones not
included as vertebral elements; not only do they fail to show their
typical characters by their constant presence; but, when present, they
exhibit no persistent marks of individuality. The central element may
be ossified from six, four, three, or two points; or it may have no
separate point of ossification at all: and similarly with various of
the peripheral elements. The whole group of bones forming the “ideal
typical vertebra” may severally have their one or more ossific centres;
or they may, as in a mammal’s tail, lose their individualities in a
single bone ossified from one or two points.

       *       *       *       *       *

Another fact which seems very difficult to reconcile with the
hypothesis of an “ideal typical vertebra,” is the not infrequent
presence of some of the typical elements in duplicate. Not only, as
we have seen, may they severally be absent, but they may severally be
present in greater number than they should be. When we see, in the
ideal diagram, one centrum, two neurapophyses, two pleurapophyses,
two hæmapophyses, one neural spine, and one hæmal spine, we naturally
expect to find them always bearing to each other these numerical
relations. Though we may not be greatly surprised by the absence
of some of them, we are hardly prepared to find others multiplied.
Yet such cases are common. Thus the neural spine “is double in the
anterior vertebræ of some fishes” (p. 98). Again, in the abdominal
region of extinct saurians, and in crocodiles, “the freely-suspended
hæmapophyses are compounded of two or more overlapping bony pieces”
(p. 100). Yet again, at p. 99, we read--“I have observed some of the
expanded pleurapophyses in the great _Testudo elephantopus_ ossified
from two centres, and the resulting divisions continuing distinct, but
united by suture.” Once more “the neurapophyses, which do not advance
beyond the cartilaginous stage in the sturgeon, consist in that fish
of two distinct pieces of cartilage; and the anterior pleurapophyses
also consist of two or more cartilages, set end on end” (p. 91). And
elsewhere referring to this structure, he says:--

   “Vegetative repetition of perivertebral parts not only manifests
   itself in the composite neurapophyses and pleurapophyses, but
   in a small accessory (interneural) cartilage, at the fore and
   back part of the base of the neurapophysis; and by a similar
   (interhæmal) one at the fore and back part of most of the
   parapophyses” (p. 87).

Thus the neural and hæmal spines, the neurapophyses, the
pleurapophyses, the hæmapophyses, may severally consist of two or more
pieces. This is not all: the like is true even of the centrums.

   “In _Heptanchus_ (_Squalus cinereus_) the vertebral centres are
   feebly and vegetatively marked out by numerous slender rings
   of hard cartilage in the notochordal capsule, the number of
   vertebræ being more definitely indicated by the neurapophyses
   and parapophyses.... In the piked dog-fish (_Acanthias_) and the
   spotted dog-fish (_Scyllium_) the vertebral centres coincide in
   number with the neural arches” (p. 87).

Is it not strange that the pattern-vertebra should be so little adhered
to, that each of its single typical pieces may be transformed into two
or three?

But there are still more startling departures from the alleged type.
The numerical relations of the elements vary not only in this way, but
in the opposite way. A given part may be present not only in greater
number than it should be, but also in less. In the tails of homocercal
fishes, the centrums “are rendered by centripetal shortening and bony
confluence fewer in number than the persistent, neural, and hæmal
arches of that part”--that is, there is only a fraction of a centrum to
each vertebra. Nay, even this is not the most heteroclite structure.
Paradoxical as it may seem, there are cases in which the same vertebral
element is, considered under different aspects, at once in excess and
defect. Speaking of the hæmal spine, Professor Owen says:--

   “The horizontal extension of this vertebral element is
   sometimes accompanied by a median division, or in other
   words, it is ossified from two lateral centres; this is seen
   in the development of parts of the human sternum; the same
   vegetative character is constant in the broader thoracic
   hæmal spines of birds; though, sometimes, as _e.g._, in the
   struthionidæ, _ossification extends from the same lateral centre
   lengthwise--i.e., forwards and backwards, calcifying the connate
   cartilaginous homologues of halves of four or five hæmal spines,
   before these finally coalesce with their fellows at the median
   line_” (p. 101).

So that the sternum of the ostrich, which according to the hypothesis,
should, in its cartilaginous stage, have consisted of _four or five
transverse_ pieces, answering to the vertebral segments, and should
have been ossified from four or five centres, one to each cartilaginous
piece, shows not a trace of this structure; but instead, consists
of _two longitudinal_ pieces of cartilage, each ossified from one
centre, and finally coalescing on the median line. These four or five
hæmal spines have at the same time doubled their individualities
transversely, and entirely lost them longitudinally!

       *       *       *       *       *

There still remains to be considered the test of relative position.
It might be held that, spite of all the foregoing anomalies, if the
typical parts of the vertebræ always stood towards each other in the
same relations--always preserved the same connexions, something like a
case would be made out. Doubtless, relative position is an important
point; and it is one on which Professor Owen manifestly places great
dependence. In his discussion of “moot cases of special homology,” it
is the general test to which he appeals. The typical natures of the
alisphenoid, the mastoid, the orbito-sphenoid, the prefrontal, the
malar, the squamosal, &c. he determines almost wholly by reference to
the adjacent nerve-perforations and the articulations with neighbouring
bones (see pp. 19 to 72): the general form of the argument being--This
bone is to be classed as such or such, _because_ it is connected
thus and thus with these others, which are so and so. Moreover, by
putting forth an “ideal typical vertebra,” consisting of a number of
elements standing towards each other in certain definite arrangement,
this persistency of relative position is manifestly alleged. The
essential attribute of this group of bones, considered as a typical
group, is the constancy in the connexions of its parts: change the
connexions, and the type is changed. But the constancy of relative
position thus tacitly asserted, and appealed to as a conclusive test
in “moot cases of special homology,” is clearly negatived by Professor
Owen’s own facts. For instance, in the “ideal typical vertebra,” the
hæmal arch is represented as formed by the two hæmapophyses and the
hæmal spine; but at p. 91 we are told that

   “The contracted hæmal arch in the caudal region of the body may
   be formed by different elements of the typical vertebra: _e.g._,
   by the parapophyses (fishes generally); by the pleurapophyses
   (lepidosiren); by both parapophyses and pleurapophyses (_Sudis_,
   _Lepidosteus_), and by hæmapophyses, shortened and directly
   articulated with the centrums (reptiles and mammals).”

And further, in the thorax of reptiles, birds, and mammals, “the
hæmapophyses are removed from the centrum, and are articulated to
the distal ends of the pleurapophyses; the bony hoop being completed
by the intercalation of the hæmal spine” (p. 82). So that there are
_five_ different ways in which the hæmal arch may be formed--four
modes of attachment of the parts different from that shown in the
typical diagram! Nor is this all. The pleurapophyses “may be quite
detached from their proper segment, and suspended to the hæmal arch of
another vertebra;” as we have already seen, the entire hæmal arch may
be detached and removed to a distance, sometimes reaching the length
of twenty-seven vertebræ; and, even more remarkable, the ventral fins
of some fishes, which theoretically belong to the pelvic arch, are so
much advanced forward as to be articulated to the scapular arch--“the
ischium elongating to join the coracoid.” With these admissions it
seems to us that relative position and connexions cannot be appealed to
as tests of homology, nor as evidence of any original type of vertebra.

In no class of facts, then, do we find a good foundation for the
hypothesis of an “ideal typical vertebra.” There is no one conceivable
attribute of this archetypal form which is habitually realised by
actual vertebræ. The alleged group of true vertebral elements is not
distinguished in any specified way from bones not included in it. Its
members have various degrees of inconstancy; are rarely all present
together; and no one of them is essential. They are severally developed
in no uniform way: each of them may arise either out of a separate
piece of cartilage, or out of a piece continuous with that of some
other element; and each may be ossified from many independent points,
from one, or from none. Not only may their respective individualities
be lost by absence, or by confluence with others; but they may be
doubled, or tripled, or halved, or may be multiplied in one direction
and lost in another. The entire group of typical elements may coalesce
into one simple bone representing the whole vertebra; and even, as in
the terminal piece of a bird’s tail, half-a-dozen vertebræ, with all
their many elements, may become entirely lost in a single mass. Lastly,
the respective elements, when present, have no fixity of relative
position: sundry of them are found articulated to various others than
those with which they are typically connected; they are frequently
displaced and attached to neighbouring vertebræ; and they are even
removed to quite remote parts of the skeleton. It seems to us that if
this want of congruity with the facts does not disprove the hypothesis,
no such hypothesis admits of disproof.

       *       *       *       *       *

Unsatisfactory as is the evidence in the case of the trunk and tail
vertebræ, to which we have hitherto confined ourselves, it is far worse
in the case of the alleged cranial vertebræ. The mere fact that those
who have contended for the vertebrate structure of the skull, have
differed so astonishingly in their special interpretations of it, is
enough to warrant great doubt as to the general truth of their theory.
From Professor Owen’s history of the doctrine of general homology,
we gather that Duméril wrote upon “la tête considérée comme _une_
vertèbre;” that Kielmeyer, “instead of calling the skull a vertebra,
said each vertebra might be called a skull;” that Oken recognized in
the skull _three_ vertebræ and a rudiment; that Professor Owen himself
makes out _four_ vertebræ; that Goethe’s idea, adopted and developed
by Carus, was, that the skull is composed of _six_ vertebræ; and that
Geoffrey St. Hilaire divided it into _seven_. Does not the fact that
different comparative anatomists have arranged the same group of bones
into _one_, _three_, _four_, _six_, and _seven_ vertebral segments,
show that the mode of determination is arbitrary, and the conclusions
arrived at fanciful? May we not properly entertain great doubts as to
any one scheme being more valid than the others? And if out of these
conflicting schemes we are asked to accept one, ought we not to accept
it only on the production of some thoroughly conclusive proof--some
rigorous test showing irrefragably that the others must be wrong and
this alone right? Evidently where such contradictory opinions have been
formed by so many competent judges, we ought, before deciding in favour
of one of them, to have a clearness of demonstration much exceeding
that required in any ordinary case. Let us see whether Professor Owen
supplies us with any such clearness of demonstration.

To bring the first or occipital segment of the skull into
correspondence with the “ideal typical vertebra,” Professor Owen
argues, in the case of the fish, that the parapophyses are _displaced_,
and wedged between the neurapophyses and the neural spine--removed
from the hæmal arch and built into the upper part of the neural arch.
Further, he considers that the pleurapophyses are _teleologically
compound_. And then, in all the higher vertebrata, he alleges that
the hæmal arch is _separated_ from its centrum, taken to a distance,
and transformed into the scapular arch. Add to which, he says that
in mammals the displaced parapophyses are mere processes of the
neurapophyses (p. 133): these vertebral elements, typically belonging
to the lower part of the centrum, and in nearly all cases confluent
with it, are not only removed to the far ends of elements placed above
the centrum, but have become exogenous parts of them!

Conformity of the second or parietal segment of the cranium with the
pattern-vertebra, is produced thus:--The petrosals are _excluded_
as being partially-ossified sense-capsules, not forming parts of the
true vertebral system, but belonging to the “splanchno-skeleton.” A
centrum is _artificially_ obtained by sawing in two the bone which
serves in common as centrum to this and the preceding segment; and
this though it is admitted that in fishes, where their individualities
ought to be best seen, these two hypothetical centrums are not simply
coalescent, but connate. Next, a similar _arbitrary bisection_ is
made of certain elements of the hæmal arches. And then, “the principle
of _vegetative repetition_ is still more manifest in this arch
than in the occipital one:” each pleurapophysis is double; each
hæmapophysis is double; and the hæmal spine consists of six pieces!

The interpretation of the third and fourth segments being of the same
general character, need not be detailed. The only point calling for
remark being, that in addition to the above various modes of getting
over anomalies, we find certain bones referred to the _dermo-skeleton_.

Now it seems to us, that even supposing no antagonist interpretations
had been given, an hypothesis reconcilable with the facts only by
the aid of so many questionable devices, could not be considered
satisfactory; and that when, as in this case, various comparative
anatomists have contended for other interpretations, the character
of this one is certainly not of a kind to warrant the rejection of
the others in its favour; but rather of a kind to make us doubt
the possibility of all such interpretations. The question which
naturally arises is, whether by proceeding after this fashion,
groups of bones might not be arranged into endless typical forms.
If, when a given element was not in its place, we were at liberty
to consider it as _suppressed_, or _connate_ with some neighbouring
element, or _removed_ to some more or less distant position;--if,
on finding a bone in excess, we might consider it, now as part of
the _dermo-skeleton_, now as part of the _splanchno-skeleton_, now
as _transplanted_ from its typical position, now as resulting from
_vegetative repetition_, and now as a bone _teleologically compound_
(for these last two are intrinsically different, though often used by
Professor Owen as equivalents);--if, in other cases, a bone might be
regarded as _spurious_ (p. 91), or again as having _usurped_ the place
of another;--if, we say, these various liberties were allowed us, we
should not despair of reconciling the facts with various diagrammatic
types besides that adopted by Professor Owen.

When, in 1851, we attended a course of Professor Owen’s lectures
on Comparative Osteology, beginning though we did in the attitude
of discipleship, our scepticism grew as we listened, and reached
its climax when we came to the skull; the reduction of which to the
vertebrate structure, reminded us very much of the interpretation of
prophecy. The delivery, at the Royal Society, of the Croonian Lecture
for 1858, in which Professor Huxley, confirming the statements of
several German anatomists, has shown that the facts of embryology do
not countenance Professor Owen’s views respecting the formation of the
cranium, has induced us to reconsider the vertebral theory as a whole.
Closer examination of Professor Owen’s doctrines, as set forth in his
works, has certainly not removed the scepticism generated years ago
by his lectures. On the contrary, that scepticism has deepened into
disbelief. And we venture to think that the evidence above cited shows
this disbelief to be warranted.

       *       *       *       *       *

There remains the question--What general views are we to take
respecting the vertebrate structure? If the hypothesis of an “ideal
typical vertebra” is not justified by the facts, how are we to
understand that degree of similarity which vertebræ display?

We believe the explanation is not far to seek. All that our space will
here allow, is a brief indication of what seems to us the natural view
of the matter.

Professor Owen, in common with other comparative anatomists, regards
the divergences of individual vertebræ from the average form, as due
to adaptive modifications. If here one vertebral element is largely
developed, while elsewhere it is small--if now the form, now the
position, now the degree of coalescence, of a given part varies; it
is that the local requirements have involved this change. The entire
teaching of comparative osteology implies that differences in the
conditions of the respective vertebræ necessitate differences in their
structures.

Now, it seems to us that the first step towards a right conception
of the phenomena, is to recognize this general law in its converse
application. If vertebræ are unlike in proportion to the unlikeness
of their circumstances, then, by implication, they will be like in
proportion to the likeness of their circumstances. While successive
segments of the same skeleton, and of different skeletons, are all in
some respects more or less differently acted on by incident forces,
and are therefore required to be more or less different; they are all,
in other respects, similarly acted on by incident forces, and are
therefore required to be more or less similar. It is impossible to
deny that if differences in the mechanical functions of the vertebræ
involve differences in their forms; then, community in their mechanical
functions, must involve community in their forms. And as we know that
throughout the _Vertebrata_ generally, and in each vertebrate animal,
the vertebræ, amid all their varying circumstances, _have_ a certain
community of function, it follows necessarily that they will have a
certain general resemblance--there will recur that average shape which
has suggested the notion of a pattern-vertebra.

A glance at the facts at once shows their harmony with this conclusion.
In an eel or a snake, where the bodily actions are such as to involve
great homogeneity in the mechanical conditions of the vertebræ, the
series of them is comparatively homogeneous. On the contrary, in a
mammal or a bird, where there is considerable heterogeneity in their
circumstances, their similarity is no longer so great. And if, instead
of comparing the vertebral columns of different animals, we compare
the successive vertebræ of any one animal, we recognize the same law.
In the segments of an individual spine, where is there the greatest
divergence from the common mechanical conditions? and where may we
therefore expect to find the widest departure from the average form?
Obviously at the two extremities. And accordingly it is at the two
extremities that the ordinary structure is lost.

Still clearer becomes the truth of this view, when we consider the
genesis of the vertebral column as displayed throughout the ascending
grades of the _Vertebrata_. In its first embryonic stage, the spine
is an undivided column of flexible substance. In the early fishes,
while some of the peripheral elements of the vertebræ were marked out,
the central axis was still a continuous unossified cord. And thus we
have good reason for thinking that in the primitive vertebrate animal,
as in the existing _Amphioxus_, the notochord was persistent. The
production of a higher, more powerful, more active creature of the same
type, by whatever method it is conceived to have taken place, involved
a change in the notochordal structure. Greater muscular endowments
presupposed a firmer internal fulcrum --a less yielding central axis.
On the other hand, for the central axis to have become firmer while
remaining continuous, would have entailed a stiffness incompatible
with the creature’s movements. Hence, increasing density of the
central axis necessarily went hand in hand with its segmentation: for
strength, ossification was required; for flexibility, division into
parts. The production of vertebræ resulting thus, there obviously
would arise among them a general likeness, due to the similarity in
their mechanical conditions, and more especially the muscular forces
bearing on them. And then observe, lastly, that where, as in the head,
the terminal position and the less space for development of muscles,
entailed smaller lateral bendings, the segmentation would naturally
be less decided, less regular, and would be lost as we approached the
front of the head.

But, it may be replied, this hypothesis does not explain all the
facts. It does not tell us why a bone whose function in a given animal
requires it to be solid, is formed not of a single piece, but by the
coalescence of several pieces, which in other creatures are separate;
it does not account for the frequent manifestations of unity of plan
in defiance of teleological requirements. This is quite true. But it
is not true, as Professor Owen argues respecting such cases, that
“if the principle of special adaptation fails to explain them, and
we reject the idea that these correspondences are manifestations of
some archetypal exemplar, on which it has pleased the Creator to frame
certain of his living creatures, there remains only the alternative
that the organic atoms have concurred fortuitously to produce such
harmony.” This is not the only alternative: there is another, which
Professor Owen has overlooked. It is a perfectly tenable supposition
that all higher vertebrate forms have arisen by _the superposing of
adaptations upon adaptations_. Either of the two antagonist cosmogonies
consists with this supposition. If, on the one hand, we conceive
species to have resulted from acts of special creation; then it is
quite a fair assumption that to produce a higher vertebrate animal,
the Creator did not begin afresh, but took a lower vertebrate animal,
and so far modified its pre-existing parts as to fit them for the
new requirements; in which case the original structure would show
itself through the superposed modifications. If, on the other hand, we
conceive species to have resulted by gradual differentiations under the
influence of changed conditions; then, it would manifestly follow that
the higher, heterogeneous forms, would bear traces of the lower and
more homogeneous forms from which they were evolved.

Thus, besides finding that the hypothesis of an “ideal typical
vertebra” is irreconcilable with the facts, we find that the facts are
interpretable without gratuitous assumptions. The average community of
form which vertebræ display, is explicable as resulting from natural
causes. And those typical similarities which are traceable under
adaptive modifications, must obviously exist if, throughout creation in
general, there has gone on that continuous superposing of modifications
upon modifications which goes on in every unfolding organism.

       *       *       *       *       *

[I might with propriety have added to the foregoing criticisms, the
remark that Professor Owen has indirectly conferred a great benefit by
the elaborate investigations he has made with the view of establishing
his hypothesis. He has himself very conclusively proved that the
teleological interpretation is quite irreconcilable with the facts.
In gathering together evidence in support of his own conception of
archetypal forms, he has disclosed adverse evidence which I think shows
his conception to be untenable. The result is that the field is left
clear for the hypothesis of Evolution as the only tenable one.]




                              APPENDIX C.

      [_From the_ TRANSACTIONS OF THE LINNEAN SOCIETY, VOL. XXV.]


   XV. _On Circulation and the Formation of Wood in Plants. By_
   HERBERT SPENCER, _Esq. Communicated by_ GEORGE BUSK, _Esq.,
   F.R.S., Sec. L.S._

                         Read March 1st, 1866.

Opinions respecting the functions of the vascular tissues in plants
appear to make but little progress towards agreement. The supposition
that these vessels and strings of partially-united cells, lined with
spiral, annular, reticulated, or other frameworks, are carriers of the
plant-juices, is objected to on the ground that they often contain
air: as the presence of air arrests the movement of blood through
arteries and veins, its presence in the ducts of stems and petioles
is assumed to unfit them as channels for sap. On the other hand, that
these structures have a respiratory office, as some have thought, is
certainly not more tenable, since, if the presence of air in them
negatives the belief that their function is to distribute liquid, the
presence of liquid in them equally negatives the belief that their
function is to distribute air. Nor can any better defence be made for
the hypothesis which I find propounded, that these parts serve “to give
strength to the parenchyma.” Tubes with fenestrated and reticulated
internal skeletons have, indeed, some power of supporting the tissue
through which they pass; but tubes lined with spiral threads can yield
extremely little support, while tubes lined with annuli, or spirals
alternating with annuli, can yield no support whatever. Though all
these types of internal framework are more or less efficient for
preventing closure by lateral pressure, they are some of them quite
useless for holding up the mass through which the vessels pass; and the
best of them are for this purpose mechanically inferior to the simple
cylinder. The same quantity of matter made into a continuous tube would
be more effective in giving stiffness to the cellular tissue around it.

In the absence of any feasible alternative, the hypothesis that these
vessels are distributors of sap claims reconsideration. The objections
are not, I think, so serious as they seem. The habitual presence of
air in the ducts that traverse wood, can scarcely be held anomalous if
when the wood is formed their function ceases. The canals which ramify
through a Stag’s horn, contain air after the Stag’s horn is fully
developed; but it is not thereby rendered doubtful whether it is the
function of arteries to convey blood. Again, that air should frequently
be found even in the vessels of petioles and leaves, will not appear
remarkable when we call to mind the conditions to which a leaf is
subject. Evaporation is going on from it. The thinner liquids pass
by osmose out of the vessels into the tissues containing the liquids
thickened by evaporation. And as the vessels are thus continually
drained, a draught is made upon the liquid contained in the stem and
roots. Suppose that this draught is unusually great, or suppose that
around the roots there exists no adequate supply of moisture. A state
of capillary tension must result--a tendency of the liquid to pass into
the leaves resisted below by liquid cohesion. Now, had the vessels
impermeable coats, only their upper extremities would under these
conditions be slowly emptied. But their coats, in common with all the
surrounding tissues, are permeable by air. Hence, under this state
of capillary tension, air will enter; and as the upper ends of the
tubes, being both smaller in diameter and less porous than the lower,
will retain the liquids with greater tenacity, the air will enter the
wider and more porous tubes below--the ducts of the stem and branches.
Thus the entrance of air no more proves that these ducts are not
sap-carriers, than does the emptiness of tropical river-beds in the dry
season prove that they are not channels for water. There is, however, a
difficulty which seems more serious. It is said that air, when present
in these minute canals, must be a great obstacle to the movement of sap
through them. The investigations of Jamin have shown that bubbles in a
capillary tube resist the passage of liquid, and that their resistance
becomes very great when the bubbles are numerous--reaching, in some
experiments, as much as three atmospheres. Nevertheless the inference
that any such resistance is offered by the air-bubbles in the vessels
of a plant, is, I think, an erroneous one. What happens in a capillary
tube having impervious sides, with which these experiments were made,
will by no means happen in a capillary tube having pervious sides.
Any pressure brought to bear on the column of liquid contained in the
porous duct of a plant, must quickly cause the expulsion of a contained
air-bubble through the minute openings in the coats of the duct. The
greater molecular mobility of gases than liquids, implies that air will
pass out far more readily than sap. Whilst, therefore, a slight tension
on the column of sap will cause it to part and the air to enter, a
slight pressure upon it will force out the air and reunite the divided
parts of the column.

To obtain data for an opinion on this vexed question, I have lately
been experimenting on the absorption of dyes by plants. So far as I can
learn, experiments of this kind have most, if not all of them, been
made on stems, and, as it would seem from the results, on stems so far
developed as to contain all their characteristic structures. The first
experiments I made myself were on such parts, and yielded evidence that
served but little to elucidate matters. It was only after trying like
experiments with leaves of different ages and different characters,
and with undeveloped axes, as well as with axes of special kinds, that
comprehensible results were reached; and it then became manifest that
the appearances presented by ordinary stems when thus tested, are in a
great degree misleading. Let me briefly indicate the differences.

If an adult shoot of a tree or shrub be cut off, and have its lower
end placed in an alumed decoction of logwood or a dilute solution of
magenta,[69] the dye will, in the course of a few hours, ascend to a
distance varying according to the rate of evaporation from the leaves.
On making longitudinal sections of the part traversed by it, the dye is
found to have penetrated extensive tracts of the woody tissue; and on
making transverse sections, the openings of the ducts appear as empty
spaces in the midst of a deeply-coloured prosenchyma. It would thus
seem that the liquid is carried up the denser parts of the vascular
bundles; neglecting the cambium layer, neglecting the central pith,
and neglecting the spiral vessels of the medullary sheath. Apparently
the substance of the wood has afforded the readiest channel. When,
however, we examine these appearances critically, we find reasons for
doubting this conclusion. If a transverse section of the lower part,
into which the dye passed first and has remained longest, be compared
with a transverse section of the part which the dye has but just
reached, a marked difference is visible. In the one case the whole of
the dense tissue is stained; in the other case it is not. This uneven
distribution of stain in the part which the dye has incompletely
permeated is not at random; it admits of definite description. A
tolerably regular continuous ring of colour distinguishes the outer
part of the wood from the inner mass, implying a passage of liquid
up the elongated cells next the cambium layer. And the inner mass is
coloured more round the mouths of the pitted ducts than elsewhere: the
dense tissue is darkest close to the edges of these ducts; the colour
fades away gradually on receding from their edges; there is most colour
where there are several ducts together; and the dense tissue which
is fully dyed for some space, is that which lies between two or more
ducts. These are indications that while the layer of pitted cells
next the cambium has served as a channel for part of the liquid, the
rest has ascended the pitted ducts, and oozed out of these into the
prosenchyma around. And this conclusion is confirmed by the contrast
between the appearances of the lowest part of a shoot under different
conditions. For if, instead of allowing the dye time for oozing through
the prosenchyma, the end of the shoot be just dipped into the dye and
taken out again, we find, on making transverse sections of the part
into which the dye has been rapidly taken up, that, though it has
diffused to some distance round the ducts, it has left tracts of wood
between the ducts uncoloured--a difference which would not exist had
the ascent been through the substance of the wood. Even still stronger
is the confirmation obtained by using one dye after another. If a shoot
that has absorbed magenta for an hour be placed for five minutes in the
logwood decoction, transverse sections of it taken at a short distance
from its end show the mouths of the ducts surrounded by dark stains in
the midst of the much wider red stains.

Based on these comparisons only, the inference pointed out has little
weight; but its weight is increased by the results of experiments on
quite young shoots, and shoots that develope very little wood. The
behaviour of these corresponds perfectly with the expectation that a
liquid will ascend capillary tubes in preference to simple cellular
tissue or tissue not differentiated into continuous canals. The
vascular bundles of the medullary sheath are here the only channels
which the coloured liquid takes. In sections of the parts up to which
the dye has but just reached, the spiral, fenestrated, scalariform,
or other vessels contained in these bundles are alone coloured, and
lower down it is only after some hours that such an exudation of dye
takes place as suffices partially to colour the other substances of the
bundle. Further, it is to be noted that at the terminations of shoots,
where the vessels are but incompletely formed out of irregularly-joined
fibrous cells which still retain their original shapes, the dye runs up
the incipient vessels and does not colour in the smallest degree the
surrounding tissue.

Experiments with leaves bring out parallel facts. On placing in a dye
a petiole of an adult leaf of a tree, and putting it before the fire
to accelerate evaporation, the dye will be found to ascend the mid-rib
and veins at various rates, up even to a foot per hour. At first it
is confined to the vessels; but by the time it has reached the point
of the leaf, it will commonly be seen that at the lower part it has
diffused itself into the sheaths of the vessels. In a quite young leaf
from the same shoot, we find a much more rigorous restriction of the
dye to the vessels. On making oblique sections of its petiole, mid-rib,
and veins, the vessels have the appearance of groups of sharply
defined coloured rods imbedded in the green prosenchyma; and this
marked contrast continues with scarcely an appreciable change after
plenty of time has been allowed for exudation.

The facts thus grouped and thus contrasted seem, at first sight, to
imply that while they are young the coats of these ramifying canals
lined with spiral or allied structures are not readily permeable,
but that, becoming porous as they grow old, they allow the liquids
they carry to escape with increasing facility; and hence a possible
interpretation of the fact that, in the older parts, the staining of
the tissue around the vessels is so rapid as to suggest that the dye
has ascended directly through this tissue, whereas in the younger
parts the reverse appearance necessitates the reverse conclusion. But
now, is this difference determined by difference of age, or is it
otherwise determined? The evidence as presented in ordinary stems and
leaves shows us that the parts of the vascular system at which there
is a rapid escape of dye are not simply older parts, but are parts
where a deposit of woody matter is taking place. Is it, then, that
the increasing permeability of the ducts, instead of being directly
associated with their increasing age, is directly associated with the
increasing deposit of dense substance around them?

To get proof that this last connexion is the true one, we have but to
take a class of cases in which wood is formed only to a small extent.
In such cases experiments show us a far more general and continued
limitation of the dye to the vessels. Ordinary herbs and vegetables,
when contrasted with shrubs and trees, illustrate this; as instance
the petioles of Celery, or of the common Dock, and the leaves of
Cabbages or Turnips. And then in very succulent plants, such as
_Bryophyllum calycinum_, _Kalanchoë rotundifolia_, the various species
of _Crassula_, _Cotyledon_, _Kleinia_, and others of like habit, the
ducts of old and young leaves alike retain the dye very persistently:
the concomitant in these cases being the small amount of prosenchyma
around the ducts, or the small amount of deposit in it, or both. More
conclusive yet is the evidence which meets us when we turn from very
succulent leaves to very succulent axes. The tender young shoots of
_Kleinia ante-euphorbium_, or _Euphorbia Mauritanica_, which for many
inches of their lengths have scarcely any ligneous fibres, show us
scarcely any escape of the coloured liquid from the vessels of the
medullary sheath. So, too, is it with _Stapelia Buffonia_, a plant of
another order, having soft swollen axes. And then we have a repetition
of the like connexion of facts throughout the _Cactaceæ_: the most
succulent showing us the smallest permeability of the vessels. In
two species of _Rhipsalis_, in two species of _Cereus_, and in two
species of _Mammillaria_, which I have tried, I have found this so.
_Mammillaria gracilis_ may be named as exemplifying the relation under
its extreme form. Into one of these small spheroidal masses, the dye
ascends through the large bundles of spiral or annular ducts, or cells
partially united into such ducts, colouring them deeply, and leaving
the feebly-marked sheath of prosenchyma, together with the surrounding
watery cellular tissue, perfectly uncoloured.

The most conclusive evidence, however, is furnished by those _Cactaceæ_
in which the transition from succulent to dense tissue takes place
variably, according as local circumstances determine. _Opuntia_ yields
good examples. If a piece of it including one of the joints at which
wood is beginning to form, be allowed to absorb a coloured liquid,
the liquid, running up the irregular bundles of vessels and into many
of their minute ramifications, is restricted to these where they pass
through the parenchyma forming the mass of the stem; but near the
joints the hardened tissue around the vessels is coloured. In one of
these fleshy growths we get clear evidence that the escape of the
dye has no immediate dependence on the age of the vessels, since, in
parts of the stem that are alike in age, some of the vessels retain
their contents while others do not. Nay, we even find that the younger
vessels are more pervious than the older ones, if round the younger
ones there is a formation of wood.

Thus, then, is confirmed the inference before drawn, that in ordinary
stems the staining of the wood by an ascending coloured liquid is due,
not to the passage of the coloured liquid up the substance of the wood,
but to the permeability of its ducts and such of its pitted cells as
are united into irregular canals. And the facts showing this, at the
same time indicate with tolerable clearness the process by which wood
is formed. What in these cases is seen to take place with a dye, may
be fairly presumed to take place with sap. Where the dye exudes but
slowly, we may infer that the sap exudes but slowly; and it is a fair
inference that where the dye leaks rapidly out of the vessels, the sap
does the same. Inferring, thus, that where-ever there is a considerable
formation of wood there is a considerable escape of the sap, we see
in the one the result of the other. The thickening of the prosenchyma
is proportionate to the quantity of nutritive liquid passing into it;
and this nutritive liquid passes into it from the vessels, ducts, and
irregular canals it surrounds.

But an objection is made to such experiments as the foregoing, and to
all the inferences drawn from them. It is said that portions of plants
cut off and thus treated, have their physiological actions arrested,
or so changed as may render the results misleading; and it is said
that when detached shoots and leaves have their cut ends placed in
solutions, the open mouths of their vessels and ducts are directly
presented with the liquids to be absorbed, which does not happen in
their natural states. Further, making these objections look serious, it
is alleged that when solutions are absorbed through the roots, quite
different results are obtained: the absorbed matters are found in the
tissues and not in the vessels. Clearly, were the experiments yielding
these adverse results conducted in unobjectionable ways, the conclusion
implied by them would negative the conclusions above drawn. But these
experiments are no less objectionable than those to which they are
opposed. Such mineral matters as salts of iron, solutions of which
have in some cases been supplied to the roots for their absorption,
are obviously so unlike the matters ordinarily absorbed, that they
are likely to interfere fatally with the physiological actions. If
experiments of this kind are made by immersing the roots in a dye,
there is, besides the difficulty that the mineral mordant contained
by the dye is injurious to the plant, the further difficulty that the
colouring matter, being seized by the substances for which it has an
affinity, is left behind in the first layers of root-tissues passed
through, and that the decolorized water passing up into the plant is
not traceable. To be conclusive, then, an experiment on absorption
through roots must be made with some solution which will not seriously
interfere with the plant’s vital processes, and which will not have
its distinctive element left behind. To fulfil these requirements I
adopted the following method. Having imbedded a well-soaked broad-bean
in moist sand, contained in an inverted cone of cardboard with its
apex cut off for the radicle to come through--having placed this in
a wide-mouthed dwarf bottle, partly filled with water, so that the
protruding radicle dipped into the water--and having waited until the
young bean had a shoot some three or more inches high, and a cluster
of secondary rootlets from an inch to an inch and a-half long--I
supplied for its absorption a simple decoction of logwood, which,
being a vegetal matter, was not likely to do it much harm, and which,
being without a mordant, would not leave its suspended colour in the
first tissues passed through. To avoid any possible injury, I did not
remove the plant from the bottle, but slightly raising the cone out of
its neck, I poured away the water through the crevice and then poured
in the logwood decoction; so that there could have been no broken end
or abraded surface of a rootlet through which the decoction might
enter. Being prepared with some chloride of tin as a mordant, I cut
off, after some three hours, one of the lowest leaves, expecting that
the application of the mordant to the cut surface would bring out
the characteristic colour if the logwood decoction had risen to that
height. I got no reaction, however. But after eight hours I found, on
cutting off another leaf, that the vessels of its petiole were made
visible as dark streaks by the colour with which they were charged--a
colour differing, as was to be expected, from that of the logwood
decoction, which spontaneously changes even by simple exposure. It was
then too late in the day to pursue the observations; but next morning
the vessels of the whole plant, as far as the petioles of its highest
unfolded leaves, were full of the colouring matter; and on applying
chloride of tin to the cut surfaces, the vessels assumed that purplish
red which this mordant produces when directly mixed with the logwood
decoction. Subsequently, when one of the cotyledons was cut open
by Prof. Oliver, to whom, in company with Dr. Hooker, I showed the
specimen, we found that the whole of its vascular system was filled
with the decoction, which everywhere gave the characteristic reaction.
And it became manifest that the liquid absorbed through the rootlets,
in the central vessels of which it was similarly traceable, had part
of it passed directly up the vessels of the axis, while part of it
had passed through other vessels into the cotyledon, out of which, no
doubt, the liquid ordinarily so carried returns charged with a supply
of the stored nutriment. I have since obtained a verification by
varying the method. Digging up some young plants (Marigolds happened to
afford the best choice) with large masses of soil round them, placing
them in water, so as gradually to detach the soil without injuring the
rootlets, planting them afresh in a flower-pot full of washed sand,
and then, after a few days, watering them with a logwood decoction, I
found, as before, that in less than twenty-four hours the colouring
matter had run up into the vessels of the leaves. Though the reaction
produced by the mordant was not so strong as before, it was marked
enough to be quite unquestionable.

As these experiments were so conducted that there was no access to
the vessels except through the natural channels, and as the vital
actions of the plants were so little interfered with that at the end
of twenty-four hours they showed no traces of disturbance, I think the
results must be held conclusive.

Taking it, then, as a fact that in plants possessing them the vessels
and ducts are the channels through which sap is distributed, we come
now to the further question--What determines the varying permeability
of the walls of the vessels and ducts, and the consequent varying
formation of wood? To this question I believe the true reply is--The
exposure of the parts to intermittent mechanical strains, actual or
potential, or both. By actual strains I of course mean those which the
plant experiences in the course of its individual life. By potential
strains I mean those which the form, attitude, and circumstances
common to its kind involve, and which its inherited structure is
adapted to meet. In plants with stems, petioles, and leaves, having
tolerably constant attitudes, the increasing porosity of the tubes and
consequent deposit of dense tissue takes place in anticipation of the
strains to which the parts of the individual are liable, but takes
place at parts which have been habitually subject to such strains
in ancestral individuals. But though in such plants the tendency to
repeat that distribution of dense tissue caused by mechanical actions
on past generations, goes on irrespective of the mechanical actions
to which the developing individual is subject, these direct actions,
while they greatly aid the assumption of the typical structure, are
the sole causes of those deviations in the relative thickenings of
parts which distinguish the individual from others of its kind. And
then, in certain irregularly growing plants, such as Cactuses and
Euphorbias, where the strains fall on parts that do not correspond in
successive individuals, we distinctly trace a direct relation between
the degrees of strain and the rates of these changes which result in
dense tissue. I will not occupy space in detailing the evidence of this
relation, which is conspicuous in the orders named, but will pass to
the question--What are the physical processes by which intermittent
mechanical strains produce this deposit of resistant substance at
places where it is needed to meet the strains? We have not to seek far
for an answer. If a trunk, a bough, a shoot, or a petiole, is bent
by a gust of wind, the substance of its convex side is subject to
longitudinal tension: the substance of its concave side being at the
same time compressed. This is the primary mechanical effect. There is,
however, a secondary mechanical effect, which here chiefly concerns
us. That bend by which the tissues of the convex side are stretched,
also produces lateral compression of them. Buttoning on a tight
glove and then closing the hand, will make this necessity clear: the
leather, while it is strained along the backs of the fingers, presses
with considerable force on the knuckles. It is demonstrable that the
tensions of the outer layer of a mass made convex by bending, must,
by composition of forces, produce at every point a resultant at right
angles to the layer beneath it; that, similarly, the joint tensions
of these two layers must throw a pressure on the next deeper layer;
and so on. Hence, if at some little distance beneath the surface of a
stem, twig, or leaf-stalk, there exist longitudinal tubes, these tubes
must be squeezed each time the side of the branch they are placed on
becomes convex. Modifying the illustration just drawn from the clenched
hand will make this clear. When, on forcibly grasping something, the
skin is drawn tightly over the back of the hand, the whitening of
the knuckles shows how the blood is expelled from the vessels below
the surface by the pressure of the tightened skin. If, then, the
sap-vessels must be thus compressed, what will happen to the liquid
they contain? It will move away along the lines of least resistance.
Part, and probably the greater part, will escape lengthways from the
place of greatest pressure: some of it being expelled downwards, and
some of it upwards. But, at the same time, part of it will be likely
to ooze through the walls of the tubes. If these walls are so perfect
as to permit the passage of liquid only by osmose, it may still be
inferred that the osmose will increase under pressure; and probably,
under recurrent pressure, the places at which the osmotic current
passes most readily will become more and more permeable, until they
eventually form pores. At any rate it is manifest that where pores
and slits exist, whether thus formed or formed in any other way, the
escape of sap into the adjacent tissue at each bend will become easy
and rapid. What further must happen? When the branch or shoot recoils,
the vessels on the side that was convex, being relieved from pressure,
will tend to resume their previous diameters; and will be helped to do
this by the elasticity of the surrounding tissue, as well as by those
spiral, annular, and allied structures which they contain. But this
resumption of their previous diameters must cause an immediate rush
of sap back into them. Whence will it come? Not to any considerable
extent from the surrounding tissues into which part of it has been
squeezed, seeing that the resistance to the return of liquid through
small pores will be greater than the resistance to its return along the
vessels themselves. Manifestly the sap which was thrust up and down
the vessels from the place of compression will return--the quantities
returning from above and from below varying, as we shall hereafter
see, according to circumstances. But this is not all. From some side a
greater quantity must come back than was sent away; for the amount that
has escaped out of the tube into the prosenchyma has to be replaced.
Thus during the time when the side of the branch or twig becomes
concave, more sap returns from above or below than was expelled upwards
or downwards during the previous compression. The refilled vessels,
when the next bend renders their side convex, again have part of their
contents forced through their parietes, and are again refilled in the
same way. There is thus set up a draught of sap to the place where
these intermittent strains are going on, an exudation proportionate
to the frequency and intensity of the strains, and a proportionate
nutrition or thickening of the wood-cells, fitting them to resist the
strains. A rude idea of this action may be obtained by grasping in one
hand a damp sponge, having its lower end in water, while holding a
piece of blotting-paper in contact with its upper end, and then giving
the sponge repeated squeezes. At each squeeze some of the water will
be sent into the blotting-paper; at each relaxation the sponge will
refill from below, to give another portion of its contents to the
blotting-paper when again squeezed.

But how does this explanation apply to roots? If the formation of
wood is due to intermittent transverse strains, such as are produced
in the aërial parts of upright plants by the wind, how does it happen
that woody matter is deposited in roots, where there are no lateral
oscillations, no transverse strains? The answer is, that longitudinal
strains also are capable of causing the effects described. It is
true that perfectly straight fibres united into a bundle and pulled
lengthways would not exert on one another any lateral pressure, and
would not laterally compress any similarly-straight canals running
along with them. But if the fibres united into a bundle are variously
bent or twisted, they cannot be longitudinally strained without
compressing one another and structures imbedded in them. It needs but
to watch a wet rope drawn tight by a capstan, to see that an action
like that which squeezes the water out of its strands, will squeeze the
sap out of the vessels of a root into the surrounding tissue, as often
as the root is pulled by the swaying of the plant it belongs to. Here,
too, as before, the vessels will refill when the pull intermits; and
so, in the roots as in the branches, this rude pumping process will
produce a growth of hard tissue proportionate to the stress to be borne.

These conclusions are supported by the evidence which exceptional
cases supply. If intermittent mechanical strains thus cause the
formation of wood where wood is found, then where it is not found,
there should be an absence of intermittent mechanical strains. There
is such an absence. Vascular plants characterized by little or no
deposit of dense substance, are those having vessels so conditioned
that no considerable pressures are borne by them. The more succulent a
petiole or leaf becomes, the more do the effects of transverse strains
fall on its outer layers of cells. Its mechanical support is chiefly
derived from the ability of these minute vesicles, full of liquid, to
resist bursting and tearing under the compressions and tensions they
are exposed to. And just as fast as this change from a thin leaf or
foot-stalk to a thick one entails increasing stress on the superficial
tissue, so fast does it diminish the stress on the internally-seated
vascular tissue. The succulent leaf cannot be swayed about by the wind
as much as an ordinary leaf; and such small bends as can be given to
it and its foot-stalk are prevented from affecting in any considerable
degree the tubes running through its interior. Hence the retentiveness
of the vessels in these fleshy leaves, as shown by the small exudation
of dye; and hence the small thickening of their surrounding prosenchyma
by woody deposit. Still more conspicuously is this connexion of facts
shown when, from the soft thick leaves before named and such others as
those of _Echeveria_, _Rochea_, _Pereskia_, we turn to the thick leaves
that have strong exo-skeletons. _Gasteria_ serves as an illustration.
The leathery or horny skin here evidently bears the entire weight of
the leaf, and is so stiff as to prevent any oscillation. Here, then,
the vessels running inside are protected from all mechanical stress;
and accordingly we find that the cells surrounding them are not
appreciably thickened.

Equally clear, and more striking because more obviously exceptional,
is the evidence given by succulent stems which are leafless. _Stapelia
Buffonia_, having soft procumbent axes not liable to be bent backwards
and forwards in any considerable degree by the wind, has, ramifying
through its tissue, vessels that allow but an extremely slow escape
of dye and have unthickened sheaths. Such of the Euphorbias as have
acquired the fleshy character while retaining the arborescent growth,
like _Euphorbia Canariensis_, teach us the same truth in another way.
In them the formation of wood around the vessels is inconspicuous
where the intermittent strains are but slight; but it is conspicuous
at those joints on which lateral oscillations of the attached branches
throw great extensions and compressions of tissue. Throughout
the _Cactaceæ_ we find varied examples of the alleged relation.
_Mammillaria_ furnishes a very marked one. The substance of one of
these globular masses, resting on the ground, admits of no bending from
side to side; and accordingly its large bundles of spiral and annular
vessels, or partially-united cells, have very feebly-marked sheaths
not at all thickened. In such types as _Cereus_ and _Opuntia_ we see,
as in the Euphorbias, that where little stress falls on the vessels,
little deposit takes place around them; while there is much deposit
where there is much stress. Here let me add a confirmation obtained
since writing the above. After observing among the Cactuses the very
manifest relation between strain and the formation of wood, I inquired
of Mr. Croucher, the intelligent foreman of the Cactus-house at Kew,
whether he found this relation a constant one. He replied that he did,
and that he had frequently tested it by artificially subjecting parts
of them to strains. Neglecting at the time to inquire how he had done
this, it afterwards occurred to me that if he had so done it as to
cause constant strains, the observed result would not tell in favour of
the foregoing interpretation. Subsequently, however, I learned that he
had produced the strains by placing the plants in inclined attitudes--a
method which, by permitting oscillations of the strained joints,
allowed the strains to intermit. And then, making the proof conclusive,
Mr. Croucher volunteered the statement that where he had produced
constant strains by tying, no formation of wood took place.

Aberrant growths of another class display the same relations of
phenomena. Take first the underground stems, such as the Potato and
the Artichoke. The vessels which run through these, slowly take up
the dye without letting it pass to any considerable extent into the
surrounding tissues.[70] Only after an interval of many hours does the
prosenchyma become stained in some places. Here, as before, an absence
of rapid exudation accompanies an absence of woody deposit; and both
these go along with the absence of intermittent strains. Take again the
fleshy roots. The Turnip, the Carrot, and the Beetroot, have vessels
that retain very persistently the coloured liquids they take up. And
differing in this, as these roots do, from ordinary roots, we see
that they also differ from them in not being woody, and in not being
appreciably subject to the usual mechanical actions. In these cases,
as in the others, parts that ordinarily become dense, deviate from
this typical character when they are not exposed to those forces which
produce dense tissue by increasing the extravasation of sap.

To complete the proof that such a relation exists, let me add the
results of some experiments on equal and similarly-developed parts,
kept respectively at rest and in motion. I have tested the effects on
large petioles, on herbaceous shoots, and on woody shoots. If two such
petioles as those of Rhubarb, with their leaves attached, have their
cut ends inserted in bottles of dye, and the one be bent backwards
and forwards while the other remains motionless, there arises, after
the lapse of an hour, scarcely any difference in the states of their
vessels: a certain proportion of these are in both cases charged with
the dye, and little exudation has been produced by the motion. Here,
however, it is to be observed that the causes of exudation are scarcely
operative; the vascular bundles are distributed all through the mass
of the petiole, which is formed of soft watery tissue; and they are,
therefore, not so circumstanced as to be effectually compressed by the
bends. In herbaceous stems, such as those of the Jerusalem Artichoke
and of the Foxglove, an effect scarcely more decided is produced; and
here, too, when we seek a reason, we find it in the non-fulfilment of
the mechanical conditions; for the vascular bundles are not so seated
between a tough layer of bark and a solid core as to be compressed at
each bend. When, however, we come to experiment upon woody shoots, we
meet with conspicuous effects, though by no means uniformly. In some
cases oscillations produce immense amounts of exudation--parallel
transverse sections of the compared shoots showing that where, in the
one that has been at rest, there are spots of colour round but a few
pitted ducts, in the one that has been kept in motion the substance of
the wood is soaked almost uniformly through with dye. In other cases,
especially where there is much undifferentiated tissue remaining, the
exudation is not very marked. The difference appears to depend on
the quantity of liquid contained in the shoot. If its substance is
relatively dry, the exudation is great; but it is comparatively small
if all the tissues are fully charged with sap. This contrast of results
is one which contemplation of the mechanical actions will lead us to
expect.

And now, with these facts to aid our interpretation, let us return to
ordinary stems. If the upper end of a growing shoot, the prosenchyma
of which is but little thickened, be allowed to imbibe the dye, the
vessels of its medullary sheath alone become charged; and from them
there takes place but a slow oozing. If a like experiment be tried
with a lower part of the shoot, where the wood in course of formation
has its inner boundary marked but not its outer boundary, we find
that the pitted ducts, and more especially the inner ones, come into
play. And then lower still, where the wood has its periphery defined
and its histological characters decided, the appearances show that
the tissue forming its outer surface begins to take a leading part
in the transmission of liquid. What now is the explanation of these
changes, mechanically considered? In the young soft part of the shoot,
as in all normal and abnormal growths that have not formed wood, the
channels for the passage of sap are the spiral, annular, fenestrated,
or reticulated vessels. These vessels, here included in the bundles
of the medullary sheath, are, in common with the tissues around
them, subject, by the bendings of the shoot, to slight intermittent
compressions, and, especially the outermost of them, are thus forced
to give the prosenchyma an extra supply of nutritive liquid. The
thickening of the prosenchyma, spreading laterally as well as outwards
from each bundle of the medullary sheath, goes on until it meets the
thickenings that spread from the other bundles; and there is so formed
an irregular cylinder of hardened tissue, surrounding the medulla and
the vascular bundles of its sheath. As soon as this happens, these
vascular bundles become, to a considerable extent, shielded from the
effects of transverse strains, since the tensions and compressions
chiefly fall on the developing wood outside of them. Clearly, too, the
greatest stress must be felt by the outer layer of the developing wood:
being further removed from the neutral axis, it must be subject to
severer strains at each bend; and lying between the bark and the layer
of wood first formed, it must be most exposed to lateral compressions.
Among the elongated cells of this outer layer, some unite to form the
pitted ducts. Being, as we see, better circumstanced mechanically,
they become greater carriers of sap than the original vessels, and,
in consequence of this, as well as in consequence of their relative
proximity, become the sources of nutrition to the still more external
layers of wood-cells. The same causes and the same effects hold with
each new indurated coat deposited round the previously indurated coats.

This description may be thought to go far towards justifying the
current views respecting the course taken by the sap. But the
justification is more apparent than real. In the first place, the
implication here is that the sap-carrying function is at first
discharged entirely by the vessels of the medullary sheath, and
that they cease to discharge this function only as fast as they are
relatively incapacitated by their mechanical circumstances. And the
second implication is, that it is not the wood itself, but the more
or less continuous canals formed in it, which are the subsequent
sap-distributors. This, though readily made clear by microscopic
examination of the large pitted ducts in a partially lignified shoot
that has absorbed the dye, is less manifestly true of the peripheral
layer of sap-carrying tissue finally formed. But it is really true
here. For this layer, though nominally a layer of wood, is practically
a layer of inosculating vessels. It is formed out of irregular lines
and networks of elongated pitted cells, obliquely united by their
ends. Examination of them after absorption of a dye, shows that it is
only along the continuous channels they unite to form that the current
has passed. But the essentially vascular character of this outer and
latest-formed layer of the alburnum is best seen in the fact that
the vascular systems of new axes take their rise from it, and form
with it continuous canals. If a shoot of last year in which growth
is recommencing, be cut lengthways after it has imbibed a dye, clear
proof is obtained that the passage of the dye into a lateral bud takes
place from this outermost layer of pitted cells, and that the channels
taken by the dye through the new tissue are composed of cells that pass
through modified forms into the spiral vessels of the new medullary
sheath. This transition may be still more clearly traced in a terminal
bud that continues the line of last year’s shoot. A longitudinal
section of this shows that the vessels of the new medullary sheath do
not obtain their sap from the vessels of last year’s sheath (which, as
shown by the non-absorption of dye, have become inactive), but that
their supplies are obtained from those inosculating canals formed out
of last year’s outermost layer of prosenchyma, and that between the
component cells of this and those of the new vascular system there are
all gradations of structure.[71]

It is not the aim of the foregoing reasoning to show that mechanical
actions are the sole causes of the formation of dense tissue in
plants. Dense tissue is in many cases formed where no such causes
have come into play--as, for example, in thorns and in the shells
of nuts. Here the natural selection of variations can alone have
operated. It is manifest, too, that even those supporting structures
the building up of which is above ascribed to intermittent strains,
may, in the individual plant of a species that ordinarily has them, be
developed to a great extent when intermittent strains are prevented.
We see this in trees that are artificially supported by nailing to
walls; and we also see a kindred fact in natural climbers. Though in
these cases the formation of wood is obviously less than it would be
were the stem and branches habitually moved about by the wind, it
nevertheless goes on. Clearly the tendency of the plant to repeat the
structure of its type (in the one case the structure of its species;
and in the other case that of the order from which it has diverged in
becoming a climber) is here almost the sole cause of wood formation.
But though in plants so circumstanced intermittent mechanical strains
have little or no direct share, it may still be true, and I believe
is true, that intermittent mechanical strains are the original cause;
for, as before hinted, the typical structure which the individual
thus repeats irrespective of its own conditions, is interpretable
as a typical structure that is itself the product of these actions
and reactions between the plant and its environment. Grant the
inheritance of functionally-produced modifications; grant that natural
selection will always co-operate in such way as to favour those
individuals and families in which functionally-produced modifications
have progressed most advantageously; and it will follow that this
mechanically-caused formation of dense substance, accumulating from
generation to generation by the survival of the fittest, will result
in an organic habit of forming dense tissue at the required places.
The deposit arising from exudation at the places of greatest strain,
recurring from generation to generation at the same places, will come
to be reproduced in anticipation of strain, and will continue to
be reproduced for a long time after a changed habit of the species
prevents the strain--eventually, however, decreasing, both through
functional inactivity and natural selection, to the point at which it
is in equilibrium with the requirement.

Another side of the general question may now be considered. We have
seen how, by intermittent pressures on capillary vessels and ducts
and inosculating canals, there must be produced a draught of sap
towards the point of compression to replace the sap squeezed out. But
we have still to inquire what will be the effect on the distribution
of sap throughout the plant as a whole. It was concluded that out of
the compressed vessels the greater part of the liquid would escape
longitudinally--the longitudinal resistance to movement being least.
In every case the probabilities are infinity to one against the
resistances being equal upwards and downwards. Always, then, more sap
will be expelled in one direction than in the other. But in whichever
direction least sap is expelled, from that same direction most sap will
return when the vessels are relieved from pressure--the force which is
powerful in arresting the back current in that direction being the same
force which is powerful in producing a forward current. Ordinarily, the
more abundant supply of liquid being from below, there will result an
upward current. At each bend a portion of the contents will be squeezed
out through the sides of the vessels--a portion will be squeezed
downwards, reversing the current ascending from the roots, but soon
stopped by its resistance; while a larger portion will be squeezed
upwards towards the extremities of the vessels, where consumption and
loss are most rapid. At each recoil the vessels will be replenished,
chiefly by the repressed upward current; and at the next bend more of
it will be thrust onwards than backwards. Hence we have everywhere in
action a kind of rude force-pump, worked by the wind; and we see how
sap may thus be raised to a height far beyond that to which it could be
raised by capillary action, aided by osmose and evaporation.

Thus far, however, the argument proceeds on the assumption that there
is liquid enough to replenish every time the vessels subject to
this process. But suppose the supply fails--suppose the roots have
exhausted the surrounding stock of moisture. Evidently the vessels thus
repeatedly having their contents squeezed out into the surrounding
tissue, cannot go on refilling themselves from other vessels without
tending to empty the vascular system. On the one hand, evaporation from
the leaves causing a draught on the capillary tubes that end in them,
continually generates a capillary tension upwards; while, on the other
hand, the vessels below, expanding after their sap has been squeezed
out, produce a tension both upwards and downwards towards the point
of loss. Were the limiting membranes of the vessels impermeable, the
movement of sap would, under these conditions, soon be arrested. But
these membranes are permeable; and the surrounding tissues readily
permit the passage of air. This state of tension, then, will cause an
entrance of air into the tubes; the columns of liquid they contain
will be interrupted by bubbles. It seems, indeed, not improbable that
this entrance of air may take place even when there is a good supply
of liquid, if the mechanical strains are so violent and the exudation
so rapid that the currents cannot refill the half-emptied vessels
with sufficient rapidity. And in this case the intruding air may
possibly play the same part as that contained in the air-chamber of a
force-pump--tending, by moderating the violence of the jets, and by
equalizing the strains, to prevent rupture of the apparatus. Of course
when the supply of liquid becomes adequate, and the strains not too
violent, these bubbles will be expelled as readily as they entered.

Here, as before, let me add the conclusive proof furnished by a direct
experiment. To ascertain the amount of this propulsive action, I took
from the same tree, a Laurel, two equal shoots, and placing them in
the same dye, subjected them to conditions that were alike in all
respects save that of motion: while one remained at rest, the other
was bent backwards and forwards, now by switching and now by straining
with the fingers. After the lapse of an hour, I found that the dye had
ascended the oscillating shoot three times as far as it had ascended
the stationary shoot--this result being an average from several trials.
Similar trials brought out similar effects in other structures. The
various petioles and herbaceous shoots experimented upon for the
purpose of ascertaining the amount of exudation produced by transverse
strains, showed also the amount of longitudinal movement. It was
observable that the height ascended by the dye was in all cases greater
where there had been oscillation than where there had been rest--the
difference, however, being much less marked in succulent structures
than in woody ones.

It need scarcely be said that this mechanical action is not here
assigned as the sole cause of circulation, but as a cause co-operating
with others, and helping others to produce effects that could not
otherwise be produced. Trees growing in conservatories afford us
abundant proof that sap is raised to considerable heights by other
forces. Though it is notorious that trees so circumstanced do not
thrive unless, through open sashes, they are frequently subject
to breezes sufficient to make their parts oscillate, yet there is
evidently a circulation that goes on without mechanical aid. The
_causes_ of circulation are those actions only which disturb the liquid
equilibrium in a plant, by permanently abstracting water or sap from
some part of it; and of these the first is the absorption of materials
for the formation of new tissue in growing parts; the second is the
loss by evaporation, mainly through adult leaves; and the third is the
loss by extravasation, through compressed vessels. Only so far as it
produces this last, can mechanical strain be regarded as truly a cause
of circulation. All the other actions concerned must be classed as
_aids_ to circulation--as facilitating that re-distribution of liquid
that continually restores the equilibrium continually disturbed; and
of these capillary action may be named as the first, osmose as the
second, and the propulsive effect of mechanical strains as the third.
The first two of these aids are doubtless capable by themselves of
producing a large part of the observed result--more of the observed
result than is at first sight manifest; for there is an important
indirect effect of osmotic action which appears to be overlooked.
Osmose does not aid circulation only by setting up, within the plant,
exchange currents between the more dense and the less dense solutions
in different parts of it; but it aids circulation much more by
producing distention of the plant as a whole. In consequence of the
average contrast in density between the water outside of the plant and
the sap inside of it, the constant tendency is for the plant to absorb
a quantity in excess of its capacity, and so to produce distention
and erection of its tissues. It is because of this that the drooping
plant raises itself when watered; for capillary action alone could only
refill its tissues without changing their attitudes. And it is because
of this that juicy plants with collapsible structures bleed so rapidly
when cut, not only from the cut surface of the rooted part, but from
the cut surface of the detached part--the elastic tissues tending to
press out the liquid which distends them. And manifestly if osmose
serves thus to maintain a state of distention throughout a plant,
it indirectly furthers circulation; since immediately evaporation
or growth at any part, by abstracting liquid from the neighbouring
tissues, begins to diminish the liquid pressure within such tissues,
the distended structures throughout the rest of the plant thrust
their liquid contents towards the place of diminished pressure. This,
indeed, may very possibly be the most efficient of the agencies at
work. Remembering how great is the distention producible by osmotic
absorption--great enough to burst a bladder--it is clear that the
force with which the distended tissues of a plant urge forward the sap
to places of consumption, is probably very great. We must therefore
regard the aid which mechanical strains give as being one of several.
Oscillations help directly to restore any disturbed liquid equilibrium;
and they also help indirectly, by facilitating the re-distribution
caused by capillary action and the process just described; but in the
absence of oscillations the equilibrium may still be restored, though
less rapidly and within narrower limits of distance.

One half of the problem of the circulation, however, has been left out
of sight. Thus far our inquiry has been, how the ascending current
of sap is produced. There remains the rationale of the descending
current. What forces cause it, and through what tissues it takes place,
are questions to which no satisfactory answers have been given. That
the descent is due to gravitation, as some allege, is difficult to
conceive, since, as gravitation acts equally on all liquid columns
contained in the stem, it is not easy to see why it should produce
downward movements in some while permitting upward movements in
others--unless, indeed, there existed descending tubes too wide
to admit of much capillary action, which there do not. Moreover,
gravitation is clearly inadequate to cause currents towards the roots
out of branches that droop to the ground. Here the gravitation of the
contained liquid columns must nearly balance that of the connected
columns in the stem, leaving no appreciable force to cause motion. Nor
does there seem much probability in the assumption that the route of
the descending sap is through the cambium layer, since experiments on
the absorption of dyes prove that simple cellular tissue is a very bad
conductor of liquids: their movement through it does not take place
with one-fiftieth of the rapidity with which it takes place through
vessels.[72]

Of course the defence for these hypotheses is, that there must be a
downward current, which must have a course and a cause; and the very
natural assumption has been that the course and the cause must be other
than those which produce the ascending current. Nevertheless there
is an alternative supposition to which the foregoing considerations
introduce us. It is quite possible for the same vascular system
to serve as a channel for movement in opposite directions at
different times. We have among animals well-known cases in which the
blood-vessels carry a current first in one direction and then, after a
brief pause, in the reverse direction. And there seems an _à priori_
probability that, lowly-organized as they are, plants are more likely
to have distributing appliances of this imperfect kind than to have
two sets of channels for two simultaneous currents. If, led by this
suspicion, we inquire whether among the forces which unite to produce
movements of sap, there are any variations or intermissions capable of
determining the currents in different directions, we quickly discover
that there are such, and that the hypothesis of an alternating motion
of the sap, now centrifugal and now centripetal, through the same
vessels, has good warrant. What are the several forces at work? First
may be set down that tendency existing in every part of a plant to
expand into its typical form, and to absorb nutritive liquids in doing
this. The resulting competition for sap will, other things being
equal, cause currents towards the most rapidly-growing parts--towards
unfolding shoots and leaves, but not towards adult leaves. Next we
have evaporation, acting more on the adult leaves than on those which
are in the bud, or but partially developed. This evaporation is both
regularly and irregularly intermittent. Depending chiefly on the
action of the sun, it is, in fine weather, greatly checked or wholly
arrested every evening; and in cloudy weather must be much retarded
during the day. Further, every hygrometric variation, as well as every
variation in the movement of the air, must vary the evaporation.
This chief action, therefore, which, by continually emptying the
ends of the capillary tubes, makes upward currents possible, is one
which intermits every night, and every day is strong or feeble as
circumstances determine. Then, in the third place, we have this rude
pumping process above described, going on with greater vigour when the
wind is violent, and with less vigour when it is gentle--drawing liquid
_towards_ different parts according to their degrees of oscillation,
and _from_ different parts according as they can most readily furnish
it. And now let us ask what must result under changing conditions
from these variously-conflicting and conspiring forces. When a warm
sunshine, causing rapid evaporation, is emptying the vessels of the
leaves, the osmotic and capillary actions that refill them will be
continually aided by the pumping action of the swaying petioles,
twigs, and branches, provided their oscillations are moderate. Under
these conditions the current of sap, moving in the direction of least
resistance, will set towards the leaves. But what will happen when the
sun sets? There is now nothing to determine currents either upwards
or downwards, except the relative rates of growth in the parts and
the relative demands set up by the oscillations; and the oscillations
acting alone, will draw sap to the oscillating parts as much from above
as from below. If the resistance to be overcome by a current setting
back from the leaves is less than the resistance to be overcome by a
current setting up from the roots, then a current will set back from
the leaves. Now it is, I think, tolerably manifest that in the swaying
twigs and minor branches, less force will be required to overcome the
inertia of the short columns of liquid between them and the leaves than
to overcome the inertia of the long columns between them and the roots.
Hence during the night, as also at other times when evaporation is not
going on, the sap will be drawn out of the leaves into the adjacent
supporting parts; and their nutrition will be increased. If the wind is
strong enough to produce a swaying of the thicker branches, the back
current will extend to them also; and a further strengthening will
result from their absorption of the elaborated sap. And when the great
branches and the stem are bent backwards and forwards by a gale, they
too will share in the nutrition. It may at first sight seem that these
parts, being nearer to the roots than to the leaves, will draw their
supplies from the roots only. But the quantity which the roots can
furnish is insufficient to meet so great a demand. Under the conditions
described, the exudation of sap from the vessels will be very great,
and the draught of liquid required to refill them, not satisfied by
that which the root-fibres can take in, will extend to the leaves.
Thus sap will flow to the several parts according to their respective
degrees of activity--to the leaves while light and heat enable them to
discharge their functions, and back to the twigs, branches, stem, and
roots when these become active and the leaves inactive, or when their
activity dominates over that of the leaves. And this distribution of
nutriment, varying with the varying activities of the parts, is just
such a distribution as we know must be required to keep up the organic
balance.

To this explanation it may be objected that it does not account for
the downward current of sap in plants that are sheltered. The stem and
roots of a drawing-room Geranium display a thickening which implies
that nutritive matters have descended from the leaves, although there
are none of those oscillations by which the sap is said to be drawn
downwards as well as upwards. The reply is, that the stem and roots
tend to repeat their typical structures, and that the absorption of
sap for the formation of their respective dense tissues, is here the
force which determines the descent. Indeed it must be borne in mind
that the mechanical strains and the pumping process which they keep
up, as well as the distention caused by osmose, do not in themselves
produce a current either upwards or downwards: they simply help to move
the sap towards that place where there is the most rapid abstraction
of it--the place towards which its motion is least resisted. Whether
there is oscillation or whether there is not, the physiological demands
of the different parts of the plant determine the direction of the
current; and all which the oscillations and the distention do is to
facilitate the supply of these demands. Just as much, therefore,
in a plant at rest as in a plant in motion, the current will set
downwards when the function of the leaves is arrested, and when there
is nothing to resist that abstraction of sap caused by the tendency
of the stem- and root-tissues to assume their typical structures. To
which admission, however, it must be added that since this typical
structure assumed, though imperfectly assumed, by the hothouse plant,
is itself interpretable as the inherited effect of external mechanical
actions on its ancestors, we may still consider the current set up by
the assumption of the typical structure to be indirectly due to such
actions.

Interesting evidence of another order here demands notice. In the
course of experiments on the absorption of dyes by leaves, it happened
that in making sections parallel to the plane of a leaf, with the
view of separating its middle layer containing the vessels, I came
upon some structures that were new to me. These structures, where they
are present, form the terminations of the vascular system. They are
masses of irregular and imperfectly united fibrous cells, such as those
out of which vessels are developed; and they are sometimes slender,
sometimes bulky--usually, however, being more or less club-shaped. In
transverse sections of leaves their distinctive characters are not
shown: they are taken for the smaller veins. It is only by carefully
slicing away the surface of a leaf until we come down to that part
which contains them, that we get any idea of their nature. Fig. 1
represents a specimen taken from a leaf of _Euphorbia neriifolia_.
Occupying one of the interspaces of the ultimate venous network, it
consists of a spirally-lined duct or set of ducts, which connects with
the neighbouring vein a cluster of half-reticulated, half-scalariform
cells. These cells have projections, many of them tapering, that insert
themselves into the adjacent intercellular spaces, thus producing
an extensive surface of contact between the organ and the imbedding
tissues. A further trait is, that the ensheathing prosenchyma is
either but little developed or wholly absent; and consequently this
expanded vascular structure, especially at its end, comes immediately
in contact with the tissues concerned in assimilation. The leaf of
_Euphorbia neriifolia_ is a very fleshy one; and in it these organs
are distributed through a compact, though watery, cellular mass. But
in any leaf of the ordinary type which possesses them, they lie in
the network-parenchyma composing its lower layer; and wherever they
occur in this layer its cells unite to enclose them. This arrangement
is shown in fig. 2, representing a sample from the Caoutchouc-leaf,
as seen with the upper part of its envelope removed; and it is shown
still more clearly in a sample from the leaf of _Panax Lessonii_,
fig. 3. Figures 4 and 5 represent, without their sheaths, other such
organs from the leaves of _Panax Lessonii_ and _Clusia flava_. Some
relation seems to exist between their forms and the thicknesses of the
layers in which they lie. Certain very thick leaves, such as those of
_Clusia flava_, have them less abundantly distributed than is usual,
but more massive. Where the parenchyma is developed not to so great an
extreme, though still largely, as in the leaves of Holly, _Aucuba_,
_Camellia_, they are not so bulky; and in thinner leaves, like those
of Privet, Elder, &c., they become longer and less conspicuously
club-shaped. Some adaptations to their respective positions seem
implied by these modifications; and we may naturally expect that in
many thin leaves these free ends, becoming still narrower, lose the
distinctive and suggestive characters possessed by those shown in the
diagrams. Relations of this kind are not regular, however. In various
other genera, members of which I have examined, as _Rhus_, _Viburnum_,
_Griselinia_, _Brexia_, _Botryodendron_, _Pereskia_, the variations in
the bulk and form of these structures are not directly determined by
the spaces which the leaves allow: obviously there are other modifying
causes. It should be added that while these expanded free extremities
graduate into tapering free extremities, not differing from ordinary
vessels, they also pass insensibly into the ordinary inosculations.
Occasionally, along with numerous free endings, there occur loops; and
from such loops there are transitions to the ultimate meshes of the
veins.

These organs are by no means common to all leaves. In many that afford
ample spaces for them they are not to be found. So far as I have
observed, they are absent from the thick leaves of plants which form
very little wood. In _Sempervivum_, in _Echeveria_, in _Bryophyllum_,
they do not appear to exist; and I have been unable to discover them in
_Kalanchoë rotundifolia_, in _Kleinia ante-euphorbium_ and _ficoides_,
in the several species of _Crassula_, and in other succulent plants. It
may be added that they are not absolutely confined to leaves, but occur
in stems that have assumed the functions of leaves. At least I have
found, in the green parenchyma of _Opuntia_, organs that are analogous
though much more rudely and irregularly formed. In other parts, too,
that have usurped the leaf-function, they occur, as in the phyllodes of
the Australian Acacias. These have them abundantly developed; and it
is interesting to observe that here, where the two vertically-placed
surfaces of the flattened-out petiole are equally adapted to the
assimilative function, there exist two layers of these expanded
vascular terminations, one applied to the inner surface of each layer
of parenchyma.

Considering the structures and positions of these organs, as well as
the natures of the plants possessing them, may we not form a shrewd
suspicion respecting their function? Is it not probable that they
facilitate absorption of the juices carried back from the leaf for
the nutrition of the stem and roots? They are admirably adapted for
performing this office. Their component fibrous cells, having angles
insinuated between the cells of the parenchyma, are shaped just as they
should be for taking up its contents; and the absence of sheathing
tissue between them and the parenchyma facilitates the passage of the
elaborated liquids. Moreover there is the fact that they are allied to
organs which obviously have absorbent functions. I am indebted to Dr.
Hooker for pointing out the figures of two such organs in the “Icones
Anatomicæ” of Link. One of them is from the end of a dicotyledonous
root-fibre, and the other is from the prothallus of a young Fern. In
each case a cluster of fibrous cells, seated at a place from which
liquid has to be drawn, is connected by vessels with the parts to which
liquid has to be carried. There can scarcely be a doubt, then, that in
both cases absorption is effected through them. I have met with another
such organ, more elaborately constructed, but evidently adapted to the
same office, in the common Turnip-root. As shown by the end view and
longitudinal section in figs. 6 and 7, this organ consists of rings of
fenestrated cells, arranged with varying degrees of regularity into a
funnel, ordinarily having its apex directed towards the central mass
of the Turnip, with which it has, in some cases at least, a traceable
connexion by a canal. Presenting as it does an external porous surface
terminating one of the branches of the vascular system, each of these
organs is well fitted for taking up with rapidity the nutriment laid
by in the Turnip-root, and used by the plant when it sends up its
flower-stalk. Nor does even this exhaust the analogies. The cotyledons
of the young bean, experimented upon as before described, furnished
other examples of such structures, exactly in the places where, if
they are absorbents, we might expect to find them. Amid the branchings
and inosculations of the vascular layer running through the mass of
nutriment deposited in each cotyledon, there are conspicuous free
terminations that are club-shaped, and prove to be composed, like those
in leaves, of irregularly formed and clustered fibrous cells; and some
of them, diverging from the plane of the vascular layer, dip down into
the mass of starch and albumen which the young plant has to utilize,
and which these structures can have no other function but to take up.

Besides being so well fitted for absorption, and besides being
similar to organs which we cannot doubt are absorbents, these
vascular terminations in leaves afford us yet another evidence of
their functions. They are seated in a tissue so arranged as specially
to facilitate the abstraction of liquid. The centripetal movement
of the sap must be set up by a force that is comparatively feeble,
since, the parietes of the ducts being porous, air will enter if the
tension on the contained columns becomes considerable. Hence it is
needful that the exit of sap from the leaves should meet with very
little resistance. Now were it not for an adjustment presently to
be described, it would meet with great resistance, notwithstanding
the peculiar fitness of these organs to take it in. Liquid cannot be
drawn out of any closed cavity without producing a collapse of the
cavity’s sides; and if its sides are not readily collapsible, there
must be a corresponding resistance to the abstraction of liquid from
it. Clearly the like must happen if the liquid is to be drawn out of
a tissue which cannot either diminish in bulk bodily or allow its
components individually to diminish in bulk. In an ordinary leaf, the
upper layer of parenchyma, formed as it is of closely-packed cells
that are without interspaces, and are everywhere held fast within
their framework of veins, can neither contract easily as a mass, nor
allow its separate cells to do so. Quite otherwise is it with the
network-parenchyma below. The long cells of this, united merely by
their ends and having their flexible sides surrounded by air, may
severally have their contents considerably increased and decreased
without offering appreciable resistances: and the network-tissue
which they form will, at the same time, be capable of undergoing
slight expansions and contractions of its thickness. In this layer
occur these organs that are so obviously fitted for absorption. Here
we find them in direct communication with its system of collapsible
cells. The probability appears to be, that when the current sets into
the leaf, it passes through the vessels and their sheaths chiefly into
the upper layer of cells (this upper layer having a larger surface
of contact with the veins than the lower layer, and being the seat
of more active processes); and that the juices of the upper layer,
enriched by the assimilated matters, pass into the network-parenchyma,
which serves as a reservoir from which they are from time to time
drawn for the nutrition of the rest of the plant, when the actions
determine the downward current. Should it be asked what happens where
the absorbents, instead of being inserted in a network-parenchyma,
are, as in the leaves of _Euphorbia neriifolia_, inserted in a solid
parenchyma, the reply is, that such a parenchyma, though not furnished
with systematically arranged air-chambers, nevertheless contains air
in its intercellular spaces; and that when there occurs a draught upon
its contents, the expansion of this air and the entrance of more from
without, quickly supply the place of the abstracted liquid.

If then, returning to the general argument, we conclude that these
expanded terminations of the vascular system in leaves are absorbent
organs, we find a further confirmation of the views set forth
respecting the alternating movement of the sap along the same channels.
These spongioles of the leaves, like the spongioles of the roots, being
appliances by which liquid is taken up to be carried into the mass
of the plant, we are obliged to regard the vessels that end in these
spongioles of the leaves as being the channels of the down current
whenever it is produced. If the elaborated sap is abstracted from the
leaves by these absorbents, then we have no alternative but to suppose
that, having entered the vascular system, the elaborated sap descends
through it. And seeing how, by the help of these special terminations,
it becomes possible for the same vessels to carry back a quality of sap
unlike that which they bring up, we are enabled to understand tolerably
well how this rhythmical movement produces a downward transfer of
materials for growth.

       *       *       *       *       *

The several lines of argument may now be brought together; and along
with them may be woven up such evidences as remain. Let me first point
out the variety of questions to which the hypothesis supplies answers.

It is required to account for the ascent of sap to a height beyond that
to which capillary action can raise it. This ascent is accounted for
by the propulsive action of transverse strains, joined with that of
osmotic distention. A cause has to be assigned for that rise of sap
which, in the spring, while yet there is no considerable evaporation to
aid it, goes on with a power which capillarity does not explain. The
co-operation of the same two agencies is assignable for this result
also.[73] The circumstance that vessels and ducts here contain sap and
there contain air, and at the same place contain at different seasons
now air and now sap is a fact calling for explanation. An explanation
is furnished by these mechanical actions which involve the entrance
or expulsion of air according to the supply of liquid. That vessels
and ducts which were originally active sap-carriers go completely out
of use, and have their function discharged by other vessels or ducts,
is an anomaly that has to be solved. Again, we are supplied with a
solution: these deserted vessels and ducts are those which, by the
formation of dense tissue outside of them, become so circumstanced that
they cannot be compressed as they originally were. A channel has to be
found for the downward current of sap, which, on any other hypothesis
than the foregoing, must be a channel separate from that taken by the
upward current; and yet no good evidence of a separate channel has
been pointed out. Here, however, the difficulty disappears, since one
channel suffices for the current alternating upwards and downwards
according to the conditions. Moreover there has to be found a force
producing or facilitating the downward current, capable even of drawing
sap out of drooping branches; and no such force is forthcoming. The
hypothesis set forth dispenses with this necessity; under the recurring
change of conditions, the same distention and oscillation which before
raised the sap to the places of consumption, now bring it down to the
places of consumption. A physical process has to be pointed out by
which the material that forms dense tissue is deposited at the places
where it is wanted, rather than at other places. This physical process
the hypothesis indicates. It is requisite to find an explanation of the
fact that, when plants ordinarily swayed about by the wind are grown
indoors, the formation of wood is so much diminished that they become
abnormally slender. Of this an explanation is supplied. Yet a further
fact to be interpreted is, that in the same individual plant homologous
parts, which, according to the type of the plant, should be equally
woody, become much thicker one than another if subject to greater
mechanical stress. And of this too an interpretation is similarly
afforded.

Now the sufficiency of the assigned actions to account for so many
phenomena not otherwise explained, would be strong evidence that the
rationale is the true one, even were it of a purely hypothetical
kind. How strong, then, becomes the reason for believing it the true
one when we remember that the actions alleged demonstrably go on in
the way asserted. They are ever operating before our eyes; and that
they produce the effects in question is a conclusion deducible from
mechanical principles, a conclusion established by induction, and a
conclusion verified by experiment. These three orders of proof may be
briefly summed up as follows.

That plants which have to raise themselves above the earth’s surface,
and to withstand the actions of the wind, must have a power of
developing supporting structure, is an _à priori_ conclusion which may
be safely drawn. It is an equally safe _à priori_ conclusion, that if
the supporting structure, either as a whole or in any of its parts, has
to adapt itself to the particular strains which the individual plant is
subject to by its particular circumstances, there must be at work some
process by which the strength of the supporting structure is everywhere
brought into equilibrium with the forces it has to bear. Though the
typical distribution of supporting structure in each kind of plant may
be explained teleologically by those whom teleological explanations
satisfy; and though otherwise this typical distribution may be ascribed
to natural selection acting apart from any directly adaptive process;
yet it is manifest that those departures from the typical distribution
which fit the parts of each plant to their special conditions are
explicable neither teleologically nor by natural selection. We are,
therefore, compelled to admit that, if in each plant there goes on
a balancing of the particular strains by the particular strengths,
there must be a physical or physico-chemical process by which the
adjustments of the two are effected. Meanwhile we are equally compelled
to admit, _à priori_, that the mechanical actions to be resisted,
themselves affect the internal tissues in such ways as to further the
increase of that dense substance by which they are resisted. It is
demonstrable that bending the petioles, shoots, and stems must compress
the vessels beneath their surfaces, and increase the exudation of
nutritive matters from them, and must do this actively in proportion
as the bends are great and frequent; so that while, on the one hand,
it is a necessary deduction that, if the parts of each plant are to be
severally strengthened according to the several strains, there must
be some direct connexion between strains and strengths, it is, on the
other hand, a necessary deduction from mechanical principles that the
strains do act in such ways as to aid the increase of the strengths.
How a like correspondence between two _à priori_ arguments holds in
the case of the circulation, needs not to be shown in detail. It will
suffice to remind the reader that while the raising of sap to heights
beyond the limit of capillarity implies some force to effect it, we
have in the osmotic distention and the intermittent compressions caused
by transverse strains, forces which, under the conditions, cannot but
tend to effect it; and similarly with the requirement for a downward
current, and the production of a downward current.

Among the inductive proofs we find a kindred agreement. Different
individuals of the same species, and different parts of the same
individual, do strengthen in different degrees; and there is a clearly
traceable connexion between their strengthenings and the intermittent
strains they are exposed to. This evidence, derived from contrasts
between growths on the same plant or on plants of the same type, is
enforced by evidence derived from contrasts between plants of different
types. The deficiency of woody tissue which we see in plants called
succulent, is accompanied by a bulkiness of the parts which prevents
any considerable oscillations; and this character is also habitually
accompanied by a dwarfed growth. When, leaving these relations as
displayed externally, we examine them internally, we find the facts
uniting to show, by their agreements and differences, that between
the compression of the sap-canals and the production of wood there
is a direct relation. We have the facts, that in each plant, and in
every new part of each plant, the formation of sap-canals precedes
the formation of wood; that the deposit of woody matter, when it
begins, takes place around these sap-canals, and afterwards around
the new sap-canals successively developed; that this formation of
wood around the sap-canals takes place where the coats of the canals
are demonstrably permeable, and that the amount of wood formation is
proportionate to the permeability. And then that the permeability and
extravasation of sap occur wherever, in the individual or in the type,
there are intermittent compressions, is proved alike by ordinary cases
and by exceptional cases. In the one class of cases we see that the
deposit of wood round the vessels begins to take place when they come
into positions that subject them to intermittent compressions, while it
ceases when they become shielded from compressions. And in the other
class of cases, where, from the beginning, the vessels are shielded
from compression by surrounding fleshy tissue, there is a permanent
absence of wood formation.

To which complete agreement between the deductive and inductive
inferences has to be added the direct proof supplied by experiments. It
is put beyond doubt by experiment that the liquids absorbed by plants
are distributed to their different parts through their vessels--at
first by the spiral or allied vessels originally developed, and then by
the better-placed ducts formed later. By experiment it is demonstrated
that the intermittent compressions caused by oscillations urge the
sap along the vessels and ducts. And it is also experimentally proved
that the same intermittent compressions produce exudation of sap from
vessels and ducts into the surrounding tissue.

That the processes here described, acting through all past time, have
sufficed of themselves to develope the supporting and distributing
structures of plants, is not alleged. What share the natural selection
of variations distinguished as spontaneous, has had in establishing
them, is a question which remains to be discussed. Whether acting
alone natural selection would have sufficed to evolve these vascular
and resisting tissues, I do not profess to say. That it has been a
co-operating cause, I take to be self-evident: it must all along
have furthered the action of any other cause, by preserving the
individuals on which such other cause had acted most favourably.
Seeing, however, the conclusive proof which we have that another cause
has been in action--certainly on individuals, and, in all probability,
by inheritance on races--we may most philosophically ascribe the
genesis of these internal structures to this cause, and regard natural
selection as having here played the part of an accelerator.


                         EXPLANATION OF PLATE.

Fig. 1. Absorbent organ from the leaf of _Euphorbia neriifolia_.
The cluster of fibrous cells forming one of the terminations of the
vascular system is here imbedded in a solid parenchyma.

Fig. 2. A structure of analogous kind from the leaf of _Ficus
elastica_. Here the expanded terminations of the vessels are imbedded
in the network-parenchyma, the cells of which unite to form envelopes
for them.

Fig. 3. Shows on a larger scale one of these absorbents from the leaf
of _Panax Lessonii_. In this figure is clearly seen the way in which
the cells of the network-parenchyma unite into a closely-fitting case
for the spiral cells.

Fig. 4. Represents a much more massive absorbent from the same leaf,
the surrounding tissues being omitted.

Fig. 5. Similarly represents, without its sheath, an absorbent from the
leaf of _Clusia flava_.

Fig. 6. End view of an absorbent organ from the root of a Turnip. It is
taken from the outermost layer of vessels. Its funnel-shaped interior
is drawn as it presents itself when looked at from the outside of this
layer, its narrow end being directed towards the centre of the Turnip.

Fig. 7. A longitudinal section through the axis of another such organ,
showing its annuli of reticulated cells when cut through. The cellular
tissue which fills the interior is supposed to be removed.

Fig. 8. A less developed absorbent, showing its approximate connexion
with a duct. In their simplest forms, these structures consist of only
two fenestrated cells, with their ends bent round so as to meet. Such
types occur in the central mass of the Turnip, where the vascular
system is relatively imperfect. Besides the comparatively regular forms
of these absorbents, there are forms composed of amorphous masses
of fenestrated cells. It should be added that both the regular and
irregular kinds are very variable in their numbers: in some turnips
they are abundant, and in others scarcely to be found. Possibly their
presence depends on the age of the Turnip. Judging from the period
during which my investigations were made, namely winter and early
spring, I suspect that they are developed only in preparation for
sending up the flower-stalk.

  [Illustration: Figs.1–8.]

Let me add that experiments on circulation in plants made during the
state of inactivity, when it is to be presumed that the vessels and
tissues contain but little gap, are much more successful than those
made in the summer. It would seem that when the tissues are fully
charged with sap the taking up of dyes is comparatively slow and the
above-described effects are not so easily demonstrable.

       *       *       *       *       *

[An expert writes concerning this essay:--“I have not attempted to
annotate critically this paper. There is no doubt that many of your
conclusions are perfectly sound, particularly those relating to the
passage of crude sap through the _cavities_ of the elements of the
wood, though the opinion that the actual passage was through the walls
very generally held till about 12 years ago.”]




                              APPENDIX D.

                 ON THE ORIGIN OF THE VERTEBRATE TYPE.


   [_When studying the development of the vertebrate skeleton,
   there occurred to me the following idea respecting the possible
   origin of the notochord. I was eventually led to omit the few
   pages of Appendix in which I had expressed this idea, because
   it was unsupported by developmental evidence. The developmental
   evidence recently discovered, however, has led Professor Haeckel
   and others to analogous views respecting the affiliation of the_
   Vertebrata _on the_ Molluscoida. _Having fortunately preserved a
   proof of the suppressed pages, I am able now to add them. With
   the omission of a superfluous paragraph, they are reprinted
   verbatim from this proof, which dates back to the autumn of
   1865, at which time the chapter on “The Shapes of Vertebrate
   Skeletons” was written._--December, 1869.]

The general argument contained in Chap. XVI. of Part IV., I have
thought it undesirable to implicate with any conception more
speculative than those essential to it; and to avoid so implicating it,
I transfer to this place an hypothesis respecting the derivation of the
rudimentary vertebrate structure, which appears to me worth considering.

Among those molluscoid animals with which the lowest vertebrate animal
has sundry traits in common, it very generally happens that while the
adult is stationary the larva is locomotive. The locomotion of the
larva is effected by the undulations of a tail. In shape and movement
one of these young Ascidians is not altogether unlike a Tadpole.
And as the tail of the Tadpole disappears when its function comes
to be fulfilled by limbs; so the Ascidian larva’s tail disappears
when fixation of the larva renders it useless. This disappearance of
the tail, however, is not without exception. The _Appendicularia_
is an Ascidian which retains its tail throughout life; and by its
aid continues throughout life to swim about. Now this tail of the
_Appendicularia_ has a very suggestive structure. It is long, tapering
to a point, and flattened. From end to end there runs a mid-rib, which
appears to be an imbedded gelatinous rod, not unlike a notochord.
Extending along the two sides of this mid-rib, are bundles of muscular
fibres; and its top bears a gangliated nervous thread, giving off, at
intervals, branches to the muscular fibres. In the _Appendicularia_
this tail, which is inserted at the lower part of the back, is bent
forwards, so as not to be adapted for propelling the body of the animal
head foremost; but the homologous tails of the larval Ascidians are
directed backwards, so as to produce forward movement. If we suppose
a type like the _Appendicularia_ in the structure and insertion of
its permanent tail, but resembling the larval forms in the direction
of its tail, it is, I think, not difficult to see that functional
adaptation joined with natural selection, might readily produce a
type approximating to that whose origin we are considering. It is a
fair assumption that an habitually-locomotive creature would profit
by increased power of locomotion. This granted, it follows that
such further development of the tail-structures as might arise from
enhanced function, and such better distribution of them as spontaneous
variation might from time to time initiate, would be perpetuated. What
must be the accompanying changes? The more vigorous action of such an
appendage implies a firmer insertion into the body; and this would be
effected by the prolongation forwards of the central axis of the tail
into the creature’s back. As fast as there progressed this fusion of
the increasingly-powerful tail with the body, the body would begin to
partake of its oscillations; and at the same time that the resistant
axis of the tail advanced along the dorsal region, its accompanying
muscular fibres would spread over the sides of the body: gradually
taking such modified directions and insertions as their new conditions
rendered most advantageous. Without further explanation, those who
examine drawings of the structures described, will, I think, see that
in such a way a tail homologous with that of the _Appendicularia_,
would be likely, in the course of that development required for its
greater efficiency, gradually to encroach on the body, until its
mid-rib became the dorsal axis, its gangliated nerve-thread the spinal
chord, and its muscular fibres the myocommata. Such a development of an
appendage into a dominant part of the organism, though at first sight
a startling supposition, is not without plenty of parallels: instance
the way in which the cerebral ganglia, originally mere adjuncts of the
spinal chord, eventually become the great centres of the nervous system
to which the spinal chord is quite subordinate; or instance the way in
which the limbs, small and inconspicuous in fishes, become, in Man,
masses which, taken together, outweigh the trunk. It may be added that
these familiar cases have a further appropriateness; for they exhibit
higher degrees of that same increasing dominance of the organs of
external relation, which the hypothesis itself implies.

Of course, if the rudimentary vertebrate apparatus thus grew into,
and spread over, a molluscoid visceral system, the formation of the
notochord under the action of alternating transverse strains, did not
take place as suggested in § 255; but it does not therefore follow
that its differentiation from surrounding tissues was not mechanically
initiated in the way described. For what was said in that section
respecting the effects of lateral bendings of the body, equally
applies to lateral bendings of the tail; and as fast as the developing
tail encroached on the body, the body would become implicated in the
transverse strains, and the differentiation would advance forwards
under the influences originally alleged. Obviously, too, though the
lateral muscular masses would in this case have a different history;
yet the segmentation of them would be eventually determined by the
assigned causes. For as fast as the strata of contractile fibres,
developing somewhat in advance of the dorsal axis, spread along the
sides, they would come under the influence of the alternate flexions;
and while, by survival of the fittest, their parts became adjusted
in direction, their segmentation would, as before, accompany their
increasing massiveness. The actions and reactions due to lateral
undulations would still, therefore, be the causes of differentiation,
with which natural selection would co-operate.




                             APPENDIX D 2.

                          THE ANNULOSE TYPE.


The production of a segmental structure by undulatory movements,
suggested in Appendix D, as also in B (first published in 1858) as
explaining the vertebral column, has been recently suggested by Prof.
Korschelt as the cause of that segmentation of the annulose type which
gives the name to it. He espouses a--

   “view which is based upon the assumption that at first an
   unsegmented, elongated ancestral form was produced by terminal
   growth, whereupon the entire body became separated at once into
   a large number of segments by a re-arrangement of the individual
   organs. This assumption is supported by the consideration that
   with the lateral sinuous movement of the body, and with the
   rigidity of the tissues caused by increasing differentiation,
   the formation of alternating regions of greater and less
   motility was of considerable advantage to the individual, and
   rendered possible a further elongation of the body. The first
   cause for the appearance of metameric segmentation would then be
   sought in the manner of locomotion and in mechanical conditions.
   However, this latter view is not supported in any way by
   embryology.” (_Embryology of Invertebrates_, Part I, pp. 349–50.)

I venture to think the confession that this view “is not supported in
any way by embryology” should be joined with the confession that it
is at variance with that abstract embryology which comprehends the
process of development in general. The assumption that there took
place “a re-arrangement of the individual organs” of “an unsegmented,
elongated ancestral form,” in such wise that the organs, previously
single, presently became multiple, so that instead of one organ of each
kind there were substituted many organs of each kind, is inconsistent
with the general law of evolution, organic and other--implies not
integration but disintegration. Everywhere the advance is from many
like parts performing like functions to relatively few unlike parts
performing unlike functions. The higher forms of the annulose type
itself show this. Compare a myriapod and a crab. In the one we have
not only a great number of similar segments bearing similar limbs,
but we have in each segment a dilatation of the main blood-vessel--a
rudimentary heart--a swollen portion of the nerve cord--a small
ganglion--and so on; whereas in the other, besides relatively few
segments and few limbs (sundry of them extremely unlike the rest) we
have a vascular system concentrated into a central heart with arteries
and a concentrated nervous system, such that the great ganglia in the
integrated carapace immensely subordinate the ganglia of the remaining
segments; and similarly with the other organs. Now unless it be denied
that these highest decapods have been evolved from low types akin to
myriapods in composition, it must be admitted that the progress has
been from a string of many like segments with similar sets of organs
to a group of relatively-few unlike segments with dissimilar sets of
organs. If so we cannot rationally deny that the progress has been
of this nature up from the lowest annelid, instead of having been,
as Prof. Korschelt’s hypothesis implies, of opposite nature at the
beginning.

In a preceding passage a clear recognition of the normal course of
development occurs. In opposing the view set forth in §§ 205–7 of this
work, Prof. Korschelt says:--

   “It seems scarcely favourable to this theory that the degree
   of independence which the individual segments present is
   comparatively slight. The most important organs (nervous system,
   body musculature, blood-vascular system) show themselves to be
   single fundaments of the entire body, and are also developed as
   such even though they also exhibit evidences of metamerism. Even
   the excretory canals may give up their segmental isolation and
   become united to one another by means of longitudinal canals.”
   (_Ib._ p. 348.)

On turning back to § 206, the reader will, I think, demur to the
assertion that the independence is “comparatively slight”; seeing
that, as in _Ctenodrilus_, a single segment sometimes becomes separate
and reproduces other segments to form a new series. Instead of
admitting that “the most important organs” “show themselves to be
single fundaments of the entire body,” it may be held, contrariwise,
that their original independence in each segment is masked only to
the degree involved by their co-operation as parts of a compound
organism. But chiefly I remark that when it is said that “the excretory
canals may give up their segmental isolation and become united” by
“longitudinal canals,” there is a clear confession that the isolation
of these organs was original and their union superinduced--an
implication that the course of evolution is as I have described it, and
at variance with the course of evolution assumed by Prof. Korschelt.

Yet another incongruity is involved in his interpretation. He writes:--

   “Just as in the consideration of the tapeworm chain we were
   induced by the comparison with unsegmented forms to refer the
   entire chain to an unsegmented individual, and, on the other
   hand, to see in the proglottis, not a complete individual, but
   only the abstricted hinder portion of the body of the Cestode,
   in the same manner, and with much more reason, we adhere to the
   individuality of the Annelid body.” (P. 349.)

And then on the preceding page, referring to the composition of the
Annelid body, he says:--“The most natural comparisons are those
with the tapeworm chain and with the strobila of the Scyphomedusæ.”
Now since it is here assumed that the tapeworm and the strobila are
analogous in composition, it is implied that the detached proglottis
and the detached medusa are analogous; and hence if we are to regard
the proglottis as “not a complete individual but only the abstricted
hinder portion of the body of the Cestode,” then we must similarly
regard the medusa as not a complete individual, but only the abstricted
hinder portion of the strobila. This commits us to the strange
conclusion that whereas individuality is ascribed to the original
simple polyp, and by and by to the partially-segmented strobila, though
these are without special senses and with only rudiments of muscular
and nervous systems, individuality is denied to the detached medusa,
which has organs of sense, a distinct nervo-muscular system and a
considerable power of locomotion, as well as a generative system:
traits which in other cases characterize developed individuals. Here
also, then, there seems to be an inversion of the ordinary conception.

This conception of the proglottis and the medusa is, I see, accepted by
some as tenable. But if we accept it we must accept also an analogous
conception, which will I think be regarded as untenable. It is that
supplied by the _Aphides_. From an egg proceeds a series of sexless and
wingless females, and at the end of the series there come winged males
and females with resulting gamic reproduction. If instead of forming
a discrete series the imperfect females formed a concrete series,
the members of which could individually feed without being detached
from one another, as the segments of a tapeworm can, the parallelism
would be complete; and then, according to the view in question,
we should have to regard the perfect males and females eventually
arising, not as individuals but as terminal portions of the series,
containing generative products and having wings for the dispersion of
them--locomotive egg-bearing segments of the chain. Whoever espouses
this view must hold either that the first imperfect female of the
series was the individual or that the entire string of them constituted
the individual (in conformity with a view once propounded by Prof.
Huxley). But he must do more than this. Since the _Aphides_ have
descended from some winged species of the order _Hemiptera_, he must
hold that among those remote ancestors each particular fly, male or
female, was an individual; but that when abundant food and inert life
led to the partheno-genetic habit, and to chains of sexless forms, the
males and females eventually produced at the end of each chain, though,
like their remote ancestors, possessed of procreative organs and wings,
are not individuals.

[Some memoranda bearing on the question here discussed, mislaid at the
time when the chapter dealing with it was revised, have been discovered
in time for utilization in this appendix.]

One of my critics says:--

   “You have overstated the case in your favour: the alimentary
   canal does not, as you suggest, show a segmentation
   corresponding to that of the other organs in Annelids. Either
   it is a simple uniform tube, or else its differentiations
   (pharynx, œsophagus, crop, intestine) are quite independent of
   the repetition of the somites.”

In presence of statements made in works of authority, this objection
greatly surprises me. I meet with the descriptive word “moniliform”
applied to the intestine in some Annelids, and then in the Text Book of
Claus, translated and edited by Sedgwick, it is said, concerning the
alimentary canal in the _Annelida_:--

   “This is followed by the gastric region of the gut, which
   occupies the greatest portion of the length of the body, and
   is either regularly constricted in correspondence with the
   segments, or possesses lateral diverticula.” (P. 365.)

And again on p. 369 it is said:--

   “The intestine usually preserves the same structure in its
   entire length and is divided by regular constrictions into
   a number of divisions or chambers, which correspond to the
   segments and dilate again into lateral diverticula and cæca.”

The alimentary canal thus presents the segmental character as clearly
as consists with fulfilment of its function. If the successive segments
are co-operating units of a compound animal having but one mouth,
then, necessarily, the gut cannot be completely cut into parts, each
answering to a segment, for there could be, in that case, no passage
for the food. If the portion of the intestine belonging to each segment
has a conspicuous dilatation, or has a cæcum on each side, it exhibits
the segmental character as much as the physical requirements permit. So
far from being at variance with the hypothesis, its structure exhibits
a verification of it.

The next objection runs as follows:--

   “Then, again, the ovaries and testes do not exhibit a
   corresponding segmentation. When it is allowable to speak of
   ovary or testis at all as in _Lumbricus_, we find that in
   the case of both organs we have at most two pairs.”

It seems to me that the distribution of the generative organs in
a comparatively-developed member of the Annelid type, is not the
question. We have to ask what it is in undeveloped members of that
type. Among them the repetition of generative parts is in some cases
just what the theory implies. Thus in Claus I read:--“In the marine
_Chætopoda_, the ova or spermatozoa originate on the body-wall from
cells of the peritoneal membrane, either in the anterior segments
alone or along the whole length of the body.” So that in these last
cases there are, in all the segments, parts from which arise generative
products. The fact that these parts are not definite ovaries and testes
is irrelevant. Ovaries and testes are developed generative structures,
and in the order of evolution are preceded by undeveloped ones; and the
fact that these undeveloped ones are found in little-developed members
of the type conforms perfectly to the hypothesis. [I may remark in
passing that here is a good illustration of that process of evolution
which, in the above speculation of Prof. Korschelt, is supposed to be
inverted: many dispersed, similar, and indefinite parts, are integrated
into a few localized and definite parts.]

In continuation the critic above quoted says:--“My position is that
the repetition of segments in an Annelid is a phenomenon of the same
nature as the repetition of hairs in a Mammal or of scutes in a
Reptile”, and he proceeds to give instances of repetitions of organs
in other types, as of the reproductive structures and excretory
system in the young Dog-fish or of the ovaries in _Amphioxus_. These
examples do not seem to me relevant. No parallelism exists between
the repetition of a particular organ in an animal, and the repetition
of an entire cluster of organs constituting a physiological whole.
The repetitions of the ovaries in _Amphioxus_ and of the excretory
system in a young Dog-fish, occur without threatening to divide into
similar parts the entire organism. But the segmental repetitions in
an annulose creature implicate the structures at large, and would, if
pushed a little further, result in separate creatures. The segment
of a low Annelid contains alimentary, vascular, nervous, excretory,
reproductive, sensory and locomotive organs--all the organs required
for carrying on life, save certain organs of external relation which
its position excludes. When there is shown some vertebrate animal, or
proto-vertebrate animal, that is divisible into parts each of which is
in great measure physiologically independent, I shall feel obliged to
abandon my position.

       *       *       *       *       *

While this appendix is in hand I have received from another expert,
whose view is in general agreement with my own, a letter containing the
following passage:--

   “You will see that Dohrn’s theory was the antithesis of your
   own view of vertebrate structure, namely that the vertebræ
   were formed by the segmentation, from mechanical causes of a
   body originally simple. This view of yours has been confirmed
   by later researches, which have shown that the most primitive
   forms allied to the Vertebrates, possessing the essential
   organs, viz., gill-slits, notochord, and dorsal nerve cord,
   are not segmented animals, like Annelids and _Crustacea_, but
   simple animals, having at most three regions, not exactly
   corresponding to segments. These primitive unsegmented forms are
   Ascidian tadpoles, _Balanoglossus_, and certain other primitive
   forms. The embryology of Vertebrates also proves that they are
   originally simple and not segmented animals, especially the
   fact that there is originally one pronephric duct or primitive
   kidney.”

Nevertheless there survives a leaning towards the notion of a segmental
origin of the _Vertebrata_. But the repetitions of organs named in
support of this notion have, I think, no more relation to the genesis
of the vertebrate type than the multiplication of vertebræ in a snake
has relation to the genesis of the vertebral column.




                              APPENDIX E.

                THE SHAPES AND ARRANGEMENTS OF FLOWERS.


In Part IV., Chapter X., under the title of “The Shapes of Flowers,” I
have, after describing their several kinds of symmetry, as habitually
related to their positions, made some remarks by way of interpretation.
The truth that flowers exhibit a radial symmetry when they are so
placed as to be equally affected all round by incident forces, having
been exemplified, and also the truth that they assume a bilateral
symmetry when they are so placed that their two sides are conditioned
in ways different from the ways in which their upper and lower
parts are conditioned; I have gone on to inquire (in § 234) by what
causes such modifications of form are produced. I have stated that,
originally, I inclined to ascribe them entirely to differences in the
relations of the parts to physical forces--light, heat, gravitation,
etc.; but that I found sundry facts stood in the way of this
interpretation. And I have said that “Mr. Darwin’s investigations into
the fertilization of Orchids led me to take into account an unnoticed
agency.” Continuing to recognize the physical forces as factors having
some influence, I have concluded that the most important factor is
the action of insects; which, aiding most the fertilization of those
flowers which most facilitate their entrance, produce, in course of
generations, a form of flower specially adapted to the special position.

Though still adhering to this interpretation, I have since found reason
to think that the original interpretation contains a larger portion
of truth than I supposed at the time when I was led thus to revise
it. While staying at Mürren, in Switzerland, in 1872, I observed
some modifications in a species of Gentian, which proved to me that
the action of incident physical forces on flowers is, in some cases,
very rapid and decided. The species furnishing this evidence was the
_Gentiana Asclepiadea_; which I found in a copse formed of bushes
that were here wide apart and there close together. In some places not
near to the bushes, the individuals of the species grew vertically; in
other places, partially shaded, their inclined shoots curved in such
directions as to get the most light; and in other cases their shoots
were led to take directions almost or quite horizontal. That, along
with these modifications in the directions of their shoots, there went
adjustments in the attitudes of their leaves, was a fact not specially
worthy of remark; for plants placed inside the windows of houses
habitually show us that leaves quickly bend themselves into attitudes
giving them the greatest amounts of light. But the fact which attracted
my attention was, that the flowers changed their attitudes in an
equally-marked manner. The radial distribution passed into a bilateral
distribution with the greatest readiness. Comparison of the annexed
figures will show the character of this change.

Figure I. represents part of a vertically-growing shoot. This
belonged to an individual growing unimpeded by bushes, and getting
light on all sides. Here it is observable that the pairs of leaves,
placed alternately in directions transverse to one another--one pair
pointing, say, north and south, and the next pair pointing east and
west--maintain, taking them in the aggregate, a radial distribution;
and it is also observable that the alternate pairs of flowers are
similarly arranged.

Figure II. is a sketch from a shoot which leaned towards one side,
and of which the higher part, as it bent more and more, got its upper
side more and more differently conditioned from its lower side. Here
we find that not only the leaves, but also the flowers, have adjusted
themselves to the changed conditions. The leaves of the lowest pair
hang out in the normal way, on the opposite sides of the axis, so that
a plane passing through their surfaces will cut the axis transversely;
and their two axillary flower-buds, _c_ and _d_, are similarly placed
on opposite sides of the axis. But at the other part of the shoot, we
see both that the leaves have adjusted themselves so that their planes,
no longer cutting the axis transversely, keep a fit adjustment with
respect to the light; and also that the flowers, no longer on opposite
sides of the axis, have bent round to the upper side, as at _a_ and _b_.

Figure III. shows us this re-arrangement carried still further. The
shoot it represents was growing in a direction nearly horizontal, and
therefore receiving the light only on one side. And here, besides
seeing that the leaves have so adjusted themselves that they all lie in
approximately the same plane, which is parallel to the axis instead of
transverse to it, we see that the two pairs of flower-buds have both
come round to the upper side of the axis. So that in this shoot, the
original radial symmetry in the arrangement of leaves and flowers, is
completely changed into a bilateral symmetry.

  [Illustration: Figs. 1–3.]

These facts do not, it is true, prove any modification in the forms
of the flowers themselves: they only prove modification in the grouping
of the flowers. But beyond showing, as they do conclusively, how
readily a bilateral arrangement of flowers is producible out of an
arrangement that was not bilateral, by the action of light, etc.;
they give increased probability to the belief that changes in the
shapes of flowers are producible by the same agencies. Doubtless this
change in the attitudes of the flower-buds is due to the action of
light on their calyces and peduncles more than to its action on their
unfolding corollas. But along with an action so decided on the growth
of these sheathing and supporting organs containing chlorophyll, it
is scarcely probable that there is _no_ action on the growth of the
petals, containing other colouring matter; considering that in both
cases the development of the colouring matter depends on the action of
light, and considering also the effect of light on petals, familiarly
shown by their opening and closing. And if even but a small effect is
producible on the growth of the corolla, then it is to be expected that
light will be an agent in changing the form of the corolla, when the
attitude of the flower causes its parts to be differently exposed. For
a small effect on the individual flower will become a great effect in
the flowers of remote descendants; provided the changed attitudes of
the flowers preserve considerable constancy throughout the succession
of individuals.

Be this as it may, however, the facts I have here described, which
I doubt not other observers have seen paralleled in other plants,
are instructive, as showing how quickly certain metamorphoses are
produced, and as implying the easy establishment of such metamorphoses
as permanent characters in a species, if the modifying conditions
become permanent. The changes of arrangement I have pointed out, do not
become permanent in this species because its individuals are variously
affected by the modifying forces: on some they do not act at all,
on some a little, on some much; and even on the same individual the
different shoots are quite differently affected. But if the habit of
this plant were greatly changed--if, for instance, by spreading into
habitats yielding abundant nutriment, the plant became very luxuriant,
and, multiplying its branches, grew shrub-like; it is clear that, being
shaded by one another, these branches would be habitually circumstanced
in a way like that which we here see produces bilateralness in the
distribution of the flowers, if not in the flowers themselves; and
being thus permanently affected, would become permanently bilateral.
Accumulating by inheritance, what is here only an individual
peculiarity, would become a peculiarity of the species--a specific
character.




                              APPENDIX F.

               PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS.


There has recently come before me a fact which has a significant
bearing on the hypothesis of Constitutional units: serving, indeed, to
give an apparently conclusive proof of its truth. Before stating it,
however, I may with advantage re-state the several evidences already
assigned in support of it.

       *       *       *       *       *

1. First comes the _à priori_ reason. These units in the germ of an
organism which cause development into a special structure, cannot be
_chemical_ units--cannot be simply molecules of proteid substance in
one or other of its forms; since these are not special to any type of
creature but common to all creatures. Nor can they be what we may call
_morphological_ units--the cells or protoplasts; because in the early
stages of development the cells of one organism are indistinguishable
from those of others, and because were cells the units of composition
there could be no interpretation of what are called unicellular
organisms--nothing to account for the innumerable varieties of them.
Hence, of necessity, the structural elements of which each organism is
built, being neither proteid molecules nor cells, must be something
between them: probably some complex combination of different isomeric
forms of proteids.

       *       *       *       *       *

2. That units of such natures are the essential components of each
species of organism, is shown by the fact that in low types of
creatures, little differentiated into special tissues, any considerable
portion of the body will, when separated, begin to assume the structure
proper to the species--a truth recently shown afresh by Prof. T.
H. Morgan’s experiments on the regeneration of _Planaria maculata_
(already referred to in § 206) showing that various fragments cut out
develop into new individuals, and that when, being too small they die
before doing this, there is always an abortive attempt to assume the
specific structure.

       *       *       *       *       *

3. This truth that a portion of undifferentiated tissue, if adequate in
quantity, assumes the structure of the type, illustrating as it does
the proclivity of the constitutional units towards the structure of
the species, allies itself with the phenomena of both agamogenesis and
gamogenesis. The first of these shows us how a fissiparously-detached
portion of the parental tissue takes on the same form as the parent;
and the second shows how those small detached portions distinguished
as sperm-cell and germ-cell also, when united and supplied with the
needful materials, do the same thing.

       *       *       *       *       *

4. But the set of phenomena following the union of sperm-cell and
germ-cell differ in a certain way from those which follow when a
_gemma_ or other unfertilized portion of parental tissue is detached.
The incomprehensibleness of this difference as otherwise contemplated,
and the partial comprehensibleness of it when joined with the
hypothesis of physiological units, furnish a further support for the
hypothesis.

The familiar truth learnt by the tyro in algebra that an apparent
solution which contains the unknown quantity is no solution, is a
truth apt to be overlooked in other spheres than the algebraic. An
illustration is supplied by the answer once given in Parliament to the
question “What is an Archdeacon?”--“One who discharges archidiaconal
functions.” But science as well as daily life furnishes examples.
When it is said by Engelmann, Hensen, Hertwig, and Maupas that “the
essential end of sexuality is rejuvenescence, that is, the restoration
of growth-energy,” we have another instance of an explanation which
explains nothing. What is the phenomenon to be explained? That
unfolding of an organism from a germ which displays growth-energy. And
what is the explanation? The giving of fresh growth-energy. The unknown
quantity “growth-energy” is contained in the explanation proposed.
There exists no conception of “juvenescence” save that derived from
observing developing plants and animals; and if “re” be prefixed, no
interpretation is thereby given to the unexplained thing “juvenescence.”

Coleridge somewhere comments on a source of fallacy which he calls the
“hypostasis of a relation”--the changing of a relation into a thing.
The plumber who tells you that water rises in a pump “by suction”
supplies an instance. Having assumed suction to be an agent, he
thinks that he understands how the piston does its work. Some of the
explanations given of fertilization supply further instances. When it
is said that sexual union has for its end “to give increased vigour
to all the vital processes,” it is tacitly implied that vigour is a
something--a something which can be given. But now, in the first place,
it is only by the hypostasis of a relation that we are led to think
of vigour as a thing. Vigour is a state--that state of a living body
which enables it to give out much motion. What enables it to do this?
The presence in it of abundant molecules containing much molecular
motion which can be transformed into molar motion: the transformation
being effected by the falling of these molecules into their simpler
and relatively-inert components, which are thereupon excreted.
Energy-containing matter is used up, and more energy or vigour can be
given only by supplying more such matter. How then can the union of
two nuclei--those of the sperm-cell and germ-cell--give vigour? Only
an infinitesimal portion of vigour in the sense above explained exists
in either, and the union of them leaves it still infinitesimal. And
then, even supposing the vigour to be an entity and to be appreciable
in quantity, how could it go on producing that immense combination
of physiological actions seen in the unfolding of the germ into an
organism? and how could it go on producing the physiological actions of
an adult organism during a whole century?

May we not then say that these proposed explanations leave the question
where it was--are nominal solutions, not real solutions?

       *       *       *       *       *

5. But the hypothesis of constitutional units furnishes, if not a
satisfactory answer yet, something in the nature of an answer--a true
cause; that is to say, a cause actually known to us as operating in
other cases. In § 92 it was pointed out that in proportion as units
are similar, there may be built up from them an aggregate which is
relatively stable, and that along with increasing dissimilarity the
stability of the aggregate decreases. It was inferred that if a group
of constitutional units belonging to one individual which have become
moulded into relatively exact congruity with the organism and with
one another by long co-operation, are mingled with some belonging
to another individual which, differently circumstanced, has become
somewhat different in itself and in its units, then the mass formed by
the union of the two groups will be relatively unstable--relatively
modifiable by incident forces. Whereas in either organism, no longer
perpetually changed in the relations of its parts by growth, there is
an approach towards equilibrium between the whole and its components,
the components contributed by the two to form a germ, being slightly
unlike one another, will not form a group in a state of equilibrium.
The group they form will be capable of easy change by incident forces;
and they will so be rendered free to follow their proclivities towards
the typical form of the species. Inferring this we must also infer that
so long as these two sets of slightly different units are not exposed
to any constant forces tending to coerce them into the same form, there
will continue to exist in the nuclei of all descendant cells this same
relative instability and consequent plasticity.

Such evidence as we have verifies this interpretation. There is
first the universal fact that development of the germ begins when
it is exposed to an incident force--heat--the undulations of which,
increasing the oscillations of the mixed units, give them greater
freedom to arrange themselves in conformity with their type. We see
this alike when spring warmth makes a seed germinate and when the
warmth of a sitting hen sets up organization in her eggs. Heat frees
the molecules of inorganic matter from local restraints and, as we see
in molten metal, lets them yield to other forces; and similarly in this
organic matter, the units are made free to follow their proclivities.
Then, secondly, there comes the evidence from comparisons between the
effects of mixing constitutional units differing in various degrees.
Let the cluster of mixed units be derived from animals that are
ordinally distinct. Nothing happens. The units each contributes tend to
arrange themselves after the parental type. Hence a conflict between
the tendencies towards two markedly unlike structures, and no structure
arises. Suppose the mixed units come from two kindred species--say
horse and ass. The structures which they respectively tend to form,
being in their main characters alike, there is such co-operation
as produces a working organism but an organism in certain respects
imperfect--a mule. Suppose, again, the units come from two varieties
of the same species. A perfect organism results, and, as shown by Mr.
Darwin when detailing the effects of crossing, an unusually vigorous
organism. The units being more unlike than those belonging to the same
variety, the instability of the germ-plasm is unusually great, and
the transformations which constitute development and action become
unusually active. When, as in ordinary cases, the units are supplied by
members of the same variety who have not been made very much alike by
their antecedents, there follows the usual amount of organic vigour.
Coming now to the results of breeding in-and-in--breeding between
individuals whose constitutions (_i.e._ constitutional units) have for
generations been growing more alike in the absence of crossing with
other stirps--we see that diminution of organic vigour is displayed:
there is a decrease in the rate of physiological change. Finally, on
coming to a closer relationship, as in marriages between cousins, in
whom the constitutional units are more than commonly alike, we see
there frequently follows either barrenness or the production of feeble
offspring.

All these facts, then, are congruous with the hypothesis that the
use of fertilization is the mixing of unlike units, and consequent
production of plasticity. Leaving out cases in which the unlikenesses
are so great as wholly to prevent co-operation among the units, the
degree of vigour, that is, the activity of physiological change, is
great where the unlikeness is great and diminishes with the approach
towards likeness.

       *       *       *       *       *

6. The existence of constitutional units seems otherwise necessarily
implied. I refer to the fact that no organism is a homogeneous mean
between its parents but consists of a mixture of parts, some following
one parent and some the other. Among illustrations of this the most
conspicuous are those yielded by the variously-mixed colours of
hair or feathers. Horses, cattle, dogs, cats, hens, pigeons display
these mixtures: colours in one place like the mother and in another
place like the father. As the internal organs are invisible, and as
visible organs have indefinite shapes and graduate indefinitely into
adjacent ones, the mixture of traits is elsewhere less conspicuous; but
occasional marked cases (especially in malformations) leave no doubt
that it pervades the entire organism.

This peculiarity of transmission seems necessarily to imply that there
are distinct units derived from the two parents, and that in the course
of development there is more or less segregation of them--those of
the one origin predominating so far in some places as to give special
likeness to one parent, and those derived from the other doing the like
in other places. All which interpretation is impossible unless the
hypothesis of constitutional units be admitted.

       *       *       *       *       *

7. I come at length to the special evidence referred to at the outset.
It is evidence of the same nature as that just assigned, but carried
to a higher stage. It is furnished not by the segregation of traits
derived from two parents of the same variety, but is furnished by the
segregation of traits derived from parents of different varieties. In
articles on “Bud Variations or Sports” (_Gardener’s Chronicle_, 1891)
Dr. Masters gives various examples of the separation or unmixing of
ancestral constitutions. Mr. Noble formed a hybrid between _Clematis
Jackmani_ and _C. patens_. One of these varieties flowers in the autumn
on new wood, while the other flowers in the spring on old wood; and
the result is that flowers of two kinds, quite unlike, are produced at
different parts of the year, and that by pruning so as to cut away one
or other set of shoots, the plant may be made to produce exclusively
for the time being one or other sort of flower.

   “Another very interesting case of unmixing, or, if it be
   preferred, of partial mixture, is afforded by Neubert’s
   Berberis. This is a hybrid between the evergreen pinnate-leaved
   Mahonia and the deciduous simple-leaved Berberis vulgaris, and
   it bears leaves some of which are intermediate in appearance,
   while others are much like those of one or other of its parents.

   “A not uncommon illustration of a similar kind, is the
   production of a Peach and a Nectarine on the same branch, and we
   have just learnt from Canon Ellacombe that some of the Berlin
   Hellebores show evidence of their hybrid nature by occasionally
   producing foliage [and flowers?] of the two parents separately
   from the same root-stock.

   “In addition to the cases given above, we may here cite a few
   more which have come under our notice, such as a Chrysanthemum,
   half the florets of which are of one colour, half of another.
   A hybrid Calanthe, showing a similar piebald variation, is
   shown in Fig. 14. A very curious case was that of the Narcissus
   received from Mr. Walker, and in which flowers of two distinct
   varieties sprang from the same bulb. Grapes not uncommonly show
   their crossed origin by presenting a striped appearance, one
   stripe being of one colour, one of another, as may also be seen
   in the Orange, Apple, Lemon, and Currant.”

Thus, however the germ-plasm is constituted its essential components
cannot be all alike. Before there can be this dissociation of ancestral
characters, there must be in the germ-plasm different elements capable
of being dissociated. This single fact seems to compel us to assume
constitutional units.




                              APPENDIX G.

         THE INHERITANCE OF FUNCTIONALLY-CAUSED MODIFICATIONS.


In Part II, Chapter X^A, I have confessed that the process by which
a structure changed by use or disuse affects the sperm-cells or
germ-cells whence arise descendants, is unimaginable: without, however,
inferring that therefore such a process does not exist. With others
it seems different. Some three years ago the following expression of
opinion came to me from a zoological expert:--

   “Many zoologists--most of us here at Cambridge--are intensely
   opposed to the doctrine of the inheritability of acquired
   variations. Even assuming that the developmental power of a
   germ is determined by its molecular structure (and I for one
   would question this--Driesch and his school when they find that
   they can squeeze a developing egg into all sorts of shapes
   without altering the final result, that one blastomere in an
   egg which has divided into 8 is still able to reproduce a whole
   embryo--question it also), we still fail to conceive any means
   by which, for instance, a change in the development of a muscle
   or nerve can effect a corresponding change in that part of the
   germ which is destined to produce a corresponding part in the
   descendant.”

Here it will be observed that belief in the inheritance of structural
effects wrought by use and disuse, is rejected because of inability “to
conceive any means” by which the modifications produced in an organ can
effect a correlated modification in the germ of a descendant: failure
to conceive is the test. The implication is that some alternative
hypothesis is accepted because the correlating of a variation in an
organ with a corresponding germ-variation is effected by a means which
_is_ conceivable. This is the hypothesis of Weismann. Concerning its
conceivability I have, in the chapter just named, already written as
follows:--

   “If we follow Prof. Weismann we are led into an astounding
   supposition. He admits that every variable part must have a
   special determinant, and that this results in the assumption of
   over two hundred thousand for the four wings of a butterfly.
   Let us ask what must happen in the case of a peacock’s feather.
   On looking at the eye near its end, we see that the minute
   processes on the edge of each lateral thread must have been in
   some way exactly adjusted, in colour and position, so as to fall
   into line with the processes on adjacent threads: otherwise the
   symmetrical arrangement of coloured rings would be impossible.
   Each of these processes, then, being an independent variable,
   must have had its particular determinant. Now there are about
   300 threads on the shaft of a large feather, and each of them
   bears on the average 1,600 processes, making for the whole
   feather 480,000 of these processes. For one feather alone there
   must have been 480,000 determinants, and for the whole tail
   many millions. And these, along with the determinants for the
   detailed parts of all the other feathers, and for the variable
   components of all organs forming the body at large, must have
   been contained in the microscopic head of a spermatozoon!” [And
   each of them must, throughout all the complex developmental
   processes, have preserved the ability to find its way to the
   exact place where it was wanted!]

If my Cambridge correspondent is able to conceive this process implied
by the hypothesis of Weismann, I can only say that he has an enviable
power of imagination.

       *       *       *       *       *

But now comes the strange fact that an impossibility of thought implied
by Weismann’s hypothesis does not cause rejection of it, but yet is
urged as a reason for rejecting an alternative hypothesis which does
not imply it. One objector cannot conceive that “a change in the
development of a muscle or nerve can effect a corresponding change in
that part of the germ which is destined to _produce a corresponding
part_ in the descendant”; and another objector says it is “very hard to
believe” that a functionally-changed organ will so affect spermatozoa
and ova that “_one particular part of them_ will be so altered that
the organisms which grow up from them will be able to present the
same modification on the application of a different stimulus.” It is
tacitly assumed by both that, as in the hypothesis of Weismann so in
the counter-hypothesis, a particular part of the germ-plasm gives
origin to a particular part of the developed organism. But nothing of
the kind is implied. The nature of the counter-hypothesis (at any rate
as held by me) is entirely misapprehended. Anyone who turns back to
the chapters in the first volume where the conception of physiological
units (or constitutional units) was set forth, or who re-reads the
foregoing appendix, will see that there is altogether excluded any idea
of correlation between certain parts of the germ and certain parts of
the resulting organism. The units are supposed to be all alike, and
during the progressive embryological changes local groups of them are
supposed to take on different forms and structures under the combined
forces, general and local, brought to bear on them. This conception
is necessitated by all the evidence. The fact disclosed by the
experiments of Driesch, Wilson, and Chabry, that from fractions of an
ovum structures may be obtained like that obtained from the whole ovum,
only smaller, necessitates it. The fact that any sufficiently large
fragment of a polyp or planarian, no matter from what part of the body
taken, will develop into a complete polyp or planarian necessitates
it. The fact that from an undifferentiated portion of a plant, even
so small as a scale, a complete plant may arise necessitates it. And
it is necessitated by the fact that among plants, roots are produced
by imbedded shoots and shoots by roots, as well as by the fact that
low animals, such as hydroids, if deprived of both head and root,
will develop a head from the root part and a root from the head
part, if their respective conditions are inverted. All this evidence
shows conclusively that the component units of each species, whether
existing in the germ or in the developed organism, are, when not yet
differentiated by local conditions, all alike, and that the notion of
special parts of the germ-plasm correlated with special parts of the
resulting organism, is entirely alien to the hypothesis.

“But how do the units of a modified organ affect the units of the
germ in such wise that these produce an inherited modification of
the organ?” will be asked. This difficulty has been dealt with in
§§ 97_d_, 97_e_, where the analogy between the social organism and
the individual organism has been brought in aid: serving, if not to
furnish a conception, yet to furnish an adumbration. Regarding citizens
as the units of an unfolding society, say a colony, it was pointed
out that the nature they inherit from a mother-society gives them a
proclivity towards a society of like structure, the traits of which
are progressively assumed as the colony grows sufficiently large to
make them possible. At the same time it was pointed out that while the
influence of the entire aggregate on the individuals is seen in this
forming of them into a society of the inherited type, the influences of
local circumstances, and of individuals on one another, in each group,
make them differentiate into appropriate social structures, taking on
fit occupations and industries: the implication being that in virtue
of their inherited natures they all have partial capacities for the
various activities they undertake; so that an immigrant clerk sets up
a tavern, a compositor takes to carpentering, and a university man
rides after cattle or is employed on a sheep farm. Evidence was given
in that place, as in the above paragraph, that the constitutional units
of an organism similarly have all of them potentialities for taking
on this or that structure and mode of action which local conditions
determine. It was further argued that as citizens are continually
being remoulded by their society into congruity with it, and, if
circumstances change them, tend to remould their society; so in the
individual organism, there is this reciprocal action of the whole on
the units and of the units on the whole. Hence it was inferred that the
modified units in any modified part tend to diffuse modifications like
their own through the units at large: being aided by the circulation of
protoplasm, as suggested in §§ 54_d_ and 97_f_. And it was urged that,
however inconceivably complex such a process may be, yet it seems not
incredible when we recognise the probability that an organism is more
or less permeable to undulations propagated by its molecules: Rontgen
rays giving warrant. If such units throughout the tissues may take in
and send out ethereal waves which bring it into rhythmical relations
with others of its kind and tend to produce congruity, it becomes,
if not conceivable still supposable, that throughout the circulating
protoplasm there goes on a continual harmonization of its components--a
moulding of each by all and of all by each. Should it be said that such
a process is too marvellous to be reasonably assumed, the reply is that
it is not more marvellous than heredity itself, which, were it not
familiar to us, would be thought incredible.

       *       *       *       *       *

But as I have said in the place referred to--“At last then we are
obliged to admit that the actual organizing process transcends
conception. It is not enough to say that we cannot know it; we must say
that we cannot even conceive it:” can only conceive the possibility of
a suggested interpretation.

Hence we have to rely upon evidences of other kinds. Among these, some
which I think dispose absolutely of the fashionable hypothesis while
they harmonize with the opposed hypothesis, have now to be named. That
their implication should not have been generally recognized would have
seemed to me incomprehensible were it not that I have myself only now
observed this implication. The facts are these:--

   “Verlot mentions a gardener who could distinguish 150 kinds
   of camellia, when not in flower; and it has been positively
   asserted that the famous old Dutch florist Voorhelm, who
   kept above 1,200 varieties of the hyacinth, was hardly ever
   deceived in knowing each variety by the bulb alone. Hence we
   must conclude that the bulbs of the hyacinth and the branches
   and leaves of the camellia, though appearing to an unpractised
   eye absolutely undistinguishable, yet really differ.” (Darwin,
   _Variation of Animals and Plants, &c._, vol. ii, p. 251.)

More recently testimony to like effect has been given by Dr. Maxwell
Masters, and has already been quoted by me in a note to § 286 in
illustration of another truth. He says concerning such variations:--

   “To the untrained eye, the primordial differences noted are
   often very slight; even the botanist, unless his attention
   be specially directed to the matter, fails to see minute
   differences which are perceptible enough to the raiser or his
   workmen.... These apparently trifling morphological differences
   are often associated with physiological variations which render
   some varieties, say of wheat, much better enabled to resist
   mildew and disease generally than others. Some, again, prove to
   be better adapted for certain soils or for some climates than
   others; some are less liable to injury from predatory birds than
   others, and so on.”

In his _Vegetable Teratology_, p. 493, Dr. Masters names another
fact having a like implication--the fact that among seedling stocks
which have not yet flowered, those which will produce double flowers
are distinguishable. He says:--

   “This separation of the single from the double-flowered plants,
   M. Chatié tells us is not so difficult as might be supposed. The
   single stocks, he explains, have deep green leaves (glabrous in
   certain species), rounded at the top, the heart being in the
   form of a shuttlecock, and the plant stout and thick-set in its
   general aspect, while the plants yielding double flowers have
   very long leaves of a light green colour, hairy and curled at
   the edges, the heart consisting of whitish leaves, curved so
   that they enclose it completely.”

What is the general truth implied? Clearly that there exists no such
thing as an independent local variation. Some marked change in the form
or colour of a flower or a fruit draws attention; and, being a change
which interests the florist or gardener, pecuniarily or otherwise,
not only draws attention but usually monopolizes attention: the
natural impression produced being that this variation stands there by
itself--is without relation to variations elsewhere. But now it turns
out that there are concomitant variations all over the plant. Even in
underground bulbs certain appreciable differences go along with certain
conspicuous differences in the flowers. And if along with a striking
change in a flower which the florist contemplates, there go changes
all over the plant not obvious to careless observers but visible to
him, we must infer that there are everywhere minute differences which
even the florist cannot perceive: the whole constitution of the plant
has diverged in some measure from the constitutions of kindred plants.
Every local variation implies a change pervading the entire organism,
manifested in concomitant variations everywhere else.

If so, what becomes of the hypothesis of determinants--the hypothesis
that there is a special element in the germ-plasm which results in a
special local modification in the adult organism? That there are no
facts supporting it has been all along manifest; but now it is manifest
that the facts directly contradict it.

At the same time it may be remarked that while the facts are wholly
incongruous with the hypothesis of determinants and its accompanying
elaborate speculation, they are not incongruous with the alternative
hypothesis. Impossible though it may be to imagine the natures of
those ultimate units peculiar to each species, which have proclivities
towards the particular form of organization characterizing it, yet
that a change of structure arising in one part of the organism is
accompanied by multitudinous changes of structure in other parts of
the organism, is not only congruous with the belief that there exist
such constitutional units, but yields it distinct support. For if, as
above argued, a conspicuous local variation is not the result of any
modification of units special to the locality, but is the result of
a modification of the units at large, then it must happen that such
modification must have its effects on all other parts of the organism;
so that there cannot fail to result all those small concomitant
variations above indicated.

May we not also say that it becomes less incomprehensible that
structural changes caused by use and disuse are inherited? If, as
we see, a local variation spontaneously arising is accompanied by
multitudinous other local variations, implying a necessary correlation
between each local variation and the general constitution of the
organism; then it may be argued that if a marked change of function in
an organ causes increase or decrease of it, this general correlation
implies that there must be a reciprocal reaction between the part and
the whole, tending to re-establish their congruity. The constitution
at large will in so far be changed, and along with its change will go
corresponding changes in the sperm-cells and germ-cells.

       *       *       *       *       *

Finally let me add, not another argument, but another fact of
observation, of the kind which opponents demand, but which, when they
are from time to time furnished, are severally pooh-poohed as not
enough. Each of them is spoken of as a solitary fact and slighted as
inadequate; and when by and by another is named, this is treated in
the same way; so that the facts which if brought together would be
recognized as sufficient are never brought together. That to which I
refer is set forth in a pamphlet by M. Leo Errera, Professor at the
University of Brussels, entitled “Hérédite d’un Caractère acquis chez
un Champignon pluricellulaire;” being an account of experiments of
Dr. Hunger, at the Botanical Institute in Brussels. First enumerating
various instances of adaptations to climate, as those of plants which,
fitted to northern regions, preserve their constitutional rapidity
of growth and seeding when brought south, and do this for several
generations, he goes on to detail the culture-experiments of M. Hunger,
and sums up the results of these in the following words:--

   “On déduit de là que:

   “1^o Les conidies d’_Aspergillus niger_ sont adaptées à la
   concentration du milieu où a vécu l’individu qui les porte; cet
   effet est encore plus marqué après deux générations passées dans
   un milieu donné (Expér. I et II);

   “2^o II s’agit d’une véritable adaptation et non pas simplement
   d’un accroissement de vigueur chez les conidies provenant des
   liquides concentrés, car ces mêmes conidies germent moins
   rapidement et donnent des plantes moins vigoureuses que
   les conidies normales lorsqu’on les sème de nouveau sur le
   milieu-type: en s’adaptant aux liquides concentrés, elles se
   sont _désadaptées_ du liquide normal (Expér. III);

   “3^o Une génération passée sur le liquide normal n’efface pas
   l’influence d’une ou de deux générations antérieures passées sur
   une liquide plus concentré (Expér. IV).

   “Tous ces résultats concordent: _ils montrent une légère, mais
   incontestable transmission héréditaire de l’adaptation au
   milieu_.”




                            SUBJECT-INDEX.

   (For this Index as it appeared in previous editions the
   Author is indebted to F. HOWARD COLLINS, Esq., of Edgbaston,
   Birmingham. It has now been adjusted to suit the present revised
   and enlarged edition.)


    Acacia, foliar organs, =II=, 41, 264.

    _Acalephæ_: environment, =I=, 105;
      water in, =I=, 173.

    _Acari_: special creation and effects of, =I=, 428;
      direct transformations, =I=, 706;
      segmentation, =II=, 111.

    _Acorus calamus_, agamic propagation, =I=, 642.

    Acquired characters, inheritance of: functionally-produced
        modifications in plants and animals, =I=, 307–13, 318, 526,
        541, 562, 692–5; =II=, 618–22;
      conceivability of, on the hypothesis of physiological units, =I=,
          368–71, 695; =II=, 618–22;
      diminution of jaw, =I=, 541–2, 693;
      current views on, =I=, 559–60;
      cessation of selection, =I=, 560–3;
      Eimer’s theory of orthogenesis, =I=, 560;
      species differentiation, =I=, 573;
      location of mammalian testes, =I=, 573;
      tactual perceptiveness, =I=, 602–8, 633, 665, 666, 672–3, 692;
      blindness of cave-animals, =I=, 612–3, 647–9;
      co-adaptation of co-operative parts, =I=, 621, 663–5;
      transmission of disease, =I=, 622–3;
      hypothesis supported by telegony, =I=, 624–8, 644–6, 649–50;
      views of Darwin and neo-Darwinists, =I=, 630, 685, 690;
      why facts in support are meagre, =I=, 632;
      degradation of little toe, =I=, 652–3, 673;
      neuter forms of social insects, =I=, 658–9, 663–4, 670, 675;
      degenerated instinct in ants, =I=, 660–2;
      rudimentary limbs of whale, =I=, 669, 692;
      importance of question, =I=, 672, 690;
      monstrous development of honey-ants, =I=, 683–4;
      osteology of Punjabis, =I=, 689;
      summary of evidences in support, =I=, 692–5;
      genesis of vertebrate skull, =II=, 227;
      false joints, =II=, 371, 372;
      conceivability of rival hypotheses, =II=, 618–22;
      adaptation to environment in _Aspergillus_, =II=, 623.

    Acrogens, the term, =II=, 55–6.
      (See _Archegoniateæ_.)

    _Actinophrys_: a primary aggregate, =II=, 76;
      genesis, =II=, 452.

    _Actinozoa_: multiaxial development, =I=, 166;
      waste and repair, =I=, 213, 219;
      differentiation, =I=, 391;
      parasitism, =I=, 397;
      integration, =II=, 92;
      symmetry, =II=, 189, 192;
      growth and genesis, =II=, 444.

    Activity: the principle of, the essential element in Life, =I=,
        113, 114, 122;
      not inherent in living matter, =I=, 120;
      nutrition and genesis, _résumé_, =II=, 497–9;
      and evolution, =II=, 501–4.

    Adaptation: general truths, =I=, 227–33, 233–5;
      botanical, =I=, 227;
      physiological, =I=, 228–33;
      psychological, =I=, 229, 230–3;
      structural, functional, and interdependence, =I=, 235–9, 240–1,
          318;
      social and organic stability, =I=, 240–2;
      _résumé_, =I=, 242–3;
      to varied media, =I=, 479–81, 489, 556;
      multiplication of effects, =I=, 512–3, 550;
      direct equilibration, =I=, 522–3;
      natural selection and equilibration, =I=, 530–5;
      non-adaptive specific characters, =I=, 565;
      time required for effecting, =I=, 565–6;
      an obstacle to re-adaptation, =II=, 11;
      of skin and skeleton, =II=, 215, 217;
      outer tissue, =II=, 312–4, 387;
      skin and mucous membrane differentiation, =II=, 321–2, 389;
      vascular system, =II=, 343–4;
      osseous, =II=, 352;
      muscular, =II=, 368–9, 391;
      persistence of force and physiological, =II=, 394;
      of reproductive activity to conditions, =II=, 411–6;
      vertebræ development, =II=, 563–6.
      (_See also_ Co-adaptation.)

    Africa, effect of climate on inhabitants, =I=, 30.

    Agamogenesis: alternation with gamogenesis, =I=, 266–7, 272–3,
        284–94, 336, 592; =II=, 415;
      parallelism in karyokinesis, =I=, 267–8;
      a process of disintegration, =I=, 276–7;
      conditions determining its continuance, =I=, 284–94, 295–7, 330;
      physiological units, =I=, 351; =II=, 613;
      spontaneous fission, =I=, 582, 584–7, 589–92, 595–6, 599;
      remarkable extent of, under favourable conditions, =I=, 591–2,
          640–1;
      in _Actinozoa_, =II=, 92;
      in _Hydrozoa_, =II=, 102;
      in _Annelida_, =II=, 103;
      innutrition, =II=, 179–80.

    _Agaricinæ_, =II=, 139, 257.

    Agassiz, L. J. R., zoological classification, =I=, 380.

    Aggregates, Animal and Plant (_see_ Morphology).

    Agility, a vital attribute, =I=, 578.

    Agrimony, floral symmetry, =II=, 42, 167, 170.

    Air, in vegetal tissues, =II=, 567–8, 583, 591, 593.

    “Air plants,” =I=, 208.

    Albumen: properties, =I=, 12;
      Lieberkühn’s formula, =I=, 13;
      diffusibility, =I=, 19;
      in organic tissues, =I=, 41.

    Alcohols, properties, =I=, 10–12.

    _Algæ_: reproduction, and the dynamic element in life, =I=, 118–9;
      multicentral development, =I=, 163, 164;
      axial development, =I=, 165;
      locomotive powers of minute forms, =I=, 196;
      uniform tissue and function, =I=, 200, 586;
      gamogenesis, =I=, 271, 279, 280, 283; =II=, 448, 449, 450;
      fertility, =I=, 582; =II=, 440, 441;
      fission, =I=, 584, 585;
      unicellular forms, =II=, 22;
      integration in _Confervoideæ_ and _Conjugateæ_, =II=, 25;
      pseudo-foliar and axial development, =II=, 28–33, 57;
      foliar development, =II=, 76, 91;
      branch symmetry, =II=, 145;
      cell metamorphoses, =II=, 176;
      tissue differentiation, =II=, 244, 246, 251, 252, 256, 272,
          385–6;
      adaptation of reproductive activity to conditions, =II=, 289;
      integration, =II=, 292;
      indefiniteness, =II=, 295;
      genesis and development, =II=, 463.

    Alimentary canal: metabolic processes and agents, =I=, 68–9, 74;
      structural traits, =I=, 192;
      progressive development, =I=, 195;
      relation to environment, =I=, 196;
      function, =I=, 205;
      segmentation in annelids, =II=, 125;
      differentiation, =II=, 301, 302, 321–2, 323–5, 389;
      specializations in birds, =II=, 325;
      in ruminants, =II=, 327–9;
      differentiation of liver, =II=, 329–33;
      muscularity, =II=, 364.

    Allotropism: of organic constituents, =I=, 4, 9;
      muscular action, =I=, 59.

    Alloys, melting point of, =I=, 339.

    Alternation of generations, misleading application of term, =II=,
        84.
      (_See_ Agamogenesis _and_ Gamogenesis.)

    Amitosis, occurrence of, in morbid tissues, =I=, 264.

    Ammonia: properties, =I=, 7, 9;
      nerve stimulation, =I=, 55.

    _Amœba_: central development, =I=, 163;
      a primary aggregate, =II=, 86;
      symmetry of encysted, =II=, 186;
      symbiosis, =II=, 400.

    _Amphibia_: classification of, =I=, 392;
      embryonic respiratory system, =I=, 457;
      structure and media, =I=, 483;
      limb locomotion, =II=, 15;
      segmentation, =II=, 122, 225;
      outer tissues, =II=, 311;
      respiration, =II=, 334, 338;
      Owen on skeleton, =II=, 552, 557, 558.

    _Amphioxus_: separation of segmentation spheres of egg, =I=, 691;
      embryogeny, =II=, 121;
      local segmentation, =II=, 125–7, 605;
      genesis of vertebrate axis, =II=, 213–6, 218, 222;
      development, =II=, 564.

    _Amphipnous cuchia_, vascular air-sacs, =II=, 337.

    _Anabas scandens_, the climbing fish, =I=, 480, 483.

    _Anacharis_ (see _Eloidea_).

    Anæsthetics, diverse effects of, =I=, 55.

    _Angræcum_, assimilative function of root, =II=, 255.

    “Animal Spirits,” vitalism and, =I=, 115.

    Animals: nutrition and molecular re-arrangement, =I=, 36–7;
      nitrogenous character, =I=, 39–41;
      sensible motion, =I=, 57;
      metabolism, =I=, 62–77;
      multiplication of energies, =I=, 75;
      contrasted traits of plants and, =I=, 196;
      what is an individual? =I=, 246–7;
      solar influence, =I=, 500, 556;
      geologic changes affecting, =I=, 501–4, 549, 550, 556;
      interdependence with plants, =I=, 504–6, 514; =II=, 398–401;
      complexity of influences affecting, =I=, 506;
      geographical isolation and origin of species, =I=, 568–9;
      vital attributes, =I=, 577–9;
      distribution and antiquity of plant and animal types, =II=, 297;
      mutual dependence of organisms at large, =II=, 397–408;
      hypothetical plant-animal type, =II=, 397;
      progressive increase of size, =II=, 401;
      laws of multiplication, =II=, 411–6;
      rhythm in numbers, =II=, 419;
      law of weights and dimensions, =II=, 434.

    Animals, domesticated: variation, =I=, 324, 326, 560, 563, 693;
      interbreeding, =I=, 345–7, 354; =II=, 615;
      pure and mixed breeds, =I=, 354, 625.

    _Annelida_: phosphorescence, =I=, 50;
      axial development, =I=, 165, 166;
      integration, =I=, 363;
      larval forms and phylogeny, =I=, 447; =II=, 115;
      segmental fission, =I=, 588–9;
      segmentation, =I=, 98–101, 103–4, 602–5; =II=, 107–9, 125–7;
      lateral gemmation, =II=, 105;
      embryogeny, =II=, 119;
      bilateral symmetry, =II=, 197–200;
      genesis, =II=, 444, 453.

    _Annulosa_: regeneration, =I=, 361–2;
      distinctive traits, =I=, 392;
      origin of type, =II=, 98–110, 602–6;
      unit of composition, =II=, 105;
      application of term, =II=, 111;
      vertebrate symmetry compared, =II=, 203–6;
      segmental differentiation, =II=, 207–9;
      unintegrated function in _Planaria_, =II=, 373;
      development and genesis, =II=, 464;
      nutrition and genesis, =II=, 490.
      (See also _Annelida_ and _Arthropoda_.)

    Anthropomorphism, former prevalence of, =I=, 419.

    Ants: utilization of aphids, =I=, 660–1; =II=, 403, 405;
      nest-mates, =II=, 405;
      castes in social species, =I=, 658–9, 670, 675;
      loss of self-feeding instinct in Amazons, =I=, 660–1, 663–4;
      monstrous development of Honey-ants, =I=, 683;
      bulk and fecundity, =II=, 492.
      (See also _Termites_.)

    _Aphis_: individuality, =I=, 249, 250; =II=, 603;
      parthenogenesis, =I=, 274–5, 289;
      fertility, =I=, 582, 640–1; =II=, 476, 490;
      utilized by ants, =I=, 660–1; =II=, 403, 405;
      over-multiplication checked by lady-bird, =II=, 406.

    Aquatic animals, large size attained by, =I=, 156.

    _Arachnida_: avoidance of danger, =I=, 92;
      oviparous homogenesis, =I=, 271;
      segmentation, =I=, 469; =II=, 113, 114;
      integration and homology, =II=, 111, 121;
      bilateral symmetry, =II=, 198.

    _Arcella_: symmetry, =II=, 186;
      outer tissue differentiation, =II=, 309.

    _Archegoniateæ_: morphological composition, =II=, 32–5;
      growth and development, =II=, 50–6;
      tubular structure, =II=, 58, 62;
      alternating generation not distinctive, =II=, 84;
      asymmetry and environment, =II=, 140;
      integration, =II=, 293, 296;
      individuation and genesis, =II=, 441, 451, 463.

    Archenteron: primitive externality, =II=, 301;
      formation of cœlom, =II=, 302.

    _Archiannelida_: segmentation, =II=, 125.

    _Arenicola marina_: polytrochal larvæ, =II=, 109.

    Arm: embryogeny of human, =I=, 169;
      vicarious use of, =I=, 209.

    Army, morphological analogy, =II=, 6.

    Arteries (_see_ Vascular System).

    _Arthropoda_: uniaxial development, =I=, 165;
      protoplasmic continuity, =I=, 190, 629;
      excursiveness, =I=, 481;
      limb locomotion, =II=, 15;
      integration and homology, =II=, 111–4, 121;
      bilateral symmetry, =II=, 197–200;
      genesis, =II=, 445, 453.

    Ascidians: multiaxial development, =I=, 165, 166;
      functional differentiation, =I=, 202;
      composite individuality of _Doliolum_, =I=, 247;
      self-fertilization, =I=, 342;
      integration, =II=, 94, 96, 97;
      symmetry, =II=, 194;
      origin of vertebrate type, =II=, 194, 598, 605.

    _Ascomycetes_, reproduction, =II=, 450.

    Assimilation: compared with reasoning, =I=, 81–7;
      a trait of vitality, =I=, 577.

    _Asteroidea_, radial symmetry, =II=, 196.

    Astronomy: growth of celestial bodies, =I=, 135;
      Schleiden on individuality, =I=, 245;
      evolution, =I=, 432, 435;
      classification of stars, =I=, 444;
      rhythm of, and organic change, =I=, 499–501, 557;
      law of equilibration, =I=, 519–20;
      co-operation of structure and function, =II=, 3.

    Atavism: occurrence of, =I=, 305–6, 314;
      digital variation, =I=, 321–3.

    Atoms: use of term, =I=, 6, 31;
      ethereal undulations and oscillations, =I=, 31–5.

    Australia: settler’s usages, =I=, 364;
      ratio of jaw to skull in natives, =I=, 541.

    Axillary buds, origin and development, =II=, 65–8.

    Axis: “neutral” of mechanics, =II=, 210;
      genesis of vertebrate, =II=, 212–6, 224–7.


    _Bacteria_: fission, =I=, 270;
      non-nucleated, =II=, 20;
      rate of increase, =II=, 443.

    Baer, K. E. von: embryological formula, =I=, 171, 172, 451, 453,
        461, 466;
      zoological classification, =I=, 383;
      on animal transitions, =I=, 480.

    _Balanophoræ_, inner tissue, =II=, 274.

    Bark: varied development, =II=, 247–9;
      physiological differentiation, =II=, 249–50, 258, 386.

    _Basidiomycetes_, reproduction, =II=, 450.

    Bat, infertility of, =II=, 473.

    Bates, H. W., protective mimicry of butterflies, =I=, 398.

    _Batrachia_ (see _Amphibia_).

    Bean, vascular system, =II=, 573, 591.

    Beaver, tail and co-adapted structures, =I=, 616.

    Bees (_see_ Insects).

    _Begoniaceæ_: multiplication =I=, 224, 317, 442;
      individuality, =I=, 251;
      development from scales, =I=, 282;
      symmetry, =II=, 159, 166;
      development, =II=, 271.

    Berkeley, M. J., indefiniteness of mosses and ferns, =II=, 296.

    Bile, arrest of excretion, =I=, 209.

    Bilirubin and biliverdine, function of, =II=, 330, 333.

    Biology: definition and divisions, =I=, 124–5;
      organic structural phenomena, =I=, 125–7;
      also functional, =I=, 127–9;
      actions and reactions of function and structure, =I=, 129–30;
      genesis, =I=, 130–1;
      limited knowledge of, =I=, 131;
      evolution, =I=, 432, 434;
      sociological analogies (_see_ Sociology).

    Biophors, Weismann’s germ-plasm units (_see_ Weismann).

    Birds: flesh-eating and grain-eating contrasted, =I=, 68;
      growth and expenditure of force, =I=, 142;
      size of egg and adult, =I=, 144;
      limitations on flight, =I=, 155;
      self-mobility, =I=, 175;
      temperature, =I=, 176;
      functional and structural differentiation, =I=, 201;
      food of starving pigeon, =I=, 215;
      viviparousness, =I=, 271;
      heredity and pigeon breeding, =I=, 305;
      atavism in pigeon, =I=, 314;
      osseous variation in pigeon, =I=, 321;
      classification, =I=, 392;
      migrations and change of habits, =I=, 399, 402, 500;
      distribution in time, =I=, 410;
      Darwin on petrels, =I=, 455;
      rudimentary teeth, =I=, 457;
      vertebræ, =I=, 471; =II=, 564;
      feather development, =I=, 473;
      habits of water ouzel, =I=, 485;
      egg shells and direct equilibration, =I=, 526;
      bones of waders and direct equilibration, =I=, 527;
      fertility and nervous development, =I=, 598;
      cellular continuity, =I=, 629;
      adaptation of structure to environment, =II=, 12;
      sexual selection, =II=, 269;
      wing spurs, =II=, 313;
      outer tissue differentiation, =II=, 314–5, 387;
      alimentary canal development, =II=, 325, 327;
      muscular colour and activity, =II=, 365–9;
      nutrition, =II=, 433;
      cost of genesis, =II=, 436;
      growth and genesis, =II=, 454, 458;
      heat expenditure and genesis, =II=, 468–9, 474;
      activity and genesis, =II=, 470–2, 474;
      contrasted mammalian fertility, =II=, 470;
      eggs of wild and tame, =II=, 478;
      fertility of blackbird and linnet compared, =II=, 503;
      Owen on skeleton of, =II=, 559, 560, 561.

    Bischoff, embryogeny of human arm, =I=, 169.

    Bison, modifications entailed by increased weight of head, =I=,
        512.

    Blackbird, contrasted with linnet in development, =II=, 503.

    Blainville, de, definition of life, =I=, 79, 93.

    Blastosphere, independence of cells in Echinoderm larvæ, =I=, 185.

    Blastula, definition of life and formation of, =I=, 112.

    Blood: similarity of iron peroxide, =I=, 17;
      metabolic processes, =I=, 69;
      segregation of abnormal constituents, =I=, 180;
      protozoon life of corpuscles, =I=, 186–7;
      morbid changes, =I=, 221, 701;
      assimilative power and organic repair, =I=, 221–2;
      respiratory tissue differentiation, =II=, 310–1;
      pressure in mammals, =II=, 340.
      (_See also_ Vascular System.)

    Blow-fly, Weismann on nutrition and genesis in, =I=, 678–9.

    Boers, Cape, habits and fertility, =II=, 508.

    Boismont, A. B. de, on human fertility, =II=, 511.

    Bone: growth and function, =I=, 151;
      adaptability, =I=, 230; =II=, 217–8;
      function and weight, =I=, 308, 693;
      mammalian cervical vertebræ, =I=, 394;
      evolution and vertebral column, =I=, 470–1;
      partial development, =I=, 473;
      size of head as influencing, =I=, 512, 536–9;
      direct equilibration and strength, =I=, 527;
      natural selection and co-adaptations, =I=, 614–21, 674, 677;
      rudimentary limbs of whale, =I=, 668, 685, 692;
      inheritance of acquired modifications in Punjabis, =I=, 689;
      skull development, =II=, 222;
      theory of supernumerary, =II=, 223;
      Cope on origin of vertebrate osseous system, =II=, 225–7;
      differentiation, =II=, 344–56;
      false joints, =II=, 370–2;
      Owen’s theory of vertebrate skeleton, =II=, 548–66.

    Book-worm, food of, =I=, 77.

    Born, G., experiments on frog larvæ, =I=, 365.

    Botany, biological classification, =I=, 124, 125.
      (_See_ Plants.)

    _Bothriocephalus_, development, =II=, 490.

    _Botryllidæ_: development, =I=, 166;
      independence of components, =I=, 247;
      agamogenesis, =I=, 641.

    Bower, Prof., on alternation of generations, =II=, 84.

    _Brachiopoda_, rude vascular system, =II=, 340.

    Bradbury, J. B., on vaso-dilators, =I=, 55.

    Brain: natural selection and mental evolution, =I=, 553;
      analysis of substance, =I=, 596;
      weight in higher animals, =I=, 598–9;
      size in civilized and uncivilized, =II=, 530.

    Branches (_see_ Morphology).

    _Branchiæ_ (_see_ Respiratory System).

    Brass, effect of antimony on, =I=, 121.

    Bread, diamagnetism, =I=, 370.

    Breeding: heredity, =I=, 304–5;
      in-and-in, =I=, 344–7, 353; =II=, 615;
      pure and mixed, =I=, 354, 625.

    Bricks, changed equilibrium shown by, =I=, 38, 42.

    Brodie, T. G., cell chemistry, =I=, 260.

    Brownell, Miss J. L., on birth-rate in United States, =II=, 520.

    Brown-Séquard, on inherited epilepsy, =I=, 312, 624.

    _Bryophyllum_, peculiar proliferation, =II=, 295.

    _Bryophyta_, large size attained by some, =I=, 138.

    _Bryozoa_, gemmation, =I=, 588.

    Budding (_see_ Gemmation).

    Buds: development, =I=, 167–8;
      theories of heredity and cauline, =I=, 358–9, 360;
      axillary, =II=, 65–9;
      effects of nutrition, =II=, 73–4.

    Butterfly: protective mimicry, =I=, 398;
      instance of tame, =I=, 684.


    Cabbage, varieties of, =I=, 302.

    _Cactaceæ_: foliar and axial development, =II=, 47–9;
      differentiation in, =II=, 258, 276, 282;
      vascular system, =II=, 282;
      dye permeability and circulation, =II=, 571, 572;
      wood formation, =II=, 575, 577, 578, 580.

    “Callus,” budding from, =I=, 358, 359.

    Camel, natural selection and hump of, =I=, 534.

    Canadians, French, fertility of, =II=, 509.

    Cancer, the definition of life, =I=, 111;
      œsophageal, =II=, 324;
      and vascular system, =II=, 343.

    Caoutchouc, leaf-structure, =II=, 589.

    Capillaries (_see_ Vascular System).

    Capillarity, and vegetal vascular system, =II=, 279–80, 286, 568,
        570, 585, 587, 592–6.

    Carbohydrates: instability, =I=, 10;
      the term “hydro-carbon,” _ib._;
      molecular changes in, =I=, 42–3;
      organic transformation, =I=, 43, 48;
      metabolic processes, =I=, 63–77, 262–3; =II=, 362.

    Carbon: properties, =I=, 3–5, 20;
      compounds, =I=, 6, 7, 9, 10–12, 13, 24–5.

    Carbonic acid (carbon dioxide): properties, =I=, 6, 7, 9;
      in animal and plant functions, =I=, 62, 214; =II=, 398;
      diffusibility, =II=, 331.

    Carbonic oxide, properties, =I=, 6.

    Carnivores: nitrogenous food, =I=, 47, 68;
      katabolic process, =I=, 71;
      restricted environment, =I=, 396;
      their beneficial effects on animal life, =II=, 405–6.

    Carpenter, W. B.: on functional specialization, =I=, 208;
      reproduction of seaweed, =I=, 582;
      vegetal cell multiplication, =I=, 585;
      structure and multiplication of compound organisms, =I=, 586–9;
      on fundamental traits of sex, =I=, 595;
      nutritive system of invertebrates, =I=, 595;
      _Macrocystis_, =II=, 450;
      nutrition and reproductive function, =II=, 460.

    Cartilage (_see_ Bone).

    Castration, effect of, on growth, =II=, 459.

    “Castration parasitaire,” Julin on, =II=, 493–6.

    Catalysis, and vital metamorphosis, =I=, 39, 43.

    Cattell, McKeen, on tactual perceptiveness, =I=, 666.

    _Caulerpa_, simulation of higher plant-forms, =II=, 22.

    Cave-animals, degeneration of eyes, =I=, 309, 612–3, 614, 647–9,
        693.

    Cell, the: incomprehensibility of forces at work in, =I=, 118;
      protoplasts and their traits, =I=, 181;
      the cell-theory, =I=, 184, 252; =II=, 17–21, 85;
      differentiation, =I=, 188–9, 194;
      the continuity of protoplasm, =I=, 190–2, 194, 628–30; =II=, 21;
      its structure, =I=, 253–5;
      function of centrosome, =I=, 254–5, 257;
      structure and function of nucleus, =I=, 255–6, 258–9;
      karyokinesis, =I=, 257–8;
      function of chromatin, =I=, 259–65;
      fertilization and function of polar bodies, =I=, 266–8;
      theories of heredity based on theory, =I=, 356;
      Weismann’s differentiation into reproductive and somatic, =I=,
          622, 628–30, 633–44;
      nucleus absent or dispersed, =II=, 20, 85;
      morphological differentiation, =II=, 175–7;
      animal morphology, =II=, 228–30;
      morphological summary, =II=, 233;
      vegetal tissue differentiation, =II=, 249–50, 386;
      vascular development, =II=, 279–84, 389.

    Centipede, bilateral symmetry, =II=, 198–200.

    _Cephalopoda_: bilateral symmetry, =II=, 203;
      vascular system, =II=, 341.

    _Cercariæ_ (see _Distoma_).

    _Cereus_, tissue differentiation, =II=, 276, 283.

    Cesalpino, =I=, 377.

    _Cestoda_ (see _Entozoa_).

    _Chætopoda_, segmentation, =II=, 98, 103, 605.

    _Chaja_, wing spurs, =II=, 313.

    Change, and definition of life, =I=, 81–90, 113.

    Charles, R. H., on inheritance of acquired modifications in
        leg-bones of Punjabis, =I=, 689.

    Chatié, on single and double stocks, =II=, 622.

    Chemistry: properties of organic elements, =I=, 3–5, 20, 22;
      of diatomic compounds, =I=, 7–10;
      tri-atomic, =I=, 10–12;
      poly-atomic, =I=, 12–13, 25;
      traits of evolution, =I=, 23–4;
      ethereal undulations and atomic oscillation, =I=, 31–6;
      chemical affinity and organic change, =I=, 36–7, 38–43;
      oxidation and generation of heat, =I=, 46–9, 60;
      generation of nerve force, =I=, 52, 60;
      metabolism, =I=, 62–77;
      physiology and organic, =I=, 127;
      flesh constituents, =I=, 154;
      composition of organisms and environment, =I=, 173;
      organic development and differential assimilation, =I=, 179–80;
      chemical units, =I=, 225; =II=, 612;
      primitive ideas of elements, =I=, 417;
      evolution of organic compounds, =I=, 696–701, 703.

    Chestnut, leaf symmetry, =II=, 149, 153.

    _Chiton_: simulation of segmentation, =II=, 116, 118;
      symmetry, =II=, 202.

    Chlorophyll: function, =I=, 65; =II=, 263;
      nutrition and absence of, =II=, 74;
      constitution, =II=, 262;
      symbiotic presence in animals, =II=, 400.

    _Chondracanthus gibbosus_, enormous development of reproductive
        system, =II=, 487.

    _Chordata_, affinities, =I=, 466.

    Chromatin (_see_ Cell).

    Circle, the, and evolution hypothesis, =I=, 433.

    Circulation (_see_ Vascular System).

    _Cirrhipedia_: Darwin on retrograde development, =I=, 458;
      remarkable transformation in _Sacculina_, =II=, 494–5.

    Civilization, human evolution and genesis, =II=, 529–31.

    _Cladophora_: integration, =II=, 25;
      axial development, =II=, 28.

    Classification: subjective conception, =I=, 78;
      two purposes of, =I=, 374;
      a gradual process, =I=, 375;
      botanical, =I=, 377–80, 389–90;
      zoological, =I=, 380–9;
      incomplete equivalence of groups, =I=, 389, 445–6, 448, 555, 572;
      group attributes, =I=, 390–3;
      the truths interpreted, =I=, 393–4;
      ethnologic and linguistic evolution, =I=, 441–6;
      organic evolution, =I=, 443, 447, 555;
      differences in kind and degree, =I=, 444–6;
      antecedent structural similarity, =I=, 447, 448–9;
      Von Baer’s formula, =I=, 451–4, 555;
      organic, not uniserial, =II=, 115.

    _Classification of the Sciences, The_, and evolution and
        dissolution, =II=, 5.

    Claus, C, on segmentation in Annelids and Chætopods, =II=, 605.

    Clover: flower and axial development, =II=, 45;
      symmetry, =II=, 152.

    Co-adaptation of co-operative parts: principles underlying, =I=,
        234–5, 511–3, 514–5;
      slow operation of the process, =I=, 236;
      sociological analogy, =I=, 237–40;
      reversion under original conditions, =I=, 240;
      the analogy continued, _ib._;
      the case of bison’s head, =I=, 512;
      natural selection an inadequate explanation, =I=, 535, 614–21,
          692;
      Romanes on “cessation of selection” as effecting, =I=, 560,
          561–2;
      Weismann’s theories, =I=, 560–3, 663–5, 670, 674–5;
      natural selection and economy of growth, =I=, 562;
      physiological processes involved, =I=, 566–7;
      Wallace’s argument from artificial selection, =I=, 615;
      what are co-operative parts? =I=, 616–7;
      “intra-selection” examined, =I=, 676–8.

    Coal, social effects of supply, =I=, 238–9, 241.

    Cocoa-nut, growth and fertility, =II=, 457.

    Coccospheres: vital problem presented by protective structures,
        =I=, 119;
      imbricated plates, =I=, 182.

    Cockroach, ousting of European species, =I=, 399.

    Cod: ova of, =II=, 435;
      growth and fertility, =II=, 454.

    _Codium_: symmetry, =II=, 136;
      tissue differentiation, =II=, 246.

    _Cœlenterata_: rudimentary contractile organs, =I=, 58;
      vital changes in polyp, =I=, 95;
      axial development, =I=, 165, 166;
      environment and structure, =I=, 173;
      self-mobility, =I=, 175; =II=, 14, 15;
      functional differentiation, =I=, 201, 391;
      inactivity and waste, =I=, 213;
      reparative power, =I=, 219, 224;
      individuality, =I=, 246, 247, 250;
      heterogenesis, =I=, 273, 277, 296;
      negative disintegration in _Hydrozoa_, =I=, 276, 587;
      reproductive tissue, =I=, 281;
      differentiation in _Hydrozoa_, =I=, 391;
      classificatory value, =I=, 446;
      regeneration of fragments, =II=, 90;
      integration, =II=, 90, 102, 105, 124;
      gemmation, =II=, 91;
      tertiary aggregation, =II=, 92, 95, 124;
      molluscan affinities, =II=, 115;
      radial symmetry, =II=, 188;
      symmetry of compound, =II=, 192–3;
      segmental differentiation, =II=, 207;
      physiological differentiation in _Hydra_ and analogy, =II=, 300;
      ciliation of blastula, =II=, 301;
      tissue reduplication, =II=, 301–2, 389;
      outer tissue differentiation, =II=, 309;
      osmosis in _Hydra_, =II=, 339;
      vascular system in _Hydra_, =II=, 340, 376;
      functional co-ordination, =II=, 376;
      symbiosis, =II=, 400;
      asexual genesis, =II=, 443–4;
      growth and sexual genesis, =II=, 452;
      development and genesis, =II=, 462;
      nutrition and genesis, =II=, 476.

    Cœlom, origin and function, =II=, 302–3.

    Collins, F. Howard, jaws and teeth of savages and civilized, =I=,
        541.

    Colloids: T. Graham on, =I=, 15–8;
      diffusibility, =I=, 18–21;
      organic, =I=, 21, 25, 26;
      pliability and elasticity, =I=, 27;
      capillary affinity, =I=, 28;
      isomerism, =I=, 59;
      instability, =I=, 350;
      molecular mobility and diffusibility, =II=, 331;
      instability of, and nerve differentiation, =II=, 356–61;
      and muscular tissue, =II=, 361–4.

    Colonies, autogenous development and parallel in heredity, =I=,
        366–8; =II=, 620.

    Colour: sensation of, =I=, 54;
      phœnogamic, =II=, 75, 265–6;
      light and vegetal, =II=, 261–2;
      floral fertilization, =II=, 267–9;
      sexual selection, =II=, 269;
      activity and muscular, =II=, 365–9;
      physiological units and mixture of, in offspring, =II=, 616, 617.

    Commensalism, organic Integration as displayed in, =II=, 402–4.

    _Compositæ_: floral symmetry, =II=, 173.

    Condor, weight of, =I=, 155.

    _Confervoideæ_, =I=, 279, 280; =II=, 25, 28, 449.
      (See _Algæ_.)

    _Conjugateæ_, =II=, 449.
      (See _Algæ_.)

    Conjugation, in _Algæ_, =I=, 279;
      in _Protozoa_, =I=, 280; =II=, 452;
      can fission persist without? =I=, 637;
      relation to growth, =II=, 449.

    Connective tissue, Hertwig’s classification, =I=, 189.

    Constitutional units, =I=, 369.
      (_See_ Physiological Units.)

    Consumption, hereditary transmission, =I=, 307.

    Co-ordination of actions (_see_ Life).

    Cope, E. D., on origin of vertebrate structure, =II=, 225–7.

    _Cormophyta_: slight internal differentiation, =II=, 273;
      vascular system, =II=, 280.

    _Corpuscula tactus_, their function, =I=, 75.

    Correspondence, use of word, =I=, 97.
      (_See_ Life.)

    Cousin-marriages, =I=, 345; =II=, 615.

    Cow: what prompts her to mumble a bone? =I=, 120.

    Cow-parsnip (see _Heracleum_).

    Crab (see _Crustacea_).

    Creation (_see_ Special creation).

    _Crinoidea_, symmetry, =II=, 195–6.

    Crocodile, continuous growth, =I=, 154, 292.

    Crookes, Sir W., hypothetical chemical unit “protyle,” =I=, 22, 23.

    _Cruciferæ_, floral symmetry, =II=, 164, 171.

    _Crustacea_: locomotion of lobster, =I=, 175;
      regeneration of limbs, =I=, 224, 360, 589; =II=, 76;
      homogenesis, =I=, 271;
      genesis and nutrition in _Daphnidæ_, =I=, 290–1;
      growth and genesis, =I=, 292;
      degeneration of eye in cave-inhabiting, =I=, 309, 614, 648;
      hermit-crab parasite, =I=, 397;
      changes of media, =I=, 401, 481–2;
      retrograde development in cirripedes, =I=, 458;
      segmentation, =I=, 468–9; =II=, 114;
      Darwin on jaws and legs, =I=, 471;
      survival of cirripedes, =I=, 517;
      integration and homology, =II=, 111–4, 121, 603;
      bilateral symmetry, =II=, 198–201;
      eyes, =II=, 318;
      dermal structure of hermit-crab, =II=, 322, 387;
      fertility, =II=, 453;
      nutrition and genesis in parasitic species, =II=, 487;
      “castration parasitaire,” =II=, 493–6.

    Crystalloids: Prof. Graham on, =I=, 15–8;
      diffusibility, =I=, 18–21;
      organic, =I=, 21–2, 26.

    Crystals: simulation of life in “storm glass,” =I=, 96;
      growth, =I=, 135–7, 577;
      segregation, =I=, 179, 221, 223;
      equilibration, =I=, 337;
      physiological units and polarity, =I=, 701–6;
      time and formation, =II=, 77.

    _Ctenodrilus_, segmental individuality, =II=, 103, 603, 604.

    Cube, bilateral symmetry, =II=, 132.

    Cunningham, J. T., =I=, vi; =II=, vi;
      on non-adaptive specific characters, =I=, 565;
      food of blow-fly larvæ, =I=, 678;
      arthropod segmentation, =II=, 114;
      egg-production of Conger, =II=, 425.

    Cuttle-fish, Individuality of _Hectocotylus_, =I=, 250.

    Cuvier, zoological classification, =I=, 381.

    Cyanogen, properties, =I=, 7, 9.

    _Cyclichthys_, dermal structure, =II=, 306.


    Dalyell, Sir J., regeneration in _Dasychone_, =I=, 361;
      propagation of _Hydra_, =II=, 476.

    _Daphnidæ_, heterogenesis and nutrition, =I=, 290–1.

    Darwin, C: _Origin of Species_, =I=, 129; =II=, 528;
      natural selection and function, =I=, 308–9, 693;
      atavism, =I=, 314;
      osseous variations in pigeons, =I=, 321;
      plant variation and domestication, =I=, 325;
      “spontaneous variation,” =I=, 328, 697;
      floral fertilization, =I=, 340; =II=, 168, 267, 407, 608;
      intercrossing and self-fertilization, =I=, 344, 345;
      intercrossing =I=, 347, 611, 669;
      his theory of pangenesis examined, =I=, 356–62, 370, 372;
      plant-fertilization and distribution, =I=, 397;
      habits of birds, =I=, 400;
      distribution and natural barriers, =I=, 402, 476;
      disappearance and non-reappearance of species, =I=, 406;
      distribution in time and space, =I=, 410;
      linguistic classification, =I=, 442;
      classification of organisms, =I=, 443;
      classification and descent, =I=, 448;
      on petrels, =I=, 455;
      suppression of organs, =I=, 457;
      development of _Cirrhipedia_, =I=, 438;
      jaws and legs of _Crustacea_, =I=, 471;
      aborted organs, =I=, 474, 563;
      relations of species in Galapagos archipelago, =I=, 478;
      opinions of E. Darwin and Lamarck, =I=, 491;
      the term “survival of the fittest,” =I=, 530;
      Indirect equilibration by natural selection, =I=, 530–5;
      inheritance of acquired characters, =I=, 535–42, 560, 630, 685,
          690;
      Wallace on natural selection in man, =I=, 553;
      misleading connotations of term “natural selection,” =I=, 609,
          695;
      caste gradations and jaws of driver ants, =I=, 658;
      attachment of climbing plants, =II=, 276–7;
      vegetal fructification, =II=, 294;
      earth-worm, =II=, 402;
      animal sterility and domestication, =II=, 480, 483;
      variation in hyacinth and camellia, =II=, 621.

    Darwin, Dr. E., modifiability of organisms, =I=, 490, 492–7.

    Death: an arrest of vital correspondence, =I=, 102;
      only limit to vegetal growth, =I=, 153;
      cessation of co-ordination of actions, =I=, 578, 579;
      Weismann’s hypothesis, =I=, 636–8;
      physiological integration, =II=, 374, 392;
      cause of natural, =II=, 413;
      relation to births, =II=, 417.

    Definiteness: of vital change, =I=, 87–90, 106, 109;
      developmental, =I=, 178;
      functional, =I=, 212;
      segregation of evolution, =I=, 514–6.

    Definition, difficulties of, =I=, 78; =II=, 17.

    Degeneracy, morphological obscurations due to, =II=, 12, 13.

    _Dendrobium_ (_see_ Orchids).

    _Desmidiaceæ_: unicellular, =II=, 21;
      linear and central aggregation, =II=, 23;
      natural selection and symmetry, =II=, 134, 133;
      morphological differentiation, =II=, 177;
      tissue, =II=, 244;
      genesis, =II=, 440, 449.

    Determinants, Weismann’s germ-plasm units (_see_ Germ-plasm).

    Development: an increase of structure, =I=, 162; =II=, 461;
      primarily central, =I=, 162, 166;
      uni- and multicentral, =I=, 163–4, 166–7;
      axial, =I=, 164, 167;
      uni- and multiaxial, =I=, 165–6;
      a change to coherent definite heterogeneity, =I=, 167–70, 179;
      Von Baer’s formula, =I=, 171–2;
      individual differentiation from environment, =I=, 172–8;
      cell-formation, =I=, 225;
      discontinuous, and agamogenesis, =I=, 275;
      Prof. Huxley’s classification, =I=, 276;
      sociological parallel to autogenous, =I=, 364–8; =II=, 620;
      retrograde, =I=, 457–8;
      inequalities among co-operative parts, =I=, 617;
      “heterochrony,” =I=, 655;
      continuous and discontinuous vegetal, =II=, 52;
      summary of physiological, =II=, 384–90;
      nutrition and genesis, _résumé_, =II=, 497–9;
      evolution, =II=, 501–5;
      commencement of genesis, =II=, 506;
      of vertebrate limbs, =II=, 553.
      (_See also_ Multiplication.)

    _Development Hypothesis, The_, =I=, 417.

    Dialects (_see_ Language).

    Dialysis, and diffusibillty. =I=, 19, 20.

    Diastase, decomposition of, =I=, 38, 40.

    _Diatomaceæ_: tissue, =II=, 244;
      genesis, =II=, 440, 448.

    Diatomic compounds (_see_ Chemistry).

    Dicotyledons: growth, =I=, 139, 143; =II=, 63–4, 69–72, 78, 82–3;
      uniaxial development, =I=, 165;
      stem and leaf functions, =II=, 257;
      mechanical stress and wood formation, =II=, 277;
      growth and genesis, =II=, 451.

    Differentiation (_see_ Morphology _and_ Physiology).

    _Difflugia_: primary aggregate, =II=, 86–7;
      symmetry, =II=, 186;
      outer tissue differentiation, =II=, 309.

    Diffusion, of colloids and crystalloids, =I=, 18–20; =II=, 331.

    Digestion: action of nitrogenous compounds, =I=, 69;
      obesity, =II=, 480–4;
      fertility, =II=, 514.

    Dimorphism: floral, =I=, 534;
      sexual, in parasites, =I=, 315;
      social insects (_see_ Insects).

    Dinosaurs, size of, =I=, 139.

    _Diphyes_: individuality, =I=, 246;
      symmetry, =II=, 192.

    Disease: segregation of blood constituents, =I=, 179;
      changes in blood from, =I=, 221, 701;
      heredity, =I=, 306–7, 312–3, 622–3;
      belief in supernatural origin, =I=, 419;
      parasitism and special creation, =I=, 427;
      morbid products as specific characters, =I=, 567;
      telegony, =I=, 646;
      dermal structure, =II=, 306;
      intestinal muscular hypertrophy, =II=, 325;
      indigestion and alimentary canal development, =II=, 328;
      jaundice and bilirubin, =II=, 330;
      localization of excretion, =II=, 331;
      membranes in inflammatory, =II=, 343;
      osseous differentiation in rickets, =II=, 352;
      fatty degeneration, =II=, 482.

    Disintegration, physiological (_see_ Physiology).

    _Distoma_: metagenesis, =I=, 273–4;
      disintegration of genesis, =I=, 276;
      cycle of generations, =II=, 489.

    Distribution: physical limits, =I=, 396;
      organic environment, =I=, 396–8;
      parasitic conditions, =I=, 397–8;
      simultaneity of agencies affecting, =I=, 398;
      mutual encroachments of species, =I=, 398–401, 477, 489;
      facts disproving pre-adaptation to habitats, =I=, 401–3, 411–2;
      of animals and plants in time, =I=, 404–11, 412;
      ousting of native species in New Zealand, =I=, 477;
      local influences, =I=, 477–9, 489;
      through varied media, =I=, 479–85, 489, 556;
      past and present organic forms, =I=, 485–9, 556;
      complex organization and, =II=, 296–7.

    Division of labour, physiological (_see_ Labour).

    Dog: contrasted lives of tortoise and, =I=, 103, 104;
      inherited habits, =I=, 309, 573;
      abnormal digits, =I=, 324;
      interbreeding of divergent varieties, =I=, 565;
      decrease of jaw, =I=, 615, 693;
      telegony, =I=, 645;
      conditions affecting fertility, =II=, 474, 479.

    Dohrn, theory of vertebrate structure, =II=, 606.

    _Doliolum_, combination of individualities, =I=, 247.

    Domestication (_see_ Animals).

    Doubleday, E., on nutrition of genesis, =II=, 510–2.

    Driesch, separation of segmentation spheres of _Echinus_ ovum, =I=,
        691; =II=, 618.

    Dropsy (_see_ Disease).

    _Drosera_: individuality, =I=, 251;
      proliferous growth, =II=, 75.

    Du Bois-Reymond, E. H., electricity from muscles and nerves, =I=,
        50.

    Dumas, antithesis of animals and plants, =I=, 62.

    Dwarfs, Hindu family of, =I=, 316.


    Ear, development of vertebrate, =II=, 318, 320.

    Earth, climatic rhythm and organic change, =I=, 499–501, 557.

    Earth-worm: bilateral symmetry, =II=, 199, 200;
      mould production, =II=, 402.

    _Echinococcus_ (see _Entozoa_).

    _Echinodermata_: independence of blastosphere cells, =I=, 185;
      protoplasmic continuity in embryos, =I=, 190;
      separation of segmentation spheres of ovum, =I=, 691; =II=, 618;
      symmetry, =II=, 191, 195–6.

    Economy: of growth in natural selection, =I=, 536, 562;
      a trait of organic evolution, =II=, 501, 504.

    Ectoderm: functional differentiation, =I=, 202, 203;
      functional vicariousness, =I=, 209;
      reproductive function, =I=, 281.

    Effects, Multiplication of: variation, =I=, 329–30, 333;
      organic evolution, =I=, 511–4, 515, 517, 549, 557; =II=, 405–6;
      morphological development, =II=, 7–9, 234;
      physiological differentiation, =II=, 390–1, 392.

    Eggs (_see_ Embryology).

    Eimer, T., theory of orthogenesis, =I=, 563–4.

    _Elasmobranchii_: protoplasmic continuity, =I=, 629;
      segmentation, =II=, 126.

    Electricity: genesis in organic matter, =I=, 50–2, 60;
      muscular action, =I=, 59;
      incomprehensibility, =I=, 121.

    Elephant: fertility, =I=, 583, 599; =II=, 459, 506;
      cerebro-spinal system, =I=, 598, 599.

    Elk, Irish, horns and correlated parts, =I=, 537, 674.

    _Eloidea canadensis_: individuality, =I=, 248;
      enormous agamic multiplication, =I=, 642.

    Elongation, and locomotion in animals, =II=, 15.

    Embryology: as aiding biology, =I=, 125–6;
      simulated growth, =I=, 136;
      initial and final organic bulks, =I=, 143, 158, 161;
      fœtal flesh constituents, =I=, 154;
      human arm development, =I=, 169;
      Von Baer’s formula, =I=, 170–2, 451–4, 466;
      embryonic heat, =I=, 177;
      spherical organic form, =I=, 177;
      unit-life in multicellular organisms, =I=, 185–6;
      functional differentiation, =I=, 203;
      individuality, =I=, 246–7;
      unspecialized reproductive tissue, =I=, 279–83, 317;
      changes following impregnation, =I=, 283–4;
      nutrition and vegetal growth, =I=, 285–8, 295–7;
      and animal growth, =I=, 289–94, 295–7;
      physiological units and heredity, =I=, 317–9;
      variation and parental functional condition, =I=, 324;
      uterine environment, =I=, 327–8;
      physiological units and variation, =I=, 330–4, 458;
      fertilized and unfertilized ova, =I=, 340–1;
      hermaphrodism, =I=, 341–2, 344;
      sociological parallel, =I=, 366–8;
      evolution hypothesis, =I=, 434, 436, 453, 454, 555;
      petrel development, =I=, 455;
      substitution and suppression of organs, =I=, 456–8, 466, 472–3;
      structural proclivities of physiological units, =I=, 458;
      abridgment of stages, =I=, 458–9, 464;
      disappearance of intermediate forms, =I=, 459–60, 463;
      pre-adaptation, =I=, 461–3;
      discrimination of species in early stages, =I=, 461;
      anomalous persistence of ancestral traits, =I=, 463–5;
      phylogeny, =I=, 486;
      egg-shell function, =I=, 527;
      genesis of grades in social insects, =I=, 654–6, 658–9, 679–80;
      determination of sex, =I=, 657;
      order of development qualified by needs, =I=, 679;
      osteology of Punjabis, =I=, 689;
      direct transformations and physiological units, =I=, 706;
      transformation of blastema, =II=, 20;
      arrest of growth and innutrition, =II=, 73;
      development of segmented animals, =II=, 100–2, 602;
      adaptive vertebrate segmentation, =II=, 118–23, 124, 223–4,
          605–6;
      animal cell morphology, =II=, 228;
      primary differentiations of germinal layers, =II=, 300–2;
      lung development, =II=, 333–4;
      mammalian ova-maturation, =II=, 342–3;
      movements of ova, =II=, 356, 363;
      modifications in mole, =II=, 391;
      genesis and nutrition, =II=, 424, 425;
      fish ova, =II=, 435, 454;
      cost of genesis, =II=, 435–6;
      number of birds’ eggs, =II=, 454–6, 478;
      heat and genesis, =II=, 468, 474;
      activity and genesis in birds, =II=, 470–2, 474;
      vertebrate limb development, =II=, 553;
      ossification in vertebrates, =II=, 556;
      Owen’s vertebrate theory, =II=, 563;
      development of vertebræ, =II=, 564.
      (_See also_ Multiplication.)

    Embryology of conceptions, =I=, 451.

    Emigrants, type of organization among, =I=, 364; =II=, 620.

    Endoderm: functional differentiation, =I=, 202, 203;
      functional vicariousness, =I=, 209.

    Endogen, application of term, =II=, 62, 78, 82.
      (_See_ Monocotyledons.)

    Energy: evolution of, in animals, =I=, 71–7;
      organic growth and expenditure, =I=, 141;
      functional transfer, =I=, 201–6;
      chromatin as the source of, in karyokinesis, =II=, 261–5.
      (_See also_ Force.)

    _Entozoa_: metagenesis, =I=, 273, 641;
      self-fertilization, =I=, 342;
      distribution, =I=, 398;
      and special creation, =I=, 428;
      fission in simple types, =I=, 584;
      nutrition and genesis, =I=, 641; =II=, 488;
      direct transformation, =I=, 706;
      integration, =II=, 102;
      segmentation, =II=, 107, 108;
      interdependence and organic integration, =II=, 404.

    Environment: degree of life and complexity of, =I=, 104–7;
      relation to organic structure and function, =I=, 172–8; =II=,
          12–5;
      adaptation to varied media an evidence of evolution, =I=, 479–81,
          556;
      influence of solar system, =I=, 500, 556;
      inherited adaptation to, =II=, 623.

    _Eolis_, branchiæ, =II=, 118.

    Epidermis (_see_ Skin).

    Epilepsy: definition of life and movements in, =I=, 112;
      heredity, =I=, 312.

    Epithelium: ciliated, =I=, 187;
      Hertwig’s classification, =I=, 189;
      reproductive function, =I=, 280;
      “pavement” and “cylinder,” =II=, 229.

    _Epizoa_: distribution, =I=, 398;
      special creation and effects of, =I=, 428;
      interdependence and organic integration, =II=, 404;
      nutrition and genesis, =II=, 487.

    Equilibration: variation and law of, =I=, 326, 334;
      molecular arrangement, =I=, 337–45;
      of organic change, =I=, 348, 347, 557;
      direct and indirect, =I=, 519–22, 573;
      adaptation by direct, =I=, 522–3, 551, 557;
      nutrition, defence, and fertilization of plants, =I=, 523–5;
      direct of animals, =I=, 525–8, 551, 557;
      natural selection and indirect, =I=, 530–4, 552, 557;
      of natural selection, =I=, 543–7, 552–3, 557;
      increasing importance of direct, =I=, 553;
      of forces acting on species, =I=, 571–2; =II=, 417–20;
      phenomena not accounted for by, =I=, 573;
      tissue differentiation, =II=, 245;
      genesis of nervous system, =II=, 307–8;
      functional, =II=, 391–4;
      laws of multiplication, =II=, 411–6;
      in human and social evolution, =II=, 537.
      (_See also_ Acquired characters _and_ Natural selection.)

    Errera, L., on inherited adaptation to environment in Aspergillus,
        =II=, 623.

    Ethnology: heredity, =I=, 303–4, 310;
      plasticity of mixed races, =I=, 354;
      primitive ideas, =I=, 417;
      evolution and classification, =I=, 441–3, 446;
      natural selection, =I=, 553.

    _Euphorbiaceæ_: foliar and axial development, =II=, 47–8;
      physiological differentiation, =II=, 258;
      dye permeability and circulation, =II=, 571;
      wood formation, =II=, 575, 577, 578;
      foliar vascular system, =II=, 589–92, 596.

    Evaporation: organic change, =I=, 28;
      vegetal circulation, =II=, 587.

    Evolution: chemical elements and compounds, =I=, 22–4, 67;
      primordial form of living matter, =I=, 63–4, 181; =II=, 21–2;
      definitions of life, =I=, 107–10;
      growth the primary trait of, =I=, 135;
      comprehends growth and development, =I=, 162;
      illustrations in development, =I=, 167–70, 178–9;
      progressive structural differentiation, =I=, 181–4, 192–6, 211–2;
      life before organization, =I=, 210;
      heterogeneity of function, =I=, 211;
      stability of species, =I=, 242, 515, 518;
      individuality, =I=, 247;
      cell-organization, =I=, 262;
      genesis, heredity, and variation resulting from, =I=, 354–5;
      period required for organic, =I=, 407, 565–6;
      contrasted with special creation hypothesis, =I=, 415, 431–40;
      derivation of hypothesis, =I=, 431, 439, 554;
      increasing belief in, =I=, 431–3, 439;
      experiences supporting conceivability, =I=, 433–5, 439;
      direct evidence, =I=, 435–7, 439;
      malevolence not implied by, =I=, 437–9;
      evidence from classification, =I=, 443, 444, 449, 466, 555;
      embryology, =I=, 451–3, 466;
      substitution and suppression of organs, =I=, 456–8, 466, 472–3;
      insect segmentation, =I=, 468–9;
      vertebral column development, =I=, 470–2;
      rudimentary organs, =I=, 472–5;
      adaptation to varied media, =I=, 479–85, 556;
      growth of the theory of organic, =I=, 490–8;
      instability of the homogeneous, a cause, =I=, 509–11, 516, 550;
      multiplication of effects, =I=, 511–14, 517–8, 550; =II=, 405;
      segregation, and heterogeneity and definiteness of, =I=, 514–8,
          550;
      natural selection and general doctrine of, =I=, 543–8, 557;
      factors tabulated, =I=, 551;
      inductive evidences summarized, =I=, 555–6;
      surviving disbelief in France, =I=, 559;
      current theories of organic, =I=, 559–74;
      Eimer’s theory of orthogenesis, =I=, 563–4;
      Gulick on monotypic and polytypic, =I=, 569;
      phenomena unexplained by theories, =I=, 573–4;
      inorganic and the _System of Philosophy_, =I=, 696;
      “spontaneous generation,” =I=, 696–701, 702;
      dissolution and problems of morphology, =II=, 4–6;
      morphology and formula, =II=, 7–9, 231–5;
      difficulties of definition, =II=, 17;
      cell-doctrine, =II=, 17–21, 85;
      unicellular origin of plants, =II=, 21–2;
      _résumé_ of plant-morphology, =II=, 78–80;
      origin and differentiation of phænogamic type, =II=, 83;
      physiological problems, =II=, 239–43;
      tissue differentiation, =II=, 244–6, 385;
      integration of organic world, =II=, 396, 406;
      race and individual multiplication, =II=, 428–30;
      declining fertility and human, =II=, 431, 529–30;
      individuation, genesis, and, =II=, 501–5;
      human life, prospective, =II=, 522–5;
      forces influencing human, =II=, 525–8;
      future of population, =II=, 532–7;
      self-sufficingness of, =II=, 537;
      vertebral, =II=, 563–6.

    Excretion: genesis of organs of, =II=, 303;
      localization of, =II=, 331–3.

    Exogen, application of term, =II=, 82.
      (_See_ Dicotyledons.)

    Expenditure (_see_ Multiplication).

    Eye, the: molecular transformations in visual process, =I=, 75–6;
      progressive development, =I=, 195; =II=, 317–9;
      waste and repair, =I=, 218;
      transmitted defects, =I=, 306, 311, 694;
      degeneration in cave-animals, =I=, 309, 612–3, 614, 647–9, 693;
      late development in insects, =I=, 658;
      migration in flat fishes, =II=, 205.


    Fabre, J. H., nutrition and sex in _Osmia tricornis_, =I=, 657.

    False joints, =I=, 230;
      theories of heredity and, =I=, 362, 364; =II=, 371–2.

    Fats, the: physical and chemical properties, =I=, 10–12;
      non-nitrogenous, =I=, 41;
      action of bile, =II=, 330.

    Fatty degeneration, and failing vitality, =I=, 41.

    Feathers, development, =I=, 474; =II=, 314–6.

    Feet, heredity and size, =I=, 311.

    Ferments, changes and nitrogenous character of, =I=, 38.

    Ferns: foliar development and nutrition, =II=, 76;
      inner tissue differentiation, =II=, 273;
      indefiniteness, =II=, 296;
      genesis, =II=, 441, 463.

    Fertility, the General Law of Animal, =I=, 577–601.
      (_See_ Multiplication.)

    Fertilization: unit-life of generative elements, =I=, 185–6;
      the function of chromatin, =I=, 260, 263–5;
      extrusion of polar bodies, =I=, 266–8;
      nature and functions of generative elements, =I=, 279–83, 317,
          334, 342, 593–7;
      differentiation and variation effected by, =I=, 330–2;
      the essential object of, =I=, 340–1; =II=, 614–6;
      hermaphrodism and self-, =I=, 341–2;
      crossing and its effects, =I=, 343–7;
      isolation of species in respect of, =I=, 570;
      floral (_see_ Flowers).

    _Ficus_, foliar structure, =II=, 589, 596.

    Fingers: embryogeny of human, =I=, 169;
      heredity and abnormal, =I=, 305, 314, 321–3;
      autogenous development of supernumerary, =I=, 363;
      rudimentary, =I=, 473.

    Fishes: sizes of ova and adult, =I=, 143–4;
      growth of pike, =I=, 154, 292;
      size and environment, =I=, 156;
      temperature, =I=, 174;
      self-mobility, =I=, 175;
      continuity of blastomeres, =I=, 214; =II=, 327;
      genesis, =I=, 271; =II=, 435, 436;
      conditions affecting genesis, =I=, 292–3, 583, 598, 599; =II=,
          454;
      classification, =I=, 392;
      change of media, =I=, 401, 480;
      distribution in time, =I=, 408–9;
      climbing species, =I=, 480, 482;
      migrations, =I=, 500;
      dermal structure, =I=, 526; =II=, 305–6, 315, 387;
      Cunningham on non-adaptive specific characters, =I=, 565;
      elongation and locomotion, =II=, 15;
      segmentation, =II=, 122, 225;
      bilateral symmetry, =II=, 203–5;
      eyes of _Pleuronectidæ_, =II=, 205;
      genesis of vertebrate axis, =II=, 212–6, 218–21, 225;
      ossification of paleozoic, =II=, 218;
      respiratory organs, =II=, 334–8;
      activity and muscular colour, =II=, 365–9;
      Owen on skeleton, =II=, 552, 557, 558–60, 562, 564.

    Fission (_see_ Agamogenesis).

    Flint, Austin, on telegony, =I=, 644.

    Flounder, symmetry and eyes, =II=, 205.

    Flower, Sir W., on ferret, =II=, 480.

    Flowers: pollen propulsion in orchids, =I=, 57;
      nature of reproductive elements, =I=, 283;
      insect fertilization, =I=, 340, 525; =II=, 168, 174, 267, 407;
      self- and mutual fertilization, =I=, 342–5, 570;
      Darwin on homologies, =I=, 472;
      direct equilibration and fertilization, =I=, 524–5;
      dimorphism, =I=, 534;
      foliar homology of petals, =II=, 43–6;
      symmetry, =II=, 132, 161, 162–4, 170, 174, 608;
      fertilization and symmetry, =II=, 164–70;
      clusters and components, =II=, 170–4;
      nutrition and inflorescence, =II=, 179–80, 541–2, 546–7;
      tissue differentiation, =II=, 265–9;
      separation of ancestral traits in hybrids, =II=, 616–7.

    Fly, beneficial parasitism, =II=, 406.

    Food (_see_ Nutrition).

    Food-cavity, genesis and development of, =I=, 188, 195.

    _Foraminifera_: form, =I=, 173;
      primary aggregate, =II=, 87, 124;
      progressing integration, =II=, 89–90, 124.

    Force: action on like and unlike units, =I=, 5;
      expenditure and organic growth, =I=, 149–54, 161;
      functional accumulation, transfer, and expenditure, =I=, 198–9,
          201–3, 391;
      waste and expenditure, =I=, 214–5;
      distribution during strain, =II=, 209–12.
      (_See also_ Energy, _and_ Persistence of Force.)

    Fossils (_see_ Palæontology).

    Foster, Sir M., on storage of glycogen, =I=, 70, 74;
      Increase of weight in hybernating dormouse, =I=, 214.

    Fowls (see _Gallinaceæ_).

    Foxglove: abnormal development, =I=, 287; =II=, 46;
      floral distribution, =II=, 141;
      nutrition and growth, =II=, 179.

    France: surviving disbelief in organic evolution, =I=, 559;
      rate of multiplication, =II=, 509, 512.

    Frankland, Sir E., on isomerism of protein, =I=, 700.

    Fraser, Col. A. T., on family of Hindu dwarfs, =I=, 316.

    Fries, E., multiplication of _Reticularia_, =I=, 582; =II=, 450.

    Frog: vitality of detached heart, =I=, 111;
      of larval fragments, =I=, 365.

    Fry, Sir E., on alternation of generations, =II=, 84.

    _Fuci_: cell multiplication, =II=, 27;
      undifferentiated outer tissue, =II=, 256.

    Function: as a basis of classification, =I=, 124–9, 129–31;
      simultaneous progress of structure and, =I=, 197, 211;
      divisions of, =I=, 198–200, 391;
      correlative complexity of structure, =I=, 200, 210–1;
      progressive differentiations, =I=, 201–4;
      concomitant integration, =I=, 205–8;
      specialization and vicariousness, =I=, 208–10;
      formula of evolution, =I=, 211;
      diminished ability and overwork, =I=, 215–6;
      growth and increased, =I=, 228–33, 234–5;
      interdependence of social and organic, =I=, 237–9, 240–2;
      structure and heredity, =I=, 306–13, 318–9 (_see_ Acquired
          Characters);
      aids natural selection, =I=, 308;
      organic interdependence, =I=, 318–9;
      parental condition and variation, =I=, 324, 326;
      variation and altered, =I=, 325–6, 333–4;
      as causing variation, =I=, 334–5;
      effect on physiological units, =I=, 353–4; =II=, 620;
      zoological classification, =I=, 391–3;
      multiplication of effects, =I=, 512;
      law of equilibration, =I=, 519–22, 557;
      correlation of changes in, =I=, 529;
      structural effects of changing, =I=, 541–2;
      structural co-operation, =II=, 3, 217;
      vicarious vegetal, =II=, 270;
      vicariousness and specialization, =II=, 293;
      epidermic structure, =II=, 312–4, 387;
      structure and muscular, =II=, 369, 391;
      adaptive bone-structures, =II=, 370–1;
      equilibration and adaptation, =II=, 392;
      persistence of force and adaptation, =II=, 394.
      (_See also_ Physiology.)

    _Fungi_: nitrogenous character, =I=, 40;
      development, =I=, 163, 164, 165;
      conjugation, =I=, 279; =II=, 449;
      fission, =I=, 584, 585;
      integration, =II=, 24–5, 293;
      symmetry, =II=, 137–40, 146;
      puff-ball tissue, =II=, 246, 252, 386;
      tissue differentiation, =II=, 256;
      inner tissue, =II=, 279;
      indefiniteness, =II=, 295;
      growth and genesis, =II=, 459;
      nutrition and genesis, =II=, 487.


    _Gallinaceæ_: conditions affecting fertility, =II=, 454–5, 469,
        471;
      masculine traits of old hens, =II=, 495.

    Galls: definition of life and, =I=, 111;
      Hertwig on, =I=, 690.

    Galton, F., on variation outside the mean, =I=, 669.

    Gamogenesis: homogenesis, =I=, 270, 271, 336;
      heterogenesis, =I=, 270, 336;
      independence of offspring, =I=, 278;
      reproductive tissue, =I=, 279–84;
      vegetal nutrition, =I=, 285–8, 293; =II=, 39;
      animal nutrition, =I=, 289–94, 297;
      when and why does it recur? =I=, 294–7, 336–40;
      effect on species, =I=, 347–9;
      leaf formation, =II=, 39;
      alternating generation in liverworts, =II=, 80–4;
      molluscan homogenesis, =II=, 116, 117–8;
      vertebrate, =II=, 118;
      growth, =II=, 266.
      (_See also_ Fertilization, _and_ Multiplication.)

    _Gasteropoda_ (see _Mollusca_).

    Geddes and Thompson, on the determination of sex, =I=, 657.

    Gelatine, nutritive value of, =I=, 77.

    Gemmation: and genesis, =I=, 272–6;
      theories of heredity and, =I=, 361;
      annulose, =II=, 100–5, 106.

    Generalization, impossibility of perfect, =I=, 450.

    Generation, and genesis: the words, =I=, 269.

    Genesis (_see_ Multiplication).

    _Gentiana_: floral arrangement, =II=, 608–11.

    Genus: indefinite value, =I=, 389, 446;
      instability of homogeneous and heterogeneity of, =I=, 509–11,
          515, 517–8, 550, 557.

    Geology: growth displayed in, =I=, 135, 136;
      distribution in time, =I=, 404–11, 412;
      special creation, =I=, 419, 426;
      evolution, =I=, 432, 437;
      record congruous with evolution, =I=, 485–9, 556;
      organic influence of changes, =I=, 501–3, 549, 550, 557;
      climatic influence of changes, =I=, 503;
      time required for organic evolution, =I=, 565–6;
      rise of insect and plant relations, =II=, 407;
      human evolution and changes, =II=, 534.

    Geometry, evolution illustrated by, =I=, 433–4.

    Germ-cell: unspecialized nature, =I=, 279–83, 317;
      dissimilarity, =I=, 330, 332, 334, 342;
      equilibrium, =I=, 340.
      (_See also_ Fertilization.)

    Germ-plasm, Weismann’s theory of, =I=, 357–8;
      inconsistent with plant embryogeny, =I=, 359;
      regeneration of lost limbs, =I=, 362;
      variations in peacock’s tail feather, =I=, 372, 695; =II=, 618–9;
      alleged differentiation of reproductive and somatic cells, =I=,
          622, 628–30, 633–44, 646;
      origin of variations in neuter insects, =I=, 659, 663–5, 671,
          675;
      correlated variations in stag, =I=, 677;
      insuperable difficulties, =I=, 682;
      conceivability of hypothesis, =I=, 695; =II=, 619;
      correlated variations in cultivated plants, =II=, 621–2.

    Ghost-theory, Vitalism and, =I=, 114.

    Giraffe, co-adaptation of structures, =I=, 615.

    Gizzard, development of birds, =II=, 320.

    Glass, molecular re-arrangement, =I=, 337, 352, 704.

    Glove, strain analogy, =II=, 575.

    Glycogen, In animal metabolism, =I=, 70, 72.

    Goethe, J. W. von: foliar homology, =II=, 43–4, 543, 544;
      archetypal hypothesis, =II=, 122;
      vegetal fructification and nutrition, =II=, 180;
      theory of supernumerary bones, =II=, 223;
      on the skull, =II=, 561.

    Gold, effect of bismuth on, =I=, 121.

    Gorilla, callosities, =II=, 312.

    Gould, J., _Birds of Australia_, =II=, 469.

    Gout (_see_ Disease).

    Grafting, Born’s experiments with frog larvæ, =I=, 365.

    Graham, T., properties of water, =I=, 9; =II=, 359;
      colloids and crystalloids, =I=, 15–8; =II=, 356;
      their diffusibility, =I=, 18–20;
      sapid and insipid substances, =I=, 53.

    _Graminæ_: foliar surfaces, =II=, 61, 263;
      floral symmetry, =II=, 165;
      physiological differentiation, =II=, 257.

    Graminivores, food contrasted with that of carnivores, =I=, 68.

    Grassi, on food habits of _Termites_, =I=, 686.

    Gravity: its ultimate incomprehensibility, =I=, 121;
      vegetal circulation, =II=, 586.
      (_See also_ Specific Gravity.)

    _Gregarina_: central development, =I=, 163;
      primary aggregate, =II=, 87;
      symmetry, =II=, 186.

    Grimaux, on artificial proteids, =I=, 39.

    Growth: organic and inorganic, =I=, 135–7;
      simulation of, =I=, 136;
      limits to, =I=, 137, 155–7;
      structural complexity, =I=, 138–40, 145–7, 161;
      nutrition, =I=, 140, 147–9, 161;
      expenditure of energy, =I=, 141–3, 161;
      initial and final bulks, =I=, 143–4, 157–60, 161;
      final arrest of, =I=, 149–55, 639;
      where unceasing, =I=, 154;
      _résumé_ with generalizations, =I=, 161;
      defined, =I=, 162; =II=, 461;
      increased function, =I=, 228–33, 234–5;
      functional interdependence, =I=, 235–9, 240;
      nutrition and vegetal, =I=, 293, 294–7, 336; =II=, 39;
      heterogenesis and animal nutrition, =I=, 289–93, 296, 336;
      homo- and heterogenesis and natural selection, =I=, 294–8;
      of acrogens, =II=, 56;
      cylindrical form of vegetal, =II=, 56–64;
      endogenous, =II=, 60–2, 78;
      exogenous, =II=, 63–4, 78;
      plant differentiation, =II=, 129–131;
      tissue differentiation, =II=, 370;
      formation of adaptive bone-structures, =II=, 370–2;
      progressive increase of size with evolution, =II=, 401–2;
      vegetal, and asexual genesis, =II=, 439–42;
      animal, and asexual genesis, =II=, 442–5;
      antagonistic to asexual genesis, =II=, 446;
      vegetal and sexual genesis, =II=, 448–51;
      animal and sexual genesis, =II=, 452–6, 495;
      antagonistic to sexual genesis, =II=, 457–8;
      nutrition and genesis, _résumé_, =II=, 497–9;
      evolution and, =II=, 501–5;
      commencement of genesis, =II=, 506;
      fertilization and restoration of growth-energy, =II=, 613.

    Gulick, T.: on monotypic and polytypic evolution, =I=, 569;
      physiological selection, =I=, 569–71.

    Gunpowder, nitrogenous instability, =I=, 8, 43.

    _Gymnotus_, electricity of, =I=, 51.

    _Gyrodactylus elegans_, rapid succession of generations, =I=, 641;
        =II=, 488.


    Habit, change of, in plants, =I=, 308.

    Hæmal, term applied to female element, =I=, 594–5.

    Hairs: non-conductors of heat, =I=, 526;
      vegetal, and natural selection, =I=, 532;
      development, =II=, 314–6;
      tactual organs, =II=, 317.

    Hand: embryogeny, =I=, 169;
      heredity and size of, =I=, 311;
      distribution of veins, =I=, 364.

    Hardy, W. B., =I=, vii; =II=, vi.

    Hare: activity and muscular colour, =II=, 365;
      expenditure and genesis, =II=, 472.

    Hart, J. A., on “Parasol” ants, =I=, 687–8.

    Havilland, G. D., collection of Termites, =I=, 687.

    Haystack, chemical action in, =I=, 74.

    Head, structural influence of size, =I=, 512, 537.

    Hearing: the sense of, =I=, 54;
      multiplying agencies, =I=, 75.

    Heart (_see_ Vascular System).

    Heat: action on di- and tri-atomic compounds, =I=, 7–8, 10–12, 23,
        24;
      on colloids and crystalloids, =I=, 26;
      organic changes from evaporation, =I=, 29;
      chemical decomposition by, =I=, 33;
      organic oxidation, =I=, 46–9, 60;
      growth and organic, =I=, 152–3;
      animal, vegetal, and environment, =I=, 174–5, 177;
      alloy melting points, =I=, 339;
      organic effects of rhythm in terrestrial, =I=, 498, 557;
      effect on physiological units, =I=, 705;
      respiration in fishes, =II=, 337;
      animal preservation, =II=, 434;
      vertebrate expenditure and genesis, =II=, 468–9, 474;
      insect genesis, =II=, 476;
      seasonal variations and genesis, =II=, 484–5;
      in germination, =II=, 615.

    Hebrew idea of creation, =I=, 421.

    _Hectocotylus_, individuality, =I=, 250.

    Hellin, D., on multiparity and twin-births, =II=, 457.

    Hen, what prompts her to pick up egg-shell fragments? =I=, 120.

    Henslow, Rev. G., inheritance of functionally-produced changes,
        =I=, 560.

    _Hepaticæ_: Schleiden on, =II=, 51, 52;
      continuous and discontinuous development, =II=, 52;
      phyletic homologies, =II=, 80–4;
      meaning of so-called alternating generation, =II=, 84;
      vascular system, =II=, 280;
      genesis and development, =II=, 463.

    Heredity: structural modification, =I=, 232;
      function of cell-nucleus in, =I=, 258–59;
      general truths, =I=, 301–4;
      transmission of congenital peculiarities, =I=, 304–7;
      structure and altered function, =I=, 307–13, 318–9 (_see also_
          Acquired Characters);
      atavism, or recurrence of ancestral traits, =I=, 314;
      sex limitation, =I=, 314–6;
      physiological units, _résumé_, =I=, 350–5; =II=, 612–6;
      Darwin’s and Weismann’s theories examined, =I=, 356 _et seq._,
          559–61; =II=, 622;
      true theory must include plants, =I=, 358;
      inadequacy of theory of physiological units, =I=, 360–1;
      sociological parallel, =I=, 366–8;
      natural selection (_q. v._), =I=, 545–7, 553, 557;
      ethnology and natural selection, =I=, 553;
      unsolved problems, =I=, 573–4;
      mutilations, =I=, 631;
      ultimate process incomprehensible, =I=, 695;
      cell-doctrine, =II=, 19;
      physiological development, =II=, 242;
      wood formation, =II=, 287;
      tissue differentiation, =II=, 304, 312–4;
      respiratory system, =II=, 311;
      osseous differentiation, =II=, 351;
      muscular adaptation, =II=, 367;
      persistence of force and physiological adaptation, =II=, 394;
      vegetal vascular system, =II=, 574, 582, 588, 596.

    Hermaphrodism, =I=, 340–3.

    Hertwig, O.: on Weismann’s germ-plasm theory, =I=, 690;
      cell characters, =I=, 691;
      meaning of fertilization, =II=, 613.

    Hertwig, R., classification of tissues, =I=, 189.

    Heterochrony of development, =I=, 655.

    Heterogeneity: in chemical evolution, =I=, 23–4;
      of vital changes, =I=, 84–90;
      of development, =I=, 170, 178;
      functional, =I=, 204–8, 211–2;
      of organic matter, =I=, 350–5;
      organic and instability of homogeneous, =I=, 509–11, 517, 549,
          557;
      segregation accompanying, =I=, 514–6, 517–8, 550.

    Heterogenesis: occurrence, =I=, 270, 272–5, 336;
      animal nutrition, =I=, 289–91, 295–7;
      natural selection, =I=, 295–8;
      heredity, =I=, 301.

    Hindus: food, =I=, 68;
      dwarf family, =I=, 316.

    Histology (_see_ Physiology).

    Hofmeister, sporophytic generation of Archegoniates, =II=, 80.

    Hollyhock, floral symmetry, =II=, 167, 169, 170.

    Homogeneous, instability of the: variation, =I=, 330, 334, 342;
      evolution, =I=, 509–11, 517, 549, 557;
      morphological development, =II=, 7–9, 234;
      direction of vegetal growth, =II=, 181;
      radial symmetry, =II=, 190;
      physiological differentiation, =II=, 384, 392.

    Homogenesis (_see_ Gamogenesis).

    Homology, simulation of, by analogy, =II=, 14.

    Hooker, Sir J. D., =I=, ix;
      European plants in New Zealand, =I=, 477;
      plant distribution, =I=, 479;
      adaptation of plants to varied media, =I=, 484;
      plant growth, =II=, 56;
      _Balanophoræ_ and _Raffiesiaceæ_, =II=, 274;
      structural complexity, =II=, 295, 297;
      relative antiquity and distribution of plants and animals, =II=,
          297;
      bean vascular system, =II=, 574.

    Hooker, Sir W., on fructification in _Jungermanniaceæ_, =II=, 52.

    Horns, natural selection and correlated variation, =I=, 537, 567,
        674, 677.

    Horse: ancestral types, =I=, 409;
      fertility, =I=, 598;
      weight of brain, =I=, 599;
      quagga markings, =I=, 624, 627.

    Husbandry, co-ordination of actions in, =I=, 96, 579.

    Hutchinson, Sir J., hereditary syphilis, =I=, 623.

    Huxley, T. H., =I=, ix;
      “continuous” and “discontinuous” development, =I=, 164;
      classification of development, =I=, 276;
      hermaphrodism, =I=, 344;
      zoological classification, =I=, 383;
      on “Persistent Types,” =I=, 408–9;
      ancestral equine types, _ib._;
      segmentation of articulates, =I=, 468–9; =II=, 113;
      agamic multiplication of _Aphis_ and _Entozoa_, =I=, 640–1; =II=,
          476;
      cell-doctrine, =II=, 21;
      vertebrate embryo, =II=, 119, 120;
      molluscan symmetry, =II=, 202;
      tegumentary organs, =II=, 314, 315;
      vertebrate sensory organs, =II=, 318, 319;
      _Chondracanthus_, =II=, 487;
      Owen’s vertebrate theory, =II=, 563.

    Hyacinth: lateral spike, =II=, 42;
      symmetry, =II=, 141, 162.

    Hybernation, waste and repair in, =I=, 214–5.

    Hybrids, separation of ancestral traits in, =II=, 616–7.

    Hydro-carbons: properties, =I=, 6–9;
      the term carbo-hydrates (_q. v._), =I=, 10.

    Hydrochloric acid, in gastric juice, =I=, 69.

    Hydrogen: chemical and physical properties, =I=, 3–5;
      compounds, =I=, 6, 8, 9, 10–12, 12–13.

    _Hydrozoa_ (see _Cœlenterata_).

    _Hymenoptera_ (_see_ Insects).

    Hypertrophy (_see_ Disease).

    Hypospadias, telegonic transmission, =I=, 646.

    Hypostasis of a relation, exemplified in explanations of
        fertilization, =II=, 613.


    Ideas (_see_ Psychology).

    Individuality: the botanical, =I=, 244–6;
      the zoological, =I=, 246–7;
      the fertilized germ product, =I=, 248–9;
      definition of life, =I=, 250.

    Individuation: and genesis, =I=, 583–4; =II=, 428–30, 499;
      total cost, =II=, 435–7;
      genesis and evolution, =II=, 501–5, 529, 530.

    _Infusoria_: functional specialization, =I=, 391;
      primary aggregate, =II=, 87;
      asymmetry, =II=, 187, 188;
      differentiation, =II=, 299, 385;
      genesis, =II=, 442, 446, 452.

    Injuries, repair of animal, =I=, 219, 222–4, 316; =II=, 102, 611.

    Insanity, inherited, =I=, 314.

    Insects: temperature, =I=, 47, 174;
      phosphorescence, =I=, 49;
      self-mobility, =I=, 175;
      parthenogenesis, =I=, 274–5, 277, 294, 592, 640;
      growth and reproduction, =I=, 292;
      species distribution determined by presence of, =I=, 396–7;
      eyes of cave-inhabiting, =I=, 309, 612–3, 614, 647–9, 693;
      persistent types, =I=, 408;
      retrograde development, =I=, 458;
      segmentation, =I=, 468–9; =II=, 114;
      aborted organs, =I=, 474;
      East Indian distribution, =I=, 478;
      floral fertilization, =I=, 525; =II=, 168–9, 406–7, 608;
      appliances for cleaning antennæ, =I=, 651;
      eyes, =I=, 658; =II=, 318;
      integration and homology, =II=, 111–3, 121;
      bilateral symmetry, =II=, 198;
      sexual selection, =II=, 269;
      eyes, =II=, 318;
      environment, =II=, 433;
      cost of genesis, =II=, 436, 437;
      development and genesis, =II=, 461;
      nutrition and genesis, =II=, 476, 490–2.

    Insects, Social, origin of caste gradations in, =I=, 654–65, 670,
        674, 675, 678–84, 686–8.

    Instability of the homogeneous (_see_ Homogeneous).

    Instinct: organic evolution and co-ordination of, in mason-wasp,
        =I=, 574;
      a vital attribute, =I=, 578;
      loss of self-feeding, in Amazon ants, =I=, 660–1, 663–4.

    Integration: in chemical evolution, =I=, 23;
      morphological composition, =II=, 4–6;
      arthropod, =II=, 111–4, 121;
      physiological, in plants, =II=, 292–5, 295–8, 390;
      of organic world, =II=, 396–408;
      genesis, =II=, 424, 426–9.

    Intelligence, a vital attribute, =I=, 579.

    Internodes: varied development, =II=, 45;
      nutrition and length, =II=, 178–9.

    Intestine (_see_ Alimentary Canal).

    Intra-selection, Roux’s theory of, =I=, 562, 676–8.

    Irish, nutrition and genesis, =II=, 510.

    Iron: colloidal form of peroxide, =I=, 17, 20;
      molecular re-arrangement, =I=, 337, 704;
      vegetal absorption, =II=, 573.

    Iron industry, interdependence of social function, =I=, 237–41.

    Isolation, and species differentiation, =I=, 568–9.

    Isomerism: of organic constituents, =I=, 4, 9, 25;
      tri- and poly-atomic compounds, =I=, 11, 13, 25;
      muscular action, =I=, 59;
      organic evolution, =I=, 700, 703;
      differentiation of nerve tissue, =II=, 356–60, 361;
      of muscular tissue, =II=, 361–4.


    Jackson, J. Hughlings, on inheritance of nervous peculiarities,
        =I=, 313, 694.

    Jaundice (_see_ Disease).

    Jaws, of uncivilized and civilized, =I=, 541–2, 612, 693.

    Johnson, G. Lindsey, on inherited myopia, =I=, 694.

    Jones, T. Rymer, on fission, =I=, 585, 590.

    Julin, C., on “castration parasitaire” in Crustaceans, =II=, 493–6.

    _Jungermanniaceæ_: morphology, =II=, 33–4;
      relations of high and low types, =II=, 35, 55;
      continuous and discontinuous development, =II=, 52–5, 92;
      tubular structure, =II=, 58, 62;
      proliferous growth, =II=, 67, 91;
      colour, =II=, 75, 265;
      symmetry, =II=, 140;
      fertility and growth, =II=, 441.

    Jussieu, A. de, plant classification, =I=, 378.


    Karyokinesis, =I=, 257, 259, 263–5.

    Kerner, A., on cauline buds, =I=, 358;
      plant classification in _Natural History of Plants_, =I=, 378–9.

    Kidd, Benj., his acceptance of Weismannism, =I=, 690.

    Kitto, Dr., his visual memory and deafness, =I=, 230.

    Klebs, on _Hydrodictyon_, =I=, 288;
      _Vaucheria_, =II=, 84.

    Klein, E., multiplication of _Bacteria_, =II=, 443.

    Korschelt, E., annulose segmentation, =II=, 103, 601–3, 605;
      _Arenicola_ larvæ, =II=, 109.


    Labour, physiological division of, =I=, 204, 207, 591; =II=, 373;
      its meaning and Weismann’s fallacious interpretation, =I=, 634–5.

    Lacaze-Duthiers, on origin of annulose type, =II=, 110.

    Lamarck: zoological classification, =I=, 382;
      opinions of E. Darwin and, =I=, 491, 493–7;
      neo-Darwinists and, =I=, 630–1.

    _Laminariaceæ_: pseudo-foliar and axial development, =II=, 30;
      tissue, =II=, 247, 256, 272.

    Language: and evolution, =I=, 442, 444, 446;
      perceptiveness of tongue-tip, =I=, 607.

    Lankester, Sir E. Ray, absence of nucleus in _Archerina_, =I=, 183;
      diversity of _Protozoa_, _ib._;
      zoological classification, =I=, 387;
      blindness of cave-animals, =I=, 647–8, 649.

    Laugh, definition of life and, =I=, 112.

    Laurel, leaves of, =II=, 149, 249.

    Leaves: growth of shoot, =I=, 168;
      development and aggregation, =II=, 37–42, 76;
      stem-like stalks, =II=, 41;
      homologies, =II=, 42, 75–7, 83;
      nutrition and compound, =II=, 42;
      foliar and axial development, =II=, 46–50, 541–7;
      “adnate,” =II=, 58;
      proliferous growth, =II=, 67, 91;
      nutrition and development, =II=, 76–8;
      symmetry, and of branches, =II=, 148–50, 151;
      size and distribution of leaflets, =II=, 152–5;
      transition from compound to simple, =II=, 155–8;
      unsymmetrical form, =II=, 158–9;
      natural selection and distribution, =II=, 179;
      morphological summary, =II=, 234–5;
      tissue differentiation, =II=, 247;
      distribution, =II=, 249;
      outer tissues of stem and, =II=, 256–9, 270, 386;
      distribution of stomata, =II=, 260–1;
      wax deposit on, =II=, 260, 261;
      light and colour, =II=, 261–2;
      superficial differentiation, =II=, 263–5, 270, 387;
      abortive in parasitic plants, =II=, 274;
      submerged, in aquatic plants, =II=, 274–5;
      inner tissue differentiation, =II=, 278, 388;
      vascular tissue differentiation, =II=, 286, 288, 388;
      dye absorption and circulation, =II=, 570–4, 577;
      vascular system, =II=, 588–92, 596;
      arrangement, =II=, 608–11.

    _Lepidoptera_ (_see_ Insects).

    _Lepidosiren_: ossification, =II=, 218;
      respiration, =II=, 338;
      skeleton, =II=, 553, 555, 560.

    _Lepidosteus_: armour, =I=, 526;
      air-bladder, =II=, 334.

    Leroy-Beaulieu, Pierre, on Australian miners’ usages, =I=, 364.

    _Lessonia_: Hooker on growth, =II=, 56;
      branch symmetry, =II=, 146.

    Lewes, G. H., definition of life, =I=, 80.

    Lichens: tissue, =I=, 586;
      cell multiplication, =II=, 27;
      Hooker on growth, =II=, 56;
      tubular structure, =II=, 57;
      integration, =II=, 293;
      dual nature, =II=, 399;
      reproduction, =II=, 450.

    Liebig, Baron, nitrogenous food stuffs, =I=, 47–8.

    Life: co-ordination of actions, =I=, 79, 89, 577–80;
      defined by Schelling, =I=, 78, 178;
      Richeraud, =I=, 79;
      De Blainville, =I=, 79, 93;
      Lewes, =I=, 80;
      definition yielded by contrasting most unlike kinds, =I=, 81–8;
      changes showing, =I=, 91;
      vital actions and environment, =I=, 92–3;
      resulting addition to conception, =I=, 93, 326;
      Comte’s definition, =I=, 93;
      correspondence of external and internal relations, =I=, 93–6,
          100; =II=, 523;
      continuous adjustment of such relations, =I=, 99;
      completeness proportionate to correspondence, =I=, 101–4, 109,
          349;
      length and complexity, =I=, 103;
      complexity of environment and degree of, =I=, 104–6;
      definitions of evolution and, =I=, 107–10;
      deficiencies of formula, =I=, 112–3;
      activity the essential element, =I=, 113;
      hypothesis of independent vital principle examined, =I=, 114–7;
      difficulties of physico-chemical theory, =I=, 117–20;
      ultimate incomprehensibility, =I=, 120–3, 373;
      validity of conclusions reached, =I=, 123;
      is organization produced by? =I=, 197;
      precedes organization, =I=, 210;
      definitions of individuality and, =I=, 250;
      effect of incident forces on, =I=, 348–9, 355;
      length in individuals and species, =I=, 422;
      equilibration of, =I=, 547, 557;
      final formulation of definition, =I=, 580;
      co-ordination of actions and sexual differentiation, =I=, 593;
      “absolute” commencement of, =I=, 699, 702;
      integration and augmentation, =II=, 426;
      prospective human, =II=, 522–5.

    Light: influence on organisms, =I=, 30–6; =II=, 433;
      nitrogenous plants, =I=, 40;
      organic phosphorescence, =I=, 49;
      heliotropism, =I=, 92; =II=, 160;
      effects on organic matter, =I=, 149;
      plant adaptation, =I=, 227;
      rhythmical variation of, and organic life, =I=, 499, 557;
      vegetal influences, =II=, 130, 131, 147, 149, 158;
      influence on flowers, =II=, 167–8, 608–11;
      vegetal tissue differentiation, =II=, 253–5, 258, 259;
      action on leaves, =II=, 260–4;
      on plant vascular system, =II=, 288, 297, 586;
      development of sensory organs, =II=, 320.

    _Liliaceæ_, floral symmetry, =II=, 170.

    Lime, leaf forms, =II=, 158, 159.

    Lindley, J., plant classification, =I=, 377.

    Linnæus, C., classificatory system, =I=, 377, 380.

    Linnet, contrasted with blackbird in development, =II=, 503.

    Liver: metabolic processes, =I=, 69, 70;
      vitality of excised, =I=, 111;
      development, =II=, 329–33.

    Liver-fluke (see _Distoma_).

    Liverworts (see _Hepaticæ_).

    Lizard, regeneration of lost tail, =I=, 360.

    Locomotion (_see_ Motion).

    Logic, reasoning and definition of life, =I=, 81–6.

    Logwood, vegetal staining, =II=, 569–74, 577–81, 584.

    Longevity, and complexity of life, =I=, 102–3.

    Lubbock, Sir J.: on growth and genesis in insects and crustaceans,
        =I=, 292;
      aquatic flies, =I=, 400.

    Lungs (_see_ Respiratory System).

    Lymphatic system: amœboid cells, =I=, 187;
      structural traits, =I=, 192, 193.


    MacBride, E. W., =I=, vi; =II=, vi;
      zoological phyla, =I=, 386–7;
      arthropod segmentation, =II=, 114;
      ctenidia of slug, =II=, 117;
      conjugation of _Paramæcium_, =II=, 452.

    _Macrocystis pyrifera_, gigantic seaweed, =I=, 121.

    Magenta, vegetal staining, =II=, 569–74, 577–81, 584.

    Magnetism: muscular action, =I=, 59;
      incomprehensibility, =I=, 121.

    Maillet, B. de, modifiability of organisms, =I=, 490, 496.

    _Mammalia_: temperature and molecular change, =I=, 30;
      nutrition and growth, =I=, 141;
      expenditure of force, =I=, 142, 156;
      flesh constituents, =I=, 154;
      temperature, =I=, 174, 177;
      self-mobility, =I=, 175;
      functional and structural differentiation, =I=, 201;
      heart-function, =I=, 206;
      viviparous homogenesis, =I=, 271;
      variation and uterine environment, =I=, 327;
      classification, =I=, 392;
      cervical vertebræ, =I=, 394; =II=, 564;
      aquatic types, =I=, 400;
      fossil remains and rate of evolution, =I=, 407;
      ancient and modern forms contrasted, =I=, 408–10;
      embryonic respiratory system, =I=, 456;
      suppression of teeth, =I=, 457;
      arrested development, =I=, 473–4;
      simulated homologies, =I=, 485;
      natural selection and inactive parts, =I=, 534;
      re-development of rudimentary organs, =I=, 563;
      location of testes and current theories, =I=, 573;
      fertility and development, =I=, 583; =II=, 465;
      fertility and nervous development, =I=, 598–9;
      locomotion and elongated form, =II=, 15;
      symmetry, =II=, 204;
      tegumentary structure, =II=, 314;
      circulation, =II=, 340;
      vascularity and ova-maturation, =II=, 342–3;
      activity and muscular colour, =II=, 365–9;
      functional integration, =II=, 375;
      outer tissue differentiation, =II=, 387;
      growth and genesis, =II=, 456, 459;
      comparative fertility, =II=, 465, 470;
      heat expenditure and genesis, =II=, 467–9;
      activity and fertility, =II=, 472;
      nutrition and genesis, =II=, 479–80.

    Man: effect of climate on vigour, =I=, 30;
      flesh and grain eaters compared, =I=, 68;
      longevity and life, =I=, 103;
      complex environment, =I=, 105;
      embryogeny of arm, =I=, 169;
      fertility and conditions affecting it, =I=, 300, 570, 583; =II=,
          484, 506–21;
      inheritance of functionally-produced changes, =I=, 310–3, 541,
          605, 608, 612, 652, 673, 689, 693–4;
      heredity and sex, =I=, 315–6;
      function of bilirubin, =I=, 330;
      cousin-marriages, =I=, 346; =II=, 615;
      primitive notions, =I=, 417–9;
      inutility of _Appendix vermiformis_, =I=, 474;
      diminution of jaw, =I=, 541, 612, 693;
      co-ordination of actions greatest in, =I=, 579;
      fundamental traits of sex, =I=, 594–7;
      obesity, =I=, 594;
      substance and weight of brain, =I=, 596, 599;
      distribution of tactual perceptiveness, =I=, 602–8, 665–6, 672–3,
          692;
      telegony, =I=, 625, 644–5;
      degradation of little toe, =I=, 652, 673;
      transmitted osteological peculiarities of Punjabis, =I=, 689;
      traits of twin-bearing women, =II=, 457;
      comparative mammalian fertility, =II=, 465;
      future evolution, =II=, 522–37.
      (_See also_ Language _and_ Sociology.)

    Manatee, nailless paddles, =I=, 473.

    Manx cats, =I=, 303.

    _Marchantiaceæ_: symmetry, =II=, 140;
      outer tissue differentiation, =II=, 252.

    Marmot, hybernation and waste, =I=, 214–5.

    Marriage (_see_ Multiplication).

    Marsh, O. C., on telegony, =I=, 644.

    Masters, M. T., on foliar homology, =II=, 46–7;
      selection of inconspicuous variations in plants, =II=, 298, 621;
      separation of ancestral constitutions in plant hybrids, =II=,
          616;
      single and double stocks, =II=, 622.

    Matter, incomprehensibility of interactions, =I=, 121–2.

    Mechanics: transverse strains, =II=, 209–12;
      genesis of vertebrate axis, =II=, 212–6, 216–8, 224, 225–7;
      osseous differentiation, =II=, 345–51;
      disintegrated motion, =II=, 375;
      analogy from locomotive, =II=, 517–9;
      future human evolution, =II=, 524;
      strain and vegetal structure, =II=, 574–88, 592–6.

    _Medusæ_: contractile functions, =I=, 58; =II=, 374;
      individuality, =I=, 248;
      heterogenesis, =I=, 273;
      fertility, =I=, 582;
      strobilization, =I=, 592;
      symmetry, =II=, 188–91.

    Mehnert, E., on feet of pentadactyle vertebrates, =I=, 461.

    Mensel’s salt, temperature and isomerism, =I=, 77.

    Metabolism: antithesis between plants and animals, =I=, 62–3;
      evolution hypothesis and primordial, =I=, 63–4;
      in plants, =I=, 64–7;
      animals, =I=, 67–77;
      nervo-muscular activities, =I=, 71–7;
      summary, =I=, 77;
      cell processes, =I=, 261.

    Metals: remarkable interactions of some, =I=, 121;
      melting of alloys, =I=, 339;
      atomic re-arrangement, =I=, 352.

    Metamerism (_see_ Segmentation).

    _Metazoa_: cellular structure, =I=, 184, 194; =II=, 21;
      subordination of units, =I=, 185–7;
      general characters of tissues, =I=, 188–9;
      protoplasmic continuity, =I=, 190–2, 194, 628;
      genesis of food-cavity and visual organ, =I=, 195;
      Weismann’s differentiation theory, =I=, 637–43.

    Meteorology: non-vital changes shown in, =I=, 82, 84;
      crystallization of “storm glass,” =I=, 96;
      special creation, =I=, 419;
      rhythm in, and organic change, =I=, 499–501, 557;
      variations due to geologic change, =I=, 503.

    _Microstomida_, segmental reproduction, =II=, 102.

    Migration: of animal species, =I=, 396–401, 411;
      solar influences, =I=, 500;
      part played by, in organic evolution, =I=, 568;
      causes of, =II=, 533–4.

    Milk, heat and supply of, =II=, 468.

    Milne-Edwards, H., “physiological division of labour,” =I=, 204;
      Weismann’s erroneous application of it, =I=, 634;
      on ocular structure, =II=, 318.

    Mind (_see_ Psychology).

    Mitosis (_see_ Karyokinesis).

    Mobility, molar and molecular, =I=, 14;
      environment and self-mobility, =I=, 177.

    Mohl, on phænogamic growth, =II=, 82.

    Mole, modifications due to habits, =II=, 391.

    Molecules: mechanically considered, =I=, 14;
      stability, =I=, 337–40;
      nerve differentiation, =II=, 355–61, 379–82.

    _Mollusca_: axial development, =I=, 165;
      genesis, =I=, 271; =II=, 444;
      hermaphrodism, =I=, 341;
      classificatory traits, =I=, 392;
      distribution in time, =I=, 405, 408, 410, 446–7;
      trochophore and its relationships, =I=, 447; =II=, 108, 109, 115;
      development, =I=, 460;
      amphibious and terrestrial, =I=, 481;
      indirect equilibration, =I=, 534;
      secondary aggregation, =II=, 115–7;
      symmetry, =II=, 201–3;
      outer tissue, =II=, 310, 387;
      alimentary system, =II=, 325;
      vascular system, =II=, 340–1.

    _Molluscoida_, =II=, 598.
      (See _Polyzoa_ and _Tunicata_.)

    Monocotyledons: growth, =I=, 138, 139, 143;
      uniaxial development, =I=, 165;
      cotyledonous germination and endogenous growth, =II=, 59–62,
          69–72, 82–3, 181–2;
      absence of helical phyllotaxy in _Ravenala_, =II=, 182;
      surface contrasts, =II=, 257;
      outer leaf tissue, =II=, 263;
      wood formation, =II=, 278;
      growth and genesis, =II=, 451.

    Monstrosities, in plants, =II=, 78, 541, 546;
      vertebrate, =II=, 118.

    Morgan, T. H., on regeneration of _Planaria_, =II=, 102, 611.

    Morphology: facts comprised in, =I=, 125–6;
      morphological units, =I=, 190–2, 225;
      rudimentary organs, =I=, 472–5, 556;
      structural and functional co-operation, =II=, 3, 239;
      integration, =II=, 4–6, 181–96;
      change of shape, =II=, 6;
      formula of evolution, =II=, 7–9;
      as interpreted by phylogeny, =II=, 10–6;
      evolution and cell-doctrine, =II=, 17–21.

    Morphology, Animal: evolution and segmentation of _Articulata_,
        =I=, 468–9;
      vertebral column development, =I=, 470;
      simulated homologies, =II=, 14–5;
      primary aggregates, =II=, 85–8, 123–4;
      secondary, =II=, 88–91, 124;
      tertiary, =II=, 91–3;
      integration and independence of individuality, =II=, 93–9, 124;
      annulose segmentation, =II=, 98–101, 106–10, 125–7, 602–7;
      progressive annulose integration, =II=, 100–5, 111–5, 121, 124,
          223;
      unintegrated molluscan form, =II=, 115–7;
      vertebrate segmentation and integration, =II=, 117–23, 124–7,
          223–4, 602, 606–7;
      motion and symmetry, =II=, 183–5;
      symmetry of primary and secondary aggregates, =II=, 186, 187–91;
      of compound _Cœlenterata_, =II=, 192–4;
      simulation of plant shapes, =II=, 192;
      symmetry of _Polyzoa_ and _Tunicata_, =II=, 194;
      of _Platyhelminthes_ and _Echinoderms_, =II=, 195–7;
      of _Annulosa_, =II=, 197–201;
      of molluscs, =II=, 201–3;
      of vertebrates, =II=, 203–6, 208;
      similarity of animal and plant, =II=, 206;
      cell-shapes, =II=, 228–30;
      evolution and generalizations summarized, =II=, 231–5.
      (_See also_ Structure.)

    Morphology, Vegetal: simulated homologies, =II=, 13–4;
      unicellular plants, =II=, 21;
      aggregation and integration, =II=, 22–6, 78–9;
      pseudo-foliar development, =II=, 26–8;
      pseudo-axial, =II=, 28–9;
      pseudo-foliar and axial, =II=, 30–2;
      composition of Archegoniates, =II=, 33–5;
      leaf development and aggregation, =II=, 37–42, 75–8;
      foliar homologies, =II=, 42–6, 75–8;
      foliar and axial development, =II=, 46–50, 541–7;
      growth and development of Archegoniates, =II=, 50–6;
      of Phænogams, =II=, 56–64, 78–80;
      axillary bud development, =II=, 65–9;
      phænogamic modes of growth, =II=, 69–72;
      homologies, =II=, 73–5, 80–4;
      development of foliar into axial organs, =II=, 75–8;
      _résumé_, =II=, 78–80;
      criticisms and replies, =II=, 80–4;
      can plant shapes be formulated? =II=, 128;
      growth and differentiation, =II=, 129–31;
      kinds of symmetry, =II=, 131–3;
      symmetry of primary aggregates, =II=, 134–7;
      of secondary, =II=, 137–40;
      tertiary, =II=, 140–3;
      symmetry and environing influences, =II=, 143–4;
      symmetry of branches, =II=, 145–8;
      leaf and branch symmetry, =II=, 148–50;
      phænogamic unit homology, =II=, 151;
      size and distribution of leaflets, =II=, 152–5;
      transition from compound to simple leaves, =II=, 155–8;
      unsymmetrical leaf development, =II=, 158–9;
      differentiation of homologous units, =II=, 159–60;
      floral symmetry, =II=, 161–74;
      cell-differentiation and metamorphosis, =II=, 175–7;
      nutrition and differentiation, =II=, 178;
      and inflorescence, =II=, 179;
      helical growth of phænogams, =II=, 180–1;
      summary of symmetry, =II=, 234;
      stress and structure, =II=, 275–9, 388.
      (_See also_ Structure.)

    Morton, Lord, quagga-marked foal, =I=, 624.

    Moser, impressions produced by light on metals, =I=, 352.

    Mosses: varied development, =II=, 501, 52;
      homologies, =II=, 80, 81;
      indefiniteness, =II=, 296;
      multiplication, =II=, 441.

    Moth, clothes, food of larva, =I=, 77.

    Motion: organic, and environment, =I=, 75–7, 175–8, 196;
      of animals and waste, =I=, 214, 220;
      simulation of locomotive structures, =II=, 15.

    Motor organs, differentiation of, =I=, 262.

    Mountains: climatic effects, =I=, 504;
      growth of trees on, =II=, 142.

    Mouse: fertility of, =II=, 421, 473;
      tapeworm parasitism, =II=, 490;
      compared with rat, =II=, 503–4.

    _Mucor_, =II=, 22, 123.

    Mucous membrane, differentiation, =II=, 321–2, 389.

    Multiplication: decline of fertility with evolution, =I=, 103;
        =II=, 431;
      vitalism, =I=, 116;
      phenomena classified, =I=, 130;
      the term “genesis,” =I=, 269;
      processes classified, =I=, 270–6, 336, 583;
      a process of disintegration, =I=, 276;
      reproductive tissue in gamogenesis, =I=, 278–84;
      nutrition and growth, =I=, 285–94, 295–7, 299;
      natural selection, =I=, 295–8;
      hermaphrodism, =I=, 340–4;
      in-and-in breeding, =I=, 344–7;
      physiological units, =I=, 350–5;
      law of race-maintenance, =I=, 581; =II=, 420–3, 430;
      effect of mental application, =I=, 597; =II=, 511–4, 516–9, 530;
      individuation antagonistic to, =I=, 598–600; =II=, 428–30, 435–7,
          499, 501–5;
      checks put by carnivores on, =II=, 405;
      four factors in rate of, =II=, 416, 435;
      destructive and preservative forces, =II=, 417–20;
      rhythm of species, =II=, 419;
      nutrition and disintegration of, =II=, 424, 425, 430;
      integration and genesis, =II=, 426–8;
      influence of environment, =II=, 432–3;
      and variations of expenditure, =II=, 433–5;
      growth and asexual genesis, =II=, 439–46;
      asexual and sexual distinguished, =II=, 448;
      sexual genesis and growth, =II=, 448–58, 495;
      and development, =II=, 461–5;
      plant expenditure, =II=, 467;
      animal expenditure, =II=, 468–72;
      nutrition in plants, =II=, 475, 511;
      in animals, =II=, 476–84, 511;
      seasonal variations, =II=, 484–5;
      nutrition, _résumé_, =II=, 486, 497–9;
      nutrition and parasitic, =II=, 486–90;
      reversion to agamogenesis, =II=, 490–2;
      human fertility, =II=, 506–10;
      Doubleday on, =II=, 510–2;
      civilized and uncivilized, =II=, 514–6;
      human evolution and decline in, =II=, 529–31;
      the future of population, =II=, 532–7;
      equilibration and evolution, =II=, 537.

    Muscle: electrical contrasts, =I=, 50;
      action of, =I=, 59;
      metabolism, =I=, 70, 71–4;
      definition of life and actions of, =I=, 112–3;
      growth and function, =I=, 151, 155;
      development, =I=, 170;
      Hertwig’s classification of tissues, =I=, 189;
      functional differentiation, =I=, 203–4;
      waste and repair, =I=, 215–7;
      modifiability and adaptability, =I=, 228–9, 230, 232;
      correlated variations, =I=, 536–9, 614–21, 676, 693;
      resistance to strains, =I=, 639;
      action on bones in Punjabis, =I=, 689;
      differentiation, =II=, 361–9;
      activity and colour, =II=, 365–9;
      integration, =II=, 376, 382;
      equilibration in action, =II=, 393;
      activity and fertility in birds, =II=, 470–2;
      future human evolution, =II=, 523;
      origin of vertebrate type, =II=, 598–600.

    Music: limited adaptability of voice and ear, =I=, 231;
      inheritance of faculty, =I=, 311–2, 694.

    Mutilations, the question of their inheritance, =I=, 631.

    _Mycetozoa_, growth and reproduction, =I=, 298–9.

    Myocommata (myotomes), and vertebrate skeleton, =II=, 216, 217–8,
        222.

    Myopia, inheritance of, =I=, 306, 694.

    _Myrianida fasciata_, =I=, 361; =II=, 445.

    _Myriapoda_: gemmation, =I=, 589;
      segmentation, =I=, 590; =II=, 113, 114, 601;
      degenerated eyes of cave-inhabiting, =I=, 649;
      integration and homology, =II=, 111–4;
      genesis, =II=, 445.

    _Myxothallophyta_, =I=, 378.


    Nails, mammalian, =I=, 473.

    _Nais_: regeneration of detached parts, =I=, 219, 361.

    Narcissus, separation of ancestral traits in hybrids, =II=, 617.

    Natural selection: structural modification, =I=, 211;
      in cell processes, =I=, 263–4;
      multiplication, =I=, 295–8;
      aided by function, =I=, 308–10;
      special creation, =I=, 426–7;
      the term “survival of the fittest,” =I=, 530;
      indirect equilibration, =I=, 530–5, 552–3, 557, 571;
      changes unexplained by, =I=, 535–42, 571; =II=, 371;
      tendency to economy, =I=, 536, 562;
      decrease of jaw, =I=, 541, 693;
      general doctrine of evolution, =I=, 543–8, 557;
      unceasing operation, =I=, 552;
      human races, =I=, 553;
      current views, =I=, 559–60;
      panmixia and cessation of selection, =I=, 560–3;
      intra-selection, =I=, 562, 676–8;
      Eimer’s theory of orthogenesis, =I=, 564;
      Mr. Cunningham’s criticism, =I=, 565–6;
      location of mammalian testes, =I=, 573;
      co-ordinated instincts of mason-wasp, =I=, 574;
      tactual perceptiveness, =I=, 603–8, 633, 646, 665, 671, 672, 692;
      erroneously identified with artificial selection, =I=, 609, 695;
      reversed selection, =I=, 611;
      blindness of cave-animals, =I=, 613, 614, 647–8, 693;
      co-adaptation of co-operative parts, =I=, 614, 621, 663–5, 670,
          674, 675, 689, 692;
      where operative, =I=, 632;
      Weismann on conceivability of process, =I=, 651;
      degeneration of little toe, =I=, 652–3, 673;
      genesis of caste gradations In social insects, =I=, 654–60, 663,
          670, 675, 684;
      self-feeding instinct in ants, =I=, 660–2, 670;
      rudimentary organs, =I=, 667–9, 671, 692;
      horns of stag, =I=, 676–8, 692;
      musical faculty, =I=, 694;
      the neo-Darwinian position reviewed, =I=, 694–5;
      vegetal nutrition, =II=, 51–2;
      upright vegetal growth, =II=, 56–7;
      endogenous growth, =II=, 57–8;
      exogenous, =II=, 64;
      _Navicula_ symmetry, =II=, 135;
      foliar, =II=, 158;
      foliar distribution, =II=, 167, 179;
      floral fertilization and symmetry, =II=, 168–70, 608–11;
      helical phænogamic growth, =II=, 181;
      _Echinodermata_ and bilateral symmetry, =II=, 197;
      vertebrate structure, =II=, 214–20, 227;
      phænogamic tissue differentiation, =II=, 248;
      physiological differentiation, =II=, 252, 256;
      rootlets of ivy, =II=, 254;
      stomata and foliar surfaces, =II=, 261, 262;
      floral fertilization, =II=, 268–9;
      sexual selection, =II=, 269;
      vegetal tissue differentiation, =II=, 279;
      wood formation, =II=, 287–8, 290;
      animal tissue differentiation, =II=, 304–8;
      evolution of nervous system, =II=, 307–8;
      respiratory system, =II=, 311;
      dermal callosities, =II=, 312–4;
      sensory organ complexities, =II=, 321;
      skin and mucous membrane differentiation, =II=, 322;
      localization of excretion, =II=, 333;
      respiratory organs of fishes, =II=, 335–8;
      heart and vascular system, =II=, 341, 344;
      osseous differentiation, =II=, 355;
      also muscular, =II=, 363, 368–9;
      “false joints,” =II=, 371;
      insect nutrition and genesis, =II=, 499;
      economics of evolution, =II=, 501–5;
      author’s enunciation of survival of the fittest in 1852, =II=,
          528–9;
      evils of interference with, in man, =II=, 532–3;
      vegetal tissue formation, =II=, 582, 594–6;
      origin of vertebrate type, =II=, 599.

    Nature, more complex than supposed, =I=, 252, 450.

    _Navicula_, symmetry, =II=, 134–5.

    “Nebular Hypothesis,” =I=, 23.

    Negation, inconceivability of, the ultimate test of truth, =I=,
        675.

    Negroes, telegony In United States, =I=, 644–5.

    _Nemertidæ_: continuing vitality of pilidium, =I=, 250;
      bilateral symmetry, =II=, 195.

    Neo-Darwinists, and Lamarck, =I=, 630;
      their position reviewed, =I=, 694–5.

    Nerves: electrical phenomena, =I=, 51;
      generation of nerve force, =I=, 52–6, 60;
      _corpuscula tactus_, =I=, 75;
      Hertwig’s classification of tissues, =I=, 189;
      structural traits, =I=, 192, 193;
      environment and structure, =I=, 196;
      differentiation, =I=, 203; =II=, 355–61;
      vasomotor system, =I=, 206;
      vicarious function, =I=, 209;
      activity and waste, =I=, 216;
      adaptability, =I=, 229, 232, 236;
      parallelism in cell processes, =I=, 260–2;
      heredity, =I=, 313;
      effects of severance, =I=, 349;
      relative development in men and women, =I=, 594;
      analysis of brain substance, =I=, 596;
      individuation and development of, =I=, 598, 599, 600;
      distribution of tactual perceptiveness, =I=, 603–8, 633, 646,
          665–6, 671, 672, 692;
      alleged costliness of tissue, =I=, 662;
      instinct degeneration in ants, _ib._;
      “sensation areas,” =I=, 666;
      segmentation in Annelids, =II=, 125;
      ectodermal derivation, =II=, 303–4;
      co-operating factors in evolution of, =II=, 307–8;
      differentiation from muscle, =II=, 363.
      (_See also_ Psychology.)

    Nervousness, hereditary transmission, =I=, 307.

    Neurine, =I=, 594, 597.

    Neuter insects (_see_ Insects).

    New Zealand: invasion of alien species, =I=, 477;
      kinship of past and present forms, =I=, 489.

    Nitrogen: properties, =I=, 3–5, 20, 24;
      compounds and their properties, =I=, 6, 8, 9, 12–14, 25–6, 39,
          41, 42–3; =II=, 250;
      organic importance, =I=, 42–3;
      evolution of heat and oxidation, =I=, 47;
      violent organic effects of compounds, =I=, 54–5;
      function in metabolism, =I=, 63–4, 66, 68–76;
      presence in protoplasm, =I=, 66;
      action in digestion, =I=, 69;
      fat accumulation and fertility, =II=, 483.

    Nitro-glycerine, violent effects of, =I=, 55, 122.

    Notochord: segmentation, =II=, 125, 218–22;
      formation, =II=, 217–8, 600;
      in Permian vertebrates, =II=, 225.

    Noumenon, life not manifested as, =I=, 580.

    Nuclein, =II=, 21.

    Nucleus: central development, =I=, 163;
      in simple organisms, =I=, 183;
      phenomena exhibited by, =I=, 255–8;
      current hypotheses of function, =I=, 258–9;
      properties and function of chromatin, =I=, 259–65;
      fusion in fertilization, =I=, 283–4;
      function in unicellular reproduction, =I=, 595–6;
      absence of, =II=, 20–1;
      diffused form, =II=, 85;
      macro- and micronucleus in conjugation, =II=, 452.

    Nutrition: organic molecular re-arrangement, =I=, 36;
      nitrogenous and non-nitrogenous, =I=, 47–8, 68, 71–4, 77; =II=,
          362;
      food assimilation and reasoning, =I=, 81;
      needful for vital change, =I=, 94;
      relation to growth, =I=, 140, 143, 144, 147–9, 150, 157, 161;
      expenditure of energy, =I=, 157, 391;
      fluid, =I=, 208;
      vegetal fructification, =I=, 267; =II=, 266;
      vegetal growth and genesis, =I=, 293, 294–7, 336;
      animal growth and genesis, =I=, 289–93, 295–7, 336;
      conditions qualifying antagonism of growth and genesis, =I=, 299;
      competition among parts of an organism for, =I=, 562, 566, 676;
      sex differentiation, =I=, 594–5;
      cell multiplication, =I=, 638;
      differentiation of neuter insects, =I=, 655–60, 670, 674, 686–8;
      monstrous ant forms, =I=, 683–4;
      leaf development, =II=, 39, 42, 73–8;
      vegetal development, =II=, 51–2, 178, 276;
      axillary buds, =II=, 65–9, 73–4;
      effect on animal aggregation, =II=, 93;
      internodes and inflorescence, =II=, 178–80;
      helical phænogamic growth, =II=, 181;
      action of bile, =II=, 330;
      osseous development, =II=, 349, 353;
      genesis, =II=, 419, 422, 427, 435–7, 452;
      parental loss in feeding young, =II=, 424, 429;
      diverse sources, =II=, 433;
      Carpenter on reproduction and, =II=, 460;
      animal development and genesis, =II=, 465;
      expenditure and genesis, =II=, 468;
      variations of genesis, =II=, 475–80, 511;
      obesity and genesis, =II=, 480–4, 511;
      general doctrine of genesis, =II=, 486;
      genesis and vegetal parasitism, =II=, 486;
      also animal, =II=, 487–90, 495;
      insect genesis, =II=, 490–2;
      genesis, _résumé_, =II=, 497–9;
      and evolution, =II=, 501–4;
      of blackbird and linnet, =II=, 503;
      genesis in human race, =II=, 508–10, 514–6;
      Doubleday on, =II=, 510–2;
      future human evolution, =II=, 526, 531;
      floral monstrosities, =II=, 542, 546, 547.


    Obesity, nutrition and genesis, =II=, 480–4, 511.

    Odoriferous glands, natural selection and, =I=, 534.

    Odours: floral fertilization, =II=, 268–9;
      animal protection, =II=, 434.

    Offspring: parental loss entailed by nurture, =II=, 424, 429;
      influence of age on, =II=, 507.

    Oken, L., archetypal hypothesis, =II=, 122;
      theory of supernumerary bones, =II=, 223;
      on the skull, =II=, 561.

    Oliver, F. W., classification of plants, =I=, 378–9.

    _Ophryotrocha puerilis_, ciliation of segments, =II=, 109.

    Orchids: pollen propulsion, =I=, 57;
      leaf formation in _Dendrobium_, =II=, 60–1;
      aërial roots and physiological differentiation, =II=, 255, 257;
      foliar surface, =II=, 264.

    Organic matter: properties of elements, =I=, 3–5, 22;
      of compounds, =I=, 5–13, 25;
      molar and molecular mobility, =I=, 12–14;
      colloid and crystalloid form, =I=, 15–8, 25;
      their diffusibility, =I=, 18–21, 26;
      extreme complexity, =I=, 21;
      laws of evolution and genesis of, =I=, 22–4;
      modifiability, =I=, 27, 44;
      capillarity and osmosis, =I=, 28;
      effects of heat, =I=, 29;
      of light, =I=, 30–4;
      nitrogenous, =I=, 39–43;
      oxidation and evolution of heat, =I=, 46, 60;
      genesis of electricity, =I=, 50–2, 60;
      sensible motions in, =I=, 59;
      transformations and persistence of force, =I=, 61;
      metabolism, =I=, 62–77;
      artificial production of compounds, =I=, 64;
      contrasted with inorganic matter, =I=, 107–8;
      incomprehensibility of vital changes in, =I=, 122;
      instability, =I=, 149, 508;
      phosphorus in cell-organization, =I=, 260–1;
      heterogeneity, =I=, 350–5;
      “spontaneous generation” and evolution of, =I=, 696–701;
      cell-doctrine and evolution of, =II=, 17–21.

    Organization (_see_ Structure).

    Ormerod, Dr., on sex and nutrition in wasps, =I=, 656.

    Orthogenesis, Eimer’s theory of, =I=, 563–4.

    Osmosis: organic effects, =I=, 28, 29;
      in animals, =I=, 58;
      in vascular system, =II=, 339;
      in vegetal tissue, =II=, 568, 575, 577, 585, 592–6.

    Osteology (_see_ Bone).

    Ovum (_see_ Egg _and_ Fertilization).

    Owen, Sir R.: metagenesis and parthenogenesis, =I=, 273–4;
      fossil mammals, =I=, 410;
      human parasites, =I=, 427;
      continuous operation of creative power, =I=, 492;
      fission in _Infusoria_, =I=, 584, 585, 595–6;
      parthenogenesis, =I=, 592;
      theory of vertebrate skeleton, =II=, 123, 548–66;
      theory of supernumerary bones, =II=, 223;
      Eschricht on _Ascaris_, =II=, 488.

    _Oxalis_: radial symmetry, =II=, 152;
      foliar surface, =II=, 264.

    Oxen: comparison with sheep, =I=, 158, 160;
      cerebro-spinal system, =I=, 508.

    Oxidation (_see_ Oxygen).

    Oxygen: properties, =I=, 3–5, 20, 22;
      compounds, =I=, 6–7, 10–13, 22, 24–5;
      a crystalloid, =I=, 21;
      combining power and atomic weight, =I=, 33;
      organic change from, =I=, 37;
      heat generation, =I=, 46–9;
      phosphorescence, =I=, 49;
      nerve force dependent on, =I=, 53;
      animal metabolism, =I=, 72, 73;
      necessary to animal life, =I=, 94–5, 577;
      activity and amount inhaled, =I=, 214.


    Packard, A. S., on eyes of cave-animals, =I=, 648–9, 693.

    Paget, Sir J., blood changes in small-pox and scarlatina, =I=, 221,
        701.

    Palæontology: distribution in time, =I=, 404–11, 412;
      special creation, =I=, 425;
      congruity with evolution hypothesis, =I=, 485–9, 556;
      relations of present to extinct species, =II=, 10–11;
      scarcity of remains, =II=, 34–5;
      secondary thickening in plants, =II=, 56;
      Cope on osteology of Permian Vertebrates, =II=, 225–6.

    Pangenesis, Darwin’s theory of, =I=, 356, 357, 359, 360, 362, 372.

    Panmixia, Weismann’s hypothesis of: Its relation to Romanes’
        “cessation of selection,” =I=, 560;
      alleged selective process denied, =I=, 561–3, 667, 685;
      distribution of tactual perceptiveness, =I=, 608;
      rudimentary eyes of cave fauna, =I=, 612–3, 647;
      Romanes on process, =I=, 649, 667;
      degeneration of self-feeding instinct In Amazon ants, =I=, 660–2,
          670;
      rudimentary limbs of whale, =I=, 668–9, 685;
      a pure speculation, =I=, 671;
      markings on leg-bones of Punjabis, =I=, 689.

    _Paramœcium_: parasite infesting, =I=, 427;
      reproduction, =II=, 443, 452.

    Parasites: sexual dimorphism, =I=, 315;
      limits to distribution, =I=, 397;
      special creation and, =I=, 427–9, 438;
      retrograde development, =I=, 457; =II=, 12;
      aphis and ant, =I=, 660–1; =II=, 403, 405;
      as an integrating agency, =II=, 402–4;
      its comparative recency, =II=, 404;
      nutrition and genesis in vegetal, =II=, 486;
      in animal, =II=, 487–90, 493;
      “castration parasitaire” in crustaceans, =II=, 493–6.

    Parasol Ants, origin of classes, =I=, 687–8.

    Parthenogenesis: occurrence, =I=, 274–5;
      alternating with gamogenesis, =I=, 289–91;
      Owen on, =I=, 592;
      laws of multiplication, =II=, 415;
      in articulate animals, =II=, 445.

    Pasteur, L., silkworm diseases, =I=, 622–3.

    Peacock: theories of heredity and structure of tail feather, =I=,
        372–3, 695; =II=, 618–9.

    Pear, foreright shoots, =I=, 287.

    Peloria: in _gloxinia_, =II=, 166;
      phænogams, =II=, 180.

    Penguin, dermal structure, =II=, 314.

    Pepsin, =I=, 69.

    Pericyclic fibres of monocotyledons, =II=, 278.

    _Peripatus capensis_, protoplasmic continuity, =I=, 629.

    Peri-visceral sac, function and differentiation, =I=, 391.

    Perkin, W. H., =I=, vi.

    Persistence of force, corollaries from: properties of compounds,
        =I=, 3;
      organic transformation, =I=, 60;
      growth, =I=, 150;
      organic energy, =I=, 220;
      variation, =I=, 335;
      genesis, heredity, and variation, =I=, 354–5;
      morphological summary, =II=, 235;
      vegetal tissue differentiation, =II=, 245;
      physiological development, =II=, 394.

    Petals: foliar homology, =II=, 43–6;
      “adnate,” =II=, 58.

    Petrels, Darwin on, =I=, 455.

    Phænogams: production of spermatozoids, =I=, 186;
      morphological composition, =II=, 37–79;
      leaf transitions, =II=, 37–42;
      foliar homologies, =II=, 42–9;
      origin of type, =II=, 49–84;
      vertical growth, =II=, 56–64;
      axillary buds, =II=, 66;
      cotyledonous germination and endogenous growth, =II=, 69–72;
      axial homologies, =II=, 73–5;
      irregular development, =II=, 75–8;
      degree of composition, =II=, 78;
      reproductive homology, =II=, 80–4;
      uni- and multiaxial symmetry, =II=, 141–3;
      unit of composition, =II=, 151;
      helical growth, =II=, 181;
      secondary thickening, =II=, 247;
      tissue and leaf differentiation, =II=, 247–9, 387;
      also bark and cambium, =II=, 249–50, 386;
      also outer tissue, =II=, 252, 256–9, 270, 386–7;
      wax deposit on leaves, =II=, 260–2;
      differentiation of inner tissues, =II=, 273–5, 388;
      vascular system development, =II=, 280–4, 388;
      integration, =II=, 293–5, 296, 390;
      insect fertilization, =II=, 407;
      multiplication, =II=, 441, 442;
      genesis and growth, =II=, 451, 457;
      and development, =II=, 464;
      and nutrition, =II=, 476, 477, 511;
      substitution of axial for foliar organs, =II=, 541–7.

    Phenomenon, life manifested as, =I=, 580.

    Philology (_see_ Language).

    _Phoronis_, individuality, =II=, 444.

    Phosphorescence, organic, =I=, 49.

    Phosphorus: allotropic, =I=, 4;
      in cell physiology, =I=, 259–62;
      cerebral activity, =I=, 596–7;
      organic evolution, =I=, 703.

    Photogenes, visibility of, =I=, 218.

    Phylogeny: as interpreting morphology, =II=, 10–12;
      difficulties of affiliation, =II=, 34–5.
      (_See_ Embryology _and_ Evolution.)

    Physiological Units: definition, =I=, 226;
      genesis, =I=, 280–1, 316;
      heredity, =I=, 315–9;
      variation, =I=, 330, 331–2, 333; =II=, 619;
      stability, =I=, 340; =II=, 614;
      self-fertilization, =I=, 342–4, 353;
      interbreeding, =I=, 345, 353; =II=, 615;
      recapitulation of hypothesis, =I=, 350–5; =II=, 612–7;
      structural proclivities, =I=, 362, 364, 369–71; =II=, 613, 622;
      sociological analogy, =I=, 364; =II=, 620;
      complexity in organized types, =I=, 368–70;
      re-named “constitutional units,” =I=, 369;
      telegony, =I=, 650;
      “mechanical theory,” =I=, 701–6;
      morphological development, =II=, 7–9;
      cell-doctrine, =II=, 17–21;
      development, =II=, 76;
      “false joints,” =II=, 371–2;
      dissociation of ancestral traits in hybrids, =II=, 616–7;
      inheritance of acquired characters, =II=, 618–23.

    Physiological division of labour (_see_ Labour).

    Physiological Selection, =I=, 569–71.

    Physiology: and psychology, =I=, 127;
      subdivisions, =I=, 128;
      vicarious function, =I=, 208;
      primitive interpretations, =I=, 417;
      multiplication of effects exemplified, =I=, 512; =II=, 390;
      relations to morphology, =II=, 3, 239–41;
      evolutionary interpretation of phenomena, =II=, 241–5, 384–95;
      ultimate inconceivability of processes, =II=, 372;
      correlated integration and differentiation, =II=, 373.

    Physiology, Animal: metabolism, =I=, 67–77;
      vertebrate internal symmetry, =II=, 108;
      tissue differentiation in _Protozoa_, =II=, 299, 385;
      primary tissue differentiation, =II=, 300–2, 382, 389;
      natural selection and tissue differentiation, =II=, 304–8;
      outer tissue in _Cœlenterata_, =II=, 309–10;
      respiratory organs, =II=, 310–1, 333–8;
      differentiation of animal epidermic tissue, =II=, 312–4, 387;
      development of tegumentary organs, =II=, 314–6;
      of sensory, =II=, 317–20;
      inner and outer tissue transition, =II=, 321–2, 389;
      alimentary canal differentiation, =II=, 323–5;
      gizzard development in birds, =II=, 325;
      alimentary canal of ruminants, =II=, 327–9;
      differentiation of liver, =II=, 329–33;
      of animal vascular system, =II=, 339–44;
      of osseous system, =II=, 344–55;
      of nerve tissue, =II=, 355–61;
      of muscle, =II=, 361–9;
      differentiation and integration, =II=, 373–6;
      in vascular system, =II=, 376–9, 383;
      in nerves, =II=, 379–82;
      origin of development, =II=, 384;
      differentiation and instability of homogeneous, =II=, 384–9, 392;
      summary of development, =II=, 384–94;
      multiplication of effects, =II=, 390–1, 392;
      equilibration, =II=, 391–4.
      (_See also_ Function.)

    Physiology, Plant: metabolism, =I=, 62–7;
      tissue differentiation in secondary aggregates, =II=, 246, 385;
      in phænogams, =II=, 247–9, 386;
      in bark and cambium, =II=, 249–50, 386;
      in free and fixed surfaces, =II=, 251–6, 270, 386;
      outer stem and leaf tissue, =II=, 256–9, 270, 386;
      superficial differentiation in leaves, =II=, 260–4, 270, 387;
      floral tissue differentiation, =II=, 265–9;
      outer tissue, _résumé_, =II=, 270;
      inner tissue differentiation, =II=, 273–5, 388;
      supporting tissue, =II=, 275–9, 285–8, 388;
      vascular system development, =II=, 273–5, 279–84, 285–8, 388;
      inner tissue, summary, =II=, 288–91, 388;
      integration, =II=, 292–8;
      differentiation and instability of homogeneous, =II=, 384–9, 392;
      multiplication of effects, =II=, 390–1, 392;
      equilibration, =II=, 391–4;
      circulation and wood formation, =II=, 564–97;
      dye permeability, =II=, 569–74, 577–81, 584, 586.
      (_See also_ Function.)

    Pickering, J. W., on artificial proteids, =I=, 39.

    Pig: colour of muscles, =I=, 365–6;
      telegony, =I=, 627;
      fertility of domestic and wild sow, =II=, 479–80.

    Pigeons: food of starving, =I=, 215;
      heredity and variation, =I=, 305, 321, 615;
      atavism, =I=, 314;
      fertility, =II=, 471–2, 478.

    Pike, unceasing growth, =I=, 154, 292.

    Pique-gouffe, commensal relations with buffalo, =II=, 403.

    _Plagiochila_, evolution of stem, =II=, 62.

    _Planaria_: integration, =II=, 101–2;
      Morgan on regeneration, =II=, 102, 611;
      segmentation, =II=, 107;
      symmetry, =II=, 195;
      unintegrated function, =II=, 373.

    Plants: influence of heat, =I=, 29;
      effect of solar rays, =I=, 31–6, 500, 557;
      chemical composition, =I=, 40–1;
      heat generation, =I=, 47;
      phosphorescence, =I=, 49;
      electricity, =I=, 51;
      sensible motion, =I=, 56–7, 58;
      metabolism, =I=, 62–7, 70;
      vital changes, =I=, 86, 87, 91, 94;
      simulation by crystals, =I=, 96;
      vital adjustments, =I=, 102;
      length and complexity of life, =I=, 103–4;
      biological classification, =I=, 125;
      growth, =I=, 136, 138, 140, 143, 145–9, 153, 160–1; =II=, 401–2;
      development, =I=, 163–5, 167–70, 272;
      weight, temperature, and self-mobility, =I=, 174;
      function, =I=, 174–8;
      structure, =I=, 194–6; =II=, 21;
      animal structure contrasted, =I=, 196;
      function and structure, =I=, 200;
      vicarious function, =I=, 208–9;
      waste and repair, =I=, 213, 220;
      physiological units, =I=, 225–6, 317, 360;
      adaptation, =I=, 227;
      what is an individual? =I=, 244–6, 250–1;
      genesis, =I=, 270, 271, 272–3, 274, 276–8, 279–85;
      relation of nutrition to growth and genesis, =I=, 284–9, 294,
          295–300, 642; =II=, 39;
      ovule homologues, =I=, 288;
      natural selection, =I=, 294–8, 532, 533; =II=, 51;
      heredity, =I=, 301–4, 308, 358–60;
      variation, =I=, 320, 323–4, 325–6;
      fertilization, =I=, 340–5;
      classification, =I=, 377–80, 389–90;
      distribution, =I=, 396–400, 401–3, 404–12, 478–9, 556;
      special creation and parasitism, =I=, 428;
      evolution hypothesis, =I=, 434, 443, 449–50;
      rudimentary organs, =I=, 474, 475, 556;
      varied media, =I=, 484; =II=, 32;
      alien and native species in New Zealand, =I=, 477;
      E. Darwin and Lamarck on evolution of, =I=, 490–8;
      geologic changes affecting, =I=, 501–3, 557;
      interdependence of animals and, =I=, 504–6, 514; =II=, 398;
      complexity of influences affecting, =I=, 506;
      direct equilibration, =I=, 523–5;
      indirect, =I=, 532, 533;
      seed distribution, =I=, 546;
      wood development, =II=, 285–7, 289, 567–97;
      interdependence, =II=, 402–3, 404;
      insect relations, =II=, 406–7;
      adaptation and multiplication, =II=, 411–6;
      rhythm in numbers, =II=, 419;
      growth and asexual genesis, =II=, 439–42;
      growth and sexual genesis, =II=, 448–51;
      expenditure, =II=, 467;
      horticulture, nutrition, and genesis, =II=, 477;
      tree development, =II=, 553;
      circulation and wood formation, =II=, 567–92;
      dye permeability and circulation, =II=, 569–74, 577–81, 584, 586;
      _résumé_ on circulation and wood formation, =II=, 592–7.
      (_See also_ Multiplication, Morphology, _and_ Physiology.)

    Plasmodium, dissolution of, =I=, 185.

    Plato, ἰδέα of, =II=, 550.

    _Platyhelminthes_: transverse fission, =II=, 101;
      segmented and non-segmented types, =II=, 102, 107;
      symmetry, =II=, 195, 197;
      multiplication and growth, =II=, 488–9.

    Plethora, fertility and, =II=, 480–4, 511.

    _Pleurococcaceæ_, unicellular form, =II=, 21, 134.

    _Pleuronectidæ_: symmetry and location of eyes, =II=, 205;
      outer tissue, =II=, 387.

    _Plumatella_: metagenesis, =I=, 277;
      symmetry, =II=, 195.

    _Podostemaceæ_, undeveloped circulatory system, =II=, 274.

    Polar bodies, hypothesis concerning extrusion of, =I=, 266–8.

    Polarity, organic, of physiological units, =I=, 226, 315, 317, 332,
        350–1, 701–6.

    Polyatomic compounds (_see_ Chemistry).

    _Polychætæ_, anomalous development in _Myrianida_, =I=, 361.

    _Polycytharia_, integration, =II=, 90, 124.

    _Polygastrica_, aggregation, =I=, 586.

    Polymerism: of compounds, =I=, 9, 11, 25;
      nerve tissue, =II=, 356.

    _Polypori_, symmetry and environment, =II=, 139.

    Polyps (see _Cœlenterata_).

    _Polyzoa_: size, =I=, 140;
      multiaxial development, =I=, 165;
      structural indefiniteness, =I=, 173;
      functional differentiation, =I=, 202;
      trochophoral kinship, =I=, 447;
      integration, =II=, 93–4, 96, 124;
      symmetry, =II=, 194, 207;
      vascular system, =II=, 340;
      gemmation, =II=, 444.

    Poor Laws, and natural selection, =II=, 532.

    _Population, A Theory of_, =I=, 265, 577–601; =II=, 411.

    Potato: simulated growth, =I=, 136;
      vicarious function of tuber, =I=, 209; =II=, 255;
      sub-species, =I=, 302;
      dye absorption, =II=, 279.

    Preservation: fertility and self-, =I=, 581; =II=, 423, 430;
      nutrition, =II=, 493.

    “Progress; its Law and Cause,” theory of species differentiation,
        =I=, 568.

    Projectiles, factors in flight of, =I=, 450–1.

    Proteids: metabolic function, =I=, 67, 68, 69, 72, 76;
      complexity of molecule, =I=, 122.

    Protein: evolution, =I=, 23, 24;
      isomerism, =I=, 700, 703, 704.

    _Proteus_, degeneration of eye, =I=, 613.

    _Protodrilus_, intestine segmentation, =II=, 125.

    _Protophyta_: internal movements, =I=, 56;
      limit of growth, =I=, 138;
      development, =I=, 164;
      structure, =I=, 173, 181–3;
      self-mobility, =I=, 175;
      individuality, =I=, 245;
      multiplication, =I=, 270, 276, 279, 581, 584–5; =II=, 439, 462;
      genesis and nutrition, =I=, 295;
      unicellular, =II=, 21;
      central aggregation, =II=, 24;
      symmetry, =II=, 134;
      tissues, =II=, 244, 249;
      primary differentiation, =II=, 385;
      primordial type, =II=, 398;
      symbiosis, =II=, 400.

    Protoplasm: self-increasing function of primordial, =I=, 63–4;
      plant metabolism, =I=, 65–7;
      complexity, =I=, 122, 253–5;
      differentiation in simple organisms, =I=, 182–3;
      continuity and inter-circulation, =I=, 190–2, 371, 629; =II=, 21,
          620;
      “streaming,” =I=, 253;
      structure, =I=, 253–5.
      (_See also_ Cell.)

    _Protozoa_: inorganic components, =I=, 17;
      locomotion, =I=, 58, 175; =II=, 14;
      vital changes shown by, =I=, 94;
      limitation of growth, =I=, 138;
      development, =I=, 164;
      structure, =I=, 173, 181–3;
      incipient differentiation, =I=, 198, 391; =II=, 299, 309;
      multiplication, =I=, 270, 276, 279, 280, 582, 584; =II=, 442,
          451–2;
      genesis and nutrition, =I=, 295;
      distribution, =I=, 396;
      parasites infesting, =I=, 427;
      Weismann’s hypothesis of immortality, =I=, 637;
      “spontaneous generation,” =I=, 697–701;
      non-nucleated, =II=, 20;
      primary aggregate, =II=, 86–7, 124;
      progressing integration, =II=, 89–91, 124;
      symmetry, =II=, 186;
      primordial plant-animal type, =II=, 397–8;
      symbiosis, =II=, 400.

    Protyle, hypothetical chemical unit, =I=, 22, 23.

    Pseud-axial development, vegetal, =II=, 28–9, 30.

    Pseudo-foliar development, vegetal, =II=, 26–8, 30.

    _Psychidæ_: parthenogenesis, =I=, 275;
      sexual dimorphism, =I=, 683.

    Psychology: reasoning and definition of life, =I=, 81–8;
      correspondence shown by _recognition_, =I=, 95;
      contrasted with physiology, =I=, 127;
      departments of, =I=, 127–8;
      vicarious function, =I=, 209;
      waste and repair in sensory organs, =I=, 217;
      sensory adaptability, =I=, 229, 231, 232;
      inheritance of sensory defects, =I=, 306;
      musical talent, =I=, 311–2;
      intellectual progress and special creation hypothesis, =I=, 417;
      special creation a pseud-idea, =I=, 420, 429, 433, 554;
      legitimacy of evolution hypothesis, =I=, 433–5, 439, 554;
      embryology of ideas, =I=, 450, 457;
      persistent formative power unrepresentable, =I=, 492;
      E. Darwin’s and Lamarck’s theory of desires, =I=, 494;
      natural selection and brain evolution, =I=, 553;
      genesis and cerebral activity, =I=, 594; =II=, 512–4, 516–9, 530;
      heredity and distribution of tactual perceptiveness, =I=, 602–8,
          646, 665–6, 672, 692;
      inconceivability of the negation, =I=, 675;
      vitiation of evidence, =II=, 88;
      repetition and perception, =II=, 143;
      differentiation of sensory organs, =II=, 317–20;
      differentiation of nerve tissue, =II=, 355–61;
      functional integration, =II=, 376;
      also integration, =II=, 380–2;
      equilibration of nerve discharge, =II=, 393;
      human fertility and nerve development, =II=, 466, 532;
      future human evolution, =II=, 523–5, 527;
      human evolution and genesis, =II=, 529–31;
      future mental development, =II=, 535;
      origin of vertebrate type, =II=, 598–600.

    _Pteridophyta_: size attained by, =I=, 138, 139;
      homologies, =II=, 80–1, 82;
      frond surface differentiation, =II=, 260.

    _Pteropoda_: bilateral symmetry, =II=, 201;
      dermal respiration, =II=, 310.

    Ptyaline, metabolic function, =I=, 69.

    Punjabis, inheritance of acquired osteological peculiarities, =I=,
        689.

    _Pyrosomidæ_: phosphorescence, =I=, 47;
      integration, =I=, 588; =II=, 97.


    Quagga, telegonic transmission of markings to offspring of mare,
        =I=, 624, 627, 646.

    Quills, development, =II=, 314–6.


    Rabbit: activity and muscle colour, =II=, 365;
      over-running checked by weasels, =II=, 405;
      expenditure and genesis, =II=, 472.

    Radial, definition, =II=, 148.

    _Radiolaria_: unicentral development, =I=, 163;
      secondary aggregation, =II=, 88;
      symmetry, =II=, 187.

    _Radula_, development of roots from leaflets, =II=, 34.

    _Rafflesiaceæ_: homogenesis, =I=, 272;
      tissue differentiation, =II=, 274;
      nutrition and genesis, =II=, 486.

    Rat (see _Rodentia_).

    Rathke, H., on vertebrate embryo, =II=, 119.

    Ray, J., plant classification, =I=, 378.

    Reasoning, compared with assimilation, =I=, 81–7.

    Recapitulation, embryological, =I=, 453.

    Regeneration (_see_ Repair).

    Rejuvenescence, and sexual fertilization, =I=, 637; =II=, 613.

    Remak, R., vertebrate embryo, =II=, 120.

    Repair: continuity of, =I=, 216–9;
      animal injuries, =I=, 219, 222–4; =II=, 102, 611;
      deductive interpretation, =I=, 221–2;
      theories of heredity and regenerative phenomena, =I=, 360–1.

    Repetition of like parts, =II=, 126.

    Reproduction (_see_ Multiplication).

    _Reptilia_: growth and expenditure of force, =I=, 142;
      sizes of ova and adult, =I=, 144;
      longevity of crocodile, =I=, 154;
      temperature, =I=, 174;
      waste, =I=, 214;
      distinctive characters, =I=, 392;
      distribution in time, =I=, 409, 412;
      vertebral segmentation, =I=, 470;
      rudimentary limbs of snakes, =I=, 473;
      fertility and development, =I=, 583, 598, 599;
      regeneration, =I=, 589;
      elongated form, =II=, 15;
      supernumerary vertebræ, =II=, 123, 564;
      bilateral symmetry, =II=, 203, 204;
      Cope on segmentation in extinct, =II=, 225, 226;
      activity and muscular colour, =II=, 365;
      functional integration, =II=, 375;
      outer tissue differentiation, =II=, 387;
      Owen on skeleton, =II=, 560.

    Resistance of media to locomotion, =II=, 15.

    Respiratory System: effect of light, =I=, 31;
      organic re-arrangement, =I=, 37;
      cutaneous, =I=, 209;
      air-cells of lungs, =I=, 254;
      embryonic branchiæ of salamander, =I=, 457;
      differentiation, =II=, 310–1, 333–8;
      physiological integration, =II=, 374–5, 382;
      vascular differentiation and integration, =II=, 377.

    Retrograde metamorphoses, in animals, =II=, 12.

    Retzius, G., superficial nerve-endings, =I=, 666.

    Reversed Selection, =I=, 611, 612.

    Rhabdospheres, calcareous armour and dynamic element in life, =I=,
        119.

    Rhizoids, foliar expansions, =II=, 50.

    _Rhizopoda_: structure, =I=, 173;
      undifferentiated function, =I=, 200;
      a primary aggregate, =II=, 86;
      symmetry, =II=, 186;
      tissue differentiated, =II=, 299, 385;
      motion of sarcode, =II=, 356;
      symbiosis, =II=, 400.

    Rhythm: astronomic and organic, =I=, 499, 557;
      law of equilibration, =I=, 520–1;
      in multiplication, =II=, 419.

    Richeraud, Baron A., definition of life, =I=, 79.

    Riley, C. V., on telegony, =I=, 645;
      _Termites_, =I=, 680, 681;
      pouch of Honey-ants, =I=, 684.

    _Rodentia_: incursions, =I=, 399;
      American types, =I=, 403;
      fertility and development, =I=, 583, 599.

    Rivinus, plant classification, =I=, 377.

    Rokitansky, on false joints, =I=, 230.

    Romanes, G. J.: on “cessation of selection,” =I=, 560–2;
      isolation and species differentiation, =I=, 569;
      “physiological selection,” =I=, 569–71;
      panmixia, =I=, 649, 667;
      influence of a previous sire on progeny, =I=, 649.

    Röntgen rays, =I=, 121; =II=, 621.

    Roots: developed from leaflets, =II=, 34;
      physiological differentiation, =II=, 253–5, 270;
      nutrition from leaves, =II=, 274;
      size and function, =II=, 276.

    _Rotiferæ_: latent vitality of desiccated, =I=, 117;
      trochopore, =II=, 108, 109;
      molluscan relationship, =II=, 115;
      fertility and size, =II=, 453, 459.

    Roux, W.: “intra-selection,” =I=, 676;
      functional adaptation, =II=, 354.

    Rudimentary organs: the definition of life and, =I=, 112;
      natural selection and eyes of cave fauna, =I=, 309, 612–4, 647–9,
          693;
      evolution hypothesis, =I=, 472–5, 556;
      limbs of whale, =I=, 668–9, 685, 693.

    Ruminants, alimentary canal development, =II=, 327–9.


    Salamander, embryonic branchiæ, =I=, 457.

    _Salmonidæ_, reproduction and growth, =I=, 291–3; =II=, 454.

    _Salpidæ_: heterogenesis, =I=, 272, 277;
      integration, =I=, 588; =II=, 97.

    Sap (_see_ Vascular system).

    _Sarcina_: central aggregation, =II=, 24;
      fertility, =II=, 440.

    Savage, Dr., on “Heredity and Neurosis,” =I=, 313.

    _Scenedesmus_, individuation, =II=, 24.

    Scent: natural selection and keenness of, =I=, 610;
      floral fertilization, =II=, 268–9;
      animal protection, =II=, 434.

    Schelling, E. W. J. von, definition of life, =I=, 78, 178.

    Schleiden, J. M., on individuality, =I=, 245;
      on liverworts, =II=, 50, 52;
      algal indefiniteness, =II=, 296.

    Science, complex revelations of, =I=, 252, 369, 450.

    _Scyphomedusæ_, strobilization, =II=, 108.

    Sea: changes and movements in, =I=, 83;
      life in, lower than terrestrial, =I=, 104;
      distribution, =I=, 396, 517;
      change of media caused by, =I=, 481;
      geologic influence, =I=, 502.

    Seals: nail-bearing toes, =I=, 473;
      _vibrissæ_, =II=, 317.

    Seasons: reproductive periodicity, =I=, 299;
      variations of genesis with, =II=, 484–5.

    Sedgwick, Adam: on continuity of protoplasm in animals, =I=, 190,
        629; =II=, 21;
      zoological classification, =I=, 387;
      discrimination of species in embryonic stages, =I=, 461;
      persistence of ancestral traits, =I=, 463–4;
      Archiannelidan segmentation, =II=, 109.

    Sedgwick, Wm.: heredity and sex, =I=, 305, 314;
      telegonic transmission of hypospadias, =I=, 646.

    Seeds: nitrogenous, =I=, 40;
      temperature of germinating, =I=, 47; =II=, 615;
      vitalism and latent vitality of, =I=, 116–7;
      variation in environment, =I=, 327;
      natural selection among, =I=, 532.

    Segmentation (metameric): special creation hypothesis, =I=, 468–9;
      Huxley on number of somites in higher articulates, _ib._;
      in annulose animals, =II=, 98–110, 111–5, 601–5;
      simulated molluscan, =II=, 116;
      in vertebrates, =II=, 125–7, 225–7, 606–7;
      in elasmobranchs, =II=, 126.

    Segregation: of growth, =I=, 136;
      of like units, =I=, 179;
      organic repair, =I=, 221;
      variation, =I=, 331, 334;
      heterogeneity, and definiteness of evolution, =I=, 514–6, 517–8;
      morphological development, =II=, 7–9;
      physiological units, =II=, 616.

    Self-fertilization, animal and vegetal, =I=, 341–4, 353.

    Senses, the (_see_ Psychology).

    Sex: in Ascidian colonies, =I=, 247;
      limitation of heredity by, =I=, 314–6;
      correlated traits, =I=, 371–2, 513;
      nutrition and determination of, in social insects, =I=, 655–60,
          678–84, 686–9;
      neural and hæmal traits, =I=, 683;
      differentiation of organs, =II=, 303;
      castration and growth, =II=, 459;
      Julin on “castration parasitaire” in crustaceans, =II=, 493–6;
      the object of fertilization, =II=, 613.
      (_See also_ Fertilization.)

    Sexual Selection (_see_ Natural Selection).

    Sharp, D.: on insect somites, =I=, 469;
      food habits of _Termites_, =I=, 686–7.

    Sheep: contrasted with oxen, =I=, 158, 160;
      crossing of English and French breeds, =I=, 625;
      nutrition and genesis, =II=, 480.

    Sherrington, Prof., on effects of nerve severance, =I=, 349.

    Ship-building, interdependence of social functions, =I=, 237–9,
        241.

    Shipley, A. E.: segmentation of _Microstomida_, =II=, 102;
      _Protodrilus_, =II=, 125.

    Silica, colloid and crystalloid, =I=, 16.

    Silicic acid: properties, =I=, 16;
      isomerism, =I=, 59.

    Silicon, allotropic, =I=, 4.

    Silkworm disease, =I=, 622–3.

    Simulation: of homology by analogy, =II=, 14, 485;
      of segmented structure by molluscs, =II=, 116.

    _Siphonophora_, specialization of component polyps, =II=, 95.

    _Sirenia_, simulated fish form, =I=, 485.

    Size (_see_ Growth).

    Skeleton, vertebrate (see _Vertebrata_).

    Skin: respiratory function, =I=, 209;
      adaptability, =I=, 228; =II=, 312–4, 387;
      transmitted peculiarities, =I=, 306;
      Wallace on distribution of sensitiveness, =I=, 646–7;
      differentiation, =II=, 215, 217, 304–7;
      tegumentary development, =II=, 314–6, 387;
      differentiation of sensory organs, =II=, 317–20;
      and mucous membrane, =II=, 303–4, 321–2, 389.

    “Skin friction,” and locomotion of aquatic animals, =I=, 156.

    Skull (see _Vertebrata_).

    Sleep, repair favoured by, =I=, 216.

    Small-pox, blood changes from, =I=, 221.

    Smith, Prof. W., on fertility of _diatomaceæ_, =II=, 440.

    Smith, W. P., on telegony in calves and foals, =I=, 645.

    Smith, W. W., on habits of Tetramorium, =I=, 660.

    Snakes (see _Reptilia_).

    “Social organism,” author’s essay on, =I=, 363, 676.

    Sociology: environment and degree of life, =I=, 105–6;
      functional differentiation, =I=, 204;
      division of labour, =I=, 207, 363–4, 367;
      functional interdependence, =I=, 237–9, 240–2;
      autogenous development of units in colonies, =I=, 364, 367–8;
          =II=, 620;
      belief in social evolution, =I=, 432;
      natural selection, =I=, 553; =II=, 532;
      integration and differentiation, =II=, 378–9;
      effects of population, =II=, 535–6;
      equilibration, =II=, 537.

    Soil, dependence of plant evolution on, =II=, 402.

    _Solanum jasminoides_, organs of attachment, =II=, 276.

    Solar system, autogenous development illustrated by distribution
        of forces in, =I=, 366.

    Sole, symmetry and location of eyes, =II=, 205.

    Soma-plasm, Weismann’s theory of differentiation from germ-plasm,
        =I=, 357, 622, 628–30, 633–44.

    Somites (_see_ Segmentation).

    Special creation: and evolution, =I=, 412, 415, 431;
      improbabilities, =I=, 418–9, 430, 439, 554;
      inconceivability, =I=, 420, 429, 431, 554;
      of individuals and species, =I=, 421–4;
      the implication of beneficence, =I=, 425–9;
      summary, =I=, 429, 554;
      Von Baer’s formula, =I=, 451–6;
      vertebrate skeleton, =II=, 551, 556, 565.

    Species: adaptation and stability, =I=, 242;
      hereditary transmission, =I=, 301–4;
      variation in wild and cultivated, =I=, 323–5, 326, 693;
      gamogenesis and life of, =I=, 347–9;
      physiological units, =I=, 362, 364, 369–71, 458; =II=, 613;
      indefiniteness, =I=, 389, 445, 572;
      special creation, =I=, 422–4;
      instability of homogeneous, and differentiation of, =I=, 509–11,
          515, 517–8, 550, 557;
      persistence of, =I=, 516, 518; =II=, 10–11;
      natural selection and equilibration, =I=, 543–8, 553, 557;
      non-adaptive characters, =I=, 565;
      morbid products as marks of, =I=, 567;
      migration and isolation as causes of differentiation, =I=, 568–9;
      increasing multiformity of aggregate, =II=, 396.

    Specific gravity, of organisms and environment, =I=, 174, 177.

    Spermatozoa}
    Sperm-cell } (_see_ Fertilization).

    Sphere: tendency of units to form, =I=, 15;
      the embryonic form, =I=, 177;
      symmetry, =II=, 131.

    Spheroid, symmetry, =II=, 132.

    Spiders (see _Arachnida_).

    Spine (see _Vertebrata_).

    Sponge: structure and dynamic element in life, =I=, 119;
      multicentral development, =I=, 164;
      units and aggregate, =I=, 185;
      reproductive tissue, =I=, 283;
      integration, =I=, 586; =II=, 90, 383;
      physiological differentiation, =II=, 300, 386;
      development and genesis, =II=, 463;
      analogy from, =II=, 576.

    Spontaneous generation: and heterogenesis, =I=, 270;
      and evolution, =I=, 696–701, 703.

    Stag, horns and correlated structures, =I=, 567, 670, 676–7, 692.

    Stamens, and foliar homology, =II=, 44.

    Starches: properties, =I=, 11;
      transformations, =I=, 66, 68, 69, 70; =II=, 593.

    Star-fishes (see _Asteroidea_).

    Statoblasts, of _Plumatella_, =I=, 277.

    Steenstrup, on “Alternate Generation,” =I=, 592.

    Sterility (_see_ Multiplication).

    Stickleback: ova, =II=, 454;
      _bothriocephalus_ in, =II=, 490.

    Stomach (_see_ Alimentary canal).

    Stomata, distribution, =II=, 260–1.

    Straight line, and evolution hypothesis, =I=, 433.

    Strain: compression and tension of, =I=, 151; =II=, 209–12;
      relation to mass, =I=, 155–7;
      vegetal structure, =II=, 574–88, 592–6;
      origin of vertebrate type, =II=, 600.

    Strawberry: multiaxial development, =I=, 166;
      multiplication, =II=, 441.

    Strength, a vital attribute, =I=, 578.

    Structure: appliances for generating motion, =I=, 75–7;
      biological classification, =I=, 125–7, 129;
      size and organic, =I=, 137;
      growth and complexity, =I=, 138, 145, 161;
      relation to environment, =I=, 172–8, 195–6;
      of unicellular organisms, =I=, 181–3;
      multicellular, =I=, 183–96;
      Hertwig’s classification of tissues, =I=, 189;
      continuity of units, =I=, 190–2;
      systems of organs, =I=, 192;
      division into universal and particular, =I=, 193–4;
      general truths, =I=, 194–5;
      plant and animal, contrasted, =I=, 195–6;
      precedence of function or, =I=, 197, 211;
      correlative complexity of function and, =I=, 200, 211;
      progressive concomitant differentiation, =I=, 201–4;
      physiological units, =I=, 225–6, 362, 364, 369–71; =II=, 613;
      social and organic interdependence, =I=, 235–42;
      varied by function, =I=, 334, 535; =II=, 217 (_see_ Acquired
          Characters);
      zoological classification, =I=, 390–2;
      equilibration, =I=, 521, 557;
      progress of, and genesis, =I=, 590–1; =II=, 462;
      co-operation with function, =II=, 3;
      evolution and increased, =II=, 4;
      retrograde metamorphosis, =II=, 12;
      simulated homologies, =II=, 13–14;
      earliest organic forms, =II=, 19;
      cylindrical vegetal, =II=, 57–62;
      permanence and complexity, =II=, 295, 296;
      function and epidermic, =II=, 312–4, 387;
      and muscular, =II=, 369, 391;
      adaptation and equilibration, =II=, 392;
      persistence of force and physiological adaptation, =II=, 394;
      evolution, =II=, 501–4.
      (_See also_ Morphology.)

    Struggle, for nutriment among components of an organism, =I=, 562,
        676;
      for existence (_see_ Natural Selection).

    Struthers, Sir J.: on heredity, =I=, 305, 314;
      digital variation, =I=, 321;
      rudimentary limbs of whale, =I=, 668.

    Strychnine, effects of, =I=, 54, 55.

    Sturgeon, size of ova and adult, =I=, 144.

    Sugars: properties, =I=, 10–11;
      transformations, =I=, 38, 40, 66, 69, 70; =II=, 593.

    Suicide, hereditary tendency to, =I=, 307.

    Sulphur: allotropic, =I=, 4, 59;
      organic evolution, =I=, 703.

    Sun (_see_ Light).

    Survival of the Fittest, the expression, =I=, 530, 610.
      (_See_ Natural Selection.)

    Swan, vertebræ of neck, =II=, 123.

    Swiftness, a vital attribute, =I=, 578.

    _Syllis ramosa_, lateral branching, =I=, 166, 361; =II=, 105, 108.

    Symbiosis, =II=, 399, 400.

    Symmetry (_see_ Morphology).

    Syphilis, hereditary transmission, =I=, 623.


    Tactual, Perceptiveness, heredity and the distribution of, =I=,
        602–8, 633, 665, 666, 672, 692.

    _Tænia_ (see _Entozoa_).

    Tansley, A. G., =I=, vi; =II=, vi;
      adaptation of reproductive activity to conditions in _Algæ_, =I=,
          288–9;
      shapes of _Caulerpa_, =II=, 22;
      stem-thickening in extinct Thallophytes, =II=, 56;
      natural selection and leaf-distribution, =II=, 179.

    Tape-worm (see _Entozoa_).

    Taste, dependent on chemical action, =I=, 54.

    Teeth: hereditary transmission, =I=, 306;
      suppression of mammalian, =I=, 457;
      of uncivilized and civilized, =I=, 541, 693.

    Tegumentary organs, origin of, =I=, 314–6.

    Telegony, or the Influence of a previous sire on offspring, =I=,
        624–7, 644–6, 649–50.

    Temperature (_see_ Heat).

    Tension (_see_ Strain).

    _Termites_: fertility, =I=, 583; =II=, 493;
      late development of sexual organs, =I=, 680;
      nutrition and differentiation of forms, =I=, 681.

    _Tetramorium_, utilization of aphides by, =I=, 660–1.

    _Thallophyta_: size, =I=, 138, 139;
      low co-ordination of parts, =I=, 164;
      pseudo-foliar, =II=, 28;
      “transition place,” =II=, 30;
      simulation of higher types, =II=, 32;
      secondary thickening in extinct species, =II=, 56;
      sexual and asexual genesis, =II=, 84.
      (See also _Algæ_.)

    Tickling, physiology of, =I=, 76.

    Tide (_see_ Sea).

    Time, as a factor in growth, =II=, 77.

    Tissue, Hertwig’s classification, =I=, 189.
      (_See_ Physiology.)

    Tongue, perceptiveness of tip, =I=, 606–8, 665, 672–3.

    Tortoise: contrasted life of dog and, =I=, 103–4;
      natural selection and carapace, =I=, 534.

    “Transcendental Physiology,” =I=, 176.

    Tree, as symbolizing phylogeny, =I=, 428, 452–3.
      (_See_ Plants.)

    _Trematoda_: agamogenesis, =I=, 277;
      parasitism, =I=, 428;
      alternate generation, =I=, 592.

    Trembley, A., on the polyp, =I=, 223.

    Trichinosis, in Germany, =I=, 428.

    Trochophore, phyletic relationships shown by, =I=, 447; =II=,
        108–9.

    _Tubicolæ_: development, =II=, 100;
      bilateral symmetry, =II=, 197.

    _Tunicata_: gemmation, =I=, 588; =II=, 445;
      alternate generation, =I=, 592;
      integration, =II=, 93–4;
      tertiary aggregation, =II=, 124;
      symmetry, =II=, 194–5.

    Tunny, size of ova and adult, =I=, 144.

    _Turbellaria_: segmentation, =II=, 102;
      symbiosis, =II=, 400.

    Turnip: chlorophyll in roots, =I=, 209; =II=, 254;
      vascular system, =II=, 281, 284, 578, 591, 596.

    Twins: similarity of, =I=, 324;
      traits of women bearing, =II=, 457.

    “Types, persistent,” Huxley on, =I=, 408.


    Ulcer, dermal structure, =II=, 306.

    Ultimate Reality, incomprehensibility of, =I=, 120.

    _Ulva_: cell multiplication, =II=, 26;
      outer tissue, =II=, 256.

    _Umbelliferæ_: floral symmetry, =II=, 171;
      axial and foliar organs, =II=, 541–6.

    United States: cases of telegony, =I=, 644–5;
      birth-rate, =II=, 520.

    Units: differentiation and dissimilarity, =I=, 20;
      “protyle,” =I=, 22–3;
      shapes in higher types, =I=, 164;
      differential assimilation, =I=, 180;
      primordial organic, =I=, 181;
      morphological composition, =I=, 184–7, 194, 252; =II=, 5, 7–9,
          21, 79, 85–6;
      segregation and organic repair, =I=, 221–2, 222–6;
      chemical, morphological, and physiological, =I=, 225–6; =II=,
          612;
      stability, =I=, 339;
      instability and heterogeneity of organic, =I=, 350;
      Darwin’s gemmules, =I=, 356–60, 362, 372;
      Weissmann’s germ-plasm (_q. v._) _ib._;
      sociological comparison, =I=, 363–8;
      specific proclivities in embryogeny, =I=, 458;
      phænogamic, =II=, 73, 151;
      annulose, =II=, 105;
      incident force and homologous, =II=, 159;
      morphological summary, =II=, 233.
      (_See also_ Physiological Units.)

    “Universal Postulate,” =I=, 675.

    Unsymmetrical, definition, =II=, 131.

    Urea, muscular energy and excretion, =I=, 72.


    Van Beneden, P. J., on _Tænia_, =II=, 103.

    Variation: digital, =I=, 331;
      effects of parental conditions, =I=, 324;
      of altered function, =I=, 325, 334, 693;
      dissimilarity of initial conditions, =I=, 327–32, 333;
      “spontaneous,” =I=, 328, 513, 697; =II=, 529;
      persistence of force, =I=, 335;
      physiological units, =I=, 348–54, 360, 369, 371–3; =II=, 614–7,
          622–3;
      Weismann’s germ-plasm theory, =I=, 357–8, 372–3, 671, 677; =II=,
          622;
      equilibration and vegetal, =I=, 523–5;
      Weismann’s panmixia theory, =I=, 561–3, 649, 667–9, 671, 685;
      reproductive organs, =I=, 570;
      natural selection and concomitant, =I=, 614–21, 653, 664, 674,
          692;
      and disused organs, =I=, 648, 668;
      plus and minus. =I=, 667, 685;
      Masters on correlated, in plants, =II=, 298, 621–2;
      equilibration of favourable, =II=, 394.

    Vascular System: effects of vegeto-alkalies, =I=, 55;
      nutrition, =I=, 146, 148;
      embryonic development, =I=, 169;
      structural traits, =I=, 192, 193;
      function, =I=, 199;
      of Ascidians. =I=, 202;
      functional differentiation and integration, =I=, 205–6;
      organic repair, =I=, 217, 221–2;
      effect of function, =I=, 229, 234–5, 236;
      equilibration, =I=, 535;
      community in compound organisms, =I=, 588;
      development of vegetal, =II=, 273–5, 279–84, 285–8, 388;
      differentiation of, summary, =II=, 288–90, 388;
      differentiation of animal, =II=, 339–44;
      osseous development, =II=, 347–51;
      muscularity, =II=, 364;
      muscular colour, =II=, 365–9;
      heart-motor apparatus, =II=, 374;
      differentiation and integration in animal, =II=, 376–9, 383;
      wood formation, =II=, 567–92;
      _résumé_ of wood formation, =II=, 592–7.

    _Vaucheria_, reproduction, =I=, 279, 289.

    Vegetative System, co-ordination of actions in, =I=, 578.

    Vegeto-alkalies, physiological effects of, =I=, 54–5.

    Velocity, of moving bodies, =II=, 219–20.

    _Vertebrata_: size, =I=, 139;
      size at birth and maturity, =I=, 144;
      axial structure, =I=, 165;
      embryonic development and self-mobility, =I=, 175;
      functional differentiation, =I=, 206, 591;
      reparative power, =I=, 219, 223, 589;
      homogenesis universal, =I=, 271;
      distinctive traits, =I=, 392; =II=, 35;
      distribution in time, =I=, 408;
      classificatory value, =I=, 446;
      embryonic mammalian respiratory system, =I=, 456;
      embryological pre-adaptation, =I=, 461;
      evolution and vertebral column, =I=, 470;
      rudimentary organs, =I=, 473;
      evolution and varied media, =I=, 479–85;
      size of head and vertebræ, =I=, 512, 537;
      segregation and evolution of vertebræ, =I=, 515;
      fertility and development, =I=, 583, 598–9;
      Weismann on reproductive cells, =I=, 635;
      limb locomotion, =II=, 15;
      adaptive segmentation, =II=, 117–23, 125–7, 223, 602, 605–7;
      supernumerary vertebræ, =II=, 123;
      bilateral symmetry, =II=, 203–6;
      internal organic symmetry, =II=, 208;
      genesis of rudimentary axis, =II=, 212–6;
      natural selection and genesis of structure, =II=, 216, 227;
      origin of notochord, =II=, 216–8;
      spinal segmentation, =II=, 218–22, 224;
      skull development, =II=, 222, 227;
      _résumé_ of axis development, =II=, 224;
      Cope on author’s theory, =II=, 225–7;
      nerve differentiation, =II=, 304;
      sensory organs, =II=, 318;
      air-chambers, =II=, 334;
      osseous differentiation, =II=, 344–55;
      activity and muscular colour, =II=, 365–9;
      heart-motor apparatus, =II=, 374;
      cost of genesis, =II=, 436;
      agamogenesis unknown, =II=, 445;
      growth and genesis, =II=, 454;
      heat expenditure and genesis, =II=, 468–9, 474;
      Owen, theory of skeleton, =II=, 548–66;
      evolution of vertebræ, =II=, 563–6;
      origin of type, =II=, 598–600.

    _Vestiges of Creation_, =I=, 491.

    Vibrissæ, function of, =I=, 75.

    Vitalism, hypothesis examined, =I=, 114–7.

    Vittadini, C., on silkworm disease, =I=, 622–3.

    Viviparons genesis, =I=, 271, 274–5, 278.

    Voice, correlated sexual traits, =I=, 371–2.

    Volcano, definition of life and, =I=, 85, 89.

    _Volvocineæ_: unicentral development, =I=, 163;
      individuality, =I=, 245;
      disintegration of genesis, =I=, 276, 587;
      spherical aggregation, =II=, 24;
      symmetry, =II=, 137, 187;
      fertility, =II=, 441.

    Vomiting, alimentary canal development, =II=, 328.

    _Vorticella_: secondary aggregate, =II=, 90;
      symmetry, =II=, 188.


    Wallace, A. R.: “The Origin of the Human Races,” =I=, 553;
      the expression “Survival of the Fittest,” =I=, 530;
      his association of natural with artificial selection, =I=, 609;
      co-adaptation in giraffe, =I=, 615;
      skin sensitiveness, =I=, 646.

    Wasp: co-ordination of instincts in Mason-, =I=, 574, 679–80;
      genesis of worker, =I=, 654–7.

    Waste, animal, =I=, 69, 213–5, 228;
      relation to activity, =I=, 196, 220–1;
      in plants, =I=, 213, 220.

    Water: properties, =I=, 7, 9;
      colloidal affinity for, =I=, 28;
      organic change from, =I=, 29;
      organic need for, =I=, 147;
      proportion in mammalian adult and fœtus, =I=, 154;
      motion through, =I=, 156;
      organic development and environment, =I=, 173, 177, 479;
      terrestrial organisms inhabiting, =I=, 400;
      adaptation of organisms to change of media, =I=, 479–85;
      vegetal tissue differentiation, =II=, 253;
      molecular re-arrangement, =II=, 359;
      colloidal contraction, =II=, 361–2.

    Water-weed, American, invasion of, =I=, 399.

    Watts, Dr., on _The Principles of Biology_, =I=, ix.

    Wax, foliar deposit, =II=, 260–1.

    Weber, on tactual discriminativeness, =I=, 602.

    Weight: relation to environment of organic, =I=, 174, 177;
      varying as cube of dimensions, =I=, 151; =II=, 434, 470.

    Weismann, Aug.: reproductive tissue in _Medusæ_, =I=, 281;
      in _Daphnidæ_, =I=, 290;
      his theory of the differentiated germ-plasm and its fundamental
          units, =I=, 357, 622–3, 628–30, 633–44, 646; =II=, 618–9,
          622;
      the alleged differentiation and plant-phenomena, =I=, 359–60;
      and regenerative processes, =I=, 360;
      false joints, =I=, 362;
      implied complexity of determinants, =I=, 370;
      theory inadequate to explain correlation of sexual traits, =I=,
          372;
      and variations in peacock’s tail feather, =I=, 372–3, 695; =II=,
          618;
      his view of natural selection as sole factor in organic evolution,
          =I=, 559;
      the doctrine of panmixia, =I=, 561–3, 612, 632, 649, 667–9, 671,
          685, 689;
      arguments against inheritance of acquired characters, =I=, 612–3,
          651–65, 669–71;
      blindness of cave-animals, =I=, 613;
      current acceptance of his views, =I=, 631, 690;
      cannot explain the process of natural selection, =I=, 651;
      the degradation of the little toe in man, =I=, 652, 669, 673;
      caste gradations of social insects, =I=, 654, 658–65, 670, 675,
          678–84, 685;
      food-seeking instinct in Amazon ants, =I=, 660, 670;
      the co-adaptation of co-operative parts, =I=, 663–4, 670, 674,
          675, 676;
      tactual discriminativeness, =I=, 665, 672;
      intra-selection, =I=, 676–8;
      effect of nutrition on fertility of blow-fly, =I=, 678–9.

    Whale: weight of brain, =I=, 599;
      rudimentary limbs, =I=, 668–9, 685, 693.

    Wheat, adaptive variations, =II=, 298.

    Whistling, definition of life and, =I=, 112.

    White-Cooper, Mr., on inheritance of abnormal vision, =I=, 306.

    Willow, nutrition and growth, =I=, 294.

    Wilson, E. B.: composition of chromatin, =I=, 260;
      separation of segmentation spheres of _Amphioxus_ ovum, =I=, 691.

    Wind: and vegetal bilateral symmetry, =II=, 142;
      and inner vegetal tissue differentiation, =II=, 275–9, 285, 288,
          388;
      and proliferation of _Bryophyllum_, =II=, 295;
      and vegetal sap movement, =II=, 583, 584, 587;
      _résumé_, 592–6.

    Wolff, C.: vegetal fructification and nutrition, =I=, 283; =II=,
        179–80;
      vegetal vascular system, =II=, 283.

    Women (_see_ Man).

    Wood (_see_ Plants).


    Yeast: fermentation, =I=, 38;
      fertility, =I=, 581; =II=, 440;
      linear aggregation, =I=, 587; =II=, 23.


    Zebra marks in horses, =I=, 314.

    Zoology, classification, =I=, 124–5, 380–9.

    Zoophytes, structural indefiniteness, =I=, 173.

    Zoospores, unit-life of, =I=, 185.

    Zygote, of conjugating _Algæ_, =I=, 283.


                               THE END.


FOOTNOTES:

[1] It seems needful here to say, that allusion is made in this
paragraph to a proposition respecting the ultimate natures of Evolution
and Dissolution, which is contained in an essay on _The Classification
of the Sciences_, published in March, 1864. When the opportunity
comes, I hope to make the definition there arrived at, the basis of a
re-organization of the second part of _First Principles_: giving to
that work a higher development, and a greater cohesion, than it at
present possesses. [The intention here indicated was duly carried out
in 1867.]

[2] Let me here refer those who are interested in this question,
to Prof. Huxley’s criticism on the cell-doctrine, published in the
_Medico-Chirurgical Review_ in 1853.

A critic who thinks the above statements are “rather misleading”
admits that the lowest types of organisms yield them support, saying
that “there are certainly masses of protoplasm containing many nuclei,
but no trace of cellular structure, in both animals and plants. Such
non-cellular masses may exist during development and later become
separated up into cells, but there are certain low organisms in which
such masses exist in the adult state. They are called by some botanists
non-cellular, by others multi-nucleate cells. Clearly the difference
lies in the criteria of a cell. There are also some _Protozoa_, and
the _Bacteria_, in which no nucleus has certainly been demonstrated.
But it is usual to consider the bodies of such organisms as cells
nevertheless, and it is supposed that such cells represent a stage of
development in which the nucleus has not yet been evolved, though the
chemical substance ‘nuclein’ has been formed in some of them.”

Perhaps it will be most correct to say that, excluding the minute,
non-nucleated organisms, all the higher organisms--_Metazoa_ and
_Metaphyta_--are composed throughout of cells, or of tissues originally
cellular, or of materials which have in the course of development been
derived from cells. It must, however, be borne in mind that, according
to sundry leading biologists, cells in the strict sense are not the
immediate products either of the primitive fissions or of subsequent
fissions; but that the multiplying so-called cells are nucleated masses
of protoplasm which remain connected by strands of protoplasm, and
which acquire limiting membranes by a secondary process. So that, in
the view of Mr. Adam Sedgwick and others, the substance of an organism
is in fact a continuous mass of vacuolated protoplasm.

[3] In further illustration, Mr. Tansley names the fact that in
the genus _Caulerpa_ we have extremely complicated forms often of
considerable size produced in the same way. The various species
simulate very perfectly the members of different groups among the
higher plants, such as Horse-tails, Mosses, Cactuses, Conifers and the
like.

[4] It may be objected that in _Cladophora_ the separate compartments
of the thallus severally contain many nuclei, making it doubtful
whether they descend from uni-nucleate cells. If, however, they do not
they simply illustrate another form of integration.

[5] The great mass of early ancestral types--plant and
animal--consisting of soft tissues, have left no remains whatever, and
we have no reason to suppose that those which left remains fell within
the direct ancestral lines of any existing forms. Contrariwise, we have
reason to suppose that they fell within lines of evolution out of which
the lines ending in existing forms diverged. We must therefore infer
that the difficulties of affiliation which arise if we contemplate
divergent types now existing, would not arise if we had before us all
the early intermediate types. The Mammalia differ in sundry respects
from all other kinds of Vertebrata--Fishes, Reptiles, Birds; and if
the absence of hair, mammæ, and two occipital condyles, in these other
vertebrates were taken to imply a fundamental distinction, it might, in
the absence of any known fossil links, be inferred that the Mammalia
belonged to a separate phylum. But these differences are not held to
negative the assumed relationship. Similarly among plants. We must
not reject an hypothesis respecting a certain supposed type, because
the existing types it must have been akin to present traits which it
could not have had. We are justified in assuming, within limits, a
hypothetical type, unlike existing types in traits of some importance.
Hence results the answer to a criticism passed on the above argument,
that it implies relations between the undeveloped and developed forms
of the _Jungermanniaceæ_ such as the facts do not show us. This
objection is met on remembering that the types in which the supposed
transition took place disappeared myriads of years ago.

[6] There is much force in the criticism passed on the above paragraph,
and by implication on some preceding paragraphs, that though in plants
which tend to produce compound leaves the production is largely
dependent on the supply of nutriment, yet the unqualified statement of
this relation as a general one, is negatived by the existence of plants
which bear only simple leaves, however much high nutrition causes
growth. But mostly valid though this objection is, it is probably not
universally valid. I am led to say this by what occasionally occurs
in flowers. The flowering stem of the Hyacinth is single; but I have
seen a cultivated Hyacinth in which one of the flowers had developed
into a lateral spike. Still more striking evidence was once supplied
to me by Agrimony. All samples of this plant previously seen had
single flowering spikes, but some years ago I met with one, extremely
luxuriant, in which some flowers of the primitive spike were replaced
by lateral spikes; and I am not sure that some of these, again, did
not bear lateral spikes. Now if in plants which, in probably millions
of cases, have their flowering stems single, excessive nutrition
changes certain of their flowers into new spikes, it is a reasonable
supposition that in like manner plants which are thought invariably to
bear only single leaves, will, under kindred conditions, bear compound
leaves.

[7] See _British and Foreign Medico-Chirurgical Review_ for January,
1862.

[8] Schleiden, who chooses to regard as an axis that which Mr.
Berkeley, with more obvious truth, calls a mid-rib, says:--“The flat
stem of the Liverworts presents many varieties, consisting frequently
of one simple layer of thin-walled cells, or it exhibits in its
axis the elements of the ordinary stem.” This passage exemplifies
the wholly gratuitous hypotheses which men will sometimes espouse,
to escape hypotheses they dislike. Schleiden, with the positiveness
characteristic of him, asserts the primordial distinction between
axial organs and foliar organs. In the higher Archegoniates he sees an
undeniable stem. In the lower Archegoniates, clearly allied to them
by their fructification, there is no structure having the remotest
resemblance to a stem. But to save his hypothesis, Schleiden calls
that “a flat stem,” which is obviously a structure in which stem and
leaf are not differentiated. He is the more to be blamed for this
unphilosophical assumption, since he is merciless in his strictures on
the unphilosophical assumptions of other botanists.

[9] To this interpretation it is objected that “the more-developed
_Jungermanniaceæ_” do not appear to have arisen from the lower forms
of _Jungermanniaceæ_--that is to say, from such lower forms as are now
existing. It may, however, be contended that this fact does not exclude
the interpretation given; since the higher forms may well have been
evolved, not from any of the lower forms we now know, but from lower
forms which have become extinct. This, indeed, is the implication of
the evolutionary process as pointed out in the note to Chap. I. If then
we assume some early type of intermediate structure, the explanation
may not improbably hold.

[10] I am indebted to Dr. Hooker for pointing out further facts
supporting this view. In his _Flora Antarctica_, he describes the genus
_Lessonia_ (see Fig. 37), and especially _L. ovata_, as having a mode
of growth simulating that of the dicotyledonous trees, not only in
general form but in internal structure. The tall vertical stem thickens
as it grows, by the periodical addition of layers to its periphery.
That even Thallophytes should thus, under certain conditions, present
a transversely-increasing axis, shows that there is nothing absolutely
characteristic of Phanerogams in their habit of stem-thickening. Mr.
Tansley gives me further verification by the statement that “it is
also now certain that members of the _Equisetineæ_ and _Lycopodineæ_,
as well as some Ferns which flourished in Carboniferous times, had
secondary thickening in their stems quite comparable to that of modern
Dicotyledonous trees.”

[11] See note at the end of the chapter.

[12] Since this paragraph was put in type [this refers to the first
edition], I have observed that in some varieties of _Cineraria_, as
probably in other plants, a single individual furnishes all these forms
of leaves--all gradations between unstipulated leaves on long petioles,
and leaves that embrace the axis. It may be added that the distribution
of these various forms is quite in harmony with the rationale above
given.

[13] Since these figures were put on the block, it has occurred to
me that the relations would be still clearer, were the primary frond
represented as not taking part in these processes of modification,
which have been described as giving rise to the erect form; as,
indeed, the rooting of its under surface will prevent it from doing
in any considerable degree. In such case, each of the Figs. 111 to
117, should have a horizontal rooted frond at its base, homologous
with the pro-embryo among Acrogens. This primary frond would then more
manifestly stand in the same relation to the rest, as the cotyledon
does to the plumule--both by position, and as a supplier of nutriment.
Fig. 117_a_, which I am enabled to add, shows that this would complete
the interpretation. Of the dicotyledonous series, it is needful to add
no further explanation than that the difference in habit of growth,
will permit the second frond to root itself as well as the first; and
so to become an additional source of nutriment, similarly circumstanced
to the first and equal with it.

[14] How the element of time modifies the result, is shown by the
familiar fact that crystals rapidly formed are small, and become
relatively large when left to form more slowly. If the quantity of
molecules contained in a solution is relatively great, so that the
mutual polarities of the molecules crowded together in every place
throughout the solution are intense, there arises a crystalline
aggregation around local axes; whereas, in proportion as the local
action of molecules on one another is rendered less intense by their
wider dispersion, they become relatively more subordinate to the forces
exerted on them by the larger aggregates of molecules that are at
greater distances, and thus are left to arrange themselves round fewer
axes into larger crystals.

[15] It is objected that these transformations should be much
commoner than they are, were they caused solely by the variations of
nutrition described. The reply is that they are comparatively rare
in uncultivated plants, where such variations are not frequent. The
occurrence of them is chiefly among cultivated plants which, being
artificially manured, are specially liable to immense accessions of
nutriment, caused now by sudden supplies of fertilizing matters, and
now by sudden arrival of the roots at such matters already deposited in
the soil. It is to these great _changes_ of nutrition, especially apt
to take place in gardens, that these monstrosities are ascribed; and it
seems to me that they are as frequent as may be expected.

[16] Since this paragraph was published in 1865, much has been learned
concerning cell-structure, as is shown in Chapter VI^A of Part I.
While some assert that there exist portions of living protoplasm
without nuclei, others assert that a nucleus is in every case present,
and that where it does not exist in a definite aggregated form it
exists in a dispersed form. As remarked in the chapter named, “the
evidence is somewhat strained to justify this dogma.” Words are taken
in their non-natural senses, if one which connotes an individualized
body is applied to the widely-diffused components of such a body; and
this perverting of proper meanings leads to obscuration of what may
perhaps be an essential truth. As argued in the chapter named (§§
74_e_, 74_f_), nuclear matter is, as shown by its chemical character,
an extremely unstable substance, the molecular changes of which,
perpetually going on, initiate shocks, producing changes all around.
In the earlier stages of cell-evolution this unstable substance is
dispersed throughout the cytoplasm; whereas in the more advanced stages
it is gathered together in one mass. If so, instead of saying there is
a dispersed nucleus we should say there are the materials of a nucleus
not yet integrated.

[17] This statement seems at variance with the figure; but the figure
is very inaccurate. Its inaccuracy curiously illustrates the vitiation
of evidence. When I saw the drawing on the block, I pointed out to
the draughtsman, that he had made the surrounding curves much more
obviously related to the contained bodies, than they were in the
original (in Dr. Carpenter’s _Foraminifera_); and having looked on
while he in great measure remedied this defect, thought no further
care was needed. Now, however, on seeing the figure in the printer’s
proof, I find that the engraver, swayed by the same supposition as the
draughtsman that such a relation was meant to be shown, has made his
lines represent it still more decidedly than those of the draughtsman
before they were corrected. Thus, vague linear representations, like
vague verbal ones, are apt to grow more definite when repeated.
Hypothesis warps perceptions as it warps thoughts.

[18] Though the subdivision into chambers of the shell does not
correspond to the subdivision into cell-units it may still be held
that since in the solitary types the subdivision of the nucleus is
followed by formation of new individuals which separate, and since in
the compound types the subdivision of the nucleus is followed by growth
and formation of new chambers, the compound type must be regarded as an
aggregate of the second order.

[19] A critic says the question is “what are the forces internal
or external which produce union or separation.” A proximate reply
is--degree of nutrition. As in a plant new individuals or rudiments of
them are cast off where nutrition is failing, so in a compound animal.
The connecting part dwindles if it ceases to carry nutriment.

[20] It has been pointed out that I have here understated the evidence
of physiological integration. An instance of it among _Hydrozoa_ is
shown in Fig. 151, but by a strange oversight I have forgotten to
name the various cases furnished by the _Siphonophora_ in which the
individual polypes of a compound aggregate are greatly specialized in
adaptation to different functions.

[21] Recently Mr. T. H. Morgan has made elaborate experiments which
show that _Planaria Maculata_ may be cut into many pieces from various
parts and of various shapes--even a slice out of the side--and each, if
not too small, will produce a perfect animal.

[22] Since this was written in 1865 there has come to light evidence
more completely to the point than any at that time known. In the
subdivision of _Platyhelminthes_ known as _Turbellaria_, there are
some, the _Microstomida_ which, by a process of segmentation form
“chains of 4, then 8, then 16, and sometimes even 32 individuals.”
“Each forms a mouth [lateral] and for some time the chain persists, but
the individuals ultimately become sexually matured and then separate.”
(Shipley, _Zoology of the Invertebrata_, p. 92.) Here it should be
remarked that the lateral mouths enable the members of a string to feed
separately, and that nutrition not being interfered with they doubtless
gain some advantage by temporary maintenance of their union--probably
in creeping.

[23] I find that the reasons for regarding the segment of a _Tænia_
as answering to an individual of the second order of aggregation, are
much stronger than I supposed when writing the above. Van Beneden
says:--“Le Proglottis (segment) ayant acquis tout son développement,
se détache ordinairement de la colonie et continue encore à croître
dans l’intestin du même animal; il change même souvent de forme et
semble doué d’une nouvelle vie; ses angles s’effacent, tout le corps
s’arrondit, et il nage comme une Planaire au milieu des muscosités
intestinales.”

[24] Though this was doubtful in 1865 it is no longer doubtful. In
an individual _Ctenodrilus monostylus_, which multiplies by dividing
and subdividing itself, “parts arise which are destitute of both head
and anus and at times consist of only a single segment.” In another
species, _C. pardalis_, there is separation into many segments; and
each segment before separating forms a budding zone out of which other
segments are afterwards produced, completing the animal (Korschelt and
Heider, _Embryology_, i, 301–2).

[25] In place of those originally here instanced about which there
are disputes, I may give an undoubted one described by McIntosh, the
_Syllis ramosa_, a species of chætopod living in hexactinellid sponges
from the Arafura Sea, which branches laterally repeatedly so as to
extend in all directions through the canals of the sponge. In most
cases the buds terminate in oval segments with two long cirri each.
But male and female buds were found, provided each with a head, and
containing ovaries and testes. Sometimes these sexual buds had become
separate from the branched stock.

[26] The name _Annulosa_, once used to embrace the _Annelida_ and
_Arthropoda_, has of late ceased to be used. It seems to me better than
_Appendiculata_, both as being more obviously descriptive and as being
more exclusive.

[27] The fusion of the segments forming the Arthropod head and the
extreme changes, or perhaps in some cases disappearances, of their
appendages, put great difficulties in the way of identification;
so that there are differences of opinion respecting the number of
included segments. Prof. MacBride writes:--“It is highly probable that
a primary head (præoral lobe or præstomium) has been derived from
annelid ancestors, but the secondary fusion of body-segments with this
head, in other words the formation of a secondary head, has gone on
independently in the different classes of the phylum _Arthropoda_,
viz., _Arachnida_, _Crustacea_, and _Tracheata_ (including Insects and
Myriapods). Judged by the number of appendages (which gives an inferior
limit) the head of a malacostracous Crustacean consists of præstomium
and 8 segments; the head of an insect of præstomium and 4 segments; the
head of a Myriapod of præstomium and 3 segments; and the head of an
Arachnid of præstomium and 3 segments.” Again, the comment of Mr. J. T.
Cunningham is:--“According to Claus and most modern authorities there
are only 5 segments in the head of an Arthropod, the eyes not counting
as appendages; and further it should be noted that the second pair of
antennæ are wanting in Insects.”

Of course difference of opinion respecting the number of somites in the
head involves difference of opinion respecting the number constituting
the entire body, which, in the higher Arthropods, is said by some to
be 19 and by others 20. But those who thus differ in detail, agree in
regarding all the segments of head and body as homologous, and this is
the essential point with which we are here concerned.

[28] Prof. MacBride corrects this statement by saying that “The
ctenidia or gills (which in _Mollusca_ generally are represented only
by a single pair) are here represented by a large number of pairs;
they do not, however, correspond in either number or position to the
shell plates.” It may, I think, be contended that if these had any
morphological significance, they would not differ in arrangement from
the shell plates, and would not be limited to this special type of
Mollusc.

[29] Though it is alleged that at a later stage the posterior part
of the skull is formed by fusion of divisions which are assumed to
represent vertebræ, yet it is admitted that the anterior part of the
skull never shows any signs of such division. Moreover in both parts
the bones show no trace of primitive segmentation.

[30] See note at the end of the chapter.

[31] A qualifying fact should be named. When the production of
vertebral segments has become constitutionally established, so that
there is an innate tendency to form them, there arises a liability to
form supernumerary ones; and this, from time to time recurring, may
lengthen the series, as in the body of a snake or the neck of a swan.
This qualification, however, affects equally the hypothesis of an ideal
type and the hypothesis of mechanical genesis.

[32] Here and throughout, the word _radial_ is applied equally to the
spiral and the whorled structures. These, as being alike on all sides,
are similarly distinguished from arrangements that are alike on two
sides only.

[33] It should be added that this change of distribution is not due to
change in the relative positions of the insertions of the leaves but to
their twistings.

[34] We may note that some of these leaves, as those of the Lime,
furnish indications of the ratio which exists between the effects of
individual circumstances and those of typical tendencies. On the one
hand, the leaves borne by these drooping branches of the Lime are
with hardly an exception unsymmetrical more or less decidedly, even
in positions where the causes of unsymmetry are not in action: a fact
showing us the repetition of the type irrespective of the conditions.
On the other hand, the degree of deviation from symmetry is extremely
variable, even on the same shoot: a fact proving that the circumstances
of the individual leaf are influential in modifying its form. But the
most striking evidence of this direct modification is afforded by the
suckers of the Lime. Growing, as these do, in approximately upright
attitudes, the leaves they bear do not stand to one another in the way
above described, and the causes of unsymmetry are not in action; and
here, though there is a general leaning to the unsymmetrical form, a
large proportion of the leaves become quite symmetrical.

[35] It was by an observation on the forms of leaves, that I was first
led to the views set forth in the preceding and succeeding chapters
on the morphological differentiation of plants and animals. In the
year 1851, during a country ramble in which the structures of plants
had been a topic of conversation with a friend--Mr. G. H. Lewes--I
happened to pick up the leaf of a buttercup, and, drawing it by its
foot-stalk through my fingers so as to thrust together its deeply-cleft
divisions, observed that its palmate and almost radial form was changed
into a bilateral one; and that were the divisions to grow together in
this new position, an ordinary bilateral leaf would result. Joining
this observation with the familiar fact that leaves, in common with
the larger members of plants, habitually turn themselves to the
light, it occurred to me that a natural change in the circumstances
of the leaf might readily cause such a modification of form as that
which I had produced artificially. If, as they often do with plants,
soil and climate were greatly to change the habit of the buttercup,
making it branched and shrub-like; and if these palmate leaves were
thus much overshadowed by one another; would not the inner segments
of the leaves grow towards the periphery of the plant where the light
was greatest, and so change the palmate form into a more decidedly
bilateral form? Immediately I began to look round for evidence of the
relation between the forms of leaves and the general characters of the
plants they belong to; and soon found some signs of connexion. Certain
anomalies, or seeming anomalies, however, prevented me from then
pursuing the inquiry much further. But consideration cleared up these
difficulties; and the idea afterwards widened into the general doctrine
here elaborated. Occupation with other things prevented me from giving
expression to this general doctrine until Jan. 1859; when I published
an outline of it in the _Medico-Chirugical Review_.

[36] It is objected to the above interpretation that “many flowers of
sizes intermediate between the Hollyhock and the Agrimony are radially
symmetrical and yet grow sideways. I may mention various _Liliaceæ_,
e.g. _Chlorophytum_, _Eucomis_, _Muscari_, _Anthericum_. _Sagittaria_,
also, has many of its flowers in this position. Further, if the higher
insects alight on flowers in a definite way, as they do, the parts
of the flower must bear different relations to the visiting insect,
however large, so that flowers unvisited ought all to be zygomorphic.”
My reply is that in the sense which here concerns us, the different
petals of the Hollyhock-flower do not bear different relations to the
visiting insect; since, practically, the upper and lateral petals
bear no physical relations at all: in so far as the visiting bee is
concerned they are non-existent. The argument implies that change in
the form of a flower from the radial to the bilateral is likely to take
place only when the contact-relations of the petals to the visiting
insect, are such as to make some forms facilitate its action more than
others; and the large petals of the Hollyhock cannot facilitate its
action at all. In respect of the _Liliaceæ_ instanced, it is needful
to inquire whether the structures are such that this alleged cause of
bilateral symmetry can come into play.

[37] I had intended here to insert a figure exhibiting these
differences; but as the Cow-parsnip does not flower till July, and as
I can find no drawing of the umbel which adequately represents its
details, I am obliged to take another instance.

[38] It has been pointed out to me that “the extreme development of the
corolla so often found in the outer flowers or on the outer side of the
outer flowers in closely-packed inflorescences, associated as it often
is with disappearance of stamens or carpels or both, is usually put
down to specialization of these outer flowers for attractive purposes.
Since the whole inflorescence is increased in conspicuousness by such
a modification, it is supposed that natural selection favoured those
plants which sacrificed a portion of their seed-bearing capacity for
the supposed greater advantage of securing more insect visits.” But
granting this interpretation, it may still be held that increase of
attractiveness due to increase of area must be achieved by florets at
the periphery, and that their ability to achieve it depends on their
having an outer, unoccupied, space which the inner florets have not;
so that, though in a more indirect way, their different development is
determined by different exposure to conditions.

[39] One of my critics writes:--“This chapter might of course be
enormously extended, not only as in the preceding ones by citation of
quite similar cases, but by the introduction of fresh groups of cases.”

[40] Natural selection may have operated in establishing a
constitutional tendency to other sudden abridgments. Mr. Tansley
alleges that this is a part-cause of the varying distribution of
leaves. He says:--“I have myself made some observations on the length
of internodes in the Beech, and am satisfied that it follows quite
other laws, connected with the suitable disposition of the leaves on
the branch. Although I have not had the opportunity of following up
this line of work so as in any way to generalize the results, I suspect
that ‘indirect equilibration’ is a widespread cause of such variation.”

[41] It is but just to the memory of Wolff, here to point out that
he was immensely in advance of Goethe in his rationale of these
metamorphoses. Whatever greater elaboration Goethe gave to the theory
considered as an induction, seems to me more than counter-balanced
by the irrationality of his deductive interpretation; which unites
mediæval physiology with Platonic philosophy. A dominant idea with him
is that leaves exist for the purpose of carrying off crude juices--that
“as long as there are crude juices to be carried off, the plant must
be provided with organs competent to effect the task”; that while “the
less pure fluids are got rid of, purer ones are introduced” and that
“if nourishment is withheld, that operation of nature (flowering) is
facilitated and hastened; the organs of the nodes (leaves) become
more refined in texture, the action of the purified juices becomes
stronger, and the transformation of parts having now become possible,
takes place without delay.” This being the proximate explanation, the
ultimate explanation is, that Nature wishes to form flowers--that when
a plant flowers it “attains the end prescribed to it by nature”; and
that so “Nature at length attains her object.” Instead of vitiating
his induction by a teleology that is as unwarranted in its assigned
object as in its assigned means, Wolff ascribes the phenomena to a
cause which, whether sufficient or not, is strictly scientific in its
character. Variation of nutrition is unquestionably a “true cause” of
variation in plant-structure. We have here no imaginary action of a
fictitious agency; but an ascertained action of a known agency.

[42] The _Natural History Review_ for July, 1865, contained an article
on the doctrine of morphological composition set forth in the foregoing
Chaps. I. to III. In this article, which unites exposition and
criticism in a way that is unhappily not common with reviewers, it is
suggested that the spiral structure may be caused by natural selection.
When this article appeared, the foregoing five pages were standing over
in type, as surplus from No. 14, issued in June, 1865.

[43] A verifying comment on this paragraph runs as follows:--“In the
Hypotricha Infusoria, which creep over solid surfaces, there is a
differentiation between ventral and dorsal surface and an approach to
bilateral symmetry. The ventral surface is provided with movable cilia,
the dorsal with immobile setæ.”

[44] Criticisms on the above passage have shown the need for naming
sundry complications. These complications chiefly, if not wholly,
arise from changes in modes of life--changes from the locomotive to
the stationary, and from the stationary to the locomotive. Referring
to my statement that (ignoring the spherical) the radial type is the
lowest and must be taken as antecedent to the bilateral type, it is
alleged that all existing “radial animals above Protozoa are probably
derived from free-swimming, bilaterally-symmetrical animals.” If this
is intended to include the planulæ of the hydroid polyps, then it seems
rather a straining of the evidence. These locomotive embryos, described
as severally having the structure of a gastrula with a closed mouth,
can be said to show bilateralness only because the first two tentacles
make their appearance on opposite sides of the mouth--a bilateralness
which lasts only till two other tentacles make their appearance in a
plane at right angles, so giving the radial structure. I think the
criticism applies only to cases furnished by Echinoderms. The larvæ
of these creatures have bilaterally-symmetrical structures, which
they retain as long as they swim about and which such of them as fix
themselves lose by becoming similarly related to conditions all round:
the radial structure being retained by those types which, becoming
subsequently detached, move about miscellaneously. But, as happens
in some of the Sea-urchins and still more among the Holothurians,
the structure is again made bilaterally-symmetrical by a locomotive
life pursued with one end foremost. Should it be contended that the
conditions and the forms are reciprocally influential--that either may
initiate the other, it still remains unquestionable that ordinarily the
conditions are the antecedents, as is so abundantly shown by plants.

[45] Should it be proved that the Ascidian is a degraded vertebrate,
then the argument will be strengthened; since loss of bilateral
symmetry has gone along with change to asymmetrical conditions.

[46] A critical comment made on this sentence runs as follows:--“The
aërial roots of most epiphytic orchids contain chlorophyll in their
cortex throughout their length, but the cortex being covered by a
‘velamen’ of air-containing cells which break up and reflect incident
light, the green colour is not visible through this opaque coat. When
moistened the cells of the velamen take up water and the green colour
immediately shows through. Such roots do not however possess stomata.
The roots of certain species of _Angræcum_, however, contain the whole
of the assimilating tissue of the plant.”

[47] The current doctrine that chlorophyll is _the_ special substance
concerned in vegetal assimilation, either as an agent or as an
incidental product, must be taken with considerable qualification.
Besides the fact that among the _Algæ_ there are many red and brown
kinds which thrive; and besides the fact that among the lower
Archegoniates there are species which are purple or chocolate-coloured;
there is the fact that Phænogams are not all green. We have the
Copper-Beech, we have the black-purple _Coleus Verschaffeltii_, and we
have the red variety of Cabbage, which seems to flourish as well as
the other varieties. Chlorophyll, then, must be regarded simply as the
most general of the colouring matters found in those parts of plants in
which assimilation is being effected by the agency of light. Though it
is always present _along with_ the red and brown pigments, yet there is
much evidence to show that these are the actual assimilative pigments.

[48] This seems as fit a place as any for noting the fact, that the
greater part of what we call beauty in the organic world, is in some
way dependent on the sexual relation. It is not only so with the
colours and odours of flowers. It is so, too, with the brilliant
plumage of birds; and it is probable that the colours of the more
conspicuous insects are in part similarly determined. The remarkable
circumstance is, that these characteristics, which have originated
by furthering the production of the best offspring, while they are
naturally those which render the organisms possessing them attractive
to one another, directly or indirectly, should also be those which are
so generally attractive to us--those without which the fields and woods
would lose half their charm. It is interesting, too, to observe how the
conception of human beauty is in a considerable degree thus originated.
And the trite observation that the element of beauty which grows out of
the sexual relation is so predominant in æsthetic products--in music,
in the drama, in fiction, in poetry--gains a new meaning when we see
how deep down in organic nature this connexion extends.

[49] Students of vegetal physiology, familiar with the controversies
respecting sundry points dealt with in this chapter, will probably be
surprised to find taken for granted in it, propositions which they have
habitually regarded as open to doubt. Hence it seems needful to say
that the conclusions here set forth, have resulted from investigations
undertaken for the purpose of forming opinions on several unsettled
questions which I had to treat, but which I could find in books no
adequate data for treating. The details of these investigations, and
the entire argument of which this chapter is partly an abstract, will
be found in Appendix C.

[50] To this implied inference it is objected that “excess of nutritive
material does not necessarily lead to correspondingly increased
growth.” My reply is that a concomitant factor is activity of the
tissue, and that in its absence growth is not to be expected.

[51] In recent years (since 1890) Prof. Wilhelm Roux, in essays on
functional adaptation, has set forth some views akin to the foregoing
in respect to the general belief they imply, though differing in
respect of the physiological processes he indicates. The following
relevant passage has been translated for me from an article of his in
the _Real-Encyclopädie der gesammten Heilkunde_:--“A more complete
theory of functional adaptation by the author is founded on the
assumption that the ‘functional’ stimulus, or ‘the act of exercising
the function’ (in muscles and glands), and especially, in the case of
bones, the concussion and tension caused by stress and strain, exert
a ‘trophic’ stimulus on the cells, in consequence of which, and along
with an increased absorption of nutriment, they grow and eventually
increase (or the osteoblasts at the point of greater stimulus form more
bone); while, conversely, with continued inactivity, by absence of
these stimuli the nourishment of the cell declines so that the waste is
insufficiently replaced (or otherwise that the bone-substance gradually
loses its power of resistance to the osteoblasts formed as a result of
inactivity”).

[52] An outline of the doctrine set forth in the following chapters,
was originally published in the _Westminster Review_ for April, 1852,
under the title--_A Theory of Population deduced from the General Law
of Animal Fertility_; and was shortly afterwards republished with a
prefatory note stating that it must be accepted as a sketch which I
hoped at some future time to elaborate. In now revising and completing
it, I have omitted a non-essential part of the argument, while I
have expanded the remainder by adding to the number of facts put in
evidence, by meeting objections which want of space before obliged
me to pass over, and by drawing various secondary conclusions. The
original paper, with omissions, will be found in Appendix A to Volume I
of this work.

[53] I was here thinking only of the cases which are general among
insects, but it seems that vertebrate animals, too, furnish cases. Mr.
Cunningham writes:--“There is a curious instance of this in the Conger:
the female grows to 6 or 7 feet long and a weight of 60 lbs. and
upwards and then ceases to feed for 6 months while the eggs develop,
and when the eggs are shed dies.”

[54] I say “normal” for the purpose of excluding not only morbid
growths but excess of fat.

[55] To meet a possible criticism it should be remarked that this
calculation assumes that the power of asexual reproduction is not
exhausted by the end of the month. It has been found that “the
successive fissions of _Paramœcium_ cannot continue indefinitely. After
some hundreds of generations the products of fission are small, have no
mouth, and die unless before this they have been allowed to conjugate
with individuals of another brood.” It may, however, be fairly taken
for granted that “some hundreds of generations” would take longer than
a month.

[56] Even this number is far exceeded. Dr. Edward Klein, in a lecture
he gave at the Royal Institution on June 2, 1898, asserted that 246
bacteria in a cubic centimetre of nutritive liquid would multiply to
20,000,000 in the course of twenty-four hours: a rate which, at the end
of the _third_ day, would give, as the offspring of one individual,
537,367,797,000,000.

[57] It has since been shown that in _Myrianida fasciata_ as many as
29 attached groups exist. See _Cambridge Natural History_, Vol. II,
_Worms, Rotifers and Polyzoa_, p. 280.

[58] To this passage Prof. MacBride appends the remark:--“This is quite
proven now, and the statement as it stands is quite correct; but far
better and more minutely worked out cases are to be found amongst the
_Infusoria_. In _Paramœcium_ for example, there are normally present a
large macronucleus and a small micronucleus lying alongside of it. When
two individuals adhere preparatory to conjugation, the macronucleus
breaks up into fragments which are absorbed: the micronucleus--which
has some time previously divided into two--begins to break up further
and eventually forms eight bodies; all of these except one disappear;
this last piece then divides into two; of these two one represents
a male genital cell, for it passes over into the body of the other
_Paramœcium_ and fuses with one of the two corresponding nuclei there;
thus each of the two individuals which adhere fertilizes the other.
The two individuals then separate and the nucleus (result of fusion of
male and female nuclei) in each divides into four. Of these, two move
to one end of the animal and two to the other. The animal then divides
into two transversely--each of the products thus having two nuclei
which form the micro-and macronucleus of it. Thus it appears that the
function of sexual union is simply to give increased vigour to all the
vital processes _including fission_. Since as mentioned above (p. 443)
if it is prevented, the products of fission are eventually unable to
feed themselves.”

[59] A passage translated for me from the German may be here given
in verification. Dr. Dionys Hellin in an essay on the origin of
Multiparity and Twin-births, refers to the thesis above set forth, and
says that “the fact that it is generally women of small growth who bear
twins is in complete agreement with it.” He adds that “Puech is right
in his opinion that twin pregnancies are a direct result of relatively
large ovaries (_i.e._, in comparison with the whole body). He has
observed that for the same size of body the ovarium of a pluriparous
animal is always of greater volume than that of a uniparous animal ...
a sow has ovaries as large as a cow’s; but while the latter bears only
one calf [at a time], the sow brings forth 6–15 at each litter. Even
in animals of the same species but belonging to different races these
relations may be verified,” _e.g._, Barbary sheep and ordinary sheep.

[60] When, after having held for some years the general doctrine
elaborated in these chapters, I agreed, early in 1852, to prepare an
outline of it for the _Westminster Review_, I consulted, among other
works, the just-issued third edition of Dr. Carpenter’s _Principles
of Physiology, General and Comparative_--seeking in it for facts
illustrating the different degrees of fertility of different organisms,
I met with a passage, quoted above in § 339, which seemed tacitly
to assert that individual aggrandizement is at variance with the
propagation of the race; but nowhere found a distinct enunciation of
this truth. I did not then read the Chapter entitled “General View
of the Functions,” which held out no promise of such evidence as I
was looking for. But on since referring to this chapter, I discovered
in it the definite statement that--“there is a certain degree of
antagonism between the Nutritive and Reproductive functions, the
one being executed at the expense of the other. The reproductive
apparatus derives the materials of its operations through the nutritive
system, and is entirely dependent upon it for the continuance of its
function. If, therefore, it be in a state of excessive activity, it
will necessarily draw off from the individual fabric some portion
of the aliment destined for its maintenance. It may be universally
observed that, when the nutritive functions are particularly active
in supporting the _individual_, the reproductive system is in a
corresponding degree undeveloped,--and _vice versâ_.” P. 592.

[61] The climate, the locality, and the kind of food, are of course all
factors; and hence, probably, the differences between the statements of
different authorities concerning these several cases. Prof. MacBride
writes:--

“According to Flower (_Mammals, Living and Extinct_) the Ferret is a
domesticated variety of the common polecat, which has 3 to 8 young.
Darwin (_Animals and Plants_) says that the wild sow often breeds twice
a year and produces a litter of 4 to 8, and sometimes even 12. The
domestic sow breeds twice and would breed oftener if permitted, and if
any good at all produces 8 in litter.”

[62] It is worth while inquiring whether unfitness of the food given
to them, is not the chief cause of that sterility which, as Mr.
Darwin says, “is the great bar to the domestication of animals.” He
remarks that “when animals and plants are removed from their natural
conditions, they are extremely liable to have their reproductive
systems seriously affected.” Possibly the relative or absolute arrest
of genesis, is less due to a direct effect on the reproductive
system, than to a changed nutrition of which the reproductive system
most clearly shows the results. The matters required for forming an
embryo are in a greater proportion nitrogenous than are the matters
required for maintaining an adult. Hence, an animal forced to live on
insufficiently-nitrogenized food, may have its surplus for reproduction
cut off, but still have a sufficiency to keep its own tissues in
repair, and appear to be in good health--meanwhile increasing in bulk
from excess of the non-nitrogenous matters it eats.

[63] Huxley, _Anatomy of Invertebrated Animals_, p. 274.

[64] Shipley, _Zoology of Invertebrata_, p. 112.

[65] I am told that “Wagner, who described the larva, found that it
bored into the bark of trees. It attacks also the wheat plant, and is
a most destructive parasite.” Apparently this statement is at variance
with the foregoing inference. It is clear, however, that since these
heaps of nitrogenous refuse in which it has been found are artificial
and recent, they cannot be its natural habitats; and it seems not
improbable that these larvæ, suddenly supplied with a more nutritive
food in unlimited amount, may have as a consequence acquired this habit
of agamogenetic multiplication which did not characterize the species
under its natural conditions and relatively low nutrition.

[66] This is exactly the reverse of Mr. Doubleday’s doctrine; which is
that throughout both the animal and vegetable kingdoms, “over-feeding
checks increase; whilst, on the other hand, a limited or deficient
nutriment stimulates and adds to it.” Or, as he elsewhere says--“Be the
range of the natural power to increase in any species what it may, the
_plethoric_ state invariably checks it, and the _deplethoric_ state
invariably develops it; and this happens in the exact ratio of the
intensity and completeness of each state, until each state be carried
so far as to bring about the actual death of the animal or plant
itself.”

I have space here only to indicate the misinterpretations on which Mr.
Doubleday has based his argument.

In the first place, he has confounded normal plethora with what I have,
in § 355, distinguished as abnormal plethora. The cases of infertility
accompanying fatness, which he cites in proof that over-feeding checks
increase, are not cases of high nutrition properly so-called; but
cases of such defective absorption or assimilation as constitutes
low nutrition. In Chap. IX, abundant proof was given that a truly
plethoric state is an unusually fertile state. It may be added that
much of the evidence by which Mr. Doubleday seeks to show that among
men, highly-fed classes are infertile classes, may be out-balanced by
counter-evidence. Many years ago Mr. G. H. Lewes pointed this out:
extracting from a book on the peerage, the names of 16 peers who had,
at that time, 186 children; giving an average of 11·6 in a family.

Mr. Doubleday insists much on the support given to his theory by the
barrenness of very luxuriant plants, and the fruitfulness produced
in plants by depletion. Had he been aware that the change from
barrenness to fruitfulness in plants, is a change from agamogenesis to
gamogenesis--had it been as well known at the time when he wrote as it
is now, that a tree which goes on putting out sexless shoots, is thus
producing new individuals; and that when it begins to bear fruit, it
simply begins to produce new individuals after another manner--he would
have perceived that facts of this class do not tell in his favour.

In the law which Mr. Doubleday alleges, he sees a guarantee for the
maintenance of species. He argues that the plethoric state of the
individuals constituting any race of organisms, presupposes conditions
so favourable to life that the race can be in no danger; and that
rapidity of multiplication becomes needless. Conversely, he argues
that a deplethoric state implies unfavourable conditions--implies,
consequently, unusual mortality; that is--implies a necessity for
increased fertility to prevent the race from dying out. It may be
readily shown, however, that such an arrangement would be the reverse
of self-adjusting. Suppose a species, too numerous for its food, to
be in the resulting deplethoric state. It will, according to Mr.
Doubleday, become unusually fertile; and the next generation will be
more numerous rather than less numerous. For, by the hypothesis, the
unusual fertility due to the deplethoric state, is the cause of undue
increase of population. But if the next generation is more numerous
while the supply of food has not increased in proportion, then this
next generation will be in a still more deplethoric state, and will be
still more fertile. Thus there will go on an ever-increasing rate of
multiplication, and an ever-decreasing share of food, for each person,
until the species disappears. Suppose, on the other hand, the members
of a species to be in an unusually plethoric state. Their rate of
multiplication, ordinarily sufficient to maintain their numbers, will
become insufficient to maintain their numbers. In the next generation,
therefore, there will be fewer to eat the already abundant food, which
becoming relatively still more abundant, will render the fewer members
of the species still more plethoric, and still less fertile, than their
parents. And the actions and reactions continuing, the species will
presently die out from absolute barrenness.

[67] A good deal of this chapter retains its original form; and the
above paragraph is reprinted verbatim from the _Westminster Review_
for April, 1852, in which the views developed in the foregoing hundred
pages were first sketched out. This paragraph shows how near one may
be to a great generalization without seeing it. Though the struggle
for life is the alleged motive force; though the process of natural
selection is recognized; and though to it is ascribed a share in the
evolution of a higher type; yet the conception is not that which Mr.
Darwin has worked out with such wonderful skill and knowledge. In the
first place, natural selection is here described only as furthering
direct adaptation--only as aiding progress by the preservation of
individuals in whom functionally-produced modifications have gone on
most favourably. In the second place, there is no trace of the idea
that natural selection may by co-operation with the cause assigned, or
with other causes, produce _divergences_ of structure; and of course,
in the absence of this idea, there is no implication that natural
selection has anything to do with the origin of species. And in the
third place, the all-important factor of variation--“spontaneous,”
or incidental as we may otherwise call it--is wholly ignored. Though
use and disuse are, I think, much more potent causes of organic
modification than Mr. Darwin supposes--though, while pursuing the
inquiry in detail, I have been led to believe that direct equilibration
has played a more active part even than I had myself at one time
thought; yet I hold Mr. Darwin to have shown beyond question, that a
great part of the facts--perhaps the greater part--are explicable only
as resulting from the survival of individuals which have deviated in
some indirectly-caused way from the ancestral type. Thus, the above
paragraph contains merely a passing recognition of the selective
process; and indicates no suspicion of the enormous range of its
effects, or of the conditions under which a large part of its effects
are produced.

[68] For the information of those who may wish to examine metamorphoses
of these kinds, I may here state that I have found nearly all the
examples described, in the neighbourhood of the sea--the last-named,
on the shore of Locheil, near Fort William. Whether it is that I have
sought more diligently for cases when in such localities, or whether
it is that the sea-air favours that excessive nutrition whence these
transformations result, I am unable to say.

[69] These two dyes have affinities for different components of the
tissues, and may be advantageously used in different cases. Magenta
is rapidly taken up by woody matter and other secondary deposits;
while logwood colours the cell-membranes, and takes but reluctantly
to the substances seized by magenta. By trying both of them on the
same structure, we may guard ourselves against any error arising from
selective combination.

[70] Those who repeat these experiments must be prepared for great
irregularities in the rates of absorption. Succulent structures in
general absorb much more slowly than others, and sometimes will
scarcely take up the dye at all. The differences between different
structures, and the same structure at different times, probably depend
on the degrees in which the tissues are charged with liquid and the
rates at which they are losing it by evaporation.

[71] It may be added here that, on considering the mechanical actions
that must go on, we are enabled in some measure to understand both
how such inosculating channels are initiated, and how the structures
of their component cells are explicable. What must happen to one of
these elongated prosenchyma-cells if, in the course of its development,
it is subject to intermittent compressions? Its squeezed-out liquid
while partially escaping laterally, will more largely escape upwards
and downwards; and while repeated lateral escape will tend to form
lateral channels communicating with laterally-adjacent cells, repeated
longitudinal escape will tend to form channels communicating with
longitudinally-adjacent cells--so producing continuous though irregular
longitudinal canals. Meanwhile each cell into and out of which the
nutritive liquid is from time to time squeezed through small openings
in its walls, cannot thicken internally in an even manner: deposition
will be interfered with by the passage of the currents through
the pores. The rush to or from each pore will tend to maintain a
funnel-shaped depression in the deposit around; and the opening from
cell to cell will so acquire just that shape which the microscope
shows up--two hollow cones with their apices meeting at the point
where the cell-membranes are in contact. Moreover, as confirming this
interpretation, it may be remarked that we are thus supplied with a
reason for the differences of shape between these passages from one
pitted cell to another, and the analogous passages that exist between
cells otherwise formed and otherwise conditioned. In the cells of the
medulla, and others which are but little exposed to compression, the
passages are severally formed more like a tube with two trumpet-mouths,
one in each cell. This is just the form which might be expected where
the nutritive fluid passes from cell to cell in moderate currents,
and not by the violent rushes caused by intermittent pressures. Of
course it is not meant that in each individual cell these structures
are determined by these mechanical actions. The facts clearly negative
any such conclusion, showing us, as they in many cases do, that these
structures are assumed in advance of these mechanical actions. The
implication is, that such mechanical actions initiated modifications
that have, with the aid of natural selection, been accumulated
from generation to generation; until, in conformity with ordinary
embryological laws, the cells of the parts exposed to such actions
assume these special structures irrespective of the actions--the
actions, however, still serving to aid and complete the assumption of
the inherited type.

[72] Some exceptions to this occur in plants that have retrograded
in the character of their tissues towards the simpler vegetal types.
Certain very succulent leaves, such as those of _Sempervivum_, in which
the cellular tissue is immensely developed in comparison with the
vascular tissue, seem to have resumed to a considerable extent what
we must regard as the primitive form of vegetal circulation--simple
absorption from cell to cell. These, when they have lost much of their
water, will take up the dye to some distance through their general
substance, or rather through its interstices, even neglecting the
vessels. At other times, in the same leaves, the vessels will become
charged while comparatively little absorption takes place through the
cellular tissue. Even in these exceptional cases, however, the movement
through cellular tissue is nothing like as fast as the movement through
vessels.

[73] It seems probable, however, that osmotic distention is here,
especially, the more important of the two factors. The rising of the
sap in spring may indirectly result, like the sprouting of the seed,
from the transformation of starch into sugar. During germination, this
change of an oxy-hydro-carbon from an insoluble into a soluble form,
leads to rapid endosmose; consequently to great distention of the seed;
and therefore to a force which thrusts the contained liquids into the
plumule and radicle, and gives them power to displace the soil in their
way: it sets up an active internal movement when neither evaporation
nor the change which light produces can be operative. And similarly,
if, in the spring, the starch stored-up in the roots of a tree passes
into the form of sugar, the unusual osmotic absorption that arises will
cause an unusual distention--a distention which, being resisted by the
tough bark of the roots and stem, will result in a powerful upward
thrust of the contained liquid.


Transcriber’s Note:

1. Obvious printers’, spelling and punctuation errors have been
silently corrected.

2. Where appropriate, original spelling has been retained.

3. Both hyphenated and non-hyphenated versions of the same words have been
retained where deemed appropriate.

4. Superscripts are represented using the caret character, e.g. D^r.

5. Italics are shown as _xxx_, bold print is shown as =xxx=.