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1228    1859, First Edition
22764   1860, Second Edition
2009    1872, Sixth Edition, considered the definitive edition.




On the Origin of Species

BY MEANS OF NATURAL SELECTION,

OR THE PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.

By Charles Darwin, M.A., F.R.S.,

Author of “The Descent of Man,” etc., etc.

Sixth London Edition, with all Additions and Corrections.




“But with regard to the material world, we can at least go so far as
this—we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by
the establishment of general laws.”

WHEWELL: _Bridgewater Treatise_.


“The only distinct meaning of the word ‘natural’ is _stated_, _fixed_
or _settled;_ since what is natural as much requires and presupposes an
intelligent agent to render it so, _i.e._, to effect it continually or
at stated times, as what is supernatural or miraculous does to effect
it for once.”

BUTLER: _Analogy of Revealed Religion_.


“To conclude, therefore, let no man out of a weak conceit of sobriety,
or an ill-applied moderation, think or maintain, that a man can search
too far or be too well studied in the book of God’s word, or in the
book of God’s works; divinity or philosophy; but rather let men
endeavour an endless progress or proficience in both.”

BACON: _Advancement of Learning_.




AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF
SPECIES, PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS
WORK.


I will here give a brief sketch of the progress of opinion on the
Origin of Species. Until recently the great majority of naturalists
believed that species were immutable productions, and had been
separately created. This view has been ably maintained by many authors.
Some few naturalists, on the other hand, have believed that species
undergo modification, and that the existing forms of life are the
descendants by true generation of pre existing forms. Passing over
allusions to the subject in the classical writers,[1] the first author
who in modern times has treated it in a scientific spirit was Buffon.
But as his opinions fluctuated greatly at different periods, and as he
does not enter on the causes or means of the transformation of species,
I need not here enter on details.

 [1] Aristotle, in his “Physicæ Auscultationes” (lib.2, cap.8, s.2),
 after remarking that rain does not fall in order to make the corn
 grow, any more than it falls to spoil the farmer’s corn when threshed
 out of doors, applies the same argument to organisation; and adds (as
 translated by Mr. Clair Grece, who first pointed out the passage to
 me), “So what hinders the different parts (of the body) from having
 this merely accidental relation in nature? as the teeth, for example,
 grow by necessity, the front ones sharp, adapted for dividing, and the
 grinders flat, and serviceable for masticating the food; since they
 were not made for the sake of this, but it was the result of accident.
 And in like manner as to other parts in which there appears to exist
 an adaptation to an end. Wheresoever, therefore, all things together
 (that is all the parts of one whole) happened like as if they were
 made for the sake of something, these were preserved, having been
 appropriately constituted by an internal spontaneity; and whatsoever
 things were not thus constituted, perished and still perish.” We here
 see the principle of natural selection shadowed forth, but how little
 Aristotle fully comprehended the principle, is shown by his remarks on
 the formation of the teeth.


Lamarck was the first man whose conclusions on the subject excited much
attention. This justly celebrated naturalist first published his views
in 1801; he much enlarged them in 1809 in his “Philosophie Zoologique”,
and subsequently, 1815, in the Introduction to his “Hist. Nat. des
Animaux sans Vertébres”. In these works he up holds the doctrine that
all species, including man, are descended from other species. He first
did the eminent service of arousing attention to the probability of all
change in the organic, as well as in the inorganic world, being the
result of law, and not of miraculous interposition. Lamarck seems to
have been chiefly led to his conclusion on the gradual change of
species, by the difficulty of distinguishing species and varieties, by
the almost perfect gradation of forms in certain groups, and by the
analogy of domestic productions. With respect to the means of
modification, he attributed something to the direct action of the
physical conditions of life, something to the crossing of already
existing forms, and much to use and disuse, that is, to the effects of
habit. To this latter agency he seems to attribute all the beautiful
adaptations in nature; such as the long neck of the giraffe for
browsing on the branches of trees. But he likewise believed in a law of
progressive development, and as all the forms of life thus tend to
progress, in order to account for the existence at the present day of
simple productions, he maintains that such forms are now spontaneously
generated.[2]

 [2] I have taken the date of the first publication of Lamarck from
 Isidore Geoffroy Saint-Hilaire’s (“Hist. Nat. Générale”, tom. ii. page
 405, 1859) excellent history of opinion on this subject. In this work
 a full account is given of Buffon’s conclusions on the same subject.
 It is curious how largely my grandfather, Dr. Erasmus Darwin,
 anticipated the views and erroneous grounds of opinion of Lamarck in
 his “Zoonomia” (vol. i. pages 500-510), published in 1794. According
 to Isid. Geoffroy there is no doubt that Goethe was an extreme
 partisan of similar views, as shown in the introduction to a work
 written in 1794 and 1795, but not published till long afterward; he
 has pointedly remarked (“Goethe als Naturforscher”, von Dr. Karl
 Meding, s. 34) that the future question for naturalists will be how,
 for instance, cattle got their horns and not for what they are used.
 It is rather a singular instance of the manner in which similar views
 arise at about the same time, that Goethe in Germany, Dr. Darwin in
 England, and Geoffroy Saint-Hilaire (as we shall immediately see) in
 France, came to the same conclusion on the origin of species, in the
 years 1794-5.


Geoffroy Saint-Hilaire, as is stated in his “Life”, written by his son,
suspected, as early as 1795, that what we call species are various
degenerations of the same type. It was not until 1828 that he published
his conviction that the same forms have not been perpetuated since the
origin of all things. Geoffroy seems to have relied chiefly on the
conditions of life, or the “_monde ambiant_” as the cause of change. He
was cautious in drawing conclusions, and did not believe that existing
species are now undergoing modification; and, as his son adds, “C’est
donc un problème à réserver entièrement à l’avenir, supposé même que
l’avenir doive avoir prise sur lui.”

In 1813 Dr. W.C. Wells read before the Royal Society “An Account of a
White Female, part of whose skin resembles that of a Negro”; but his
paper was not published until his famous “Two Essays upon Dew and
Single Vision” appeared in 1818. In this paper he distinctly recognises
the principle of natural selection, and this is the first recognition
which has been indicated; but he applies it only to the races of man,
and to certain characters alone. After remarking that negroes and
mulattoes enjoy an immunity from certain tropical diseases, he
observes, firstly, that all animals tend to vary in some degree, and,
secondly, that agriculturists improve their domesticated animals by
selection; and then, he adds, but what is done in this latter case “by
art, seems to be done with equal efficacy, though more slowly, by
nature, in the formation of varieties of mankind, fitted for the
country which they inhabit. Of the accidental varieties of man, which
would occur among the first few and scattered inhabitants of the middle
regions of Africa, some one would be better fitted than others to bear
the diseases of the country. This race would consequently multiply,
while the others would decrease; not only from their in ability to
sustain the attacks of disease, but from their incapacity of contending
with their more vigorous neighbours. The colour of this vigorous race I
take for granted, from what has been already said, would be dark. But
the same disposition to form varieties still existing, a darker and a
darker race would in the course of time occur: and as the darkest would
be the best fitted for the climate, this would at length become the
most prevalent, if not the only race, in the particular country in
which it had originated.” He then extends these same views to the white
inhabitants of colder climates. I am indebted to Mr. Rowley, of the
United States, for having called my attention, through Mr. Brace, to
the above passage of Dr. Wells’ work.

The Hon. and Rev. W. Herbert, afterward Dean of Manchester, in the
fourth volume of the “Horticultural Transactions”, 1822, and in his
work on the “Amaryllidaceæ” (1837, pages 19, 339), declares that
“horticultural experiments have established, beyond the possibility of
refutation, that botanical species are only a higher and more permanent
class of varieties.” He extends the same view to animals. The dean
believes that single species of each genus were created in an
originally highly plastic condition, and that these have produced,
chiefly by inter-crossing, but likewise by variation, all our existing
species.

In 1826 Professor Grant, in the concluding paragraph in his well-known
paper (“Edinburgh Philosophical Journal”, vol. XIV, page 283) on the
Spongilla, clearly declares his belief that species are descended from
other species, and that they become improved in the course of
modification. This same view was given in his Fifty-fifth Lecture,
published in the “Lancet” in 1834.

In 1831 Mr. Patrick Matthew published his work on “Naval Timber and
Arboriculture”, in which he gives precisely the same view on the origin
of species as that (presently to be alluded to) propounded by Mr.
Wallace and myself in the “Linnean Journal”, and as that enlarged in
the present volume. Unfortunately the view was given by Mr. Matthew
very briefly in scattered passages in an appendix to a work on a
different subject, so that it remained unnoticed until Mr. Matthew
himself drew attention to it in the “Gardeners’ Chronicle”, on April 7,
1860. The differences of Mr. Matthew’s views from mine are not of much
importance: he seems to consider that the world was nearly depopulated
at successive periods, and then restocked; and he gives as an
alternative, that new forms may be generated “without the presence of
any mold or germ of former aggregates.” I am not sure that I understand
some passages; but it seems that he attributes much influence to the
direct action of the conditions of life. He clearly saw, however, the
full force of the principle of natural selection.

The celebrated geologist and naturalist, Von Buch, in his excellent
“Description Physique des Isles Canaries” (1836, page 147), clearly
expresses his belief that varieties slowly become changed into
permanent species, which are no longer capable of intercrossing.

Rafinesque, in his “New Flora of North America”, published in 1836,
wrote (page 6) as follows: “All species might have been varieties once,
and many varieties are gradually becoming species by assuming constant
and peculiar characters;” but further on (page 18) he adds, “except the
original types or ancestors of the genus.”

In 1843-44 Professor Haldeman (“Boston Journal of Nat. Hist. U.
States”, vol. iv, page 468) has ably given the arguments for and
against the hypothesis of the development and modification of species:
he seems to lean toward the side of change.

The “Vestiges of Creation” appeared in 1844. In the tenth and much
improved edition (1853) the anonymous author says (page 155): “The
proposition determined on after much consideration is, that the several
series of animated beings, from the simplest and oldest up to the
highest and most recent, are, under the providence of God, the results,
_first_, of an impulse which has been imparted to the forms of life,
advancing them, in definite times, by generation, through grades of
organisation terminating in the highest dicotyledons and vertebrata,
these grades being few in number, and generally marked by intervals of
organic character, which we find to be a practical difficulty in
ascertaining affinities; _second_, of another impulse connected with
the vital forces, tending, in the course of generations, to modify
organic structures in accordance with external circumstances, as food,
the nature of the habitat, and the meteoric agencies, these being the
‘adaptations’ of the natural theologian.” The author apparently
believes that organisation progresses by sudden leaps, but that the
effects produced by the conditions of life are gradual. He argues with
much force on general grounds that species are not immutable
productions. But I cannot see how the two supposed “impulses” account
in a scientific sense for the numerous and beautiful coadaptations
which we see throughout nature; I cannot see that we thus gain any
insight how, for instance, a woodpecker has become adapted to its
peculiar habits of life. The work, from its powerful and brilliant
style, though displaying in the early editions little accurate
knowledge and a great want of scientific caution, immediately had a
very wide circulation. In my opinion it has done excellent service in
this country in calling attention to the subject, in removing
prejudice, and in thus preparing the ground for the reception of
analogous views.

In 1846 the veteran geologist M.J. d’Omalius d’Halloy published in an
excellent though short paper (“Bulletins de l’Acad. Roy. Bruxelles”,
tom. xiii, page 581) his opinion that it is more probable that new
species have been produced by descent with modification than that they
have been separately created: the author first promulgated this opinion
in 1831.

Professor Owen, in 1849 (“Nature of Limbs”, page 86), wrote as follows:
“The archetypal idea was manifested in the flesh under diverse such
modifications, upon this planet, long prior to the existence of those
animal species that actually exemplify it. To what natural laws or
secondary causes the orderly succession and progression of such organic
phenomena may have been committed, we, as yet, are ignorant.” In his
address to the British Association, in 1858, he speaks (page li) of
“the axiom of the continuous operation of creative power, or of the
ordained becoming of living things.” Further on (page xc), after
referring to geographical distribution, he adds, “These phenomena shake
our confidence in the conclusion that the Apteryx of New Zealand and
the Red Grouse of England were distinct creations in and for those
islands respectively. Always, also, it may be well to bear in mind that
by the word ‘creation’ the zoologist means ‘a process he knows not
what.’” He amplifies this idea by adding that when such cases as that
of the Red Grouse are “enumerated by the zoologist as evidence of
distinct creation of the bird in and for such islands, he chiefly
expresses that he knows not how the Red Grouse came to be there, and
there exclusively; signifying also, by this mode of expressing such
ignorance, his belief that both the bird and the islands owed their
origin to a great first Creative Cause.” If we interpret these
sentences given in the same address, one by the other, it appears that
this eminent philosopher felt in 1858 his confidence shaken that the
Apteryx and the Red Grouse first appeared in their respective homes “he
knew not how,” or by some process “he knew not what.”

This Address was delivered after the papers by Mr. Wallace and myself
on the Origin of Species, presently to be referred to, had been read
before the Linnean Society. When the first edition of this work was
published, I was so completely deceived, as were many others, by such
expressions as “the continuous operation of creative power,” that I
included Professor Owen with other palæontologists as being firmly
convinced of the immutability of species; but it appears (“Anat. of
Vertebrates”, vol. iii, page 796) that this was on my part a
preposterous error. In the last edition of this work I inferred, and
the inference still seems to me perfectly just, from a passage
beginning with the words “no doubt the type-form,” &c.(Ibid., vol. i,
page xxxv), that Professor Owen admitted that natural selection may
have done something in the formation of a new species; but this it
appears (Ibid., vol. iii. page 798) is inaccurate and without evidence.
I also gave some extracts from a correspondence between Professor Owen
and the editor of the “London Review”, from which it appeared manifest
to the editor as well as to myself, that Professor Owen claimed to have
promulgated the theory of natural selection before I had done so; and I
expressed my surprise and satisfaction at this announcement; but as far
as it is possible to understand certain recently published passages
(Ibid., vol. iii. page 798) I have either partially or wholly again
fallen into error. It is consolatory to me that others find Professor
Owen’s controversial writings as difficult to understand and to
reconcile with each other, as I do. As far as the mere enunciation of
the principle of natural selection is concerned, it is quite immaterial
whether or not Professor Owen preceded me, for both of us, as shown in
this historical sketch, were long ago preceded by Dr. Wells and Mr.
Matthews.

M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850
(of which a Résumé appeared in the “Revue et Mag. de Zoolog.”, Jan.,
1851), briefly gives his reason for believing that specific characters
“sont fixés, pour chaque espèce, tant qu’elle se perpétue au milieu des
mêmes circonstances: ils se modifient, si les circonstances ambiantes
viennent à changer. En résumé, _l’observation_ des animaux sauvages
démontre deja la variabilité _limitée_ des espèces. Les _expériences_
sur les animaux sauvages devenus domestiques, et sur les animaux
domestiques redevenus sauvages, la démontrent plus clairment encore.
Ces mêmes expériences prouvent, de plus, que les différences produites
peuvent être de _valeur générique_.” In his “Hist. Nat. Générale” (tom.
ii, page 430, 1859) he amplifies analogous conclusions.

From a circular lately issued it appears that Dr. Freke, in 1851
(“Dublin Medical Press”, page 322), propounded the doctrine that all
organic beings have descended from one primordial form. His grounds of
belief and treatment of the subject are wholly different from mine; but
as Dr. Freke has now (1861) published his Essay on the “Origin of
Species by means of Organic Affinity”, the difficult attempt to give
any idea of his views would be superfluous on my part.

Mr. Herbert Spencer, in an Essay (originally published in the “Leader”,
March, 1852, and republished in his “Essays”, in 1858), has contrasted
the theories of the Creation and the Development of organic beings with
remarkable skill and force. He argues from the analogy of domestic
productions, from the changes which the embryos of many species
undergo, from the difficulty of distinguishing species and varieties,
and from the principle of general gradation, that species have been
modified; and he attributes the modification to the change of
circumstances. The author (1855) has also treated Psychology on the
principle of the necessary acquirement of each mental power and
capacity by gradation.

In 1852 M. Naudin, a distinguished botanist, expressly stated, in an
admirable paper on the Origin of Species (“Revue Horticole”, page 102;
since partly republished in the “Nouvelles Archives du Muséum”, tom. i,
p. 171), his belief that species are formed in an analogous manner as
varieties are under cultivation; and the latter process he attributes
to man’s power of selection. But he does not show how selection acts
under nature. He believes, like Dean Herbert, that species, when
nascent, were more plastic than at present. He lays weight on what he
calls the principle of finality, “puissance mystérieuse, indéterminée;
fatalité pour les uns; pour les autres volonté providentielle, dont
l’action incessante sur les êtres vivantes détermine, à toutes les
époques de l’existence du monde, la forme, le volume, et la durée de
chacun d’eux, en raison de sa destinée dans l’ordre de choses dont il
fait partie. C’est cette puissance qui harmonise chaque membre à
l’ensemble, en l’appropriant à la fonction qu’il doit remplir dans
l’organisme général de la nature, fonction qui est pour lui sa raison
d’être.”[3]

 [3] From references in Bronn’s “Untersuchungen über die
 Entwickelungs-Gesetze”, it appears that the celebrated botanist and
 palæontologist Unger published, in 1852, his belief that species
 undergo development and modification. Dalton, likewise, in Pander and
 Dalton’s work on Fossil Sloths, expressed, in 1821, a similar belief.
 Similar views have, as is well known, been maintained by Oken in his
 mystical “Natur-Philosophie”. From other references in Godron’s work
 “Sur l’Espèce”, it seems that Bory St. Vincent, Burdach, Poiret and
 Fries, have all admitted that new species are continually being
 produced.
    I may add, that of the thirty-four authors named in this Historical
    Sketch, who believe in the modification of species, or at least
    disbelieve in separate acts of creation, twenty-seven have written
    on special branches of natural history or geology.


In 1853 a celebrated geologist, Count Keyserling (“Bulletin de la Soc.
Geolog.”, 2nd Ser., tom. x, page 357), suggested that as new diseases,
supposed to have been caused by some miasma have arisen and spread over
the world, so at certain periods the germs of existing species may have
been chemically affected by circumambient molecules of a particular
nature, and thus have given rise to new forms.

In this same year, 1853, Dr. Schaaffhausen published an excellent
pamphlet (“Verhand. des Naturhist. Vereins der Preuss. Rheinlands”,
&c.), in which he maintains the development of organic forms on the
earth. He infers that many species have kept true for long periods,
whereas a few have become modified. The distinction of species he
explains by the destruction of intermediate graduated forms. “Thus
living plants and animals are not separated from the extinct by new
creations, but are to be regarded as their descendants through
continued reproduction.”

A well-known French botanist, M. Lecoq, writes in 1854 (“Etudes sur
Géograph.” Bot. tom. i, page 250), “On voit que nos recherches sur la
fixité ou la variation de l’espéce, nous conduisent directement aux
idées émises par deux hommes justement célèbres, Geoffroy Saint-Hilaire
et Goethe.” Some other passages scattered through M. Lecoq’s large work
make it a little doubtful how far he extends his views on the
modification of species.

The “Philosophy of Creation” has been treated in a masterly manner by
the Rev. Baden Powell, in his “Essays on the Unity of Worlds”, 1855.
Nothing can be more striking than the manner in which he shows that the
introduction of new species is “a regular, not a casual phenomenon,”
or, as Sir John Herschel expresses it, “a natural in contradistinction
to a miraculous process.”

The third volume of the “Journal of the Linnean Society” contains
papers, read July 1, 1858, by Mr. Wallace and myself, in which, as
stated in the introductory remarks to this volume, the theory of
Natural Selection is promulgated by Mr. Wallace with admirable force
and clearness.

Von Baer, toward whom all zoologists feel so profound a respect,
expressed about the year 1859 (see Prof. Rudolph Wagner,
“Zoologisch-Anthropologische Untersuchungen”, 1861, s. 51) his
conviction, chiefly grounded on the laws of geographical distribution,
that forms now perfectly distinct have descended from a single
parent-form.

In June, 1859, Professor Huxley gave a lecture before the Royal
Institution on the ‘Persistent Types of Animal Life’. Referring to such
cases, he remarks, “It is difficult to comprehend the meaning of such
facts as these, if we suppose that each species of animal and plant, or
each great type of organisation, was formed and placed upon the surface
of the globe at long intervals by a distinct act of creative power; and
it is well to recollect that such an assumption is as unsupported by
tradition or revelation as it is opposed to the general analogy of
nature. If, on the other hand, we view ‘Persistent Types’ in relation
to that hypothesis which supposes the species living at any time to be
the result of the gradual modification of pre-existing species, a
hypothesis which, though unproven, and sadly damaged by some of its
supporters, is yet the only one to which physiology lends any
countenance; their existence would seem to show that the amount of
modification which living beings have undergone during geological time
is but very small in relation to the whole series of changes which they
have suffered.”

In December, 1859, Dr. Hooker published his “Introduction to the
Australian Flora”. In the first part of this great work he admits the
truth of the descent and modification of species, and supports this
doctrine by many original observations.

The first edition of this work was published on November 24, 1859, and
the second edition on January 7, 1860.




Contents


AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES
INTRODUCTION.

CHAPTER I. VARIATION UNDER DOMESTICATION
CHAPTER II. VARIATION UNDER NATURE
CHAPTER III. STRUGGLE FOR EXISTENCE
CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST
CHAPTER V. LAWS OF VARIATION
CHAPTER VI. DIFFICULTIES OF THE THEORY
CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION
CHAPTER VIII. INSTINCT
CHAPTER IX. HYBRIDISM
CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD
CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS
CHAPTER XII. GEOGRAPHICAL DISTRIBUTION
CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION—continued
CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS
CHAPTER XV. RECAPITULATION AND CONCLUSION

GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.
INDEX.




DETAILED CONTENTS.


INTRODUCTION

CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of Variability—Effects of Habit and the use or disuse of
Parts—Correlated Variation—Inheritance—Character of Domestic
Varieties—Difficulty of distinguishing between Varieties and
Species—Origin of Domestic Varieties from one or more Species—Domestic
Pigeons, their Differences and Origin—Principles of Selection,
anciently followed, their Effects—Methodical and Unconscious
Selection—Unknown Origin of our Domestic Productions—Circumstances
favourable to Man’s power of Selection.

CHAPTER II.
VARIATION UNDER NATURE.
Variability—Individual Differences—Doubtful species—Wide ranging, much
diffused, and common species, vary most—Species of the larger genera in
each country vary more frequently than the species of the smaller
genera—Many of the species of the larger genera resemble varieties in
being very closely, but unequally, related to each other, and in having
restricted ranges.

CHAPTER III.
STRUGGLE FOR EXISTENCE.
Its bearing on natural selection—The term used in a wide
sense—Geometrical ratio of increase—Rapid increase of naturalised
animals and plants—Nature of the checks to increase—Competition
universal—Effects of climate—Protection from the number of
individuals—Complex relations of all animals and plants throughout
nature—Struggle for life most severe between individuals and varieties
of the same species; often severe between species of the same genus—The
relation of organism to organism the most important of all relations.

CHAPTER IV.
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
Natural Selection—its power compared with man’s selection—its power on
characters of trifling importance—its power at all ages and on both
sexes—Sexual Selection—On the generality of intercrosses between
individuals of the same species—Circumstances favourable and
unfavourable to the results of Natural Selection, namely,
intercrossing, isolation, number of individuals—Slow action—Extinction
caused by Natural Selection—Divergence of Character, related to the
diversity of inhabitants of any small area and to naturalisation—Action
of Natural Selection, through Divergence of Character and Extinction,
on the descendants from a common parent—Explains the Grouping of all
organic beings—Advance in organisation—Low forms preserved—Convergence
of character—Indefinite multiplication of species—Summary.

CHAPTER V.
LAWS OF VARIATION.
Effects of changed conditions—Use and disuse, combined with natural
selection; organs of flight and of vision—Acclimatisation—Correlated
variation—Compensation and economy of growth—False
correlations—Multiple, rudimentary, and lowly organised structures
variable—Parts developed in an unusual manner are highly variable;
specific characters more variable than generic; secondary sexual
characters variable—Species of the same genus vary in an analogous
manner—Reversions to long-lost characters—Summary.

CHAPTER VI.
DIFFICULTIES OF THE THEORY.
Difficulties of the theory of descent with modification—Absence or
rarity of transitional varieties—Transitions in habits of
life—Diversified habits in the same species—Species with habits widely
different from those of their allies—Organs of extreme perfection—Modes
of transition—Cases of difficulty—Natura non facit saltum—Organs of
small importance—Organs not in all cases absolutely perfect—The law of
Unity of Type and of the Conditions of Existence embraced by the theory
of Natural Selection.

CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.
Longevity—Modifications not necessarily simultaneous—Modifications
apparently of no direct service—Progressive development—Characters of
small functional importance, the most constant—Supposed incompetence of
natural selection to account for the incipient stages of useful
structures—Causes which interfere with the acquisition through natural
selection of useful structures—Gradations of structure with changed
functions—Widely different organs in members of the same class,
developed from one and the same source—Reasons for disbelieving in
great and abrupt modifications.

CHAPTER VIII.
INSTINCT.
Instincts comparable with habits, but different in their
origin—Instincts graduated—Aphides and ants—Instincts variable—Domestic
instincts, their origin—Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees—Slave-making ants—Hive-bee, its cell-making
instinct—Changes of instinct and structure not necessarily
simultaneous—Difficulties on the theory of the Natural Selection of
instincts—Neuter or sterile insects—Summary.

CHAPTER IX.
HYBRIDISM.
Distinction between the sterility of first crosses and of
hybrids—Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication—Laws governing the sterility of
hybrids—Sterility not a special endowment, but incidental on other
differences, not accumulated by natural selection—Causes of the
sterility of first crosses and of hybrids—Parallelism between the
effects of changed conditions of life and of crossing—Dimorphism and
Trimorphism—Fertility of varieties when crossed and of their mongrel
offspring not universal—Hybrids and mongrels compared independently of
their fertility—Summary.

CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day—On the
nature of extinct intermediate varieties; on their number—On the lapse
of time, as inferred from the rate of denudation and of deposition—On
the lapse of time as estimated in years—On the poorness of our
palæontological collections—On the intermittence of geological
formations—On the denudation of granitic areas—On the absence of
intermediate varieties in any one formation—On the sudden appearance of
groups of species—On their sudden appearance in the lowest known
fossiliferous strata—Antiquity of the habitable earth.

CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species—On their different
rates of change—Species once lost do not reappear—Groups of species
follow the same general rules in their appearance and disappearance as
do single species—On extinction—On simultaneous changes in the forms of
life throughout the world—On the affinities of extinct species to each
other and to living species—On the state of development of ancient
forms—On the succession of the same types within the same areas—Summary
of preceding and present chapter.

CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions—Importance of barriers—Affinity of the productions of the
same continent—Centres of creation—Means of dispersal by changes of
climate and of the level of the land, and by occasional means—Dispersal
during the Glacial period—Alternate Glacial periods in the north and
south.

CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION—_continued_.
Distribution of fresh-water productions—On the inhabitants of oceanic
islands—Absence of Batrachians and of terrestrial Mammals—On the
relation of the inhabitants of islands to those of the nearest
mainland—On colonisation from the nearest source with subsequent
modification—Summary of the last and present chapter.

CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS:
MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS.
Classification, groups subordinate to groups—Natural system—Rules and
difficulties in classification, explained on the theory of descent with
modification—Classification of varieties—Descent always used in
classification—Analogical or adaptive characters—Affinities, general,
complex and radiating—Extinction separates and defines
groups—Morphology, between members of the same class, between parts of
the same individual—Embryology, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding
age—Rudimentary Organs; their origin explained—Summary.

CHAPTER XV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the objections to the theory of Natural
Selection—Recapitulation of the general and special circumstances in
its favour—Causes of the general belief in the immutability of
species—How far the theory of Natural Selection may be extended—Effects
of its adoption on the study of Natural history—Concluding remarks.

GLOSSARY OF SCIENTIFIC TERMS.

INDEX.




ORIGIN OF SPECIES.




INTRODUCTION.


When on board H.M.S. Beagle, as naturalist, I was much struck with
certain facts in the distribution of the organic beings inhabiting
South America, and in the geological relations of the present to the
past inhabitants of that continent. These facts, as will be seen in the
latter chapters of this volume, seemed to throw some light on the
origin of species—that mystery of mysteries, as it has been called by
one of our greatest philosophers. On my return home, it occurred to me,
in 1837, that something might perhaps be made out on this question by
patiently accumulating and reflecting on all sorts of facts which could
possibly have any bearing on it. After five years’ work I allowed
myself to speculate on the subject, and drew up some short notes; these
I enlarged in 1844 into a sketch of the conclusions, which then seemed
to me probable: from that period to the present day I have steadily
pursued the same object. I hope that I may be excused for entering on
these personal details, as I give them to show that I have not been
hasty in coming to a decision.

My work is now (1859) nearly finished; but as it will take me many more
years to complete it, and as my health is far from strong, I have been
urged to publish this abstract. I have more especially been induced to
do this, as Mr. Wallace, who is now studying the natural history of the
Malay Archipelago, has arrived at almost exactly the same general
conclusions that I have on the origin of species. In 1858 he sent me a
memoir on this subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it is published
in the third volume of the Journal of that Society. Sir C. Lyell and
Dr. Hooker, who both knew of my work—the latter having read my sketch
of 1844—honoured me by thinking it advisable to publish, with Mr.
Wallace’s excellent memoir, some brief extracts from my manuscripts.

This abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my accuracy.
No doubt errors may have crept in, though I hope I have always been
cautious in trusting to good authorities alone. I can here give only
the general conclusions at which I have arrived, with a few facts in
illustration, but which, I hope, in most cases will suffice. No one can
feel more sensible than I do of the necessity of hereafter publishing
in detail all the facts, with references, on which my conclusions have
been grounded; and I hope in a future work to do this. For I am well
aware that scarcely a single point is discussed in this volume on which
facts cannot be adduced, often apparently leading to conclusions
directly opposite to those at which I have arrived. A fair result can
be obtained only by fully stating and balancing the facts and arguments
on both sides of each question; and this is here impossible.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however, let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who, for the last fifteen years, has aided
me in every possible way by his large stores of knowledge and his
excellent judgment.

In considering the origin of species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that species had not been independently created, but had
descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species, inhabiting this world have
been modified, so as to acquire that perfection of structure and
coadaptation which justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, &c.,
as the only possible cause of variation. In one limited sense, as we
shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance, of
the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
mistletoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant
itself.

It is, therefore, of the highest importance to gain a clear insight
into the means of modification and coadaptation. At the commencement of
my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been
disappointed; in this and in all other perplexing cases I have
invariably found that our knowledge, imperfect though it be, of
variation under domestication, afforded the best and safest clue. I may
venture to express my conviction of the high value of such studies,
although they have been very commonly neglected by naturalists.

From these considerations, I shall devote the first chapter of this
abstract to variation under domestication. We shall thus see that a
large amount of hereditary modification is at least possible; and, what
is equally or more important, we shall see how great is the power of
man in accumulating by his selection successive slight variations. I
will then pass on to the variability of species in a state of nature;
but I shall, unfortunately, be compelled to treat this subject far too
briefly, as it can be treated properly only by giving long catalogues
of facts. We shall, however, be enabled to discuss what circumstances
are most favourable to variation. In the next chapter the struggle for
existence among all organic beings throughout the world, which
inevitably follows from the high geometrical ratio of their increase,
will be considered. This is the doctrine of Malthus, applied to the
whole animal and vegetable kingdoms. As many more individuals of each
species are born than can possibly survive; and as, consequently, there
is a frequently recurring struggle for existence, it follows that any
being, if it vary however slightly in any manner profitable to itself,
under the complex and sometimes varying conditions of life, will have a
better chance of surviving, and thus be _naturally selected_. From the
strong principle of inheritance, any selected variety will tend to
propagate its new and modified form.

This fundamental subject of natural selection will be treated at some
length in the fourth chapter; and we shall then see how natural
selection almost inevitably causes much extinction of the less improved
forms of life, and leads to what I have called divergence of character.
In the next chapter I shall discuss the complex and little known laws
of variation. In the five succeeding chapters, the most apparent and
gravest difficulties in accepting the theory will be given: namely,
first, the difficulties of transitions, or how a simple being or a
simple organ can be changed and perfected into a highly developed being
or into an elaborately constructed organ; secondly the subject of
instinct, or the mental powers of animals; thirdly, hybridism, or the
infertility of species and the fertility of varieties when
intercrossed; and fourthly, the imperfection of the geological record.
In the next chapter I shall consider the geological succession of
organic beings throughout time; in the twelfth and thirteenth, their
geographical distribution throughout space; in the fourteenth, their
classification or mutual affinities, both when mature and in an
embryonic condition. In the last chapter I shall give a brief
recapitulation of the whole work, and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he make due allowance
for our profound ignorance in regard to the mutual relations of the
many beings which live around us. Who can explain why one species
ranges widely and is very numerous, and why another allied species has
a narrow range and is rare? Yet these relations are of the highest
importance, for they determine the present welfare and, as I believe,
the future success and modification of every inhabitant of this world.
Still less do we know of the mutual relations of the innumerable
inhabitants of the world during the many past geological epochs in its
history. Although much remains obscure, and will long remain obscure, I
can entertain no doubt, after the most deliberate study and
dispassionate judgment of which I am capable, that the view which most
naturalists until recently entertained, and which I formerly
entertained—namely, that each species has been independently created—is
erroneous. I am fully convinced that species are not immutable; but
that those belonging to what are called the same genera are lineal
descendants of some other and generally extinct species, in the same
manner as the acknowledged varieties of any one species are the
descendants of that species. Furthermore, I am convinced that natural
selection has been the most important, but not the exclusive, means of
modification.




CHAPTER I.
VARIATION UNDER DOMESTICATION.


Causes of Variability—Effects of Habit and the use and disuse of
Parts—Correlated Variation—Inheritance—Character of Domestic
Varieties—Difficulty of distinguishing between Varieties and
Species—Origin of Domestic Varieties from one or more Species—Domestic
Pigeons, their Differences and Origin—Principles of Selection,
anciently followed, their Effects—Methodical and Unconscious
Selection—Unknown Origin of our Domestic Productions—Circumstances
favourable to Man’s power of Selection.


_Causes of Variability._


When we compare the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us is, that they generally differ more from each other than do
the individuals of any one species or variety in a state of nature. And
if we reflect on the vast diversity of the plants and animals which
have been cultivated, and which have varied during all ages under the
most different climates and treatment, we are driven to conclude that
this great variability is due to our domestic productions having been
raised under conditions of life not so uniform as, and somewhat
different from, those to which the parent species had been exposed
under nature. There is, also, some probability in the view propounded
by Andrew Knight, that this variability may be partly connected with
excess of food. It seems clear that organic beings must be exposed
during several generations to new conditions to cause any great amount
of variation; and that, when the organisation has once begun to vary,
it generally continues varying for many generations. No case is on
record of a variable organism ceasing to vary under cultivation. Our
oldest cultivated plants, such as wheat, still yield new varieties: our
oldest domesticated animals are still capable of rapid improvement or
modification.

As far as I am able to judge, after long attending to the subject, the
conditions of life appear to act in two ways—directly on the whole
organisation or on certain parts alone and in directly by affecting the
reproductive system. With respect to the direct action, we must bear in
mind that in every case, as Professor Weismann has lately insisted, and
as I have incidently shown in my work on “Variation under
Domestication,” there are two factors: namely, the nature of the
organism and the nature of the conditions. The former seems to be much
the more important; for nearly similar variations sometimes arise
under, as far as we can judge, dissimilar conditions; and, on the other
hand, dissimilar variations arise under conditions which appear to be
nearly uniform. The effects on the offspring are either definite or in
definite. They may be considered as definite when all or nearly all the
offspring of individuals exposed to certain conditions during several
generations are modified in the same manner. It is extremely difficult
to come to any conclusion in regard to the extent of the changes which
have been thus definitely induced. There can, however, be little doubt
about many slight changes, such as size from the amount of food, colour
from the nature of the food, thickness of the skin and hair from
climate, &c. Each of the endless variations which we see in the plumage
of our fowls must have had some efficient cause; and if the same cause
were to act uniformly during a long series of generations on many
individuals, all probably would be modified in the same manner. Such
facts as the complex and extraordinary out growths which variably
follow from the insertion of a minute drop of poison by a
gall-producing insect, shows us what singular modifications might
result in the case of plants from a chemical change in the nature of
the sap.

In definite variability is a much more common result of changed
conditions than definite variability, and has probably played a more
important part in the formation of our domestic races. We see in
definite variability in the endless slight peculiarities which
distinguish the individuals of the same species, and which cannot be
accounted for by inheritance from either parent or from some more
remote ancestor. Even strongly-marked differences occasionally appear
in the young of the same litter, and in seedlings from the same
seed-capsule. At long intervals of time, out of millions of individuals
reared in the same country and fed on nearly the same food, deviations
of structure so strongly pronounced as to deserve to be called
monstrosities arise; but monstrosities cannot be separated by any
distinct line from slighter variations. All such changes of structure,
whether extremely slight or strongly marked, which appear among many
individuals living together, may be considered as the in definite
effects of the conditions of life on each individual organism, in
nearly the same manner as the chill effects different men in an in
definite manner, according to their state of body or constitution,
causing coughs or colds, rheumatism, or inflammation of various organs.

With respect to what I have called the in direct action of changed
conditions, namely, through the reproductive system of being affected,
we may infer that variability is thus induced, partly from the fact of
this system being extremely sensitive to any change in the conditions,
and partly from the similarity, as Kölreuter and others have remarked,
between the variability which follows from the crossing of distinct
species, and that which may be observed with plants and animals when
reared under new or unnatural conditions. Many facts clearly show how
eminently susceptible the reproductive system is to very slight changes
in the surrounding conditions. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely
under confinement, even when the male and female unite. How many
animals there are which will not breed, though kept in an almost free
state in their native country! This is generally, but erroneously
attributed to vitiated instincts. Many cultivated plants display the
utmost vigour, and yet rarely or never seed! In some few cases it has
been discovered that a very trifling change, such as a little more or
less water at some particular period of growth, will determine whether
or not a plant will produce seeds. I cannot here give the details which
I have collected and elsewhere published on this curious subject; but
to show how singular the laws are which determine the reproduction of
animals under confinement, I may mention that carnivorous animals, even
from the tropics, breed in this country pretty freely under
confinement, with the exception of the plantigrades or bear family,
which seldom produce young; whereas, carnivorous birds, with the rarest
exception, hardly ever lay fertile eggs. Many exotic plants have pollen
utterly worthless, in the same condition as in the most sterile
hybrids. When, on the one hand, we see domesticated animals and plants,
though often weak and sickly, breeding freely under confinement; and
when, on the other hand, we see individuals, though taken young from a
state of nature perfectly tamed, long-lived, and healthy (of which I
could give numerous instances), yet having their reproductive system so
seriously affected by unperceived causes as to fail to act, we need not
be surprised at this system, when it does act under confinement, acting
irregularly, and producing offspring somewhat unlike their parents. I
may add that as some organisms breed freely under the most unnatural
conditions—for instance, rabbits and ferrets kept in hutches—showing
that their reproductive organs are not easily affected; so will some
animals and plants withstand domestication or cultivation, and vary
very slightly—perhaps hardly more than in a state of nature.

Some naturalists have maintained that all variations are connected with
the act of sexual reproduction; but this is certainly an error; for I
have given in another work a long list of “sporting plants;” as they
are called by gardeners; that is, of plants which have suddenly
produced a single bud with a new and sometimes widely different
character from that of the other buds on the same plant. These bud
variations, as they may be named, can be propagated by grafts, offsets,
&c., and sometimes by seed. They occur rarely under nature, but are far
from rare under culture. As a single bud out of many thousands produced
year after year on the same tree under uniform conditions, has been
known suddenly to assume a new character; and as buds on distinct
trees, growing under different conditions, have sometimes yielded
nearly the same variety—for instance, buds on peach-trees producing
nectarines, and buds on common roses producing moss-roses—we clearly
see that the nature of the conditions is of subordinate importance in
comparison with the nature of the organism in determining each
particular form of variation; perhaps of not more importance than the
nature of the spark, by which a mass of combustible matter is ignited,
has in determining the nature of the flames.

_Effects of Habit and of the Use or Disuse of Parts; Correlated
Variation; Inheritance._


Changed habits produce an inherited effect as in the period of the
flowering of plants when transported from one climate to another. With
animals the increased use or disuse of parts has had a more marked
influence; thus I find in the domestic duck that the bones of the wing
weigh less and the bones of the leg more, in proportion to the whole
skeleton, than do the same bones in the wild duck; and this change may
be safely attributed to the domestic duck flying much less, and walking
more, than its wild parents. The great and inherited development of the
udders in cows and goats in countries where they are habitually milked,
in comparison with these organs in other countries, is probably another
instance of the effects of use. Not one of our domestic animals can be
named which has not in some country drooping ears; and the view which
has been suggested that the drooping is due to disuse of the muscles of
the ear, from the animals being seldom much alarmed, seems probable.

Many laws regulate variation, some few of which can be dimly seen, and
will hereafter be briefly discussed. I will here only allude to what
may be called correlated variation. Important changes in the embryo or
larva will probably entail changes in the mature animal. In
monstrosities, the correlations between quite distinct parts are very
curious; and many instances are given in Isidore Geoffroy St. Hilaire’s
great work on this subject. Breeders believe that long limbs are almost
always accompanied by an elongated head. Some instances of correlation
are quite whimsical; thus cats which are entirely white and have blue
eyes are generally deaf; but it has been lately stated by Mr. Tait that
this is confined to the males. Colour and constitutional peculiarities
go together, of which many remarkable cases could be given among
animals and plants. From facts collected by Heusinger, it appears that
white sheep and pigs are injured by certain plants, while dark-coloured
individuals escape: Professor Wyman has recently communicated to me a
good illustration of this fact; on asking some farmers in Virginia how
it was that all their pigs were black, they informed him that the pigs
ate the paint-root (Lachnanthes), which coloured their bones pink, and
which caused the hoofs of all but the black varieties to drop off; and
one of the “crackers” (_i.e._ Virginia squatters) added, “we select the
black members of a litter for raising, as they alone have a good chance
of living.” Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many
horns; pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly modify unintentionally other
parts of the structure, owing to the mysterious laws of correlation.

The results of the various, unknown, or but dimly understood laws of
variation are infinitely complex and diversified. It is well worth
while carefully to study the several treatises on some of our old
cultivated plants, as on the hyacinth, potato, even the dahlia, &c.;
and it is really surprising to note the endless points of structure and
constitution in which the varieties and sub-varieties differ slightly
from each other. The whole organisation seems to have become plastic,
and departs in a slight degree from that of the parental type.

Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both those
of slight and those of considerable physiological importance, are
endless. Dr. Prosper Lucas’ treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance; that like produces like is his fundamental
belief: doubts have been thrown on this principle only by theoretical
writers. When any deviation of structure often appears, and we see it
in the father and child, we cannot tell whether it may not be due to
the same cause having acted on both; but when among individuals,
apparently exposed to the same conditions, any very rare deviation, due
to some extraordinary combination of circumstances, appears in the
parent—say, once among several million individuals—and it reappears in
the child, the mere doctrine of chances almost compels us to attribute
its reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, &c., appearing in several members
of the same family. If strange and rare deviations of structure are
truly inherited, less strange and commoner deviations may be freely
admitted to be inheritable. Perhaps the correct way of viewing the
whole subject would be, to look at the inheritance of every character
whatever as the rule, and non-inheritance as the anomaly.

The laws governing inheritance are for the most part unknown; no one
can say why the same peculiarity in different individuals of the same
species, or in different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characteristics to its
grandfather or grandmother or more remote ancestor; why a peculiarity
is often transmitted from one sex to both sexes, or to one sex alone,
more commonly but not exclusively to the like sex. It is a fact of some
importance to us, that peculiarities appearing in the males of our
domestic breeds are often transmitted, either exclusively or in a much
greater degree, to the males alone. A much more important rule, which I
think may be trusted, is that, at whatever period of life a peculiarity
first appears, it tends to reappear in the offspring at a corresponding
age, though sometimes earlier. In many cases this could not be
otherwise; thus the inherited peculiarities in the horns of cattle
could appear only in the offspring when nearly mature; peculiarities in
the silk-worm are known to appear at the corresponding caterpillar or
cocoon stage. But hereditary diseases and some other facts make me
believe that the rule has a wider extension, and that, when there is no
apparent reason why a peculiarity should appear at any particular age,
yet that it does tend to appear in the offspring at the same period at
which it first appeared in the parent. I believe this rule to be of the
highest importance in explaining the laws of embryology. These remarks
are of course confined to the first _appearance_ of the peculiarity,
and not to the primary cause which may have acted on the ovules or on
the male element; in nearly the same manner as the increased length of
the horns in the offspring from a short-horned cow by a long-horned
bull, though appearing late in life, is clearly due to the male
element.

Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists—namely, that our domestic
varieties, when run wild, gradually but invariably revert in character
to their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be great
difficulty in proving its truth: we may safely conclude that very many
of the most strongly marked domestic varieties could not possibly live
in a wild state. In many cases we do not know what the aboriginal stock
was, and so could not tell whether or not nearly perfect reversion had
ensued. It would be necessary, in order to prevent the effects of
intercrossing, that only a single variety should be turned loose in its
new home. Nevertheless, as our varieties certainly do occasionally
revert in some of their characters to ancestral forms, it seems to me
not improbable that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance, of
the cabbage, in very poor soil—in which case, however, some effect
would have to be attributed to the _definite_ action of the poor
soil—that they would, to a large extent, or even wholly, revert to the
wild aboriginal stock. Whether or not the experiment would succeed is
not of great importance for our line of argument; for by the experiment
itself the conditions of life are changed. If it could be shown that
our domestic varieties manifested a strong tendency to reversion—that
is, to lose their acquired characters, while kept under the same
conditions and while kept in a considerable body, so that free
intercrossing might check, by blending together, any slight deviations
in their structure, in such case, I grant that we could deduce nothing
from domestic varieties in regard to species. But there is not a shadow
of evidence in favour of this view: to assert that we could not breed
our cart and race-horses, long and short-horned cattle, and poultry of
various breeds, and esculent vegetables, for an unlimited number of
generations, would be opposed to all experience.

_Character of Domestic Varieties; difficulty of distinguishing between
Varieties and Species; origin of Domestic Varieties from one or more
Species._


When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races often have
a somewhat monstrous character; by which I mean, that, although
differing from each other and from other species of the same genus, in
several trifling respects, they often differ in an extreme degree in
some one part, both when compared one with another, and more especially
when compared with the species under nature to which they are nearest
allied. With these exceptions (and with that of the perfect fertility
of varieties when crossed—a subject hereafter to be discussed),
domestic races of the same species differ from each other in the same
manner as do the closely allied species of the same genus in a state of
nature, but the differences in most cases are less in degree. This must
be admitted as true, for the domestic races of many animals and plants
have been ranked by some competent judges as the descendants of
aboriginally distinct species, and by other competent judges as mere
varieties. If any well marked distinction existed between a domestic
race and a species, this source of doubt would not so perpetually
recur. It has often been stated that domestic races do not differ from
each other in characters of generic value. It can be shown that this
statement is not correct; but naturalists differ much in determining
what characters are of generic value; all such valuations being at
present empirical. When it is explained how genera originate under
nature, it will be seen that we have no right to expect often to find a
generic amount of difference in our domesticated races.

In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are descended from one or several parent species. This
point, if it could be cleared up, would be interesting; if, for
instance, it could be shown that the greyhound, bloodhound, terrier,
spaniel and bull-dog, which we all know propagate their kind truly,
were the offspring of any single species, then such facts would have
great weight in making us doubt about the immutability of the many
closely allied natural species—for instance, of the many
foxes—inhabiting the different quarters of the world. I do not believe,
as we shall presently see, that the whole amount of difference between
the several breeds of the dog has been produced under domestication; I
believe that a small part of the difference is due to their being
descended from distinct species. In the case of strongly marked races
of some other domesticated species, there is presumptive or even strong
evidence that all are descended from a single wild stock.

It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed
an animal, whether it would vary in succeeding generations, and whether
it would endure other climates? Has the little variability of the ass
and goose, or the small power of endurance of warmth by the reindeer,
or of cold by the common camel, prevented their domestication? I cannot
doubt that if other animals and plants, equal in number to our
domesticated productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could be made to
breed for an equal number of generations under domestication, they
would on an average vary as largely as the parent species of our
existing domesticated productions have varied.

In the case of most of our anciently domesticated animals and plants,
it is not possible to come to any definite conclusion, whether they are
descended from one or several wild species. The argument mainly relied
on by those who believe in the multiple origin of our domestic animals
is, that we find in the most ancient times, on the monuments of Egypt,
and in the lake-habitations of Switzerland, much diversity in the
breeds; and that some of these ancient breeds closely resemble, or are
even identical with, those still existing. But this only throws far
backward the history of civilisation, and shows that animals were
domesticated at a much earlier period than has hitherto been supposed.
The lake-inhabitants of Switzerland cultivated several kinds of wheat
and barley, the pea, the poppy for oil and flax; and they possessed
several domesticated animals. They also carried on commerce with other
nations. All this clearly shows, as Heer has remarked, that they had at
this early age progressed considerably in civilisation; and this again
implies a long continued previous period of less advanced civilisation,
during which the domesticated animals, kept by different tribes in
different districts, might have varied and given rise to distinct
races. Since the discovery of flint tools in the superficial formations
of many parts of the world, all geologists believe that barbarian men
existed at an enormously remote period; and we know that at the present
day there is hardly a tribe so barbarous as not to have domesticated at
least the dog.

The origin of most of our domestic animals will probably forever remain
vague. But I may here state that, looking to the domestic dogs of the
whole world, I have, after a laborious collection of all known facts,
come to the conclusion that several wild species of Canidæ have been
tamed, and that their blood, in some cases mingled together, flows in
the veins of our domestic breeds. In regard to sheep and goats I can
form no decided opinion. From facts communicated to me by Mr. Blyth, on
the habits, voice, constitution and structure of the humped Indian
cattle, it is almost certain that they are descended from a different
aboriginal stock from our European cattle; and some competent judges
believe that these latter have had two or three wild progenitors,
whether or not these deserve to be called species. This conclusion, as
well as that of the specific distinction between the humped and common
cattle, may, indeed, be looked upon as established by the admirable
researches of Professor Rütimeyer. With respect to horses, from reasons
which I cannot here give, I am doubtfully inclined to believe, in
opposition to several authors, that all the races belong to the same
species. Having kept nearly all the English breeds of the fowl alive,
having bred and crossed them, and examined their skeletons, it appears
to me almost certain that all are the descendants of the wild Indian
fowl, Gallus bankiva; and this is the conclusion of Mr. Blyth, and of
others who have studied this bird in India. In regard to ducks and
rabbits, some breeds of which differ much from each other, the evidence
is clear that they are all descended from the common duck and wild
rabbit.

The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats, in Europe alone, and
several even within Great Britain. One author believes that there
formerly existed eleven wild species of sheep peculiar to Great
Britain! When we bear in mind that Britain has now not one peculiar
mammal, and France but few distinct from those of Germany, and so with
Hungary, Spain, &c., but that each of these kingdoms possesses several
peculiar breeds of cattle, sheep, &c., we must admit that many domestic
breeds must have originated in Europe; for whence otherwise could they
have been derived? So it is in India. Even in the case of the breeds of
the domestic dog throughout the world, which I admit are descended from
several wild species, it cannot be doubted that there has been an
immense amount of inherited variation; for who will believe that
animals closely resembling the Italian greyhound, the bloodhound, the
bull-dog, pug-dog, or Blenheim spaniel, &c.—so unlike all wild
Canidæ—ever existed in a state of nature? It has often been loosely
said that all our races of dogs have been produced by the crossing of a
few aboriginal species; but by crossing we can only get forms in some
degree intermediate between their parents; and if we account for our
several domestic races by this process, we must admit the former
existence of the most extreme forms, as the Italian greyhound,
bloodhound, bull-dog, &c., in the wild state. Moreover, the possibility
of making distinct races by crossing has been greatly exaggerated. Many
cases are on record showing that a race may be modified by occasional
crosses if aided by the careful selection of the individuals which
present the desired character; but to obtain a race intermediate
between two quite distinct races would be very difficult. Sir J.
Sebright expressly experimented with this object and failed. The
offspring from the first cross between two pure breeds is tolerably and
sometimes (as I have found with pigeons) quite uniform in character,
and every thing seems simple enough; but when these mongrels are
crossed one with another for several generations, hardly two of them
are alike, and then the difficulty of the task becomes manifest.

_Breeds of the Domestic Pigeon, their Differences and Origin._


Believing that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured
with skins from several quarters of the world, more especially by the
Hon. W. Elliot from India, and by the Hon. C. Murray from Persia. Many
treatises in different languages have been published on pigeons, and
some of them are very important, as being of considerable antiquity. I
have associated with several eminent fanciers, and have been permitted
to join two of the London Pigeon Clubs. The diversity of the breeds is
something astonishing. Compare the English carrier and the short-faced
tumbler, and see the wonderful difference in their beaks, entailing
corresponding differences in their skulls. The carrier, more especially
the male bird, is also remarkable from the wonderful development of the
carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a
wide gape of mouth. The short-faced tumbler has a beak in outline
almost like that of a finch; and the common tumbler has the singular
inherited habit of flying at a great height in a compact flock, and
tumbling in the air head over heels. The runt is a bird of great size,
with long, massive beak and large feet; some of the sub-breeds of runts
have very long necks, others very long wings and tails, others
singularly short tails. The barb is allied to the carrier, but, instead
of a long beak, has a very short and broad one. The pouter has a much
elongated body, wings, and legs; and its enormously developed crop,
which it glories in inflating, may well excite astonishment and even
laughter. The turbit has a short and conical beak, with a line of
reversed feathers down the breast; and it has the habit of continually
expanding, slightly, the upper part of the œsophagus. The Jacobin has
the feathers so much reversed along the back of the neck that they form
a hood, and it has, proportionally to its size, elongated wing and tail
feathers. The trumpeter and laugher, as their names express, utter a
very different coo from the other breeds. The fantail has thirty or
even forty tail-feathers, instead of twelve or fourteen, the normal
number in all the members of the great pigeon family: these feathers
are kept expanded and are carried so erect that in good birds the head
and tail touch: the oil-gland is quite aborted. Several other less
distinct breeds might be specified.

In the skeletons of the several breeds, the development of the bones of
the face, in length and breadth and curvature, differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The caudal and sacral vertebræ
vary in number; as does the number of the ribs, together with their
relative breadth and the presence of processes. The size and shape of
the apertures in the sternum are highly variable; so is the degree of
divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the
eyelids, of the orifice of the nostrils, of the tongue (not always in
strict correlation with the length of beak), the size of the crop and
of the upper part of the œsophagus; the development and abortion of the
oil-gland; the number of the primary wing and caudal feathers; the
relative length of the wing and tail to each other and to the body; the
relative length of the leg and foot; the number of scutellæ on the
toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage
is acquired varies, as does the state of the down with which the
nestling birds are clothed when hatched. The shape and size of the eggs
vary. The manner of flight, and in some breeds the voice and
disposition, differ remarkably. Lastly, in certain breeds, the males
and females have come to differ in a slight degree from each other.

Altogether at least a score of pigeons might be chosen, which, if shown
to an ornithologist, and he were told that they were wild birds, would
certainly be ranked by him as well-defined species. Moreover, I do not
believe that any ornithologist would in this case place the English
carrier, the short-faced tumbler, the runt, the barb, pouter, and
fantail in the same genus; more especially as in each of these breeds
several truly-inherited sub-breeds, or species, as he would call them,
could be shown him.

Great as are the differences between the breeds of the pigeon, I am
fully convinced that the common opinion of naturalists is correct,
namely, that all are descended from the rock-pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the
several breeds are not varieties, and have not proceeded from the
rock-pigeon, they must have descended from at least seven or eight
aboriginal stocks; for it is impossible to make the present domestic
breeds by the crossing of any lesser number: how, for instance, could a
pouter be produced by crossing two breeds unless one of the
parent-stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock-pigeons, that is, they did
not breed or willingly perch on trees. But besides C. livia, with its
geographical sub-species, only two or three other species of
rock-pigeons are known; and these have not any of the characters of the
domestic breeds. Hence the supposed aboriginal stocks must either still
exist in the countries where they were originally domesticated, and yet
be unknown to ornithologists; and this, considering their size, habits
and remarkable characters, seems improbable; or they must have become
extinct in the wild state. But birds breeding on precipices, and good
flyers, are unlikely to be exterminated; and the common rock-pigeon,
which has the same habits with the domestic breeds, has not been
exterminated even on several of the smaller British islets, or on the
shores of the Mediterranean. Hence the supposed extermination of so
many species having similar habits with the rock-pigeon seems a very
rash assumption. Moreover, the several above-named domesticated breeds
have been transported to all parts of the world, and, therefore, some
of them must have been carried back again into their native country;
but not one has become wild or feral, though the dovecot-pigeon, which
is the rock-pigeon in a very slightly altered state, has become feral
in several places. Again, all recent experience shows that it is
difficult to get wild animals to breed freely under domestication; yet
on the hypothesis of the multiple origin of our pigeons, it must be
assumed that at least seven or eight species were so thoroughly
domesticated in ancient times by half-civilized man, as to be quite
prolific under confinement.

An argument of great weight, and applicable in several other cases, is,
that the above-specified breeds, though agreeing generally with the
wild rock-pigeon in constitution, habits, voice, colouring, and in most
parts of their structure, yet are certainly highly abnormal in other
parts; we may look in vain through the whole great family of Columbidæ
for a beak like that of the English carrier, or that of the short-faced
tumbler, or barb; for reversed feathers like those of the Jacobin; for
a crop like that of the pouter; for tail-feathers like those of the
fantail. Hence it must be assumed, not only that half-civilized man
succeeded in thoroughly domesticating several species, but that he
intentionally or by chance picked out extraordinarily abnormal species;
and further, that these very species have since all become extinct or
unknown. So many strange contingencies are improbable in the highest
degree.

Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, with white loins;
but the Indian sub-species, C. intermedia of Strickland, has this part
bluish. The tail has a terminal dark bar, with the outer feathers
externally edged at the base with white. The wings have two black bars.
Some semi-domestic breeds, and some truly wild breeds, have, besides
the two black bars, the wings chequered with black. These several marks
do not occur together in any other species of the whole family. Now, in
every one of the domestic breeds, taking thoroughly well-bred birds,
all the above marks, even to the white edging of the outer
tail-feathers, sometimes concur perfectly developed. Moreover, when
birds belonging to two or more distinct breeds are crossed, none of
which are blue or have any of the above-specified marks, the mongrel
offspring are very apt suddenly to acquire these characters. To give
one instance out of several which I have observed: I crossed some white
fantails, which breed very true, with some black barbs—and it so
happens that blue varieties of barbs are so rare that I never heard of
an instance in England; and the mongrels were black, brown and mottled.
I also crossed a barb with a spot, which is a white bird with a red
tail and red spot on the forehead, and which notoriously breeds very
true; the mongrels were dusky and mottled. I then crossed one of the
mongrel barb-fantails with a mongrel barb-spot, and they produced a
bird of as beautiful a blue colour, with the white loins, double black
wing-bar, and barred and white-edged tail-feathers, as any wild
rock-pigeon! We can understand these facts, on the well-known principle
of reversion to ancestral characters, if all the domestic breeds are
descended from the rock-pigeon. But if we deny this, we must make one
of the two following highly improbable suppositions. Either, first,
that all the several imagined aboriginal stocks were coloured and
marked like the rock-pigeon, although no other existing species is thus
coloured and marked, so that in each separate breed there might be a
tendency to revert to the very same colours and markings. Or, secondly,
that each breed, even the purest, has within a dozen, or at most within
a score, of generations, been crossed by the rock-pigeon: I say within
a dozen or twenty generations, for no instance is known of crossed
descendants reverting to an ancestor of foreign blood, removed by a
greater number of generations. In a breed which has been crossed only
once the tendency to revert to any character derived from such a cross
will naturally become less and less, as in each succeeding generation
there will be less of the foreign blood; but when there has been no
cross, and there is a tendency in the breed to revert to a character
which was lost during some former generation, this tendency, for all
that we can see to the contrary, may be transmitted undiminished for an
indefinite number of generations. These two distinct cases of reversion
are often confounded together by those who have written on inheritance.

Lastly, the hybrids or mongrels from between all the breeds of the
pigeon are perfectly fertile, as I can state from my own observations,
purposely made, on the most distinct breeds. Now, hardly any cases have
been ascertained with certainty of hybrids from two quite distinct
species of animals being perfectly fertile. Some authors believe that
long-continued domestication eliminates this strong tendency to
sterility in species. From the history of the dog, and of some other
domestic animals, this conclusion is probably quite correct, if applied
to species closely related to each other. But to extend it so far as to
suppose that species, aboriginally as distinct as carriers, tumblers,
pouters, and fantails now are, should yield offspring perfectly
fertile, _inter se_, seems to me rash in the extreme.

From these several reasons, namely, the improbability of man having
formerly made seven or eight supposed species of pigeons to breed
freely under domestication—these supposed species being quite unknown
in a wild state, and their not having become anywhere feral—these
species presenting certain very abnormal characters, as compared with
all other Columbidæ, though so like the rock-pigeon in most other
respects—the occasional reappearance of the blue colour and various
black marks in all the breeds, both when kept pure and when crossed—and
lastly, the mongrel offspring being perfectly fertile—from these
several reasons, taken together, we may safely conclude that all our
domestic breeds are descended from the rock-pigeon or Columba livia
with its geographical sub-species.

In favour of this view, I may add, firstly, that the wild C. livia has
been found capable of domestication in Europe and in India; and that it
agrees in habits and in a great number of points of structure with all
the domestic breeds. Secondly, that although an English carrier or a
short-faced tumbler differs immensely in certain characters from the
rock-pigeon, yet that by comparing the several sub-breeds of these two
races, more especially those brought from distant countries, we can
make, between them and the rock-pigeon, an almost perfect series; so we
can in some other cases, but not with all the breeds. Thirdly, those
characters which are mainly distinctive of each breed are in each
eminently variable, for instance, the wattle and length of beak of the
carrier, the shortness of that of the tumbler, and the number of
tail-feathers in the fantail; and the explanation of this fact will be
obvious when we treat of selection. Fourthly, pigeons have been watched
and tended with the utmost care, and loved by many people. They have
been domesticated for thousands of years in several quarters of the
world; the earliest known record of pigeons is in the fifth Ægyptian
dynasty, about 3000 B.C., as was pointed out to me by Professor
Lepsius; but Mr. Birch informs me that pigeons are given in a bill of
fare in the previous dynasty. In the time of the Romans, as we hear
from Pliny, immense prices were given for pigeons; “nay, they are come
to this pass, that they can reckon up their pedigree and race.” Pigeons
were much valued by Akber Khan in India, about the year 1600; never
less than 20,000 pigeons were taken with the court. “The monarchs of
Iran and Turan sent him some very rare birds;” and, continues the
courtly historian, “His Majesty, by crossing the breeds, which method
was never practised before, has improved them astonishingly.” About
this same period the Dutch were as eager about pigeons as were the old
Romans. The paramount importance of these considerations in explaining
the immense amount of variation which pigeons have undergone, will
likewise be obvious when we treat of Selection. We shall then, also,
see how it is that the several breeds so often have a somewhat
monstrous character. It is also a most favourable circumstance for the
production of distinct breeds, that male and female pigeons can be
easily mated for life; and thus different breeds can be kept together
in the same aviary.

I have discussed the probable origin of domestic pigeons at some, yet
quite insufficient, length; because when I first kept pigeons and
watched the several kinds, well knowing how truly they breed, I felt
fully as much difficulty in believing that since they had been
domesticated they had all proceeded from a common parent, as any
naturalist could in coming to a similar conclusion in regard to the
many species of finches, or other groups of birds, in nature. One
circumstance has struck me much; namely, that nearly all the breeders
of the various domestic animals and the cultivators of plants, with
whom I have conversed, or whose treatises I have read, are firmly
convinced that the several breeds to which each has attended, are
descended from so many aboriginally distinct species. Ask, as I have
asked, a celebrated raiser of Hereford cattle, whether his cattle might
not have descended from Long-horns, or both from a common parent-stock,
and he will laugh you to scorn. I have never met a pigeon, or poultry,
or duck, or rabbit fancier, who was not fully convinced that each main
breed was descended from a distinct species. Van Mons, in his treatise
on pears and apples, shows how utterly he disbelieves that the several
sorts, for instance a Ribston-pippin or Codlin-apple, could ever have
proceeded from the seeds of the same tree. Innumerable other examples
could be given. The explanation, I think, is simple: from
long-continued study they are strongly impressed with the differences
between the several races; and though they well know that each race
varies slightly, for they win their prizes by selecting such slight
differences, yet they ignore all general arguments, and refuse to sum
up in their minds slight differences accumulated during many successive
generations. May not those naturalists who, knowing far less of the
laws of inheritance than does the breeder, and knowing no more than he
does of the intermediate links in the long lines of descent, yet admit
that many of our domestic races are descended from the same parents—may
they not learn a lesson of caution, when they deride the idea of
species in a state of nature being lineal descendants of other species?

_Principles of Selection anciently followed, and their Effects._


Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some effect
may be attributed to the direct and definite action of the external
conditions of life, and some to habit; but he would be a bold man who
would account by such agencies for the differences between a dray and
race-horse, a greyhound and bloodhound, a carrier and tumbler pigeon.
One of the most remarkable features in our domesticated races is that
we see in them adaptation, not indeed to the animal’s or plant’s own
good, but to man’s use or fancy. Some variations useful to him have
probably arisen suddenly, or by one step; many botanists, for instance,
believe that the fuller’s teasel, with its hooks, which can not be
rivalled by any mechanical contrivance, is only a variety of the wild
Dipsacus; and this amount of change may have suddenly arisen in a
seedling. So it has probably been with the turnspit dog; and this is
known to have been the case with the ancon sheep. But when we compare
the dray-horse and race-horse, the dromedary and camel, the various
breeds of sheep fitted either for cultivated land or mountain pasture,
with the wool of one breed good for one purpose, and that of another
breed for another purpose; when we compare the many breeds of dogs,
each good for man in different ways; when we compare the game-cock, so
pertinacious in battle, with other breeds so little quarrelsome, with
“everlasting layers” which never desire to sit, and with the bantam so
small and elegant; when we compare the host of agricultural, culinary,
orchard, and flower-garden races of plants, most useful to man at
different seasons and for different purposes, or so beautiful in his
eyes, we must, I think, look further than to mere variability. We can
not suppose that all the breeds were suddenly produced as perfect and
as useful as we now see them; indeed, in many cases, we know that this
has not been their history. The key is man’s power of accumulative
selection: nature gives successive variations; man adds them up in
certain directions useful to him. In this sense he may be said to have
made for himself useful breeds.

The great power of this principle of selection is not hypothetical. It
is certain that several of our eminent breeders have, even within a
single lifetime, modified to a large extent their breeds of cattle and
sheep. In order fully to realise what they have done it is almost
necessary to read several of the many treatises devoted to this
subject, and to inspect the animals. Breeders habitually speak of an
animal’s organisation as something plastic, which they can model almost
as they please. If I had space I could quote numerous passages to this
effect from highly competent authorities. Youatt, who was probably
better acquainted with the works of agriculturalists than almost any
other individual, and who was himself a very good judge of animals,
speaks of the principle of selection as “that which enables the
agriculturist, not only to modify the character of his flock, but to
change it altogether. It is the magician’s wand, by means of which he
may summon into life whatever form and mould he pleases.” Lord
Somerville, speaking of what breeders have done for sheep, says: “It
would seem as if they had chalked out upon a wall a form perfect in
itself, and then had given it existence.” In Saxony the importance of
the principle of selection in regard to merino sheep is so fully
recognised, that men follow it as a trade: the sheep are placed on a
table and are studied, like a picture by a connoisseur; this is done
three times at intervals of months, and the sheep are each time marked
and classed, so that the very best may ultimately be selected for
breeding.

What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds; all the best breeders
are strongly opposed to this practice, except sometimes among closely
allied sub-breeds. And when a cross has been made, the closest
selection is far more indispensable even than in ordinary cases. If
selection consisted merely in separating some very distinct variety and
breeding from it, the principle would be so obvious as hardly to be
worth notice; but its importance consists in the great effect produced
by the accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated eye—differences
which I for one have vainly attempted to appreciate. Not one man in a
thousand has accuracy of eye and judgment sufficient to become an
eminent breeder. If gifted with these qualities, and he studies his
subject for years, and devotes his lifetime to it with indomitable
perseverance, he will succeed, and may make great improvements; if he
wants any of these qualities, he will assuredly fail. Few would readily
believe in the natural capacity and years of practice requisite to
become even a skilful pigeon-fancier.

The same principles are followed by horticulturists; but the variations
are here often more abrupt. No one supposes that our choicest
productions have been produced by a single variation from the
aboriginal stock. We have proofs that this is not so in several cases
in which exact records have been kept; thus, to give a very trifling
instance, the steadily-increasing size of the common gooseberry may be
quoted. We see an astonishing improvement in many florists’ flowers,
when the flowers of the present day are compared with drawings made
only twenty or thirty years ago. When a race of plants is once pretty
well established, the seed-raisers do not pick out the best plants, but
merely go over their seed-beds, and pull up the “rogues,” as they call
the plants that deviate from the proper standard. With animals this
kind of selection is, in fact, likewise followed; for hardly any one is
so careless as to breed from his worst animals.

In regard to plants, there is another means of observing the
accumulated effects of selection—namely, by comparing the diversity of
flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever
part is valued, in the kitchen-garden, in comparison with the flowers
of the same varieties; and the diversity of fruit of the same species
in the orchard, in comparison with the leaves and flowers of the same
set of varieties. See how different the leaves of the cabbage are, and
how extremely alike the flowers; how unlike the flowers of the
heartsease are, and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, colour, shape, and
hairiness, and yet the flowers present very slight differences. It is
not that the varieties which differ largely in some one point do not
differ at all in other points; this is hardly ever—I speak after
careful observation—perhaps never, the case. The law of correlated
variation, the importance of which should never be overlooked, will
ensure some differences; but, as a general rule, it cannot be doubted
that the continued selection of slight variations, either in the
leaves, the flowers, or the fruit, will produce races differing from
each other chiefly in these characters.

It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century;
it has certainly been more attended to of late years, and many
treatises have been published on the subject; and the result has been,
in a corresponding degree, rapid and important. But it is very far from
true that the principle is a modern discovery. I could give several
references to works of high antiquity, in which the full importance of
the principle is acknowledged. In rude and barbarous periods of English
history choice animals were often imported, and laws were passed to
prevent their exportation: the destruction of horses under a certain
size was ordered, and this may be compared to the “roguing” of plants
by nurserymen. The principle of selection I find distinctly given in an
ancient Chinese encyclopædia. Explicit rules are laid down by some of
the Roman classical writers. From passages in Genesis, it is clear that
the colour of domestic animals was at that early period attended to.
Savages now sometimes cross their dogs with wild canine animals, to
improve the breed, and they formerly did so, as is attested by passages
in Pliny. The savages in South Africa match their draught cattle by
colour, as do some of the Esquimaux their teams of dogs. Livingstone
states that good domestic breeds are highly valued by the negroes in
the interior of Africa who have not associated with Europeans. Some of
these facts do not show actual selection, but they show that the
breeding of domestic animals was carefully attended to in ancient
times, and is now attended to by the lowest savages. It would, indeed,
have been a strange fact, had attention not been paid to breeding, for
the inheritance of good and bad qualities is so obvious.

_Unconscious Selection._


At the present time, eminent breeders try by methodical selection, with
a distinct object in view, to make a new strain or sub-breed, superior
to anything of the kind in the country. But, for our purpose, a form of
selection, which may be called unconscious, and which results from
every one trying to possess and breed from the best individual animals,
is more important. Thus, a man who intends keeping pointers naturally
tries to get as good dogs as he can, and afterwards breeds from his own
best dogs, but he has no wish or expectation of permanently altering
the breed. Nevertheless we may infer that this process, continued
during centuries, would improve and modify any breed, in the same way
as Bakewell, Collins, &c., by this very same process, only carried on
more methodically, did greatly modify, even during their lifetimes, the
forms and qualities of their cattle. Slow and insensible changes of
this kind could never be recognised unless actual measurements or
careful drawings of the breeds in question have been made long ago,
which may serve for comparison. In some cases, however, unchanged, or
but little changed, individuals of the same breed exist in less
civilised districts, where the breed has been less improved. There is
reason to believe that King Charles’ spaniel has been unconsciously
modified to a large extent since the time of that monarch. Some highly
competent authorities are convinced that the setter is directly derived
from the spaniel, and has probably been slowly altered from it. It is
known that the English pointer has been greatly changed within the last
century, and in this case the change has, it is believed, been chiefly
effected by crosses with the foxhound; but what concerns us is, that
the change has been effected unconsciously and gradually, and yet so
effectually that, though the old Spanish pointer certainly came from
Spain, Mr. Borrow has not seen, as I am informed by him, any native dog
in Spain like our pointer.

By a similar process of selection, and by careful training, English
race-horses have come to surpass in fleetness and size the parent
Arabs, so that the latter, by the regulations for the Goodwood Races,
are favoured in the weights which they carry. Lord Spencer and others
have shown how the cattle of England have increased in weight and in
early maturity, compared with the stock formerly kept in this country.
By comparing the accounts given in various old treatises of the former
and present state of carrier and tumbler pigeons in Britain, India, and
Persia, we can trace the stages through which they have insensibly
passed, and come to differ so greatly from the rock-pigeon.

Youatt gives an excellent illustration of the effects of a course of
selection which may be considered as unconscious, in so far that the
breeders could never have expected, or even wished, to produce the
result which ensued—namely, the production of the distinct strains. The
two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as
Mr. Youatt remarks, “Have been purely bred from the original stock of
Mr. Bakewell for upwards of fifty years. There is not a suspicion
existing in the mind of any one at all acquainted with the subject that
the owner of either of them has deviated in any one instance from the
pure blood of Mr. Bakewell’s flock, and yet the difference between the
sheep possessed by these two gentlemen is so great that they have the
appearance of being quite different varieties.”

If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one
animal particularly useful to them, for any special purpose, would be
carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case there
would be a kind of unconscious selection going on. We see the value set
on animals even by the barbarians of Tierra del Fuego, by their killing
and devouring their old women, in times of dearth, as of less value
than their dogs.

In plants the same gradual process of improvement through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked at their first appearance as
distinct varieties, and whether or not two or more species or races
have become blended together by crossing, may plainly be recognised in
the increased size and beauty which we now see in the varieties of the
heartsease, rose, pelargonium, dahlia, and other plants, when compared
with the older varieties or with their parent-stocks. No one would ever
expect to get a first-rate heartsease or dahlia from the seed of a wild
plant. No one would expect to raise a first-rate melting pear from the
seed of a wild pear, though he might succeed from a poor seedling
growing wild, if it had come from a garden-stock. The pear, though
cultivated in classical times, appears, from Pliny’s description, to
have been a fruit of very inferior quality. I have seen great surprise
expressed in horticultural works at the wonderful skill of gardeners in
having produced such splendid results from such poor materials; but the
art has been simple, and, as far as the final result is concerned, has
been followed almost unconsciously. It has consisted in always
cultivating the best known variety, sowing its seeds, and, when a
slightly better variety chanced to appear, selecting it, and so
onwards. But the gardeners of the classical period, who cultivated the
best pears which they could procure, never thought what splendid fruit
we should eat; though we owe our excellent fruit in some small degree
to their having naturally chosen and preserved the best varieties they
could anywhere find.

A large amount of change, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a number of cases
we cannot recognise, and therefore do not know, the wild parent-stocks
of the plants which have been longest cultivated in our flower and
kitchen gardens. If it has taken centuries or thousands of years to
improve or modify most of our plants up to their present standard of
usefulness to man, we can understand how it is that neither Australia,
the Cape of Good Hope, nor any other region inhabited by quite
uncivilised man, has afforded us a single plant worth culture. It is
not that these countries, so rich in species, do not by a strange
chance possess the aboriginal stocks of any useful plants, but that the
native plants have not been improved by continued selection up to a
standard of perfection comparable with that acquired by the plants in
countries anciently civilised.

In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
“natural selection,” as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains why the
varieties kept by savages, as has been remarked by some authors, have
more of the character of true species than the varieties kept in
civilised countries.

On the view here given of the important part which selection by man has
played, it becomes at once obvious, how it is that our domestic races
show adaptation in their structure or in their habits to man’s wants or
fancies. We can, I think, further understand the frequently abnormal
character of our domestic races, and likewise their differences being
so great in external characters, and relatively so slight in internal
parts or organs. Man can hardly select, or only with much difficulty,
any deviation of structure excepting such as is externally visible; and
indeed he rarely cares for what is internal. He can never act by
selection, excepting on variations which are first given to him in some
slight degree by nature. No man would ever try to make a fantail till
he saw a pigeon with a tail developed in some slight degree in an
unusual manner, or a pouter till he saw a pigeon with a crop of
somewhat unusual size; and the more abnormal or unusual any character
was when it first appeared, the more likely it would be to catch his
attention. But to use such an expression as trying to make a fantail
is, I have no doubt, in most cases, utterly incorrect. The man who
first selected a pigeon with a slightly larger tail, never dreamed what
the descendants of that pigeon would become through long-continued,
partly unconscious and partly methodical, selection. Perhaps the parent
bird of all fantails had only fourteen tail-feathers somewhat expanded,
like the present Java fantail, or like individuals of other and
distinct breeds, in which as many as seventeen tail-feathers have been
counted. Perhaps the first pouter-pigeon did not inflate its crop much
more than the turbit now does the upper part of its œsophagus—a habit
which is disregarded by all fanciers, as it is not one of the points of
the breed.

Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier’s eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one’s own possession. Nor must the value which would
formerly have been set on any slight differences in the individuals of
the same species, be judged of by the value which is now set on them,
after several breeds have fairly been established. It is known that
with pigeons many slight variations now occasionally appear, but these
are rejected as faults or deviations from the standard of perfection in
each breed. The common goose has not given rise to any marked
varieties; hence the Toulouse and the common breed, which differ only
in colour, that most fleeting of characters, have lately been exhibited
as distinct at our poultry-shows.

These views appear to explain what has sometimes been noticed, namely,
that we know hardly anything about the origin or history of any of our
domestic breeds. But, in fact, a breed, like a dialect of a language,
can hardly be said to have a distinct origin. A man preserves and
breeds from an individual with some slight deviation of structure, or
takes more care than usual in matching his best animals, and thus
improves them, and the improved animals slowly spread in the immediate
neighbourhood. But they will as yet hardly have a distinct name, and
from being only slightly valued, their history will have been
disregarded. When further improved by the same slow and gradual
process, they will spread more widely, and will be recognised as
something distinct and valuable, and will then probably first receive a
provincial name. In semi-civilised countries, with little free
communication, the spreading of a new sub-breed will be a slow process.
As soon as the points of value are once acknowledged, the principle, as
I have called it, of unconscious selection will always tend—perhaps
more at one period than at another, as the breed rises or falls in
fashion—perhaps more in one district than in another, according to the
state of civilisation of the inhabitants—slowly to add to the
characteristic features of the breed, whatever they may be. But the
chance will be infinitely small of any record having been preserved of
such slow, varying, and insensible changes.

_Circumstances favourable to Man’s Power of Selection._


I will now say a few words on the circumstances, favourable or the
reverse, to man’s power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply sufficient,
with extreme care, to allow of the accumulation of a large amount of
modification in almost any desired direction. But as variations
manifestly useful or pleasing to man appear only occasionally, the
chance of their appearance will be much increased by a large number of
individuals being kept. Hence number is of the highest importance for
success. On this principle Marshall formerly remarked, with respect to
the sheep of part of Yorkshire, “As they generally belong to poor
people, and are mostly _in small lots_, they never can be improved.” On
the other hand, nurserymen, from keeping large stocks of the same
plant, are generally far more successful than amateurs in raising new
and valuable varieties. A large number of individuals of an animal or
plant can be reared only where the conditions for its propagation are
favourable. When the individuals are scanty all will be allowed to
breed, whatever their quality may be, and this will effectually prevent
selection. But probably the most important element is that the animal
or plant should be so highly valued by man, that the closest attention
is paid to even the slightest deviations in its qualities or structure.
Unless such attention be paid nothing can be effected. I have seen it
gravely remarked, that it was most fortunate that the strawberry began
to vary just when gardeners began to attend to this plant. No doubt the
strawberry had always varied since it was cultivated, but the slight
varieties had been neglected. As soon, however, as gardeners picked out
individual plants with slightly larger, earlier, or better fruit, and
raised seedlings from them, and again picked out the best seedlings and
bred from them, then (with some aid by crossing distinct species) those
many admirable varieties of the strawberry were raised which have
appeared during the last half-century.

With animals, facility in preventing crosses is an important element in
the formation of new races—at least, in a country which is already
stocked with other races. In this respect enclosure of the land plays a
part. Wandering savages or the inhabitants of open plains rarely
possess more than one breed of the same species. Pigeons can be mated
for life, and this is a great convenience to the fancier, for thus many
races may be improved and kept true, though mingled in the same aviary;
and this circumstance must have largely favoured the formation of new
breeds. Pigeons, I may add, can be propagated in great numbers and at a
very quick rate, and inferior birds may be freely rejected, as when
killed they serve for food. On the other hand, cats, from their
nocturnal rambling habits, can not be easily matched, and, although so
much valued by women and children, we rarely see a distinct breed long
kept up; such breeds as we do sometimes see are almost always imported
from some other country. Although I do not doubt that some domestic
animals vary less than others, yet the rarity or absence of distinct
breeds of the cat, the donkey, peacock, goose, &c., may be attributed
in main part to selection not having been brought into play: in cats,
from the difficulty in pairing them; in donkeys, from only a few being
kept by poor people, and little attention paid to their breeding; for
recently in certain parts of Spain and of the United States this animal
has been surprisingly modified and improved by careful selection; in
peacocks, from not being very easily reared and a large stock not kept;
in geese, from being valuable only for two purposes, food and feathers,
and more especially from no pleasure having been felt in the display of
distinct breeds; but the goose, under the conditions to which it is
exposed when domesticated, seems to have a singularly inflexible
organisation, though it has varied to a slight extent, as I have
elsewhere described.

Some authors have maintained that the amount of variation in our
domestic productions is soon reached, and can never afterward be
exceeded. It would be somewhat rash to assert that the limit has been
attained in any one case; for almost all our animals and plants have
been greatly improved in many ways within a recent period; and this
implies variation. It would be equally rash to assert that characters
now increased to their utmost limit, could not, after remaining fixed
for many centuries, again vary under new conditions of life. No doubt,
as Mr. Wallace has remarked with much truth, a limit will be at last
reached. For instance, there must be a limit to the fleetness of any
terrestrial animal, as this will be determined by the friction to be
overcome, the weight of the body to be carried, and the power of
contraction in the muscular fibres. But what concerns us is that the
domestic varieties of the same species differ from each other in almost
every character, which man has attended to and selected, more than do
the distinct species of the same genera. Isidore Geoffroy St. Hilaire
has proved this in regard to size, and so it is with colour, and
probably with the length of hair. With respect to fleetness, which
depends on many bodily characters, Eclipse was far fleeter, and a
dray-horse is comparably stronger, than any two natural species
belonging to the same genus. So with plants, the seeds of the different
varieties of the bean or maize probably differ more in size than do the
seeds of the distinct species in any one genus in the same two
families. The same remark holds good in regard to the fruit of the
several varieties of the plum, and still more strongly with the melon,
as well as in many other analogous cases.

To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and
indirectly by affecting the reproductive system. It is not probable
that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion
determine whether variations shall endure. Variability is governed by
many unknown laws, of which correlated growth is probably the most
important. Something, but how much we do not know, may be attributed to
the definite action of the conditions of life. Some, perhaps a great,
effect may be attributed to the increased use or disuse of parts. The
final result is thus rendered infinitely complex. In some cases the
intercrossing of aboriginally distinct species appears to have played
an important part in the origin of our breeds. When several breeds have
once been formed in any country, their occasional intercrossing, with
the aid of selection, has, no doubt, largely aided in the formation of
new sub-breeds; but the importance of crossing has been much
exaggerated, both in regard to animals and to those plants which are
propagated by seed. With plants which are temporarily propagated by
cuttings, buds, &c., the importance of crossing is immense; for the
cultivator may here disregard the extreme variability both of hybrids
and of mongrels, and the sterility of hybrids; but plants not
propagated by seed are of little importance to us, for their endurance
is only temporary. Over all these causes of change, the accumulative
action of selection, whether applied methodically and quickly, or
unconsciously and slowly, but more efficiently, seems to have been the
predominant power.




CHAPTER II.
VARIATION UNDER NATURE.


Variability—Individual differences—Doubtful species—Wide ranging, much
diffused, and common species, vary most—Species of the larger genera in
each country vary more frequently than the species of the smaller
genera—Many of the species of the larger genera resemble varieties in
being very closely, but unequally, related to each other, and in having
restricted ranges.


Before applying the principles arrived at in the last chapter to
organic beings in a state of nature, we must briefly discuss whether
these latter are subject to any variation. To treat this subject
properly, a long catalogue of dry facts ought to be given; but these I
shall reserve for a future work. Nor shall I here discuss the various
definitions which have been given of the term species. No one
definition has satisfied all naturalists; yet every naturalist knows
vaguely what he means when he speaks of a species. Generally the term
includes the unknown element of a distinct act of creation. The term
“variety” is almost equally difficult to define; but here community of
descent is almost universally implied, though it can rarely be proved.
We have also what are called monstrosities; but they graduate into
varieties. By a monstrosity I presume is meant some considerable
deviation of structure, generally injurious, or not useful to the
species. Some authors use the term “variation” in a technical sense, as
implying a modification directly due to the physical conditions of
life; and “variations” in this sense are supposed not to be inherited;
but who can say that the dwarfed condition of shells in the brackish
waters of the Baltic, or dwarfed plants on Alpine summits, or the
thicker fur of an animal from far northwards, would not in some cases
be inherited for at least a few generations? And in this case I presume
that the form would be called a variety.

It may be doubted whether sudden and considerable deviations of
structure, such as we occasionally see in our domestic productions,
more especially with plants, are ever permanently propagated in a state
of nature. Almost every part of every organic being is so beautifully
related to its complex conditions of life that it seems as improbable
that any part should have been suddenly produced perfect, as that a
complex machine should have been invented by man in a perfect state.
Under domestication monstrosities sometimes occur which resemble normal
structures in widely different animals. Thus pigs have occasionally
been born with a sort of proboscis, and if any wild species of the same
genus had naturally possessed a proboscis, it might have been argued
that this had appeared as a monstrosity; but I have as yet failed to
find, after diligent search, cases of monstrosities resembling normal
structures in nearly allied forms, and these alone bear on the
question. If monstrous forms of this kind ever do appear in a state of
nature and are capable of reproduction (which is not always the case),
as they occur rarely and singly, their preservation would depend on
unusually favourable circumstances. They would, also, during the first
and succeeding generations cross with the ordinary form, and thus their
abnormal character would almost inevitably be lost. But I shall have to
return in a future chapter to the preservation and perpetuation of
single or occasional variations.

_Individual Differences._


The many slight differences which appear in the offspring from the same
parents, or which it may be presumed have thus arisen, from being
observed in the individuals of the same species inhabiting the same
confined locality, may be called individual differences. No one
supposes that all the individuals of the same species are cast in the
same actual mould. These individual differences are of the highest
importance for us, for they are often inherited, as must be familiar to
every one; and they thus afford materials for natural selection to act
on and accumulate, in the same manner as man accumulates in any given
direction individual differences in his domesticated productions. These
individual differences generally affect what naturalists consider
unimportant parts; but I could show, by a long catalogue of facts, that
parts which must be called important, whether viewed under a
physiological or classificatory point of view, sometimes vary in the
individuals of the same species. I am convinced that the most
experienced naturalist would be surprised at the number of the cases of
variability, even in important parts of structure, which he could
collect on good authority, as I have collected, during a course of
years. It should be remembered that systematists are far from being
pleased at finding variability in important characters, and that there
are not many men who will laboriously examine internal and important
organs, and compare them in many specimens of the same species. It
would never have been expected that the branching of the main nerves
close to the great central ganglion of an insect would have been
variable in the same species; it might have been thought that changes
of this nature could have been effected only by slow degrees; yet Sir
J. Lubbock has shown a degree of variability in these main nerves in
Coccus, which may almost be compared to the irregular branching of the
stem of a tree. This philosophical naturalist, I may add, has also
shown that the muscles in the larvæ of certain insects are far from
uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically rank
those parts as important (as some few naturalists have honestly
confessed) which do not vary; and, under this point of view, no
instance will ever be found of an important part varying; but under any
other point of view many instances assuredly can be given.

There is one point connected with individual differences which is
extremely perplexing: I refer to those genera which have been called
“protean” or “polymorphic,” in which species present an inordinate
amount of variation. With respect to many of these forms, hardly two
naturalists agree whether to rank them as species or as varieties. We
may instance Rubus, Rosa, and Hieracium among plants, several genera of
insects, and of Brachiopod shells. In most polymorphic genera some of
the species have fixed and definite characters. Genera which are
polymorphic in one country seem to be, with a few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variability is independent of the
conditions of life. I am inclined to suspect that we see, at least in
some of these polymorphic genera, variations which are of no service or
disservice to the species, and which consequently have not been seized
on and rendered definite by natural selection, as hereafter to be
explained.

Individuals of the same species often present, as is known to every
one, great differences of structure, independently of variation, as in
the two sexes of various animals, in the two or three castes of sterile
females or workers among insects, and in the immature and larval states
of many of the lower animals. There are, also, cases of dimorphism and
trimorphism, both with animals and plants. Thus, Mr. Wallace, who has
lately called attention to the subject, has shown that the females of
certain species of butterflies, in the Malayan Archipelago, regularly
appear under two or even three conspicuously distinct forms, not
connected by intermediate varieties. Fritz Müller has described
analogous but more extraordinary cases with the males of certain
Brazilian Crustaceans: thus, the male of a Tanais regularly occurs
under two distinct forms; one of these has strong and differently
shaped pincers, and the other has antennæ much more abundantly
furnished with smelling-hairs. Although in most of these cases, the two
or three forms, both with animals and plants, are not now connected by
intermediate gradations, it is possible that they were once thus
connected. Mr. Wallace, for instance, describes a certain butterfly
which presents in the same island a great range of varieties connected
by intermediate links, and the extreme links of the chain closely
resemble the two forms of an allied dimorphic species inhabiting
another part of the Malay Archipelago. Thus also with ants, the several
worker-castes are generally quite distinct; but in some cases, as we
shall hereafter see, the castes are connected together by finely
graduated varieties. So it is, as I have myself observed, with some
dimorphic plants. It certainly at first appears a highly remarkable
fact that the same female butterfly should have the power of producing
at the same time three distinct female forms and a male; and that an
hermaphrodite plant should produce from the same seed-capsule three
distinct hermaphrodite forms, bearing three different kinds of females
and three or even six different kinds of males. Nevertheless these
cases are only exaggerations of the common fact that the female
produces offspring of two sexes which sometimes differ from each other
in a wonderful manner.

_Doubtful Species._


The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so
closely linked to them by intermediate gradations, that naturalists do
not like to rank them as distinct species, are in several respects the
most important for us. We have every reason to believe that many of
these doubtful and closely allied forms have permanently retained their
characters for a long time; for as long, as far as we know, as have
good and true species. Practically, when a naturalist can unite by
means of intermediate links any two forms, he treats the one as a
variety of the other, ranking the most common, but sometimes the one
first described as the species, and the other as the variety. But cases
of great difficulty, which I will not here enumerate, sometimes arise
in deciding whether or not to rank one form as a variety of another,
even when they are closely connected by intermediate links; nor will
the commonly assumed hybrid nature of the intermediate forms always
remove the difficulty. In very many cases, however, one form is ranked
as a variety of another, not because the intermediate links have
actually been found, but because analogy leads the observer to suppose
either that they do now somewhere exist, or may formerly have existed;
and here a wide door for the entry of doubt and conjecture is opened.

Hence, in determining whether a form should be ranked as a species or a
variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species
by at least some competent judges.

That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France, or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H.C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties, but
which have all been ranked by botanists as species; and in making this
list he has omitted many trifling varieties, but which nevertheless
have been ranked by some botanists as species, and he has entirely
omitted several highly polymorphic genera. Under genera, including the
most polymorphic forms, Mr. Babington gives 251 species, whereas Mr.
Bentham gives only 112—a difference of 139 doubtful forms! Among
animals which unite for each birth, and which are highly locomotive,
doubtful forms, ranked by one zoologist as a species and by another as
a variety, can rarely be found within the same country, but are common
in separated areas. How many of the birds and insects in North America
and Europe, which differ very slightly from each other, have been
ranked by one eminent naturalist as undoubted species, and by another
as varieties, or, as they are often called, geographical races! Mr.
Wallace, in several valuable papers on the various animals, especially
on the Lepidoptera, inhabiting the islands of the great Malayan
Archipelago, shows that they may be classed under four heads, namely,
as variable forms, as local forms, as geographical races or
sub-species, and as true representative species. The first or variable
forms vary much within the limits of the same island. The local forms
are moderately constant and distinct in each separate island; but when
all from the several islands are compared together, the differences are
seen to be so slight and graduated that it is impossible to define or
describe them, though at the same time the extreme forms are
sufficiently distinct. The geographical races or sub-species are local
forms completely fixed and isolated; but as they do not differ from
each other by strongly marked and important characters, “There is no
possible test but individual opinion to determine which of them shall
be considered as species and which as varieties.” Lastly,
representative species fill the same place in the natural economy of
each island as do the local forms and sub-species; but as they are
distinguished from each other by a greater amount of difference than
that between the local forms and sub-species, they are almost
universally ranked by naturalists as true species. Nevertheless, no
certain criterion can possibly be given by which variable forms, local
forms, sub species and representative species can be recognised.

Many years ago, when comparing, and seeing others compare, the birds
from the closely neighbouring islands of the Galapagos Archipelago, one
with another, and with those from the American mainland, I was much
struck how entirely vague and arbitrary is the distinction between
species and varieties. On the islets of the little Madeira group there
are many insects which are characterized as varieties in Mr.
Wollaston’s admirable work, but which would certainly be ranked as
distinct species by many entomologists. Even Ireland has a few animals,
now generally regarded as varieties, but which have been ranked as
species by some zoologists. Several experienced ornithologists consider
our British red grouse as only a strongly marked race of a Norwegian
species, whereas the greater number rank it as an undoubted species
peculiar to Great Britain. A wide distance between the homes of two
doubtful forms leads many naturalists to rank them as distinct species;
but what distance, it has been well asked, will suffice if that between
America and Europe is ample, will that between Europe and the Azores,
or Madeira, or the Canaries, or between the several islets of these
small archipelagos, be sufficient?

Mr. B.D. Walsh, a distinguished entomologist of the United States, has
described what he calls Phytophagic varieties and Phytophagic species.
Most vegetable-feeding insects live on one kind of plant or on one
group of plants; some feed indiscriminately on many kinds, but do not
in consequence vary. In several cases, however, insects found living on
different plants, have been observed by Mr. Walsh to present in their
larval or mature state, or in both states, slight, though constant
differences in colour, size, or in the nature of their secretions. In
some instances the males alone, in other instances, both males and
females, have been observed thus to differ in a slight degree. When the
differences are rather more strongly marked, and when both sexes and
all ages are affected, the forms are ranked by all entomologists as
good species. But no observer can determine for another, even if he can
do so for himself, which of these Phytophagic forms ought to be called
species and which varieties. Mr. Walsh ranks the forms which it may be
supposed would freely intercross, as varieties; and those which appear
to have lost this power, as species. As the differences depend on the
insects having long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found.
The naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily
occurs with closely allied organisms, which inhabit distinct continents
or islands. When, on the other hand, an animal or plant ranges over the
same continent, or inhabits many islands in the same archipelago, and
presents different forms in the different areas, there is always a good
chance that intermediate forms will be discovered which will link
together the extreme states; and these are then degraded to the rank of
varieties.

Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of
specific value; and when the same identical form is met with in two
distant countries, or in two geological formations, they believe that
two distinct species are hidden under the same dress. The term species
thus comes to be a mere useless abstraction, implying and assuming a
separate act of creation. It is certain that many forms, considered by
highly competent judges to be varieties, resemble species so completely
in character that they have been thus ranked by other highly competent
judges. But to discuss whether they ought to be called species or
varieties, before any definition of these terms has been generally
accepted, is vainly to beat the air.

Many of the cases of strongly marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, &c., have
been brought to bear in the attempt to determine their rank; but space
does not here permit me to discuss them. Close investigation, in many
cases, will no doubt bring naturalists to agree how to rank doubtful
forms. Yet it must be confessed that it is in the best known countries
that we find the greatest number of them. I have been struck with the
fact that if any animal or plant in a state of nature be highly useful
to man, or from any cause closely attracts his attention, varieties of
it will almost universally be found recorded. These varieties,
moreover, will often be ranked by some authors as species. Look at the
common oak, how closely it has been studied; yet a German author makes
more than a dozen species out of forms, which are almost universally
considered by other botanists to be varieties; and in this country the
highest botanical authorities and practical men can be quoted to show
that the sessile and pedunculated oaks are either good and distinct
species or mere varieties.

I may here allude to a remarkable memoir lately published by A. de
Candolle, on the oaks of the whole world. No one ever had more ample
materials for the discrimination of the species, or could have worked
on them with more zeal and sagacity. He first gives in detail all the
many points of structure which vary in the several species, and
estimates numerically the relative frequency of the variations. He
specifies above a dozen characters which may be found varying even on
the same branch, sometimes according to age or development, sometimes
without any assignable reason. Such characters are not of course of
specific value, but they are, as Asa Gray has remarked in commenting on
this memoir, such as generally enter into specific definitions. De
Candolle then goes on to say that he gives the rank of species to the
forms that differ by characters never varying on the same tree, and
never found connected by intermediate states. After this discussion,
the result of so much labour, he emphatically remarks: “They are
mistaken, who repeat that the greater part of our species are clearly
limited, and that the doubtful species are in a feeble minority. This
seemed to be true, so long as a genus was imperfectly known, and its
species were founded upon a few specimens, that is to say, were
provisional. Just as we come to know them better, intermediate forms
flow in, and doubts as to specific limits augment.” He also adds that
it is the best known species which present the greatest number of
spontaneous varieties and sub-varieties. Thus Quercus robur has
twenty-eight varieties, all of which, excepting six, are clustered
round three sub-species, namely Q. pedunculata, sessiliflora and
pubescens. The forms which connect these three sub-species are
comparatively rare; and, as Asa Gray again remarks, if these connecting
forms which are now rare were to become totally extinct the three
sub-species would hold exactly the same relation to each other as do
the four or five provisionally admitted species which closely surround
the typical Quercus robur. Finally, De Candolle admits that out of the
300 species, which will be enumerated in his Prodromus as belonging to
the oak family, at least two-thirds are provisional species, that is,
are not known strictly to fulfil the definition above given of a true
species. It should be added that De Candolle no longer believes that
species are immutable creations, but concludes that the derivative
theory is the most natural one, “and the most accordant with the known
facts in palæontology, geographical botany and zoology, of anatomical
structure and classification.”

When a young naturalist commences the study of a group of organisms
quite unknown to him he is at first much perplexed in determining what
differences to consider as specific and what as varietal; for he knows
nothing of the amount and kind of variation to which the group is
subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country he will soon make up his mind how to rank most of the doubtful
forms. His general tendency will be to make many species, for he will
become impressed, just like the pigeon or poultry fancier before
alluded to, with the amount of difference in the forms which he is
continually studying; and he has little general knowledge of analogical
variation in other groups and in other countries by which to correct
his first impressions. As he extends the range of his observations he
will meet with more cases of difficulty; for he will encounter a
greater number of closely-allied forms. But if his observations be
widely extended he will in the end generally be able to make up his own
mind; but he will succeed in this at the expense of admitting much
variation, and the truth of this admission will often be disputed by
other naturalists. When he comes to study allied forms brought from
countries not now continuous, in which case he cannot hope to find
intermediate links, he will be compelled to trust almost entirely to
analogy, and his difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn between
species and sub-species—that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at, the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other by an insensible series; and a series impresses
the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to the
systematist, as of the highest importance for us, as being the first
step towards such slight varieties as are barely thought worth
recording in works on natural history. And I look at varieties which
are in any degree more distinct and permanent, as steps towards more
strongly marked and permanent varieties; and at the latter, as leading
to sub-species, and then to species. The passage from one stage of
difference to another may, in many cases, be the simple result of the
nature of the organism and of the different physical conditions to
which it has long been exposed; but with respect to the more important
and adaptive characters, the passage from one stage of difference to
another may be safely attributed to the cumulative action of natural
selection, hereafter to be explained, and to the effects of the
increased use or disuse of parts. A well-marked variety may therefore
be called an incipient species; but whether this belief is justifiable
must be judged by the weight of the various facts and considerations to
be given throughout this work.

It need not be supposed that all varieties or incipient species attain
the rank of species. They may become extinct, or they may endure as
varieties for very long periods, as has been shown to be the case by
Mr. Wollaston with the varieties of certain fossil land-shells in
Madeira, and with plants by Gaston de Saporta. If a variety were to
flourish so as to exceed in numbers the parent species, it would then
rank as the species, and the species as the variety; or it might come
to supplant and exterminate the parent species; or both might co-exist,
and both rank as independent species. But we shall hereafter return to
this subject.

From these remarks it will be seen that I look at the term species as
one arbitrarily given, for the sake of convenience, to a set of
individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied
arbitrarily, for convenience sake.

_Wide-ranging, much-diffused, and common Species vary most._


Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H.C.
Watson, to whom I am much indebted for valuable advice and assistance
on this subject, soon convinced me that there were many difficulties,
as did subsequently Dr. Hooker, even in stronger terms. I shall reserve
for a future work the discussion of these difficulties, and the tables
of the proportional numbers of the varying species. Dr. Hooker permits
me to add that after having carefully read my manuscript, and examined
the tables, he thinks that the following statements are fairly well
established. The whole subject, however, treated as it necessarily here
is with much brevity, is rather perplexing, and allusions cannot be
avoided to the “struggle for existence,” “divergence of character,” and
other questions, hereafter to be discussed.

Alphonse de Candolle and others have shown that plants which have very
wide ranges generally present varieties; and this might have been
expected, as they are exposed to diverse physical conditions, and as
they come into competition (which, as we shall hereafter see, is a far
more important circumstance) with different sets of organic beings. But
my tables further show that, in any limited country, the species which
are the most common, that is abound most in individuals, and the
species which are most widely diffused within their own country (and
this is a different consideration from wide range, and to a certain
extent from commonness), oftenest give rise to varieties sufficiently
well-marked to have been recorded in botanical works. Hence it is the
most flourishing, or, as they may be called, the dominant species—those
which range widely, are the most diffused in their own country, and are
the most numerous in individuals—which oftenest produce well-marked
varieties, or, as I consider them, incipient species. And this,
perhaps, might have been anticipated; for, as varieties, in order to
become in any degree permanent, necessarily have to struggle with the
other inhabitants of the country, the species which are already
dominant will be the most likely to yield offspring, which, though in
some slight degree modified, still inherit those advantages that
enabled their parents to become dominant over their compatriots. In
these remarks on predominence, it should be understood that reference
is made only to the forms which come into competition with each other,
and more especially to the members of the same genus or class having
nearly similar habits of life. With respect to the number of
individuals or commonness of species, the comparison of course relates
only to the members of the same group. One of the higher plants may be
said to be dominant if it be more numerous in individuals and more
widely diffused than the other plants of the same country, which live
under nearly the same conditions. A plant of this kind is not the less
dominant because some conferva inhabiting the water or some parasitic
fungus is infinitely more numerous in individuals, and more widely
diffused. But if the conferva or parasitic fungus exceeds its allies in
the above respects, it will then be dominant within its own class.

_Species of the Larger Genera in each Country vary more Frequently than
the Species of the Smaller Genera._


If the plants inhabiting a country as described in any Flora, be
divided into two equal masses, all those in the larger genera (_i.e._,
those including many species) being placed on one side, and all those
in the smaller genera on the other side, the former will be found to
include a somewhat larger number of the very common and much diffused
or dominant species. This might have been anticipated, for the mere
fact of many species of the same genus inhabiting any country, shows
that there is something in the organic or inorganic conditions of that
country favourable to the genus; and, consequently, we might have
expected to have found in the larger genera, or those including many
species, a larger proportional number of dominant species. But so many
causes tend to obscure this result, that I am surprised that my tables
show even a small majority on the side of the larger genera. I will
here allude to only two causes of obscurity. Fresh water and
salt-loving plants generally have very wide ranges and are much
diffused, but this seems to be connected with the nature of the
stations inhabited by them, and has little or no relation to the size
of the genera to which the species belong. Again, plants low in the
scale of organisation are generally much more widely diffused than
plants higher in the scale; and here again there is no close relation
to the size of the genera. The cause of lowly-organised plants ranging
widely will be discussed in our chapter on Geographical Distribution.

From looking at species as only strongly marked and well-defined
varieties, I was led to anticipate that the species of the larger
genera in each country would oftener present varieties, than the
species of the smaller genera; for wherever many closely related
species (_i.e._, species of the same genus) have been formed, many
varieties or incipient species ought, as a general rule, to be now
forming. Where many large trees grow, we expect to find saplings. Where
many species of a genus have been formed through variation,
circumstances have been favourable for variation; and hence we might
expect that the circumstances would generally still be favourable to
variation. On the other hand, if we look at each species as a special
act of creation, there is no apparent reason why more varieties should
occur in a group having many species, than in one having few.

To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has
invariably proved to be the case that a larger proportion of the
species on the side of the larger genera presented varieties, than on
the side of the smaller genera. Moreover, the species of the large
genera which present any varieties, invariably present a larger average
number of varieties than do the species of the small genera. Both these
results follow when another division is made, and when all the least
genera, with from only one to four species, are altogether excluded
from the tables. These facts are of plain signification on the view
that species are only strongly marked and permanent varieties; for
wherever many species of the same genus have been formed, or where, if
we may use the expression, the manufactory of species has been active,
we ought generally to find the manufactory still in action, more
especially as we have every reason to believe the process of
manufacturing new species to be a slow one. And this certainly holds
true if varieties be looked at as incipient species; for my tables
clearly show, as a general rule, that, wherever many species of a genus
have been formed, the species of that genus present a number of
varieties, that is, of incipient species, beyond the average. It is not
that all large genera are now varying much, and are thus increasing in
the number of their species, or that no small genera are now varying
and increasing; for if this had been so, it would have been fatal to my
theory; inasmuch as geology plainly tells us that small genera have in
the lapse of time often increased greatly in size; and that large
genera have often come to their maxima, declined, and disappeared. All
that we want to show is, that where many species of a genus have been
formed, on an average many are still forming; and this certainly holds
good.

_Many of the Species included within the Larger Genera resemble
Varieties in being very closely, but unequally, related to each other,
and in having restricted ranges._


There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and when intermediate links have not been found between
doubtful forms, naturalists are compelled to come to a determination by
the amount of difference between them, judging by analogy whether or
not the amount suffices to raise one or both to the rank of species.
Hence the amount of difference is one very important criterion in
settling whether two forms should be ranked as species or varieties.
Now Fries has remarked in regard to plants, and Westwood in regard to
insects, that in large genera the amount of difference between the
species is often exceedingly small. I have endeavoured to test this
numerically by averages, and, as far as my imperfect results go, they
confirm the view. I have also consulted some sagacious and experienced
observers, and, after deliberation, they concur in this view. In this
respect, therefore, the species of the larger genera resemble
varieties, more than do the species of the smaller genera. Or the case
may be put in another way, and it may be said, that in the larger
genera, in which a number of varieties or incipient species greater
than the average are now manufacturing, many of the species already
manufactured still to a certain extent resemble varieties, for they
differ from each other by a less than the usual amount of difference.

Moreover, the species of the larger genera are related to each other,
in the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around
other species. And what are varieties but groups of forms, unequally
related to each other, and clustered round certain forms—that is, round
their parent-species. Undoubtedly there is one most important point of
difference between varieties and species, namely, that the amount of
difference between varieties, when compared with each other or with
their parent-species, is much less than that between the species of the
same genus. But when we come to discuss the principle, as I call it, of
divergence of character, we shall see how this may be explained, and
how the lesser differences between varieties tend to increase into the
greater differences between species.

There is one other point which is worth notice. Varieties generally
have much restricted ranges. This statement is indeed scarcely more
than a truism, for if a variety were found to have a wider range than
that of its supposed parent-species, their denominations would be
reversed. But there is reason to believe that the species which are
very closely allied to other species, and in so far resemble varieties,
often have much restricted ranges. For instance, Mr. H.C. Watson has
marked for me in the well-sifted London catalogue of Plants (4th
edition) sixty-three plants which are therein ranked as species, but
which he considers as so closely allied to other species as to be of
doubtful value: these sixty-three reputed species range on an average
over 6.9 of the provinces into which Mr. Watson has divided Great
Britain. Now, in this same catalogue, fifty-three acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas,
the species to which these varieties belong range over 14.3 provinces.
So that the acknowledged varieties have very nearly the same restricted
average range, as have the closely allied forms, marked for me by Mr.
Watson as doubtful species, but which are almost universally ranked by
British botanists as good and true species.

_Summary._


Finally, varieties cannot be distinguished from species—except, first,
by the discovery of intermediate linking forms; and, secondly, by a
certain indefinite amount of difference between them; for two forms, if
differing very little, are generally ranked as varieties,
notwithstanding that they cannot be closely connected; but the amount
of difference considered necessary to give to any two forms the rank of
species cannot be defined. In genera having more than the average
number of species in any country, the species of these genera have more
than the average number of varieties. In large genera the species are
apt to be closely but unequally allied together, forming little
clusters round other species. Species very closely allied to other
species apparently have restricted ranges. In all these respects the
species of large genera present a strong analogy with varieties. And we
can clearly understand these analogies, if species once existed as
varieties, and thus originated; whereas, these analogies are utterly
inexplicable if species are independent creations.

We have also seen that it is the most flourishing or dominant species
of the larger genera within each class which on an average yield the
greatest number of varieties, and varieties, as we shall hereafter see,
tend to become converted into new and distinct species. Thus the larger
genera tend to become larger; and throughout nature the forms of life
which are now dominant tend to become still more dominant by leaving
many modified and dominant descendants. But, by steps hereafter to be
explained, the larger genera also tend to break up into smaller genera.
And thus, the forms of life throughout the universe become divided into
groups subordinate to groups.




CHAPTER III.
STRUGGLE FOR EXISTENCE.


Its bearing on natural selection—The term used in a wide
sense—Geometrical ratio of increase—Rapid increase of naturalised
animals and plants—Nature of the checks to increase—Competition
universal—Effects of climate—Protection from the number of
individuals—Complex relations of all animals and plants throughout
nature—Struggle for life most severe between individuals and varieties
of the same species: often severe between species of the same genus—The
relation of organism to organism the most important of all relations.


Before entering on the subject of this chapter I must make a few
preliminary remarks to show how the struggle for existence bears on
natural selection. It has been seen in the last chapter that among
organic beings in a state of nature there is some individual
variability: indeed I am not aware that this has ever been disputed. It
is immaterial for us whether a multitude of doubtful forms be called
species or sub-species or varieties; what rank, for instance, the two
or three hundred doubtful forms of British plants are entitled to hold,
if the existence of any well-marked varieties be admitted. But the mere
existence of individual variability and of some few well-marked
varieties, though necessary as the foundation for the work, helps us
but little in understanding how species arise in nature. How have all
those exquisite adaptations of one part of the organisation to another
part, and to the conditions of life and of one organic being to another
being, been perfected? We see these beautiful co-adaptations most
plainly in the woodpecker and the mistletoe; and only a little less
plainly in the humblest parasite which clings to the hairs of a
quadruped or feathers of a bird; in the structure of the beetle which
dives through the water; in the plumed seed which is wafted by the
gentlest breeze; in short, we see beautiful adaptations everywhere and
in every part of the organic world.

Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow from the struggle for life. Owing to this struggle,
variations, however slight and from whatever cause proceeding, if they
be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their
physical conditions of life, will tend to the preservation of such
individuals, and will generally be inherited by the offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term natural
selection, in order to mark its relation to man’s power of selection.
But the expression often used by Mr. Herbert Spencer, of the Survival
of the Fittest, is more accurate, and is sometimes equally convenient.
We have seen that man by selection can certainly produce great results,
and can adapt organic beings to his own uses, through the accumulation
of slight but useful variations, given to him by the hand of Nature.
But Natural Selection, we shall hereafter see, is a power incessantly
ready for action, and is as immeasurably superior to man’s feeble
efforts, as the works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for existence.
In my future work this subject will be treated, as it well deserves, at
greater length. The elder De Candolle and Lyell have largely and
philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently
the result of his great horticultural knowledge. Nothing is easier than
to admit in words the truth of the universal struggle for life, or more
difficult—at least I found it so—than constantly to bear this
conclusion in mind. Yet unless it be thoroughly engrained in the mind,
the whole economy of nature, with every fact on distribution, rarity,
abundance, extinction, and variation, will be dimly seen or quite
misunderstood. We behold the face of nature bright with gladness, we
often see superabundance of food; we do not see or we forget that the
birds which are idly singing round us mostly live on insects or seeds,
and are thus constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind, that, though food
may be now superabundant, it is not so at all seasons of each recurring
year.

_The Term, Struggle for Existence, used in a large sense._


I should premise that I use this term in a large and metaphorical
sense, including dependence of one being on another, and including
(which is more important) not only the life of the individual, but
success in leaving progeny. Two canine animals, in a time of dearth,
may be truly said to struggle with each other which shall get food and
live. But a plant on the edge of a desert is said to struggle for life
against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a thousand
seeds, of which only one of an average comes to maturity, may be more
truly said to struggle with the plants of the same and other kinds
which already clothe the ground. The mistletoe is dependent on the
apple and a few other trees, but can only in a far-fetched sense be
said to struggle with these trees, for, if too many of these parasites
grow on the same tree, it languishes and dies. But several seedling
mistletoes, growing close together on the same branch, may more truly
be said to struggle with each other. As the mistletoe is disseminated
by birds, its existence depends on them; and it may metaphorically be
said to struggle with other fruit-bearing plants, in tempting the birds
to devour and thus disseminate its seeds. In these several senses,
which pass into each other, I use for convenience sake the general term
of Struggle for Existence.

_Geometrical Ratio of Increase._


A struggle for existence inevitably follows from the high rate at which
all organic beings tend to increase. Every being, which during its
natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country
could support the product. Hence, as more individuals are produced than
can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species, or
with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with manifold
force to the whole animal and vegetable kingdoms; for in this case
there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increasing,
more or less rapidly, in numbers, all cannot do so, for the world would
not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding man
has doubled in twenty-five years, and at this rate, in less than a
thousand years, there would literally not be standing room for his
progeny. Linnæus has calculated that if an annual plant produced only
two seeds—and there is no plant so unproductive as this—and their
seedlings next year produced two, and so on, then in twenty years there
would be a million plants. The elephant is reckoned the slowest breeder
of all known animals, and I have taken some pains to estimate its
probable minimum rate of natural increase; it will be safest to assume
that it begins breeding when thirty years old, and goes on breeding
till ninety years old, bringing forth six young in the interval, and
surviving till one hundred years old; if this be so, after a period of
from 740 to 750 years there would be nearly nineteen million elephants
alive descended from the first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of
the world; if the statements of the rate of increase of slow-breeding
cattle and horses in South America, and latterly in Australia, had not
been well authenticated, they would have been incredible. So it is with
plants; cases could be given of introduced plants which have become
common throughout whole islands in a period of less than ten years.
Several of the plants, such as the cardoon and a tall thistle, which
are now the commonest over the wide plains of La Plata, clothing square
leagues of surface almost to the exclusion of every other plant, have
been introduced from Europe; and there are plants which now range in
India, as I hear from Dr. Falconer, from Cape Comorin to the Himalaya,
which have been imported from America since its discovery. In such
cases, and endless others could be given, no one supposes that the
fertility of the animals or plants has been suddenly and temporarily
increased in any sensible degree. The obvious explanation is that the
conditions of life have been highly favourable, and that there has
consequently been less destruction of the old and young and that nearly
all the young have been enabled to breed. Their geometrical ratio of
increase, the result of which never fails to be surprising, simply
explains their extraordinarily rapid increase and wide diffusion in
their new homes.

In a state of nature almost every full-grown plant annually produces
seed, and among animals there are very few which do not annually pair.
Hence we may confidently assert that all plants and animals are tending
to increase at a geometrical ratio—that all would rapidly stock every
station in which they could any how exist, and that this geometrical
tendency to increase must be checked by destruction at some period of
life. Our familiarity with the larger domestic animals tends, I think,
to mislead us; we see no great destruction falling on them, and we do
not keep in mind that thousands are annually slaughtered for food, and
that in a state of nature an equal number would have somehow to be
disposed of.

The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is, that
the slow breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two. The Fulmar
petrel lays but one egg, yet it is believed to be the most numerous
bird in the world. One fly deposits hundreds of eggs, and another, like
the hippobosca, a single one. But this difference does not determine
how many individuals of the two species can be supported in a district.
A large number of eggs is of some importance to those species which
depend on a fluctuating amount of food, for it allows them rapidly to
increase in number. But the real importance of a large number of eggs
or seeds is to make up for much destruction at some period of life; and
this period in the great majority of cases is an early one. If an
animal can in any way protect its own eggs or young, a small number may
be produced, and yet the average stock be fully kept up; but if many
eggs or young are destroyed, many must be produced or the species will
become extinct. It would suffice to keep up the full number of a tree,
which lived on an average for a thousand years, if a single seed were
produced once in a thousand years, supposing that this seed were never
destroyed and could be ensured to germinate in a fitting place; so
that, in all cases, the average number of any animal or plant depends
only indirectly on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind—never to forget that every single organic
being may be said to be striving to the utmost to increase in numbers;
that each lives by a struggle at some period of its life; that heavy
destruction inevitably falls either on the young or old during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount.

_Nature of the Checks to Increase._


The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will it tend to increase still further.
We know not exactly what the checks are even in a single instance. Nor
will this surprise any one who reflects how ignorant we are on this
head, even in regard to mankind, although so incomparably better known
than any other animal. This subject of the checks to increase has been
ably treated by several authors, and I hope in a future work to discuss
it at considerable length, more especially in regard to the feral
animals of South America. Here I will make only a few remarks, just to
recall to the reader’s mind some of the chief points. Eggs or very
young animals seem generally to suffer most, but this is not invariably
the case. With plants there is a vast destruction of seeds, but from
some observations which I have made it appears that the seedlings
suffer most from germinating in ground already thickly stocked with
other plants. Seedlings, also, are destroyed in vast numbers by various
enemies; for instance, on a piece of ground three feet long and two
wide, dug and cleared, and where there could be no choking from other
plants, I marked all the seedlings of our native weeds as they came up,
and out of 357 no less than 295 were destroyed, chiefly by slugs and
insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants gradually kill the less vigorous, though fully grown
plants; thus out of twenty species grown on a little plot of mown turf
(three feet by four) nine species perished, from the other species
being allowed to grow up freely.

The amount of food for each species, of course, gives the extreme limit
to which each can increase; but very frequently it is not the obtaining
food, but the serving as prey to other animals, which determines the
average number of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game
than at present, although hundreds of thousands of game animals are now
annually shot. On the other hand, in some cases, as with the elephant,
none are destroyed by beasts of prey; for even the tiger in India most
rarely dares to attack a young elephant protected by its dam.

Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought seem to be
the most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5
destroyed four-fifths of the birds in my own grounds; and this is a
tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle
for existence; but in so far as climate chiefly acts in reducing food,
it brings on the most severe struggle between the individuals, whether
of the same or of distinct species, which subsist on the same kind of
food. Even when climate, for instance, extreme cold, acts directly, it
will be the least vigorous individuals, or those which have got least
food through the advancing winter, which will suffer the most. When we
travel from south to north, or from a damp region to a dry, we
invariably see some species gradually getting rarer and rarer, and
finally disappearing; and the change of climate being conspicuous, we
are tempted to attribute the whole effect to its direct action. But
this is a false view; we forget that each species, even where it most
abounds, is constantly suffering enormous destruction at some period of
its life, from enemies or from competitors for the same place and food;
and if these enemies or competitors be in the least degree favoured by
any slight change of climate, they will increase in numbers; and as
each area is already fully stocked with inhabitants, the other species
must decrease. When we travel southward and see a species decreasing in
numbers, we may feel sure that the cause lies quite as much in other
species being favoured, as in this one being hurt. So it is when we
travel northward, but in a somewhat lesser degree, for the number of
species of all kinds, and therefore of competitors, decreases
northward; hence in going northward, or in ascending a mountain, we far
oftener meet with stunted forms, due to the _directly_ injurious action
of climate, than we do in proceeding southward or in descending a
mountain. When we reach the Arctic regions, or snow-capped summits, or
absolute deserts, the struggle for life is almost exclusively with the
elements.

That climate acts in main part indirectly by favouring other species we
clearly see in the prodigious number of plants which in our gardens can
perfectly well endure our climate, but which never become naturalised,
for they cannot compete with our native plants nor resist destruction
by our native animals.

When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics—at least, this
seems generally to occur with our game animals—often ensue; and here we
have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of
diffusion among the crowded animals, been disproportionally favoured:
and here comes in a sort of struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of corn
and rape-seed, &c., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during the winter; but any one who has tried knows
how troublesome it is to get seed from a few wheat or other such plants
in a garden; I have in this case lost every single seed. This view of
the necessity of a large stock of the same species for its
preservation, explains, I believe, some singular facts in nature such
as that of very rare plants being sometimes extremely abundant, in the
few spots where they do exist; and that of some social plants being
social, that is abounding in individuals, even on the extreme verge of
their range. For in such cases, we may believe, that a plant could
exist only where the conditions of its life were so favourable that
many could exist together, and thus save the species from utter
destruction. I should add that the good effects of intercrossing, and
the ill effects of close interbreeding, no doubt come into play in many
of these cases; but I will not here enlarge on this subject.

_Complex Relations of all Animals and Plants to each other in the
Struggle for Existence._


Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, interested me. In Staffordshire, on the
estate of a relation, where I had ample means of investigation, there
was a large and extremely barren heath, which had never been touched by
the hand of man; but several hundred acres of exactly the same nature
had been enclosed twenty-five years previously and planted with Scotch
fir. The change in the native vegetation of the planted part of the
heath was most remarkable, more than is generally seen in passing from
one quite different soil to another: not only the proportional numbers
of the heath-plants were wholly changed, but twelve species of plants
(not counting grasses and carices) flourished in the plantations, which
could not be found on the heath. The effect on the insects must have
been still greater, for six insectivorous birds were very common in the
plantations, which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds. Here we see
how potent has been the effect of the introduction of a single tree,
nothing whatever else having been done, with the exception of the land
having been enclosed, so that cattle could not enter. But how important
an element enclosure is, I plainly saw near Farnham, in Surrey. Here
there are extensive heaths, with a few clumps of old Scotch firs on the
distant hill-tops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so
close together that all cannot live. When I ascertained that these
young trees had not been sown or planted I was so much surprised at
their numbers that I went to several points of view, whence I could
examine hundreds of acres of the unenclosed heath, and literally I
could not see a single Scotch fir, except the old planted clumps. But
on looking closely between the stems of the heath, I found a multitude
of seedlings and little trees, which had been perpetually browsed down
by the cattle. In one square yard, at a point some hundred yards
distant from one of the old clumps, I counted thirty-two little trees;
and one of them, with twenty-six rings of growth, had, during many
years tried to raise its head above the stems of the heath, and had
failed. No wonder that, as soon as the land was enclosed, it became
thickly clothed with vigorously growing young firs. Yet the heath was
so extremely barren and so extensive that no one would ever have
imagined that cattle would have so closely and effectually searched it
for food.

Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous as
they are, must be habitually checked by some means, probably by other
parasitic insects. Hence, if certain insectivorous birds were to
decrease in Paraguay, the parasitic insects would probably increase;
and this would lessen the number of the navel-frequenting flies—then
cattle and horses would become feral, and this would certainly greatly
alter (as indeed I have observed in parts of South America) the
vegetation: this again would largely affect the insects; and this, as
we have just seen in Staffordshire, the insectivorous birds, and so
onwards in ever-increasing circles of complexity. Not that under nature
the relations will ever be as simple as this. Battle within battle must
be continually recurring with varying success; and yet in the long-run
the forces are so nicely balanced that the face of nature remains for
long periods of time uniform, though assuredly the merest trifle would
give the victory to one organic being over another. Nevertheless, so
profound is our ignorance, and so high our presumption, that we marvel
when we hear of the extinction of an organic being; and as we do not
see the cause, we invoke cataclysms to desolate the world, or invent
laws on the duration of the forms of life!

 I am tempted to give one more instance showing how plants and animals,
 remote in the scale of nature, are bound together by a web of complex
 relations. I shall hereafter have occasion to show that the exotic
 Lobelia fulgens is never visited in my garden by insects, and
 consequently, from its peculiar structure, never sets a seed. Nearly
 all our orchidaceous plants absolutely require the visits of insects
 to remove their pollen-masses and thus to fertilise them. I find from
 experiments that humble-bees are almost indispensable to the
 fertilisation of the heartsease (Viola tricolor), for other bees do
 not visit this flower. I have also found that the visits of bees are
 necessary for the fertilisation of some kinds of clover; for instance
 twenty heads of Dutch clover (Trifolium repens) yielded 2,290 seeds,
 but twenty other heads, protected from bees, produced not one. Again,
 100 heads of red clover (T. pratense) produced 2,700 seeds, but the
 same number of protected heads produced not a single seed. Humble bees
 alone visit red clover, as other bees cannot reach the nectar. It has
 been suggested that moths may fertilise the clovers; but I doubt
 whether they could do so in the case of the red clover, from their
 weight not being sufficient to depress the wing petals. Hence we may
 infer as highly probable that, if the whole genus of humble-bees
 became extinct or very rare in England, the heartsease and red clover
 would become very rare, or wholly disappear. The number of humble-bees
 in any district depends in a great measure upon the number of
 field-mice, which destroy their combs and nests; and Colonel Newman,
 who has long attended to the habits of humble-bees, believes that
 “more than two-thirds of them are thus destroyed all over England.”
 Now the number of mice is largely dependent, as every one knows, on
 the number of cats; and Colonel Newman says, “Near villages and small
 towns I have found the nests of humble-bees more numerous than
 elsewhere, which I attribute to the number of cats that destroy the
 mice.” Hence it is quite credible that the presence of a feline animal
 in large numbers in a district might determine, through the
 intervention first of mice and then of bees, the frequency of certain
 flowers in that district!

In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally the
most potent, but all will concur in determining the average number, or
even the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what
we call chance. But how false a view is this! Every one has heard that
when an American forest is cut down, a very different vegetation
springs up; but it has been observed that ancient Indian ruins in the
Southern United States, which must formerly have been cleared of trees,
now display the same beautiful diversity and proportion of kinds as in
the surrounding virgin forests. What a struggle must have gone on
during long centuries between the several kinds of trees, each annually
scattering its seeds by the thousand; what war between insect and
insect—between insects, snails, and other animals with birds and beasts
of prey—all striving to increase, all feeding on each other, or on the
trees, their seeds and seedlings, or on the other plants which first
clothed the ground and thus checked the growth of the trees. Throw up a
handful of feathers, and all fall to the ground according to definite
laws; but how simple is the problem where each shall fall compared to
that of the action and reaction of the innumerable plants and animals
which have determined, in the course of centuries, the proportional
numbers and kinds of trees now growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This
is likewise sometimes the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle will almost invariably be
most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed to
the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat
be sown together, and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally the most fertile,
will beat the others and so yield more seed, and will consequently in a
few years supplant the other varieties. To keep up a mixed stock of
even such extremely close varieties as the variously coloured
sweet-peas, they must be each year harvested separately, and the seed
then mixed in due proportion, otherwise the weaker kinds will steadily
decrease in number and disappear. So again with the varieties of sheep:
it has been asserted that certain mountain-varieties will starve out
other mountain-varieties, so that they cannot be kept together. The
same result has followed from keeping together different varieties of
the medicinal leech. It may even be doubted whether the varieties of
any of our domestic plants or animals have so exactly the same
strength, habits, and constitution, that the original proportions of a
mixed stock (crossing being prevented) could be kept up for
half-a-dozen generations, if they were allowed to struggle together, in
the same manner as beings in a state of nature, and if the seed or
young were not annually preserved in due proportion.

_Struggle for Life most severe between Individuals and Varieties of the
same Species._


As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if
they come into competition with each other, than between the species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under
the most different climates! In Russia the small Asiatic cockroach has
everywhere driven before it its great congener. In Australia the
imported hive-bee is rapidly exterminating the small, stingless native
bee. One species of charlock has been known to supplant another
species; and so in other cases. We can dimly see why the competition
should be most severe between allied forms, which fill nearly the same
place in the economy of nature; but probably in no one case could we
precisely say why one species has been victorious over another in the
great battle of life.

A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all other
organic beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which it preys. This
is obvious in the structure of the teeth and talons of the tiger; and
in that of the legs and claws of the parasite which clings to the hair
on the tiger’s body. But in the beautifully plumed seed of the
dandelion, and in the flattened and fringed legs of the water-beetle,
the relation seems at first confined to the elements of air and water.
Yet the advantage of the plumed seeds no doubt stands in the closest
relation to the land being already thickly clothed with other plants;
so that the seeds may be widely distributed and fall on unoccupied
ground. In the water-beetle, the structure of its legs, so well adapted
for diving, allows it to compete with other aquatic insects, to hunt
for its own prey, and to escape serving as prey to other animals.

The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds, as peas and
beans, when sown in the midst of long grass, it may be suspected that
the chief use of the nutriment in the seed is to favour the growth of
the seedlings, whilst struggling with other plants growing vigorously
all around.

Look at a plant in the midst of its range! Why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case
we can clearly see that if we wish in imagination to give the plant the
power of increasing in numbers, we should have to give it some
advantage over its competitors, or over the animals which prey on it.
On the confines of its geographical range, a change of constitution
with respect to climate would clearly be an advantage to our plant; but
we have reason to believe that only a few plants or animals range so
far, that they are destroyed exclusively by the rigour of the climate.
Not until we reach the extreme confines of life, in the Arctic regions
or on the borders of an utter desert, will competition cease. The land
may be extremely cold or dry, yet there will be competition between
some few species, or between the individuals of the same species, for
the warmest or dampest spots.

Hence we can see that when a plant or animal is placed in a new
country, among new competitors, the conditions of its life will
generally be changed in an essential manner, although the climate may
be exactly the same as in its former home. If its average numbers are
to increase in its new home, we should have to modify it in a different
way to what we should have had to do in its native country; for we
should have to give it some advantage over a different set of
competitors or enemies.

It is good thus to try in imagination to give any one species an
advantage over another. Probably in no single instance should we know
what to do. This ought to convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it is
difficult to acquire. All that we can do is to keep steadily in mind
that each organic being is striving to increase in a geometrical ratio;
that each, at some period of its life, during some season of the year,
during each generation, or at intervals, has to struggle for life and
to suffer great destruction. When we reflect on this struggle we may
console ourselves with the full belief that the war of nature is not
incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.




CHAPTER IV.
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.


Natural Selection—its power compared with man’s selection—its power on
characters of trifling importance—its power at all ages and on both
sexes—Sexual Selection—On the generality of intercrosses between
individuals of the same species—Circumstances favourable and
unfavourable to the results of Natural Selection, namely,
intercrossing, isolation, number of individuals—Slow action—Extinction
caused by Natural Selection—Divergence of Character, related to the
diversity of inhabitants of any small area and to naturalisation—Action
of Natural Selection, through Divergence of Character and Extinction,
on the descendants from a common parent—Explains the Grouping of all
organic beings—Advance in organisation—Low forms preserved—Convergence
of character—Indefinite multiplication of species—Summary.


How will the struggle for existence, briefly discussed in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply under
nature? I think we shall see that it can act most efficiently. Let the
endless number of slight variations and individual differences
occurring in our domestic productions, and, in a lesser degree, in
those under nature, be borne in mind; as well as the strength of the
hereditary tendency. Under domestication, it may truly be said that the
whole organisation becomes in some degree plastic. But the variability,
which we almost universally meet with in our domestic productions is
not directly produced, as Hooker and Asa Gray have well remarked, by
man; he can neither originate varieties nor prevent their occurrence;
he can only preserve and accumulate such as do occur. Unintentionally
he exposes organic beings to new and changing conditions of life, and
variability ensues; but similar changes of conditions might and do
occur under nature. Let it also be borne in mind how infinitely complex
and close-fitting are the mutual relations of all organic beings to
each other and to their physical conditions of life; and consequently
what infinitely varied diversities of structure might be of use to each
being under changing conditions of life. Can it then be thought
improbable, seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way to each being in the
great and complex battle of life, should occur in the course of many
successive generations? If such do occur, can we doubt (remembering
that many more individuals are born than can possibly survive) that
individuals having any advantage, however slight, over others, would
have the best chance of surviving and procreating their kind? On the
other hand, we may feel sure that any variation in the least degree
injurious would be rigidly destroyed. This preservation of favourable
individual differences and variations, and the destruction of those
which are injurious, I have called Natural Selection, or the Survival
of the Fittest. Variations neither useful nor injurious would not be
affected by natural selection, and would be left either a fluctuating
element, as perhaps we see in certain polymorphic species, or would
ultimately become fixed, owing to the nature of the organism and the
nature of the conditions.

Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces
variability, whereas it implies only the preservation of such
variations as arise and are beneficial to the being under its
conditions of life. No one objects to agriculturists speaking of the
potent effects of man’s selection; and in this case the individual
differences given by nature, which man for some object selects, must of
necessity first occur. Others have objected that the term selection
implies conscious choice in the animals which become modified; and it
has even been urged that, as plants have no volition, natural selection
is not applicable to them! In the literal sense of the word, no doubt,
natural selection is a false term; but who ever objected to chemists
speaking of the elective affinities of the various elements?—and yet an
acid cannot strictly be said to elect the base with which it in
preference combines. It has been said that I speak of natural selection
as an active power or Deity; but who objects to an author speaking of
the attraction of gravity as ruling the movements of the planets? Every
one knows what is meant and is implied by such metaphorical
expressions; and they are almost necessary for brevity. So again it is
difficult to avoid personifying the word Nature; but I mean by nature,
only the aggregate action and product of many natural laws, and by laws
the sequence of events as ascertained by us. With a little familiarity
such superficial objections will be forgotten.

We shall best understand the probable course of natural selection by
taking the case of a country undergoing some slight physical change,
for instance, of climate. The proportional numbers of its inhabitants
will almost immediately undergo a change, and some species will
probably become extinct. We may conclude, from what we have seen of the
intimate and complex manner in which the inhabitants of each country
are bound together, that any change in the numerical proportions of the
inhabitants, independently of the change of climate itself, would
seriously affect the others. If the country were open on its borders,
new forms would certainly immigrate, and this would likewise seriously
disturb the relations of some of the former inhabitants. Let it be
remembered how powerful the influence of a single introduced tree or
mammal has been shown to be. But in the case of an island, or of a
country partly surrounded by barriers, into which new and better
adapted forms could not freely enter, we should then have places in the
economy of nature which would assuredly be better filled up if some of
the original inhabitants were in some manner modified; for, had the
area been open to immigration, these same places would have been seized
on by intruders. In such cases, slight modifications, which in any way
favoured the individuals of any species, by better adapting them to
their altered conditions, would tend to be preserved; and natural
selection would have free scope for the work of improvement.

We have good reason to believe, as shown in the first chapter, that
changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions the changed, and
this would manifestly be favourable to natural selection, by affording
a better chance of the occurrence of profitable variations. Unless such
occur, natural selection can do nothing. Under the term of
“variations,” it must never be forgotten that mere individual
differences are included. As man can produce a great result with his
domestic animals and plants by adding up in any given direction
individual differences, so could natural selection, but far more easily
from having incomparably longer time for action. Nor do I believe that
any great physical change, as of climate, or any unusual degree of
isolation, to check immigration, is necessary in order that new and
unoccupied places should be left for natural selection to fill up by
improving some of the varying inhabitants. For as all the inhabitants
of each country are struggling together with nicely balanced forces,
extremely slight modifications in the structure or habits of one
species would often give it an advantage over others; and still further
modifications of the same kind would often still further increase the
advantage, as long as the species continued under the same conditions
of life and profited by similar means of subsistence and defence. No
country can be named in which all the native inhabitants are now so
perfectly adapted to each other and to the physical conditions under
which they live, that none of them could be still better adapted or
improved; for in all countries, the natives have been so far conquered
by naturalised productions that they have allowed some foreigners to
take firm possession of the land. And as foreigners have thus in every
country beaten some of the natives, we may safely conclude that the
natives might have been modified with advantage, so as to have better
resisted the intruders.

As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so
far as they are useful to any being. She can act on every internal
organ, on every shade of constitutional difference, on the whole
machinery of life. Man selects only for his own good; Nature only for
that of the being which she tends. Every selected character is fully
exercised by her, as is implied by the fact of their selection. Man
keeps the natives of many climates in the same country. He seldom
exercises each selected character in some peculiar and fitting manner;
he feeds a long and a short-beaked pigeon on the same food; he does not
exercise a long-backed or long-legged quadruped in any peculiar manner;
he exposes sheep with long and short wool to the same climate; does not
allow the most vigorous males to struggle for the females; he does not
rigidly destroy all inferior animals, but protects during each varying
season, as far as lies in his power, all his productions. He often
begins his selection by some half-monstrous form, or at least by some
modification prominent enough to catch the eye or to be plainly useful
to him. Under nature, the slightest differences of structure or
constitution may well turn the nicely-balanced scale in the struggle
for life, and so be preserved. How fleeting are the wishes and efforts
of man! How short his time, and consequently how poor will be his
results, compared with those accumulated by Nature during whole
geological periods! Can we wonder, then, that Nature’s productions
should be far “truer” in character than man’s productions; that they
should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship?

It may metaphorically be said that natural selection is daily and
hourly scrutinising, throughout the world, the slightest variations;
rejecting those that are bad, preserving and adding up all that are
good; silently and insensibly working, _whenever and wherever
opportunity offers_, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of time has
marked the long lapse of ages, and then so imperfect is our view into
long-past geological ages that we see only that the forms of life are
now different from what they formerly were.

In order that any great amount of modification should be effected in a
species, a variety, when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must again be preserved, and so
onward, step by step. Seeing that individual differences of the same
kind perpetually recur, this can hardly be considered as an
unwarrantable assumption. But whether it is true, we can judge only by
seeing how far the hypothesis accords with and explains the general
phenomena of nature. On the other hand, the ordinary belief that the
amount of possible variation is a strictly limited quantity, is
likewise a simple assumption.

Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to consider
as of very trifling importance, may thus be acted on. When we see
leaf-eating insects green, and bark-feeders mottled-grey; the alpine
ptarmigan white in winter, the red-grouse the colour of heather, we
must believe that these tints are of service to these birds and insects
in preserving them from danger. Grouse, if not destroyed at some period
of their lives, would increase in countless numbers; they are known to
suffer largely from birds of prey; and hawks are guided by eyesight to
their prey,—so much so that on parts of the continent persons are
warned not to keep white pigeons, as being the most liable to
destruction. Hence natural selection might be effective in giving the
proper colour to each kind of grouse, and in keeping that colour, when
once acquired, true and constant. Nor ought we to think that the
occasional destruction of an animal of any particular colour would
produce little effect; we should remember how essential it is in a
flock of white sheep to destroy a lamb with the faintest trace of
black. We have seen how the colour of hogs, which feed on the
“paint-root” in Virginia, determines whether they shall live or die. In
plants, the down on the fruit and the colour of the flesh are
considered by botanists as characters of the most trifling importance;
yet we hear from an excellent horticulturist, Downing, that in the
United States smooth-skinned fruits suffer far more from a beetle, a
Curculio, than those with down; that purple plums suffer far more from
a certain disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle which
variety, whether a smooth or downy, a yellow or a purple-fleshed fruit,
should succeed.

In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem quite unimportant, we
must not forget that climate, food, &c., have no doubt produced some
direct effect. It is also necessary to bear in mind that, owing to the
law of correlation, when one part varies and the variations are
accumulated through natural selection, other modifications, often of
the most unexpected nature, will ensue.

As we see that those variations which, under domestication, appear at
any particular period of life, tend to reappear in the offspring at the
same period; for instance, in the shape, size and flavour of the seeds
of the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the
eggs of poultry, and in the colour of the down of their chickens; in
the horns of our sheep and cattle when nearly adult; so in a state of
nature natural selection will be enabled to act on and modify organic
beings at any age, by the accumulation of variations profitable at that
age, and by their inheritance at a corresponding age. If it profit a
plant to have its seeds more and more widely disseminated by the wind,
I can see no greater difficulty in this being effected through natural
selection, than in the cotton-planter increasing and improving by
selection the down in the pods on his cotton-trees. Natural selection
may modify and adapt the larva of an insect to a score of
contingencies, wholly different from those which concern the mature
insect; and these modifications may affect, through correlation, the
structure of the adult. So, conversely, modifications in the adult may
affect the structure of the larva; but in all cases natural selection
will ensure that they shall not be injurious: for if they were so, the
species would become extinct.

Natural selection will modify the structure of the young in relation to
the parent and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the whole community; if the community profits by the selected
change. What natural selection cannot do, is to modify the structure of
one species, without giving it any advantage, for the good of another
species; and though statements to this effect may be found in works of
natural history, I cannot find one case which will bear investigation.
A structure used only once in an animal’s life, if of high importance
to it, might be modified to any extent by natural selection; for
instance, the great jaws possessed by certain insects, used exclusively
for opening the cocoon—or the hard tip to the beak of unhatched birds,
used for breaking the eggs. It has been asserted, that of the best
short-beaked tumbler-pigeons a greater number perish in the egg than
are able to get out of it; so that fanciers assist in the act of
hatching. Now, if nature had to make the beak of a full-grown pigeon
very short for the bird’s own advantage, the process of modification
would be very slow, and there would be simultaneously the most rigorous
selection of all the young birds within the egg, which had the most
powerful and hardest beaks, for all with weak beaks would inevitably
perish: or, more delicate and more easily broken shells might be
selected, the thickness of the shell being known to vary like every
other structure.

It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the
course of natural selection. For instance, a vast number of eggs or
seeds are annually devoured, and these could be modified through
natural selection only if they varied in some manner which protected
them from their enemies. Yet many of these eggs or seeds would perhaps,
if not destroyed, have yielded individuals better adapted to their
conditions of life than any of those which happened to survive. So
again a vast number of mature animals and plants, whether or not they
be the best adapted to their conditions, must be annually destroyed by
accidental causes, which would not be in the least degree mitigated by
certain changes of structure or constitution which would in other ways
be beneficial to the species. But let the destruction of the adults be
ever so heavy, if the number which can exist in any district be not
wholly kept down by such causes—or again let the destruction of eggs or
seeds be so great that only a hundredth or a thousandth part are
developed—yet of those which do survive, the best adapted individuals,
supposing that there is any variability in a favourable direction, will
tend to propagate their kind in larger numbers than the less well
adapted. If the numbers be wholly kept down by the causes just
indicated, as will often have been the case, natural selection will be
powerless in certain beneficial directions; but this is no valid
objection to its efficiency at other times and in other ways; for we
are far from having any reason to suppose that many species ever
undergo modification and improvement at the same time in the same area.

_Sexual Selection._


Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, so no doubt it will be
under nature. Thus it is rendered possible for the two sexes to be
modified through natural selection in relation to different habits of
life, as is sometimes the case; or for one sex to be modified in
relation to the other sex, as commonly occurs. This leads me to say a
few words on what I have called sexual selection. This form of
selection depends, not on a struggle for existence in relation to other
organic beings or to external conditions, but on a struggle between the
individuals of one sex, generally the males, for the possession of the
other sex. The result is not death to the unsuccessful competitor, but
few or no offspring. Sexual selection is, therefore, less rigorous than
natural selection. Generally, the most vigorous males, those which are
best fitted for their places in nature, will leave most progeny. But in
many cases victory depends not so much on general vigour, but on having
special weapons, confined to the male sex. A hornless stag or spurless
cock would have a poor chance of leaving numerous offspring. Sexual
selection, by always allowing the victor to breed, might surely give
indomitable courage, length of spur, and strength to the wing to strike
in the spurred leg, in nearly the same manner as does the brutal
cockfighter by the careful selection of his best cocks. How low in the
scale of nature the law of battle descends I know not; male alligators
have been described as fighting, bellowing, and whirling round, like
Indians in a war-dance, for the possession of the females; male salmons
have been observed fighting all day long; male stag-beetles sometimes
bear wounds from the huge mandibles of other males; the males of
certain hymenopterous insects have been frequently seen by that
inimitable observer M. Fabre, fighting for a particular female who sits
by, an apparently unconcerned beholder of the struggle, and then
retires with the conqueror. The war is, perhaps, severest between the
males of polygamous animals, and these seem oftenest provided with
special weapons. The males of carnivorous animals are already well
armed; though to them and to others, special means of defence may be
given through means of sexual selection, as the mane of the lion, and
the hooked jaw to the male salmon; for the shield may be as important
for victory as the sword or spear.

Among birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the
severest rivalry between the males of many species to attract, by
singing, the females. The rock-thrush of Guiana, birds of paradise, and
some others, congregate, and successive males display with the most
elaborate care, and show off in the best manner, their gorgeous
plumage; they likewise perform strange antics before the females,
which, standing by as spectators, at last choose the most attractive
partner. Those who have closely attended to birds in confinement well
know that they often take individual preferences and dislikes: thus Sir
R. Heron has described how a pied peacock was eminently attractive to
all his hen birds. I cannot here enter on the necessary details; but if
man can in a short time give beauty and an elegant carriage to his
bantams, according to his standard of beauty, I can see no good reason
to doubt that female birds, by selecting, during thousands of
generations, the most melodious or beautiful males, according to their
standard of beauty, might produce a marked effect. Some well-known
laws, with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can partly be explained
through the action of sexual selection on variations occurring at
different ages, and transmitted to the males alone or to both sexes at
corresponding ages; but I have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour,
or ornament, such differences have been mainly caused by sexual
selection: that is, by individual males having had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; which they have transmitted to their male
offspring alone. Yet, I would not wish to attribute all sexual
differences to this agency: for we see in our domestic animals
peculiarities arising and becoming attached to the male sex, which
apparently have not been augmented through selection by man. The tuft
of hair on the breast of the wild turkey-cock cannot be of any use, and
it is doubtful whether it can be ornamental in the eyes of the female
bird; indeed, had the tuft appeared under domestication it would have
been called a monstrosity.

_Illustrations of the Action of Natural Selection, or the Survival of
the Fittest._


In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in the
country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf was hardest
pressed for food. Under such circumstances the swiftest and slimmest
wolves have the best chance of surviving, and so be preserved or
selected, provided always that they retained strength to master their
prey at this or some other period of the year, when they were compelled
to prey on other animals. I can see no more reason to doubt that this
would be the result, than that man should be able to improve the
fleetness of his greyhounds by careful and methodical selection, or by
that kind of unconscious selection which follows from each man trying
to keep the best dogs without any thought of modifying the breed. I may
add that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains, in the United States, one with a
light greyhound-like form, which pursues deer, and the other more
bulky, with shorter legs, which more frequently attacks the shepherd’s
flocks.

Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and
almost nightly catching woodcocks or snipes. The tendency to catch rats
rather than mice is known to be inherited. Now, if any slight innate
change of habit or of structure benefited an individual wolf, it would
have the best chance of surviving and of leaving offspring. Some of its
young would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would
either supplant or coexist with the parent-form of wolf. Or, again, the
wolves inhabiting a mountainous district, and those frequenting the
lowlands, would naturally be forced to hunt different prey; and from
the continued preservation of the individuals best fitted for the two
sites, two varieties might slowly be formed. These varieties would
cross and blend where they met; but to this subject of intercrossing we
shall soon have to return. I may add, that, according to Mr. Pierce,
there are two varieties of the wolf inhabiting the Catskill Mountains
in the United States, one with a light greyhound-like form, which
pursues deer, and the other more bulky, with shorter legs, which more
frequently attacks the shepherd’s flocks.

It should be observed that in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked
variation having been preserved. In former editions of this work I
sometimes spoke as if this latter alternative had frequently occurred.
I saw the great importance of individual differences, and this led me
fully to discuss the results of unconscious selection by man, which
depends on the preservation of all the more or less valuable
individuals, and on the destruction of the worst. I saw, also, that the
preservation in a state of nature of any occasional deviation of
structure, such as a monstrosity, would be a rare event; and that, if
at first preserved, it would generally be lost by subsequent
intercrossing with ordinary individuals. Nevertheless, until reading an
able and valuable article in the “North British Review” (1867), I did
not appreciate how rarely single variations, whether slight or strongly
marked, could be perpetuated. The author takes the case of a pair of
animals, producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an average
survive to pro-create their kind. This is rather an extreme estimate
for most of the higher animals, but by no means so for many of the
lower organisms. He then shows that if a single individual were born,
which varied in some manner, giving it twice as good a chance of life
as that of the other individuals, yet the chances would be strongly
against its survival. Supposing it to survive and to breed, and that
half its young inherited the favourable variation; still, as the
Reviewer goes onto show, the young would have only a slightly better
chance of surviving and breeding; and this chance would go on
decreasing in the succeeding generations. The justice of these remarks
cannot, I think, be disputed. If, for instance, a bird of some kind
could procure its food more easily by having its beak curved, and if
one were born with its beak strongly curved, and which consequently
flourished, nevertheless there would be a very poor chance of this one
individual perpetuating its kind to the exclusion of the common form;
but there can hardly be a doubt, judging by what we see taking place
under domestication, that this result would follow from the
preservation during many generations of a large number of individuals
with more or less strongly curved beaks, and from the destruction of a
still larger number with the straightest beaks.

It should not, however, be overlooked that certain rather strongly
marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organisation being
similarly acted on—of which fact numerous instances could be given with
our domestic productions. In such cases, if the varying individual did
not actually transmit to its offspring its newly-acquired character, it
would undoubtedly transmit to them, as long as the existing conditions
remained the same, a still stronger tendency to vary in the same
manner. There can also be little doubt that the tendency to vary in the
same manner has often been so strong that all the individuals of the
same species have been similarly modified without the aid of any form
of selection. Or only a third, fifth, or tenth part of the individuals
may have been thus affected, of which fact several instances could be
given. Thus Graba estimates that about one-fifth of the guillemots in
the Faroe Islands consist of a variety so well marked, that it was
formerly ranked as a distinct species under the name of Uria lacrymans.
In cases of this kind, if the variation were of a beneficial nature,
the original form would soon be supplanted by the modified form,
through the survival of the fittest.

To the effects of intercrossing in eliminating variations of all kinds,
I shall have to recur; but it may be here remarked that most animals
and plants keep to their proper homes, and do not needlessly wander
about; we see this even with migratory birds, which almost always
return to the same spot. Consequently each newly-formed variety would
generally be at first local, as seems to be the common rule with
varieties in a state of nature; so that similarly modified individuals
would soon exist in a small body together, and would often breed
together. If the new variety were successful in its battle for life, it
would slowly spread from a central district, competing with and
conquering the unchanged individuals on the margins of an
ever-increasing circle.

It may be worth while to give another and more complex illustration of
the action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the
sap: this is effected, for instance, by glands at the base of the
stipules in some Leguminosæ, and at the backs of the leaves of the
common laurel. This juice, though small in quantity, is greedily sought
by insects; but their visits do not in any way benefit the plant. Now,
let us suppose that the juice or nectar was excreted from the inside of
the flowers of a certain number of plants of any species. Insects in
seeking the nectar would get dusted with pollen, and would often
transport it from one flower to another. The flowers of two distinct
individuals of the same species would thus get crossed; and the act of
crossing, as can be fully proved, gives rise to vigorous seedlings,
which consequently would have the best chance of flourishing and
surviving. The plants which produced flowers with the largest glands or
nectaries, excreting most nectar, would oftenest be visited by insects,
and would oftenest be crossed; and so in the long-run would gain the
upper hand and form a local variety. The flowers, also, which had their
stamens and pistils placed, in relation to the size and habits of the
particular insect which visited them, so as to favour in any degree the
transportal of the pollen, would likewise be favoured. We might have
taken the case of insects visiting flowers for the sake of collecting
pollen instead of nectar; and as pollen is formed for the sole purpose
of fertilisation, its destruction appears to be a simple loss to the
plant; yet if a little pollen were carried, at first occasionally and
then habitually, by the pollen-devouring insects from flower to flower,
and a cross thus effected, although nine-tenths of the pollen were
destroyed it might still be a great gain to the plant to be thus
robbed; and the individuals which produced more and more pollen, and
had larger anthers, would be selected.

When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their
part, regularly carry pollen from flower to flower; and that they do
this effectually I could easily show by many striking facts. I will
give only one, as likewise illustrating one step in the separation of
the sexes of plants. Some holly-trees bear only male flowers, which
have four stamens producing a rather small quantity of pollen, and a
rudimentary pistil; other holly-trees bear only female flowers; these
have a full-sized pistil, and four stamens with shrivelled anthers, in
which not a grain of pollen can be detected. Having found a female tree
exactly sixty yards from a male tree, I put the stigmas of twenty
flowers, taken from different branches, under the microscope, and on
all, without exception, there were a few pollen-grains, and on some a
profusion. As the wind had set for several days from the female to the
male tree, the pollen could not thus have been carried. The weather had
been cold and boisterous and therefore not favourable to bees,
nevertheless every female flower which I examined had been effectually
fertilised by the bees, which had flown from tree to tree in search of
nectar. But to return to our imaginary case; as soon as the plant had
been rendered so highly attractive to insects that pollen was regularly
carried from flower to flower, another process might commence. No
naturalist doubts the advantage of what has been called the
“physiological division of labour;” hence we may believe that it would
be advantageous to a plant to produce stamens alone in one flower or on
one whole plant, and pistils alone in another flower or on another
plant. In plants under culture and placed under new conditions of life,
sometimes the male organs and sometimes the female organs become more
or less impotent; now if we suppose this to occur in ever so slight a
degree under nature, then, as pollen is already carried regularly from
flower to flower, and as a more complete separation of the sexes of our
plant would be advantageous on the principle of the division of labour,
individuals with this tendency more and more increased, would be
continually favoured or selected, until at last a complete separation
of the sexes might be effected. It would take up too much space to show
the various steps, through dimorphism and other means, by which the
separation of the sexes in plants of various kinds is apparently now in
progress; but I may add that some of the species of holly in North
America are, according to Asa Gray, in an exactly intermediate
condition, or, as he expresses it, are more or less dioeciously
polygamous.

Let us now turn to the nectar-feeding insects; we may suppose the plant
of which we have been slowly increasing the nectar by continued
selection, to be a common plant; and that certain insects depended in
main part on its nectar for food. I could give many facts showing how
anxious bees are to save time: for instance, their habit of cutting
holes and sucking the nectar at the bases of certain flowers, which
with a very little more trouble they can enter by the mouth. Bearing
such facts in mind, it may be believed that under certain circumstances
individual differences in the curvature or length of the proboscis,
&c., too slight to be appreciated by us, might profit a bee or other
insect, so that certain individuals would be able to obtain their food
more quickly than others; and thus the communities to which they
belonged would flourish and throw off many swarms inheriting the same
peculiarities. The tubes of the corolla of the common red or incarnate
clovers (Trifolium pratense and incarnatum) do not on a hasty glance
appear to differ in length; yet the hive-bee can easily suck the nectar
out of the incarnate clover, but not out of the common red clover,
which is visited by humble-bees alone; so that whole fields of the red
clover offer in vain an abundant supply of precious nectar to the
hive-bee. That this nectar is much liked by the hive-bee is certain;
for I have repeatedly seen, but only in the autumn, many hive-bees
sucking the flowers through holes bitten in the base of the tube by
humble bees. The difference in the length of the corolla in the two
kinds of clover, which determines the visits of the hive-bee, must be
very trifling; for I have been assured that when red clover has been
mown, the flowers of the second crop are somewhat smaller, and that
these are visited by many hive-bees. I do not know whether this
statement is accurate; nor whether another published statement can be
trusted, namely, that the Ligurian bee, which is generally considered a
mere variety of the common hive-bee, and which freely crosses with it,
is able to reach and suck the nectar of the red clover. Thus, in a
country where this kind of clover abounded, it might be a great
advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, as the fertility of this
clover absolutely depends on bees visiting the flowers, if humble-bees
were to become rare in any country, it might be a great advantage to
the plant to have a shorter or more deeply divided corolla, so that the
hive-bees should be enabled to suck its flowers. Thus I can understand
how a flower and a bee might slowly become, either simultaneously or
one after the other, modified and adapted to each other in the most
perfect manner, by the continued preservation of all the individuals
which presented slight deviations of structure mutually favourable to
each other.

I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which
were first urged against Sir Charles Lyell’s noble views on “the modern
changes of the earth, as illustrative of geology;” but we now seldom
hear the agencies which we see still at work, spoken of as trifling and
insignificant, when used in explaining the excavation of the deepest
valleys or the formation of long lines of inland cliffs. Natural
selection acts only by the preservation and accumulation of small
inherited modifications, each profitable to the preserved being; and as
modern geology has almost banished such views as the excavation of a
great valley by a single diluvial wave, so will natural selection
banish the belief of the continued creation of new organic beings, or
of any great and sudden modification in their structure.

_On the Intercrossing of Individuals._


I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two
individuals must always (with the exception of the curious and not well
understood cases of parthenogenesis) unite for each birth; but in the
case of hermaphrodites this is far from obvious. Nevertheless there is
reason to believe that with all hermaphrodites two individuals, either
occasionally or habitually, concur for the reproduction of their kind.
This view was long ago doubtfully suggested by Sprengel, Knight and
Kölreuter. We shall presently see its importance; but I must here treat
the subject with extreme brevity, though I have the materials prepared
for an ample discussion. All vertebrate animals, all insects and some
other large groups of animals, pair for each birth. Modern research has
much diminished the number of supposed hermaphrodites and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals
ever concur in reproduction? As it is impossible here to enter on
details, I must trust to some general considerations alone.

In the first place, I have collected so large a body of facts, and made
so many experiments, showing, in accordance with the almost universal
belief of breeders, that with animals and plants a cross between
different varieties, or between individuals of the same variety but of
another strain, gives vigour and fertility to the offspring; and on the
other hand, that _close_ interbreeding diminishes vigour and fertility;
that these facts alone incline me to believe that it is a general law
of nature that no organic being fertilises itself for a perpetuity of
generations; but that a cross with another individual is
occasionally—perhaps at long intervals of time—indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any
other view are inexplicable. Every hybridizer knows how unfavourable
exposure to wet is to the fertilisation of a flower, yet what a
multitude of flowers have their anthers and stigmas fully exposed to
the weather! If an occasional cross be indispensable, notwithstanding
that the plant’s own anthers and pistil stand so near each other as
almost to ensure self-fertilisation, the fullest freedom for the
entrance of pollen from another individual will explain the above state
of exposure of the organs. Many flowers, on the other hand, have their
organs of fructification closely enclosed, as in the great
papilionaceous or pea-family; but these almost invariably present
beautiful and curious adaptations in relation to the visits of insects.
So necessary are the visits of bees to many papilionaceous flowers,
that their fertility is greatly diminished if these visits be
prevented. Now, it is scarcely possible for insects to fly from flower
to flower, and not to carry pollen from one to the other, to the great
good of the plant. Insects act like a camel-hair pencil, and it is
sufficient, to ensure fertilisation, just to touch with the same brush
the anthers of one flower and then the stigma of another; but it must
not be supposed that bees would thus produce a multitude of hybrids
between distinct species; for if a plant’s own pollen and that from
another species are placed on the same stigma, the former is so
prepotent that it invariably and completely destroys, as has been shown
by Gärtner, the influence of the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but the agency of insects is often required to cause the
stamens to spring forward, as Kölreuter has shown to be the case with
the barberry; and in this very genus, which seems to have a special
contrivance for self-fertilisation, it is well known that, if
closely-allied forms or varieties are planted near each other, it is
hardly possible to raise pure seedlings, so largely do they naturally
cross. In numerous other cases, far from self-fertilisation being
favoured, there are special contrivances which effectually prevent the
stigma receiving pollen from its own flower, as I could show from the
works of Sprengel and others, as well as from my own observations: for
instance, in Lobelia fulgens, there is a really beautiful and elaborate
contrivance by which all the infinitely numerous pollen-granules are
swept out of the conjoined anthers of each flower, before the stigma of
that individual flower is ready to receive them; and as this flower is
never visited, at least in my garden, by insects, it never sets a seed,
though by placing pollen from one flower on the stigma of another, I
raise plenty of seedlings. Another species of Lobelia, which is visited
by bees, seeds freely in my garden. In very many other cases, though
there is no special mechanical contrivance to prevent the stigma
receiving pollen from the same flower, yet, as Sprengel, and more
recently Hildebrand and others have shown, and as I can confirm, either
the anthers burst before the stigma is ready for fertilisation, or the
stigma is ready before the pollen of that flower is ready, so that
these so-named dichogamous plants have in fact separated sexes, and
must habitually be crossed. So it is with the reciprocally dimorphic
and trimorphic plants previously alluded to. How strange are these
facts! How strange that the pollen and stigmatic surface of the same
flower, though placed so close together, as if for the very purpose of
self-fertilisation, should be in so many cases mutually useless to each
other! How simply are these facts explained on the view of an
occasional cross with a distinct individual being advantageous or
indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority of the
seedlings thus raised turn out, as I found, mongrels: for instance, I
raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind,
and some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens but by
those of the many other flowers on the same plant; and the pollen of
each flower readily gets on its stigma without insect agency; for I
have found that plants carefully protected from insects produce the
full number of pods. How, then, comes it that such a vast number of the
seedlings are mongrelized? It must arise from the pollen of a distinct
_variety_ having a prepotent effect over the flower’s own pollen; and
that this is part of the general law of good being derived from the
intercrossing of distinct individuals of the same species. When
distinct _species_ are crossed the case is reversed, for a plant’s own
pollen is always prepotent over foreign pollen; but to this subject we
shall return in a future chapter.

In the case of a large tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at
most only from flower to flower on the same tree; and flowers on the
same tree can be considered as distinct individuals only in a limited
sense. I believe this objection to be valid, but that nature has
largely provided against it by giving to trees a strong tendency to
bear flowers with separated sexes. When the sexes are separated,
although the male and female flowers may be produced on the same tree,
pollen must be regularly carried from flower to flower; and this will
give a better chance of pollen being occasionally carried from tree to
tree. That trees belonging to all orders have their sexes more often
separated than other plants, I find to be the case in this country; and
at my request Dr. Hooker tabulated the trees of New Zealand, and Dr.
Asa Gray those of the United States, and the result was as I
anticipated. On the other hand, Dr. Hooker informs me that the rule
does not hold good in Australia: but if most of the Australian trees
are dichogamous, the same result would follow as if they bore flowers
with separated sexes. I have made these few remarks on trees simply to
call attention to the subject.

Turning for a brief space to animals: various terrestrial species are
hermaphrodites, such as the land-mollusca and earth-worms; but these
all pair. As yet I have not found a single terrestrial animal which can
fertilise itself. This remarkable fact, which offers so strong a
contrast with terrestrial plants, is intelligible on the view of an
occasional cross being indispensable; for owing to the nature of the
fertilising element there are no means, analogous to the action of
insects and of the wind with plants, by which an occasional cross could
be effected with terrestrial animals without the concurrence of two
individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here the currents of water offer an obvious means
for an occasional cross. As in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities, namely,
Professor Huxley, to discover a single hermaphrodite animal with the
organs of reproduction so perfectly enclosed that access from without,
and the occasional influence of a distinct individual, can be shown to
be physically impossible. Cirripedes long appeared to me to present,
under this point of view, a case of great difficulty; but I have been
enabled, by a fortunate chance, to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, both
with animals and plants, some species of the same family and even of
the same genus, though agreeing closely with each other in their whole
organisation, are hermaphrodites, and some unisexual. But if, in fact,
all hermaphrodites do occasionally intercross, the difference between
them and unisexual species is, as far as function is concerned, very
small.

From these several considerations and from the many special facts which
I have collected, but which I am unable here to give, it appears that
with animals and plants an occasional intercross between distinct
individuals is a very general, if not universal, law of nature.

_Circumstances favourable for the production of new forms through
Natural Selection._


This is an extremely intricate subject. A great amount of variability,
under which term individual differences are always included, will
evidently be favourable. A large number of individuals, by giving a
better chance within any given period for the appearance of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, a highly important element of success.
Though nature grants long periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic
beings are striving to seize on each place in the economy of nature, if
any one species does not become modified and improved in a
corresponding degree with its competitors it will be exterminated.
Unless favourable variations be inherited by some at least of the
offspring, nothing can be effected by natural selection. The tendency
to reversion may often check or prevent the work; but as this tendency
has not prevented man from forming by selection numerous domestic
races, why should it prevail against natural selection?

In the case of methodical selection, a breeder selects for some
definite object, and if the individuals be allowed freely to
intercross, his work will completely fail. But when many men, without
intending to alter the breed, have a nearly common standard of
perfection, and all try to procure and breed from the best animals,
improvement surely but slowly follows from this unconscious process of
selection, notwithstanding that there is no separation of selected
individuals. Thus it will be under nature; for within a confined area,
with some place in the natural polity not perfectly occupied, all the
individuals varying in the right direction, though in different
degrees, will tend to be preserved. But if the area be large, its
several districts will almost certainly present different conditions of
life; and then, if the same species undergoes modification in different
districts, the newly formed varieties will intercross on the confines
of each. But we shall see in the sixth chapter that intermediate
varieties, inhabiting intermediate districts, will in the long run
generally be supplanted by one of the adjoining varieties.
Intercrossing will chiefly affect those animals which unite for each
birth and wander much, and which do not breed at a very quick rate.
Hence with animals of this nature, for instance birds, varieties will
generally be confined to separated countries; and this I find to be the
case. With hermaphrodite organisms which cross only occasionally, and
likewise with animals which unite for each birth, but which wander
little and can increase at a rapid rate, a new and improved variety
might be quickly formed on any one spot, and might there maintain
itself in a body and afterward spread, so that the individuals of the
new variety would chiefly cross together. On this principle nurserymen
always prefer saving seed from a large body of plants, as the chance of
intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not
propagate rapidly, we must not assume that free intercrossing would
always eliminate the effects of natural selection; for I can bring
forward a considerable body of facts showing that within the same area
two varieties of the same animal may long remain distinct, from
haunting different stations, from breeding at slightly different
seasons, or from the individuals of each variety preferring to pair
together.

Intercrossing plays a very important part in nature by keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but, as already stated,
we have reason to believe that occasional intercrosses take place with
all animals and plants. Even if these take place only at long intervals
of time, the young thus produced will gain so much in vigour and
fertility over the offspring from long-continued self-fertilisation,
that they will have a better chance of surviving and propagating their
kind; and thus in the long run the influence of crosses, even at rare
intervals, will be great. With respect to organic beings extremely low
in the scale, which do not propagate sexually, nor conjugate, and which
cannot possibly intercross, uniformity of character can be retained by
them under the same conditions of life, only through the principle of
inheritance, and through natural selection which will destroy any
individuals departing from the proper type. If the conditions of life
change and the form undergoes modification, uniformity of character can
be given to the modified offspring, solely by natural selection
preserving similar favourable variations.

Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all the
varying individuals of the same species in the same manner.
Intercrossing with the inhabitants of the surrounding districts, will
also be thus prevented. Moritz Wagner has lately published an
interesting essay on this subject, and has shown that the service
rendered by isolation in preventing crosses between newly-formed
varieties is probably greater even than I supposed. But from reasons
already assigned I can by no means agree with this naturalist, that
migration and isolation are necessary elements for the formation of new
species. The importance of isolation is likewise great in preventing,
after any physical change in the conditions, such as of climate,
elevation of the land, &c., the immigration of better adapted
organisms; and thus new places in the natural economy of the district
will be left open to be filled up by the modification of the old
inhabitants. Lastly, isolation will give time for a new variety to be
improved at a slow rate; and this may sometimes be of much importance.
If, however, an isolated area be very small, either from being
surrounded by barriers, or from having very peculiar physical
conditions, the total number of the inhabitants will be small; and this
will retard the production of new species through natural selection, by
decreasing the chances of favourable variations arising.

The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously
asserted that the element of time has been assumed by me to play an
all-important part in modifying species, as if all the forms of life
were necessarily undergoing change through some innate law. Lapse of
time is only so far important, and its importance in this respect is
great, that it gives a better chance of beneficial variations arising
and of their being selected, accumulated, and fixed. It likewise tends
to increase the direct action of the physical conditions of life, in
relation to the constitution of each organism.

If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the number
of the species inhabiting it is small, as we shall see in our chapter
on Geographical Distribution; yet of these species a very large
proportion are endemic,—that is, have been produced there and nowhere
else in the world. Hence an oceanic island at first sight seems to have
been highly favourable for the production of new species. But we may
thus deceive ourselves, for to ascertain whether a small isolated area,
or a large open area like a continent, has been most favourable for the
production of new organic forms, we ought to make the comparison within
equal times; and this we are incapable of doing.

Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area
is still more important, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will there be a
better chance of favourable variations, arising from the large number
of individuals of the same species there supported, but the conditions
of life are much more complex from the large number of already existing
species; and if some of these many species become modified and
improved, others will have to be improved in a corresponding degree, or
they will be exterminated. Each new form, also, as soon as it has been
much improved, will be able to spread over the open and continuous
area, and will thus come into competition with many other forms.
Moreover, great areas, though now continuous, will often, owing to
former oscillations of level, have existed in a broken condition, so
that the good effects of isolation will generally, to a certain extent,
have concurred. Finally, I conclude that, although small isolated areas
have been in some respects highly favourable for the production of new
species, yet that the course of modification will generally have been
more rapid on large areas; and what is more important, that the new
forms produced on large areas, which already have been victorious over
many competitors, will be those that will spread most widely, and will
give rise to the greatest number of new varieties and species. They
will thus play a more important part in the changing history of the
organic world.

In accordance with this view, we can, perhaps, understand some facts
which will be again alluded to in our chapter on Geographical
Distribution; for instance, the fact of the productions of the smaller
continent of Australia now yielding before those of the larger
Europæo-Asiatic area. Thus, also, it is that continental productions
have everywhere become so largely naturalised on islands. On a small
island, the race for life will have been less severe, and there will
have been less modification and less extermination. Hence, we can
understand how it is that the flora of Madeira, according to Oswald
Heer, resembles to a certain extent the extinct tertiary flora of
Europe. All fresh water basins, taken together, make a small area
compared with that of the sea or of the land. Consequently, the
competition between fresh water productions will have been less severe
than elsewhere; new forms will have been more slowly produced, and old
forms more slowly exterminated. And it is in fresh water basins that we
find seven genera of Ganoid fishes, remnants of a once preponderant
order: and in fresh water we find some of the most anomalous forms now
known in the world, as the Ornithorhynchus and Lepidosiren, which, like
fossils, connect to a certain extent orders at present widely separated
in the natural scale. These anomalous forms may be called living
fossils; they have endured to the present day, from having inhabited a
confined area, and from having been exposed to less varied, and
therefore less severe, competition.

To sum up, as far as the extreme intricacy of the subject permits, the
circumstances favourable and unfavourable for the production of new
species through natural selection. I conclude that for terrestrial
productions a large continental area, which has undergone many
oscillations of level, will have been the most favourable for the
production of many new forms of life, fitted to endure for a long time
and to spread widely. While the area existed as a continent the
inhabitants will have been numerous in individuals and kinds, and will
have been subjected to severe competition. When converted by subsidence
into large separate islands there will still have existed many
individuals of the same species on each island: intercrossing on the
confines of the range of each new species will have been checked: after
physical changes of any kind immigration will have been prevented, so
that new places in the polity of each island will have had to be filled
up by the modification of the old inhabitants; and time will have been
allowed for the varieties in each to become well modified and
perfected. When, by renewed elevation, the islands were reconverted
into a continental area, there will again have been very severe
competition; the most favoured or improved varieties will have been
enabled to spread; there will have been much extinction of the less
improved forms, and the relative proportional numbers of the various
inhabitants of the reunited continent will again have been changed; and
again there will have been a fair field for natural selection to
improve still further the inhabitants, and thus to produce new species.

That natural selection generally act with extreme slowness I fully
admit. It can act only when there are places in the natural polity of a
district which can be better occupied by the modification of some of
its existing inhabitants. The occurrence of such places will often
depend on physical changes, which generally take place very slowly, and
on the immigration of better adapted forms being prevented. As some few
of the old inhabitants become modified the mutual relations of others
will often be disturbed; and this will create new places, ready to be
filled up by better adapted forms; but all this will take place very
slowly. Although all the individuals of the same species differ in some
slight degree from each other, it would often be long before
differences of the right nature in various parts of the organisation
might occur. The result would often be greatly retarded by free
intercrossing. Many will exclaim that these several causes are amply
sufficient to neutralise the power of natural selection. I do not
believe so. But I do believe that natural selection will generally act
very slowly, only at long intervals of time, and only on a few of the
inhabitants of the same region. I further believe that these slow,
intermittent results accord well with what geology tells us of the rate
and manner at which the inhabitants of the world have changed.

Slow though the process of selection may be, if feeble man can do much
by artificial selection, I can see no limit to the amount of change, to
the beauty and complexity of the coadaptations between all organic
beings, one with another and with their physical conditions of life,
which may have been effected in the long course of time through
nature’s power of selection, that is by the survival of the fittest.

_Extinction caused by Natural Selection._


This subject will be more fully discussed in our chapter on Geology;
but it must here be alluded to from being intimately connected with
natural selection. Natural selection acts solely through the
preservation of variations in some way advantageous, which consequently
endure. Owing to the high geometrical rate of increase of all organic
beings, each area is already fully stocked with inhabitants, and it
follows from this, that as the favoured forms increase in number, so,
generally, will the less favoured decrease and become rare. Rarity, as
geology tells us, is the precursor to extinction. We can see that any
form which is represented by few individuals will run a good chance of
utter extinction, during great fluctuations in the nature or the
seasons, or from a temporary increase in the number of its enemies. But
we may go further than this; for as new forms are produced, unless we
admit that specific forms can go on indefinitely increasing in number,
many old forms must become extinct. That the number of specific forms
has not indefinitely increased, geology plainly tells us; and we shall
presently attempt to show why it is that the number of species
throughout the world has not become immeasurably great.

We have seen that the species which are most numerous in individuals
have the best chance of producing favourable variations within any
given period. We have evidence of this, in the facts stated in the
second chapter, showing that it is the common and diffused or dominant
species which offer the greatest number of recorded varieties. Hence,
rare species will be less quickly modified or improved within any given
period; they will consequently be beaten in the race for life by the
modified and improved descendants of the commoner species.

From these several considerations I think it inevitably follows, that
as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct. The
forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,—varieties of the same species, and species of the
same genus or related genera,—which, from having nearly the same
structure, constitution and habits, generally come into the severest
competition with each other. Consequently, each new variety or species,
during the progress of its formation, will generally press hardest on
its nearest kindred, and tend to exterminate them. We see the same
process of extermination among our domesticated productions, through
the selection of improved forms by man. Many curious instances could be
given showing how quickly new breeds of cattle, sheep and other
animals, and varieties of flowers, take the place of older and inferior
kinds. In Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these “were swept
away by the short-horns” (I quote the words of an agricultural writer)
“as if by some murderous pestilence.”

_Divergence of Character._


The principle, which I have designated by this term, is of high
importance, and explains, as I believe, several important facts. In the
first place, varieties, even strongly-marked ones, though having
somewhat of the character of species—as is shown by the hopeless doubts
in many cases how to rank them—yet certainly differ far less from each
other than do good and distinct species. Nevertheless according to my
view, varieties are species in the process of formation, or are, as I
have called them, incipient species. How, then, does the lesser
difference between varieties become augmented into the greater
difference between species? That this does habitually happen, we must
infer from most of the innumerable species throughout nature presenting
well-marked differences; whereas varieties, the supposed prototypes and
parents of future well-marked species, present slight and ill-defined
differences. Mere chance, as we may call it, might cause one variety to
differ in some character from its parents, and the offspring of this
variety again to differ from its parent in the very same character and
in a greater degree; but this alone would never account for so habitual
and large a degree of difference as that between the species of the
same genus.

As has always been my practice, I have sought light on this head from
our domestic productions. We shall here find something analogous. It
will be admitted that the production of races so different as
short-horn and Hereford cattle, race and cart horses, the several
breeds of pigeons, &c., could never have been effected by the mere
chance accumulation of similar variations during many successive
generations. In practice, a fancier is, for instance, struck by a
pigeon having a slightly shorter beak; another fancier is struck by a
pigeon having a rather longer beak; and on the acknowledged principle
that “fanciers do not and will not admire a medium standard, but like
extremes,” they both go on (as has actually occurred with the
sub-breeds of the tumbler-pigeon) choosing and breeding from birds with
longer and longer beaks, or with shorter and shorter beaks. Again, we
may suppose that at an early period of history, the men of one nation
or district required swifter horses, while those of another required
stronger and bulkier horses. The early differences would be very
slight; but, in the course of time, from the continued selection of
swifter horses in the one case, and of stronger ones in the other, the
differences would become greater, and would be noted as forming two
sub-breeds. Ultimately after the lapse of centuries, these sub-breeds
would become converted into two well-established and distinct breeds.
As the differences became greater, the inferior animals with
intermediate characters, being neither very swift nor very strong,
would not have been used for breeding, and will thus have tended to
disappear. Here, then, we see in man’s productions the action of what
may be called the principle of divergence, causing differences, at
first barely appreciable, steadily to increase, and the breeds to
diverge in character, both from each other and from their common
parent.

But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently (though it was a long
time before I saw how), from the simple circumstance that the more
diversified the descendants from any one species become in structure,
constitution, and habits, by so much will they be better enabled to
seize on many and widely diversified places in the polity of nature,
and so be enabled to increase in numbers.

We can clearly discern this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can
be supported in any country has long ago arrived at its full average.
If its natural power of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in conditions) only
by its varying descendants seizing on places at present occupied by
other animals: some of them, for instance, being enabled to feed on new
kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animals become, the more places they
will be enabled to occupy. What applies to one animal will apply
throughout all time to all animals—that is, if they vary—for otherwise
natural selection can effect nothing. So it will be with plants. It has
been experimentally proved, that if a plot of ground be sown with one
species of grass, and a similar plot be sown with several distinct
genera of grasses, a greater number of plants and a greater weight of
dry herbage can be raised in the latter than in the former case. The
same has been found to hold good when one variety and several mixed
varieties of wheat have been sown on equal spaces of ground. Hence, if
any one species of grass were to go on varying, and the varieties were
continually selected which differed from each other in the same manner,
though in a very slight degree, as do the distinct species and genera
of grasses, a greater number of individual plants of this species,
including its modified descendants, would succeed in living on the same
piece of ground. And we know that each species and each variety of
grass is annually sowing almost countless seeds; and is thus striving,
as it may be said, to the utmost to increase in number. Consequently,
in the course of many thousand generations, the most distinct varieties
of any one species of grass would have the best chance of succeeding
and of increasing in numbers, and thus of supplanting the less distinct
varieties; and varieties, when rendered very distinct from each other,
take the rank of species.

The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be very severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets: also in small ponds of fresh water. Farmers find
that they can raise more food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any
small piece of ground, could live on it (supposing its nature not to be
in any way peculiar), and may be said to be striving to the utmost to
live there; but, it is seen, that where they come into the closest
competition, the advantages of diversification of structure, with the
accompanying differences of habit and constitution, determine that the
inhabitants, which thus jostle each other most closely, shall, as a
general rule, belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through
man’s agency in foreign lands. It might have been expected that the
plants which would succeed in becoming naturalised in any land would
generally have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for their own
country. It might also, perhaps, have been expected that naturalised
plants would have belonged to a few groups more especially adapted to
certain stations in their new homes. But the case is very different;
and Alph. de Candolle has well remarked, in his great and admirable
work, that floras gain by naturalisation, proportionally with the
number of the native genera and species, far more in new genera than in
new species. To give a single instance: in the last edition of Dr. Asa
Gray’s “Manual of the Flora of the Northern United States,” 260
naturalised plants are enumerated, and these belong to 162 genera. We
thus see that these naturalised plants are of a highly diversified
nature. They differ, moreover, to a large extent, from the indigenes,
for out of the 162 naturalised genera, no less than 100 genera are not
there indigenous, and thus a large proportional addition is made to the
genera now living in the United States.

By considering the nature of the plants or animals which have in any
country struggled successfully with the indigenes, and have there
become naturalised, we may gain some crude idea in what manner some of
the natives would have had to be modified in order to gain an advantage
over their compatriots; and we may at least infer that diversification
of structure, amounting to new generic differences, would be profitable
to them.

The advantage of diversification of structure in the inhabitants of the
same region is, in fact, the same as that of the physiological division
of labour in the organs of the same individual body—a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws
most nutriment from these substances. So in the general economy of any
land, the more widely and perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves. A set of
animals, with their organisation but little diversified, could hardly
compete with a set more perfectly diversified in structure. It may be
doubted, for instance, whether the Australian marsupials, which are
divided into groups differing but little from each other, and feebly
representing, as Mr. Waterhouse and others have remarked, our
carnivorous, ruminant, and rodent mammals, could successfully compete
with these well-developed orders. In the Australian mammals, we see the
process of diversification in an early and incomplete stage of
development.

_The Probable Effects of the Action of Natural Selection through
Divergence of Character and Extinction, on the Descendants of a Common
Ancestor._


After the foregoing discussion, which has been much compressed, we may
assume that the modified descendants of any one species will succeed so
much the better as they become more diversified in structure, and are
thus enabled to encroach on places occupied by other beings. Now let us
see how this principle of benefit being derived from divergence of
character, combined with the principles of natural selection and of
extinction, tends to act.

The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal
distances. I have said a large genus, because as we saw in the second
chapter, on an average more species vary in large genera than in small
genera; and the varying species of the large genera present a greater
number of varieties. We have, also, seen that the species, which are
the commonest and most widely-diffused, vary more than do the rare and
restricted species. Let (A) be a common, widely-diffused, and varying
species, belonging to a genus large in its own country. The branching
and diverging dotted lines of unequal lengths proceeding from (A), may
represent its varying offspring. The variations are supposed to be
extremely slight, but of the most diversified nature; they are not
supposed all to appear simultaneously, but often after long intervals
of time; nor are they all supposed to endure for equal periods. Only
those variations which are in some way profitable will be preserved or
naturally selected. And here the importance of the principle of benefit
derived from divergence of character comes in; for this will generally
lead to the most different or divergent variations (represented by the
outer dotted lines) being preserved and accumulated by natural
selection. When a dotted line reaches one of the horizontal lines, and
is there marked by a small numbered letter, a sufficient amount of
variation is supposed to have been accumulated to form it into a fairly
well-marked variety, such as would be thought worthy of record in a
systematic work.


[Illustration]

The intervals between the horizontal lines in the diagram, may
represent each a thousand or more generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely _a_1 and _m_1. These two varieties will
generally still be exposed to the same conditions which made their
parents variable, and the tendency to variability is in itself
hereditary; consequently they will likewise tend to vary, and commonly
in nearly the same manner as did their parents. Moreover, these two
varieties, being only slightly modified forms, will tend to inherit
those advantages which made their parent (A) more numerous than most of
the other inhabitants of the same country; they will also partake of
those more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country. And all
these circumstances are favourable to the production of new varieties.

If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety _a_2, which will, owing to the
principle of divergence, differ more from (A) than did variety _a_1.
Variety _m_1 is supposed to have produced two varieties, namely _m_2
and _s_2, differing from each other, and more considerably from their
common parent (A). We may continue the process by similar steps for any
length of time; some of the varieties, after each thousand generations,
producing only a single variety, but in a more and more modified
condition, some producing two or three varieties, and some failing to
produce any. Thus the varieties or modified descendants of the common
parent (A), will generally go on increasing in number and diverging in
character. In the diagram the process is represented up to the
ten-thousandth generation, and under a condensed and simplified form up
to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes
on so regularly as is represented in the diagram, though in itself made
somewhat irregular, nor that it goes on continuously; it is far more
probable that each form remains for long periods unaltered, and then
again undergoes modification. Nor do I suppose that the most divergent
varieties are invariably preserved: a medium form may often long
endure, and may or may not produce more than one modified descendant;
for natural selection will always act according to the nature of the
places which are either unoccupied or not perfectly occupied by other
beings; and this will depend on infinitely complex relations. But as a
general rule, the more diversified in structure the descendants from
any one species can be rendered, the more places they will be enabled
to seize on, and the more their modified progeny will increase. In our
diagram the line of succession is broken at regular intervals by small
numbered letters marking the successive forms which have become
sufficiently distinct to be recorded as varieties. But these breaks are
imaginary, and might have been inserted anywhere, after intervals long
enough to allow the accumulation of a considerable amount of divergent
variation.

As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same
advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in
character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later and
more highly improved branches in the lines of descent, will, it is
probable, often take the place of, and so destroy, the earlier and less
improved branches: this is represented in the diagram by some of the
lower branches not reaching to the upper horizontal lines. In some
cases no doubt the process of modification will be confined to a single
line of descent, and the number of modified descendants will not be
increased; although the amount of divergent modification may have been
augmented. This case would be represented in the diagram, if all the
lines proceeding from (A) were removed, excepting that from _a_1 to
_a_10. In the same way the English racehorse and English pointer have
apparently both gone on slowly diverging in character from their
original stocks, without either having given off any fresh branches or
races.

After ten thousand generations, species (A) is supposed to have
produced three forms, _a_10, _f_10, and _m_10, which, from having
diverged in character during the successive generations, will have come
to differ largely, but perhaps unequally, from each other and from
their common parent. If we suppose the amount of change between each
horizontal line in our diagram to be excessively small, these three
forms may still be only well-marked varieties; but we have only to
suppose the steps in the process of modification to be more numerous or
greater in amount, to convert these three forms into doubtful or at
least into well-defined species: thus the diagram illustrates the steps
by which the small differences distinguishing varieties are increased
into the larger differences distinguishing species. By continuing the
same process for a greater number of generations (as shown in the
diagram in a condensed and simplified manner), we get eight species,
marked by the letters between _a_14 and _m_14, all descended from (A).
Thus, as I believe, species are multiplied and genera are formed.

In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced,
by analogous steps, after ten thousand generations, either two
well-marked varieties (_w_10 and _z_10) or two species, according to
the amount of change supposed to be represented between the horizontal
lines. After fourteen thousand generations, six new species, marked by
the letters _n_14 to _z_14, are supposed to have been produced. In any
genus, the species which are already very different in character from
each other, will generally tend to produce the greatest number of
modified descendants; for these will have the best chance of seizing on
new and widely different places in the polity of nature: hence in the
diagram I have chosen the extreme species (A), and the nearly extreme
species (I), as those which have largely varied, and have given rise to
new varieties and species. The other nine species (marked by capital
letters) of our original genus, may for long but unequal periods
continue to transmit unaltered descendants; and this is shown in the
diagram by the dotted lines unequally prolonged upwards.

But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency
in the improved descendants of any one species to supplant and
exterminate in each stage of descent their predecessors and their
original progenitor. For it should be remembered that the competition
will generally be most severe between those forms which are most nearly
related to each other in habits, constitution and structure. Hence all
the intermediate forms between the earlier and later states, that is
between the less and more improved states of a the same species, as
well as the original parent-species itself, will generally tend to
become extinct. So it probably will be with many whole collateral lines
of descent, which will be conquered by later and improved lines. If,
however, the modified offspring of a species get into some distinct
country, or become quickly adapted to some quite new station, in which
offspring and progenitor do not come into competition, both may
continue to exist.

If, then, our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have
become extinct, being replaced by eight new species (_a_14 to _m_14);
and species (I) will be replaced by six (_n_14 to _z_14) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally
the case in nature; species (A) being more nearly related to B, C, and
D than to the other species; and species (I) more to G, H, K, L, than
to the others. These two species (A and I), were also supposed to be
very common and widely diffused species, so that they must originally
have had some advantage over most of the other species of the genus.
Their modified descendants, fourteen in number at the
fourteen-thousandth generation, will probably have inherited some of
the same advantages: they have also been modified and improved in a
diversified manner at each stage of descent, so as to have become
adapted to many related places in the natural economy of their country.
It seems, therefore, extremely probable that they will have taken the
places of, and thus exterminated, not only their parents (A) and (I),
but likewise some of the original species which were most nearly
related to their parents. Hence very few of the original species will
have transmitted offspring to the fourteen-thousandth generation. We
may suppose that only one (F) of the two species (E and F) which were
least closely related to the other nine original species, has
transmitted descendants to this late stage of descent.

The new species in our diagram, descended from the original eleven
species, will now be fifteen in number. Owing to the divergent tendency
of natural selection, the extreme amount of difference in character
between species _a_14 and _z_14 will be much greater than that between
the most distinct of the original eleven species. The new species,
moreover, will be allied to each other in a widely different manner. Of
the eight descendants from (A) the three marked _a_14, _q_14, _p_14,
will be nearly related from having recently branched off from _a_10;
_b_14 and _f_14, from having diverged at an earlier period from _a_5,
will be in some degree distinct from the three first-named species; and
lastly, _o_14, _e_14, and _m_14, will be nearly related one to the
other, but, from having diverged at the first commencement of the
process of modification, will be widely different from the other five
species, and may constitute a sub-genus or a distinct genus.

The six descendants from (I) will form two sub-genera or genera. But as
the original species (I) differed largely from (A), standing nearly at
the extreme end of the original genus, the six descendants from (I)
will, owing to inheritance alone, differ considerably from the eight
descendants from (A); the two groups, moreover, are supposed to have
gone on diverging in different directions. The intermediate species,
also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, except (F), extinct, and
have left no descendants. Hence the six new species descended from (I),
and the eight descendants from (A), will have to be ranked as very
distinct genera, or even as distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by
descent with modification, from two or more species of the same genus.
And the two or more parent-species are supposed to be descended from
some one species of an earlier genus. In our diagram this is indicated
by the broken lines beneath the capital letters, converging in
sub-branches downwards towards a single point; this point represents a
species, the supposed progenitor of our several new sub-genera and
genera.

It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character,
but to have retained the form of (F), either unaltered or altered only
in a slight degree. In this case its affinities to the other fourteen
new species will be of a curious and circuitous nature. Being descended
from a form that stood between the parent-species (A) and (I), now
supposed to be extinct and unknown, it will be in some degree
intermediate in character between the two groups descended from these
two species. But as these two groups have gone on diverging in
character from the type of their parents, the new species (F14) will
not be directly intermediate between them, but rather between types of
the two groups; and every naturalist will be able to call such cases
before his mind.

In the diagram each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
more generations; it may also represent a section of the successive
strata of the earth’s crust including extinct remains. We shall, when
we come to our chapter on geology, have to refer again to this subject,
and I think we shall then see that the diagram throws light on the
affinities of extinct beings, which, though generally belonging to the
same orders, families, or genera, with those now living, yet are often,
in some degree, intermediate in character between existing groups; and
we can understand this fact, for the extinct species lived at various
remote epochs when the branching lines of descent had diverged less.

I see no reason to limit the process of modification, as now explained,
to the formation of genera alone. If, in the diagram, we suppose the
amount of change represented by each successive group of diverging
dotted lines to be great, the forms marked _a_14 to _p_14, those marked
_b_14 and _f_14, and those marked _o_14 to _m_14, will form three very
distinct genera. We shall also have two very distinct genera descended
from (I), differing widely from the descendants of (A). These two
groups of genera will thus form two distinct families, or orders,
according to the amount of divergent modification supposed to be
represented in the diagram. And the two new families, or orders, are
descended from two species of the original genus; and these are
supposed to be descended from some still more ancient and unknown form.

We have seen that in each country it is the species belonging to the
larger genera which oftenest present varieties or incipient species.
This, indeed, might have been expected; for as natural selection acts
through one form having some advantage over other forms in the struggle
for existence, it will chiefly act on those which already have some
advantage; and the largeness of any group shows that its species have
inherited from a common ancestor some advantage in common. Hence, the
struggle for the production of new and modified descendants will mainly
lie between the larger groups, which are all trying to increase in
number. One large group will slowly conquer another large group, reduce
its number, and thus lessen its chance of further variation and
improvement. Within the same large group, the later and more highly
perfected sub-groups, from branching out and seizing on many new places
in the polity of nature, will constantly tend to supplant and destroy
the earlier and less improved sub-groups. Small and broken groups and
sub-groups will finally disappear. Looking to the future, we can
predict that the groups of organic beings which are now large and
triumphant, and which are least broken up, that is, which have as yet
suffered least extinction, will, for a long period, continue to
increase. But which groups will ultimately prevail, no man can predict;
for we know that many groups, formerly most extensively developed, have
now become extinct. Looking still more remotely to the future, we may
predict that, owing to the continued and steady increase of the larger
groups, a multitude of smaller groups will become utterly extinct, and
leave no modified descendants; and consequently that, of the species
living at any one period, extremely few will transmit descendants to a
remote futurity. I shall have to return to this subject in the chapter
on classification, but I may add that as, according to this view,
extremely few of the more ancient species have transmitted descendants
to the present day, and, as all the descendants of the same species
form a class, we can understand how it is that there exist so few
classes in each main division of the animal and vegetable kingdoms.
Although few of the most ancient species have left modified
descendants, yet, at remote geological periods, the earth may have been
almost as well peopled with species of many genera, families, orders
and classes, as at the present day.

_On the Degree to which Organisation tends to advance._


Natural selection acts exclusively by the preservation and accumulation
of variations, which are beneficial under the organic and inorganic
conditions to which each creature is exposed at all periods of life.
The ultimate result is that each creature tends to become more and more
improved in relation to its conditions. This improvement inevitably
leads to the gradual advancement of the organisation of the greater
number of living beings throughout the world. But here we enter on a
very intricate subject, for naturalists have not defined to each
other’s satisfaction what is meant by an advance in organisation. Among
the vertebrata the degree of intellect and an approach in structure to
man clearly come into play. It might be thought that the amount of
change which the various parts and organs pass through in their
development from embryo to maturity would suffice as a standard of
comparison; but there are cases, as with certain parasitic crustaceans,
in which several parts of the structure become less perfect, so that
the mature animal cannot be called higher than its larva. Von Baer’s
standard seems the most widely applicable and the best, namely, the
amount of differentiation of the parts of the same organic being, in
the adult state, as I should be inclined to add, and their
specialisation for different functions; or, as Milne Edwards would
express it, the completeness of the division of physiological labour.
But we shall see how obscure this subject is if we look, for instance,
to fishes, among which some naturalists rank those as highest which,
like the sharks, approach nearest to amphibians; while other
naturalists rank the common bony or teleostean fishes as the highest,
inasmuch as they are most strictly fish-like, and differ most from the
other vertebrate classes. We see still more plainly the obscurity of
the subject by turning to plants, among which the standard of intellect
is of course quite excluded; and here some botanists rank those plants
as highest which have every organ, as sepals, petals, stamens and
pistils, fully developed in each flower; whereas other botanists,
probably with more truth, look at the plants which have their several
organs much modified and reduced in number as the highest.

If we take as the standard of high organisation, the amount of
differentiation and specialisation of the several organs in each being
when adult (and this will include the advancement of the brain for
intellectual purposes), natural selection clearly leads towards this
standard: for all physiologists admit that the specialisation of
organs, inasmuch as in this state they perform their functions better,
is an advantage to each being; and hence the accumulation of variations
tending towards specialisation is within the scope of natural
selection. On the other hand, we can see, bearing in mind that all
organic beings are striving to increase at a high ratio and to seize on
every unoccupied or less well occupied place in the economy of nature,
that it is quite possible for natural selection gradually to fit a
being to a situation in which several organs would be superfluous or
useless: in such cases there would be retrogression in the scale of
organisation. Whether organisation on the whole has actually advanced
from the remotest geological periods to the present day will be more
conveniently discussed in our chapter on Geological Succession.

But it may be objected that if all organic beings thus tend to rise in
the scale, how is it that throughout the world a multitude of the
lowest forms still exist; and how is it that in each great class some
forms are far more highly developed than others? Why have not the more
highly developed forms every where supplanted and exterminated the
lower? Lamarck, who believed in an innate and inevitable tendency
towards perfection in all organic beings, seems to have felt this
difficulty so strongly that he was led to suppose that new and simple
forms are continually being produced by spontaneous generation. Science
has not as yet proved the truth of this belief, whatever the future may
reveal. On our theory the continued existence of lowly organisms offers
no difficulty; for natural selection, or the survival of the fittest,
does not necessarily include progressive development—it only takes
advantage of such variations as arise and are beneficial to each
creature under its complex relations of life. And it may be asked what
advantage, as far as we can see, would it be to an infusorian
animalcule—to an intestinal worm—or even to an earth-worm, to be highly
organised. If it were no advantage, these forms would be left, by
natural selection, unimproved or but little improved, and might remain
for indefinite ages in their present lowly condition. And geology tells
us that some of the lowest forms, as the infusoria and rhizopods, have
remained for an enormous period in nearly their present state. But to
suppose that most of the many now existing low forms have not in the
least advanced since the first dawn of life would be extremely rash;
for every naturalist who has dissected some of the beings now ranked as
very low in the scale, must have been struck with their really wondrous
and beautiful organisation.

Nearly the same remarks are applicable, if we look to the different
grades of organisation within the same great group; for instance, in
the vertebrata, to the co-existence of mammals and fish—among mammalia,
to the co-existence of man and the ornithorhynchus—among fishes, to the
co-existence of the shark and the lancelet (Amphioxus), which latter
fish in the extreme simplicity of its structure approaches the
invertebrate classes. But mammals and fish hardly come into competition
with each other; the advancement of the whole class of mammals, or of
certain members in this class, to the highest grade would not lead to
their taking the place of fishes. Physiologists believe that the brain
must be bathed by warm blood to be highly active, and this requires
aërial respiration; so that warm-blooded mammals when inhabiting the
water lie under a disadvantage in having to come continually to the
surface to breathe. With fishes, members of the shark family would not
tend to supplant the lancelet; for the lancelet, as I hear from Fritz
Müller, has as sole companion and competitor on the barren sandy shore
of South Brazil, an anomalous annelid. The three lowest orders of
mammals, namely, marsupials, edentata, and rodents, co-exist in South
America in the same region with numerous monkeys, and probably
interfere little with each other. Although organisation, on the whole,
may have advanced and be still advancing throughout the world, yet the
scale will always present many degrees of perfection; for the high
advancement of certain whole classes, or of certain members of each
class, does not at all necessarily lead to the extinction of those
groups with which they do not enter into close competition. In some
cases, as we shall hereafter see, lowly organised forms appear to have
been preserved to the present day, from inhabiting confined or peculiar
stations, where they have been subjected to less severe competition,
and where their scanty numbers have retarded the chance of favourable
variations arising.

Finally, I believe that many lowly organised forms now exist throughout
the world, from various causes. In some cases variations or individual
differences of a favourable nature may never have arisen for natural
selection to act on and accumulate. In no case, probably, has time
sufficed for the utmost possible amount of development. In some few
cases there has been what we must call retrogression or organisation.
But the main cause lies in the fact that under very simple conditions
of life a high organisation would be of no service—possibly would be of
actual disservice, as being of a more delicate nature, and more liable
to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may
believe, presented the simplest structure, how, it has been asked,
could the first step in the advancement or differentiation of parts
have arisen? Mr. Herbert Spencer would probably answer that, as soon as
simple unicellular organisms came by growth or division to be
compounded of several cells, or became attached to any supporting
surface, his law “that homologous units of any order become
differentiated in proportion as their relations to incident forces
become different” would come into action. But as we have no facts to
guide us, speculation on the subject is almost useless. It is, however,
an error to suppose that there would be no struggle for existence, and,
consequently, no natural selection, until many forms had been produced:
variations in a single species inhabiting an isolated station might be
beneficial, and thus the whole mass of individuals might be modified,
or two distinct forms might arise. But, as I remarked towards the close
of the introduction, no one ought to feel surprise at much remaining as
yet unexplained on the origin of species, if we make due allowance for
our profound ignorance on the mutual relations of the inhabitants of
the world at the present time, and still more so during past ages.

_Convergence of Character._


Mr. H.C. Watson thinks that I have overrated the importance of
divergence of character (in which, however, he apparently believes),
and that convergence, as it may be called, has likewise played a part.
If two species belonging to two distinct though allied genera, had both
produced a large number of new and divergent forms, it is conceivable
that these might approach each other so closely that they would have
all to be classed under the same genus; and thus the descendants of two
distinct genera would converge into one. But it would in most cases be
extremely rash to attribute to convergence a close and general
similarity of structure in the modified descendants of widely distinct
forms. The shape of a crystal is determined solely by the molecular
forces, and it is not surprising that dissimilar substances should
sometimes assume the same form; but with organic beings we should bear
in mind that the form of each depends on an infinitude of complex
relations, namely on the variations which have arisen, these being due
to causes far too intricate to be followed out—on the nature of the
variations which have been preserved or selected, and this depends on
the surrounding physical conditions, and in a still higher degree on
the surrounding organisms with which each being has come into
competition—and lastly, on inheritance (in itself a fluctuating
element) from innumerable progenitors, all of which have had their
forms determined through equally complex relations. It is incredible
that the descendants of two organisms, which had originally differed in
a marked manner, should ever afterwards converge so closely as to lead
to a near approach to identity throughout their whole organisation. If
this had occurred, we should meet with the same form, independently of
genetic connection, recurring in widely separated geological
formations; and the balance of evidence is opposed to any such an
admission.

Mr. Watson has also objected that the continued action of natural
selection, together with divergence of character, would tend to make an
indefinite number of specific forms. As far as mere inorganic
conditions are concerned, it seems probable that a sufficient number of
species would soon become adapted to all considerable diversities of
heat, moisture, &c.; but I fully admit that the mutual relations of
organic beings are more important; and as the number of species in any
country goes on increasing, the organic conditions of life must become
more and more complex. Consequently there seems at first no limit to
the amount of profitable diversification of structure, and therefore no
limit to the number of species which might be produced. We do not know
that even the most prolific area is fully stocked with specific forms:
at the Cape of Good Hope and in Australia, which support such an
astonishing number of species, many European plants have become
naturalised. But geology shows us, that from an early part of the
tertiary period the number of species of shells, and that from the
middle part of this same period, the number of mammals has not greatly
or at all increased. What then checks an indefinite increase in the
number of species? The amount of life (I do not mean the number of
specific forms) supported on an area must have a limit, depending so
largely as it does on physical conditions; therefore, if an area be
inhabited by very many species, each or nearly each species will be
represented by few individuals; and such species will be liable to
extermination from accidental fluctuations in the nature of the seasons
or in the number of their enemies. The process of extermination in such
cases would be rapid, whereas the production of new species must always
be slow. Imagine the extreme case of as many species as individuals in
England, and the first severe winter or very dry summer would
exterminate thousands on thousands of species. Rare species, and each
species will become rare if the number of species in any country
becomes indefinitely increased, will, on the principal often explained,
present within a given period few favourable variations; consequently,
the process of giving birth to new specific forms would thus be
retarded. When any species becomes very rare, close interbreeding will
help to exterminate it; authors have thought that this comes into play
in accounting for the deterioration of the aurochs in Lithuania, of red
deer in Scotland and of bears in Norway, &c. Lastly, and this I am
inclined to think is the most important element, a dominant species,
which has already beaten many competitors in its own home, will tend to
spread and supplant many others. Alph. de Candolle has shown that those
species which spread widely tend generally to spread _very_ widely,
consequently they will tend to supplant and exterminate several species
in several areas, and thus check the inordinate increase of specific
forms throughout the world. Dr. Hooker has recently shown that in the
southeast corner of Australia, where, apparently, there are many
invaders from different quarters of the globe, the endemic Australian
species have been greatly reduced in number. How much weight to
attribute to these several considerations I will not pretend to say;
but conjointly they must limit in each country the tendency to an
indefinite augmentation of specific forms.

_Summary of Chapter._


If under changing conditions of life organic beings present individual
differences in almost every part of their structure, and this cannot be
disputed; if there be, owing to their geometrical rate of increase, a
severe struggle for life at some age, season or year, and this
certainly cannot be disputed; then, considering the infinite complexity
of the relations of all organic beings to each other and to their
conditions of life, causing an infinite diversity in structure,
constitution, and habits, to be advantageous to them, it would be a
most extraordinary fact if no variations had ever occurred useful to
each being’s own welfare, in the same manner as so many variations have
occurred useful to man. But if variations useful to any organic being
ever do occur, assuredly individuals thus characterised will have the
best chance of being preserved in the struggle for life; and from the
strong principle of inheritance, these will tend to produce offspring
similarly characterised. This principle of preservation, or the
survival of the fittest, I have called Natural Selection. It leads to
the improvement of each creature in relation to its organic and
inorganic conditions of life; and consequently, in most cases, to what
must be regarded as an advance in organisation. Nevertheless, low and
simple forms will long endure if well fitted for their simple
conditions of life.

Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young as easily as the
adult. Among many animals sexual selection will have given its aid to
ordinary selection by assuring to the most vigorous and best adapted
males the greatest number of offspring. Sexual selection will also give
characters useful to the males alone in their struggles or rivalry with
other males; and these characters will be transmitted to one sex or to
both sexes, according to the form of inheritance which prevails.

Whether natural selection has really thus acted in adapting the various
forms of life to their several conditions and stations, must be judged
by the general tenour and balance of evidence given in the following
chapters. But we have already seen how it entails extinction; and how
largely extinction has acted in the world’s history, geology plainly
declares. Natural selection, also, leads to divergence of character;
for the more organic beings diverge in structure, habits and
constitution, by so much the more can a large number be supported on
the area, of which we see proof by looking to the inhabitants of any
small spot, and to the productions naturalised in foreign lands.
Therefore, during the modification of the descendants of any one
species, and during the incessant struggle of all species to increase
in numbers, the more diversified the descendants become, the better
will be their chance of success in the battle for life. Thus the small
differences distinguishing varieties of the same species, steadily tend
to increase, till they equal the greater differences between species of
the same genus, or even of distinct genera.

We have seen that it is the common, the widely diffused, and widely
ranging species, belonging to the larger genera within each class,
which vary most; and these tend to transmit to their modified offspring
that superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, the nature of the affinities, and
the generally well defined distinctions between the innumerable organic
beings in each class throughout the world, may be explained. It is a
truly wonderful fact—the wonder of which we are apt to overlook from
familiarity—that all animals and all plants throughout all time and
space should be related to each other in groups, subordinate to groups,
in the manner which we everywhere behold—namely, varieties of the same
species most closely related, species of the same genus less closely
and unequally related, forming sections and sub-genera, species of
distinct genera much less closely related, and genera related in
different degrees, forming sub-families, families, orders, sub-classes,
and classes. The several subordinate groups in any class cannot be
ranked in a single file, but seem clustered round points, and these
round other points, and so on in almost endless cycles. If species had
been independently created, no explanation would have been possible of
this kind of classification; but it is explained through inheritance
and the complex action of natural selection, entailing extinction and
divergence of character, as we have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during former years may represent the long succession of
extinct species. At each period of growth all the growing twigs have
tried to branch out on all sides, and to overtop and kill the
surrounding twigs and branches, in the same manner as species and
groups of species have at all times overmastered other species in the
great battle for life. The limbs divided into great branches, and these
into lesser and lesser branches, were themselves once, when the tree
was young, budding twigs; and this connexion of the former and present
buds by ramifying branches may well represent the classification of all
extinct and living species in groups subordinate to groups. Of the many
twigs which flourished when the tree was a mere bush, only two or
three, now grown into great branches, yet survive and bear the other
branches; so with the species which lived during long-past geological
periods, very few have left living and modified descendants. From the
first growth of the tree, many a limb and branch has decayed and
dropped off; and these fallen branches of various sizes may represent
those whole orders, families, and genera which have now no living
representatives, and which are known to us only in a fossil state. As
we here and there see a thin, straggling branch springing from a fork
low down in a tree, and which by some chance has been favoured and is
still alive on its summit, so we occasionally see an animal like the
Ornithorhynchus or Lepidosiren, which in some small degree connects by
its affinities two large branches of life, and which has apparently
been saved from fatal competition by having inhabited a protected
station. As buds give rise by growth to fresh buds, and these, if
vigorous, branch out and overtop on all sides many a feebler branch, so
by generation I believe it has been with the great Tree of Life, which
fills with its dead and broken branches the crust of the earth, and
covers the surface with its ever-branching and beautiful ramifications.




CHAPTER V.
LAWS OF VARIATION.


Effects of changed conditions—Use and disuse, combined with natural
selection; organs of flight and of vision—Acclimatisation—Correlated
variation—Compensation and economy of growth—False
correlations—Multiple, rudimentary, and lowly organised structures
variable—Parts developed in an unusual manner are highly variable:
specific characters more variable than generic: secondary sexual
characters variable—Species of the same genus vary in an analogous
manner—Reversions to long-lost characters—Summary.


I have hitherto sometimes spoken as if the variations—so common and
multiform with organic beings under domestication, and in a lesser
degree with those under nature—were due to chance. This, of course is a
wholly incorrect expression, but it serves to acknowledge plainly our
ignorance of the cause of each particular variation. Some authors
believe it to be as much the function of the reproductive system to
produce individual differences, or slight deviations of structure, as
to make the child like its parents. But the fact of variations and
monstrosities occurring much more frequently under domestication than
under nature, and the greater variability of species having wide ranges
than of those with restricted ranges, lead to the conclusion that
variability is generally related to the conditions of life to which
each species has been exposed during several successive generations. In
the first chapter I attempted to show that changed conditions act in
two ways, directly on the whole organisation or on certain parts alone,
and indirectly through the reproductive system. In all cases there are
two factors, the nature of the organism, which is much the most
important of the two, and the nature of the conditions. The direct
action of changed conditions leads to definite or indefinite results.
In the latter case the organisation seems to become plastic, and we
have much fluctuating variability. In the former case the nature of the
organism is such that it yields readily, when subjected to certain
conditions, and all, or nearly all, the individuals become modified in
the same way.

It is very difficult to decide how far changed conditions, such as of
climate, food, &c., have acted in a definite manner. There is reason to
believe that in the course of time the effects have been greater than
can be proved by clear evidence. But we may safely conclude that the
innumerable complex co-adaptations of structure, which we see
throughout nature between various organic beings, cannot be attributed
simply to such action. In the following cases the conditions seem to
have produced some slight definite effect: E. Forbes asserts that
shells at their southern limit, and when living in shallow water, are
more brightly coloured than those of the same species from further
north or from a greater depth; but this certainly does not always hold
good. Mr. Gould believes that birds of the same species are more
brightly coloured under a clear atmosphere, than when living near the
coast or on islands; and Wollaston is convinced that residence near the
sea affects the colours of insects. Moquin-Tandon gives a list of
plants which, when growing near the sea-shore, have their leaves in
some degree fleshy, though not elsewhere fleshy. These slightly varying
organisms are interesting in as far as they present characters
analogous to those possessed by the species which are confined to
similar conditions.

When a variation is of the slightest use to any being, we cannot tell
how much to attribute to the accumulative action of natural selection,
and how much to the definite action of the conditions of life. Thus, it
is well known to furriers that animals of the same species have thicker
and better fur the further north they live; but who can tell how much
of this difference may be due to the warmest-clad individuals having
been favoured and preserved during many generations, and how much to
the action of the severe climate? For it would appear that climate has
some direct action on the hair of our domestic quadrupeds.

Instances could be given of similar varieties being produced from the
same species under external conditions of life as different as can well
be conceived; and, on the other hand, of dissimilar varieties being
produced under apparently the same external conditions. Again,
innumerable instances are known to every naturalist, of species keeping
true, or not varying at all, although living under the most opposite
climates. Such considerations as these incline me to lay less weight on
the direct action of the surrounding conditions, than on a tendency to
vary, due to causes of which we are quite ignorant.

In one sense the conditions of life may be said, not only to cause
variability, either directly or indirectly, but likewise to include
natural selection, for the conditions determine whether this or that
variety shall survive. But when man is the selecting agent, we clearly
see that the two elements of change are distinct; variability is in
some manner excited, but it is the will of man which accumulates the
variations in certain direction; and it is this latter agency which
answers to the survival of the fittest under nature.

_Effects of the increased Use and Disuse of Parts, as controlled by
Natural Selection._


From the facts alluded to in the first chapter, I think there can be no
doubt that use in our domestic animals has strengthened and enlarged
certain parts, and disuse diminished them; and that such modifications
are inherited. Under free nature we have no standard of comparison by
which to judge of the effects of long-continued use or disuse, for we
know not the parent-forms; but many animals possess structures which
can be best explained by the effects of disuse. As Professor Owen has
remarked, there is no greater anomaly in nature than a bird that cannot
fly; yet there are several in this state. The logger-headed duck of
South America can only flap along the surface of the water, and has its
wings in nearly the same condition as the domestic Aylesbury duck: it
is a remarkable fact that the young birds, according to Mr. Cunningham,
can fly, while the adults have lost this power. As the larger
ground-feeding birds seldom take flight except to escape danger, it is
probable that the nearly wingless condition of several birds, now
inhabiting or which lately inhabited several oceanic islands, tenanted
by no beasts of prey, has been caused by disuse. The ostrich indeed
inhabits continents, and is exposed to danger from which it cannot
escape by flight, but it can defend itself, by kicking its enemies, as
efficiently as many quadrupeds. We may believe that the progenitor of
the ostrich genus had habits like those of the bustard, and that, as
the size and weight of its body were increased during successive
generations, its legs were used more and its wings less, until they
became incapable of flight.

Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are often
broken off; he examined seventeen specimens in his own collection, and
not one had even a relic left. In the Onites apelles the tarsi are so
habitually lost that the insect has been described as not having them.
In some other genera they are present, but in a rudimentary condition.
In the Ateuchus or sacred beetle of the Egyptians, they are totally
deficient. The evidence that accidental mutilations can be inherited is
at present not decisive; but the remarkable cases observed by
Brown-Sequard in guinea-pigs, of the inherited effects of operations,
should make us cautious in denying this tendency. Hence, it will
perhaps be safest to look at the entire absence of the anterior tarsi
in Ateuchus, and their rudimentary condition in some other genera, not
as cases of inherited mutilations, but as due to the effects of
long-continued disuse; for as many dung-feeding beetles are generally
found with their tarsi lost, this must happen early in life; therefore
the tarsi cannot be of much importance or be much used by these
insects.

In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species (but more are now known) inhabiting Madeira, are so far
deficient in wings that they cannot fly; and that, of the twenty-nine
endemic genera, no less than twenty-three have all their species in
this condition! Several facts, namely, that beetles in many parts of
the world are very frequently blown to sea and perish; that the beetles
in Madeira, as observed by Mr. Wollaston, lie much concealed, until the
wind lulls and the sun shines; that the proportion of wingless beetles
is larger on the exposed Desertas than in Madeira itself; and
especially the extraordinary fact, so strongly insisted on by Mr.
Wollaston, that certain large groups of beetles, elsewhere excessively
numerous, which absolutely require the use of their wings, are here
almost entirely absent. These several considerations make me believe
that the wingless condition of so many Madeira beetles is mainly due to
the action of natural selection, combined probably with disuse. For
during many successive generations each individual beetle which flew
least, either from its wings having been ever so little less perfectly
developed or from indolent habit, will have had the best chance of
surviving from not being blown out to sea; and, on the other hand,
those beetles which most readily took to flight would oftenest have
been blown to sea, and thus destroyed.

The insects in Madeira which are not ground-feeders, and which, as
certain flower-feeding coleoptera and lepidoptera, must habitually use
their wings to gain their subsistence, have, as Mr. Wollaston suspects,
their wings not at all reduced, but even enlarged. This is quite
compatible with the action of natural selection. For when a new insect
first arrived on the island, the tendency of natural selection to
enlarge or to reduce the wings, would depend on whether a greater
number of individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never flying. As with
mariners shipwrecked near a coast, it would have been better for the
good swimmers if they had been able to swim still further, whereas it
would have been better for the bad swimmers if they had not been able
to swim at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered by skin and fur. This state
of the eyes is probably due to gradual reduction from disuse, but aided
perhaps by natural selection. In South America, a burrowing rodent, the
tuco-tuco, or Ctenomys, is even more subterranean in its habits than
the mole; and I was assured by a Spaniard, who had often caught them,
that they were frequently blind. One which I kept alive was certainly
in this condition, the cause, as appeared on dissection, having been
inflammation of the nictitating membrane. As frequent inflammation of
the eyes must be injurious to any animal, and as eyes are certainly not
necessary to animals having subterranean habits, a reduction in their
size, with the adhesion of the eyelids and growth of fur over them,
might in such case be an advantage; and if so, natural selection would
aid the effects of disuse.

It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Carniola and Kentucky, are blind.
In some of the crabs the foot-stalk for the eye remains, though the eye
is gone; the stand for the telescope is there, though the telescope
with its glasses has been lost. As it is difficult to imagine that
eyes, though useless, could be in any way injurious to animals living
in darkness, their loss may be attributed to disuse. In one of the
blind animals, namely, the cave-rat (Neotoma), two of which were
captured by Professor Silliman at above half a mile distance from the
mouth of the cave, and therefore not in the profoundest depths, the
eyes were lustrous and of large size; and these animals, as I am
informed by Professor Silliman, after having been exposed for about a
month to a graduated light, acquired a dim perception of objects.

It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that, in
accordance with the old view of the blind animals having been
separately created for the American and European caverns, very close
similarity in their organisation and affinities might have been
expected. This is certainly not the case if we look at the two whole
faunas; with respect to the insects alone, Schiödte has remarked: “We
are accordingly prevented from considering the entire phenomenon in any
other light than something purely local, and the similarity which is
exhibited in a few forms between the Mammoth Cave (in Kentucky) and the
caves in Carniola, otherwise than as a very plain expression of that
analogy which subsists generally between the fauna of Europe and of
North America.” On my view we must suppose that American animals,
having in most cases ordinary powers of vision, slowly migrated by
successive generations from the outer world into the deeper and deeper
recesses of the Kentucky caves, as did European animals into the caves
of Europe. We have some evidence of this gradation of habit; for, as
Schiödte remarks: “We accordingly look upon the subterranean faunas as
small ramifications which have penetrated into the earth from the
geographically limited faunas of the adjacent tracts, and which, as
they extended themselves into darkness, have been accommodated to
surrounding circumstances. Animals not far remote from ordinary forms,
prepare the transition from light to darkness. Next follow those that
are constructed for twilight; and, last of all, those destined for
total darkness, and whose formation is quite peculiar.” These remarks
of Schiödte’s it should be understood, apply not to the same, but to
distinct species. By the time that an animal had reached, after
numberless generations, the deepest recesses, disuse will on this view
have more or less perfectly obliterated its eyes, and natural selection
will often have effected other changes, such as an increase in the
length of the antennæ or palpi, as a compensation for blindness.
Notwithstanding such modifications, we might expect still to see in the
cave-animals of America, affinities to the other inhabitants of that
continent, and in those of Europe to the inhabitants of the European
continent. And this is the case with some of the American cave-animals,
as I hear from Professor Dana; and some of the European cave-insects
are very closely allied to those of the surrounding country. It would
be difficult to give any rational explanation of the affinities of the
blind cave-animals to the other inhabitants of the two continents on
the ordinary view of their independent creation. That several of the
inhabitants of the caves of the Old and New Worlds should be closely
related, we might expect from the well-known relationship of most of
their other productions. As a blind species of Bathyscia is found in
abundance on shady rocks far from caves, the loss of vision in the cave
species of this one genus has probably had no relation to its dark
habitation; for it is natural that an insect already deprived of vision
should readily become adapted to dark caverns. Another blind genus
(Anophthalmus) offers this remarkable peculiarity, that the species, as
Mr. Murray observes, have not as yet been found anywhere except in
caves; yet those which inhabit the several caves of Europe and America
are distinct; but it is possible that the progenitors of these several
species, while they were furnished with eyes, may formerly have ranged
over both continents, and then have become extinct, excepting in their
present secluded abodes. Far from feeling surprise that some of the
cave-animals should be very anomalous, as Agassiz has remarked in
regard to the blind fish, the Amblyopsis, and as is the case with the
blind Proteus, with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the scanty inhabitants of
these dark abodes will have been exposed.

_Acclimatisation._


Habit is hereditary with plants, as in the period of flowering, in the
time of sleep, in the amount of rain requisite for seeds to germinate,
&c., and this leads me to say a few words on acclimatisation. As it is
extremely common for distinct species belonging to the same genus to
inhabit hot and cold countries, if it be true that all the species of
the same genus are descended from a single parent-form, acclimatisation
must be readily effected during a long course of descent. It is
notorious that each species is adapted to the climate of its own home:
species from an arctic or even from a temperate region cannot endure a
tropical climate, or conversely. So again, many succulent plants cannot
endure a damp climate. But the degree of adaptation of species to the
climates under which they live is often overrated. We may infer this
from our frequent inability to predict whether or not an imported plant
will endure our climate, and from the number of plants and animals
brought from different countries which are here perfectly healthy. We
have reason to believe that species in a state of nature are closely
limited in their ranges by the competition of other organic beings
quite as much as, or more than, by adaptation to particular climates.
But whether or not this adaptation is in most cases very close, we have
evidence with some few plants, of their becoming, to a certain extent,
naturally habituated to different temperatures; that is, they become
acclimatised: thus the pines and rhododendrons, raised from seed
collected by Dr. Hooker from the same species growing at different
heights on the Himalayas, were found to possess in this country
different constitutional powers of resisting cold. Mr. Thwaites informs
me that he has observed similar facts in Ceylon; analogous observations
have been made by Mr. H.C. Watson on European species of plants brought
from the Azores to England; and I could give other cases. In regard to
animals, several authentic instances could be adduced of species having
largely extended, within historical times, their range from warmer to
colder latitudes, and conversely; but we do not positively know that
these animals were strictly adapted to their native climate, though in
all ordinary cases we assume such to be the case; nor do we know that
they have subsequently become specially acclimatised to their new
homes, so as to be better fitted for them than they were at first.

As we may infer that our domestic animals were originally chosen by
uncivilised man because they were useful, and because they bred readily
under confinement, and not because they were subsequently found capable
of far-extended transportation, the common and extraordinary capacity
in our domestic animals of not only withstanding the most different
climates, but of being perfectly fertile (a far severer test) under
them, may be used as an argument that a large proportion of other
animals now in a state of nature could easily be brought to bear widely
different climates. We must not, however, push the foregoing argument
too far, on account of the probable origin of some of our domestic
animals from several wild stocks: the blood, for instance, of a
tropical and arctic wolf may perhaps be mingled in our domestic breeds.
The rat and mouse cannot be considered as domestic animals, but they
have been transported by man to many parts of the world, and now have a
far wider range than any other rodent; for they live under the cold
climate of Faroe in the north and of the Falklands in the south, and on
many an island in the torrid zones. Hence adaptation to any special
climate may be looked at as a quality readily grafted on an innate wide
flexibility of constitution, common to most animals. On this view, the
capacity of enduring the most different climates by man himself and by
his domestic animals, and the fact of the extinct elephant and
rhinoceros having formerly endured a glacial climate, whereas the
living species are now all tropical or sub-tropical in their habits,
ought not to be looked at as anomalies, but as examples of a very
common flexibility of constitution, brought, under peculiar
circumstances, into action.

How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is an obscure question. That habit or custom has some
influence, I must believe, both from analogy and from the incessant
advice given in agricultural works, even in the ancient Encyclopædias
of China, to be very cautious in transporting animals from one district
to another. And as it is not likely that man should have succeeded in
selecting so many breeds and sub-breeds with constitutions specially
fitted for their own districts, the result must, I think, be due to
habit. On the other hand, natural selection would inevitably tend to
preserve those individuals which were born with constitutions best
adapted to any country which they inhabited. In treatises on many kinds
of cultivated plants, certain varieties are said to withstand certain
climates better than others; this is strikingly shown in works on
fruit-trees published in the United States, in which certain varieties
are habitually recommended for the northern and others for the southern
states; and as most of these varieties are of recent origin, they
cannot owe their constitutional differences to habit. The case of the
Jerusalem artichoke, which is never propagated in England by seed, and
of which, consequently, new varieties have not been produced, has even
been advanced, as proving that acclimatisation cannot be effected, for
it is now as tender as ever it was! The case, also, of the kidney-bean
has been often cited for a similar purpose, and with much greater
weight; but until some one will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by
frost, and then collect seed from the few survivors, with care to
prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to
have been even tried. Nor let it be supposed that differences in the
constitution of seedling kidney-beans never appear, for an account has
been published how much more hardy some seedlings are than others; and
of this fact I have myself observed striking instances.

On the whole, we may conclude that habit, or use and disuse, have, in
some cases, played a considerable part in the modification of the
constitution and structure; but that the effects have often been
largely combined with, and sometimes overmastered by, the natural
selection of innate variations.

_Correlated Variation_


I mean by this expression that the whole organisation is so tied
together, during its growth and development, that when slight
variations in any one part occur and are accumulated through natural
selection, other parts become modified. This is a very important
subject, most imperfectly understood, and no doubt wholly different
classes of facts may be here easily confounded together. We shall
presently see that simple inheritance often gives the false appearance
of correlation. One of the most obvious real cases is, that variations
of structure arising in the young or larvæ naturally tend to affect the
structure of the mature animal. The several parts which are homologous,
and which, at an early embryonic period, are identical in structure,
and which are necessarily exposed to similar conditions, seem eminently
liable to vary in a like manner: we see this in the right and left
sides of the body varying in the same manner; in the front and hind
legs, and even in the jaws and limbs, varying together, for the lower
jaw is believed by some anatomists to be homologous with the limbs.
These tendencies, I do not doubt, may be mastered more or less
completely by natural selection: thus a family of stags once existed
with an antler only on one side; and if this had been of any great use
to the breed, it might probably have been rendered permanent by natural
selection.

Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants: and nothing is more common than
the union of homologous parts in normal structures, as in the union of
the petals into a tube. Hard parts seem to affect the form of adjoining
soft parts; it is believed by some authors that with birds the
diversity in the shape of the pelvis causes the remarkable diversity in
the shape of the kidneys. Others believe that the shape of the pelvis
in the human mother influences by pressure the shape of the head of the
child. In snakes, according to Schlegel, the shape of the body and the
manner of swallowing determine the position and form of several of the
most important viscera.

The nature of the bond is frequently quite obscure. M. Is. Geoffroy St.
Hilaire has forcibly remarked that certain malconformations frequently,
and that others rarely, coexist without our being able to assign any
reason. What can be more singular than the relation in cats between
complete whiteness and blue eyes with deafness, or between the
tortoise-shell colour and the female sex; or in pigeons, between their
feathered feet and skin betwixt the outer toes, or between the presence
of more or less down on the young pigeon when first hatched, with the
future colour of its plumage; or, again, the relation between the hair
and the teeth in the naked Turkish dog, though here no doubt homology
comes into play? With respect to this latter case of correlation, I
think it can hardly be accidental that the two orders of mammals which
are most abnormal in their dermal covering, viz., Cetacea (whales) and
Edentata (armadilloes, scaly ant-eaters, &c.), are likewise on the
whole the most abnormal in their teeth, but there are so many
exceptions to this rule, as Mr. Mivart has remarked, that it has little
value.

I know of no case better adapted to show the importance of the laws of
correlation and variation, independently of utility, and therefore of
natural selection, than that of the difference between the outer and
inner flowers in some Compositous and Umbelliferous plants. Everyone is
familiar with the difference between the ray and central florets of,
for instance, the daisy, and this difference is often accompanied with
the partial or complete abortion of the reproductive organs. But in
some of these plants the seeds also differ in shape and sculpture.
These differences have sometimes been attributed to the pressure of the
involucra on the florets, or to their mutual pressure, and the shape of
the seeds in the ray-florets of some Compositæ countenances this idea;
but with the Umbelliferæ it is by no means, as Dr. Hooker informs me,
the species with the densest heads which most frequently differ in
their inner and outer flowers. It might have been thought that the
development of the ray-petals, by drawing nourishment from the
reproductive organs causes their abortion; but this can hardly be the
sole case, for in some Compositæ the seeds of the outer and inner
florets differ, without any difference in the corolla. Possibly these
several differences may be connected with the different flow of
nutriment towards the central and external flowers. We know, at least,
that with irregular flowers those nearest to the axis are most subject
to peloria, that is to become abnormally symmetrical. I may add, as an
instance of this fact, and as a striking case of correlation, that in
many pelargoniums the two upper petals in the central flower of the
truss often lose their patches of darker colour; and when this occurs,
the adherent nectary is quite aborted, the central flower thus becoming
peloric or regular. When the colour is absent from only one of the two
upper petals, the nectary is not quite aborted but is much shortened.

With respect to the development of the corolla, Sprengel’s idea that
the ray-florets serve to attract insects, whose agency is highly
advantageous, or necessary for the fertilisation of these plants, is
highly probable; and if so, natural selection may have come into play.
But with respect to the seeds, it seems impossible that their
differences in shape, which are not always correlated with any
difference in the corolla, can be in any way beneficial; yet in the
Umbelliferæ these differences are of such apparent importance—the seeds
being sometimes orthospermous in the exterior flowers and cœlospermous
in the central flowers—that the elder De Candolle founded his main
divisions in the order on such characters. Hence modifications of
structure, viewed by systematists as of high value, may be wholly due
to the laws of variation and correlation, without being, as far as we
can judge, of the slightest service to the species.

We may often falsely attribute to correlated variation structures which
are common to whole groups of species, and which in truth are simply
due to inheritance; for an ancient progenitor may have acquired through
natural selection some one modification in structure, and, after
thousands of generations, some other and independent modification; and
these two modifications, having been transmitted to a whole group of
descendants with diverse habits, would naturally be thought to be in
some necessary manner correlated. Some other correlations are
apparently due to the manner in which natural selection can alone act.
For instance, Alph. De Candolle has remarked that winged seeds are
never found in fruits which do not open; I should explain this rule by
the impossibility of seeds gradually becoming winged through natural
selection, unless the capsules were open; for in this case alone could
the seeds, which were a little better adapted to be wafted by the wind,
gain an advantage over others less well fitted for wide dispersal.

_Compensation and Economy of Growth._


The elder Geoffroy and Goethe propounded, at about the same time, their
law of compensation or balancement of growth; or, as Goethe expressed
it, “in order to spend on one side, nature is forced to economise on
the other side.” I think this holds true to a certain extent with our
domestic productions: if nourishment flows to one part or organ in
excess, it rarely flows, at least in excess, to another part; thus it
is difficult to get a cow to give much milk and to fatten readily. The
same varieties of the cabbage do not yield abundant and nutritious
foliage and a copious supply of oil-bearing seeds. When the seeds in
our fruits become atrophied, the fruit itself gains largely in size and
quality. In our poultry, a large tuft of feathers on the head is
generally accompanied by a diminished comb, and a large beard by
diminished wattles. With species in a state of nature it can hardly be
maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not,
however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoining
part being reduced by the same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the
excess of growth in another and adjoining part.

I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying
to economise in every part of the organisation. If under changed
conditions of life a structure, before useful, becomes less useful, its
diminution will be favoured, for it will profit the individual not to
have its nutriment wasted in building up a useless structure. I can
thus only understand a fact with which I was much struck when examining
cirripedes, and of which many other instances could be given: namely,
that when a cirripede is parasitic within another cirripede and is thus
protected, it loses more or less completely its own shell or carapace.
This is the case with the male Ibla, and in a truly extraordinary
manner with the Proteolepas: for the carapace in all other cirripedes
consists of the three highly important anterior segments of the head
enormously developed, and furnished with great nerves and muscles; but
in the parasitic and protected Proteolepas, the whole anterior part of
the head is reduced to the merest rudiment attached to the bases of the
prehensile antennæ. Now the saving of a large and complex structure,
when rendered superfluous, would be a decided advantage to each
successive individual of the species; for in the struggle for life to
which every animal is exposed, each would have a better chance of
supporting itself, by less nutriment being wasted.

Thus, as I believe, natural selection will tend in the long run to
reduce any part of the organisation, as soon as it becomes, through
changed habits, superfluous, without by any means causing some other
part to be largely developed in a corresponding degree. And conversely,
that natural selection may perfectly well succeed in largely developing
an organ without requiring as a necessary compensation the reduction of
some adjoining part.

_Multiple, Rudimentary, and Lowly-organised Structures are Variable._


It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
with varieties and species, that when any part or organ is repeated
many times in the same individual (as the vertebræ in snakes, and the
stamens in polyandrous flowers) the number is variable; whereas the
number of the same part or organ, when it occurs in lesser numbers, is
constant. The same author as well as some botanists, have further
remarked that multiple parts are extremely liable to vary in structure.
As “vegetative repetition,” to use Professor Owen’s expression, is a
sign of low organisation; the foregoing statements accord with the
common opinion of naturalists, that beings which stand low in the scale
of nature are more variable than those which are higher. I presume that
lowness here means that the several parts of the organisation have been
but little specialised for particular functions; and as long as the
same part has to perform diversified work, we can perhaps see why it
should remain variable, that is, why natural selection should not have
preserved or rejected each little deviation of form so carefully as
when the part has to serve for some one special purpose. In the same
way that a knife which has to cut all sorts of things may be of almost
any shape; whilst a tool for some particular purpose must be of some
particular shape. Natural selection, it should never be forgotten, can
act solely through and for the advantage of each being.

Rudimentary parts, as is generally admitted, are apt to be highly
variable. We shall have to recur to this subject; and I will here only
add that their variability seems to result from their uselessness, and
consequently from natural selection having had no power to check
deviations in their structure.

_A Part developed in any Species in an extraordinary degree or manner,
in comparison with the same part in allied Species, tends to be highly
variable._


Several years ago I was much struck by a remark to the above effect
made by Mr. Waterhouse. Professor Owen, also, seems to have come to a
nearly similar conclusion. It is hopeless to attempt to convince any
one of the truth of the above proposition without giving the long array
of facts which I have collected, and which cannot possibly be here
introduced. I can only state my conviction that it is a rule of high
generality. I am aware of several causes of error, but I hope that I
have made due allowances for them. It should be understood that the
rule by no means applies to any part, however unusually developed,
unless it be unusually developed in one species or in a few species in
comparison with the same part in many closely allied species. Thus, the
wing of the bat is a most abnormal structure in the class of mammals;
but the rule would not apply here, because the whole group of bats
possesses wings; it would apply only if some one species had wings
developed in a remarkable manner in comparison with the other species
of the same genus. The rule applies very strongly in the case of
secondary sexual characters, when displayed in any unusual manner. The
term, secondary sexual characters, used by Hunter, relates to
characters which are attached to one sex, but are not directly
connected with the act of reproduction. The rule applies to males and
females; but more rarely to females, as they seldom offer remarkable
secondary sexual characters. The rule being so plainly applicable in
the case of secondary sexual characters, may be due to the great
variability of these characters, whether or not displayed in any
unusual manner—of which fact I think there can be little doubt. But
that our rule is not confined to secondary sexual characters is clearly
shown in the case of hermaphrodite cirripedes; I particularly attended
to Mr. Waterhouse’s remark, whilst investigating this Order, and I am
fully convinced that the rule almost always holds good. I shall, in a
future work, give a list of all the more remarkable cases. I will here
give only one, as it illustrates the rule in its largest application.
The opercular valves of sessile cirripedes (rock barnacles) are, in
every sense of the word, very important structures, and they differ
extremely little even in distinct genera; but in the several species of
one genus, Pyrgoma, these valves present a marvellous amount of
diversification; the homologous valves in the different species being
sometimes wholly unlike in shape; and the amount of variation in the
individuals of the same species is so great that it is no exaggeration
to state that the varieties of the same species differ more from each
other in the characters derived from these important organs, than do
the species belonging to other distinct genera.

As with birds the individuals of the same species, inhabiting the same
country, vary extremely little, I have particularly attended to them;
and the rule certainly seems to hold good in this class. I cannot make
out that it applies to plants, and this would have seriously shaken my
belief in its truth, had not the great variability in plants made it
particularly difficult to compare their relative degrees of
variability.

When we see any part or organ developed in a remarkable degree or
manner in a species, the fair presumption is that it is of high
importance to that species: nevertheless it is in this case eminently
liable to variation. Why should this be so? On the view that each
species has been independently created, with all its parts as we now
see them, I can see no explanation. But on the view that groups of
species are descended from some other species, and have been modified
through natural selection, I think we can obtain some light. First let
me make some preliminary remarks. If, in our domestic animals, any part
or the whole animal be neglected, and no selection be applied, that
part (for instance, the comb in the Dorking fowl) or the whole breed
will cease to have a uniform character: and the breed may be said to be
degenerating. In rudimentary organs, and in those which have been but
little specialised for any particular purpose, and perhaps in
polymorphic groups, we see a nearly parallel case; for in such cases
natural selection either has not or cannot come into full play, and
thus the organisation is left in a fluctuating condition. But what here
more particularly concerns us is, that those points in our domestic
animals, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at
the individuals of the same breed of the pigeon; and see what a
prodigious amount of difference there is in the beak of tumblers, in
the beak and wattle of carriers, in the carriage and tail of fantails,
&c., these being the points now mainly attended to by English fanciers.
Even in the same sub-breed, as in that of the short-faced tumbler, it
is notoriously difficult to breed nearly perfect birds, many departing
widely from the standard. There may truly be said to be a constant
struggle going on between, on the one hand, the tendency to reversion
to a less perfect state, as well as an innate tendency to new
variations, and, on the other hand, the power of steady selection to
keep the breed true. In the long run selection gains the day, and we do
not expect to fail so completely as to breed a bird as coarse as a
common tumbler pigeon from a good short-faced strain. But as long as
selection is rapidly going on, much variability in the parts undergoing
modification may always be expected.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has undergone
an extraordinary amount of modification since the period when the
several species branched off from the common progenitor of the genus.
This period will seldom be remote in any extreme degree, as species
rarely endure for more than one geological period. An extraordinary
amount of modification implies an unusually large and long-continued
amount of variability, which has continually been accumulated by
natural selection for the benefit of the species. But as the
variability of the extraordinarily developed part or organ has been so
great and long-continued within a period not excessively remote, we
might, as a general rule, still expect to find more variability in such
parts than in other parts of the organisation which have remained for a
much longer period nearly constant. And this, I am convinced, is the
case. That the struggle between natural selection on the one hand, and
the tendency to reversion and variability on the other hand, will in
the course of time cease; and that the most abnormally developed organs
may be made constant, I see no reason to doubt. Hence, when an organ,
however abnormal it may be, has been transmitted in approximately the
same condition to many modified descendants, as in the case of the wing
of the bat, it must have existed, according to our theory, for an
immense period in nearly the same state; and thus it has come not to be
more variable than any other structure. It is only in those cases in
which the modification has been comparatively recent and
extraordinarily great that we ought to find the _generative
variability_, as it may be called, still present in a high degree. For
in this case the variability will seldom as yet have been fixed by the
continued selection of the individuals varying in the required manner
and degree, and by the continued rejection of those tending to revert
to a former and less modified condition.

_Specific Characters more Variable than Generic Characters._


The principle discussed under the last heading may be applied to our
present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is meant: if
in a large genus of plants some species had blue flowers and some had
red, the colour would be only a specific character, and no one would be
surprised at one of the blue species varying into red, or conversely;
but if all the species had blue flowers, the colour would become a
generic character, and its variation would be a more unusual
circumstance. I have chosen this example because the explanation which
most naturalists would advance is not here applicable, namely, that
specific characters are more variable than generic, because they are
taken from parts of less physiological importance than those commonly
used for classing genera. I believe this explanation is partly, yet
only indirectly, true; I shall, however, have to return to this point
in the chapter on Classification. It would be almost superfluous to
adduce evidence in support of the statement, that ordinary specific
characters are more variable than generic; but with respect to
important characters, I have repeatedly noticed in works on natural
history, that when an author remarks with surprise that some important
organ or part, which is generally very constant throughout a large
group of species, _differs_ considerably in closely-allied species, it
is often _variable_ in the individuals of the same species. And this
fact shows that a character, which is generally of generic value, when
it sinks in value and becomes only of specific value, often becomes
variable, though its physiological importance may remain the same.
Something of the same kind applies to monstrosities: at least Is.
Geoffroy St. Hilaire apparently entertains no doubt, that the more an
organ normally differs in the different species of the same group, the
more subject it is to anomalies in the individuals.

On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same part
in other independently created species of the same genus, be more
variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view that species are only strongly marked and fixed varieties, we
might expect often to find them still continuing to vary in those parts
of their structure which have varied within a moderately recent period,
and which have thus come to differ. Or to state the case in another
manner: the points in which all the species of a genus resemble each
other, and in which they differ from allied genera, are called generic
characters; and these characters may be attributed to inheritance from
a common progenitor, for it can rarely have happened that natural
selection will have modified several distinct species, fitted to more
or less widely different habits, in exactly the same manner: and as
these so-called generic characters have been inherited from before the
period when the several species first branched off from their common
progenitor, and subsequently have not varied or come to differ in any
degree, or only in a slight degree, it is not probable that they should
vary at the present day. On the other hand, the points in which species
differ from other species of the same genus are called specific
characters; and as these specific characters have varied and come to
differ since the period when the species branched off from a common
progenitor, it is probable that they should still often be in some
degree variable—at least more variable than those parts of the
organisation which have for a very long period remained constant.

_Secondary Sexual Characters Variable._—I think it will be admitted by
naturalists, without my entering on details, that secondary sexual
characters are highly variable. It will also be admitted that species
of the same group differ from each other more widely in their secondary
sexual characters, than in other parts of their organisation; compare,
for instance, the amount of difference between the males of
gallinaceous birds, in which secondary sexual characters are strongly
displayed, with the amount of difference between the females. The cause
of the original variability of these characters is not manifest; but we
can see why they should not have been rendered as constant and uniform
as others, for they are accumulated by sexual selection, which is less
rigid in its action than ordinary selection, as it does not entail
death, but only gives fewer offspring to the less favoured males.
Whatever the cause may be of the variability of secondary sexual
characters, as they are highly variable, sexual selection will have had
a wide scope for action, and may thus have succeeded in giving to the
species of the same group a greater amount of difference in these than
in other respects.

It is a remarkable fact, that the secondary differences between the two
sexes of the same species are generally displayed in the very same
parts of the organisation in which the species of the same genus differ
from each other. Of this fact I will give in illustration the first two
instances which happen to stand on my list; and as the differences in
these cases are of a very unusual nature, the relation can hardly be
accidental. The same number of joints in the tarsi is a character
common to very large groups of beetles, but in the Engidæ, as Westwood
has remarked, the number varies greatly and the number likewise differs
in the two sexes of the same species. Again in the fossorial
hymenoptera, the neuration of the wings is a character of the highest
importance, because common to large groups; but in certain genera the
neuration differs in the different species, and likewise in the two
sexes of the same species. Sir J. Lubbock has recently remarked, that
several minute crustaceans offer excellent illustrations of this law.
“In Pontella, for instance, the sexual characters are afforded mainly
by the anterior antennæ and by the fifth pair of legs: the specific
differences also are principally given by these organs.” This relation
has a clear meaning on my view: I look at all the species of the same
genus as having as certainly descended from the same progenitor, as
have the two sexes of any one species. Consequently, whatever part of
the structure of the common progenitor, or of its early descendants,
became variable; variations of this part would, it is highly probable,
be taken advantage of by natural and sexual selection, in order to fit
the several places in the economy of nature, and likewise to fit the
two sexes of the same species to each other, or to fit the males to
struggle with other males for the possession of the females.

Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the species;
that the frequent extreme variability of any part which is developed in
a species in an extraordinary manner in comparison with the same part
in its congeners; and the slight degree of variability in a part,
however extraordinarily it may be developed, if it be common to a whole
group of species; that the great variability of secondary sexual
characters and their great difference in closely allied species; that
secondary sexual and ordinary specific differences are generally
displayed in the same parts of the organisation, are all principles
closely connected together. All being mainly due to the species of the
same group being the descendants of a common progenitor, from whom they
have inherited much in common, to parts which have recently and largely
varied being more likely still to go on varying than parts which have
long been inherited and have not varied, to natural selection having
more or less completely, according to the lapse of time, overmastered
the tendency to reversion and to further variability, to sexual
selection being less rigid than ordinary selection, and to variations
in the same parts having been accumulated by natural and sexual
selection, and thus having been adapted for secondary sexual, and for
ordinary purposes.

_Distinct Species present analogous Variations, so that a Variety of
one Species often assumes a Character Proper to an allied Species, or
reverts to some of the Characters of an early Progenitor._—These
propositions will be most readily understood by looking to our domestic
races. The most distinct breeds of the pigeon, in countries widely
apart, present sub-varieties with reversed feathers on the head, and
with feathers on the feet, characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more
distinct races. The frequent presence of fourteen or even sixteen
tail-feathers in the pouter may be considered as a variation
representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a common
parent the same constitution and tendency to variation, when acted on
by similar unknown influences. In the vegetable kingdom we have a case
of analogous variation, in the enlarged stems, or as commonly called
roots, of the Swedish turnip and ruta-baga, plants which several
botanists rank as varieties produced by cultivation from a common
parent: if this be not so, the case will then be one of analogous
variation in two so-called distinct species; and to these a third may
be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants,
not to the vera causa of community of descent, and a consequent
tendency to vary in a like manner, but to three separate yet closely
related acts of creation. Many similar cases of analogous variation
have been observed by Naudin in the great gourd family, and by various
authors in our cereals. Similar cases occurring with insects under
natural conditions have lately been discussed with much ability by Mr.
Walsh, who has grouped them under his law of equable variability.

With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, white loins, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these
marks are characteristic of the parent rock-pigeon, I presume that no
one will doubt that this is a case of reversion, and not of a new yet
analogous variation appearing in the several breeds. We may, I think,
confidently come to this conclusion, because, as we have seen, these
coloured marks are eminently liable to appear in the crossed offspring
of two distinct and differently coloured breeds; and in this case there
is nothing in the external conditions of life to cause the reappearance
of the slaty-blue, with the several marks, beyond the influence of the
mere act of crossing on the laws of inheritance.

No doubt it is a very surprising fact that characters should reappear
after having been lost for many, probably for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show for many generations a tendency to revert
in character to the foreign breed—some say, for a dozen or even a score
of generations. After twelve generations, the proportion of blood, to
use a common expression, from one ancestor, is only 1 in 2048; and yet,
as we see, it is generally believed that a tendency to reversion is
retained by this remnant of foreign blood. In a breed which has not
been crossed, but in which _both_ parents have lost some character
which their progenitor possessed, the tendency, whether strong or weak,
to reproduce the lost character might, as was formerly remarked, for
all that we can see to the contrary, be transmitted for almost any
number of generations. When a character which has been lost in a breed,
reappears after a great number of generations, the most probable
hypothesis is, not that one individual suddenly takes after an ancestor
removed by some hundred generations, but that in each successive
generation the character in question has been lying latent, and at
last, under unknown favourable conditions, is developed. With the
barb-pigeon, for instance, which very rarely produces a blue bird, it
is probable that there is a latent tendency in each generation to
produce blue plumage. The abstract improbability of such a tendency
being transmitted through a vast number of generations, is not greater
than that of quite useless or rudimentary organs being similarly
transmitted. A mere tendency to produce a rudiment is indeed sometimes
thus inherited.

As all the species of the same genus are supposed to be descended from
a common progenitor, it might be expected that they would occasionally
vary in an analogous manner; so that the varieties of two or more
species would resemble each other, or that a variety of one species
would resemble in certain characters another and distinct species, this
other species being, according to our view, only a well-marked and
permanent variety. But characters exclusively due to analogous
variation would probably be of an unimportant nature, for the
preservation of all functionally important characters will have been
determined through natural selection, in accordance with the different
habits of the species. It might further be expected that the species of
the same genus would occasionally exhibit reversions to long-lost
characters. As, however, we do not know the common ancestor of any
natural group, we cannot distinguish between reversionary and analogous
characters. If, for instance, we did not know that the parent
rock-pigeon was not feather-footed or turn-crowned, we could not have
told, whether such characters in our domestic breeds were reversions or
only analogous variations; but we might have inferred that the blue
colour was a case of reversion from the number of the markings, which
are correlated with this tint, and which would not probably have all
appeared together from simple variation. More especially we might have
inferred this from the blue colour and the several marks so often
appearing when differently coloured breeds are crossed. Hence, although
under nature it must generally be left doubtful, what cases are
reversions to formerly existing characters, and what are new but
analogous variations, yet we ought, on our theory, sometimes to find
the varying offspring of a species assuming characters which are
already present in other members of the same group. And this
undoubtedly is the case.

The difficulty in distinguishing variable species is largely due to the
varieties mocking, as it were, other species of the same genus. A
considerable catalogue, also, could be given of forms intermediate
between two other forms, which themselves can only doubtfully be ranked
as species; and this shows, unless all these closely allied forms be
considered as independently created species, that they have in varying
assumed some of the characters of the others. But the best evidence of
analogous variations is afforded by parts or organs which are generally
constant in character, but which occasionally vary so as to resemble,
in some degree, the same part or organ in an allied species. I have
collected a long list of such cases; but here, as before, I lie under
the great disadvantage of not being able to give them. I can only
repeat that such cases certainly occur, and seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case almost certainly of reversion. The ass sometimes
has very distinct transverse bars on its legs, like those on the legs
of a zebra. It has been asserted that these are plainest in the foal,
and from inquiries which I have made, I believe this to be true. The
stripe on the shoulder is sometimes double, and is very variable in
length and outline. A white ass, but _not_ an albino, has been
described without either spinal or shoulder stripe; and these stripes
are sometimes very obscure, or actually quite lost, in dark-coloured
asses. The koulan of Pallas is said to have been seen with a double
shoulder-stripe. Mr. Blyth has seen a specimen of the hemionus with a
distinct shoulder-stripe, though it properly has none; and I have been
informed by Colonel Poole that foals of this species are generally
striped on the legs and faintly on the shoulder. The quagga, though so
plainly barred like a zebra over the body, is without bars on the legs;
but Dr. Gray has figured one specimen with very distinct zebra-like
bars on the hocks.

With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of _all_
colours; transverse bars on the legs are not rare in duns, mouse-duns,
and in one instance in a chestnut; a faint shoulder-stripe may
sometimes be seen in duns, and I have seen a trace in a bay horse. My
son made a careful examination and sketch for me of a dun Belgian
cart-horse with a double stripe on each shoulder and with leg-stripes.
I have myself seen a dun Devonshire pony, and a small dun Welsh pony
has been carefully described to me, both with _three_ parallel stripes
on each shoulder.

In the northwest part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
this breed for the Indian Government, a horse without stripes is not
considered as purely bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are often plainest in the foal; and
sometimes quite disappear in old horses. Colonel Poole has seen both
gray and bay Kattywar horses striped when first foaled. I have also
reason to suspect, from information given me by Mr. W.W. Edwards, that
with the English race-horse the spinal stripe is much commoner in the
foal than in the full-grown animal. I have myself recently bred a foal
from a bay mare (offspring of a Turkoman horse and a Flemish mare) by a
bay English race-horse. This foal, when a week old, was marked on its
hinder quarters and on its forehead with numerous very narrow, dark,
zebra-like bars, and its legs were feebly striped. All the stripes soon
disappeared completely. Without here entering on further details I may
state that I have collected cases of leg and shoulder stripes in horses
of very different breeds in various countries from Britain to Eastern
China; and from Norway in the north to the Malay Archipelago in the
south. In all parts of the world these stripes occur far oftenest in
duns and mouse-duns; by the term dun a large range of colour is
included, from one between brown and black to a close approach to cream
colour.

I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse are descended
from several aboriginal species, one of which, the dun, was striped;
and that the above-described appearances are all due to ancient crosses
with the dun stock. But this view may be safely rejected, for it is
highly improbable that the heavy Belgian cart-horse, Welsh ponies,
Norwegian cobs, the lanky Kattywar race, &c., inhabiting the most
distant parts of the world, should have all have been crossed with one
supposed aboriginal stock.

Now let us turn to the effects of crossing the several species of the
horse genus. Rollin asserts that the common mule from the ass and horse
is particularly apt to have bars on its legs; according to Mr. Gosse,
in certain parts of the United States, about nine out of ten mules have
striped legs. I once saw a mule with its legs so much striped that any
one might have thought that it was a hybrid zebra; and Mr. W.C. Martin,
in his excellent treatise on the horse, has given a figure of a similar
mule. In four coloured drawings, which I have seen, of hybrids between
the ass and zebra, the legs were much more plainly barred than the rest
of the body; and in one of them there was a double shoulder-stripe. In
Lord Morton’s famous hybrid, from a chestnut mare and male quagga, the
hybrid and even the pure offspring subsequently produced from the same
mare by a black Arabian sire, were much more plainly barred across the
legs than is even the pure quagga. Lastly, and this is another most
remarkable case, a hybrid has been figured by Dr. Gray (and he informs
me that he knows of a second case) from the ass and the hemionus; and
this hybrid, though the ass only occasionally has stripes on his legs
and the hemionus has none and has not even a shoulder-stripe,
nevertheless had all four legs barred, and had three short
shoulder-stripes, like those on the dun Devonshire and Welsh ponies,
and even had some zebra-like stripes on the sides of its face. With
respect to this last fact, I was so convinced that not even a stripe of
colour appears from what is commonly called chance, that I was led
solely from the occurrence of the face-stripes on this hybrid from the
ass and hemionus to ask Colonel Poole whether such face-stripes ever
occurred in the eminently striped Kattywar breed of horses, and was, as
we have seen, answered in the affirmative.

What now are we to say to these several facts? We see several distinct
species of the horse genus becoming, by simple variation, striped on
the legs like a zebra, or striped on the shoulders like an ass. In the
horse we see this tendency strong whenever a dun tint appears—a tint
which approaches to that of the general colouring of the other species
of the genus. The appearance of the stripes is not accompanied by any
change of form, or by any other new character. We see this tendency to
become striped most strongly displayed in hybrids from between several
of the most distinct species. Now observe the case of the several
breeds of pigeons: they are descended from a pigeon (including two or
three sub-species or geographical races) of a bluish colour, with
certain bars and other marks; and when any breed assumes by simple
variation a bluish tint, these bars and other marks invariably
reappear; but without any other change of form or character. When the
oldest and truest breeds of various colours are crossed, we see a
strong tendency for the blue tint and bars and marks to reappear in the
mongrels. I have stated that the most probable hypothesis to account
for the reappearance of very ancient characters, is—that there is a
_tendency_ in the young of each successive generation to produce the
long-lost character, and that this tendency, from unknown causes,
sometimes prevails. And we have just seen that in several species of
the horse genus the stripes are either plainer or appear more commonly
in the young than in the old. Call the breeds of pigeons, some of which
have bred true for centuries, species; and how exactly parallel is the
case with that of the species of the horse genus! For myself, I venture
confidently to look back thousands on thousands of generations, and I
see an animal striped like a zebra, but perhaps otherwise very
differently constructed, the common parent of our domestic horse
(whether or not it be descended from one or more wild stocks) of the
ass, the hemionus, quagga, and zebra.

He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like the other species
of the genus; and that each has been created with a strong tendency,
when crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents, but
other species of the genus. To admit this view is, as it seems to me,
to reject a real for an unreal, or at least for an unknown cause. It
makes the works of God a mere mockery and deception; I would almost as
soon believe with the old and ignorant cosmogonists, that fossil shells
had never lived, but had been created in stone so as to mock the shells
now living on the sea-shore.

_Summary._—Our ignorance of the laws of variation is profound. Not in
one case out of a hundred can we pretend to assign any reason why this
or that part has varied. But whenever we have the means of instituting
a comparison, the same laws appear to have acted in producing the
lesser differences between varieties of the same species, and the
greater differences between species of the same genus. Changed
conditions generally induce mere fluctuating variability, but sometimes
they cause direct and definite effects; and these may become strongly
marked in the course of time, though we have not sufficient evidence on
this head. Habit in producing constitutional peculiarities, and use in
strengthening, and disuse in weakening and diminishing organs, appear
in many cases to have been potent in their effects. Homologous parts
tend to vary in the same manner, and homologous parts tend to cohere.
Modifications in hard parts and in external parts sometimes affect
softer and internal parts. When one part is largely developed, perhaps
it tends to draw nourishment from the adjoining parts; and every part
of the structure which can be saved without detriment will be saved.
Changes of structure at an early age may affect parts subsequently
developed; and many cases of correlated variation, the nature of which
we are unable to understand, undoubtedly occur. Multiple parts are
variable in number and in structure, perhaps arising from such parts
not having been closely specialised for any particular function, so
that their modifications have not been closely checked by natural
selection. It follows probably from this same cause, that organic
beings low in the scale are more variable than those standing higher in
the scale, and which have their whole organisation more specialised.
Rudimentary organs, from being useless, are not regulated by natural
selection, and hence are variable. Specific characters—that is, the
characters which have come to differ since the several species of the
same genus branched off from a common parent—are more variable than
generic characters, or those which have long been inherited, and have
not differed within this same period. In these remarks we have referred
to special parts or organs being still variable, because they have
recently varied and thus come to differ; but we have also seen in the
second chapter that the same principle applies to the whole individual;
for in a district where many species of a genus are found—that is,
where there has been much former variation and differentiation, or
where the manufactory of new specific forms has been actively at
work—in that district and among these species, we now find, on an
average, most varieties. Secondary sexual characters are highly
variable, and such characters differ much in the species of the same
group. Variability in the same parts of the organisation has generally
been taken advantage of in giving secondary sexual differences to the
two sexes of the same species, and specific differences to the several
species of the same genus. Any part or organ developed to an
extraordinary size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have gone through an
extraordinary amount of modification since the genus arose; and thus we
can understand why it should often still be variable in a much higher
degree than other parts; for variation is a long-continued and slow
process, and natural selection will in such cases not as yet have had
time to overcome the tendency to further variability and to reversion
to a less modified state. But when a species with an extraordinarily
developed organ has become the parent of many modified
descendants—which on our view must be a very slow process, requiring a
long lapse of time—in this case, natural selection has succeeded in
giving a fixed character to the organ, in however extraordinary a
manner it may have been developed. Species inheriting nearly the same
constitution from a common parent, and exposed to similar influences,
naturally tend to present analogous variations, or these same species
may occasionally revert to some of the characters of their ancient
progenitors. Although new and important modifications may not arise
from reversion and analogous variation, such modifications will add to
the beautiful and harmonious diversity of nature.

Whatever the cause may be of each slight difference between the
offspring and their parents—and a cause for each must exist—we have
reason to believe that it is the steady accumulation of beneficial
differences which has given rise to all the more important
modifications of structure in relation to the habits of each species.




CHAPTER VI.
DIFFICULTIES OF THE THEORY.


Difficulties of the theory of descent with modification—Absence or
rarity of transitional varieties—Transitions in habits of
life—Diversified habits in the same species—Species with habits widely
different from those of their allies—Organs of extreme perfection—Modes
of transition—Cases of difficulty—Natura non facit saltum—Organs of
small importance—Organs not in all cases absolutely perfect—The law of
Unity of Type and of the Conditions of Existence embraced by the theory
of Natural Selection.


Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious
that to this day I can hardly reflect on them without being in some
degree staggered; but, to the best of my judgment, the greater number
are only apparent, and those that are real are not, I think, fatal to
the theory.

These difficulties and objections may be classed under the following
heads: First, why, if species have descended from other species by fine
gradations, do we not everywhere see innumerable transitional forms?
Why is not all nature in confusion, instead of the species being, as we
see them, well defined?

Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the
modification of some other animal with widely different habits and
structure? Can we believe that natural selection could produce, on the
one hand, an organ of trifling importance, such as the tail of a
giraffe, which serves as a fly-flapper, and, on the other hand, an
organ so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to the instinct which leads the bee to
make cells, and which has practically anticipated the discoveries of
profound mathematicians?

Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?

The two first heads will be here discussed; some miscellaneous
objections in the following chapter; Instinct and Hybridism in the two
succeeding chapters.

_On the Absence or Rarity of Transitional Varieties._—As natural
selection acts solely by the preservation of profitable modifications,
each new form will tend in a fully-stocked country to take the place
of, and finally to exterminate, its own less improved parent-form and
other less-favoured forms with which it comes into competition. Thus
extinction and natural selection go hand in hand. Hence, if we look at
each species as descended from some unknown form, both the parent and
all the transitional varieties will generally have been exterminated by
the very process of the formation and perfection of the new form.

But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in the
crust of the earth? It will be more convenient to discuss this question
in the chapter on the imperfection of the geological record; and I will
here only state that I believe the answer mainly lies in the record
being incomparably less perfect than is generally supposed. The crust
of the earth is a vast museum; but the natural collections have been
imperfectly made, and only at long intervals of time.

But it may be urged that when several closely allied species inhabit
the same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals
with closely allied or representative species, evidently filling nearly
the same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes rarer and
rarer, the other becomes more and more frequent, till the one replaces
the other. But if we compare these species where they intermingle, they
are generally as absolutely distinct from each other in every detail of
structure as are specimens taken from the metropolis inhabited by each.
By my theory these allied species are descended from a common parent;
and during the process of modification, each has become adapted to the
conditions of life of its own region, and has supplanted and
exterminated its original parent-form and all the transitional
varieties between its past and present states. Hence we ought not to
expect at the present time to meet with numerous transitional varieties
in each region, though they must have existed there, and may be
embedded there in a fossil condition. But in the intermediate region,
having intermediate conditions of life, why do we not now find
closely-linking intermediate varieties? This difficulty for a long time
quite confounded me. But I think it can be in large part explained.

In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during a
long period. Geology would lead us to believe that most continents have
been broken up into islands even during the later tertiary periods; and
in such islands distinct species might have been separately formed
without the possibility of intermediate varieties existing in the
intermediate zones. By changes in the form of the land and of climate,
marine areas now continuous must often have existed within recent times
in a far less continuous and uniform condition than at present. But I
will pass over this way of escaping from the difficulty; for I believe
that many perfectly defined species have been formed on strictly
continuous areas; though I do not doubt that the formerly broken
condition of areas now continuous, has played an important part in the
formation of new species, more especially with freely-crossing and
wandering animals.

In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a
common alpine species disappears. The same fact has been noticed by E.
Forbes in sounding the depths of the sea with the dredge. To those who
look at climate and the physical conditions of life as the
all-important elements of distribution, these facts ought to cause
surprise, as climate and height or depth graduate away insensibly. But
when we bear in mind that almost every species, even in its metropolis,
would increase immensely in numbers, were it not for other competing
species; that nearly all either prey on or serve as prey for others; in
short, that each organic being is either directly or indirectly related
in the most important manner to other organic beings—we see that the
range of the inhabitants of any country by no means exclusively depends
on insensibly changing physical conditions, but in large part on the
presence of other species, on which it lives, or by which it is
destroyed, or with which it comes into competition; and as these
species are already defined objects, not blending one into another by
insensible gradations, the range of any one species, depending as it
does on the range of others, will tend to be sharply defined. Moreover,
each species on the confines of its range, where it exists in lessened
numbers, will, during fluctuations in the number of its enemies or of
its prey, or in the nature of the seasons, be extremely liable to utter
extermination; and thus its geographical range will come to be still
more sharply defined.

As allied or representative species, when inhabiting a continuous area,
are generally distributed in such a manner that each has a wide range,
with a comparatively narrow neutral territory between them, in which
they become rather suddenly rarer and rarer; then, as varieties do not
essentially differ from species, the same rule will probably apply to
both; and if we take a varying species inhabiting a very large area, we
shall have to adapt two varieties to two large areas, and a third
variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and
lesser area; and practically, as far as I can make out, this rule holds
good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between
well-marked varieties in the genus Balanus. And it would appear from
information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston,
that generally, when varieties intermediate between two other forms
occur, they are much rarer numerically than the forms which they
connect. Now, if we may trust these facts and inferences, and conclude
that varieties linking two other varieties together generally have
existed in lesser numbers than the forms which they connect, then we
can understand why intermediate varieties should not endure for very
long periods: why, as a general rule, they should be exterminated and
disappear, sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run
a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on
both sides of it. But it is a far more important consideration, that
during the process of further modification, by which two varieties are
supposed to be converted and perfected into two distinct species, the
two which exist in larger numbers, from inhabiting larger areas, will
have a great advantage over the intermediate variety, which exists in
smaller numbers in a narrow and intermediate zone. For forms existing
in larger numbers will have a better chance, within any given period,
of presenting further favourable variations for natural selection to
seize on, than will the rarer forms which exist in lesser numbers.
Hence, the more common forms, in the race for life, will tend to beat
and supplant the less common forms, for these will be more slowly
modified and improved. It is the same principle which, as I believe,
accounts for the common species in each country, as shown in the second
chapter, presenting on an average a greater number of well-marked
varieties than do the rarer species. I may illustrate what I mean by
supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly
tract; and a third to the wide plains at the base; and that the
inhabitants are all trying with equal steadiness and skill to improve
their stocks by selection; the chances in this case will be strongly in
favour of the great holders on the mountains or on the plains improving
their breeds more quickly than the small holders on the intermediate
narrow, hilly tract; and consequently the improved mountain or plain
breed will soon take the place of the less improved hill breed; and
thus the two breeds, which originally existed in greater numbers, will
come into close contact with each other, without the interposition of
the supplanted, intermediate hill variety.

To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: first, because new varieties are very
slowly formed, for variation is a slow process, and natural selection
can do nothing until favourable individual differences or variations
occur, and until a place in the natural polity of the country can be
better filled by some modification of some one or more of its
inhabitants. And such new places will depend on slow changes of
climate, or on the occasional immigration of new inhabitants, and,
probably, in a still more important degree, on some of the old
inhabitants becoming slowly modified, with the new forms thus produced
and the old ones acting and reacting on each other. So that, in any one
region and at any one time, we ought to see only a few species
presenting slight modifications of structure in some degree permanent;
and this assuredly we do see.

Secondly, areas now continuous must often have existed within the
recent period as isolated portions, in which many forms, more
especially among the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to rank
as representative species. In this case, intermediate varieties between
the several representative species and their common parent, must
formerly have existed within each isolated portion of the land, but
these links during the process of natural selection will have been
supplanted and exterminated, so that they will no longer be found in a
living state.

Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will, it
is probable, at first have been formed in the intermediate zones, but
they will generally have had a short duration. For these intermediate
varieties will, from reasons already assigned (namely from what we know
of the actual distribution of closely allied or representative species,
and likewise of acknowledged varieties), exist in the intermediate
zones in lesser numbers than the varieties which they tend to connect.
From this cause alone the intermediate varieties will be liable to
accidental extermination; and during the process of further
modification through natural selection, they will almost certainly be
beaten and supplanted by the forms which they connect; for these, from
existing in greater numbers will, in the aggregate, present more
varieties, and thus be further improved through natural selection and
gain further advantages.

Lastly, looking not to any one time, but at all time, if my theory be
true, numberless intermediate varieties, linking closely together all
the species of the same group, must assuredly have existed; but the
very process of natural selection constantly tends, as has been so
often remarked, to exterminate the parent forms and the intermediate
links. Consequently evidence of their former existence could be found
only among fossil remains, which are preserved, as we shall attempt to
show in a future chapter, in an extremely imperfect and intermittent
record.

_On the Origin and Transition of Organic Beings with peculiar Habits
and Structure._—It has been asked by the opponents of such views as I
hold, how, for instance, could a land carnivorous animal have been
converted into one with aquatic habits; for how could the animal in its
transitional state have subsisted? It would be easy to show that there
now exist carnivorous animals presenting close intermediate grades from
strictly terrestrial to aquatic habits; and as each exists by a
struggle for life, it is clear that each must be well adapted to its
place in nature. Look at the Mustela vison of North America, which has
webbed feet, and which resembles an otter in its fur, short legs, and
form of tail; during summer this animal dives for and preys on fish,
but during the long winter it leaves the frozen waters, and preys, like
other polecats on mice and land animals. If a different case had been
taken, and it had been asked how an insectivorous quadruped could
possibly have been converted into a flying bat, the question would have
been far more difficult to answer. Yet I think such difficulties have
little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for, out
of the many striking cases which I have collected, I can give only one
or two instances of transitional habits and structures in allied
species; and of diversified habits, either constant or occasional, in
the same species. And it seems to me that nothing less than a long list
of such cases is sufficient to lessen the difficulty in any particular
case like that of the bat.

Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as
Sir J. Richardson has remarked, with the posterior part of their bodies
rather wide and with the skin on their flanks rather full, to the
so-called flying squirrels; and flying squirrels have their limbs and
even the base of the tail united by a broad expanse of skin, which
serves as a parachute and allows them to glide through the air to an
astonishing distance from tree to tree. We cannot doubt that each
structure is of use to each kind of squirrel in its own country, by
enabling it to escape birds or beasts of prey, or to collect food more
quickly, or, as there is reason to believe, to lessen the danger from
occasional falls. But it does not follow from this fact that the
structure of each squirrel is the best that it is possible to conceive
under all possible conditions. Let the climate and vegetation change,
let other competing rodents or new beasts of prey immigrate, or old
ones become modified, and all analogy would lead us to believe that
some, at least, of the squirrels would decrease in numbers or become
exterminated, unless they also become modified and improved in
structure in a corresponding manner. Therefore, I can see no
difficulty, more especially under changing conditions of life, in the
continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being propagated,
until, by the accumulated effects of this process of natural selection,
a perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was
formerly ranked among bats, but is now believed to belong to the
Insectivora. An extremely wide flank-membrane stretches from the
corners of the jaw to the tail, and includes the limbs with the
elongated fingers. This flank-membrane is furnished with an extensor
muscle. Although no graduated links of structure, fitted for gliding
through the air, now connect the Galeopithecus with the other
Insectivora, yet there is no difficulty in supposing that such links
formerly existed, and that each was developed in the same manner as
with the less perfectly gliding squirrels; each grade of structure
having been useful to its possessor. Nor can I see any insuperable
difficulty in further believing it possible that the membrane-connected
fingers and fore-arm of the Galeopithecus might have been greatly
lengthened by natural selection; and this, as far as the organs of
flight are concerned, would have converted the animal into a bat. In
certain bats in which the wing-membrane extends from the top of the
shoulder to the tail and includes the hind-legs, we perhaps see traces
of an apparatus originally fitted for gliding through the air rather
than for flight.

If about a dozen genera of birds were to become extinct, who would have
ventured to surmise that birds might have existed which used their
wings solely as flappers, like the logger headed duck (Micropterus of
Eyton); as fins in the water and as front legs on the land, like the
penguin; as sails, like the ostrich; and functionally for no purpose,
like the apteryx? Yet the structure of each of these birds is good for
it, under the conditions of life to which it is exposed, for each has
to live by a struggle: but it is not necessarily the best possible
under all possible conditions. It must not be inferred from these
remarks that any of the grades of wing-structure here alluded to, which
perhaps may all be the result of disuse, indicate the steps by which
birds actually acquired their perfect power of flight; but they serve
to show what diversified means of transition are at least possible.

Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land; and seeing that
we have flying birds and mammals, flying insects of the most
diversified types, and formerly had flying reptiles, it is conceivable
that flying-fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected, who
would have ever imagined that in an early transitional state they had
been inhabitants of the open ocean, and had used their incipient organs
of flight exclusively, so far as we know, to escape being devoured by
other fish?

When we see any structure highly perfected for any particular habit, as
the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom have
survived to the present day, for they will have been supplanted by
their successors, which were gradually rendered more perfect through
natural selection. Furthermore, we may conclude that transitional
states between structures fitted for very different habits of life will
rarely have been developed at an early period in great numbers and
under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many subordinate
forms, for taking prey of many kinds in many ways, on the land and in
the water, until their organs of flight had come to a high stage of
perfection, so as to have given them a decided advantage over other
animals in the battle for life. Hence the chance of discovering species
with transitional grades of structure in a fossil condition will always
be less, from their having existed in lesser numbers, than in the case
of species with fully developed structures.

I will now give two or three instances, both of diversified and of
changed habits, in the individuals of the same species. In either case
it would be easy for natural selection to adapt the structure of the
animal to its changed habits, or exclusively to one of its several
habits. It is, however, difficult to decide and immaterial for us,
whether habits generally change first and structure afterwards; or
whether slight modifications of structure lead to changed habits; both
probably often occurring almost simultaneously. Of cases of changed
habits it will suffice merely to allude to that of the many British
insects which now feed on exotic plants, or exclusively on artificial
substances. Of diversified habits innumerable instances could be given:
I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in
South America, hovering over one spot and then proceeding to another,
like a kestrel, and at other times standing stationary on the margin of
water, and then dashing into it like a kingfisher at a fish. In our own
country the larger titmouse (Parus major) may be seen climbing
branches, almost like a creeper; it sometimes, like a shrike, kills
small birds by blows on the head; and I have many times seen and heard
it hammering the seeds of the yew on a branch, and thus breaking them
like a nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost like a
whale, insects in the water.

As we sometimes see individuals following habits different from those
proper to their species and to the other species of the same genus, we
might expect that such individuals would occasionally give rise to new
species, having anomalous habits, and with their structure either
slightly or considerably modified from that of their type. And such
instances occur in nature. Can a more striking instance of adaptation
be given than that of a woodpecker for climbing trees and seizing
insects in the chinks of the bark? Yet in North America there are
woodpeckers which feed largely on fruit, and others with elongated
wings which chase insects on the wing. On the plains of La Plata, where
hardly a tree grows, there is a woodpecker (Colaptes campestris) which
has two toes before and two behind, a long-pointed tongue, pointed
tail-feathers, sufficiently stiff to support the bird in a vertical
position on a post, but not so stiff as in the typical wood-peckers,
and a straight, strong beak. The beak, however, is not so straight or
so strong as in the typical woodpeckers but it is strong enough to bore
into wood. Hence this Colaptes, in all the essential parts of its
structure, is a woodpecker. Even in such trifling characters as the
colouring, the harsh tone of the voice, and undulatory flight, its
close blood-relationship to our common woodpecker is plainly declared;
yet, as I can assert, not only from my own observations, but from those
of the accurate Azara, in certain large districts it does not climb
trees, and it makes its nest in holes in banks! In certain other
districts, however, this same woodpecker, as Mr. Hudson states,
frequents trees, and bores holes in the trunk for its nest. I may
mention as another illustration of the varied habits of this genus,
that a Mexican Colaptes has been described by De Saussure as boring
holes into hard wood in order to lay up a store of acorns.

Petrels are the most aërial and oceanic of birds, but, in the quiet
sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, in its manner of swimming
and of flying when made to take flight, would be mistaken by any one
for an auk or a grebe; nevertheless, it is essentially a petrel, but
with many parts of its organisation profoundly modified in relation to
its new habits of life; whereas the woodpecker of La Plata has had its
structure only slightly modified. In the case of the water-ouzel, the
acutest observer, by examining its dead body, would never have
suspected its sub-aquatic habits; yet this bird, which is allied to the
thrush family, subsists by diving,—using its wings under water and
grasping stones with its feet. All the members of the great order of
Hymenopterous insects are terrestrial, excepting the genus
Proctotrupes, which Sir John Lubbock has discovered to be aquatic in
its habits; it often enters the water and dives about by the use not of
its legs but of its wings, and remains as long as four hours beneath
the surface; yet it exhibits no modification in structure in accordance
with its abnormal habits.

He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not in agreement. What can be plainer than that
the webbed feet of ducks and geese are formed for swimming? Yet there
are upland geese with webbed feet which rarely go near the water; and
no one except Audubon, has seen the frigate-bird, which has all its
four toes webbed, alight on the surface of the ocean. On the other
hand, grebes and coots are eminently aquatic, although their toes are
only bordered by membrane. What seems plainer than that the long toes,
not furnished with membrane, of the Grallatores, are formed for walking
over swamps and floating plants. The water-hen and landrail are members
of this order, yet the first is nearly as aquatic as the coot, and the
second is nearly as terrestrial as the quail or partridge. In such
cases, and many others could be given, habits have changed without a
corresponding change of structure. The webbed feet of the upland goose
may be said to have become almost rudimentary in function, though not
in structure. In the frigate-bird, the deeply scooped membrane between
the toes shows that structure has begun to change.

He who believes in separate and innumerable acts of creation may say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one belonging to another type; but this seems
to me only restating the fact in dignified language. He who believes in
the struggle for existence and in the principle of natural selection,
will acknowledge that every organic being is constantly endeavouring to
increase in numbers; and that if any one being varies ever so little,
either in habits or structure, and thus gains an advantage over some
other inhabitant of the same country, it will seize on the place of
that inhabitant, however different that may be from its own place.
Hence it will cause him no surprise that there should be geese and
frigate-birds with webbed feet, living on the dry land and rarely
alighting on the water, that there should be long-toed corncrakes,
living in meadows instead of in swamps; that there should be
woodpeckers where hardly a tree grows; that there should be diving
thrushes and diving Hymenoptera, and petrels with the habits of auks.

_Organs of extreme Perfection and Complication._


To suppose that the eye with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I
freely confess, absurd in the highest degree. When it was first said
that the sun stood still and the world turned round, the common sense
of mankind declared the doctrine false; but the old saying of _Vox
populi, vox Dei_, as every philosopher knows, cannot be trusted in
science. Reason tells me, that if numerous gradations from a simple and
imperfect eye to one complex and perfect can be shown to exist, each
grade being useful to its possessor, as is certainly the case; if
further, the eye ever varies and the variations be inherited, as is
likewise certainly the case; and if such variations should be useful to
any animal under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed by natural
selection, though insuperable by our imagination, should not be
considered as subversive of the theory. How a nerve comes to be
sensitive to light, hardly concerns us more than how life itself
originated; but I may remark that, as some of the lowest organisms in
which nerves cannot be detected, are capable of perceiving light, it
does not seem impossible that certain sensitive elements in their
sarcode should become aggregated and developed into nerves, endowed
with this special sensibility.

In searching for the gradations through which an organ in any species
has been perfected, we ought to look exclusively to its lineal
progenitors; but this is scarcely ever possible, and we are forced to
look to other species and genera of the same group, that is to the
collateral descendants from the same parent-form, in order to see what
gradations are possible, and for the chance of some gradations having
been transmitted in an unaltered or little altered condition. But the
state of the same organ in distinct classes may incidentally throw
light on the steps by which it has been perfected.

The simplest organ which can be called an eye consists of an optic
nerve, surrounded by pigment-cells and covered by translucent skin, but
without any lens or other refractive body. We may, however, according
to M. Jourdain, descend even a step lower and find aggregates of
pigment-cells, apparently serving as organs of vision, without any
nerves, and resting merely on sarcodic tissue. Eyes of the above simple
nature are not capable of distinct vision, and serve only to
distinguish light from darkness. In certain star-fishes, small
depressions in the layer of pigment which surrounds the nerve are
filled, as described by the author just quoted, with transparent
gelatinous matter, projecting with a convex surface, like the cornea in
the higher animals. He suggests that this serves not to form an image,
but only to concentrate the luminous rays and render their perception
more easy. In this concentration of the rays we gain the first and by
far the most important step towards the formation of a true,
picture-forming eye; for we have only to place the naked extremity of
the optic nerve, which in some of the lower animals lies deeply buried
in the body, and in some near the surface, at the right distance from
the concentrating apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of
pupil, but destitute of lens or other optical contrivance. With insects
it is now known that the numerous facets on the cornea of their great
compound eyes form true lenses, and that the cones include curiously
modified nervous filaments. But these organs in the Articulata are so
much diversified that Müller formerly made three main classes with
seven subdivisions, besides a fourth main class of aggregated simple
eyes.

When we reflect on these facts, here given much too briefly, with
respect to the wide, diversified, and graduated range of structure in
the eyes of the lower animals; and when we bear in mind how small the
number of all living forms must be in comparison with those which have
become extinct, the difficulty ceases to be very great in believing
that natural selection may have converted the simple apparatus of an
optic nerve, coated with pigment and invested by transparent membrane,
into an optical instrument as perfect as is possessed by any member of
the Articulata class.

He who will go thus far, ought not to hesitate to go one step further,
if he finds on finishing this volume that large bodies of facts,
otherwise inexplicable, can be explained by the theory of modification
through natural selection; he ought to admit that a structure even as
perfect as an eagle’s eye might thus be formed, although in this case
he does not know the transitional states. It has been objected that in
order to modify the eye and still preserve it as a perfect instrument,
many changes would have to be effected simultaneously, which, it is
assumed, could not be done through natural selection; but as I have
attempted to show in my work on the variation of domestic animals, it
is not necessary to suppose that the modifications were all
simultaneous, if they were extremely slight and gradual. Different
kinds of modification would, also, serve for the same general purpose:
as Mr. Wallace has remarked, “If a lens has too short or too long a
focus, it may be amended either by an alteration of curvature, or an
alteration of density; if the curvature be irregular, and the rays do
not converge to a point, then any increased regularity of curvature
will be an improvement. So the contraction of the iris and the muscular
movements of the eye are neither of them essential to vision, but only
improvements which might have been added and perfected at any stage of
the construction of the instrument.” Within the highest division of the
animal kingdom, namely, the Vertebrata, we can start from an eye so
simple, that it consists, as in the lancelet, of a little sack of
transparent skin, furnished with a nerve and lined with pigment, but
destitute of any other apparatus. In fishes and reptiles, as Owen has
remarked, “The range of gradation of dioptric structures is very
great.” It is a significant fact that even in man, according to the
high authority of Virchow, the beautiful crystalline lens is formed in
the embryo by an accumulation of epidermic cells, lying in a sack-like
fold of the skin; and the vitreous body is formed from embryonic
subcutaneous tissue. To arrive, however, at a just conclusion regarding
the formation of the eye, with all its marvellous yet not absolutely
perfect characters, it is indispensable that the reason should conquer
the imagination; but I have felt the difficulty far to keenly to be
surprised at others hesitating to extend the principle of natural
selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope. We
know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that
the eye has been formed by a somewhat analogous process. But may not
this inference be presumptuous? Have we any right to assume that the
Creator works by intellectual powers like those of man? If we must
compare the eye to an optical instrument, we ought in imagination to
take a thick layer of transparent tissue, with spaces filled with
fluid, and with a nerve sensitive to light beneath, and then suppose
every part of this layer to be continually changing slowly in density,
so as to separate into layers of different densities and thicknesses,
placed at different distances from each other, and with the surfaces of
each layer slowly changing in form. Further we must suppose that there
is a power, represented by natural selection or the survival of the
fittest, always intently watching each slight alteration in the
transparent layers; and carefully preserving each which, under varied
circumstances, in any way or degree, tends to produce a distincter
image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better is
produced, and then the old ones to be all destroyed. In living bodies,
variation will cause the slight alteration, generation will multiply
them almost infinitely, and natural selection will pick out with
unerring skill each improvement. Let this process go on for millions of
years; and during each year on millions of individuals of many kinds;
and may we not believe that a living optical instrument might thus be
formed as superior to one of glass, as the works of the Creator are to
those of man?

_Modes of Transition._


If it could be demonstrated that any complex organ existed, which could
not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, around which, according to the theory, there has been much
extinction. Or again, if we take an organ common to all the members of
a class, for in this latter case the organ must have been originally
formed at a remote period, since which all the many members of the
class have been developed; and in order to discover the early
transitional grades through which the organ has passed, we should have
to look to very ancient ancestral forms, long since become extinct.

We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous
cases could be given among the lower animals of the same organ
performing at the same time wholly distinct functions; thus in the
larva of the dragon-fly and in the fish Cobites the alimentary canal
respires, digests, and excretes. In the Hydra, the animal may be turned
inside out, and the exterior surface will then digest and the stomach
respire. In such cases natural selection might specialise, if any
advantage were thus gained, the whole or part of an organ, which had
previously performed two functions, for one function alone, and thus by
insensible steps greatly change its nature. Many plants are known which
regularly produce at the same time differently constructed flowers; and
if such plants were to produce one kind alone, a great change would be
effected with comparative suddenness in the character of the species.
It is, however, probable that the two sorts of flowers borne by the
same plant were originally differentiated by finely graduated steps,
which may still be followed in some few cases.

Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same
function, and this is an extremely important means of transition: to
give one instance—there are fish with gills or branchiæ that breathe
the air dissolved in the water, at the same time that they breathe free
air in their swim-bladders, this latter organ being divided by highly
vascular partitions and having a ductus pneumaticus for the supply of
air. To give another instance from the vegetable kingdom: plants climb
by three distinct means, by spirally twining, by clasping a support
with their sensitive tendrils, and by the emission of aërial rootlets;
these three means are usually found in distinct groups, but some few
species exhibit two of the means, or even all three, combined in the
same individual. In all such cases one of the two organs might readily
be modified and perfected so as to perform all the work, being aided
during the progress of modification by the other organ; and then this
other organ might be modified for some other and quite distinct
purpose, or be wholly obliterated.

The illustration of the swim-bladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into
one for a widely different purpose, namely respiration. The
swim-bladder has, also, been worked in as an accessory to the auditory
organs of certain fishes. All physiologists admit that the swim-bladder
is homologous, or “ideally similar” in position and structure with the
lungs of the higher vertebrate animals: hence there is no reason to
doubt that the swim-bladder has actually been converted into lungs, or
an organ used exclusively for respiration.

According to this view it may be inferred that all vertebrate animals
with true lungs are descended by ordinary generation from an ancient
and unknown prototype which was furnished with a floating apparatus or
swim-bladder. We can thus, as I infer from Professor Owen’s interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchiæ have wholly
disappeared—but in the embryo the slits on the sides of the neck and
the loop-like course of the arteries still mark their former position.
But it is conceivable that the now utterly lost branchiæ might have
been gradually worked in by natural selection for some distinct
purpose: for instance, Landois has shown that the wings of insects are
developed from the trachea; it is therefore highly probable that in
this great class organs which once served for respiration have been
actually converted into organs for flight.

In considering transitions of organs, it is so important to bear in
mind the probability of conversion from one function to another, that I
will give another instance. Pedunculated cirripedes have two minute
folds of skin, called by me the ovigerous frena, which serve, through
the means of a sticky secretion, to retain the eggs until they are
hatched within the sack. These cirripedes have no branchiæ, the whole
surface of the body and of the sack, together with the small frena,
serving for respiration. The Balanidæ or sessile cirripedes, on the
other hand, have no ovigerous frena, the eggs lying loose at the bottom
of the sack, within the well-enclosed shell; but they have, in the same
relative position with the frena, large, much-folded membranes, which
freely communicate with the circulatory lacunæ of the sack and body,
and which have been considered by all naturalists to act as branchiæ.
Now I think no one will dispute that the ovigerous frena in the one
family are strictly homologous with the branchiæ of the other family;
indeed, they graduate into each other. Therefore it need not be doubted
that the two little folds of skin, which originally served as ovigerous
frena, but which, likewise, very slightly aided in the act of
respiration, have been gradually converted by natural selection into
branchiæ, simply through an increase in their size and the obliteration
of their adhesive glands. If all pedunculated cirripedes had become
extinct, and they have suffered far more extinction than have sessile
cirripedes, who would ever have imagined that the branchiæ in this
latter family had originally existed as organs for preventing the ova
from being washed out of the sack?

There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has
lately been insisted on by Professor Cope and others in the United
States. It is now known that some animals are capable of reproduction
at a very early age, before they have acquired their perfect
characters; and if this power became thoroughly well developed in a
species, it seems probable that the adult stage of development would
sooner or later be lost; and in this case, especially if the larva
differed much from the mature form, the character of the species would
be greatly changed and degraded. Again, not a few animals, after
arriving at maturity, go on changing in character during nearly their
whole lives. With mammals, for instance, the form of the skull is often
much altered with age, of which Dr. Murie has given some striking
instances with seals. Every one knows how the horns of stags become
more and more branched, and the plumes of some birds become more finely
developed, as they grow older. Professor Cope states that the teeth of
certain lizards change much in shape with advancing years. With
crustaceans not only many trivial, but some important parts assume a
new character, as recorded by Fritz Müller, after maturity. In all such
cases—and many could be given—if the age for reproduction were
retarded, the character of the species, at least in its adult state,
would be modified; nor is it improbable that the previous and earlier
stages of development would in some cases be hurried through and
finally lost. Whether species have often or ever been modified through
this comparatively sudden mode of transition, I can form no opinion;
but if this has occurred, it is probable that the differences between
the young and the mature, and between the mature and the old, were
primordially acquired by graduated steps.

_Special Diffculties of the Theory of Natural Selection._


Although we must be extremely cautious in concluding that any organ
could not have been produced by successive, small, transitional
gradations, yet undoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs
of fishes offer another case of special difficulty; for it is
impossible to conceive by what steps these wondrous organs have been
produced. But this is not surprising, for we do not even know of what
use they are. In the gymnotus and torpedo they no doubt serve as
powerful means of defence, and perhaps for securing prey; yet in the
ray, as observed by Matteucci, an analogous organ in the tail manifests
but little electricity, even when the animal is greatly irritated; so
little that it can hardly be of any use for the above purposes.
Moreover, in the ray, besides the organ just referred to, there is, as
Dr. R. McDonnell has shown, another organ near the head, not known to
be electrical, but which appears to be the real homologue of the
electric battery in the torpedo. It is generally admitted that there
exists between these organs and ordinary muscle a close analogy, in
intimate structure, in the distribution of the nerves, and in the
manner in which they are acted on by various reagents. It should, also,
be especially observed that muscular contraction is accompanied by an
electrical discharge; and, as Dr. Radcliffe insists, “in the electrical
apparatus of the torpedo during rest, there would seem to be a charge
in every respect like that which is met with in muscle and nerve during
the rest, and the discharge of the torpedo, instead of being peculiar,
may be only another form of the discharge which attends upon the action
of muscle and motor nerve.” Beyond this we cannot at present go in the
way of explanation; but as we know so little about the uses of these
organs, and as we know nothing about the habits and structure of the
progenitors of the existing electric fishes, it would be extremely bold
to maintain that no serviceable transitions are possible by which these
organs might have been gradually developed.

These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which
several are widely remote in their affinities. When the same organ is
found in several members of the same class, especially if in members
having very different habits of life, we may generally attribute its
presence to inheritance from a common ancestor; and its absence in some
of the members to loss through disuse or natural selection. So that, if
the electric organs had been inherited from some one ancient
progenitor, we might have expected that all electric fishes would have
been specially related to each other; but this is far from the case.
Nor does geology at all lead to the belief that most fishes formerly
possessed electric organs, which their modified descendants have now
lost. But when we look at the subject more closely, we find in the
several fishes provided with electric organs, that these are situated
in different parts of the body, that they differ in construction, as in
the arrangement of the plates, and, according to Pacini, in the process
or means by which the electricity is excited—and lastly, in being
supplied with nerves proceeding from different sources, and this is
perhaps the most important of all the differences. Hence in the several
fishes furnished with electric organs, these cannot be considered as
homologous, but only as analogous in function. Consequently there is no
reason to suppose that they have been inherited from a common
progenitor; for had this been the case they would have closely
resembled each other in all respects. Thus the difficulty of an organ,
apparently the same, arising in several remotely allied species,
disappears, leaving only the lesser yet still great difficulty: namely,
by what graduated steps these organs have been developed in each
separate group of fishes.

The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the
body, offer, under our present state of ignorance, a difficulty almost
exactly parallel with that of the electric organs. Other similar cases
could be given; for instance in plants, the very curious contrivance of
a mass of pollen-grains, borne on a foot-stalk with an adhesive gland,
is apparently the same in Orchis and Asclepias, genera almost as remote
as is possible among flowering plants; but here again the parts are not
homologous. In all cases of beings, far removed from each other in the
scale of organisation, which are furnished with similar and peculiar
organs, it will be found that although the general appearance and
function of the organs may be the same, yet fundamental differences
between them can always be detected. For instance, the eyes of
Cephalopods or cuttle-fish and of vertebrate animals appear wonderfully
alike; and in such widely sundered groups no part of this resemblance
can be due to inheritance from a common progenitor. Mr. Mivart has
advanced this case as one of special difficulty, but I am unable to see
the force of his argument. An organ for vision must be formed of
transparent tissue, and must include some sort of lens for throwing an
image at the back of a darkened chamber. Beyond this superficial
resemblance, there is hardly any real similarity between the eyes of
cuttle-fish and vertebrates, as may be seen by consulting Hensen’s
admirable memoir on these organs in the Cephalopoda. It is impossible
for me here to enter on details, but I may specify a few of the points
of difference. The crystalline lens in the higher cuttle-fish consists
of two parts, placed one behind the other like two lenses, both having
a very different structure and disposition to what occurs in the
vertebrata. The retina is wholly different, with an actual inversion of
the elemental parts, and with a large nervous ganglion included within
the membranes of the eye. The relations of the muscles are as different
as it is possible to conceive, and so in other points. Hence it is not
a little difficult to decide how far even the same terms ought to be
employed in describing the eyes of the Cephalopoda and Vertebrata. It
is, of course, open to any one to deny that the eye in either case
could have been developed through the natural selection of successive
slight variations; but if this be admitted in the one case it is
clearly possible in the other; and fundamental differences of structure
in the visual organs of two groups might have been anticipated, in
accordance with this view of their manner of formation. As two men have
sometimes independently hit on the same invention, so in the several
foregoing cases it appears that natural selection, working for the good
of each being, and taking advantage of all favourable variations, has
produced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to
inheritance from a common progenitor.

Fritz Müller, in order to test the conclusions arrived at in this
volume, has followed out with much care a nearly similar line of
argument. Several families of crustaceans include a few species,
possessing an air-breathing apparatus and fitted to live out of the
water. In two of these families, which were more especially examined by
Müller, and which are nearly related to each other, the species agree
most closely in all important characters: namely in their sense organs,
circulating systems, in the position of the tufts of hair within their
complex stomachs, and lastly in the whole structure of the
water-breathing branchiæ, even to the microscopical hooks by which they
are cleansed. Hence it might have been expected that in the few species
belonging to both families which live on the land, the equally
important air-breathing apparatus would have been the same; for why
should this one apparatus, given for the same purpose, have been made
to differ, whilst all the other important organs were closely similar,
or rather, identical.

Fritz Müller argues that this close similarity in so many points of
structure must, in accordance with the views advanced by me, be
accounted for by inheritance from a common progenitor. But as the vast
majority of the species in the above two families, as well as most
other crustaceans, are aquatic in their habits, it is improbable in the
highest degree that their common progenitor should have been adapted
for breathing air. Müller was thus led carefully to examine the
apparatus in the air-breathing species; and he found it to differ in
each in several important points, as in the position of the orifices,
in the manner in which they are opened and closed, and in some
accessory details. Now such differences are intelligible, and might
even have been expected, on the supposition that species belonging to
distinct families had slowly become adapted to live more and more out
of water, and to breathe the air. For these species, from belonging to
distinct families, would have differed to a certain extent, and in
accordance with the principle that the nature of each variation depends
on two factors, viz., the nature of the organism and that of the
surrounding conditions, their variability assuredly would not have been
exactly the same. Consequently natural selection would have had
different materials or variations to work on, in order to arrive at the
same functional result; and the structures thus acquired would almost
necessarily have differed. On the hypothesis of separate acts of
creation the whole case remains unintelligible. This line of argument
seems to have had great weight in leading Fritz Müller to accept the
views maintained by me in this volume.

Another distinguished zoologist, the late Professor Claparède, has
argued in the same manner, and has arrived at the same result. He shows
that there are parasitic mites (Acaridæ), belonging to distinct
sub-families and families, which are furnished with hair-claspers.
These organs must have been independently developed, as they could not
have been inherited from a common progenitor; and in the several groups
they are formed by the modification of the fore legs, of the hind legs,
of the maxillæ or lips, and of appendages on the under side of the hind
part of the body.

In the foregoing cases, we see the same end gained and the same
function performed, in beings not at all or only remotely allied, by
organs in appearance, though not in development, closely similar. On
the other hand, it is a common rule throughout nature that the same end
should be gained, even sometimes in the case of closely related beings,
by the most diversified means. How differently constructed is the
feathered wing of a bird and the membrane-covered wing of a bat; and
still more so the four wings of a butterfly, the two wings of a fly,
and the two wings with the elytra of a beetle. Bivalve shells are made
to open and shut, but on what a number of patterns is the hinge
constructed, from the long row of neatly interlocking teeth in a Nucula
to the simple ligament of a Mussel! Seeds are disseminated by their
minuteness, by their capsule being converted into a light balloon-like
envelope, by being embedded in pulp or flesh, formed of the most
diverse parts, and rendered nutritious, as well as conspicuously
coloured, so as to attract and be devoured by birds, by having hooks
and grapnels of many kinds and serrated awns, so as to adhere to the
fur of quadrupeds, and by being furnished with wings and plumes, as
different in shape as they are elegant in structure, so as to be wafted
by every breeze. I will give one other instance: for this subject of
the same end being gained by the most diversified means well deserves
attention. Some authors maintain that organic beings have been formed
in many ways for the sake of mere variety, almost like toys in a shop,
but such a view of nature is incredible. With plants having separated
sexes, and with those in which, though hermaphrodites, the pollen does
not spontaneously fall on the stigma, some aid is necessary for their
fertilisation. With several kinds this is effected by the
pollen-grains, which are light and incoherent, being blown by the wind
through mere chance on to the stigma; and this is the simplest plan
which can well be conceived. An almost equally simple, though very
different plan occurs in many plants in which a symmetrical flower
secretes a few drops of nectar, and is consequently visited by insects;
and these carry the pollen from the anthers to the stigma.

From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the
same manner, but entailing changes in every part of the flower. The
nectar may be stored in variously shaped receptacles, with the stamens
and pistils modified in many ways, sometimes forming trap-like
contrivances, and sometimes capable of neatly adapted movements through
irritability or elasticity. From such structures we may advance till we
come to such a case of extraordinary adaptation as that lately
described by Dr. Crüger in the Coryanthes. This orchid has part of its
labellum or lower lip hollowed out into a great bucket, into which
drops of almost pure water continually fall from two secreting horns
which stand above it; and when the bucket is half-full, the water
overflows by a spout on one side. The basal part of the labellum stands
over the bucket, and is itself hollowed out into a sort of chamber with
two lateral entrances; within this chamber there are curious fleshy
ridges. The most ingenious man, if he had not witnessed what takes
place, could never have imagined what purpose all these parts serve.
But Dr. Crüger saw crowds of large humble-bees visiting the gigantic
flowers of this orchid, not in order to suck nectar, but to gnaw off
the ridges within the chamber above the bucket; in doing this they
frequently pushed each other into the bucket, and their wings being
thus wetted they could not fly away, but were compelled to crawl out
through the passage formed by the spout or overflow. Dr. Crüger saw a
“continual procession” of bees thus crawling out of their involuntary
bath. The passage is narrow, and is roofed over by the column, so that
a bee, in forcing its way out, first rubs its back against the viscid
stigma and then against the viscid glands of the pollen-masses. The
pollen-masses are thus glued to the back of the bee which first happens
to crawl out through the passage of a lately expanded flower, and are
thus carried away. Dr. Crüger sent me a flower in spirits of wine, with
a bee which he had killed before it had quite crawled out, with a
pollen-mass still fastened to its back. When the bee, thus provided,
flies to another flower, or to the same flower a second time, and is
pushed by its comrades into the bucket and then crawls out by the
passage, the pollen-mass necessarily comes first into contact with the
viscid stigma, and adheres to it, and the flower is fertilised. Now at
last we see the full use of every part of the flower, of the
water-secreting horns of the bucket half-full of water, which prevents
the bees from flying away, and forces them to crawl out through the
spout, and rub against the properly placed viscid pollen-masses and the
viscid stigma.

The construction of the flower in another closely allied orchid,
namely, the Catasetum, is widely different, though serving the same
end; and is equally curious. Bees visit these flowers, like those of
the Coryanthes, in order to gnaw the labellum; in doing this they
inevitably touch a long, tapering, sensitive projection, or, as I have
called it, the antenna. This antenna, when touched, transmits a
sensation or vibration to a certain membrane which is instantly
ruptured; this sets free a spring by which the pollen-mass is shot
forth, like an arrow, in the right direction, and adheres by its viscid
extremity to the back of the bee. The pollen-mass of the male plant
(for the sexes are separate in this orchid) is thus carried to the
flower of the female plant, where it is brought into contact with the
stigma, which is viscid enough to break certain elastic threads, and
retain the pollen, thus effecting fertilisation.

How, it may be asked, in the foregoing and in innumerable other
instances, can we understand the graduated scale of complexity and the
multifarious means for gaining the same end. The answer no doubt is, as
already remarked, that when two forms vary, which already differ from
each other in some slight degree, the variability will not be of the
same exact nature, and consequently the results obtained through
natural selection for the same general purpose will not be the same. We
should also bear in mind that every highly developed organism has
passed through many changes; and that each modified structure tends to
be inherited, so that each modification will not readily be quite lost,
but may be again and again further altered. Hence, the structure of
each part of each species, for whatever purpose it may serve, is the
sum of many inherited changes, through which the species has passed
during its successive adaptations to changed habits and conditions of
life.

Finally, then, although in many cases it is most difficult even to
conjecture by what transitions organs could have arrived at their
present state; yet, considering how small the proportion of living and
known forms is to the extinct and unknown, I have been astonished how
rarely an organ can be named, towards which no transitional grade is
known to lead. It is certainly true, that new organs appearing as if
created for some special purpose rarely or never appear in any being;
as indeed is shown by that old, but somewhat exaggerated, canon in
natural history of “Natura non facit saltum.” We meet with this
admission in the writings of almost every experienced naturalist; or,
as Milne Edwards has well expressed it, “Nature is prodigal in variety,
but niggard in innovation.” Why, on the theory of Creation, should
there be so much variety and so little real novelty? Why should all the
parts and organs of many independent beings, each supposed to have been
separately created for its own proper place in nature, be so commonly
linked together by graduated steps? Why should not Nature take a sudden
leap from structure to structure? On the theory of natural selection,
we can clearly understand why she should not; for natural selection
acts only by taking advantage of slight successive variations; she can
never take a great and sudden leap, but must advance by the short and
sure, though slow steps.

_Organs of little apparent Importance, as affected by Natural
Selection._


As natural selection acts by life and death, by the survival of the
fittest, and by the destruction of the less well-fitted individuals, I
have sometimes felt great difficulty in understanding the origin or
formation of parts of little importance; almost as great, though of a
very different kind, as in the case of the most perfect and complex
organs.

In the first place, we are much too ignorant in regard to the whole
economy of any one organic being to say what slight modifications would
be of importance or not. In a former chapter I have given instances of
very trifling characters, such as the down on fruit and the colour of
its flesh, the colour of the skin and hair of quadrupeds, which, from
being correlated with constitutional differences, or from determining
the attacks of insects, might assuredly be acted on by natural
selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this
could have been adapted for its present purpose by successive slight
modifications, each better and better fitted, for so trifling an object
as to drive away flies; yet we should pause before being too positive
even in this case, for we know that the distribution and existence of
cattle and other animals in South America absolutely depend on their
power of resisting the attacks of insects: so that individuals which
could by any means defend themselves from these small enemies, would be
able to range into new pastures and thus gain a great advantage. It is
not that the larger quadrupeds are actually destroyed (except in some
rare cases) by flies, but they are incessantly harassed and their
strength reduced, so that they are more subject to disease, or not so
well enabled in a coming dearth to search for food, or to escape from
beasts of prey.

Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly
perfected at a former period, have been transmitted to existing species
in nearly the same state, although now of very slight use; but any
actually injurious deviations in their structure would of course have
been checked by natural selection. Seeing how important an organ of
locomotion the tail is in most aquatic animals, its general presence
and use for many purposes in so many land animals, which in their lungs
or modified swim-bladders betray their aquatic origin, may perhaps be
thus accounted for. A well-developed tail having been formed in an
aquatic animal, it might subsequently come to be worked in for all
sorts of purposes, as a fly-flapper, an organ of prehension, or as an
aid in turning, as in the case of the dog, though the aid in this
latter respect must be slight, for the hare, with hardly any tail, can
double still more quickly.

In the second place, we may easily err in attributing importance to
characters, and in believing that they have been developed through
natural selection. We must by no means overlook the effects of the
definite action of changed conditions of life, of so-called spontaneous
variations, which seem to depend in a quite subordinate degree on the
nature of the conditions, of the tendency to reversion to long-lost
characters, of the complex laws of growth, such as of correlation,
comprehension, of the pressure of one part on another, &c., and finally
of sexual selection, by which characters of use to one sex are often
gained and then transmitted more or less perfectly to the other sex,
though of no use to the sex. But structures thus indirectly gained,
although at first of no advantage to a species, may subsequently have
been taken advantage of by its modified descendants, under new
conditions of life and newly acquired habits.

If green woodpeckers alone had existed, and we did not know that there
were many black and pied kinds, I dare say that we should have thought
that the green colour was a beautiful adaptation to conceal this
tree-frequenting bird from its enemies; and consequently that it was a
character of importance, and had been acquired through natural
selection; as it is, the colour is probably in chief part due to sexual
selection. A trailing palm in the Malay Archipelago climbs the loftiest
trees by the aid of exquisitely constructed hooks clustered around the
ends of the branches, and this contrivance, no doubt, is of the highest
service to the plant; but as we see nearly similar hooks on many trees
which are not climbers, and which, as there is reason to believe from
the distribution of the thorn-bearing species in Africa and South
America, serve as a defence against browsing quadrupeds, so the spikes
on the palm may at first have been developed for this object, and
subsequently have been improved and taken advantage of by the plant, as
it underwent further modification and became a climber. The naked skin
on the head of a vulture is generally considered as a direct adaptation
for wallowing in putridity; and so it may be, or it may possibly be due
to the direct action of putrid matter; but we should be very cautious
in drawing any such inference, when we see that the skin on the head of
the clean-feeding male turkey is likewise naked. The sutures in the
skulls of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has
been taken advantage of in the parturition of the higher animals.

We are profoundly ignorant of the cause of each slight variation or
individual difference; and we are immediately made conscious of this by
reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized
countries, where there has been but little methodical selection.
Animals kept by savages in different countries often have to struggle
for their own subsistence, and are exposed to a certain extent to
natural selection, and individuals with slightly different
constitutions would succeed best under different climates. With cattle
susceptibility to the attacks of flies is correlated with colour, as is
the liability to be poisoned by certain plants; so that even colour
would be thus subjected to the action of natural selection. Some
observers are convinced that a damp climate affects the growth of the
hair, and that with the hair the horns are correlated. Mountain breeds
always differ from lowland breeds; and a mountainous country would
probably affect the hind limbs from exercising them more, and possibly
even the form of the pelvis; and then by the law of homologous
variation, the front limbs and the head would probably be affected. The
shape, also, of the pelvis might affect by pressure the shape of
certain parts of the young in the womb. The laborious breathing
necessary in high regions tends, as we have good reason to believe, to
increase the size of the chest; and again correlation would come into
play. The effects of lessened exercise, together with abundant food, on
the whole organisation is probably still more important, and this, as
H. von Nathusius has lately shown in his excellent Treatise, is
apparently one chief cause of the great modification which the breeds
of swine have undergone. But we are far too ignorant to speculate on
the relative importance of the several known and unknown causes of
variation; and I have made these remarks only to show that, if we are
unable to account for the characteristic differences of our several
domestic breeds, which nevertheless are generally admitted to have
arisen through ordinary generation from one or a few parent-stocks, we
ought not to lay too much stress on our ignorance of the precise cause
of the slight analogous differences between true species.

_Utilitarian Doctrine, how far true: Beauty, how acquired._


The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor.
They believe that many structures have been created for the sake of
beauty, to delight man or the Creator (but this latter point is beyond
the scope of scientific discussion), or for the sake of mere variety, a
view already discussed. Such doctrines, if true, would be absolutely
fatal to my theory. I fully admit that many structures are now of no
direct use to their possessors, and may never have been of any use to
their progenitors; but this does not prove that they were formed solely
for beauty or variety. No doubt the definite action of changed
conditions, and the various causes of modifications, lately specified,
have all produced an effect, probably a great effect, independently of
any advantage thus gained. But a still more important consideration is
that the chief part of the organisation of every living creature is due
to inheritance; and consequently, though each being assuredly is well
fitted for its place in nature, many structures have now no very close
and direct relation to present habits of life. Thus, we can hardly
believe that the webbed feet of the upland goose, or of the
frigate-bird, are of special use to these birds; we cannot believe that
the similar bones in the arm of the monkey, in the fore leg of the
horse, in the wing of the bat, and in the flipper of the seal, are of
special use to these animals. We may safely attribute these structures
to inheritance. But webbed feet no doubt were as useful to the
progenitor of the upland goose and of the frigate-bird, as they now are
to the most aquatic of living birds. So we may believe that the
progenitor of the seal did not possess a flipper, but a foot with five
toes fitted for walking or grasping; and we may further venture to
believe that the several bones in the limbs of the monkey, horse and
bat, were originally developed, on the principle of utility, probably
through the reduction of more numerous bones in the fin of some ancient
fish-like progenitor of the whole class. It is scarcely possible to
decide how much allowance ought to be made for such causes of change,
as the definite action of external conditions, so-called spontaneous
variations, and the complex laws of growth; but with these important
exceptions, we may conclude that the structure of every living creature
either now is, or was formerly, of some direct or indirect use to its
possessor.

With respect to the belief that organic beings have been created
beautiful for the delight of man—a belief which it has been pronounced
is subversive of my whole theory—I may first remark that the sense of
beauty obviously depends on the nature of the mind, irrespective of any
real quality in the admired object; and that the idea of what is
beautiful, is not innate or unalterable. We see this, for instance, in
the men of different races admiring an entirely different standard of
beauty in their women. If beautiful objects had been created solely for
man’s gratification, it ought to be shown that before man appeared
there was less beauty on the face of the earth than since he came on
the stage. Were the beautiful volute and cone shells of the Eocene
epoch, and the gracefully sculptured ammonites of the Secondary period,
created that man might ages afterwards admire them in his cabinet? Few
objects are more beautiful than the minute siliceous cases of the
diatomaceæ: were these created that they might be examined and admired
under the higher powers of the microscope? The beauty in this latter
case, and in many others, is apparently wholly due to symmetry of
growth. Flowers rank among the most beautiful productions of nature;
but they have been rendered conspicuous in contrast with the green
leaves, and in consequence at the same time beautiful, so that they may
be easily observed by insects. I have come to this conclusion from
finding it an invariable rule that when a flower is fertilised by the
wind it never has a gaily-coloured corolla. Several plants habitually
produce two kinds of flowers; one kind open and coloured so as to
attract insects; the other closed, not coloured, destitute of nectar,
and never visited by insects. Hence, we may conclude that, if insects
had not been developed on the face of the earth, our plants would not
have been decked with beautiful flowers, but would have produced only
such poor flowers as we see on our fir, oak, nut and ash trees, on
grasses, spinach, docks and nettles, which are all fertilised through
the agency of the wind. A similar line of argument holds good with
fruits; that a ripe strawberry or cherry is as pleasing to the eye as
to the palate—that the gaily-coloured fruit of the spindle-wood tree
and the scarlet berries of the holly are beautiful objects—will be
admitted by everyone. But this beauty serves merely as a guide to birds
and beasts, in order that the fruit may be devoured and the matured
seeds disseminated. I infer that this is the case from having as yet
found no exception to the rule that seeds are always thus disseminated
when embedded within a fruit of any kind (that is within a fleshy or
pulpy envelope), if it be coloured of any brilliant tint, or rendered
conspicuous by being white or black.

On the other hand, I willingly admit that a great number of male
animals, as all our most gorgeous birds, some fishes, reptiles, and
mammals, and a host of magnificently coloured butterflies, have been
rendered beautiful for beauty’s sake. But this has been effected
through sexual selection, that is, by the more beautiful males having
been continually preferred by the females, and not for the delight of
man. So it is with the music of birds. We may infer from all this that
a nearly similar taste for beautiful colours and for musical sounds
runs through a large part of the animal kingdom. When the female is as
beautifully coloured as the male, which is not rarely the case with
birds and butterflies, the cause apparently lies in the colours
acquired through sexual selection having been transmitted to both
sexes, instead of to the males alone. How the sense of beauty in its
simplest form—that is, the reception of a peculiar kind of pleasure
from certain colours, forms and sounds—was first developed in the mind
of man and of the lower animals, is a very obscure subject. The same
sort of difficulty is presented if we enquire how it is that certain
flavours and odours give pleasure, and others displeasure. Habit in all
these cases appears to have come to a certain extent into play; but
there must be some fundamental cause in the constitution of the nervous
system in each species.

Natural selection cannot possibly produce any modification in a species
exclusively for the good of another species; though throughout nature
one species incessantly takes advantage of, and profits by the
structures of others. But natural selection can and does often produce
structures for the direct injury of other animals, as we see in the
fang of the adder, and in the ovipositor of the ichneumon, by which its
eggs are deposited in the living bodies of other insects. If it could
be proved that any part of the structure of any one species had been
formed for the exclusive good of another species, it would annihilate
my theory, for such could not have been produced through natural
selection. Although many statements may be found in works on natural
history to this effect, I cannot find even one which seems to me of any
weight. It is admitted that the rattlesnake has a poison-fang for its
own defence and for the destruction of its prey; but some authors
suppose that at the same time it is furnished with a rattle for its own
injury, namely, to warn its prey. I would almost as soon believe that
the cat curls the end of its tail when preparing to spring, in order to
warn the doomed mouse. It is a much more probable view that the
rattlesnake uses its rattle, the cobra expands its frill and the
puff-adder swells while hissing so loudly and harshly, in order to
alarm the many birds and beasts which are known to attack even the most
venomous species. Snakes act on the same principle which makes the hen
ruffle her feathers and expand her wings when a dog approaches her
chickens. But I have not space here to enlarge on the many ways by
which animals endeavour to frighten away their enemies.

Natural selection will never produce in a being any structure more
injurious than beneficial to that being, for natural selection acts
solely by and for the good of each. No organ will be formed, as Paley
has remarked, for the purpose of causing pain or for doing an injury to
its possessor. If a fair balance be struck between the good and evil
caused by each part, each will be found on the whole advantageous.
After the lapse of time, under changing conditions of life, if any part
comes to be injurious, it will be modified; or if it be not so, the
being will become extinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than the other inhabitants of the same country
with which it comes into competition. And we see that this is the
standard of perfection attained under nature. The endemic productions
of New Zealand, for instance, are perfect, one compared with another;
but they are now rapidly yielding before the advancing legions of
plants and animals introduced from Europe. Natural selection will not
produce absolute perfection, nor do we always meet, as far as we can
judge, with this high standard under nature. The correction for the
aberration of light is said by Müller not to be perfect even in that
most perfect organ, the human eye. Helmholtz, whose judgment no one
will dispute, after describing in the strongest terms the wonderful
powers of the human eye, adds these remarkable words: “That which we
have discovered in the way of inexactness and imperfection in the
optical machine and in the image on the retina, is as nothing in
comparison with the incongruities which we have just come across in the
domain of the sensations. One might say that nature has taken delight
in accumulating contradictions in order to remove all foundation from
the theory of a pre-existing harmony between the external and internal
worlds.” If our reason leads us to admire with enthusiasm a multitude
of inimitable contrivances in nature, this same reason tells us, though
we may easily err on both sides, that some other contrivances are less
perfect. Can we consider the sting of the bee as perfect, which, when
used against many kinds of enemies, cannot be withdrawn, owing to the
backward serratures, and thus inevitably causes the death of the insect
by tearing out its viscera?

If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so many
members of the same great order, and that it has since been modified
but not perfected for its present purpose, with the poison originally
adapted for some other object, such as to produce galls, since
intensified, we can perhaps understand how it is that the use of the
sting should so often cause the insect’s own death: for if on the whole
the power of stinging be useful to the social community, it will fulfil
all the requirements of natural selection, though it may cause the
death of some few members. If we admire the truly wonderful power of
scent by which the males of many insects find their females, can we
admire the production for this single purpose of thousands of drones,
which are utterly useless to the community for any other purpose, and
which are ultimately slaughtered by their industrious and sterile
sisters? It may be difficult, but we ought to admire the savage
instinctive hatred of the queen-bee, which urges her to destroy the
young queens, her daughters, as soon as they are born, or to perish
herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter
fortunately is most rare, is all the same to the inexorable principles
of natural selection. If we admire the several ingenious contrivances
by which orchids and many other plants are fertilised through insect
agency, can we consider as equally perfect the elaboration of dense
clouds of pollen by our fir-trees, so that a few granules may be wafted
by chance on to the ovules?

_Summary: the Law of Unity of Type and of the Conditions of Existence
embraced by the Theory of Natural Selection._


We have in this chapter discussed some of the difficulties and
objections which may be urged against the theory. Many of them are
serious; but I think that in the discussion light has been thrown on
several facts, which on the belief of independent acts of creation are
utterly obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural
selection is always very slow, and at any one time acts only on a few
forms; and partly because the very process of natural selection implies
the continual supplanting and extinction of preceding and intermediate
gradations. Closely allied species, now living on a continuous area,
must often have been formed when the area was not continuous, and when
the conditions of life did not insensibly graduate away from one part
to another. When two varieties are formed in two districts of a
continuous area, an intermediate variety will often be formed, fitted
for an intermediate zone; but from reasons assigned, the intermediate
variety will usually exist in lesser numbers than the two forms which
it connects; consequently the two latter, during the course of further
modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first only glided through the air.

We have seen that a species under new conditions of life may change its
habits, or it may have diversified habits, with some very unlike those
of its nearest congeners. Hence we can understand, bearing in mind that
each organic being is trying to live wherever it can live, how it has
arisen that there are upland geese with webbed feet, ground
woodpeckers, diving thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is enough to stagger any one; yet in the
case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then under changing conditions
of life, there is no logical impossibility in the acquirement of any
conceivable degree of perfection through natural selection. In the
cases in which we know of no intermediate or transitional states, we
should be extremely cautious in concluding that none can have existed,
for the metamorphoses of many organs show what wonderful changes in
function are at least possible. For instance, a swim-bladder has
apparently been converted into an air-breathing lung. The same organ
having performed simultaneously very different functions, and then
having been in part or in whole specialised for one function; and two
distinct organs having performed at the same time the same function,
the one having been perfected whilst aided by the other, must often
have largely facilitated transitions.

We have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and independently
formed; but when such organs are closely examined, essential
differences in their structure can almost always be detected; and this
naturally follows from the principle of natural selection. On the other
hand, the common rule throughout nature is infinite diversity of
structure for gaining the same end; and this again naturally follows
from the same great principle.

In many cases we are far too ignorant to be enabled to assert that a
part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumulated
by means of natural selection. In many other cases, modifications are
probably the direct result of the laws of variation or of growth,
independently of any good having been thus gained. But even such
structures have often, as we may feel assured, been subsequently taken
advantage of, and still further modified, for the good of species under
new conditions of life. We may, also, believe that a part formerly of
high importance has frequently been retained (as the tail of an aquatic
animal by its terrestrial descendants), though it has become of such
small importance that it could not, in its present state, have been
acquired by means of natural selection.

Natural selection can produce nothing in one species for the exclusive
good or injury of another; though it may well produce parts, organs,
and excretions highly useful or even indispensable, or highly injurious
to another species, but in all cases at the same time useful to the
possessor. In each well-stocked country natural selection acts through
the competition of the inhabitants and consequently leads to success in
the battle for life, only in accordance with the standard of that
particular country. Hence the inhabitants of one country, generally the
smaller one, often yield to the inhabitants of another and generally
the larger country. For in the larger country there will have existed
more individuals, and more diversified forms, and the competition will
have been severer, and thus the standard of perfection will have been
rendered higher. Natural selection will not necessarily lead to
absolute perfection; nor, as far as we can judge by our limited
faculties, can absolute perfection be everywhere predicated.

On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, “Natura non facit
saltum.” This canon, if we look to the present inhabitants alone of the
world, is not strictly correct; but if we include all those of past
times, whether known or unknown, it must on this theory be strictly
true.

It is generally acknowledged that all organic beings have been formed
on two great laws—Unity of Type, and the Conditions of Existence. By
unity of type is meant that fundamental agreement in structure which we
see in organic beings of the same class, and which is quite independent
of their habits of life. On my theory, unity of type is explained by
unity of descent. The expression of conditions of existence, so often
insisted on by the illustrious Cuvier, is fully embraced by the
principle of natural selection. For natural selection acts by either
now adapting the varying parts of each being to its organic and
inorganic conditions of life; or by having adapted them during past
periods of time: the adaptations being aided in many cases by the
increased use or disuse of parts, being affected by the direct action
of external conditions of life, and subjected in all cases to the
several laws of growth and variation. Hence, in fact, the law of the
Conditions of Existence is the higher law; as it includes, through the
inheritance of former variations and adaptations, that of Unity of
Type.




CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.


Longevity—Modifications not necessarily simultaneous—Modifications
apparently of no direct service—Progressive development—Characters of
small functional importance, the most constant—Supposed incompetence of
natural selection to account for the incipient stages of useful
structures—Causes which interfere with the acquisition through natural
selection of useful structures—Gradations of structure with changed
functions—Widely different organs in members of the same class,
developed from one and the same source—Reasons for disbelieving in
great and abrupt modifications.


I will devote this chapter to the consideration of various
miscellaneous objections which have been advanced against my views, as
some of the previous discussions may thus be made clearer; but it would
be useless to discuss all of them, as many have been made by writers
who have not taken the trouble to understand the subject. Thus a
distinguished German naturalist has asserted that the weakest part of
my theory is, that I consider all organic beings as imperfect: what I
have really said is, that all are not as perfect as they might have
been in relation to their conditions; and this is shown to be the case
by so many native forms in many quarters of the world having yielded
their places to intruding foreigners. Nor can organic beings, even if
they were at any one time perfectly adapted to their conditions of
life, have remained so, when their conditions changed, unless they
themselves likewise changed; and no one will dispute that the physical
conditions of each country, as well as the number and kinds of its
inhabitants, have undergone many mutations.

A critic has lately insisted, with some parade of mathematical
accuracy, that longevity is a great advantage to all species, so that
he who believes in natural selection “must arrange his genealogical
tree” in such a manner that all the descendants have longer lives than
their progenitors! Cannot our critics conceive that a biennial plant or
one of the lower animals might range into a cold climate and perish
there every winter; and yet, owing to advantages gained through natural
selection, survive from year to year by means of its seeds or ova? Mr.
E. Ray Lankester has recently discussed this subject, and he concludes,
as far as its extreme complexity allows him to form a judgment, that
longevity is generally related to the standard of each species in the
scale of organisation, as well as to the amount of expenditure in
reproduction and in general activity. And these conditions have, it is
probable, been largely determined through natural selection.

It has been argued that, as none of the animals and plants of Egypt, of
which we know anything, have changed during the last three or four
thousand years, so probably have none in any part of the world. But, as
Mr. G.H. Lewes has remarked, this line of argument proves too much, for
the ancient domestic races figured on the Egyptian monuments, or
embalmed, are closely similar or even identical with those now living;
yet all naturalists admit that such races have been produced through
the modification of their original types. The many animals which have
remained unchanged since the commencement of the glacial period, would
have been an incomparably stronger case, for these have been exposed to
great changes of climate and have migrated over great distances;
whereas, in Egypt, during the last several thousand years, the
conditions of life, as far as we know, have remained absolutely
uniform. The fact of little or no modification having been effected
since the glacial period, would have been of some avail against those
who believe in an innate and necessary law of development, but is
powerless against the doctrine of natural selection or the survival of
the fittest, which implies that when variations or individual
differences of a beneficial nature happen to arise, these will be
preserved; but this will be effected only under certain favourable
circumstances.

The celebrated palæontologist, Bronn, at the close of his German
translation of this work, asks how, on the principle of natural
selection, can a variety live side by side with the parent species? If
both have become fitted for slightly different habits of life or
conditions, they might live together; and if we lay on one side
polymorphic species, in which the variability seems to be of a peculiar
nature, and all mere temporary variations, such as size, albinism, &c.,
the more permanent varieties are generally found, as far as I can
discover, inhabiting distinct stations, such as high land or low land,
dry or moist districts. Moreover, in the case of animals which wander
much about and cross freely, their varieties seem to be generally
confined to distinct regions.

Bronn also insists that distinct species never differ from each other
in single characters, but in many parts; and he asks, how it always
comes that many parts of the organisation should have been modified at
the same time through variation and natural selection? But there is no
necessity for supposing that all the parts of any being have been
simultaneously modified. The most striking modifications, excellently
adapted for some purpose, might, as was formerly remarked, be acquired
by successive variations, if slight, first in one part and then in
another; and as they would be transmitted all together, they would
appear to us as if they had been simultaneously developed. The best
answer, however, to the above objection is afforded by those domestic
races which have been modified, chiefly through man’s power of
selection, for some special purpose. Look at the race and dray-horse,
or at the greyhound and mastiff. Their whole frames, and even their
mental characteristics, have been modified; but if we could trace each
step in the history of their transformation—and the latter steps can be
traced—we should not see great and simultaneous changes, but first one
part and then another slightly modified and improved. Even when
selection has been applied by man to some one character alone—of which
our cultivated plants offer the best instances—it will invariably be
found that although this one part, whether it be the flower, fruit, or
leaves, has been greatly changed, almost all the other parts have been
slightly modified. This may be attributed partly to the principle of
correlated growth, and partly to so-called spontaneous variation.

A much more serious objection has been urged by Bronn, and recently by
Broca, namely, that many characters appear to be of no service whatever
to their possessors, and therefore cannot have been influenced through
natural selection. Bronn adduces the length of the ears and tails in
the different species of hares and mice—the complex folds of enamel in
the teeth of many animals, and a multitude of analogous cases. With
respect to plants, this subject has been discussed by Nägeli in an
admirable essay. He admits that natural selection has effected much,
but he insists that the families of plants differ chiefly from each
other in morphological characters, which appear to be quite unimportant
for the welfare of the species. He consequently believes in an innate
tendency towards progressive and more perfect development. He specifies
the arrangement of the cells in the tissues, and of the leaves on the
axis, as cases in which natural selection could not have acted. To
these may be added the numerical divisions in the parts of the flower,
the position of the ovules, the shape of the seed, when not of any use
for dissemination, &c.

There is much force in the above objection. Nevertheless, we ought, in
the first place, to be extremely cautious in pretending to decide what
structures now are, or have formerly been, of use to each species. In
the second place, it should always be borne in mind that when one part
is modified, so will be other parts, through certain dimly seen causes,
such as an increased or diminished flow of nutriment to a part, mutual
pressure, an early developed part affecting one subsequently developed,
and so forth—as well as through other causes which lead to the many
mysterious cases of correlation, which we do not in the least
understand. These agencies may be all grouped together, for the sake of
brevity, under the expression of the laws of growth. In the third
place, we have to allow for the direct and definite action of changed
conditions of life, and for so-called spontaneous variations, in which
the nature of the conditions apparently plays a quite subordinate part.
Bud-variations, such as the appearance of a moss-rose on a common rose,
or of a nectarine on a peach-tree, offer good instances of spontaneous
variations; but even in these cases, if we bear in mind the power of a
minute drop of poison in producing complex galls, we ought not to feel
too sure that the above variations are not the effect of some local
change in the nature of the sap, due to some change in the conditions.
There must be some efficient cause for each slight individual
difference, as well as for more strongly marked variations which
occasionally arise; and if the unknown cause were to act persistently,
it is almost certain that all the individuals of the species would be
similarly modified.

In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to
spontaneous variability. But it is impossible to attribute to this
cause the innumerable structures which are so well adapted to the
habits of life of each species. I can no more believe in this than that
the well-adapted form of a race-horse or greyhound, which before the
principle of selection by man was well understood, excited so much
surprise in the minds of the older naturalists, can thus be explained.

It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is
hardly necessary to observe that even in the higher and best-known
animals many structures exist, which are so highly developed that no
one doubts that they are of importance, yet their use has not been, or
has only recently been, ascertained. As Bronn gives the length of the
ears and tail in the several species of mice as instances, though
trifling ones, of differences in structure which can be of no special
use, I may mention that, according to Dr. Schöbl, the external ears of
the common mouse are supplied in an extraordinary manner with nerves,
so that they no doubt serve as tactile organs; hence the length of the
ears can hardly be quite unimportant. We shall, also, presently see
that the tail is a highly useful prehensile organ to some of the
species; and its use would be much influence by its length.

With respect to plants, to which on account of Nägeli’s essay I shall
confine myself in the following remarks, it will be admitted that the
flowers of the orchids present a multitude of curious structures, which
a few years ago would have been considered as mere morphological
differences without any special function; but they are now known to be
of the highest importance for the fertilisation of the species through
the aid of insects, and have probably been gained through natural
selection. No one until lately would have imagined that in dimorphic
and trimorphic plants the different lengths of the stamens and pistils,
and their arrangement, could have been of any service, but now we know
this to be the case.

In certain whole groups of plants the ovules stand erect, and in others
they are suspended; and within the same ovarium of some few plants, one
ovule holds the former and a second ovule the latter position. These
positions seem at first purely morphological, or of no physiological
signification; but Dr. Hooker informs me that within the same ovarium
the upper ovules alone in some cases, and in others the lower ones
alone are fertilised; and he suggests that this probably depends on the
direction in which the pollen-tubes enter the ovarium. If so, the
position of the ovules, even when one is erect and the other suspended
within the same ovarium, would follow the selection of any slight
deviations in position which favoured their fertilisation, and the
production of seed.

Several plants belonging to distinct orders habitually produce flowers
of two kinds—the one open, of the ordinary structure, the other closed
and imperfect. These two kinds of flowers sometimes differ wonderfully
in structure, yet may be seen to graduate into each other on the same
plant. The ordinary and open flowers can be intercrossed; and the
benefits which certainly are derived from this process are thus
secured. The closed and imperfect flowers are, however, manifestly of
high importance, as they yield with the utmost safety a large stock of
seed, with the expenditure of wonderfully little pollen. The two kinds
of flowers often differ much, as just stated, in structure. The petals
in the imperfect flowers almost always consist of mere rudiments, and
the pollen-grains are reduced in diameter. In Ononis columnæ five of
the alternate stamens are rudimentary; and in some species of Viola
three stamens are in this state, two retaining their proper function,
but being of very small size. In six out of thirty of the closed
flowers in an Indian violet (name unknown, for the plants have never
produced with me perfect flowers), the sepals are reduced from the
normal number of five to three. In one section of the Malpighiaceæ the
closed flowers, according to A. de Jussieu, are still further modified,
for the five stamens which stand opposite to the sepals are all
aborted, a sixth stamen standing opposite to a petal being alone
developed; and this stamen is not present in the ordinary flowers of
this species; the style is aborted; and the ovaria are reduced from
three to two. Now although natural selection may well have had the
power to prevent some of the flowers from expanding, and to reduce the
amount of pollen, when rendered by the closure of the flowers
superfluous, yet hardly any of the above special modifications can have
been thus determined, but must have followed from the laws of growth,
including the functional inactivity of parts, during the progress of
the reduction of the pollen and the closure of the flowers.

It is so necessary to appreciate the important effects of the laws of
growth, that I will give some additional cases of another kind, namely
of differences in the same part or organ, due to differences in
relative position on the same plant. In the Spanish chestnut, and in
certain fir-trees, the angles of divergence of the leaves differ,
according to Schacht, in the nearly horizontal and in the upright
branches. In the common rue and some other plants, one flower, usually
the central or terminal one, opens first, and has five sepals and
petals, and five divisions to the ovarium; while all the other flowers
on the plant are tetramerous. In the British Adoxa the uppermost flower
generally has two calyx-lobes with the other organs tetramerous, while
the surrounding flowers generally have three calyx-lobes with the other
organs pentamerous. In many Compositæ and Umbelliferæ (and in some
other plants) the circumferential flowers have their corollas much more
developed than those of the centre; and this seems often connected with
the abortion of the reproductive organs. It is a more curious fact,
previously referred to, that the achenes or seeds of the circumference
and centre sometimes differ greatly in form, colour and other
characters. In Carthamus and some other Compositæ the central achenes
alone are furnished with a pappus; and in Hyoseris the same head yields
achenes of three different forms. In certain Umbelliferæ the exterior
seeds, according to Tausch, are orthospermous, and the central one
cœlospermous, and this is a character which was considered by De
Candolle to be in other species of the highest systematic importance.
Professor Braun mentions a Fumariaceous genus, in which the flowers in
the lower part of the spike bear oval, ribbed, one-seeded nutlets; and
in the upper part of the spike, lanceolate, two-valved and two-seeded
siliques. In these several cases, with the exception of that of the
well-developed ray-florets, which are of service in making the flowers
conspicuous to insects, natural selection cannot, as far as we can
judge, have come into play, or only in a quite subordinate manner. All
these modifications follow from the relative position and inter-action
of the parts; and it can hardly be doubted that if all the flowers and
leaves on the same plant had been subjected to the same external and
internal condition, as are the flowers and leaves in certain positions,
all would have been modified in the same manner.

In numerous other cases we find modifications of structure, which are
considered by botanists to be generally of a highly important nature,
affecting only some of the flowers on the same plant, or occurring on
distinct plants, which grow close together under the same conditions.
As these variations seem of no special use to the plants, they cannot
have been influenced by natural selection. Of their cause we are quite
ignorant; we cannot even attribute them, as in the last class of cases,
to any proximate agency, such as relative position. I will give only a
few instances. It is so common to observe on the same plant, flowers
indifferently tetramerous, pentamerous, &c., that I need not give
examples; but as numerical variations are comparatively rare when the
parts are few, I may mention that, according to De Candolle, the
flowers of Papaver bracteatum offer either two sepals with four petals
(which is the common type with poppies), or three sepals with six
petals. The manner in which the petals are folded in the bud is in most
groups a very constant morphological character; but Professor Asa Gray
states that with some species of Mimulus, the æstivation is almost as
frequently that of the Rhinanthideæ as of the Antirrhinideæ, to which
latter tribe the genus belongs. Aug. St. Hilaire gives the following
cases: the genus Zanthoxylon belongs to a division of the Rutaceæ with
a single ovary, but in some species flowers may be found on the same
plant, and even in the same panicle, with either one or two ovaries. In
Helianthemum the capsule has been described as unilocular or
tri-locular; and in H. mutabile, “Une lame _plus ou moins large_,
s’étend entre le pericarpe et le placenta.” In the flowers of Saponaria
officinalis Dr. Masters has observed instances of both marginal and
free central placentation. Lastly, St. Hilaire found towards the
southern extreme of the range of Gomphia oleæformis two forms which he
did not at first doubt were distinct species, but he subsequently saw
them growing on the same bush; and he then adds, “Voilà donc dans un
même individu des loges et un style qui se rattachent tantôt à un axe
verticale et tantôt à un gynobase.”

We thus see that with plants many morphological changes may be
attributed to the laws of growth and the inter-action of parts,
independently of natural selection. But with respect to Nägeli’s
doctrine of an innate tendency towards perfection or progressive
development, can it be said in the case of these strongly pronounced
variations, that the plants have been caught in the act of progressing
towards a higher state of development? On the contrary, I should infer
from the mere fact of the parts in question differing or varying
greatly on the same plant, that such modifications were of extremely
small importance to the plants themselves, of whatever importance they
may generally be to us for our classifications. The acquisition of a
useless part can hardly be said to raise an organism in the natural
scale; and in the case of the imperfect, closed flowers, above
described, if any new principle has to be invoked, it must be one of
retrogression rather than of progression; and so it must be with many
parasitic and degraded animals. We are ignorant of the exciting cause
of the above specified modifications; but if the unknown cause were to
act almost uniformly for a length of time, we may infer that the result
would be almost uniform; and in this case all the individuals of the
species would be modified in the same manner.

From the fact of the above characters being unimportant for the welfare
of the species, any slight variations which occurred in them would not
have been accumulated and augmented through natural selection. A
structure which has been developed through long-continued selection,
when it ceases to be of service to a species, generally becomes
variable, as we see with rudimentary organs; for it will no longer be
regulated by this same power of selection. But when, from the nature of
the organism and of the conditions, modifications have been induced
which are unimportant for the welfare of the species, they may be, and
apparently often have been, transmitted in nearly the same state to
numerous, otherwise modified, descendants. It cannot have been of much
importance to the greater number of mammals, birds, or reptiles,
whether they were clothed with hair, feathers or scales; yet hair has
been transmitted to almost all mammals, feathers to all birds, and
scales to all true reptiles. A structure, whatever it may be, which is
common to many allied forms, is ranked by us as of high systematic
importance, and consequently is often assumed to be of high vital
importance to the species. Thus, as I am inclined to believe,
morphological differences, which we consider as important—such as the
arrangement of the leaves, the divisions of the flower or of the
ovarium, the position of the ovules, &c., first appeared in many cases
as fluctuating variations, which sooner or later became constant
through the nature of the organism and of the surrounding conditions,
as well as through the intercrossing of distinct individuals, but not
through natural selection; for as these morphological characters do not
affect the welfare of the species, any slight deviations in them could
not have been governed or accumulated through this latter agency. It is
a strange result which we thus arrive at, namely, that characters of
slight vital importance to the species, are the most important to the
systematist; but, as we shall hereafter see when we treat of the
genetic principle of classification, this is by no means so paradoxical
as it may at first appear.

Although we have no good evidence of the existence in organic beings of
an innate tendency towards progressive development, yet this
necessarily follows, as I have attempted to show in the fourth chapter,
through the continued action of natural selection. For the best
definition which has ever been given of a high standard of
organisation, is the degree to which the parts have been specialised or
differentiated; and natural selection tends towards this end, inasmuch
as the parts are thus enabled to perform their functions more
efficiently.

A distinguished zoologist, Mr. St. George Mivart, has recently
collected all the objections which have ever been advanced by myself
and others against the theory of natural selection, as propounded by
Mr. Wallace and myself, and has illustrated them with admirable art and
force. When thus marshalled, they make a formidable array; and as it
forms no part of Mr. Mivart’s plan to give the various facts and
considerations opposed to his conclusions, no slight effort of reason
and memory is left to the reader, who may wish to weigh the evidence on
both sides. When discussing special cases, Mr. Mivart passes over the
effects of the increased use and disuse of parts, which I have always
maintained to be highly important, and have treated in my “Variation
under Domestication” at greater length than, as I believe, any other
writer. He likewise often assumes that I attribute nothing to
variation, independently of natural selection, whereas in the work just
referred to I have collected a greater number of well-established cases
than can be found in any other work known to me. My judgment may not be
trustworthy, but after reading with care Mr. Mivart’s book, and
comparing each section with what I have said on the same head, I never
before felt so strongly convinced of the general truth of the
conclusions here arrived at, subject, of course, in so intricate a
subject, to much partial error.

All Mr. Mivart’s objections will be, or have been, considered in the
present volume. The one new point which appears to have struck many
readers is, “That natural selection is incompetent to account for the
incipient stages of useful structures.” This subject is intimately
connected with that of the gradation of the characters, often
accompanied by a change of function, for instance, the conversion of a
swim-bladder into lungs, points which were discussed in the last
chapter under two headings. Nevertheless, I will here consider in some
detail several of the cases advanced by Mr. Mivart, selecting those
which are the most illustrative, as want of space prevents me from
considering all.

The giraffe, by its lofty stature, much elongated neck, fore legs, head
and tongue, has its whole frame beautifully adapted for browsing on the
higher branches of trees. It can thus obtain food beyond the reach of
the other Ungulata or hoofed animals inhabiting the same country; and
this must be a great advantage to it during dearths. The Niata cattle
in South America show us how small a difference in structure may make,
during such periods, a great difference in preserving an animal’s life.
These cattle can browse as well as others on grass, but from the
projection of the lower jaw they cannot, during the often recurrent
droughts, browse on the twigs of trees, reeds, &c., to which food the
common cattle and horses are then driven; so that at these times the
Niatas perish, if not fed by their owners. Before coming to Mr.
Mivart’s objections, it may be well to explain once again how natural
selection will act in all ordinary cases. Man has modified some of his
animals, without necessarily having attended to special points of
structure, by simply preserving and breeding from the fleetest
individuals, as with the race-horse and greyhound, or as with the
game-cock, by breeding from the victorious birds. So under nature with
the nascent giraffe, the individuals which were the highest browsers
and were able during dearths to reach even an inch or two above the
others, will often have been preserved; for they will have roamed over
the whole country in search of food. That the individuals of the same
species often differ slightly in the relative lengths of all their
parts may be seen in many works of natural history, in which careful
measurements are given. These slight proportional differences, due to
the laws of growth and variation, are not of the slightest use or
importance to most species. But it will have been otherwise with the
nascent giraffe, considering its probable habits of life; for those
individuals which had some one part or several parts of their bodies
rather more elongated than usual, would generally have survived. These
will have intercrossed and left offspring, either inheriting the same
bodily peculiarities, or with a tendency to vary again in the same
manner; while the individuals less favoured in the same respects will
have been the most liable to perish.

We here see that there is no need to separate single pairs, as man
does, when he methodically improves a breed: natural selection will
preserve and thus separate all the superior individuals, allowing them
freely to intercross, and will destroy all the inferior individuals. By
this process long-continued, which exactly corresponds with what I have
called unconscious selection by man, combined, no doubt, in a most
important manner with the inherited effects of the increased use of
parts, it seems to me almost certain that an ordinary hoofed quadruped
might be converted into a giraffe.

To this conclusion Mr. Mivart brings forward two objections. One is
that the increased size of the body would obviously require an
increased supply of food, and he considers it as “very problematical
whether the disadvantages thence arising would not, in times of
scarcity, more than counterbalance the advantages.” But as the giraffe
does actually exist in large numbers in Africa, and as some of the
largest antelopes in the world, taller than an ox, abound there, why
should we doubt that, as far as size is concerned, intermediate
gradations could formerly have existed there, subjected as now to
severe dearths. Assuredly the being able to reach, at each stage of
increased size, to a supply of food, left untouched by the other hoofed
quadrupeds of the country, would have been of some advantage to the
nascent giraffe. Nor must we overlook the fact, that increased bulk
would act as a protection against almost all beasts of prey excepting
the lion; and against this animal, its tall neck—and the taller the
better—would, as Mr. Chauncey Wright has remarked, serve as a
watch-tower. It is from this cause, as Sir S. Baker remarks, that no
animal is more difficult to stalk than the giraffe. This animal also
uses its long neck as a means of offence or defence, by violently
swinging its head armed with stump-like horns. The preservation of each
species can rarely be determined by any one advantage, but by the union
of all, great and small.

Mr. Mivart then asks (and this is his second objection), if natural
selection be so potent, and if high browsing be so great an advantage,
why has not any other hoofed quadruped acquired a long neck and lofty
stature, besides the giraffe, and, in a lesser degree, the camel,
guanaco and macrauchenia? Or, again, why has not any member of the
group acquired a long proboscis? With respect to South Africa, which
was formerly inhabited by numerous herds of the giraffe, the answer is
not difficult, and can best be given by an illustration. In every
meadow in England, in which trees grow, we see the lower branches
trimmed or planed to an exact level by the browsing of the horses or
cattle; and what advantage would it be, for instance, to sheep, if kept
there, to acquire slightly longer necks? In every district some one
kind of animal will almost certainly be able to browse higher than the
others; and it is almost equally certain that this one kind alone could
have its neck elongated for this purpose, through natural selection and
the effects of increased use. In South Africa the competition for
browsing on the higher branches of the acacias and other trees must be
between giraffe and giraffe, and not with the other ungulate animals.

Why, in other quarters of the world, various animals belonging to this
same order have not acquired either an elongated neck or a proboscis,
cannot be distinctly answered; but it is as unreasonable to expect a
distinct answer to such a question as why some event in the history of
mankind did not occur in one country while it did in another. We are
ignorant with respect to the conditions which determine the numbers and
range of each species, and we cannot even conjecture what changes of
structure would be favourable to its increase in some new country. We
can, however, see in a general manner that various causes might have
interfered with the development of a long neck or proboscis. To reach
the foliage at a considerable height (without climbing, for which
hoofed animals are singularly ill-constructed) implies greatly
increased bulk of body; and we know that some areas support singularly
few large quadrupeds, for instance South America, though it is so
luxuriant, while South Africa abounds with them to an unparalleled
degree. Why this should be so we do not know; nor why the later
tertiary periods should have been much more favourable for their
existence than the present time. Whatever the causes may have been, we
can see that certain districts and times would have been much more
favourable than others for the development of so large a quadruped as
the giraffe.

In order that an animal should acquire some structure specially and
largely developed, it is almost indispensable that several other parts
should be modified and coadapted. Although every part of the body
varies slightly, it does not follow that the necessary parts should
always vary in the right direction and to the right degree. With the
different species of our domesticated animals we know that the parts
vary in a different manner and degree, and that some species are much
more variable than others. Even if the fitting variations did arise, it
does not follow that natural selection would be able to act on them and
produce a structure which apparently would be beneficial to the
species. For instance, if the number of individuals existing in a
country is determined chiefly through destruction by beasts of prey—by
external or internal parasites, &c.—as seems often to be the case, then
natural selection will be able to do little, or will be greatly
retarded, in modifying any particular structure for obtaining food.
Lastly, natural selection is a slow process, and the same favourable
conditions must long endure in order that any marked effect should thus
be produced. Except by assigning such general and vague reasons, we
cannot explain why, in many quarters of the world, hoofed quadrupeds
have not acquired much elongated necks or other means for browsing on
the higher branches of trees.

Objections of the same nature as the foregoing have been advanced by
many writers. In each case various causes, besides the general ones
just indicated, have probably interfered with the acquisition through
natural selection of structures, which it is thought would be
beneficial to certain species. One writer asks, why has not the ostrich
acquired the power of flight? But a moment’s reflection will show what
an enormous supply of food would be necessary to give to this bird of
the desert force to move its huge body through the air. Oceanic islands
are inhabited by bats and seals, but by no terrestrial mammals; yet as
some of these bats are peculiar species, they must have long inhabited
their present homes. Therefore Sir C. Lyell asks, and assigns certain
reasons in answer, why have not seals and bats given birth on such
islands to forms fitted to live on the land? But seals would
necessarily be first converted into terrestrial carnivorous animals of
considerable size, and bats into terrestrial insectivorous animals; for
the former there would be no prey; for the bats ground-insects would
serve as food, but these would already be largely preyed on by the
reptiles or birds, which first colonise and abound on most oceanic
islands. Gradations of structure, with each stage beneficial to a
changing species, will be favoured only under certain peculiar
conditions. A strictly terrestrial animal, by occasionally hunting for
food in shallow water, then in streams or lakes, might at last be
converted into an animal so thoroughly aquatic as to brave the open
ocean. But seals would not find on oceanic islands the conditions
favourable to their gradual reconversion into a terrestrial form. Bats,
as formerly shown, probably acquired their wings by at first gliding
through the air from tree to tree, like the so-called flying-squirrels,
for the sake of escaping from their enemies, or for avoiding falls; but
when the power of true flight had once been acquired, it would never be
reconverted back, at least for the above purposes, into the less
efficient power of gliding through the air. Bats, might, indeed, like
many birds, have had their wings greatly reduced in size, or completely
lost, through disuse; but in this case it would be necessary that they
should first have acquired the power of running quickly on the ground,
by the aid of their hind legs alone, so as to compete with birds or
other ground animals; and for such a change a bat seems singularly
ill-fitted. These conjectural remarks have been made merely to show
that a transition of structure, with each step beneficial, is a highly
complex affair; and that there is nothing strange in a transition not
having occurred in any particular case.

Lastly, more than one writer has asked why have some animals had their
mental powers more highly developed than others, as such development
would be advantageous to all? Why have not apes acquired the
intellectual powers of man? Various causes could be assigned; but as
they are conjectural, and their relative probability cannot be weighed,
it would be useless to give them. A definite answer to the latter
question ought not to be expected, seeing that no one can solve the
simpler problem, why, of two races of savages, one has risen higher in
the scale of civilisation than the other; and this apparently implies
increased brain power.

We will return to Mr. Mivart’s other objections. Insects often resemble
for the sake of protection various objects, such as green or decayed
leaves, dead twigs, bits of lichen, flowers, spines, excrement of
birds, and living insects; but to this latter point I shall hereafter
recur. The resemblance is often wonderfully close, and is not confined
to colour, but extends to form, and even to the manner in which the
insects hold themselves. The caterpillars which project motionless like
dead twigs from the bushes on which they feed, offer an excellent
instance of a resemblance of this kind. The cases of the imitation of
such objects as the excrement of birds, are rare and exceptional. On
this head, Mr. Mivart remarks, “As, according to Mr. Darwin’s theory,
there is a constant tendency to indefinite variation, and as the minute
incipient variations will be in _all directions_, they must tend to
neutralize each other, and at first to form such unstable modifications
that it is difficult, if not impossible, to see how such indefinite
oscillations of infinitesimal beginnings can ever build up a
sufficiently appreciable resemblance to a leaf, bamboo, or other
object, for natural selection to seize upon and perpetuate.”

But in all the foregoing cases the insects in their original state no
doubt presented some rude and accidental resemblance to an object
commonly found in the stations frequented by them. Nor is this at all
improbable, considering the almost infinite number of surrounding
objects and the diversity in form and colour of the hosts of insects
which exist. As some rude resemblance is necessary for the first start,
we can understand how it is that the larger and higher animals do not
(with the exception, as far as I know, of one fish) resemble for the
sake of protection special objects, but only the surface which commonly
surrounds them, and this chiefly in colour. Assuming that an insect
originally happened to resemble in some degree a dead twig or a decayed
leaf, and that it varied slightly in many ways, then all the variations
which rendered the insect at all more like any such object, and thus
favoured its escape, would be preserved, while other variations would
be neglected and ultimately lost; or, if they rendered the insect at
all less like the imitated object, they would be eliminated. There
would indeed be force in Mr. Mivart’s objection, if we were to attempt
to account for the above resemblances, independently of natural
selection, through mere fluctuating variability; but as the case stands
there is none.

Nor can I see any force in Mr. Mivart’s difficulty with respect to “the
last touches of perfection in the mimicry;” as in the case given by Mr.
Wallace, of a walking-stick insect (Ceroxylus laceratus), which
resembles “a stick grown over by a creeping moss or jungermannia.” So
close was this resemblance, that a native Dyak maintained that the
foliaceous excrescences were really moss. Insects are preyed on by
birds and other enemies whose sight is probably sharper than ours, and
every grade in resemblance which aided an insect to escape notice or
detection, would tend towards its preservation; and the more perfect
the resemblance so much the better for the insect. Considering the
nature of the differences between the species in the group which
includes the above Ceroxylus, there is nothing improbable in this
insect having varied in the irregularities on its surface, and in these
having become more or less green-coloured; for in every group the
characters which differ in the several species are the most apt to
vary, while the generic characters, or those common to all the species,
are the most constant.

The Greenland whale is one of the most wonderful animals in the world,
and the baleen, or whalebone, one of its greatest peculiarities. The
baleen consists of a row, on each side of the upper jaw, of about 300
plates or laminæ, which stand close together transversely to the longer
axis of the mouth. Within the main row there are some subsidiary rows.
The extremities and inner margins of all the plates are frayed into
stiff bristles, which clothe the whole gigantic palate, and serve to
strain or sift the water, and thus to secure the minute prey on which
these great animals subsist. The middle and longest lamina in the
Greenland whale is ten, twelve, or even fifteen feet in length; but in
the different species of Cetaceans there are gradations in length; the
middle lamina being in one species, according to Scoresby, four feet,
in another three, in another eighteen inches, and in the Balænoptera
rostrata only about nine inches in length. The quality of the whalebone
also differs in the different species.

With respect to the baleen, Mr. Mivart remarks that if it “had once
attained such a size and development as to be at all useful, then its
preservation and augmentation within serviceable limits would be
promoted by natural selection alone. But how to obtain the beginning of
such useful development?” In answer, it may be asked, why should not
the early progenitors of the whales with baleen have possessed a mouth
constructed something like the lamellated beak of a duck? Ducks, like
whales, subsist by sifting the mud and water; and the family has
sometimes been called _Criblatores_, or sifters. I hope that I may not
be misconstrued into saying that the progenitors of whales did actually
possess mouths lamellated like the beak of a duck. I wish only to show
that this is not incredible, and that the immense plates of baleen in
the Greenland whale might have been developed from such lamellæ by
finely graduated steps, each of service to its possessor.

The beak of a shoveller-duck (Spatula clypeata) is a more beautiful and
complex structure than the mouth of a whale. The upper mandible is
furnished on each side (in the specimen examined by me) with a row or
comb formed of 188 thin, elastic lamellæ, obliquely bevelled so as to
be pointed, and placed transversely to the longer axis of the mouth.
They arise from the palate, and are attached by flexible membrane to
the sides of the mandible. Those standing towards the middle are the
longest, being about one-third of an inch in length, and they project
fourteen one-hundredths of an inch beneath the edge. At their bases
there is a short subsidiary row of obliquely transverse lamellæ. In
these several respects they resemble the plates of baleen in the mouth
of a whale. But towards the extremity of the beak they differ much, as
they project inward, instead of straight downward. The entire head of
the shoveller, though incomparably less bulky, is about one-eighteenth
of the length of the head of a moderately large Balænoptera rostrata,
in which species the baleen is only nine inches long; so that if we
were to make the head of the shoveller as long as that of the
Balænoptera, the lamellæ would be six inches in length, that is,
two-thirds of the length of the baleen in this species of whale. The
lower mandible of the shoveller-duck is furnished with lamellæ of equal
length with these above, but finer; and in being thus furnished it
differs conspicuously from the lower jaw of a whale, which is destitute
of baleen. On the other hand, the extremities of these lower lamellæ
are frayed into fine bristly points, so that they thus curiously
resemble the plates of baleen. In the genus Prion, a member of the
distinct family of the Petrels, the upper mandible alone is furnished
with lamellæ, which are well developed and project beneath the margin;
so that the beak of this bird resembles in this respect the mouth of a
whale.

From the highly developed structure of the shoveller’s beak we may
proceed (as I have learned from information and specimens sent to me by
Mr. Salvin), without any great break, as far as fitness for sifting is
concerned, through the beak of the Merganetta armata, and in some
respects through that of the Aix sponsa, to the beak of the common
duck. In this latter species the lamellæ are much coarser than in the
shoveller, and are firmly attached to the sides of the mandible; they
are only about fifty in number on each side, and do not project at all
beneath the margin. They are square-topped, and are edged with
translucent, hardish tissue, as if for crushing food. The edges of the
lower mandible are crossed by numerous fine ridges, which project very
little. Although the beak is thus very inferior as a sifter to that of
a shoveller, yet this bird, as every one knows, constantly uses it for
this purpose. There are other species, as I hear from Mr. Salvin, in
which the lamellæ are considerably less developed than in the common
duck; but I do not know whether they use their beaks for sifting the
water.

Turning to another group of the same family. In the Egyptian goose
(Chenalopex) the beak closely resembles that of the common duck; but
the lamellæ are not so numerous, nor so distinct from each other, nor
do they project so much inward; yet this goose, as I am informed by Mr.
E. Bartlett, “uses its bill like a duck by throwing the water out at
the corners.” Its chief food, however, is grass, which it crops like
the common goose. In this latter bird the lamellæ of the upper mandible
are much coarser than in the common duck, almost confluent, about
twenty-seven in number on each side, and terminating upward in
teeth-like knobs. The palate is also covered with hard rounded knobs.
The edges of the lower mandible are serrated with teeth much more
prominent, coarser and sharper than in the duck. The common goose does
not sift the water, but uses its beak exclusively for tearing or
cutting herbage, for which purpose it is so well fitted that it can
crop grass closer than almost any other animal. There are other species
of geese, as I hear from Mr. Bartlett, in which the lamellæ are less
developed than in the common goose.

We thus see that a member of the duck family, with a beak constructed
like that of a common goose and adapted solely for grazing, or even a
member with a beak having less well-developed lamellæ, might be
converted by small changes into a species like the Egyptian goose—this
into one like the common duck—and, lastly, into one like the shoveller,
provided with a beak almost exclusively adapted for sifting the water;
for this bird could hardly use any part of its beak, except the hooked
tip, for seizing or tearing solid food. The beak of a goose, as I may
add, might also be converted by small changes into one provided with
prominent, recurved teeth, like those of the Merganser (a member of the
same family), serving for the widely different purpose of securing live
fish.

Returning to the whales. The Hyperoodon bidens is destitute of true
teeth in an efficient condition, but its palate is roughened, according
to Lacepede, with small unequal, hard points of horn. There is,
therefore, nothing improbable in supposing that some early Cetacean
form was provided with similar points of horn on the palate, but rather
more regularly placed, and which, like the knobs on the beak of the
goose, aided it in seizing or tearing its food. If so, it will hardly
be denied that the points might have been converted through variation
and natural selection into lamellæ as well-developed as those of the
Egyptian goose, in which case they would have been used both for
seizing objects and for sifting the water; then into lamellæ like those
of the domestic duck; and so onward, until they became as well
constructed as those of the shoveller, in which case they would have
served exclusively as a sifting apparatus. From this stage, in which
the lamellæ would be two-thirds of the length of the plates of baleen
in the Balænoptera rostrata, gradations, which may be observed in
still-existing Cetaceans, lead us onward to the enormous plates of
baleen in the Greenland whale. Nor is there the least reason to doubt
that each step in this scale might have been as serviceable to certain
ancient Cetaceans, with the functions of the parts slowly changing
during the progress of development, as are the gradations in the beaks
of the different existing members of the duck-family. We should bear in
mind that each species of duck is subjected to a severe struggle for
existence, and that the structure of every part of its frame must be
well adapted to its conditions of life.

The Pleuronectidæ, or Flat-fish, are remarkable for their asymmetrical
bodies. They rest on one side—in the greater number of species on the
left, but in some on the right side; and occasionally reversed adult
specimens occur. The lower, or resting-surface, resembles at first
sight the ventral surface of an ordinary fish; it is of a white colour,
less developed in many ways than the upper side, with the lateral fins
often of smaller size. But the eyes offer the most remarkable
peculiarity; for they are both placed on the upper side of the head.
During early youth, however, they stand opposite to each other, and the
whole body is then symmetrical, with both sides equally coloured. Soon
the eye proper to the lower side begins to glide slowly round the head
to the upper side; but does not pass right through the skull, as was
formerly thought to be the case. It is obvious that unless the lower
eye did thus travel round, it could not be used by the fish while lying
in its habitual position on one side. The lower eye would, also, have
been liable to be abraded by the sandy bottom. That the Pleuronectidæ
are admirably adapted by their flattened and asymmetrical structure for
their habits of life, is manifest from several species, such as soles,
flounders, &c., being extremely common. The chief advantages thus
gained seem to be protection from their enemies, and facility for
feeding on the ground. The different members, however, of the family
present, as Schiödte remarks, “a long series of forms exhibiting a
gradual transition from Hippoglossus pinguis, which does not in any
considerable degree alter the shape in which it leaves the ovum, to the
soles, which are entirely thrown to one side.”

Mr. Mivart has taken up this case, and remarks that a sudden
spontaneous transformation in the position of the eyes is hardly
conceivable, in which I quite agree with him. He then adds: “If the
transit was gradual, then how such transit of one eye a minute fraction
of the journey towards the other side of the head could benefit the
individual is, indeed, far from clear. It seems, even, that such an
incipient transformation must rather have been injurious.” But he might
have found an answer to this objection in the excellent observations
published in 1867 by Malm. The Pleuronectidæ, while very young and
still symmetrical, with their eyes standing on opposite sides of the
head, cannot long retain a vertical position, owing to the excessive
depth of their bodies, the small size of their lateral fins, and to
their being destitute of a swimbladder. Hence, soon growing tired,
they fall to the bottom on one side. While thus at rest they often
twist, as Malm observed, the lower eye upward, to see above them; and
they do this so vigorously that the eye is pressed hard against the
upper part of the orbit. The forehead between the eyes consequently
becomes, as could be plainly seen, temporarily contracted in breadth.
On one occasion Malm saw a young fish raise and depress the lower eye
through an angular distance of about seventy degrees.

We should remember that the skull at this early age is cartilaginous
and flexible, so that it readily yields to muscular action. It is also
known with the higher animals, even after early youth, that the skull
yields and is altered in shape, if the skin or muscles be permanently
contracted through disease or some accident. With long-eared rabbits,
if one ear flops forward and downward, its weight drags forward all the
bones of the skull on the same side, of which I have given a figure.
Malm states that the newly-hatched young of perches, salmon, and
several other symmetrical fishes, have the habit of occasionally
resting on one side at the bottom; and he has observed that they often
then strain their lower eyes so as to look upward; and their skulls are
thus rendered rather crooked. These fishes, however, are soon able to
hold themselves in a vertical position, and no permanent effect is thus
produced. With the Pleuronectidæ, on the other hand, the older they
grow the more habitually they rest on one side, owing to the increasing
flatness of their bodies, and a permanent effect is thus produced on
the form of the head, and on the position of the eyes. Judging from
analogy, the tendency to distortion would no doubt be increased through
the principle of inheritance. Schiödte believes, in opposition to some
other naturalists, that the Pleuronectidæ are not quite symmetrical
even in the embryo; and if this be so, we could understand how it is
that certain species, while young, habitually fall over and rest on the
left side, and other species on the right side. Malm adds, in
confirmation of the above view, that the adult Trachypterus arcticus,
which is not a member of the Pleuronectidæ, rests on its left side at
the bottom, and swims diagonally through the water; and in this fish,
the two sides of the head are said to be somewhat dissimilar. Our great
authority on Fishes, Dr. Günther, concludes his abstract of Malm’s
paper, by remarking that “the author gives a very simple explanation of
the abnormal condition of the Pleuronectoids.”

We thus see that the first stages of the transit of the eye from one
side of the head to the other, which Mr. Mivart considers would be
injurious, may be attributed to the habit, no doubt beneficial to the
individual and to the species, of endeavouring to look upward with both
eyes, while resting on one side at the bottom. We may also attribute to
the inherited effects of use the fact of the mouth in several kinds of
flat-fish being bent towards the lower surface, with the jaw bones
stronger and more effective on this, the eyeless side of the head, than
on the other, for the sake, as Dr. Traquair supposes, of feeding with
ease on the ground. Disuse, on the other hand, will account for the
less developed condition of the whole inferior half of the body,
including the lateral fins; though Yarrel thinks that the reduced size
of these fins is advantageous to the fish, as “there is so much less
room for their action than with the larger fins above.” Perhaps the
lesser number of teeth in the proportion of four to seven in the upper
halves of the two jaws of the plaice, to twenty-five to thirty in the
lower halves, may likewise be accounted for by disuse. From the
colourless state of the ventral surface of most fishes and of many
other animals, we may reasonably suppose that the absence of colour in
flat-fish on the side, whether it be the right or left, which is
under-most, is due to the exclusion of light. But it cannot be supposed
that the peculiar speckled appearance of the upper side of the sole, so
like the sandy bed of the sea, or the power in some species, as
recently shown by Pouchet, of changing their colour in accordance with
the surrounding surface, or the presence of bony tubercles on the upper
side of the turbot, are due to the action of the light. Here natural
selection has probably come into play, as well as in adapting the
general shape of the body of these fishes, and many other
peculiarities, to their habits of life. We should keep in mind, as I
have before insisted, that the inherited effects of the increased use
of parts, and perhaps of their disuse, will be strengthened by natural
selection. For all spontaneous variations in the right direction will
thus be preserved; as will those individuals which inherit in the
highest degree the effects of the increased and beneficial use of any
part. How much to attribute in each particular case to the effects of
use, and how much to natural selection, it seems impossible to decide.

I may give another instance of a structure which apparently owes its
origin exclusively to use or habit. The extremity of the tail in some
American monkeys has been converted into a wonderfully perfect
prehensile organ, and serves as a fifth hand. A reviewer, who agrees
with Mr. Mivart in every detail, remarks on this structure: “It is
impossible to believe that in any number of ages the first slight
incipient tendency to grasp could preserve the lives of the individuals
possessing it, or favour their chance of having and of rearing
offspring.” But there is no necessity for any such belief. Habit, and
this almost implies that some benefit great or small is thus derived,
would in all probability suffice for the work. Brehm saw the young of
an African monkey (Cercopithecus) clinging to the under surface of
their mother by their hands, and at the same time they hooked their
little tails round that of their mother. Professor Henslow kept in
confinement some harvest mice (Mus messorius) which do not possess a
structurally prehensive tail; but he frequently observed that they
curled their tails round the branches of a bush placed in the cage, and
thus aided themselves in climbing. I have received an analogous account
from Dr. Günther, who has seen a mouse thus suspend itself. If the
harvest mouse had been more strictly arboreal, it would perhaps have
had its tail rendered structurally prehensile, as is the case with some
members of the same order. Why Cercopithecus, considering its habits
while young, has not become thus provided, it would be difficult to
say. It is, however, possible that the long tail of this monkey may be
of more service to it as a balancing organ in making its prodigious
leaps, than as a prehensile organ.

The mammary glands are common to the whole class of mammals, and are
indispensable for their existence; they must, therefore, have been
developed at an extremely remote period, and we can know nothing
positively about their manner of development. Mr. Mivart asks: “Is it
conceivable that the young of any animal was ever saved from
destruction by accidentally sucking a drop of scarcely nutritious fluid
from an accidentally hypertrophied cutaneous gland of its mother? And
even if one was so, what chance was there of the perpetuation of such a
variation?” But the case is not here put fairly. It is admitted by most
evolutionists that mammals are descended from a marsupial form; and if
so, the mammary glands will have been at first developed within the
marsupial sack. In the case of the fish (Hippocampus) the eggs are
hatched, and the young are reared for a time, within a sack of this
nature; and an American naturalist, Mr. Lockwood, believes from what he
has seen of the development of the young, that they are nourished by a
secretion from the cutaneous glands of the sack. Now, with the early
progenitors of mammals, almost before they deserved to be thus
designated, is it not at least possible that the young might have been
similarly nourished? And in this case, the individuals which secreted a
fluid, in some degree or manner the most nutritious, so as to partake
of the nature of milk, would in the long run have reared a larger
number of well-nourished offspring, than would the individuals which
secreted a poorer fluid; and thus the cutaneous glands, which are the
homologues of the mammary glands, would have been improved or rendered
more effective. It accords with the widely extended principle of
specialisation, that the glands over a certain space of the sack should
have become more highly developed than the remainder; and they would
then have formed a breast, but at first without a nipple, as we see in
the Ornithorhyncus, at the base of the mammalian series. Through what
agency the glands over a certain space became more highly specialised
than the others, I will not pretend to decide, whether in part through
compensation of growth, the effects of use, or of natural selection.

The development of the mammary glands would have been of no service,
and could not have been affected through natural selection, unless the
young at the same time were able to partake of the secretion. There is
no greater difficulty in understanding how young mammals have
instinctively learned to suck the breast, than in understanding how
unhatched chickens have learned to break the egg-shell by tapping
against it with their specially adapted beaks; or how a few hours after
leaving the shell they have learned to pick up grains of food. In such
cases the most probable solution seems to be, that the habit was at
first acquired by practice at a more advanced age, and afterwards
transmitted to the offspring at an earlier age. But the young kangaroo
is said not to suck, only to cling to the nipple of its mother, who has
the power of injecting milk into the mouth of her helpless, half-formed
offspring. On this head Mr. Mivart remarks: “Did no special provision
exist, the young one must infallibly be choked by the intrusion of the
milk into the wind-pipe. But there _is_ a special provision. The larynx
is so elongated that it rises up into the posterior end of the nasal
passage, and is thus enabled to give free entrance to the air for the
lungs, while the milk passes harmlessly on each side of this elongated
larynx, and so safely attains the gullet behind it.” Mr. Mivart then
asks how did natural selection remove in the adult kangaroo (and in
most other mammals, on the assumption that they are descended from a
marsupial form), “this at least perfectly innocent and harmless
structure?” It may be suggested in answer that the voice, which is
certainly of high importance to many animals, could hardly have been
used with full force as long as the larynx entered the nasal passage;
and Professor Flower has suggested to me that this structure would have
greatly interfered with an animal swallowing solid food.

We will now turn for a short space to the lower divisions of the animal
kingdom. The Echinodermata (star-fishes, sea-urchins, &c.) are
furnished with remarkable organs, called pedicellariæ, which consist,
when well developed, of a tridactyle forceps—that is, of one formed of
three serrated arms, neatly fitting together and placed on the summit
of a flexible stem, moved by muscles. These forceps can seize firmly
hold of any object; and Alexander Agassiz has seen an Echinus or
sea-urchin rapidly passing particles of excrement from forceps to
forceps down certain lines of its body, in order that its shell should
not be fouled. But there is no doubt that besides removing dirt of all
kinds, they subserve other functions; and one of these apparently is
defence.

With respect to these organs, Mr. Mivart, as on so many previous
occasions, asks: “What would be the utility of the _first rudimentary
beginnings_ of such structures, and how could such insipient buddings
have ever preserved the life of a single Echinus?” He adds, “not even
the _sudden_ development of the snapping action would have been
beneficial without the freely movable stalk, nor could the latter have
been efficient without the snapping jaws, yet no minute, nearly
indefinite variations could simultaneously evolve these complex
co-ordinations of structure; to deny this seems to do no less than to
affirm a startling paradox.” Paradoxical as this may appear to Mr.
Mivart, tridactyle forcepses, immovably fixed at the base, but capable
of a snapping action, certainly exist on some star-fishes; and this is
intelligible if they serve, at least in part, as a means of defence.
Mr. Agassiz, to whose great kindness I am indebted for much information
on the subject, informs me that there are other star-fishes, in which
one of the three arms of the forceps is reduced to a support for the
other two; and again, other genera in which the third arm is completely
lost. In Echinoneus, the shell is described by M. Perrier as bearing
two kinds of pedicellariæ, one resembling those of Echinus, and the
other those of Spatangus; and such cases are always interesting as
affording the means of apparently sudden transitions, through the
abortion of one of the two states of an organ.

With respect to the steps by which these curious organs have been
evolved, Mr. Agassiz infers from his own researches and those of Mr.
Müller, that both in star-fishes and sea-urchins the pedicellariæ must
undoubtedly be looked at as modified spines. This may be inferred from
their manner of development in the individual, as well as from a long
and perfect series of gradations in different species and genera, from
simple granules to ordinary spines, to perfect tridactyle pedicellariæ.
The gradation extends even to the manner in which ordinary spines and
the pedicellariæ, with their supporting calcareous rods, are
articulated to the shell. In certain genera of star-fishes, “the very
combinations needed to show that the pedicellariæ are only modified
branching spines” may be found. Thus we have fixed spines, with three
equi-distant, serrated, movable branches, articulated to near their
bases; and higher up, on the same spine, three other movable branches.
Now when the latter arise from the summit of a spine they form, in
fact, a rude tridactyle pedicellariæ, and such may be seen on the same
spine together with the three lower branches. In this case the identity
in nature between the arms of the pedicellariæ and the movable branches
of a spine, is unmistakable. It is generally admitted that the ordinary
spines serve as a protection; and if so, there can be no reason to
doubt that those furnished with serrated and movable branches likewise
serve for the same purpose; and they would thus serve still more
effectively as soon as by meeting together they acted as a prehensile
or snapping apparatus. Thus every gradation, from an ordinary fixed
spine to a fixed pedicellariæ, would be of service.

In certain genera of star-fishes these organs, instead of being fixed
or borne on an immovable support, are placed on the summit of a
flexible and muscular, though short, stem; and in this case they
probably subserve some additional function besides defence. In the
sea-urchins the steps can be followed by which a fixed spine becomes
articulated to the shell, and is thus rendered movable. I wish I had
space here to give a fuller abstract of Mr. Agassiz’s interesting
observations on the development of the pedicellariæ. All possible
gradations, as he adds, may likewise be found between the pedicellariæ
of the star-fishes and the hooks of the Ophiurians, another group of
the Echinodermata; and again between the pedicellariæ of sea-urchins
and the anchors of the Holothuriæ, also belonging to the same great
class.

Certain compound animals, or zoophytes, as they have been termed,
namely the Polyzoa, are provided with curious organs called avicularia.
These differ much in structure in the different species. In their most
perfect condition they curiously resemble the head and beak of a
vulture in miniature, seated on a neck and capable of movement, as is
likewise the lower jaw or mandible. In one species observed by me, all
the avicularia on the same branch often moved simultaneously backwards
and forwards, with the lower jaw widely open, through an angle of about
90 degrees, in the course of five seconds; and their movement caused
the whole polyzoary to tremble. When the jaws are touched with a needle
they seize it so firmly that the branch can thus be shaken.

Mr. Mivart adduces this case, chiefly on account of the supposed
difficulty of organs, namely the avicularia of the Polyzoa and the
pedicellariæ of the Echinodermata, which he considers as “essentially
similar,” having been developed through natural selection in widely
distinct divisions of the animal kingdom. But, as far as structure is
concerned, I can see no similarity between tridactyle pedicellariæ and
avicularia. The latter resembles somewhat more closely the chelæ or
pincers of Crustaceans; and Mr. Mivart might have adduced with equal
appropriateness this resemblance as a special difficulty, or even their
resemblance to the head and beak of a bird. The avicularia are believed
by Mr. Busk, Dr. Smitt and Dr. Nitsche—naturalists who have carefully
studied this group—to be homologous with the zooids and their cells
which compose the zoophyte, the movable lip or lid of the cell
corresponding with the lower and movable mandible of the avicularium.
Mr. Busk, however, does not know of any gradations now existing between
a zooid and an avicularium. It is therefore impossible to conjecture by
what serviceable gradations the one could have been converted into the
other, but it by no means follows from this that such gradations have
not existed.

As the chelæ of Crustaceans resemble in some degree the avicularia of
Polyzoa, both serving as pincers, it may be worth while to show that
with the former a long series of serviceable gradations still exists.
In the first and simplest stage, the terminal segment of a limb shuts
down either on the square summit of the broad penultimate segment, or
against one whole side, and is thus enabled to catch hold of an object,
but the limb still serves as an organ of locomotion. We next find one
corner of the broad penultimate segment slightly prominent, sometimes
furnished with irregular teeth, and against these the terminal segment
shuts down. By an increase in the size of this projection, with its
shape, as well as that of the terminal segment, slightly modified and
improved, the pincers are rendered more and more perfect, until we have
at last an instrument as efficient as the chelæ of a lobster. And all
these gradations can be actually traced.

Besides the avicularia, the polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of
movement and easily excited. In one species examined by me the
vibracula were slightly curved and serrated along the outer margin, and
all of them on the same polyzoary often moved simultaneously; so that,
acting like long oars, they swept a branch rapidly across the
object-glass of my microscope. When a branch was placed on its face,
the vibracula became entangled, and they made violent efforts to free
themselves. They are supposed to serve as a defence, and may be seen,
as Mr. Busk remarks, “to sweep slowly and carefully over the surface of
the polyzoary, removing what might be noxious to the delicate
inhabitants of the cells when their tentacula are protruded.” The
avicularia, like the vibracula, probably serve for defence, but they
also catch and kill small living animals, which, it is believed, are
afterwards swept by the currents within reach of the tentacula of the
zooids. Some species are provided with avicularia and vibracula, some
with avicularia alone and a few with vibracula alone.

It is not easy to imagine two objects more widely different in
appearance than a bristle or vibraculum, and an avicularium like the
head of a bird; yet they are almost certainly homologous and have been
developed from the same common source, namely a zooid with its cell.
Hence, we can understand how it is that these organs graduate in some
cases, as I am informed by Mr. Busk, into each other. Thus, with the
avicularia of several species of Lepralia, the movable mandible is so
much produced and is so like a bristle that the presence of the upper
or fixed beak alone serves to determine its avicularian nature. The
vibracula may have been directly developed from the lips of the cells,
without having passed through the avicularian stage; but it seems more
probable that they have passed through this stage, as during the early
stages of the transformation, the other parts of the cell, with the
included zooid, could hardly have disappeared at once. In many cases
the vibracula have a grooved support at the base, which seems to
represent the fixed beak; though this support in some species is quite
absent. This view of the development of the vibracula, if trustworthy,
is interesting; for supposing that all the species provided with
avicularia had become extinct, no one with the most vivid imagination
would ever have thought that the vibracula had originally existed as
part of an organ, resembling a bird’s head, or an irregular box or
hood. It is interesting to see two such widely different organs
developed from a common origin; and as the movable lip of the cell
serves as a protection to the zooid, there is no difficulty in
believing that all the gradations, by which the lip became converted
first into the lower mandible of an avicularium, and then into an
elongated bristle, likewise served as a protection in different ways
and under different circumstances.

In the vegetable kingdom Mr. Mivart only alludes to two cases, namely
the structure of the flowers of orchids, and the movements of climbing
plants. With respect to the former, he says: “The explanation of their
_origin_ is deemed thoroughly unsatisfactory—utterly insufficient to
explain the incipient, infinitesimal beginnings of structures which are
of utility only when they are considerably developed.” As I have fully
treated this subject in another work, I will here give only a few
details on one alone of the most striking peculiarities of the flowers
of orchids, namely, their pollinia. A pollinium, when highly developed,
consists of a mass of pollen-grains, affixed to an elastic foot-stalk
or caudicle, and this to a little mass of extremely viscid matter. The
pollinia are by this means transported by insects from one flower to
the stigma of another. In some orchids there is no caudicle to the
pollen-masses, and the grains are merely tied together by fine threads;
but as these are not confined to orchids, they need not here be
considered; yet I may mention that at the base of the orchidaceous
series, in Cypripedium, we can see how the threads were probably first
developed. In other orchids the threads cohere at one end of the
pollen-masses; and this forms the first or nascent trace of a caudicle.
That this is the origin of the caudicle, even when of considerable
length and highly developed, we have good evidence in the aborted
pollen-grains which can sometimes be detected embedded within the
central and solid parts.

With respect to the second chief peculiarity, namely, the little mass
of viscid matter attached to the end of the caudicle, a long series of
gradations can be specified, each of plain service to the plant. In
most flowers belonging to other orders the stigma secretes a little
viscid matter. Now, in certain orchids similar viscid matter is
secreted, but in much larger quantities by one alone of the three
stigmas; and this stigma, perhaps in consequence of the copious
secretion, is rendered sterile. When an insect visits a flower of this
kind, it rubs off some of the viscid matter, and thus at the same time
drags away some of the pollen-grains. From this simple condition, which
differs but little from that of a multitude of common flowers, there
are endless gradations—to species in which the pollen-mass terminates
in a very short, free caudicle—to others in which the caudicle becomes
firmly attached to the viscid matter, with the sterile stigma itself
much modified. In this latter case we have a pollinium in its most
highly developed and perfect condition. He who will carefully examine
the flowers of orchids for himself will not deny the existence of the
above series of gradations—from a mass of pollen-grains merely tied
together by threads, with the stigma differing but little from that of
the ordinary flowers, to a highly complex pollinium, admirably adapted
for transportal by insects; nor will he deny that all the gradations in
the several species are admirably adapted in relation to the general
structure of each flower for its fertilisation by different insects. In
this, and in almost every other case, the enquiry may be pushed further
backwards; and it may be asked how did the stigma of an ordinary flower
become viscid, but as we do not know the full history of any one group
of beings, it is as useless to ask, as it is hopeless to attempt
answering, such questions.

We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those which I
have called leaf-climbers, and to those provided with tendrils. In
these two latter classes the stems have generally, but not always, lost
the power of twining, though they retain the power of revolving, which
the tendrils likewise possess. The gradations from leaf-climbers to
tendril bearers are wonderfully close, and certain plants may be
differently placed in either class. But in ascending the series from
simple twiners to leaf-climbers, an important quality is added, namely
sensitiveness to a touch, by which means the foot-stalks of the leaves
or flowers, or these modified and converted into tendrils, are excited
to bend round and clasp the touching object. He who will read my memoir
on these plants will, I think, admit that all the many gradations in
function and structure between simple twiners and tendril-bearers are
in each case beneficial in a high degree to the species. For instance,
it is clearly a great advantage to a twining plant to become a
leaf-climber; and it is probable that every twiner which possessed
leaves with long foot-stalks would have been developed into a
leaf-climber, if the foot-stalks had possessed in any slight degree the
requisite sensitiveness to a touch.

As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally be asked how did plants acquire
this power in an incipient degree, afterwards to be improved and
increased through natural selection. The power of twining depends,
firstly, on the stems while young being extremely flexible (but this is
a character common to many plants which are not climbers); and,
secondly, on their continually bending to all points of the compass,
one after the other in succession, in the same order. By this movement
the stems are inclined to all sides, and are made to move round and
round. As soon as the lower part of a stem strikes against any object
and is stopped, the upper part still goes on bending and revolving, and
thus necessarily twines round and up the support. The revolving
movement ceases after the early growth of each shoot. As in many widely
separated families of plants, single species and single genera possess
the power of revolving, and have thus become twiners, they must have
independently acquired it, and cannot have inherited it from a common
progenitor. Hence, I was led to predict that some slight tendency to a
movement of this kind would be found to be far from uncommon with
plants which did not climb; and that this had afforded the basis for
natural selection to work on and improve. When I made this prediction,
I knew of only one imperfect case, namely, of the young
flower-peduncles of a Maurandia which revolved slightly and
irregularly, like the stems of twining plants, but without making any
use of this habit. Soon afterwards Fritz Müller discovered that the
young stems of an Alisma and of a Linum—plants which do not climb and
are widely separated in the natural system—revolved plainly, though
irregularly, and he states that he has reason to suspect that this
occurs with some other plants. These slight movements appear to be of
no service to the plants in question; anyhow, they are not of the least
use in the way of climbing, which is the point that concerns us.
Nevertheless we can see that if the stems of these plants had been
flexible, and if under the conditions to which they are exposed it had
profited them to ascend to a height, then the habit of slightly and
irregularly revolving might have been increased and utilised through
natural selection, until they had become converted into well-developed
twining species.

With respect to the sensitiveness of the foot-stalks of the leaves and
flowers, and of tendrils, nearly the same remarks are applicable as in
the case of the revolving movements of twining plants. As a vast number
of species, belonging to widely distinct groups, are endowed with this
kind of sensitiveness, it ought to be found in a nascent condition in
many plants which have not become climbers. This is the case: I
observed that the young flower-peduncles of the above Maurandia curved
themselves a little towards the side which was touched. Morren found in
several species of Oxalis that the leaves and their foot-stalks moved,
especially after exposure to a hot sun, when they were gently and
repeatedly touched, or when the plant was shaken. I repeated these
observations on some other species of Oxalis with the same result; in
some of them the movement was distinct, but was best seen in the young
leaves; in others it was extremely slight. It is a more important fact
that according to the high authority of Hofmeister, the young shoots
and leaves of all plants move after being shaken; and with climbing
plants it is, as we know, only during the early stages of growth that
the foot-stalks and tendrils are sensitive.

It is scarcely possible that the above slight movements, due to a touch
or shake, in the young and growing organs of plants, can be of any
functional importance to them. But plants possess, in obedience to
various stimuli, powers of movement, which are of manifest importance
to them; for instance, towards and more rarely from the light—in
opposition to, and more rarely in the direction of, the attraction of
gravity. When the nerves and muscles of an animal are excited by
galvanism or by the absorption of strychnine, the consequent movements
may be called an incidental result, for the nerves and muscles have not
been rendered specially sensitive to these stimuli. So with plants it
appears that, from having the power of movement in obedience to certain
stimuli, they are excited in an incidental manner by a touch, or by
being shaken. Hence there is no great difficulty in admitting that in
the case of leaf-climbers and tendril-bearers, it is this tendency
which has been taken advantage of and increased through natural
selection. It is, however, probable, from reasons which I have assigned
in my memoir, that this will have occurred only with plants which had
already acquired the power of revolving, and had thus become twiners.

I have already endeavoured to explain how plants became twiners,
namely, by the increase of a tendency to slight and irregular revolving
movements, which were at first of no use to them; this movement, as
well as that due to a touch or shake, being the incidental result of
the power of moving, gained for other and beneficial purposes. Whether,
during the gradual development of climbing plants, natural selection
has been aided by the inherited effects of use, I will not pretend to
decide; but we know that certain periodical movements, for instance the
so-called sleep of plants, are governed by habit.

I have now considered enough, perhaps more than enough, of the cases,
selected with care by a skilful naturalist, to prove that natural
selection is incompetent to account for the incipient stages of useful
structures; and I have shown, as I hope, that there is no great
difficulty on this head. A good opportunity has thus been afforded for
enlarging a little on gradations of structure, often associated with
strange functions—an important subject, which was not treated at
sufficient length in the former editions of this work. I will now
briefly recapitulate the foregoing cases.

With the giraffe, the continued preservation of the individuals of some
extinct high-reaching ruminant, which had the longest necks, legs, &c.,
and could browse a little above the average height, and the continued
destruction of those which could not browse so high, would have
sufficed for the production of this remarkable quadruped; but the
prolonged use of all the parts, together with inheritance, will have
aided in an important manner in their co-ordination. With the many
insects which imitate various objects, there is no improbability in the
belief that an accidental resemblance to some common object was in each
case the foundation for the work of natural selection, since perfected
through the occasional preservation of slight variations which made the
resemblance at all closer; and this will have been carried on as long
as the insect continued to vary, and as long as a more and more perfect
resemblance led to its escape from sharp-sighted enemies. In certain
species of whales there is a tendency to the formation of irregular
little points of horn on the palate; and it seems to be quite within
the scope of natural selection to preserve all favourable variations,
until the points were converted, first into lamellated knobs or teeth,
like those on the beak of a goose—then into short lamellæ, like those
of the domestic ducks—and then into lamellæ, as perfect as those of the
shoveller-duck—and finally into the gigantic plates of baleen, as in
the mouth of the Greenland whale. In the family of the ducks, the
lamellæ are first used as teeth, then partly as teeth and partly as a
sifting apparatus, and at last almost exclusively for this latter
purpose.

With such structures as the above lamellæ of horn or whalebone, habit
or use can have done little or nothing, as far as we can judge, towards
their development. On the other hand, the transportal of the lower eye
of a flat-fish to the upper side of the head, and the formation of a
prehensile tail, may be attributed almost wholly to continued use,
together with inheritance. With respect to the mammæ of the higher
animals, the most probable conjecture is that primordially the
cutaneous glands over the whole surface of a marsupial sack secreted a
nutritious fluid; and that these glands were improved in function
through natural selection, and concentrated into a confined area, in
which case they would have formed a mamma. There is no more difficulty
in understanding how the branched spines of some ancient Echinoderm,
which served as a defence, became developed through natural selection
into tridactyle pedicellariæ, than in understanding the development of
the pincers of crustaceans, through slight, serviceable modifications
in the ultimate and penultimate segments of a limb, which was at first
used solely for locomotion. In the avicularia and vibracula of the
Polyzoa we have organs widely different in appearance developed from
the same source; and with the vibracula we can understand how the
successive gradations might have been of service. With the pollinia of
orchids, the threads which originally served to tie together the
pollen-grains, can be traced cohering into caudicles; and the steps can
likewise be followed by which viscid matter, such as that secreted by
the stigmas of ordinary flowers, and still subserving nearly but not
quite the same purpose, became attached to the free ends of the
caudicles—all these gradations being of manifest benefit to the plants
in question. With respect to climbing plants, I need not repeat what
has been so lately said.

It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it
would apparently have been advantageous? But it is unreasonable to
expect a precise answer to such questions, considering our ignorance of
the past history of each species, and of the conditions which at the
present day determine its numbers and range. In most cases only general
reasons, but in some few cases special reasons, can be assigned. Thus
to adapt a species to new habits of life, many co-ordinated
modifications are almost indispensable, and it may often have happened
that the requisite parts did not vary in the right manner or to the
right degree. Many species must have been prevented from increasing in
numbers through destructive agencies, which stood in no relation to
certain structures, which we imagine would have been gained through
natural selection from appearing to us advantageous to the species. In
this case, as the struggle for life did not depend on such structures,
they could not have been acquired through natural selection. In many
cases complex and long-enduring conditions, often of a peculiar nature,
are necessary for the development of a structure; and the requisite
conditions may seldom have concurred. The belief that any given
structure, which we think, often erroneously, would have been
beneficial to a species, would have been gained under all circumstances
through natural selection, is opposed to what we can understand of its
manner of action. Mr. Mivart does not deny that natural selection has
effected something; but he considers it as “demonstrably insufficient”
to account for the phenomena which I explain by its agency. His chief
arguments have now been considered, and the others will hereafter be
considered. They seem to me to partake little of the character of
demonstration, and to have little weight in comparison with those in
favour of the power of natural selection, aided by the other agencies
often specified. I am bound to add, that some of the facts and
arguments here used by me, have been advanced for the same purpose in
an able article lately published in the “Medico-Chirurgical Review.”

At the present day almost all naturalists admit evolution under some
form. Mr. Mivart believes that species change through “an internal
force or tendency,” about which it is not pretended that anything is
known. That species have a capacity for change will be admitted by all
evolutionists; but there is no need, as it seems to me, to invoke any
internal force beyond the tendency to ordinary variability, which
through the aid of selection, by man has given rise to many
well-adapted domestic races, and which, through the aid of natural
selection, would equally well give rise by graduated steps to natural
races or species. The final result will generally have been, as already
explained, an advance, but in some few cases a retrogression, in
organisation.

Mr. Mivart is further inclined to believe, and some naturalists agree
with him, that new species manifest themselves “with suddenness and by
modifications appearing at once.” For instance, he supposes that the
differences between the extinct three-toed Hipparion and the horse
arose suddenly. He thinks it difficult to believe that the wing of a
bird “was developed in any other way than by a comparatively sudden
modification of a marked and important kind;” and apparently he would
extend the same view to the wings of bats and pterodactyles. This
conclusion, which implies great breaks or discontinuity in the series,
appears to me improbable in the highest degree.

Everyone who believes in slow and gradual evolution, will of course
admit that specific changes may have been as abrupt and as great as any
single variation which we meet with under nature, or even under
domestication. But as species are more variable when domesticated or
cultivated than under their natural conditions, it is not probable that
such great and abrupt variations have often occurred under nature, as
are known occasionally to arise under domestication. Of these latter
variations several may be attributed to reversion; and the characters
which thus reappear were, it is probable, in many cases at first gained
in a gradual manner. A still greater number must be called
monstrosities, such as six-fingered men, porcupine men, Ancon sheep,
Niata cattle, &c.; and as they are widely different in character from
natural species, they throw very little light on our subject. Excluding
such cases of abrupt variations, the few which remain would at best
constitute, if found in a state of nature, doubtful species, closely
related to their parental types.

My reasons for doubting whether natural species have changed as
abruptly as have occasionally domestic races, and for entirely
disbelieving that they have changed in the wonderful manner indicated
by Mr. Mivart, are as follows. According to our experience, abrupt and
strongly marked variations occur in our domesticated productions,
singly and at rather long intervals of time. If such occurred under
nature, they would be liable, as formerly explained, to be lost by
accidental causes of destruction and by subsequent intercrossing; and
so it is known to be under domestication, unless abrupt variations of
this kind are specially preserved and separated by the care of man.
Hence, in order that a new species should suddenly appear in the manner
supposed by Mr. Mivart, it is almost necessary to believe, in
opposition to all analogy, that several wonderfully changed individuals
appeared simultaneously within the same district. This difficulty, as
in the case of unconscious selection by man, is avoided on the theory
of gradual evolution, through the preservation of a large number of
individuals, which varied more or less in any favourable direction, and
of the destruction of a large number which varied in an opposite
manner.

That many species have been evolved in an extremely gradual manner,
there can hardly be a doubt. The species and even the genera of many
large natural families are so closely allied together that it is
difficult to distinguish not a few of them. On every continent, in
proceeding from north to south, from lowland to upland, &c., we meet
with a host of closely related or representative species; as we
likewise do on certain distinct continents, which we have reason to
believe were formerly connected. But in making these and the following
remarks, I am compelled to allude to subjects hereafter to be
discussed. Look at the many outlying islands round a continent, and see
how many of their inhabitants can be raised only to the rank of
doubtful species. So it is if we look to past times, and compare the
species which have just passed away with those still living within the
same areas; or if we compare the fossil species embedded in the
sub-stages of the same geological formation. It is indeed manifest that
multitudes of species are related in the closest manner to other
species that still exist, or have lately existed; and it will hardly be
maintained that such species have been developed in an abrupt or sudden
manner. Nor should it be forgotten, when we look to the special parts
of allied species, instead of to distinct species, that numerous and
wonderfully fine gradations can be traced, connecting together widely
different structures.

Many large groups of facts are intelligible only on the principle that
species have been evolved by very small steps. For instance, the fact
that the species included in the larger genera are more closely related
to each other, and present a greater number of varieties than do the
species in the smaller genera. The former are also grouped in little
clusters, like varieties round species; and they present other
analogies with varieties, as was shown in our second chapter. On this
same principle we can understand how it is that specific characters are
more variable than generic characters; and how the parts which are
developed in an extraordinary degree or manner are more variable than
other parts of the same species. Many analogous facts, all pointing in
the same direction, could be added.

Although very many species have almost certainly been produced by steps
not greater than those separating fine varieties; yet it may be
maintained that some have been developed in a different and abrupt
manner. Such an admission, however, ought not to be made without strong
evidence being assigned. The vague and in some respects false
analogies, as they have been shown to be by Mr. Chauncey Wright, which
have been advanced in favour of this view, such as the sudden
crystallisation of inorganic substances, or the falling of a facetted
spheroid from one facet to another, hardly deserve consideration. One
class of facts, however, namely, the sudden appearance of new and
distinct forms of life in our geological formations supports at first
sight the belief in abrupt development. But the value of this evidence
depends entirely on the perfection of the geological record, in
relation to periods remote in the history of the world. If the record
is as fragmentary as many geologists strenuously assert, there is
nothing strange in new forms appearing as if suddenly developed.

Unless we admit transformations as prodigious as those advocated by Mr.
Mivart, such as the sudden development of the wings of birds or bats,
or the sudden conversion of a Hipparion into a horse, hardly any light
is thrown by the belief in abrupt modifications on the deficiency of
connecting links in our geological formations. But against the belief
in such abrupt changes, embryology enters a strong protest. It is
notorious that the wings of birds and bats, and the legs of horses or
other quadrupeds, are undistinguishable at an early embryonic period,
and that they become differentiated by insensibly fine steps.
Embryological resemblances of all kinds can be accounted for, as we
shall hereafter see, by the progenitors of our existing species having
varied after early youth, and having transmitted their newly-acquired
characters to their offspring, at a corresponding age. The embryo is
thus left almost unaffected, and serves as a record of the past
condition of the species. Hence it is that existing species during the
early stages of their development so often resemble ancient and extinct
forms belonging to the same class. On this view of the meaning of
embryological resemblances, and indeed on any view, it is incredible
that an animal should have undergone such momentous and abrupt
transformations as those above indicated, and yet should not bear even
a trace in its embryonic condition of any sudden modification, every
detail in its structure being developed by insensibly fine steps.

He who believes that some ancient form was transformed suddenly through
an internal force or tendency into, for instance, one furnished with
wings, will be almost compelled to assume, in opposition to all
analogy, that many individuals varied simultaneously. It cannot be
denied that such abrupt and great changes of structure are widely
different from those which most species apparently have undergone. He
will further be compelled to believe that many structures beautifully
adapted to all the other parts of the same creature and to the
surrounding conditions, have been suddenly produced; and of such
complex and wonderful co-adaptations, he will not be able to assign a
shadow of an explanation. He will be forced to admit that these great
and sudden transformations have left no trace of their action on the
embryo. To admit all this is, as it seems to me, to enter into the
realms of miracle, and to leave those of science.




CHAPTER VIII.
INSTINCT.


Instincts comparable with habits, but different in their
origin—Instincts graduated—Aphides and ants—Instincts variable—Domestic
instincts, their origin—Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees—Slave-making ants—Hive-bee, its cell-making
instinct—Changes of instinct and structure not necessarily
simultaneous—Difficulties of the theory of the Natural Selection of
instincts—Neuter or sterile insects—Summary.


Many instincts are so wonderful that their development will probably
appear to the reader a difficulty sufficient to overthrow my whole
theory. I may here premise, that I have nothing to do with the origin
of the mental powers, any more than I have with that of life itself. We
are concerned only with the diversities of instinct and of the other
mental faculties in animals of the same class.

I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this
term; but every one understands what is meant, when it is said that
instinct impels the cuckoo to migrate and to lay her eggs in other
birds’ nests. An action, which we ourselves require experience to
enable us to perform, when performed by an animal, more especially by a
very young one, without experience, and when performed by many
individuals in the same way, without their knowing for what purpose it
is performed, is usually said to be instinctive. But I could show that
none of these characters are universal. A little dose of judgment or
reason, as Pierre Huber expresses it, often comes into play, even with
animals low in the scale of nature.

Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, an accurate notion
of the frame of mind under which an instinctive action is performed,
but not necessarily of its origin. How unconsciously many habitual
actions are performed, indeed not rarely in direct opposition to our
conscious will! yet they may be modified by the will or reason. Habits
easily become associated with other habits, with certain periods of
time and states of the body. When once acquired, they often remain
constant throughout life. Several other points of resemblance between
instincts and habits could be pointed out. As in repeating a well-known
song, so in instincts, one action follows another by a sort of rhythm;
if a person be interrupted in a song, or in repeating anything by rote,
he is generally forced to go back to recover the habitual train of
thought: so P. Huber found it was with a caterpillar, which makes a
very complicated hammock; for if he took a caterpillar which had
completed its hammock up to, say, the sixth stage of construction, and
put it into a hammock completed up only to the third stage, the
caterpillar simply re-performed the fourth, fifth, and sixth stages of
construction. If, however, a caterpillar were taken out of a hammock
made up, for instance, to the third stage, and were put into one
finished up to the sixth stage, so that much of its work was already
done for it, far from deriving any benefit from this, it was much
embarrassed, and, in order to complete its hammock, seemed forced to
start from the third stage, where it had left off, and thus tried to
complete the already finished work.

If we suppose any habitual action to become inherited—and it can be
shown that this does sometimes happen—then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three
years old with wonderfully little practice, had played a tune with no
practice at all, be might truly be said to have done so instinctively.
But it would be a serious error to suppose that the greater number of
instincts have been acquired by habit in one generation, and then
transmitted by inheritance to succeeding generations. It can be clearly
shown that the most wonderful instincts with which we are acquainted,
namely, those of the hive-bee and of many ants, could not possibly have
been acquired by habit.

It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that was
profitable. It is thus, as I believe, that all the most complex and
wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are in many cases of
subordinate importance to the effects of the natural selection of what
may be called spontaneous variations of instincts;—that is of
variations produced by the same unknown causes which produce slight
deviations of bodily structure.

No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures,
we ought to find in nature, not the actual transitional gradations by
which each complex instinct has been acquired—for these could be found
only in the lineal ancestors of each species—but we ought to find in
the collateral lines of descent some evidence of such gradations; or we
ought at least to be able to show that gradations of some kind are
possible; and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been but little
observed, except in Europe and North America, and for no instinct being
known among extinct species, how very generally gradations, leading to
the most complex instincts, can be discovered. Changes of instinct may
sometimes be facilitated by the same species having different instincts
at different periods of life, or at different seasons of the year, or
when placed under different circumstances, &c.; in which case either
the one or the other instinct might be preserved by natural selection.
And such instances of diversity of instinct in the same species can be
shown to occur in nature.

Again, as in the case of corporeal structure, and conformably to my
theory, the instinct of each species is good for itself, but has never,
as far as we can judge, been produced for the exclusive good of others.
One of the strongest instances of an animal apparently performing an
action for the sole good of another, with which I am acquainted, is
that of aphides voluntarily yielding, as was first observed by Huber,
their sweet excretion to ants: that they do so voluntarily, the
following facts show. I removed all the ants from a group of about a
dozen aphides on a dock-plant, and prevented their attendance during
several hours. After this interval, I felt sure that the aphides would
want to excrete. I watched them for some time through a lens, but not
one excreted; I then tickled and stroked them with a hair in the same
manner, as well as I could, as the ants do with their antennæ; but not
one excreted. Afterwards, I allowed an ant to visit them, and it
immediately seemed, by its eager way of running about to be well aware
what a rich flock it had discovered; it then began to play with its
antennæ on the abdomen first of one aphis and then of another; and
each, as soon as it felt the antennæ, immediately lifted up its abdomen
and excreted a limpid drop of sweet juice, which was eagerly devoured
by the ant. Even the quite young aphides behaved in this manner,
showing that the action was instinctive, and not the result of
experience. It is certain, from the observations of Huber, that the
aphides show no dislike to the ants: if the latter be not present they
are at last compelled to eject their excretion. But as the excretion is
extremely viscid, it is no doubt a convenience to the aphides to have
it removed; therefore probably they do not excrete solely for the good
of the ants. Although there is no evidence that any animal performs an
action for the exclusive good of another species, yet each tries to
take advantage of the instincts of others, as each takes advantage of
the weaker bodily structure of other species. So again certain
instincts cannot be considered as absolutely perfect; but as details on
this and other such points are not indispensable, they may be here
passed over.

As some degree of variation in instincts under a state of nature, and
the inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to be given; but
want of space prevents me. I can only assert that instincts certainly
do vary—for instance, the migratory instinct, both in extent and
direction, and in its total loss. So it is with the nests of birds,
which vary partly in dependence on the situations chosen, and on the
nature and temperature of the country inhabited, but often from causes
wholly unknown to us. Audubon has given several remarkable cases of
differences in the nests of the same species in the northern and
southern United States. Why, it has been asked, if instinct be
variable, has it not granted to the bee “the ability to use some other
material when wax was deficient?” But what other natural material could
bees use? They will work, as I have seen, with wax hardened with
vermilion or softened with lard. Andrew Knight observed that his bees,
instead of laboriously collecting propolis, used a cement of wax and
turpentine, with which he had covered decorticated trees. It has lately
been shown that bees, instead of searching for pollen, will gladly use
a very different substance, namely, oatmeal. Fear of any particular
enemy is certainly an instinctive quality, as may be seen in nestling
birds, though it is strengthened by experience, and by the sight of
fear of the same enemy in other animals. The fear of man is slowly
acquired, as I have elsewhere shown, by the various animals which
inhabit desert islands; and we see an instance of this, even in
England, in the greater wildness of all our large birds in comparison
with our small birds; for the large birds have been most persecuted by
man. We may safely attribute the greater wildness of our large birds to
this cause; for in uninhabited islands large birds are not more fearful
than small; and the magpie, so wary in England, is tame in Norway, as
is the hooded crow in Egypt.

That the mental qualities of animals of the same kind, born in a state
of nature, vary much, could be shown by many facts. Several cases could
also be adduced of occasional and strange habits in wild animals,
which, if advantageous to the species, might have given rise, through
natural selection, to new instincts. But I am well aware that these
general statements, without the facts in detail, can produce but a
feeble effect on the reader’s mind. I can only repeat my assurance,
that I do not speak without good evidence.

_Inherited Changes of Habit or Instinct in Domesticated Animals._


The possibility, or even probability, of inherited variations of
instinct in a state of nature will be strengthened by briefly
considering a few cases under domestication. We shall thus be enabled
to see the part which habit and the selection of so-called spontaneous
variations have played in modifying the mental qualities of our
domestic animals. It is notorious how much domestic animals vary in
their mental qualities. With cats, for instance, one naturally takes to
catching rats, and another mice, and these tendencies are known to be
inherited. One cat, according to Mr. St. John, always brought home game
birds, another hares or rabbits, and another hunted on marshy ground
and almost nightly caught woodcocks or snipes. A number of curious and
authentic instances could be given of various shades of disposition and
taste, and likewise of the oddest tricks, associated with certain
frames of mind or periods of time. But let us look to the familiar case
of the breeds of dogs: it cannot be doubted that young pointers (I have
myself seen striking instances) will sometimes point and even back
other dogs the very first time that they are taken out; retrieving is
certainly in some degree inherited by retrievers; and a tendency to run
round, instead of at, a flock of sheep, by shepherd-dogs. I cannot see
that these actions, performed without experience by the young, and in
nearly the same manner by each individual, performed with eager delight
by each breed, and without the end being known—for the young pointer
can no more know that he points to aid his master, than the white
butterfly knows why she lays her eggs on the leaf of the cabbage—I
cannot see that these actions differ essentially from true instincts.
If we were to behold one kind of wolf, when young and without any
training, as soon as it scented its prey, stand motionless like a
statue, and then slowly crawl forward with a peculiar gait; and another
kind of wolf rushing round, instead of at, a herd of deer, and driving
them to a distant point, we should assuredly call these actions
instinctive. Domestic instincts, as they may be called, are certainly
far less fixed than natural instincts; but they have been acted on by
far less rigorous selection, and have been transmitted for an
incomparably shorter period, under less fixed conditions of life.

How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross
with a bull-dog has affected for many generations the courage and
obstinacy of greyhounds; and a cross with a greyhound has given to a
whole family of shepherd-dogs a tendency to hunt hares. These domestic
instincts, when thus tested by crossing, resemble natural instincts,
which in a like manner become curiously blended together, and for a
long period exhibit traces of the instincts of either parent: for
example, Le Roy describes a dog, whose great-grandfather was a wolf,
and this dog showed a trace of its wild parentage only in one way, by
not coming in a straight line to his master, when called.

Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this is
not true. No one would ever have thought of teaching, or probably could
have taught, the tumbler-pigeon to tumble—an action which, as I have
witnessed, is performed by young birds, that have never seen a pigeon
tumble. We may believe that some one pigeon showed a slight tendency to
this strange habit, and that the long-continued selection of the best
individuals in successive generations made tumblers what they now are;
and near Glasgow there are house-tumblers, as I hear from Mr. Brent,
which cannot fly eighteen inches high without going head over heels. It
may be doubted whether any one would have thought of training a dog to
point, had not some one dog naturally shown a tendency in this line;
and this is known occasionally to happen, as I once saw, in a pure
terrier: the act of pointing is probably, as many have thought, only
the exaggerated pause of an animal preparing to spring on its prey.
When the first tendency to point was once displayed, methodical
selection and the inherited effects of compulsory training in each
successive generation would soon complete the work; and unconscious
selection is still in progress, as each man tries to procure, without
intending to improve the breed, dogs which stand and hunt best. On the
other hand, habit alone in some cases has sufficed; hardly any animal
is more difficult to tame than the young of the wild rabbit; scarcely
any animal is tamer than the young of the tame rabbit; but I can hardly
suppose that domestic rabbits have often been selected for tameness
alone; so that we must attribute at least the greater part of the
inherited change from extreme wildness to extreme tameness, to habit
and long-continued close confinement.

Natural instincts are lost under domestication: a remarkable instance
of this is seen in those breeds of fowls which very rarely or never
become “broody,” that is, never wish to sit on their eggs. Familiarity
alone prevents our seeing how largely and how permanently the minds of
our domestic animals have been modified. It is scarcely possible to
doubt that the love of man has become instinctive in the dog. All
wolves, foxes, jackals and species of the cat genus, when kept tame,
are most eager to attack poultry, sheep and pigs; and this tendency has
been found incurable in dogs which have been brought home as puppies
from countries such as Tierra del Fuego and Australia, where the
savages do not keep these domestic animals. How rarely, on the other
hand, do our civilised dogs, even when quite young, require to be
taught not to attack poultry, sheep, and pigs! No doubt they
occasionally do make an attack, and are then beaten; and if not cured,
they are destroyed; so that habit and some degree of selection have
probably concurred in civilising by inheritance our dogs. On the other
hand, young chickens have lost wholly by habit, that fear of the dog
and cat which no doubt was originally instinctive in them, for I am
informed by Captain Hutton that the young chickens of the parent stock,
the Gallus bankiva, when reared in India under a hen, are at first
excessively wild. So it is with young pheasants reared in England under
a hen. It is not that chickens have lost all fear, but fear only of
dogs and cats, for if the hen gives the danger chuckle they will run
(more especially young turkeys) from under her and conceal themselves
in the surrounding grass or thickets; and this is evidently done for
the instinctive purpose of allowing, as we see in wild ground-birds,
their mother to fly away. But this instinct retained by our chickens
has become useless under domestication, for the mother-hen has almost
lost by disuse the power of flight.

Hence, we may conclude that under domestication instincts have been
acquired and natural instincts have been lost, partly by habit and
partly by man selecting and accumulating, during successive
generations, peculiar mental habits and actions, which at first
appeared from what we must in our ignorance call an accident. In some
cases compulsory habit alone has sufficed to produce inherited mental
changes; in other cases compulsory habit has done nothing, and all has
been the result of selection, pursued both methodically and
unconsciously; but in most cases habit and selection have probably
concurred.

_Special Instincts._


We shall, perhaps, best understand how instincts in a state of nature
have become modified by selection by considering a few cases. I will
select only three, namely, the instinct which leads the cuckoo to lay
her eggs in other birds’ nests; the slave-making instinct of certain
ants; and the cell-making power of the hive-bee: these two latter
instincts have generally and justly been ranked by naturalists as the
most wonderful of all known instincts.

_Instincts of the Cuckoo._—It is supposed by some naturalists that the
more immediate cause of the instinct of the cuckoo is that she lays her
eggs, not daily, but at intervals of two or three days; so that, if she
were to make her own nest and sit on her own eggs, those first laid
would have to be left for some time unincubated or there would be eggs
and young birds of different ages in the same nest. If this were the
case the process of laying and hatching might be inconveniently long,
more especially as she migrates at a very early period; and the first
hatched young would probably have to be fed by the male alone. But the
American cuckoo is in this predicament, for she makes her own nest and
has eggs and young successively hatched, all at the same time. It has
been both asserted and denied that the American cuckoo occasionally
lays her eggs in other birds’ nests; but I have lately heard from Dr.
Merrill, of Iowa, that he once found in Illinois a young cuckoo,
together with a young jay in the nest of a blue jay (Garrulus
cristatus); and as both were nearly full feathered, there could be no
mistake in their identification. I could also give several instances of
various birds which have been known occasionally to lay their eggs in
other birds’ nests. Now let us suppose that the ancient progenitor of
our European cuckoo had the habits of the American cuckoo, and that she
occasionally laid an egg in another bird’s nest. If the old bird
profited by this occasional habit through being enabled to emigrate
earlier or through any other cause; or if the young were made more
vigorous by advantage being taken of the mistaken instinct of another
species than when reared by their own mother, encumbered as she could
hardly fail to be by having eggs and young of different ages at the
same time, then the old birds or the fostered young would gain an
advantage. And analogy would lead us to believe, that the young thus
reared would be apt to follow by inheritance the occasional and
aberrant habit of their mother, and in their turn would be apt to lay
their eggs in other birds’ nests, and thus be more successful in
rearing their young. By a continued process of this nature, I believe
that the strange instinct of our cuckoo has been generated. It has,
also recently been ascertained on sufficient evidence, by Adolf Müller,
that the cuckoo occasionally lays her eggs on the bare ground, sits on
them and feeds her young. This rare event is probably a case of
reversion to the long-lost, aboriginal instinct of nidification.

It has been objected that I have not noticed other related instincts
and adaptations of structure in the cuckoo, which are spoken of as
necessarily co-ordinated. But in all cases, speculation on an instinct
known to us only in a single species, is useless, for we have hitherto
had no facts to guide us. Until recently the instincts of the European
and of the non-parasitic American cuckoo alone were known; now, owing
to Mr. Ramsay’s observations, we have learned something about three
Australian species, which lay their eggs in other birds’ nests. The
chief points to be referred to are three: first, that the common
cuckoo, with rare exceptions, lays only one egg in a nest, so that the
large and voracious young bird receives ample food. Secondly, that the
eggs are remarkably small, not exceeding those of the skylark—a bird
about one-fourth as large as the cuckoo. That the small size of the egg
is a real case of adaptation we may infer from the fact of the
mon-parasitic American cuckoo laying full-sized eggs. Thirdly, that the
young cuckoo, soon after birth, has the instinct, the strength and a
properly shaped back for ejecting its foster-brothers, which then
perish from cold and hunger. This has been boldly called a beneficent
arrangement, in order that the young cuckoo may get sufficient food,
and that its foster-brothers may perish before they had acquired much
feeling!

Turning now to the Australian species: though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs
in the same nest. In the bronze cuckoo the eggs vary greatly in size,
from eight to ten lines in length. Now, if it had been of an advantage
to this species to have laid eggs even smaller than those now laid, so
as to have deceived certain foster-parents, or, as is more probable, to
have been hatched within a shorter period (for it is asserted that
there is a relation between the size of eggs and the period of their
incubation), then there is no difficulty in believing that a race or
species might have been formed which would have laid smaller and
smaller eggs; for these would have been more safely hatched and reared.
Mr. Ramsay remarks that two of the Australian cuckoos, when they lay
their eggs in an open nest, manifest a decided preference for nests
containing eggs similar in colour to their own. The European species
apparently manifests some tendency towards a similar instinct, but not
rarely departs from it, as is shown by her laying her dull and
pale-coloured eggs in the nest of the hedge-warbler with bright
greenish-blue eggs. Had our cuckoo invariably displayed the above
instinct, it would assuredly have been added to those which it is
assumed must all have been acquired together. The eggs of the
Australian bronze cuckoo vary, according to Mr. Ramsay, to an
extraordinary degree in colour; so that in this respect, as well as in
size, natural selection might have secured and fixed any advantageous
variation.

In the case of the European cuckoo, the offspring of the foster-parents
are commonly ejected from the nest within three days after the cuckoo
is hatched; and as the latter at this age is in a most helpless
condition, Mr. Gould was formerly inclined to believe that the act of
ejection was performed by the foster-parents themselves. But he has now
received a trustworthy account of a young cuckoo which was actually
seen, while still blind and not able even to hold up its own head, in
the act of ejecting its foster-brothers. One of these was replaced in
the nest by the observer, and was again thrown out. With respect to the
means by which this strange and odious instinct was acquired, if it
were of great importance for the young cuckoo, as is probably the case,
to receive as much food as possible soon after birth, I can see no
special difficulty in its having gradually acquired, during successive
generations, the blind desire, the strength, and structure necessary
for the work of ejection; for those cuckoos which had such habits and
structure best developed would be the most securely reared. The first
step towards the acquisition of the proper instinct might have been
mere unintentional restlessness on the part of the young bird, when
somewhat advanced in age and strength; the habit having been afterwards
improved, and transmitted to an earlier age. I can see no more
difficulty in this than in the unhatched young of other birds acquiring
the instinct to break through their own shells; or than in young snakes
acquiring in their upper jaws, as Owen has remarked, a transitory sharp
tooth for cutting through the tough egg-shell. For if each part is
liable to individual variations at all ages, and the variations tend to
be inherited at a corresponding or earlier age—propositions which
cannot be disputed—then the instincts and structure of the young could
be slowly modified as surely as those of the adult; and both cases must
stand or fall together with the whole theory of natural selection.

Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the
cuckoo; and the species present an interesting gradation in the
perfection of their instincts. The sexes of Molothrus badius are stated
by an excellent observer, Mr. Hudson, sometimes to live promiscuously
together in flocks, and sometimes to pair. They either build a nest of
their own or seize on one belonging to some other bird, occasionally
throwing out the nestlings of the stranger. They either lay their eggs
in the nest thus appropriated, or oddly enough build one for themselves
on the top of it. They usually sit on their own eggs and rear their own
young; but Mr. Hudson says it is probable that they are occasionally
parasitic, for he has seen the young of this species following old
birds of a distinct kind and clamouring to be fed by them. The
parasitic habits of another species of Molothrus, the M. bonariensis,
are much more highly developed than those of the last, but are still
far from perfect. This bird, as far as it is known, invariably lays its
eggs in the nests of strangers; but it is remarkable that several
together sometimes commence to build an irregular untidy nest of their
own, placed in singular ill-adapted situations, as on the leaves of a
large thistle. They never, however, as far as Mr. Hudson has
ascertained, complete a nest for themselves. They often lay so many
eggs—from fifteen to twenty—in the same foster-nest, that few or none
can possibly be hatched. They have, moreover, the extraordinary habit
of pecking holes in the eggs, whether of their own species or of their
foster parents, which they find in the appropriated nests. They drop
also many eggs on the bare ground, which are thus wasted. A third
species, the M. pecoris of North America, has acquired instincts as
perfect as those of the cuckoo, for it never lays more than one egg in
a foster-nest, so that the young bird is securely reared. Mr. Hudson is
a strong disbeliever in evolution, but he appears to have been so much
struck by the imperfect instincts of the Molothrus bonariensis that he
quotes my words, and asks, “Must we consider these habits, not as
especially endowed or created instincts, but as small consequences of
one general law, namely, transition?”

Various birds, as has already been remarked, occasionally lay their
eggs in the nests of other birds. This habit is not very uncommon with
the Gallinaceæ, and throws some light on the singular instinct of the
ostrich. In this family several hen birds unite and lay first a few
eggs in one nest and then in another; and these are hatched by the
males. This instinct may probably be accounted for by the fact of the
hens laying a large number of eggs, but, as with the cuckoo, at
intervals of two or three days. The instinct, however, of the American
ostrich, as in the case of the Molothrus bonariensis, has not as yet
been perfected; for a surprising number of eggs lie strewed over the
plains, so that in one day’s hunting I picked up no less than twenty
lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests of
other kinds of bees. This case is more remarkable than that of the
cuckoo; for these bees have not only had their instincts but their
structure modified in accordance with their parasitic habits; for they
do not possess the pollen-collecting apparatus which would have been
indispensable if they had stored up food for their own young. Some
species of Sphegidæ (wasp-like insects) are likewise parasitic; and M.
Fabre has lately shown good reason for believing that, although the
Tachytes nigra generally makes its own burrow and stores it with
paralysed prey for its own larvæ, yet that, when this insect finds a
burrow already made and stored by another sphex, it takes advantage of
the prize, and becomes for the occasion parasitic. In this case, as
with that of the Molothrus or cuckoo, I can see no difficulty in
natural selection making an occasional habit permanent, if of advantage
to the species, and if the insect whose nest and stored food are
feloniously appropriated, be not thus exterminated.

_Slave-making instinct._—This remarkable instinct was first discovered
in the Formica (Polyerges) rufescens by Pierre Huber, a better observer
even than his celebrated father. This ant is absolutely dependent on
its slaves; without their aid, the species would certainly become
extinct in a single year. The males and fertile females do no work of
any kind, and the workers or sterile females, though most energetic and
courageous in capturing slaves, do no other work. They are incapable of
making their own nests, or of feeding their own larvæ. When the old
nest is found inconvenient, and they have to migrate, it is the slaves
which determine the migration, and actually carry their masters in
their jaws. So utterly helpless are the masters, that when Huber shut
up thirty of them without a slave, but with plenty of the food which
they like best, and with their larvæ and pupæ to stimulate them to
work, they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca),
and she instantly set to work, fed and saved the survivors; made some
cells and tended the larvæ, and put all to rights. What can be more
extraordinary than these well-ascertained facts? If we had not known of
any other slave-making ant, it would have been hopeless to speculate
how so wonderful an instinct could have been perfected.

Another species, Formica sanguinea, was likewise first discovered by P.
Huber to be a slave-making ant. This species is found in the southern
parts of England, and its habits have been attended to by Mr. F. Smith,
of the British Museum, to whom I am much indebted for information on
this and other subjects. Although fully trusting to the statements of
Huber and Mr. Smith, I tried to approach the subject in a sceptical
frame of mind, as any one may well be excused for doubting the
existence of so extraordinary an instinct as that of making slaves.
Hence, I will give the observations which I made in some little detail.
I opened fourteen nests of F. sanguinea, and found a few slaves in all.
Males and fertile females of the slave-species (F. fusca) are found
only in their own proper communities, and have never been observed in
the nests of F. sanguinea. The slaves are black and not above half the
size of their red masters, so that the contrast in their appearance is
great. When the nest is slightly disturbed, the slaves occasionally
come out, and like their masters are much agitated and defend the nest:
when the nest is much disturbed, and the larvæ and pupæ are exposed,
the slaves work energetically together with their masters in carrying
them away to a place of safety. Hence, it is clear that the slaves feel
quite at home. During the months of June and July, on three successive
years, I watched for many hours several nests in Surrey and Sussex, and
never saw a slave either leave or enter a nest. As, during these
months, the slaves are very few in number, I thought that they might
behave differently when more numerous; but Mr. Smith informs me that he
has watched the nests at various hours during May, June and August,
both in Surrey and Hampshire, and has never seen the slaves, though
present in large numbers in August, either leave or enter the nest.
Hence, he considers them as strictly household slaves. The masters, on
the other hand, may be constantly seen bringing in materials for the
nest, and food of all kinds. During the year 1860, however, in the
month of July, I came across a community with an unusually large stock
of slaves, and I observed a few slaves mingled with their masters
leaving the nest, and marching along the same road to a tall Scotch-fir
tree, twenty-five yards distant, which they ascended together, probably
in search of aphides or cocci. According to Huber, who had ample
opportunities for observation, the slaves in Switzerland habitually
work with their masters in making the nest, and they alone open and
close the doors in the morning and evening; and, as Huber expressly
states, their principal office is to search for aphides. This
difference in the usual habits of the masters and slaves in the two
countries, probably depends merely on the slaves being captured in
greater numbers in Switzerland than in England.

One day I fortunately witnessed a migration of F. sanguinea from one
nest to another, and it was a most interesting spectacle to behold the
masters carefully carrying their slaves in their jaws instead of being
carried by them, as in the case of F. rufescens. Another day my
attention was struck by about a score of the slave-makers haunting the
same spot, and evidently not in search of food; they approached and
were vigorously repulsed by an independent community of the slave
species (F. fusca); sometimes as many as three of these ants clinging
to the legs of the slave-making F. sanguinea. The latter ruthlessly
killed their small opponents and carried their dead bodies as food to
their nest, twenty-nine yards distant; but they were prevented from
getting any pupæ to rear as slaves. I then dug up a small parcel of the
pupæ of F. fusca from another nest, and put them down on a bare spot
near the place of combat; they were eagerly seized and carried off by
the tyrants, who perhaps fancied that, after all, they had been
victorious in their late combat.

At the same time I laid on the same place a small parcel of the pupæ of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of their nest. This species is sometimes,
though rarely, made into slaves, as has been described by Mr. Smith.
Although so small a species, it is very courageous, and I have seen it
ferociously attack other ants. In one instance I found to my surprise
an independent community of F. flava under a stone beneath a nest of
the slave-making F. sanguinea; and when I had accidentally disturbed
both nests, the little ants attacked their big neighbours with
surprising courage. Now I was curious to ascertain whether F. sanguinea
could distinguish the pupæ of F. fusca, which they habitually make into
slaves, from those of the little and furious F. flava, which they
rarely capture, and it was evident that they did at once distinguish
them; for we have seen that they eagerly and instantly seized the pupæ
of F. fusca, whereas they were much terrified when they came across the
pupæ, or even the earth from the nest, of F. flava, and quickly ran
away; but in about a quarter of an hour, shortly after all the little
yellow ants had crawled away, they took heart and carried off the pupæ.

One evening I visited another community of F. sanguinea, and found a
number of these ants returning home and entering their nests, carrying
the dead bodies of F. fusca (showing that it was not a migration) and
numerous pupæ. I traced a long file of ants burthened with booty, for
about forty yards back, to a very thick clump of heath, whence I saw
the last individual of F. sanguinea emerge, carrying a pupa; but I was
not able to find the desolated nest in the thick heath. The nest,
however, must have been close at hand, for two or three individuals of
F. fusca were rushing about in the greatest agitation, and one was
perched motionless with its own pupa in its mouth on the top of a spray
of heath, an image of despair over its ravaged home.

Such are the facts, though they did not need confirmation by me, in
regard to the wonderful instinct of making slaves. Let it be observed
what a contrast the instinctive habits of F. sanguinea present with
those of the continental F. rufescens. The latter does not build its
own nest, does not determine its own migrations, does not collect food
for itself or its young, and cannot even feed itself: it is absolutely
dependent on its numerous slaves. Formica sanguinea, on the other hand,
possesses much fewer slaves, and in the early part of the summer
extremely few. The masters determine when and where a new nest shall be
formed, and when they migrate, the masters carry the slaves. Both in
Switzerland and England the slaves seem to have the exclusive care of
the larvæ, and the masters alone go on slave-making expeditions. In
Switzerland the slaves and masters work together, making and bringing
materials for the nest: both, but chiefly the slaves, tend and milk as
it may be called, their aphides; and thus both collect food for the
community. In England the masters alone usually leave the nest to
collect building materials and food for themselves, their slaves and
larvæ. So that the masters in this country receive much less service
from their slaves than they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants which are not slave-makers, will, as
I have seen, carry off pupæ of other species, if scattered near their
nests, it is possible that such pupæ originally stored as food might
become developed; and the foreign ants thus unintentionally reared
would then follow their proper instincts, and do what work they could.
If their presence proved useful to the species which had seized them—if
it were more advantageous to this species, to capture workers than to
procreate them—the habit of collecting pupæ, originally for food, might
by natural selection be strengthened and rendered permanent for the
very different purpose of raising slaves. When the instinct was once
acquired, if carried out to a much less extent even than in our British
F. sanguinea, which, as we have seen, is less aided by its slaves than
the same species in Switzerland, natural selection might increase and
modify the instinct—always supposing each modification to be of use to
the species—until an ant was formed as abjectly dependent on its slaves
as is the Formica rufescens.

_Cell-making instinct of the Hive-Bee._—I will not here enter on minute
details on this subject, but will merely give an outline of the
conclusions at which I have arrived. He must be a dull man who can
examine the exquisite structure of a comb, so beautifully adapted to
its end, without enthusiastic admiration. We hear from mathematicians
that bees have practically solved a recondite problem, and have made
their cells of the proper shape to hold the greatest possible amount of
honey, with the least possible consumption of precious wax in their
construction. It has been remarked that a skilful workman, with fitting
tools and measures, would find it very difficult to make cells of wax
of the true form, though this is effected by a crowd of bees working in
a dark hive. Granting whatever instincts you please, it seems at first
quite inconceivable how they can make all the necessary angles and
planes, or even perceive when they are correctly made. But the
difficulty is not nearly so great as at first appears: all this
beautiful work can be shown, I think, to follow from a few simple
instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered
only as a modification of his theory. Let us look to the great
principle of gradation, and see whether Nature does not reveal to us
her method of work. At one end of a short series we have humble-bees,
which use their old cocoons to hold honey, sometimes adding to them
short tubes of wax, and likewise making separate and very irregular
rounded cells of wax. At the other end of the series we have the cells
of the hive-bee, placed in a double layer: each cell, as is well known,
is an hexagonal prism, with the basal edges of its six sides bevelled
so as to join an inverted pyramid, of three rhombs. These rhombs have
certain angles, and the three which form the pyramidal base of a single
cell on one side of the comb, enter into the composition of the bases
of three adjoining cells on the opposite side. In the series between
the extreme perfection of the cells of the hive-bee and the simplicity
of those of the humble-bee, we have the cells of the Mexican Melipona
domestica, carefully described and figured by Pierre Huber. The
Melipona itself is intermediate in structure between the hive and
humble bee, but more nearly related to the latter: it forms a nearly
regular waxen comb of cylindrical cells, in which the young are
hatched, and, in addition, some large cells of wax for holding honey.
These latter cells are nearly spherical and of nearly equal sizes, and
are aggregated into an irregular mass. But the important point to
notice is, that these cells are always made at that degree of nearness
to each other that they would have intersected or broken into each
other if the spheres had been completed; but this is never permitted,
the bees building perfectly flat walls of wax between the spheres which
thus tend to intersect. Hence, each cell consists of an outer spherical
portion, and of two, three, or more flat surfaces, according as the
cell adjoins two, three or more other cells. When one cell rests on
three other cells, which, from the spheres being nearly of the same
size, is very frequently and necessarily the case, the three flat
surfaces are united into a pyramid; and this pyramid, as Huber has
remarked, is manifestly a gross imitation of the three-sided pyramidal
base of the cell of the hive-bee. As in the cells of the hive-bee, so
here, the three plane surfaces in any one cell necessarily enter into
the construction of three adjoining cells. It is obvious that the
Melipona saves wax, and what is more important, labour, by this manner
of building; for the flat walls between the adjoining cells are not
double, but are of the same thickness as the outer spherical portions,
and yet each flat portion forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had
made its spheres at some given distance from each other, and had made
them of equal sizes and had arranged them symmetrically in a double
layer, the resulting structure would have been as perfect as the comb
of the hive-bee. Accordingly I wrote to Professor Miller, of Cambridge,
and this geometer has kindly read over the following statement, drawn
up from his information, and tells me that it is strictly correct:—

If a number of equal spheres be described with their centres placed in
two parallel layers; with the centre of each sphere at the distance of
radius x sqrt(2) or radius x 1.41421 (or at some lesser distance), from
the centres of the six surrounding spheres in the same layer; and at
the same distance from the centres of the adjoining spheres in the
other and parallel layer; then, if planes of intersection between the
several spheres in both layers be formed, there will result a double
layer of hexagonal prisms united together by pyramidal bases formed of
three rhombs; and the rhombs and the sides of the hexagonal prisms will
have every angle identically the same with the best measurements which
have been made of the cells of the hive-bee. But I hear from Professor
Wyman, who has made numerous careful measurements, that the accuracy of
the workmanship of the bee has been greatly exaggerated; so much so,
that whatever the typical form of the cell may be, it is rarely, if
ever, realised.

Hence we may safely conclude that, if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as
that of the hive-bee. We must suppose the Melipona to have the power of
forming her cells truly spherical, and of equal sizes; and this would
not be very surprising, seeing that she already does so to a certain
extent, and seeing what perfectly cylindrical burrows many insects make
in wood, apparently by turning round on a fixed point. We must suppose
the Melipona to arrange her cells in level layers, as she already does
her cylindrical cells; and we must further suppose, and this is the
greatest difficulty, that she can somehow judge accurately at what
distance to stand from her fellow-labourers when several are making
their spheres; but she is already so far enabled to judge of distance,
that she always describes her spheres so as to intersect to a certain
extent; and then she unites the points of intersection by perfectly
flat surfaces. By such modifications of instincts which in themselves
are not very wonderful—hardly more wonderful than those which guide a
bird to make its nest—I believe that the hive-bee has acquired, through
natural selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, rectangular strip of wax: the bees instantly began to excavate
minute circular pits in it; and as they deepened these little pits,
they made them wider and wider until they were converted into shallow
basins, appearing to the eye perfectly true or parts of a sphere, and
of about the diameter of a cell. It was most interesting to observe
that, wherever several bees had begun to excavate these basins near
together, they had begun their work at such a distance from each other
that by the time the basins had acquired the above stated width (_i.e._
about the width of an ordinary cell), and were in depth about one sixth
of the diameter of the sphere of which they formed a part, the rims of
the basins intersected or broke into each other. As soon as this
occurred, the bees ceased to excavate, and began to build up flat walls
of wax on the lines of intersection between the basins, so that each
hexagonal prism was built upon the scalloped edge of a smooth basin,
instead of on the straight edges of a three-sided pyramid as in the
case of ordinary cells.

I then put into the hive, instead of a thick, rectangular piece of wax,
a thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each
other, in the same way as before; but the ridge of wax was so thin,
that the bottoms of the basins, if they had been excavated to the same
depth as in the former experiment, would have broken into each other
from the opposite sides. The bees, however, did not suffer this to
happen, and they stopped their excavations in due time; so that the
basins, as soon as they had been a little deepened, came to have flat
bases; and these flat bases, formed by thin little plates of the
vermilion wax left ungnawed, were situated, as far as the eye could
judge, exactly along the planes of imaginary intersection between the
basins on the opposite side of the ridge of wax. In some parts, only
small portions, in other parts, large portions of a rhombic plate were
thus left between the opposed basins, but the work, from the unnatural
state of things, had not been neatly performed. The bees must have
worked at very nearly the same rate in circularly gnawing away and
deepening the basins on both sides of the ridge of vermilion wax, in
order to have thus succeeded in leaving flat plates between the basins,
by stopping work at the planes of intersection.

Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of
wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has
appeared to me that the bees do not always succeed in working at
exactly the same rate from the opposite sides; for I have noticed
half-completed rhombs at the base of a just-commenced cell, which were
slightly concave on one side, where I suppose that the bees had
excavated too quickly, and convex on the opposed side where the bees
had worked less quickly. In one well-marked instance, I put the comb
back into the hive, and allowed the bees to go on working for a short
time, and again examined the cell, and I found that the rhombic plate
had been completed, and had become _perfectly flat:_ it was absolutely
impossible, from the extreme thinness of the little plate, that they
could have effected this by gnawing away the convex side; and I suspect
that the bees in such cases stand in the opposed cells and push and
bend the ductile and warm wax (which as I have tried is easily done)
into its proper intermediate plane, and thus flatten it.

From the experiment of the ridge of vermilion wax we can see that, if
the bees were to build for themselves a thin wall of wax, they could
make their cells of the proper shape, by standing at the proper
distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing the
spheres to break into each other. Now bees, as may be clearly seen by
examining the edge of a growing comb, do make a rough, circumferential
wall or rim all round the comb; and they gnaw this away from the
opposite sides, always working circularly as they deepen each cell.
They do not make the whole three-sided pyramidal base of any one cell
at the same time, but only that one rhombic plate which stands on the
extreme growing margin, or the two plates, as the case may be; and they
never complete the upper edges of the rhombic plates, until the
hexagonal walls are commenced. Some of these statements differ from
those made by the justly celebrated elder Huber, but I am convinced of
their accuracy; and if I had space, I could show that they are
conformable with my theory.

Huber’s statement, that the very first cell is excavated out of a
little parallel-sided wall of wax, is not, as far as I have seen,
strictly correct; the first commencement having always been a little
hood of wax; but I will not here enter on details. We see how important
a part excavation plays in the construction of the cells; but it would
be a great error to suppose that the bees cannot build up a rough wall
of wax in the proper position—that is, along the plane of intersection
between two adjoining spheres. I have several specimens showing clearly
that they can do this. Even in the rude circumferential rim or wall of
wax round a growing comb, flexures may sometimes be observed,
corresponding in position to the planes of the rhombic basal plates of
future cells. But the rough wall of wax has in every case to be
finished off, by being largely gnawed away on both sides. The manner in
which the bees build is curious; they always make the first rough wall
from ten to twenty times thicker than the excessively thin finished
wall of the cell, which will ultimately be left. We shall understand
how they work, by supposing masons first to pile up a broad ridge of
cement, and then to begin cutting it away equally on both sides near
the ground, till a smooth, very thin wall is left in the middle; the
masons always piling up the cut-away cement, and adding fresh cement on
the summit of the ridge. We shall thus have a thin wall steadily
growing upward but always crowned by a gigantic coping. From all the
cells, both those just commenced and those completed, being thus
crowned by a strong coping of wax, the bees can cluster and crawl over
the comb without injuring the delicate hexagonal walls. These walls, as
Professor Miller has kindly ascertained for me, vary greatly in
thickness; being, on an average of twelve measurements made near the
border of the comb, 1/353 of an inch in thickness; whereas the basal
rhomboidal plates are thicker, nearly in the proportion of three to
two, having a mean thickness, from twenty-one measurements, of 1/229 of
an inch. By the above singular manner of building, strength is
continually given to the comb, with the utmost ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commencement
of the first cell. I was able practically to show this fact, by
covering the edges of the hexagonal walls of a single cell, or the
extreme margin of the circumferential rim of a growing comb, with an
extremely thin layer of melted vermilion wax; and I invariably found
that the colour was most delicately diffused by the bees—as delicately
as a painter could have done it with his brush—by atoms of the coloured
wax having been taken from the spot on which it had been placed, and
worked into the growing edges of the cells all round. The work of
construction seems to be a sort of balance struck between many bees,
all instinctively standing at the same relative distance from each
other, all trying to sweep equal spheres, and then building up, or
leaving ungnawed, the planes of intersection between these spheres. It
was really curious to note in cases of difficulty, as when two pieces
of comb met at an angle, how often the bees would pull down and rebuild
in different ways the same cell, sometimes recurring to a shape which
they had at first rejected.

When bees have a place on which they can stand in their proper
positions for working—for instance, on a slip of wood, placed directly
under the middle of a comb growing downwards, so that the comb has to
be built over one face of the slip—in this case the bees can lay the
foundations of one wall of a new hexagon, in its strictly proper place,
projecting beyond the other completed cells. It suffices that the bees
should be enabled to stand at their proper relative distances from each
other and from the walls of the last completed cells, and then, by
striking imaginary spheres, they can build up a wall intermediate
between two adjoining spheres; but, as far as I have seen, they never
gnaw away and finish off the angles of a cell till a large part both of
that cell and of the adjoining cells has been built. This capacity in
bees of laying down under certain circumstances a rough wall in its
proper place between two just-commenced cells, is important, as it
bears on a fact, which seems at first subversive of the foregoing
theory; namely, that the cells on the extreme margin of wasp-combs are
sometimes strictly hexagonal; but I have not space here to enter on
this subject. Nor does there seem to me any great difficulty in a
single insect (as in the case of a queen-wasp) making hexagonal cells,
if she were to work alternately on the inside and outside of two or
three cells commenced at the same time, always standing at the proper
relative distance from the parts of the cells just begun, sweeping
spheres or cylinders, and building up intermediate planes.

As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked,
how a long and graduated succession of modified architectural
instincts, all tending towards the present perfect plan of
construction, could have profited the progenitors of the hive-bee? I
think the answer is not difficult: cells constructed like those of the
bee or the wasp gain in strength, and save much in labour and space,
and in the materials of which they are constructed. With respect to the
formation of wax, it is known that bees are often hard pressed to get
sufficient nectar; and I am informed by Mr. Tegetmeier that it has been
experimentally proved that from twelve to fifteen pounds of dry sugar
are consumed by a hive of bees for the secretion of a pound of wax; so
that a prodigious quantity of fluid nectar must be collected and
consumed by the bees in a hive for the secretion of the wax necessary
for the construction of their combs. Moreover, many bees have to remain
idle for many days during the process of secretion. A large store of
honey is indispensable to support a large stock of bees during the
winter; and the security of the hive is known mainly to depend on a
large number of bees being supported. Hence the saving of wax by
largely saving honey, and the time consumed in collecting the honey,
must be an important element of success any family of bees. Of course
the success of the species may be dependent on the number of its
enemies, or parasites, or on quite distinct causes, and so be
altogether independent of the quantity of honey which the bees can
collect. But let us suppose that this latter circumstance determined,
as it probably often has determined, whether a bee allied to our
humble-bees could exist in large numbers in any country; and let us
further suppose that the community lived through the winter, and
consequently required a store of honey: there can in this case be no
doubt that it would be an advantage to our imaginary humble-bee if a
slight modification of her instincts led her to make her waxen cells
near together, so as to intersect a little; for a wall in common even
to two adjoining cells would save some little labour and wax. Hence, it
would continually be more and more advantageous to our humble-bees, if
they were to make their cells more and more regular, nearer together,
and aggregated into a mass, like the cells of the Melipona; for in this
case a large part of the bounding surface of each cell would serve to
bound the adjoining cells, and much labour and wax would be saved.
Again, from the same cause, it would be advantageous to the Melipona,
if she were to make her cells closer together, and more regular in
every way than at present; for then, as we have seen, the spherical
surfaces would wholly disappear and be replaced by plane surfaces; and
the Melipona would make a comb as perfect as that of the hive-bee.
Beyond this stage of perfection in architecture, natural selection
could not lead; for the comb of the hive-bee, as far as we can see, is
absolutely perfect in economising labour and wax.

Thus, as I believe, the most wonderful of all known instincts, that of
the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having, by slow degrees, more and more
perfectly led the bees to sweep equal spheres at a given distance from
each other in a double layer, and to build up and excavate the wax
along the planes of intersection. The bees, of course, no more knowing
that they swept their spheres at one particular distance from each
other, than they know what are the several angles of the hexagonal
prisms and of the basal rhombic plates; the motive power of the process
of natural selection having been the construction of cells of due
strength and of the proper size and shape for the larvæ, this being
effected with the greatest possible economy of labour and wax; that
individual swarm which thus made the best cells with least labour, and
least waste of honey in the secretion of wax, having succeeded best,
and having transmitted their newly-acquired economical instincts to new
swarms, which in their turn will have had the best chance of succeeding
in the struggle for existence.

_Objections to the Theory of Natural Selection as applied to Instincts:
Neuter and Sterile Insects._


It has been objected to the foregoing view of the origin of instincts
that “the variations of structure and of instinct must have been
simultaneous and accurately adjusted to each other, as a modification
in the one without an immediate corresponding change in the other would
have been fatal.” The force of this objection rests entirely on the
assumption that the changes in the instincts and structure are abrupt.
To take as an illustration the case of the larger titmouse, (Parus
major) alluded to in a previous chapter; this bird often holds the
seeds of the yew between its feet on a branch, and hammers with its
beak till it gets at the kernel. Now what special difficulty would
there be in natural selection preserving all the slight individual
variations in the shape of the beak, which were better and better
adapted to break open the seeds, until a beak was formed, as well
constructed for this purpose as that of the nuthatch, at the same time
that habit, or compulsion, or spontaneous variations of taste, led the
bird to become more and more of a seed-eater? In this case the beak is
supposed to be slowly modified by natural selection, subsequently to,
but in accordance with, slowly changing habits or taste; but let the
feet of the titmouse vary and grow larger from correlation with the
beak, or from any other unknown cause, and it is not improbable that
such larger feet would lead the bird to climb more and more until it
acquired the remarkable climbing instinct and power of the nuthatch. In
this case a gradual change of structure is supposed to lead to changed
instinctive habits. To take one more case: few instincts are more
remarkable than that which leads the swift of the Eastern Islands to
make its nest wholly of inspissated saliva. Some birds build their
nests of mud, believed to be moistened with saliva; and one of the
swifts of North America makes its nest (as I have seen) of sticks
agglutinated with saliva, and even with flakes of this substance. Is it
then very improbable that the natural selection of individual swifts,
which secreted more and more saliva, should at last produce a species
with instincts leading it to neglect other materials and to make its
nest exclusively of inspissated saliva? And so in other cases. It must,
however, be admitted that in many instances we cannot conjecture
whether it was instinct or structure which first varied.

No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection—cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate
gradations are known to exist; cases of instincts of such trifling
importance, that they could hardly have been acted on by natural
selection; cases of instincts almost identically the same in animals so
remote in the scale of nature that we cannot account for their
similarity by inheritance from a common progenitor, and consequently
must believe that they were independently acquired through natural
selection. I will not here enter on these several cases, but will
confine myself to one special difficulty, which at first appeared to me
insuperable, and actually fatal to the whole theory. I allude to the
neuters or sterile females in insect communities: for these neuters
often differ widely in instinct and in structure from both the males
and fertile females, and yet, from being sterile, they cannot propagate
their kind.

The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much
greater than that of any other striking modification of structure; for
it can be shown that some insects and other articulate animals in a
state of nature occasionally become sterile; and if such insects had
been social, and it had been profitable to the community that a number
should have been annually born capable of work, but incapable of
procreation, I can see no especial difficulty in this having been
effected through natural selection. But I must pass over this
preliminary difficulty. The great difficulty lies in the working ants
differing widely from both the males and the fertile females in
structure, as in the shape of the thorax, and in being destitute of
wings and sometimes of eyes, and in instinct. As far as instinct alone
is concerned, the wonderful difference in this respect between the
workers and the perfect females would have been better exemplified by
the hive-bee. If a working ant or other neuter insect had been an
ordinary animal, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection; namely,
by individuals having been born with slight profitable modifications,
which were inherited by the offspring, and that these again varied and
again were selected, and so onwards. But with the working ant we have
an insect differing greatly from its parents, yet absolutely sterile;
so that it could never have transmitted successively acquired
modifications of structure or instinct to its progeny. It may well be
asked how it is possible to reconcile this case with the theory of
natural selection?

First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all
sorts of differences of inherited structure which are correlated with
certain ages and with either sex. We have differences correlated not
only with one sex, but with that short period when the reproductive
system is active, as in the nuptial plumage of many birds, and in the
hooked jaws of the male salmon. We have even slight differences in the
horns of different breeds of cattle in relation to an artificially
imperfect state of the male sex; for oxen of certain breeds have longer
horns than the oxen of other breeds, relatively to the length of the
horns in both the bulls and cows of these same breeds. Hence, I can see
no great difficulty in any character becoming correlated with the
sterile condition of certain members of insect communities; the
difficulty lies in understanding how such correlated modifications of
structure could have been slowly accumulated by natural selection.

This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain
the desired end. Breeders of cattle wish the flesh and fat to be well
marbled together. An animal thus characterized has been slaughtered,
but the breeder has gone with confidence to the same stock and has
succeeded. Such faith may be placed in the power of selection that a
breed of cattle, always yielding oxen with extraordinarily long horns,
could, it is probable, be formed by carefully watching which individual
bulls and cows, when matched, produced oxen with the longest horns; and
yet no one ox would ever have propagated its kind. Here is a better and
real illustration: According to M. Verlot, some varieties of the double
annual stock, from having been long and carefully selected to the right
degree, always produce a large proportion of seedlings bearing double
and quite sterile flowers, but they likewise yield some single and
fertile plants. These latter, by which alone the variety can be
propagated, may be compared with the fertile male and female ants, and
the double sterile plants with the neuters of the same community. As
with the varieties of the stock, so with social insects, selection has
been applied to the family, and not to the individual, for the sake of
gaining a serviceable end. Hence, we may conclude that slight
modifications of structure or of instinct, correlated with the sterile
condition of certain members of the community, have proved
advantageous; consequently the fertile males and females have
flourished, and transmitted to their fertile offspring a tendency to
produce sterile members with the same modifications. This process must
have been repeated many times, until that prodigious amount of
difference between the fertile and sterile females of the same species
has been produced which we see in many social insects.

But we have not as yet touched on the acme of the difficulty; namely,
the fact that the neuters of several ants differ, not only from the
fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes.
The castes, moreover, do not generally graduate into each other, but
are perfectly well defined; being as distinct from each other as are
any two species of the same genus, or rather as any two genera of the
same family. Thus, in Eciton, there are working and soldier neuters,
with jaws and instincts extraordinarily different: in Cryptocerus, the
workers of one caste alone carry a wonderful sort of shield on their
heads, the use of which is quite unknown: in the Mexican Myrmecocystus,
the workers of one caste never leave the nest; they are fed by the
workers of another caste, and they have an enormously developed abdomen
which secretes a sort of honey, supplying the place of that excreted by
the aphides, or the domestic cattle as they may be called, which our
European ants guard and imprison.

It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate the theory. In the
simpler case of neuter insects all of one caste, which, as I believe,
have been rendered different from the fertile males and females through
natural selection, we may conclude from the analogy of ordinary
variations, that the successive, slight, profitable modifications did
not first arise in all the neuters in the same nest, but in some few
alone; and that by the survival of the communities with females which
produced most neuters having the advantageous modification, all the
neuters ultimately came to be thus characterized. According to this
view we ought occasionally to find in the same nest neuter-insects,
presenting gradations of structure; and this we do find, even not
rarely, considering how few neuter-insects out of Europe have been
carefully examined. Mr. F. Smith has shown that the neuters of several
British ants differ surprisingly from each other in size and sometimes
in colour; and that the extreme forms can be linked together by
individuals taken out of the same nest: I have myself compared perfect
gradations of this kind. It sometimes happens that the larger or the
smaller sized workers are the most numerous; or that both large and
small are numerous, while those of an intermediate size are scanty in
numbers. Formica flava has larger and smaller workers, with some few of
intermediate size; and, in this species, as Mr. F. Smith has observed,
the larger workers have simple eyes (ocelli), which, though small, can
be plainly distinguished, whereas the smaller workers have their ocelli
rudimentary. Having carefully dissected several specimens of these
workers, I can affirm that the eyes are far more rudimentary in the
smaller workers than can be accounted for merely by their
proportionately lesser size; and I fully believe, though I dare not
assert so positively, that the workers of intermediate size have their
ocelli in an exactly intermediate condition. So that here we have two
bodies of sterile workers in the same nest, differing not only in size,
but in their organs of vision, yet connected by some few members in an
intermediate condition. I may digress by adding, that if the smaller
workers had been the most useful to the community, and those males and
females had been continually selected, which produced more and more of
the smaller workers, until all the workers were in this condition; we
should then have had a species of ant with neuters in nearly the same
condition as those of Myrmica. For the workers of Myrmica have not even
rudiments of ocelli, though the male and female ants of this genus have
well-developed ocelli.

I may give one other case: so confidently did I expect occasionally to
find gradations of important structures between the different castes of
neuters in the same species, that I gladly availed myself of Mr. F.
Smith’s offer of numerous specimens from the same nest of the driver
ant (Anomma) of West Africa. The reader will perhaps best appreciate
the amount of difference in these workers by my giving, not the actual
measurements, but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building a house, of
whom many were five feet four inches high, and many sixteen feet high;
but we must in addition suppose that the larger workmen had heads four
instead of three times as big as those of the smaller men, and jaws
nearly five times as big. The jaws, moreover, of the working ants of
the several sizes differed wonderfully in shape, and in the form and
number of the teeth. But the important fact for us is that, though the
workers can be grouped into castes of different sizes, yet they
graduate insensibly into each other, as does the widely-different
structure of their jaws. I speak confidently on this latter point, as
Sir J. Lubbock made drawings for me, with the camera lucida, of the
jaws which I dissected from the workers of the several sizes. Mr.
Bates, in his interesting “Naturalist on the Amazons,” has described
analogous cases.

With these facts before me, I believe that natural selection, by acting
on the fertile ants or parents, could form a species which should
regularly produce neuters, all of large size with one form of jaw, or
all of small size with widely different jaws; or lastly, and this is
the greatest difficulty, one set of workers of one size and structure,
and simultaneously another set of workers of a different size and
structure; a graduated series having first been formed, as in the case
of the driver ant, and then the extreme forms having been produced in
greater and greater numbers, through the survival of the parents which
generated them, until none with an intermediate structure were
produced.

An analogous explanation has been given by Mr. Wallace, of the equally
complex case, of certain Malayan butterflies regularly appearing under
two or even three distinct female forms; and by Fritz Müller, of
certain Brazilian crustaceans likewise appearing under two widely
distinct male forms. But this subject need not here be discussed.

I have now explained how, I believe, the wonderful fact of two
distinctly defined castes of sterile workers existing in the same nest,
both widely different from each other and from their parents, has
originated. We can see how useful their production may have been to a
social community of ants, on the same principle that the division of
labour is useful to civilised man. Ants, however, work by inherited
instincts and by inherited organs or tools, while man works by acquired
knowledge and manufactured instruments. But I must confess, that, with
all my faith in natural selection, I should never have anticipated that
this principle could have been efficient in so high a degree, had not
the case of these neuter insects led me to this conclusion. I have,
therefore, discussed this case, at some little but wholly insufficient
length, in order to show the power of natural selection, and likewise
because this is by far the most serious special difficulty which my
theory has encountered. The case, also, is very interesting, as it
proves that with animals, as with plants, any amount of modification
may be effected by the accumulation of numerous, slight, spontaneous
variations, which are in any way profitable, without exercise or habit
having been brought into play. For peculiar habits, confined to the
workers of sterile females, however long they might be followed, could
not possibly affect the males and fertile females, which alone leave
descendants. I am surprised that no one has advanced this demonstrative
case of neuter insects, against the well-known doctrine of inherited
habit, as advanced by Lamarck.

_Summary._


I have endeavoured in this chapter briefly to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts
vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore, there is no
real difficulty, under changing conditions of life, in natural
selection accumulating to any extent slight modifications of instinct
which are in any way useful. In many cases habit or use and disuse have
probably come into play. I do not pretend that the facts given in this
chapter strengthen in any great degree my theory; but none of the cases
of difficulty, to the best of my judgment, annihilate it. On the other
hand, the fact that instincts are not always absolutely perfect and are
liable to mistakes;—that no instinct can be shown to have been produced
for the good of other animals, though animals take advantage of the
instincts of others;—that the canon in natural history, of “Natura non
facit saltum,” is applicable to instincts as well as to corporeal
structure, and is plainly explicable on the foregoing views, but is
otherwise inexplicable—all tend to corroborate the theory of natural
selection.

This theory is also strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but distinct,
species, when inhabiting distant parts of the world and living under
considerably different conditions of life, yet often retaining nearly
the same instincts. For instance, we can understand, on the principle
of inheritance, how it is that the thrush of tropical South America
lines its nest with mud, in the same peculiar manner as does our
British thrush; how it is that the Hornbills of Africa and India have
the same extraordinary instinct of plastering up and imprisoning the
females in a hole in a tree, with only a small hole left in the plaster
through which the males feed them and their young when hatched; how it
is that the male wrens (Troglodytes) of North America, build
“cock-nests,” to roost in, like the males of our Kitty-wrens,—a habit
wholly unlike that of any other known bird. Finally, it may not be a
logical deduction, but to my imagination it is far more satisfactory to
look at such instincts as the young cuckoo ejecting its
foster-brothers, ants making slaves, the larvæ of ichneumonidæ feeding
within the live bodies of caterpillars, not as specially endowed or
created instincts, but as small consequences of one general law leading
to the advancement of all organic beings—namely, multiply, vary, let
the strongest live and the weakest die.




CHAPTER IX.
HYBRIDISM.


Distinction between the sterility of first crosses and of
hybrids—Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication—Laws governing the sterility of
hybrids—Sterility not a special endowment, but incidental on other
differences, not accumulated by natural selection—Causes of the
sterility of first crosses and of hybrids—Parallelism between the
effects of changed conditions of life and of crossing—Dimorphism and
trimorphism—Fertility of varieties when crossed and of their mongrel
offspring not universal—Hybrids and mongrels compared independently of
their fertility—Summary.


The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order to
prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept
distinct had they been capable of freely crossing. The subject is in
many ways important for us, more especially as the sterility of species
when first crossed, and that of their hybrid offspring, cannot have
been acquired, as I shall show, by the preservation of successive
profitable degrees of sterility. It is an incidental result of
differences in the reproductive systems of the parent-species.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species when first crossed, and the sterility of the
hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive organs
functionally impotent, as may be clearly seen in the state of the male
element in both plants and animals; though the formative organs
themselves are perfect in structure, as far as the microscope reveals.
In the first case the two sexual elements which go to form the embryo
are perfect; in the second case they are either not at all developed,
or are imperfectly developed. This distinction is important, when the
cause of the sterility, which is common to the two cases, has to be
considered. The distinction probably has been slurred over, owing to
the sterility in both cases being looked on as a special endowment,
beyond the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to
be descended from common parents, when crossed, and likewise the
fertility of their mongrel offspring, is, with reference to my theory,
of equal importance with the sterility of species; for it seems to make
a broad and clear distinction between varieties and species.

_Degrees of Sterility._—First, for the sterility of species when
crossed and of their hybrid offspring. It is impossible to study the
several memoirs and works of those two conscientious and admirable
observers, Kölreuter and Gärtner, who almost devoted their lives to
this subject, without being deeply impressed with the high generality
of some degree of sterility. Kölreuter makes the rule universal; but
then he cuts the knot, for in ten cases in which he found two forms,
considered by most authors as distinct species, quite fertile together,
he unhesitatingly ranks them as varieties. Gärtner, also, makes the
rule equally universal; and he disputes the entire fertility of
Kölreuter’s ten cases. But in these and in many other cases, Gärtner is
obliged carefully to count the seeds, in order to show that there is
any degree of sterility. He always compares the maximum number of seeds
produced by two species when first crossed, and the maximum produced by
their hybrid offspring, with the average number produced by both pure
parent-species in a state of nature. But causes of serious error here
intervene: a plant, to be hybridised, must be castrated, and, what is
often more important, must be secluded in order to prevent pollen being
brought to it by insects from other plants. Nearly all the plants
experimented on by Gärtner were potted, and were kept in a chamber in
his house. That these processes are often injurious to the fertility of
a plant cannot be doubted; for Gärtner gives in his table about a score
of cases of plants which he castrated, and artificially fertilised with
their own pollen, and (excluding all cases such as the Leguminosæ, in
which there is an acknowledged difficulty in the manipulation) half of
these twenty plants had their fertility in some degree impaired.
Moreover, as Gärtner repeatedly crossed some forms, such as the common
red and blue pimpernels (Anagallis arvensis and coerulea), which the
best botanists rank as varieties, and found them absolutely sterile, we
may doubt whether many species are really so sterile, when
intercrossed, as he believed.

It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can be required
than that the two most experienced observers who have ever lived,
namely Kölreuter and Gärtner, arrived at diametrically opposite
conclusions in regard to some of the very same forms. It is also most
instructive to compare—but I have not space here to enter on
details—the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same observer from experiments made during
different years. It can thus be shown that neither sterility nor
fertility affords any certain distinction between species and
varieties. The evidence from this source graduates away, and is
doubtful in the same degree as is the evidence derived from other
constitutional and structural differences.

In regard to the sterility of hybrids in successive generations; though
Gärtner was enabled to rear some hybrids, carefully guarding them from
a cross with either pure parent, for six or seven, and in one case for
ten generations, yet he asserts positively that their fertility never
increases, but generally decreases greatly and suddenly. With respect
to this decrease, it may first be noticed that when any deviation in
structure or constitution is common to both parents, this is often
transmitted in an augmented degree to the offspring; and both sexual
elements in hybrid plants are already affected in some degree. But I
believe that their fertility has been diminished in nearly all these
cases by an independent cause, namely, by too close interbreeding. I
have made so many experiments and collected so many facts, showing on
the one hand that an occasional cross with a distinct individual or
variety increases the vigour and fertility of the offspring, and on the
other hand that very close interbreeding lessens their vigour and
fertility, that I cannot doubt the correctness of this conclusion.
Hybrids are seldom raised by experimentalists in great numbers; and as
the parent-species, or other allied hybrids, generally grow in the same
garden, the visits of insects must be carefully prevented during the
flowering season: hence hybrids, if left to themselves, will generally
be fertilised during each generation by pollen from the same flower;
and this would probably be injurious to their fertility, already
lessened by their hybrid origin. I am strengthened in this conviction
by a remarkable statement repeatedly made by Gärtner, namely, that if
even the less fertile hybrids be artificially fertilised with hybrid
pollen of the same kind, their fertility, notwithstanding the frequent
ill effects from manipulation, sometimes decidedly increases, and goes
on increasing. Now, in the process of artificial fertilisation, pollen
is as often taken by chance (as I know from my own experience) from the
anthers of another flower, as from the anthers of the flower itself
which is to be fertilised; so that a cross between two flowers, though
probably often on the same plant, would be thus effected. Moreover,
whenever complicated experiments are in progress, so careful an
observer as Gärtner would have castrated his hybrids, and this would
have insured in each generation a cross with pollen from a distinct
flower, either from the same plant or from another plant of the same
hybrid nature. And thus, the strange fact of an increase of fertility
in the successive generations of _artificially fertilised_ hybrids, in
contrast with those spontaneously self-fertilised, may, as I believe,
be accounted for by too close interbreeding having been avoided.

Now let us turn to the results arrived at by a third most experienced
hybridiser, namely, the Hon. and Rev. W. Herbert. He is as emphatic in
his conclusion that some hybrids are perfectly fertile—as fertile as
the pure parent-species—as are Kölreuter and Gärtner that some degree
of sterility between distinct species is a universal law of nature. He
experimented on some of the very same species as did Gärtner. The
difference in their results may, I think, be in part accounted for by
Herbert’s great horticultural skill, and by his having hot-houses at
his command. Of his many important statements I will here give only a
single one as an example, namely, that “every ovule in a pod of Crinum
capense fertilised by C. revolutum produced a plant, which I never saw
to occur in a case of its natural fecundation.” So that here we have
perfect, or even more than commonly perfect fertility, in a first cross
between two distinct species.

This case of the Crinum leads me to refer to a singular fact, namely,
that individual plants of certain species of Lobelia, Verbascum and
Passiflora, can easily be fertilised by the pollen from a distinct
species, but not by pollen from the same plant, though this pollen can
be proved to be perfectly sound by fertilising other plants or species.
In the genus Hippeastrum, in Corydalis as shown by Professor
Hildebrand, in various orchids as shown by Mr. Scott and Fritz Müller,
all the individuals are in this peculiar condition. So that with some
species, certain abnormal individuals, and in other species all the
individuals, can actually be hybridised much more readily than they can
be fertilised by pollen from the same individual plant! To give one
instance, a bulb of Hippeastrum aulicum produced four flowers; three
were fertilised by Herbert with their own pollen, and the fourth was
subsequently fertilised by the pollen of a compound hybrid descended
from three distinct species: the result was that “the ovaries of the
three first flowers soon ceased to grow, and after a few days perished
entirely, whereas the pod impregnated by the pollen of the hybrid made
vigorous growth and rapid progress to maturity, and bore good seed,
which vegetated freely.” Mr. Herbert tried similar experiments during
many years, and always with the same result. These cases serve to show
on what slight and mysterious causes the lesser or greater fertility of
a species sometimes depends.

The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, &c., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely
dissimilar in general habit, “reproduces itself as perfectly as if it
had been a natural species from the mountains of Chile.” I have taken
some pains to ascertain the degree of fertility of some of the complex
crosses of Rhododendrons, and I am assured that many of them are
perfectly fertile. Mr. C. Noble, for instance, informs me that he
raises stocks for grafting from a hybrid between Rhod. ponticum and
catawbiense, and that this hybrid “seeds as freely as it is possible to
imagine.” Had hybrids, when fairly treated, always gone on decreasing
in fertility in each successive generation, as Gärtner believed to be
the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrid, and such alone are
fairly treated, for by insect agency the several individuals are
allowed to cross freely with each other, and the injurious influence of
close interbreeding is thus prevented. Any one may readily convince
himself of the efficiency of insect agency by examining the flowers of
the more sterile kinds of hybrid Rhododendrons, which produce no
pollen, for he will find on their stigmas plenty of pollen brought from
other flowers.

In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that
is, if the genera of animals are as distinct from each other as are the
genera of plants, then we may infer that animals more widely distinct
in the scale of nature can be crossed more easily than in the case of
plants; but the hybrids themselves are, I think, more sterile. It
should, however, be borne in mind that, owing to few animals breeding
freely under confinement, few experiments have been fairly tried: for
instance, the canary-bird has been crossed with nine distinct species
of finches, but, as not one of these breeds freely in confinement, we
have no right to expect that the first crosses between them and the
canary, or that their hybrids, should be perfectly fertile. Again, with
respect to the fertility in successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of
the same hybrid have been raised at the same time from different
parents, so as to avoid the ill effects of close interbreeding. On the
contrary, brothers and sisters have usually been crossed in each
successive generation, in opposition to the constantly repeated
admonition of every breeder. And in this case, it is not at all
surprising that the inherent sterility in the hybrids should have gone
on increasing.

Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus, are perfectly fertile. M. Quatrefages
states that the hybrids from two moths (Bombyx cynthia and arrindia)
were proved in Paris to be fertile _inter se_ for eight generations. It
has lately been asserted that two such distinct species as the hare and
rabbit, when they can be got to breed together, produce offspring,
which are highly fertile when crossed with one of the parent-species.
The hybrids from the common and Chinese geese (A. cygnoides), species
which are so different that they are generally ranked in distinct
genera, have often bred in this country with either pure parent, and in
one single instance they have bred _inter se_. This was effected by Mr.
Eyton, who raised two hybrids from the same parents, but from different
hatches; and from these two birds he raised no less than eight hybrids
(grandchildren of the pure geese) from one nest. In India, however,
these cross-bred geese must be far more fertile; for I am assured by
two eminently capable judges, namely Mr. Blyth and Captain Hutton, that
whole flocks of these crossed geese are kept in various parts of the
country; and as they are kept for profit, where neither pure
parent-species exists, they must certainly be highly or perfectly
fertile.

With our domesticated animals, the various races when crossed together
are quite fertile; yet in many cases they are descended from two or
more wild species. From this fact we must conclude either that the
aboriginal parent-species at first produced perfectly fertile hybrids,
or that the hybrids subsequently reared under domestication became
quite fertile. This latter alternative, which was first propounded by
Pallas, seems by far the most probable, and can, indeed, hardly be
doubted. It is, for instance, almost certain that our dogs are
descended from several wild stocks; yet, with perhaps the exception of
certain indigenous domestic dogs of South America, all are quite
fertile together; but analogy makes me greatly doubt, whether the
several aboriginal species would at first have freely bred together and
have produced quite fertile hybrids. So again I have lately acquired
decisive evidence that the crossed offspring from the Indian humped and
common cattle are inter se perfectly fertile; and from the observations
by Rütimeyer on their important osteological differences, as well as
from those by Mr. Blyth on their differences in habits, voice,
constitution, &c., these two forms must be regarded as good and
distinct species. The same remarks may be extended to the two chief
races of the pig. We must, therefore, either give up the belief of the
universal sterility of species when crossed; or we must look at this
sterility in animals, not as an indelible characteristic, but as one
capable of being removed by domestication.

Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids, is an extremely general result;
but that it cannot, under our present state of knowledge, be considered
as absolutely universal.

_Laws governing the Sterility of first Crosses and of Hybrids._


We will now consider a little more in detail the laws governing the
sterility of first crosses and of hybrids. Our chief object will be to
see whether or not these laws indicate that species have been specially
endowed with this quality, in order to prevent their crossing and
blending together in utter confusion. The following conclusions are
drawn up chiefly from Gärtner’s admirable work on the hybridisation of
plants. I have taken much pains to ascertain how far they apply to
animals, and, considering how scanty our knowledge is in regard to
hybrid animals, I have been surprised to find how generally the same
rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown;
but only the barest outline of the facts can here be given. When pollen
from a plant of one family is placed on the stigma of a plant of a
distinct family, it exerts no more influence than so much inorganic
dust. From this absolute zero of fertility, the pollen of different
species applied to the stigma of some one species of the same genus,
yields a perfect gradation in the number of seeds produced, up to
nearly complete or even quite complete fertility; and, as we have seen,
in certain abnormal cases, even to an excess of fertility, beyond that
which the plant’s own pollen produces. So in hybrids themselves, there
are some which never have produced, and probably never would produce,
even with the pollen of the pure parents, a single fertile seed: but in
some of these cases a first trace of fertility may be detected, by the
pollen of one of the pure parent-species causing the flower of the
hybrid to wither earlier than it otherwise would have done; and the
early withering of the flower is well known to be a sign of incipient
fertilisation. From this extreme degree of sterility we have
self-fertilised hybrids producing a greater and greater number of seeds
up to perfect fertility.

The hybrids raised from two species which are very difficult to cross,
and which rarely produce any offspring, are generally very sterile; but
the parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced—two classes of facts which are
generally confounded together—is by no means strict. There are many
cases, in which two pure species, as in the genus Verbascum, can be
united with unusual facility, and produce numerous hybrid offspring,
yet these hybrids are remarkably sterile. On the other hand, there are
species which can be crossed very rarely, or with extreme difficulty,
but the hybrids, when at last produced, are very fertile. Even within
the limits of the same genus, for instance in Dianthus, these two
opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is that of pure species. But
the fertility of first crosses is likewise innately variable; for it is
not always the same in degree when the same two species are crossed
under the same circumstances; it depends in part upon the constitution
of the individuals which happen to have been chosen for the experiment.
So it is with hybrids, for their degree of fertility is often found to
differ greatly in the several individuals raised from seed out of the
same capsule and exposed to the same conditions.

By the term systematic affinity is meant, the general resemblance
between species in structure and constitution. Now the fertility of
first crosses, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied
species generally uniting with facility. But the correspondence between
systematic affinity and the facility of crossing is by no means strict.
A multitude of cases could be given of very closely allied species
which will not unite, or only with extreme difficulty; and on the other
hand of very distinct species which unite with the utmost facility. In
the same family there may be a genus, as Dianthus, in which very many
species can most readily be crossed; and another genus, as Silene, in
which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of
almost any other genus; but Gärtner found that N. acuminata, which is
not a particularly distinct species, obstinately failed to fertilise,
or to be fertilised, by no less than eight other species of Nicotiana.
Many analogous facts could be given.

No one has been able to point out what kind or what amount of
difference, in any recognisable character, is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences in
every part of the flower, even in the pollen, in the fruit, and in the
cotyledons, can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and fitted for
extremely different climates, can often be crossed with ease.

By a reciprocal cross between two species, I mean the case, for
instance, of a female-ass being first crossed by a stallion, and then a
mare by a male-ass: these two species may then be said to have been
reciprocally crossed. There is often the widest possible difference in
the facility of making reciprocal crosses. Such cases are highly
important, for they prove that the capacity in any two species to cross
is often completely independent of their systematic affinity, that is
of any difference in their structure or constitution, excepting in
their reproductive systems. The diversity of the result in reciprocal
crosses between the same two species was long ago observed by
Kölreuter. To give an instance: Mirabilis jalapa can easily be
fertilised by the pollen of M. longiflora, and the hybrids thus
produced are sufficiently fertile; but Kölreuter tried more than two
hundred times, during eight following years, to fertilise reciprocally
M. longiflora with the pollen of M. jalapa, and utterly failed. Several
other equally striking cases could be given. Thuret has observed the
same fact with certain sea-weeds or Fuci. Gärtner, moreover, found that
this difference of facility in making reciprocal crosses is extremely
common in a lesser degree. He has observed it even between closely
related forms (as Matthiola annua and glabra) which many botanists rank
only as varieties. It is also a remarkable fact that hybrids raised
from reciprocal crosses, though of course compounded of the very same
two species, the one species having first been used as the father and
then as the mother, though they rarely differ in external characters,
yet generally differ in fertility in a small, and occasionally in a
high degree.

Several other singular rules could be given from Gärtner: for instance,
some species have a remarkable power of crossing with other species;
other species of the same genus have a remarkable power of impressing
their likeness on their hybrid offspring; but these two powers do not
at all necessarily go together. There are certain hybrids which,
instead of having, as is usual, an intermediate character between their
two parents, always closely resemble one of them; and such hybrids,
though externally so like one of their pure parent-species, are with
rare exceptions extremely sterile. So again among hybrids which are
usually intermediate in structure between their parents, exceptional
and abnormal individuals sometimes are born, which closely resemble one
of their pure parents; and these hybrids are almost always utterly
sterile, even when the other hybrids raised from seed from the same
capsule have a considerable degree of fertility. These facts show how
completely the fertility of a hybrid may be independent of its external
resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess; that their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable; that it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross; that the
fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent; and lastly, that the
facility of making a first cross between any two species is not always
governed by their systematic affinity or degree of resemblance to each
other. This latter statement is clearly proved by the difference in the
result of reciprocal crosses between the same two species, for,
according as the one species or the other is used as the father or the
mother, there is generally some difference, and occasionally the widest
possible difference, in the facility of effecting an union. The
hybrids, moreover, produced from reciprocal crosses often differ in
fertility.

Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending
together? Why should the degree of sterility be innately variable in
the individuals of the same species? Why should some species cross with
facility and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids? Why
should there often be so great a difference in the result of a
reciprocal cross between the same two species? Why, it may even be
asked, has the production of hybrids been permitted? To grant to
species the special power of producing hybrids, and then to stop their
further propagation by different degrees of sterility, not strictly
related to the facility of the first union between their parents, seems
a strange arrangement.

The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility, both of first crosses and of hybrids,
is simply incidental or dependent on unknown differences in their
reproductive systems; the differences being of so peculiar and limited
a nature, that, in reciprocal crosses between the same two species, the
male sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction. It will
be advisable to explain a little more fully, by an example, what I mean
by sterility being incidental on other differences, and not a specially
endowed quality. As the capacity of one plant to be grafted or budded
on another is unimportant for their welfare in a state of nature, I
presume that no one will suppose that this capacity is a _specially_
endowed quality, but will admit that it is incidental on differences in
the laws of growth of the two plants. We can sometimes see the reason
why one tree will not take on another from differences in their rate of
growth, in the hardness of their wood, in the period of the flow or
nature of their sap, &c.; but in a multitude of cases we can assign no
reason whatever. Great diversity in the size of two plants, one being
woody and the other herbaceous, one being evergreen and the other
deciduous, and adaptation to widely different climates, does not always
prevent the two grafting together. As in hybridisation, so with
grafting, the capacity is limited by systematic affinity, for no one
has been able to graft together trees belonging to quite distinct
families; and, on the other hand, closely allied species and varieties
of the same species, can usually, but not invariably, be grafted with
ease. But this capacity, as in hybridisation, is by no means absolutely
governed by systematic affinity. Although many distinct genera within
the same family have been grafted together, in other cases species of
the same genus will not take on each other. The pear can be grafted far
more readily on the quince, which is ranked as a distinct genus, than
on the apple, which is a member of the same genus. Even different
varieties of the pear take with different degrees of facility on the
quince; so do different varieties of the apricot and peach on certain
varieties of the plum.

As Gärtner found that there was sometimes an innate difference in
different _individuals_ of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same two
species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting. The common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.

We have seen that the sterility of hybrids which have their
reproductive organs in an imperfect condition, is a different case from
the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct classes of cases
run to a large extent parallel. Something analogous occurs in grafting;
for Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty on
a fourth species, when thus grafted were rendered barren. On the other
hand, certain species of Sorbus, when grafted on other species, yielded
twice as much fruit as when on their own roots. We are reminded by this
latter fact of the extraordinary cases of Hippeastrum, Passiflora, &c.,
which seed much more freely when fertilised with the pollen of a
distinct species than when fertilised with pollen from the same plant.

We thus see that, although there is a clear and great difference
between the mere adhesion of grafted stocks and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and of crossing
distinct species. And as we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other as
incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of
first crosses are incidental on unknown differences in their
reproductive systems. These differences in both cases follow, to a
certain extent, as might have been expected, systematic affinity, by
which term every kind of resemblance and dissimilarity between organic
beings is attempted to be expressed. The facts by no means seem to
indicate that the greater or lesser difficulty of either grafting or
crossing various species has been a special endowment; although in the
case of crossing, the difficulty is as important for the endurance and
stability of specific forms as in the case of grafting it is
unimportant for their welfare.

_Origin and Causes of the Sterility of first Crosses and of Hybrids._


At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly
acquired through the natural selection of slightly lessened degrees of
fertility, which, like any other variation, spontaneously appeared in
certain individuals of one variety when crossed with those of another
variety. For it would clearly be advantageous to two varieties or
incipient species if they could be kept from blending, on the same
principle that, when man is selecting at the same time two varieties,
it is necessary that he should keep them separate. In the first place,
it may be remarked that species inhabiting distinct regions are often
sterile when crossed; now it could clearly have been of no advantage to
such separated species to have been rendered mutually sterile, and
consequently this could not have been effected through natural
selection; but it may perhaps be argued, that, if a species was
rendered sterile with some one compatriot, sterility with other species
would follow as a necessary contingency. In the second place, it is
almost as much opposed to the theory of natural selection as to that of
special creation, that in reciprocal crosses the male element of one
form should have been rendered utterly impotent on a second form, while
at the same time the male element of this second form is enabled freely
to fertilise the first form; for this peculiar state of the
reproductive system could hardly have been advantageous to either
species.

In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty
will be found to lie in the existence of many graduated steps, from
slightly lessened fertility to absolute sterility. It may be admitted
that it would profit an incipient species, if it were rendered in some
slight degree sterile when crossed with its parent form or with some
other variety; for thus fewer bastardised and deteriorated offspring
would be produced to commingle their blood with the new species in
process of formation. But he who will take the trouble to reflect on
the steps by which this first degree of sterility could be increased
through natural selection to that high degree which is common with so
many species, and which is universal with species which have been
differentiated to a generic or family rank, will find the subject
extraordinarily complex. After mature reflection, it seems to me that
this could not have been effected through natural selection. Take the
case of any two species which, when crossed, produced few and sterile
offspring; now, what is there which could favour the survival of those
individuals which happened to be endowed in a slightly higher degree
with mutual infertility, and which thus approached by one small step
towards absolute sterility? Yet an advance of this kind, if the theory
of natural selection be brought to bear, must have incessantly occurred
with many species, for a multitude are mutually quite barren. With
sterile neuter insects we have reason to believe that modifications in
their structure and fertility have been slowly accumulated by natural
selection, from an advantage having been thus indirectly given to the
community to which they belonged over other communities of the same
species; but an individual animal not belonging to a social community,
if rendered slightly sterile when crossed with some other variety,
would not thus itself gain any advantage or indirectly give any
advantage to the other individuals of the same variety, thus leading to
their preservation.

But it would be superfluous to discuss this question in detail: for
with plants we have conclusive evidence that the sterility of crossed
species must be due to some principle, quite independent of natural
selection. Both Gärtner and Kölreuter have proved that in genera
including numerous species, a series can be formed from species which
when crossed yield fewer and fewer seeds, to species which never
produce a single seed, but yet are affected by the pollen of certain
other species, for the germen swells. It is here manifestly impossible
to select the more sterile individuals, which have already ceased to
yield seeds; so that this acme of sterility, when the germen alone is
effected, cannot have been gained through selection; and from the laws
governing the various grades of sterility being so uniform throughout
the animal and vegetable kingdoms, we may infer that the cause,
whatever it may be, is the same or nearly the same in all cases.

We will now look a little closer at the probable nature of the
differences between species which induce sterility in first crosses and
in hybrids. In the case of first crosses, the greater or less
difficulty in effecting a union and in obtaining offspring apparently
depends on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the
case with a plant having a pistil too long for the pollen-tubes to
reach the ovarium. It has also been observed that when the pollen of
one species is placed on the stigma of a distantly allied species,
though the pollen-tubes protrude, they do not penetrate the stigmatic
surface. Again, the male element may reach the female element, but be
incapable of causing an embryo to be developed, as seems to have been
the case with some of Thuret’s experiments on Fuci. No explanation can
be given of these facts, any more than why certain trees cannot be
grafted on others. Lastly, an embryo may be developed, and then perish
at an early period. This latter alternative has not been sufficiently
attended to; but I believe, from observations communicated to me by Mr.
Hewitt, who has had great experience in hybridising pheasants and
fowls, that the early death of the embryo is a very frequent cause of
sterility in first crosses. Mr. Salter has recently given the results
of an examination of about 500 eggs produced from various crosses
between three species of Gallus and their hybrids; the majority of
these eggs had been fertilised; and in the majority of the fertilised
eggs, the embryos had either been partially developed and had then
perished, or had become nearly mature, but the young chickens had been
unable to break through the shell. Of the chickens which were born,
more than four-fifths died within the first few days, or at latest
weeks, “without any obvious cause, apparently from mere inability to
live;” so that from the 500 eggs only twelve chickens were reared. With
plants, hybridized embryos probably often perish in a like manner; at
least it is known that hybrids raised from very distinct species are
sometimes weak and dwarfed, and perish at an early age; of which fact
Max Wichura has recently given some striking cases with hybrid willows.
It may be here worth noticing that in some cases of parthenogenesis,
the embryos within the eggs of silk moths which had not been
fertilised, pass through their early stages of development and then
perish like the embryos produced by a cross between distinct species.
Until becoming acquainted with these facts, I was unwilling to believe
in the frequent early death of hybrid embryos; for hybrids, when once
born, are generally healthy and long-lived, as we see in the case of
the common mule. Hybrids, however, are differently circumstanced before
and after birth: when born and living in a country where their two
parents live, they are generally placed under suitable conditions of
life. But a hybrid partakes of only half of the nature and constitution
of its mother; it may therefore, before birth, as long as it is
nourished within its mother’s womb, or within the egg or seed produced
by the mother, be exposed to conditions in some degree unsuitable, and
consequently be liable to perish at an early period; more especially as
all very young beings are eminently sensitive to injurious or unnatural
conditions of life. But after all, the cause more probably lies in some
imperfection in the original act of impregnation, causing the embryo to
be imperfectly developed, rather than in the conditions to which it is
subsequently exposed.

In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts showing that, when animals and
plants are removed from their natural conditions, they are extremely
liable to have their reproductive systems seriously affected. This, in
fact, is the great bar to the domestication of animals. Between the
sterility thus superinduced and that of hybrids, there are many points
of similarity. In both cases the sterility is independent of general
health, and is often accompanied by excess of size or great luxuriance.
In both cases the sterility occurs in various degrees; in both, the
male element is the most liable to be affected; but sometimes the
female more than the male. In both, the tendency goes to a certain
extent with systematic affinity, for whole groups of animals and plants
are rendered impotent by the same unnatural conditions; and whole
groups of species tend to produce sterile hybrids. On the other hand,
one species in a group will sometimes resist great changes of
conditions with unimpaired fertility; and certain species in a group
will produce unusually fertile hybrids. No one can tell till he tries,
whether any particular animal will breed under confinement, or any
exotic plant seed freely under culture; nor can he tell till he tries,
whether any two species of a genus will produce more or less sterile
hybrids. Lastly, when organic beings are placed during several
generations under conditions not natural to them, they are extremely
liable to vary, which seems to be partly due to their reproductive
systems having been specially affected, though in a lesser degree than
when sterility ensues. So it is with hybrids, for their offspring in
successive generations are eminently liable to vary, as every
experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general
state of health, is affected in a very similar manner. In the one case,
the conditions of life have been disturbed, though often in so slight a
degree as to be inappreciable by us; in the other case, or that of
hybrids, the external conditions have remained the same, but the
organisation has been disturbed by two distinct structures and
constitutions, including of course the reproductive systems, having
been blended into one. For it is scarcely possible that two
organisations should be compounded into one, without some disturbance
occurring in the development, or periodical action, or mutual relations
of the different parts and organs one to another or to the conditions
of life. When hybrids are able to breed _inter se_, they transmit to
their offspring from generation to generation the same compounded
organisation, and hence we need not be surprised that their sterility,
though in some degree variable, does not diminish; it is even apt to
increase, this being generally the result, as before explained, of too
close interbreeding. The above view of the sterility of hybrids being
caused by two constitutions being compounded into one has been strongly
maintained by Max Wichura.

It must, however, be owned that we cannot understand, on the above or
any other view, several facts with respect to the sterility of hybrids;
for instance, the unequal fertility of hybrids produced from reciprocal
crosses; or the increased sterility in those hybrids which occasionally
and exceptionally resemble closely either pure parent. Nor do I pretend
that the foregoing remarks go to the root of the matter: no explanation
is offered why an organism, when placed under unnatural conditions, is
rendered sterile. All that I have attempted to show is, that in two
cases, in some respects allied, sterility is the common result—in the
one case from the conditions of life having been disturbed, in the
other case from the organisation having been disturbed by two
organisations being compounded into one.

A similar parallelism holds good with an allied yet very different
class of facts. It is an old and almost universal belief, founded on a
considerable body of evidence, which I have elsewhere given, that
slight changes in the conditions of life are beneficial to all living
things. We see this acted on by farmers and gardeners in their frequent
exchanges of seed, tubers, &c., from one soil or climate to another,
and back again. During the convalescence of animals, great benefit is
derived from almost any change in their habits of life. Again, both
with plants and animals, there is the clearest evidence that a cross
between individuals of the same species, which differ to a certain
extent, gives vigour and fertility to the offspring; and that close
interbreeding continued during several generations between the nearest
relations, if these be kept under the same conditions of life, almost
always leads to decreased size, weakness, or sterility.

Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is, crosses between the males and females of the same
species, which have been subjected to slightly different conditions, or
which have slightly varied, give vigour and fertility to the offspring.
But, as we have seen, organic beings long habituated to certain uniform
conditions under a state of nature, when subjected, as under
confinement, to a considerable change in their conditions, very
frequently are rendered more or less sterile; and we know that a cross
between two forms that have become widely or specifically different,
produce hybrids which are almost always in some degree sterile. I am
fully persuaded that this double parallelism is by no means an accident
or an illusion. He who is able to explain why the elephant, and a
multitude of other animals, are incapable of breeding when kept under
only partial confinement in their native country, will be able to
explain the primary cause of hybrids being so generally sterile. He
will at the same time be able to explain how it is that the races of
some of our domesticated animals, which have often been subjected to
new and not uniform conditions, are quite fertile together, although
they are descended from distinct species, which would probably have
been sterile if aboriginally crossed. The above two parallel series of
facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life; this principle,
according to Mr. Herbert Spencer, being that life depends on, or
consists in, the incessant action and reaction of various forces,
which, as throughout nature, are always tending towards an equilibrium;
and when this tendency is slightly disturbed by any change, the vital
forces gain in power.

_Reciprocal Dimorphism and Trimorphism._


This subject may be here briefly discussed, and will be found to throw
some light on hybridism. Several plants belonging to distinct orders
present two forms, which exist in about equal numbers and which differ
in no respect except in their reproductive organs; one form having a
long pistil with short stamens, the other a short pistil with long
stamens; the two having differently sized pollen-grains. With
trimorphic plants there are three forms likewise differing in the
lengths of their pistils and stamens, in the size and colour of the
pollen-grains, and in some other respects; and as in each of the three
forms there are two sets of stamens, the three forms possess altogether
six sets of stamens and three kinds of pistils. These organs are so
proportioned in length to each other, that half the stamens in two of
the forms stand on a level with the stigma of the third form. Now I
have shown, and the result has been confirmed by other observers, that
in order to obtain full fertility with these plants, it is necessary
that the stigma of the one form should be fertilised by pollen taken
from the stamens of corresponding height in another form. So that with
dimorphic species two unions, which may be called legitimate, are fully
fertile; and two, which may be called illegitimate, are more or less
infertile. With trimorphic species six unions are legitimate, or fully
fertile, and twelve are illegitimate, or more or less infertile.

The infertility which may be observed in various dimorphic and
trimorphic plants, when they are illegitimately fertilised, that is by
pollen taken from stamens not corresponding in height with the pistil,
differs much in degree, up to absolute and utter sterility; just in the
same manner as occurs in crossing distinct species. As the degree of
sterility in the latter case depends in an eminent degree on the
conditions of life being more or less favourable, so I have found it
with illegitimate unions. It is well known that if pollen of a distinct
species be placed on the stigma of a flower, and its own pollen be
afterwards, even after a considerable interval of time, placed on the
same stigma, its action is so strongly prepotent that it generally
annihilates the effect of the foreign pollen; so it is with the pollen
of the several forms of the same species, for legitimate pollen is
strongly prepotent over illegitimate pollen, when both are placed on
the same stigma. I ascertained this by fertilising several flowers,
first illegitimately, and twenty-four hours afterwards legitimately,
with pollen taken from a peculiarly coloured variety, and all the
seedlings were similarly coloured; this shows that the legitimate
pollen, though applied twenty-four hours subsequently, had wholly
destroyed or prevented the action of the previously applied
illegitimate pollen. Again, as in making reciprocal crosses between the
same two species, there is occasionally a great difference in the
result, so the same thing occurs with trimorphic plants; for instance,
the mid-styled form of Lythrum salicaria was illegitimately fertilised
with the greatest ease by pollen from the longer stamens of the
short-styled form, and yielded many seeds; but the latter form did not
yield a single seed when fertilised by the longer stamens of the
mid-styled form.

In all these respects, and in others which might be added, the forms of
the same undoubted species, when illegitimately united, behave in
exactly the same manner as do two distinct species when crossed. This
led me carefully to observe during four years many seedlings, raised
from several illegitimate unions. The chief result is that these
illegitimate plants, as they may be called, are not fully fertile. It
is possible to raise from dimorphic species, both long-styled and
short-styled illegitimate plants, and from trimorphic plants all three
illegitimate forms. These can then be properly united in a legitimate
manner. When this is done, there is no apparent reason why they should
not yield as many seeds as did their parents when legitimately
fertilised. But such is not the case. They are all infertile, in
various degrees; some being so utterly and incurably sterile that they
did not yield during four seasons a single seed or even seed-capsule.
The sterility of these illegitimate plants, when united with each other
in a legitimate manner, may be strictly compared with that of hybrids
when crossed _inter se_. If, on the other hand, a hybrid is crossed
with either pure parent-species, the sterility is usually much
lessened: and so it is when an illegitimate plant is fertilised by a
legitimate plant. In the same manner as the sterility of hybrids does
not always run parallel with the difficulty of making the first cross
between the two parent-species, so that sterility of certain
illegitimate plants was unusually great, while the sterility of the
union from which they were derived was by no means great. With hybrids
raised from the same seed-capsule the degree of sterility is innately
variable, so it is in a marked manner with illegitimate plants. Lastly,
many hybrids are profuse and persistent flowerers, while other and more
sterile hybrids produce few flowers, and are weak, miserable dwarfs;
exactly similar cases occur with the illegitimate offspring of various
dimorphic and trimorphic plants.

Altogether there is the closest identity in character and behaviour
between illegitimate plants and hybrids. It is hardly an exaggeration
to maintain that illegitimate plants are hybrids, produced within the
limits of the same species by the improper union of certain forms,
while ordinary hybrids are produced from an improper union between
so-called distinct species. We have also already seen that there is the
closest similarity in all respects between first illegitimate unions
and first crosses between distinct species. This will perhaps be made
more fully apparent by an illustration; we may suppose that a botanist
found two well-marked varieties (and such occur) of the long-styled
form of the trimorphic Lythrum salicaria, and that he determined to try
by crossing whether they were specifically distinct. He would find that
they yielded only about one-fifth of the proper number of seed, and
that they behaved in all the other above specified respects as if they
had been two distinct species. But to make the case sure, he would
raise plants from his supposed hybridised seed, and he would find that
the seedlings were miserably dwarfed and utterly sterile, and that they
behaved in all other respects like ordinary hybrids. He might then
maintain that he had actually proved, in accordance with the common
view, that his two varieties were as good and as distinct species as
any in the world; but he would be completely mistaken.

The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened
fertility, both in first crosses and in hybrids, is no safe criterion
of specific distinction; secondly, because we may conclude that there
is some unknown bond which connects the infertility of illegitimate
unions with that of their illegitimate offspring, and we are led to
extend the same view to first crosses and hybrids; thirdly, because we
find, and this seems to me of especial importance, that two or three
forms of the same species may exist and may differ in no respect
whatever, either in structure or in constitution, relatively to
external conditions, and yet be sterile when united in certain ways.
For we must remember that it is the union of the sexual elements of
individuals of the same form, for instance, of two long-styled forms,
which results in sterility; while it is the union of the sexual
elements proper to two distinct forms which is fertile. Hence the case
appears at first sight exactly the reverse of what occurs, in the
ordinary unions of the individuals of the same species and with crosses
between distinct species. It is, however, doubtful whether this is
really so; but I will not enlarge on this obscure subject.

We may, however, infer as probable from the consideration of dimorphic
and trimorphic plants, that the sterility of distinct species when
crossed and of their hybrid progeny, depends exclusively on the nature
of their sexual elements, and not on any difference in their structure
or general constitution. We are also led to this same conclusion by
considering reciprocal crosses, in which the male of one species cannot
be united, or can be united with great difficulty, with the female of a
second species, while the converse cross can be effected with perfect
facility. That excellent observer, Gärtner, likewise concluded that
species when crossed are sterile owing to differences confined to their
reproductive systems.

_Fertility of Varieties when Crossed, and of their Mongrel Offspring,
not universal._


It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the
latter, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. With some exceptions, presently to be given, I fully admit
that this is the rule. But the subject is surrounded by difficulties,
for, looking to varieties produced under nature, if two forms hitherto
reputed to be varieties be found in any degree sterile together, they
are at once ranked by most naturalists as species. For instance, the
blue and red pimpernel, which are considered by most botanists as
varieties, are said by Gärtner to be quite sterile when crossed, and he
consequently ranks them as undoubted species. If we thus argue in a
circle, the fertility of all varieties produced under nature will
assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in some doubt. For when it
is stated, for instance, that certain South American indigenous
domestic dogs do not readily unite with European dogs, the explanation
which will occur to everyone, and probably the true one, is that they
are descended from aboriginally distinct species. Nevertheless the
perfect fertility of so many domestic races, differing widely from each
other in appearance, for instance, those of the pigeon, or of the
cabbage, is a remarkable fact; more especially when we reflect how many
species there are, which, though resembling each other most closely,
are utterly sterile when intercrossed. Several considerations, however,
render the fertility of domestic varieties less remarkable. In the
first place, it may be observed that the amount of external difference
between two species is no sure guide to their degree of mutual
sterility, so that similar differences in the case of varieties would
be no sure guide. It is certain that with species the cause lies
exclusively in differences in their sexual constitution. Now the
varying conditions to which domesticated animals and cultivated plants
have been subjected, have had so little tendency towards modifying the
reproductive system in a manner leading to mutual sterility, that we
have good grounds for admitting the directly opposite doctrine of
Pallas, namely, that such conditions generally eliminate this tendency;
so that the domesticated descendants of species, which in their natural
state probably would have been in some degree sterile when crossed,
become perfectly fertile together. With plants, so far is cultivation
from giving a tendency towards sterility between distinct species, that
in several well-authenticated cases already alluded to, certain plants
have been affected in an opposite manner, for they have become
self-impotent, while still retaining the capacity of fertilising, and
being fertilised by, other species. If the Pallasian doctrine of the
elimination of sterility through long-continued domestication be
admitted, and it can hardly be rejected, it becomes in the highest
degree improbable that similar conditions long-continued should
likewise induce this tendency; though in certain cases, with species
having a peculiar constitution, sterility might occasionally be thus
caused. Thus, as I believe, we can understand why, with domesticated
animals, varieties have not been produced which are mutually sterile;
and why with plants only a few such cases, immediately to be given,
have been observed.

The real difficulty in our present subject is not, as it appears to me,
why domestic varieties have not become mutually infertile when crossed,
but why this has so generally occurred with natural varieties, as soon
as they have been permanently modified in a sufficient degree to take
rank as species. We are far from precisely knowing the cause; nor is
this surprising, seeing how profoundly ignorant we are in regard to the
normal and abnormal action of the reproductive system. But we can see
that species, owing to their struggle for existence with numerous
competitors, will have been exposed during long periods of time to more
uniform conditions, than have domestic varieties; and this may well
make a wide difference in the result. For we know how commonly wild
animals and plants, when taken from their natural conditions and
subjected to captivity, are rendered sterile; and the reproductive
functions of organic beings which have always lived under natural
conditions would probably in like manner be eminently sensitive to the
influence of an unnatural cross. Domesticated productions, on the other
hand, which, as shown by the mere fact of their domestication, were not
originally highly sensitive to changes in their conditions of life, and
which can now generally resist with undiminished fertility repeated
changes of conditions, might be expected to produce varieties, which
would be little liable to have their reproductive powers injuriously
affected by the act of crossing with other varieties which had
originated in a like manner.

I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is impossible to resist
the evidence of the existence of a certain amount of sterility in the
few following cases, which I will briefly abstract. The evidence is at
least as good as that from which we believe in the sterility of a
multitude of species. The evidence is also derived from hostile
witnesses, who in all other cases consider fertility and sterility as
safe criterions of specific distinction. Gärtner kept, during several
years, a dwarf kind of maize with yellow seeds, and a tall variety with
red seeds growing near each other in his garden; and although these
plants have separated sexes, they never naturally crossed. He then
fertilised thirteen flowers of the one kind with pollen of the other;
but only a single head produced any seed, and this one head produced
only five grains. Manipulation in this case could not have been
injurious, as the plants have separated sexes. No one, I believe, has
suspected that these varieties of maize are distinct species; and it is
important to notice that the hybrid plants thus raised were themselves
_perfectly_ fertile; so that even Gärtner did not venture to consider
the two varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimented on are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties, and Naudin has
come to the same conclusion.

The following case is far more remarkable, and seems at first
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness as Gärtner: namely, that the
yellow and white varieties when crossed produce less seed than the
similarly coloured varieties of the same species. Moreover, he asserts
that, when yellow and white varieties of one species are crossed with
yellow and white varieties of a _distinct_ species, more seed is
produced by the crosses between the similarly coloured flowers, than
between those which are differently coloured. Mr. Scott also has
experimented on the species and varieties of Verbascum; and although
unable to confirm Gärtner’s results on the crossing of the distinct
species, he finds that the dissimilarly coloured varieties of the same
species yield fewer seeds, in the proportion of eighty-six to 100, than
the similarly coloured varieties. Yet these varieties differ in no
respect, except in the colour of their flowers; and one variety can
sometimes be raised from the seed of another.

Kölreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact that one particular variety of
the common tobacco was more fertile than the other varieties, when
crossed with a widely distinct species. He experimented on five forms
which are commonly reputed to be varieties, and which he tested by the
severest trial, namely, by reciprocal crosses, and he found their
mongrel offspring perfectly fertile. But one of these five varieties,
when used either as the father or mother, and crossed with the
Nicotiana glutinosa, always yielded hybrids not so sterile as those
which were produced from the four other varieties when crossed with N.
glutinosa. Hence the reproductive system of this one variety must have
been in some manner and in some degree modified.

From these facts it can no longer be maintained that varieties when
crossed are invariably quite fertile. From the great difficulty of
ascertaining the infertility of varieties in a state of nature, for a
supposed variety, if proved to be infertile in any degree, would almost
universally be ranked as a species; from man attending only to external
characters in his domestic varieties, and from such varieties not
having been exposed for very long periods to uniform conditions of
life; from these several considerations we may conclude that fertility
does not constitute a fundamental distinction between varieties and
species when crossed. The general sterility of crossed species may
safely be looked at, not as a special acquirement or endowment, but as
incidental on changes of an unknown nature in their sexual elements.

_Hybrids and Mongrels compared, independently of their fertility._


Independently of the question of fertility, the offspring of species
and of varieties when crossed may be compared in several other
respects. Gärtner, whose strong wish it was to draw a distinct line
between species and varieties, could find very few, and, as it seems to
me, quite unimportant differences between the so-called hybrid
offspring of species, and the so-called mongrel offspring of varieties.
And, on the other hand, they agree most closely in many important
respects.

I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gärtner admits that hybrids from
species which have long been cultivated are often variable in the first
generation; and I have myself seen striking instances of this fact.
Gärtner further admits that hybrids between very closely allied species
are more variable than those from very distinct species; and this shows
that the difference in the degree of variability graduates away. When
mongrels and the more fertile hybrids are propagated for several
generations, an extreme amount of variability in the offspring in both
cases is notorious; but some few instances of both hybrids and mongrels
long retaining a uniform character could be given. The variability,
however, in the successive generations of mongrels is, perhaps, greater
than in hybrids.

This greater variability in mongrels than in hybrids does not seem at
all surprising. For the parents of mongrels are varieties, and mostly
domestic varieties (very few experiments having been tried on natural
varieties), and this implies that there has been recent variability;
which would often continue and would augment that arising from the act
of crossing. The slight variability of hybrids in the first generation,
in contrast with that in the succeeding generations, is a curious fact
and deserves attention. For it bears on the view which I have taken of
one of the causes of ordinary variability; namely, that the
reproductive system, from being eminently sensitive to changed
conditions of life, fails under these circumstances to perform its
proper function of producing offspring closely similar in all respects
to the parent-form. Now, hybrids in the first generation are descended
from species (excluding those long cultivated) which have not had their
reproductive systems in any way affected, and they are not variable;
but hybrids themselves have their reproductive systems seriously
affected, and their descendants are highly variable.

But to return to our comparison of mongrels and hybrids: Gärtner states
that mongrels are more liable than hybrids to revert to either parent
form; but this, if it be true, is certainly only a difference in
degree. Moreover, Gärtner expressly states that the hybrids from long
cultivated plants are more subject to reversion than hybrids from
species in their natural state; and this probably explains the singular
difference in the results arrived at by different observers. Thus Max
Wichura doubts whether hybrids ever revert to their parent forms, and
he experimented on uncultivated species of willows, while Naudin, on
the other hand, insists in the strongest terms on the almost universal
tendency to reversion in hybrids, and he experimented chiefly on
cultivated plants. Gärtner further states that when any two species,
although most closely allied to each other, are crossed with a third
species, the hybrids are widely different from each other; whereas if
two very distinct varieties of one species are crossed with another
species, the hybrids do not differ much. But this conclusion, as far as
I can make out, is founded on a single experiment; and seems directly
opposed to the results of several experiments made by Kölreuter.

Such alone are the unimportant differences which Gärtner is able to
point out between hybrid and mongrel plants. On the other hand, the
degrees and kinds of resemblance in mongrels and in hybrids to their
respective parents, more especially in hybrids produced from nearly
related species, follow, according to Gärtner the same laws. When two
species are crossed, one has sometimes a prepotent power of impressing
its likeness on the hybrid. So I believe it to be with varieties of
plants; and with animals, one variety certainly often has this
prepotent power over another variety. Hybrid plants produced from a
reciprocal cross generally resemble each other closely, and so it is
with mongrel plants from a reciprocal cross. Both hybrids and mongrels
can be reduced to either pure parent form, by repeated crosses in
successive generations with either parent.

These several remarks are apparently applicable to animals; but the
subject is here much complicated, partly owing to the existence of
secondary sexual characters; but more especially owing to prepotency in
transmitting likeness running more strongly in one sex than in the
other, both when one species is crossed with another and when one
variety is crossed with another variety. For instance, I think those
authors are right who maintain that the ass has a prepotent power over
the horse, so that both the mule and the hinny resemble more closely
the ass than the horse; but that the prepotency runs more strongly in
the male than in the female ass, so that the mule, which is an
offspring of the male ass and mare, is more like an ass than is the
hinny, which is the offspring of the female-ass and stallion.

Much stress has been laid by some authors on the supposed fact, that it
is only with mongrels that the offspring are not intermediate in
character, but closely resemble one of their parents; but this does
sometimes occur with hybrids, yet I grant much less frequently than
with mongrels. Looking to the cases which I have collected of
cross-bred animals closely resembling one parent, the resemblances seem
chiefly confined to characters almost monstrous in their nature, and
which have suddenly appeared—such as albinism, melanism, deficiency of
tail or horns, or additional fingers and toes; and do not relate to
characters which have been slowly acquired through selection. A
tendency to sudden reversions to the perfect character of either parent
would, also, be much more likely to occur with mongrels, which are
descended from varieties often suddenly produced and semi-monstrous in
character, than with hybrids, which are descended from species slowly
and naturally produced. On the whole, I entirely agree with Dr. Prosper
Lucas, who, after arranging an enormous body of facts with respect to
animals, comes to the conclusion that the laws of resemblance of the
child to its parents are the same, whether the two parents differ
little or much from each other, namely, in the union of individuals of
the same variety, or of different varieties, or of distinct species.

Independently of the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.

_Summary of Chapter._


First crosses between forms, sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that
the most careful experimentalists have arrived at diametrically
opposite conclusions in ranking forms by this test. The sterility is
innately variable in individuals of the same species, and is eminently
susceptible to action of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but
is governed by several curious and complex laws. It is generally
different, and sometimes widely different in reciprocal crosses between
the same two species. It is not always equal in degree in a first cross
and in the hybrids produced from this cross.

In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally of
an unknown nature, in their vegetative systems, so in crossing, the
greater or less facility of one species to unite with another is
incidental on unknown differences in their reproductive systems. There
is no more reason to think that species have been specially endowed
with various degrees of sterility to prevent their crossing and
blending in nature, than to think that trees have been specially
endowed with various and somewhat analogous degrees of difficulty in
being grafted together in order to prevent their inarching in our
forests.

The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it
seems to depend on several circumstances; in some instances in chief
part on the early death of the embryo. In the case of hybrids, it
apparently depends on their whole organisation having been disturbed by
being compounded from two distinct forms; the sterility being closely
allied to that which so frequently affects pure species, when exposed
to new and unnatural conditions of life. He who will explain these
latter cases will be able to explain the sterility of hybrids. This
view is strongly supported by a parallelism of another kind: namely,
that, firstly, slight changes in the conditions of life add to the
vigour and fertility of all organic beings; and secondly, that the
crossing of forms, which have been exposed to slightly different
conditions of life, or which have varied, favours the size, vigour and
fertility of their offspring. The facts given on the sterility of the
illegitimate unions of dimorphic and trimorphic plants and of their
illegitimate progeny, perhaps render it probable that some unknown bond
in all cases connects the degree of fertility of first unions with that
of their offspring. The consideration of these facts on dimorphism, as
well as of the results of reciprocal crosses, clearly leads to the
conclusion that the primary cause of the sterility of crossed species
is confined to differences in their sexual elements. But why, in the
case of distinct species, the sexual elements should so generally have
become more or less modified, leading to their mutual infertility, we
do not know; but it seems to stand in some close relation to species
having been exposed for long periods of time to nearly uniform
conditions of life.

It is not surprising that the difficulty in crossing any two species,
and the sterility of their hybrid offspring, should in most cases
correspond, even if due to distinct causes: for both depend on the
amount of difference between the species which are crossed. Nor is it
surprising that the facility of effecting a first cross, and the
fertility of the hybrids thus produced, and the capacity of being
grafted together—though this latter capacity evidently depends on
widely different circumstances—should all run, to a certain extent,
parallel with the systematic affinity of the forms subjected to
experiment; for systematic affinity includes resemblances of all kinds.

First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not, as is so often stated, invariably fertile. Nor
is this almost universal and perfect fertility surprising, when it is
remembered how liable we are to argue in a circle with respect to
varieties in a state of nature; and when we remember that the greater
number of varieties have been produced under domestication by the
selection of mere external differences, and that they have not been
long exposed to uniform conditions of life. It should also be
especially kept in mind, that long-continued domestication tends to
eliminate sterility, and is therefore little likely to induce this same
quality. Independently of the question of fertility, in all other
respects there is the closest general resemblance between hybrids and
mongrels, in their variability, in their power of absorbing each other
by repeated crosses, and in their inheritance of characters from both
parent-forms. Finally, then, although we are as ignorant of the precise
cause of the sterility of first crosses and of hybrids as we are why
animals and plants removed from their natural conditions become
sterile, yet the facts given in this chapter do not seem to me opposed
to the belief that species aboriginally existed as varieties.




CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.


On the absence of intermediate varieties at the present day—On the
nature of extinct intermediate varieties; on their number—On the lapse
of time, as inferred from the rate of denudation and of deposition
number—On the lapse of time as estimated by years—On the poorness of
our palæontological collections—On the intermittence of geological
formations—On the denudation of granitic areas—On the absence of
intermediate varieties in any one formation—On the sudden appearance of
groups of species—On their sudden appearance in the lowest known
fossiliferous strata—Antiquity of the habitable earth.


In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely, the distinctness of specific
forms and their not being blended together by innumerable transitional
links, is a very obvious difficulty. I assigned reasons why such links
do not commonly occur at the present day under the circumstances
apparently most favourable for their presence, namely, on an extensive
and continuous area with graduated physical conditions. I endeavoured
to show, that the life of each species depends in a more important
manner on the presence of other already defined organic forms, than on
climate, and, therefore, that the really governing conditions of life
do not graduate away quite insensibly like heat or moisture. I
endeavoured, also, to show that intermediate varieties, from existing
in lesser numbers than the forms which they connect, will generally be
beaten out and exterminated during the course of further modification
and improvement. The main cause, however, of innumerable intermediate
links not now occurring everywhere throughout nature depends, on the
very process of natural selection, through which new varieties
continually take the places of and supplant their parent-forms. But
just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which
have formerly existed, be truly enormous. Why then is not every
geological formation and every stratum full of such intermediate links?
Geology assuredly does not reveal any such finely graduated organic
chain; and this, perhaps, is the most obvious and serious objection
which can be urged against my theory. The explanation lies, as I
believe, in the extreme imperfection of the geological record.

In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself forms _directly_ intermediate between them. But this is a
wholly false view; we should always look for forms intermediate between
each species and a common but unknown progenitor; and the progenitor
will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons are both descended from the rock-pigeon; if we possessed all
the intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
These two breeds, moreover, have become so much modified, that, if we
had no historical or indirect evidence regarding their origin, it would
not have been possible to have determined from a mere comparison of
their structure with that of the rock-pigeon, C. livia, whether they
had descended from this species or from some other allied species, such
as C. oenas.

So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links directly intermediate between them ever existed, but between each
and an unknown common parent. The common parent will have had in its
whole organisation much general resemblance to the tapir and to the
horse; but in some points of structure may have differed considerably
from both, even perhaps more than they differ from each other. Hence,
in all such cases, we should be unable to recognise the parent-form of
any two or more species, even if we closely compared the structure of
the parent with that of its modified descendants, unless at the same
time we had a nearly perfect chain of the intermediate links.

It is just possible, by the theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case _direct_ intermediate links will have existed between
them. But such a case would imply that one form had remained for a very
long period unaltered, whilst its descendants had undergone a vast
amount of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare event;
for in all cases the new and improved forms of life tend to supplant
the old and unimproved forms.

By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the natural and domestic varieties of the
same species at the present day; and these parent-species, now
generally extinct, have in their turn been similarly connected with
more ancient forms; and so on backwards, always converging to the
common ancestor of each great class. So that the number of intermediate
and transitional links, between all living and extinct species, must
have been inconceivably great. But assuredly, if this theory be true,
such have lived upon the earth.

_On the Lapse of Time, as inferred from the rate of deposition and
extent of Denudation._


Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time cannot have
sufficed for so great an amount of organic change, all changes having
been effected slowly. It is hardly possible for me to recall to the
reader who is not a practical geologist, the facts leading the mind
feebly to comprehend the lapse of time. He who can read Sir Charles
Lyell’s grand work on the Principles of Geology, which the future
historian will recognise as having produced a revolution in natural
science, and yet does not admit how vast have been the past periods of
time, may at once close this volume. Not that it suffices to study the
Principles of Geology, or to read special treatises by different
observers on separate formations, and to mark how each author attempts
to give an inadequate idea of the duration of each formation, or even
of each stratum. We can best gain some idea of past time by knowing the
agencies at work; and learning how deeply the surface of the land has
been denuded, and how much sediment has been deposited. As Lyell has
well remarked, the extent and thickness of our sedimentary formations
are the result and the measure of the denudation which the earth’s
crust has elsewhere undergone. Therefore a man should examine for
himself the great piles of superimposed strata, and watch the rivulets
bringing down mud, and the waves wearing away the sea-cliffs, in order
to comprehend something about the duration of past time, the monuments
of which we see all around us.

It is good to wander along the coast, when formed of moderately hard
rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves eat
into them only when they are charged with sand or pebbles; for there is
good evidence that pure water effects nothing in wearing away rock. At
last the base of the cliff is undermined, huge fragments fall down, and
these remaining fixed, have to be worn away atom by atom, until after
being reduced in size they can be rolled about by the waves, and then
they are more quickly ground into pebbles, sand, or mud. But how often
do we see along the bases of retreating cliffs rounded boulders, all
thickly clothed by marine productions, showing how little they are
abraded and how seldom they are rolled about! Moreover, if we follow
for a few miles any line of rocky cliff, which is undergoing
degradation, we find that it is only here and there, along a short
length or round a promontory, that the cliffs are at the present time
suffering. The appearance of the surface and the vegetation show that
elsewhere years have elapsed since the waters washed their base.

We have, however, recently learned from the observations of Ramsay, in
the van of many excellent observers—of Jukes, Geikie, Croll and others,
that subaërial degradation is a much more important agency than
coast-action, or the power of the waves. The whole surface of the land
is exposed to the chemical action of the air and of the rainwater, with
its dissolved carbonic acid, and in colder countries to frost; the
disintegrated matter is carried down even gentle slopes during heavy
rain, and to a greater extent than might be supposed, especially in
arid districts, by the wind; it is then transported by the streams and
rivers, which, when rapid deepen their channels, and triturate the
fragments. On a rainy day, even in a gently undulating country, we see
the effects of subaërial degradation in the muddy rills which flow down
every slope. Messrs. Ramsay and Whitaker have shown, and the
observation is a most striking one, that the great lines of escarpment
in the Wealden district and those ranging across England, which
formerly were looked at as ancient sea-coasts, cannot have been thus
formed, for each line is composed of one and the same formation, while
our sea-cliffs are everywhere formed by the intersection of various
formations. This being the case, we are compelled to admit that the
escarpments owe their origin in chief part to the rocks of which they
are composed, having resisted subaërial denudation better than the
surrounding surface; this surface consequently has been gradually
lowered, with the lines of harder rock left projecting. Nothing
impresses the mind with the vast duration of time, according to our
ideas of time, more forcibly than the conviction thus gained that
subaërial agencies, which apparently have so little power, and which
seem to work so slowly, have produced great results.

When thus impressed with the slow rate at which the land is worn away
through subaërial and littoral action, it is good, in order to
appreciate the past duration of time, to consider, on the one hand, the
masses of rock which have been removed over many extensive areas, and
on the other hand the thickness of our sedimentary formations. I
remember having been much struck when viewing volcanic islands, which
have been worn by the waves and pared all round into perpendicular
cliffs of one or two thousand feet in height; for the gentle slope of
the lava-streams, due to their formerly liquid state, showed at a
glance how far the hard, rocky beds had once extended into the open
ocean. The same story is told still more plainly by faults—those great
cracks along which the strata have been upheaved on one side, or thrown
down on the other, to the height or depth of thousands of feet; for
since the crust cracked, and it makes no great difference whether the
upheaval was sudden, or, as most geologists now believe, was slow and
effected by many starts, the surface of the land has been so completely
planed down that no trace of these vast dislocations is externally
visible. The Craven fault, for instance, extends for upward of thirty
miles, and along this line the vertical displacement of the strata
varies from 600 to 3,000 feet. Professor Ramsay has published an
account of a downthrow in Anglesea of 2,300 feet; and he informs me
that he fully believes that there is one in Merionethshire of 12,000
feet; yet in these cases there is nothing on the surface of the land to
show such prodigious movements; the pile of rocks on either side of the
crack having been smoothly swept away.

On the other hand, in all parts of the world the piles of sedimentary
strata are of wonderful thickness. In the Cordillera, I estimated one
mass of conglomerate at ten thousand feet; and although conglomerates
have probably been accumulated at a quicker rate than finer sediments,
yet from being formed of worn and rounded pebbles, each of which bears
the stamp of time, they are good to show how slowly the mass must have
been heaped together. Professor Ramsay has given me the maximum
thickness, from actual measurement in most cases, of the successive
formations in _different_ parts of Great Britain; and this is the
result:—


Feet Palæozoic strata (not including igneous beds)	57,154. Secondary
strata	13,190. Tertiary strata	2,240.


that is, very nearly thirteen and three-quarters British miles. Some of
these formations, which are represented in England by thin beds, are
thousands of feet in thickness on the Continent. Moreover, between each
successive formation we have, in the opinion of most geologists, blank
periods of enormous length. So that the lofty pile of sedimentary rocks
in Britain gives but an inadequate idea of the time which has elapsed
during their accumulation. The consideration of these various facts
impresses the mind almost in the same manner as does the vain endeavour
to grapple with the idea of eternity.

Nevertheless this impression is partly false. Mr. Croll, in an
interesting paper, remarks that we do not err “in forming too great a
conception of the length of geological periods,” but in estimating them
by years. When geologists look at large and complicated phenomena, and
then at the figures representing several million years, the two produce
a totally different effect on the mind, and the figures are at once
pronounced too small. In regard to subaërial denudation, Mr. Croll
shows, by calculating the known amount of sediment annually brought
down by certain rivers, relatively to their areas of drainage, that
1,000 feet of solid rock, as it became gradually disintegrated, would
thus be removed from the mean level of the whole area in the course of
six million years. This seems an astonishing result, and some
considerations lead to the suspicion that it may be too large, but if
halved or quartered it is still very surprising. Few of us, however,
know what a million really means: Mr. Croll gives the following
illustration: Take a narrow strip of paper, eighty-three feet four
inches in length, and stretch it along the wall of a large hall; then
mark off at one end the tenth of an inch. This tenth of an inch will
represent one hundred years, and the entire strip a million years. But
let it be borne in mind, in relation to the subject of this work, what
a hundred years implies, represented as it is by a measure utterly
insignificant in a hall of the above dimensions. Several eminent
breeders, during a single lifetime, have so largely modified some of
the higher animals, which propagate their kind much more slowly than
most of the lower animals, that they have formed what well deserves to
be called a new sub-breed. Few men have attended with due care to any
one strain for more than half a century, so that a hundred years
represents the work of two breeders in succession. It is not to be
supposed that species in a state of nature ever change so quickly as
domestic animals under the guidance of methodical selection. The
comparison would be in every way fairer with the effects which follow
from unconscious selection, that is, the preservation of the most
useful or beautiful animals, with no intention of modifying the breed;
but by this process of unconscious selection, various breeds have been
sensibly changed in the course of two or three centuries.

Species, however, probably change much more slowly, and within the same
country only a few change at the same time. This slowness follows from
all the inhabitants of the same country being already so well adapted
to each other, that new places in the polity of nature do not occur
until after long intervals, due to the occurrence of physical changes
of some kind, or through the immigration of new forms. Moreover,
variations or individual differences of the right nature, by which some
of the inhabitants might be better fitted to their new places under the
altered circumstance, would not always occur at once. Unfortunately we
have no means of determining, according to the standard of years, how
long a period it takes to modify a species; but to the subject of time
we must return.

_On the Poorness of Palæontological Collections._


Now let us turn to our richest museums, and what a paltry display we
behold! That our collections are imperfect is admitted by every one.
The remark of that admirable palæontologist, Edward Forbes, should
never be forgotten, namely, that very many fossil species are known and
named from single and often broken specimens, or from a few specimens
collected on some one spot. Only a small portion of the surface of the
earth has been geologically explored, and no part with sufficient care,
as the important discoveries made every year in Europe prove. No
organism wholly soft can be preserved. Shells and bones decay and
disappear when left on the bottom of the sea, where sediment is not
accumulating. We probably take a quite erroneous view, when we assume
that sediment is being deposited over nearly the whole bed of the sea,
at a rate sufficiently quick to embed and preserve fossil remains.
Throughout an enormously large proportion of the ocean, the bright blue
tint of the water bespeaks its purity. The many cases on record of a
formation conformably covered, after an immense interval of time, by
another and later formation, without the underlying bed having suffered
in the interval any wear and tear, seem explicable only on the view of
the bottom of the sea not rarely lying for ages in an unaltered
condition. The remains which do become embedded, if in sand or gravel,
will, when the beds are upraised, generally be dissolved by the
percolation of rain water charged with carbonic acid. Some of the many
kinds of animals which live on the beach between high and low water
mark seem to be rarely preserved. For instance, the several species of
the Chthamalinæ (a sub-family of sessile cirripedes) coat the rocks all
over the world in infinite numbers: they are all strictly littoral,
with the exception of a single Mediterranean species, which inhabits
deep water and this has been found fossil in Sicily, whereas not one
other species has hitherto been found in any tertiary formation: yet it
is known that the genus Chthamalus existed during the Chalk period.
Lastly, many great deposits, requiring a vast length of time for their
accumulation, are entirely destitute of organic remains, without our
being able to assign any reason: one of the most striking instances is
that of the Flysch formation, which consists of shale and sandstone,
several thousand, occasionally even six thousand feet in thickness, and
extending for at least 300 miles from Vienna to Switzerland; and
although this great mass has been most carefully searched, no fossils,
except a few vegetable remains, have been found.

With respect to the terrestrial productions which lived during the
Secondary and Palæozoic periods, it is superfluous to state that our
evidence is fragmentary in an extreme degree. For instance, until
recently not a land-shell was known belonging to either of these vast
periods, with the exception of one species discovered by Sir C. Lyell
and Dr. Dawson in the carboniferous strata of North America; but now
land-shells have been found in the lias. In regard to mammiferous
remains, a glance at the historical table published in Lyell’s Manual,
will bring home the truth, how accidental and rare is their
preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones of
tertiary mammals have been discovered either in caves or in lacustrine
deposits; and that not a cave or true lacustrine bed is known belonging
to the age of our secondary or palæozoic formations.

But the imperfection in the geological record largely results from
another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide
intervals of time. This doctrine has been emphatically admitted by many
geologists and palæontologists, who, like E. Forbes, entirely
disbelieve in the change of species. When we see the formations
tabulated in written works, or when we follow them in nature, it is
difficult to avoid believing that they are closely consecutive. But we
know, for instance, from Sir R. Murchison’s great work on Russia, what
wide gaps there are in that country between the superimposed
formations; so it is in North America, and in many other parts of the
world. The most skilful geologist, if his attention had been confined
exclusively to these large territories, would never have suspected that
during the periods which were blank and barren in his own country,
great piles of sediment, charged with new and peculiar forms of life,
had elsewhere been accumulated. And if, in every separate territory,
hardly any idea can be formed of the length of time which has elapsed
between the consecutive formations, we may infer that this could
nowhere be ascertained. The frequent and great changes in the
mineralogical composition of consecutive formations, generally implying
great changes in the geography of the surrounding lands, whence the
sediment was derived, accord with the belief of vast intervals of time
having elapsed between each formation.

We can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other
in close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short
geological period. Along the whole west coast, which is inhabited by a
peculiar marine fauna, tertiary beds are so poorly developed that no
record of several successive and peculiar marine faunas will probably
be preserved to a distant age. A little reflection will explain why,
along the rising coast of the western side of South America, no
extensive formations with recent or tertiary remains can anywhere be
found, though the supply of sediment must for ages have been great,
from the enormous degradation of the coast rocks and from the muddy
streams entering the sea. The explanation, no doubt, is that the
littoral and sub-littoral deposits are continually worn away, as soon
as they are brought up by the slow and gradual rising of the land
within the grinding action of the coast-waves.

We may, I think, conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during
subsequent oscillations of level, as well as the subsequent subaërial
degradation. Such thick and extensive accumulations of sediment may be
formed in two ways; either in profound depths of the sea, in which case
the bottom will not be inhabited by so many and such varied forms of
life as the more shallow seas; and the mass when upraised will give an
imperfect record of the organisms which existed in the neighbourhood
during the period of its accumulation. Or sediment may be deposited to
any thickness and extent over a shallow bottom, if it continue slowly
to subside. In this latter case, as long as the rate of subsidence and
supply of sediment nearly balance each other, the sea will remain
shallow and favourable for many and varied forms, and thus a rich
fossiliferous formation, thick enough, when upraised, to resist a large
amount of denudation, may be formed.

I am convinced that nearly all our ancient formations, which are
throughout the greater part of their thickness _rich in fossils_, have
thus been formed during subsidence. Since publishing my views on this
subject in 1845, I have watched the progress of geology, and have been
surprised to note how author after author, in treating of this or that
great formation, has come to the conclusion that it was accumulated
during subsidence. I may add, that the only ancient tertiary formation
on the west coast of South America, which has been bulky enough to
resist such degradation as it has as yet suffered, but which will
hardly last to a distant geological age, was deposited during a
downward oscillation of level, and thus gained considerable thickness.

All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently, formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, will
have been formed over wide spaces during periods of subsidence, but
only where the supply of sediment was sufficient to keep the sea
shallow and to embed and preserve the remains before they had time to
decay. On the other hand, as long as the bed of the sea remained
stationary, _thick_ deposits cannot have been accumulated in the
shallow parts, which are the most favourable to life. Still less can
this have happened during the alternate periods of elevation; or, to
speak more accurately, the beds which were then accumulated will
generally have been destroyed by being upraised and brought within the
limits of the coast-action.

These remarks apply chiefly to littoral and sublittoral deposits. In
the case of an extensive and shallow sea, such as that within a large
part of the Malay Archipelago, where the depth varies from thirty or
forty to sixty fathoms, a widely extended formation might be formed
during a period of elevation, and yet not suffer excessively from
denudation during its slow upheaval; but the thickness of the formation
could not be great, for owing to the elevatory movement it would be
less than the depth in which it was formed; nor would the deposit be
much consolidated, nor be capped by overlying formations, so that it
would run a good chance of being worn away by atmospheric degradation
and by the action of the sea during subsequent oscillations of level.
It has, however, been suggested by Mr. Hopkins, that if one part of the
area, after rising and before being denuded, subsided, the deposit
formed during the rising movement, though not thick, might afterwards
become protected by fresh accumulations, and thus be preserved for a
long period.

Mr. Hopkins also expresses his belief that sedimentary beds of
considerable horizontal extent have rarely been completely destroyed.
But all geologists, excepting the few who believe that our present
metamorphic schists and plutonic rocks once formed the primordial
nucleus of the globe, will admit that these latter rocks have been
stripped of their covering to an enormous extent. For it is scarcely
possible that such rocks could have been solidified and crystallised
while uncovered; but if the metamorphic action occurred at profound
depths of the ocean, the former protecting mantle of rock may not have
been very thick. Admitting then that gneiss, mica-schist, granite,
diorite, &c., were once necessarily covered up, how can we account for
the naked and extensive areas of such rocks in many parts of the world,
except on the belief that they have subsequently been completely
denuded of all overlying strata? That such extensive areas do exist
cannot be doubted: the granitic region of Parime is described by
Humboldt as being at least nineteen times as large as Switzerland.
South of the Amazon, Boue colours an area composed of rocks of this
nature as equal to that of Spain, France, Italy, part of Germany, and
the British Islands, all conjoined. This region has not been carefully
explored, but from the concurrent testimony of travellers, the granitic
area is very large: thus Von Eschwege gives a detailed section of these
rocks, stretching from Rio de Janeiro for 260 geographical miles inland
in a straight line; and I travelled for 150 miles in another direction,
and saw nothing but granitic rocks. Numerous specimens, collected along
the whole coast, from near Rio de Janeiro to the mouth of the Plata, a
distance of 1,100 geographical miles, were examined by me, and they all
belonged to this class. Inland, along the whole northern bank of the
Plata, I saw, besides modern tertiary beds, only one small patch of
slightly metamorphosed rock, which alone could have formed a part of
the original capping of the granitic series. Turning to a well-known
region, namely, to the United States and Canada, as shown in Professor
H.D. Rogers’ beautiful map, I have estimated the areas by cutting out
and weighing the paper, and I find that the metamorphic (excluding the
“semi-metamorphic”) and granite rocks exceed, in the proportion of 19
to 12.5, the whole of the newer Palæozoic formations. In many regions
the metamorphic and granite rocks would be found much more widely
extended than they appear to be, if all the sedimentary beds were
removed which rest unconformably on them, and which could not have
formed part of the original mantle under which they were crystallised.
Hence, it is probable that in some parts of the world whole formations
have been completely denuded, with not a wreck left behind.

One remark is here worth a passing notice. During periods of elevation
the area of the land and of the adjoining shoal parts of the sea will
be increased and new stations will often be formed—all circumstances
favourable, as previously explained, for the formation of new varieties
and species; but during such periods there will generally be a blank in
the geological record. On the other hand, during subsidence, the
inhabited area and number of inhabitants will decrease (excepting on
the shores of a continent when first broken up into an archipelago),
and consequently during subsidence, though there will be much
extinction, few new varieties or species will be formed; and it is
during these very periods of subsidence that the deposits which are
richest in fossils have been accumulated.

_On the Absence of Numerous Intermediate Varieties in any Single
Formation._


From these several considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if we
confine our attention to any one formation, it becomes much more
difficult to understand why we do not therein find closely graduated
varieties between the allied species which lived at its commencement
and at its close. Several cases are on record of the same species
presenting varieties in the upper and lower parts of the same
formation. Thus Trautschold gives a number of instances with Ammonites,
and Hilgendorf has described a most curious case of ten graduated forms
of Planorbis multiformis in the successive beds of a fresh-water
formation in Switzerland. Although each formation has indisputably
required a vast number of years for its deposition, several reasons can
be given why each should not commonly include a graduated series of
links between the species which lived at its commencement and close,
but I cannot assign due proportional weight to the following
considerations.

Although each formation may mark a very long lapse of years, each
probably is short compared with the period requisite to change one
species into another. I am aware that two palæontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us from coming to
any just conclusion on this head. When we see a species first appearing
in the middle of any formation, it would be rash in the extreme to
infer that it had not elsewhere previously existed. So again, when we
find a species disappearing before the last layers have been deposited,
it would be equally rash to suppose that it then became extinct. We
forget how small the area of Europe is compared with the rest of the
world; nor have the several stages of the same formation throughout
Europe been correlated with perfect accuracy.

We may safely infer that with marine animals of all kinds there has
been a large amount of migration due to climatal and other changes; and
when we see a species first appearing in any formation, the probability
is that it only then first immigrated into that area. It is well known,
for instance, that several species appear somewhat earlier in the
palæozoic beds of North America than in those of Europe; time having
apparently been required for their migration from the American to the
European seas. In examining the latest deposits, in various quarters of
the world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now abundant
in the neighbouring sea, but are rare or absent in this particular
deposit. It is an excellent lesson to reflect on the ascertained amount
of migration of the inhabitants of Europe during the glacial epoch,
which forms only a part of one whole geological period; and likewise to
reflect on the changes of level, on the extreme change of climate, and
on the great lapse of time, all included within this same glacial
period. Yet it may be doubted whether, in any quarter of the world,
sedimentary deposits, _including fossil remains_, have gone on
accumulating within the same area during the whole of this period. It
is not, for instance, probable that sediment was deposited during the
whole of the glacial period near the mouth of the Mississippi, within
that limit of depth at which marine animals can best flourish: for we
know that great geographical changes occurred in other parts of America
during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the glacial
period shall have been upraised, organic remains will probably first
appear and disappear at different levels, owing to the migrations of
species and to geographical changes. And in the distant future, a
geologist, examining these beds, would be tempted to conclude that the
average duration of life of the embedded fossils had been less than
that of the glacial period, instead of having been really far greater,
that is, extending from before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
continuously accumulating during a long period, sufficient for the slow
process of modification; hence, the deposit must be a very thick one;
and the species undergoing change must have lived in the same district
throughout the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate only
during a period of subsidence; and to keep the depth approximately the
same, which is necessary that the same marine species may live on the
same space, the supply of sediment must nearly counterbalance the
amount of subsidence. But this same movement of subsidence will tend to
submerge the area whence the sediment is derived, and thus diminish the
supply, whilst the downward movement continues. In fact, this nearly
exact balancing between the supply of sediment and the amount of
subsidence is probably a rare contingency; for it has been observed by
more than one palæontologist that very thick deposits are usually
barren of organic remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of widely different mineralogical composition, we may
reasonably suspect that the process of deposition has been more or less
interrupted. Nor will the closest inspection of a formation give us any
idea of the length of time which its deposition may have consumed. Many
instances could be given of beds, only a few feet in thickness,
representing formations which are elsewhere thousands of feet in
thickness, and which must have required an enormous period for their
accumulation; yet no one ignorant of this fact would have even
suspected the vast lapse of time represented by the thinner formation.
Many cases could be given of the lower beds of a formation having been
upraised, denuded, submerged, and then re-covered by the upper beds of
the same formation—facts, showing what wide, yet easily overlooked,
intervals have occurred in its accumulation. In other cases we have the
plainest evidence in great fossilised trees, still standing upright as
they grew, of many long intervals of time and changes of level during
the process of deposition, which would not have been suspected, had not
the trees been preserved: thus Sir C. Lyell and Dr. Dawson found
carboniferous beds 1,400 feet thick in Nova Scotia, with ancient
root-bearing strata, one above the other, at no less than sixty-eight
different levels. Hence, when the same species occurs at the bottom,
middle, and top of a formation, the probability is that it has not
lived on the same spot during the whole period of deposition, but has
disappeared and reappeared, perhaps many times, during the same
geological period. Consequently if it were to undergo a considerable
amount of modification during the deposition of any one geological
formation, a section would not include all the fine intermediate
gradations which must on our theory have existed, but abrupt, though
perhaps slight, changes of form.

It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by the closest
intermediate gradations; and this, from the reasons just assigned, we
can seldom hope to effect in any one geological section. Supposing B
and C to be two species, and a third, A, to be found in an older and
underlying bed; even if A were strictly intermediate between B and C,
it would simply be ranked as a third and distinct species, unless at
the same time it could be closely connected by intermediate varieties
with either one or both forms. Nor should it be forgotten, as before
explained, that A might be the actual progenitor of B and C, and yet
would not necessarily be strictly intermediate between them in all
respects. So that we might obtain the parent-species and its several
modified descendants from the lower and upper beds of the same
formation, and unless we obtained numerous transitional gradations, we
should not recognise their blood-relationship, and should consequently
rank them as distinct species.

It is notorious on what excessively slight differences many
palæontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D’Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on the
theory we ought to find. Look again at the later tertiary deposits,
which include many shells believed by the majority of naturalists to be
identical with existing species; but some excellent naturalists, as
Agassiz and Pictet, maintain that all these tertiary species are
specifically distinct, though the distinction is admitted to be very
slight; so that here, unless we believe that these eminent naturalists
have been misled by their imaginations, and that these late tertiary
species really present no difference whatever from their living
representatives, or unless we admit, in opposition to the judgment of
most naturalists, that these tertiary species are all truly distinct
from the recent, we have evidence of the frequent occurrence of slight
modifications of the kind required. If we look to rather wider
intervals of time, namely, to distinct but consecutive stages of the
same great formation, we find that the embedded fossils, though
universally ranked as specifically different, yet are far more closely
related to each other than are the species found in more widely
separated formations; so that here again we have undoubted evidence of
change in the direction required by the theory; but to this latter
subject I shall return in the following chapter.

With animals and plants that propagate rapidly and do not wander much,
there is reason to suspect, as we have formerly seen, that their
varieties are generally at first local; and that such local varieties
do not spread widely and supplant their parent-form until they have
been modified and perfected in some considerable degree. According to
this view, the chance of discovering in a formation in any one country
all the early stages of transition between any two forms, is small, for
the successive changes are supposed to have been local or confined to
some one spot. Most marine animals have a wide range; and we have seen
that with plants it is those which have the widest range, that oftenest
present varieties, so that, with shells and other marine animals, it is
probable that those which had the widest range, far exceeding the
limits of the known geological formations in Europe, have oftenest
given rise, first to local varieties and ultimately to new species; and
this again would greatly lessen the chance of our being able to trace
the stages of transition in any one geological formation.

It is a more important consideration, leading to the same result, as
lately insisted on by Dr. Falconer, namely, that the period during
which each species underwent modification, though long as measured by
years, was probably short in comparison with that during which it
remained without undergoing any change.

It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties, and thus proved to be the same species, until
many specimens are collected from many places; and with fossil species
this can rarely be done. We shall, perhaps, best perceive the
improbability of our being enabled to connect species by numerous,
fine, intermediate, fossil links, by asking ourselves whether, for
instance, geologists at some future period will be able to prove that
our different breeds of cattle, sheep, horses, and dogs are descended
from a single stock or from several aboriginal stocks; or, again,
whether certain sea-shells inhabiting the shores of North America,
which are ranked by some conchologists as distinct species from their
European representatives, and by other conchologists as only varieties,
are really varieties, or are, as it is called, specifically distinct.
This could be effected by the future geologist only by his discovering
in a fossil state numerous intermediate gradations; and such success is
improbable in the highest degree.

It has been asserted over and over again, by writers who believe in the
immutability of species, that geology yields no linking forms. This
assertion, as we shall see in the next chapter, is certainly erroneous.
As Sir J. Lubbock has remarked, “Every species is a link between other
allied forms.” If we take a genus having a score of species, recent and
extinct, and destroy four-fifths of them, no one doubts that the
remainder will stand much more distinct from each other. If the extreme
forms in the genus happen to have been thus destroyed, the genus itself
will stand more distinct from other allied genera. What geological
research has not revealed, is the former existence of infinitely
numerous gradations, as fine as existing varieties, connecting together
nearly all existing and extinct species. But this ought not to be
expected; yet this has been repeatedly advanced as a most serious
objection against my views.

It may be worth while to sum up the foregoing remarks on the causes of
the imperfection of the geological record under an imaginary
illustration. The Malay Archipelago is about the size of Europe from
the North Cape to the Mediterranean, and from Britain to Russia; and
therefore equals all the geological formations which have been examined
with any accuracy, excepting those of the United States of America. I
fully agree with Mr. Godwin-Austen, that the present condition of the
Malay Archipelago, with its numerous large islands separated by wide
and shallow seas, probably represents the former state of Europe, while
most of our formations were accumulating. The Malay Archipelago is one
of the richest regions in organic beings; yet if all the species were
to be collected which have ever lived there, how imperfectly would they
represent the natural history of the world!

But we have every reason to believe that the terrestrial productions of
the archipelago would be preserved in an extremely imperfect manner in
the formations which we suppose to be there accumulating. Not many of
the strictly littoral animals, or of those which lived on naked
submarine rocks, would be embedded; and those embedded in gravel or
sand would not endure to a distant epoch. Wherever sediment did not
accumulate on the bed of the sea, or where it did not accumulate at a
sufficient rate to protect organic bodies from decay, no remains could
be preserved.

Formations rich in fossils of many kinds, and of thickness sufficient
to last to an age as distant in futurity as the secondary formations
lie in the past, would generally be formed in the archipelago only
during periods of subsidence. These periods of subsidence would be
separated from each other by immense intervals of time, during which
the area would be either stationary or rising; whilst rising, the
fossiliferous formations on the steeper shores would be destroyed,
almost as soon as accumulated, by the incessant coast-action, as we now
see on the shores of South America. Even throughout the extensive and
shallow seas within the archipelago, sedimentary beds could hardly be
accumulated of great thickness during the periods of elevation, or
become capped and protected by subsequent deposits, so as to have a
good chance of enduring to a very distant future. During the periods of
subsidence, there would probably be much extinction of life; during the
periods of elevation, there would be much variation, but the geological
record would then be less perfect.

It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would _exceed_ the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not all fully
preserved, transitional varieties would merely appear as so many new,
though closely allied species. It is also probable that each great
period of subsidence would be interrupted by oscillations of level, and
that slight climatical changes would intervene during such lengthy
periods; and in these cases the inhabitants of the archipelago would
migrate, and no closely consecutive record of their modifications could
be preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy plainly leads to
the belief that it would be chiefly these far-ranging species, though
only some of them, which would oftenest produce new varieties; and the
varieties would at first be local or confined to one place, but if
possessed of any decided advantage, or when further modified and
improved, they would slowly spread and supplant their parent-forms.
When such varieties returned to their ancient homes, as they would
differ from their former state in a nearly uniform, though perhaps
extremely slight degree, and as they would be found embedded in
slightly different sub-stages of the same formation, they would,
according to the principles followed by many palæontologists, be ranked
as new and distinct species.

If then there be some degree of truth in these remarks, we have no
right to expect to find, in our geological formations, an infinite
number of those fine transitional forms, which, on our theory, have
connected all the past and present species of the same group into one
long and branching chain of life. We ought only to look for a few
links, and such assuredly we do find—some more distantly, some more
closely, related to each other; and these links, let them be ever so
close, if found in different stages of the same formation, would, by
many palæontologists, be ranked as distinct species. But I do not
pretend that I should ever have suspected how poor was the record in
the best preserved geological sections, had not the absence of
innumerable transitional links between the species which lived at the
commencement and close of each formation, pressed so hardly on my
theory.

_On the sudden Appearance of whole Groups of allied Species._


The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palæontologists—for
instance, by Agassiz, Pictet, and Sedgwick, as a fatal objection to the
belief in the transmutation of species. If numerous species, belonging
to the same genera or families, have really started into life at once,
the fact would be fatal to the theory of evolution through natural
selection. For the development by this means of a group of forms, all
of which are descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have lived long before
their modified descendants. But we continually overrate the perfection
of the geological record, and falsely infer, because certain genera or
families have not been found beneath a certain stage, that they did not
exist before that stage. In all cases positive palæontological evidence
may be implicitly trusted; negative evidence is worthless, as
experience has so often shown. We continually forget how large the
world is, compared with the area over which our geological formations
have been carefully examined; we forget that groups of species may
elsewhere have long existed, and have slowly multiplied, before they
invaded the ancient archipelagoes of Europe and the United States. We
do not make due allowance for the enormous intervals of time which have
elapsed between our consecutive formations, longer perhaps in many
cases than the time required for the accumulation of each formation.
These intervals will have given time for the multiplication of species
from some one parent-form: and in the succeeding formation, such groups
or species will appear as if suddenly created.

I may here recall a remark formerly made, namely, that it might require
a long succession of ages to adapt an organism to some new and peculiar
line of life, for instance, to fly through the air; and consequently
that the transitional forms would often long remain confined to some
one region; but that, when this adaptation had once been effected, and
a few species had thus acquired a great advantage over other organisms,
a comparatively short time would be necessary to produce many divergent
forms, which would spread rapidly and widely throughout the world.
Professor Pictet, in his excellent Review of this work, in commenting
on early transitional forms, and taking birds as an illustration,
cannot see how the successive modifications of the anterior limbs of a
supposed prototype could possibly have been of any advantage. But look
at the penguins of the Southern Ocean; have not these birds their front
limbs in this precise intermediate state of “neither true arms nor true
wings?” Yet these birds hold their place victoriously in the battle for
life; for they exist in infinite numbers and of many kinds. I do not
suppose that we here see the real transitional grades through which the
wings of birds have passed; but what special difficulty is there in
believing that it might profit the modified descendants of the penguin,
first to become enabled to flap along the surface of the sea like the
logger-headed duck, and ultimately to rise from its surface and glide
through the air?

I will now give a few examples to illustrate the foregoing remarks, and
to show how liable we are to error in supposing that whole groups of
species have suddenly been produced. Even in so short an interval as
that between the first and second editions of Pictet’s great work on
Palæontology, published in 1844-46 and in 1853-57, the conclusions on
the first appearance and disappearance of several groups of animals
have been considerably modified; and a third edition would require
still further changes. I may recall the well-known fact that in
geological treatises, published not many years ago, mammals were always
spoken of as having abruptly come in at the commencement of the
tertiary series. And now one of the richest known accumulations of
fossil mammals belongs to the middle of the secondary series; and true
mammals have been discovered in the new red sandstone at nearly the
commencement of this great series. Cuvier used to urge that no monkey
occurred in any tertiary stratum; but now extinct species have been
discovered in India, South America and in Europe, as far back as the
miocene stage. Had it not been for the rare accident of the
preservation of footsteps in the new red sandstone of the United
States, who would have ventured to suppose that no less than at least
thirty different bird-like animals, some of gigantic size, existed
during that period? Not a fragment of bone has been discovered in these
beds. Not long ago, palæontologists maintained that the whole class of
birds came suddenly into existence during the eocene period; but now we
know, on the authority of Professor Owen, that a bird certainly lived
during the deposition of the upper greensand; and still more recently,
that strange bird, the Archeopteryx, with a long lizard-like tail,
bearing a pair of feathers on each joint, and with its wings furnished
with two free claws, has been discovered in the oolitic slates of
Solenhofen. Hardly any recent discovery shows more forcibly than this
how little we as yet know of the former inhabitants of the world.

I may give another instance, which, from having passed under my own
eyes has much struck me. In a memoir on Fossil Sessile Cirripedes, I
stated that, from the large number of existing and extinct tertiary
species; from the extraordinary abundance of the individuals of many
species all over the world, from the Arctic regions to the equator,
inhabiting various zones of depths, from the upper tidal limits to
fifty fathoms; from the perfect manner in which specimens are preserved
in the oldest tertiary beds; from the ease with which even a fragment
of a valve can be recognised; from all these circumstances, I inferred
that, had sessile cirripedes existed during the secondary periods, they
would certainly have been preserved and discovered; and as not one
species had then been discovered in beds of this age, I concluded that
this great group had been suddenly developed at the commencement of the
tertiary series. This was a sore trouble to me, adding, as I then
thought, one more instance of the abrupt appearance of a great group of
species. But my work had hardly been published, when a skilful
palæontologist, M. Bosquet, sent me a drawing of a perfect specimen of
an unmistakable sessile cirripede, which he had himself extracted from
the chalk of Belgium. And, as if to make the case as striking as
possible, this cirripede was a Chthamalus, a very common, large, and
ubiquitous genus, of which not one species has as yet been found even
in any tertiary stratum. Still more recently, a Pyrgoma, a member of a
distinct subfamily of sessile cirripedes, has been discovered by Mr.
Woodward in the upper chalk; so that we now have abundant evidence of
the existence of this group of animals during the secondary period.

The case most frequently insisted on by palæontologists of the
apparently sudden appearance of a whole group of species, is that of
the teleostean fishes, low down, according to Agassiz, in the Chalk
period. This group includes the large majority of existing species. But
certain Jurassic and Triassic forms are now commonly admitted to be
teleostean; and even some palæozoic forms have thus been classed by one
high authority. If the teleosteans had really appeared suddenly in the
northern hemisphere at the commencement of the chalk formation, the
fact would have been highly remarkable; but it would not have formed an
insuperable difficulty, unless it could likewise have been shown that
at the same period the species were suddenly and simultaneously
developed in other quarters of the world. It is almost superfluous to
remark that hardly any fossil-fish are known from south of the equator;
and by running through Pictet’s Palæontology it will be seen that very
few species are known from several formations in Europe. Some few
families of fish now have a confined range; the teleostean fishes might
formerly have had a similarly confined range, and after having been
largely developed in some one sea, have spread widely. Nor have we any
right to suppose that the seas of the world have always been so freely
open from south to north as they are at present. Even at this day, if
the Malay Archipelago were converted into land, the tropical parts of
the Indian Ocean would form a large and perfectly enclosed basin, in
which any great group of marine animals might be multiplied; and here
they would remain confined, until some of the species became adapted to
a cooler climate, and were enabled to double the southern capes of
Africa or Australia, and thus reach other and distant seas.

From these considerations, from our ignorance of the geology of other
countries beyond the confines of Europe and the United States, and from
the revolution in our palæontological knowledge effected by the
discoveries of the last dozen years, it seems to me to be about as rash
to dogmatize on the succession of organic forms throughout the world,
as it would be for a naturalist to land for five minutes on a barren
point in Australia, and then to discuss the number and range of its
productions.

_On the sudden Appearance of Groups of allied Species in the lowest
known Fossiliferous Strata._


There is another and allied difficulty, which is much more serious. I
allude to the manner in which species belonging to several of the main
divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group are descended from a single
progenitor, apply with equal force to the earliest known species. For
instance, it cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which must have
lived long before the Cambrian age, and which probably differed greatly
from any known animal. Some of the most ancient animals, as the
Nautilus, Lingula, &c., do not differ much from living species; and it
cannot on our theory be supposed, that these old species were the
progenitors of all the species belonging to the same groups which have
subsequently appeared, for they are not in any degree intermediate in
character.

Consequently, if the theory be true, it is indisputable that before the
lowest Cambrian stratum was deposited long periods elapsed, as long as,
or probably far longer than, the whole interval from the Cambrian age
to the present day; and that during these vast periods the world
swarmed with living creatures. Here we encounter a formidable
objection; for it seems doubtful whether the earth, in a fit state for
the habitation of living creatures, has lasted long enough. Sir W.
Thompson concludes that the consolidation of the crust can hardly have
occurred less than twenty or more than four hundred million years ago,
but probably not less than ninety-eight or more than two hundred
million years. These very wide limits show how doubtful the data are;
and other elements may have hereafter to be introduced into the
problem. Mr. Croll estimates that about sixty million years have
elapsed since the Cambrian period, but this, judging from the small
amount of organic change since the commencement of the Glacial epoch,
appears a very short time for the many and great mutations of life,
which have certainly occurred since the Cambrian formation; and the
previous one hundred and forty million years can hardly be considered
as sufficient for the development of the varied forms of life which
already existed during the Cambrian period. It is, however, probable,
as Sir William Thompson insists, that the world at a very early period
was subjected to more rapid and violent changes in its physical
conditions than those now occurring; and such changes would have tended
to induce changes at a corresponding rate in the organisms which then
existed.

To the question why we do not find rich fossiliferous deposits
belonging to these assumed earliest periods prior to the Cambrian
system, I can give no satisfactory answer. Several eminent geologists,
with Sir R. Murchison at their head, were until recently convinced that
we beheld in the organic remains of the lowest Silurian stratum the
first dawn of life. Other highly competent judges, as Lyell and E.
Forbes, have disputed this conclusion. We should not forget that only a
small portion of the world is known with accuracy. Not very long ago M.
Barrande added another and lower stage, abounding with new and peculiar
species, beneath the then known Silurian system; and now, still lower
down in the Lower Cambrian formation, Mr Hicks has found South Wales
beds rich in trilobites, and containing various molluscs and annelids.
The presence of phosphatic nodules and bituminous matter, even in some
of the lowest azotic rocks, probably indicates life at these periods;
and the existence of the Eozoon in the Laurentian formation of Canada
is generally admitted. There are three great series of strata beneath
the Silurian system in Canada, in the lowest of which the Eozoon is
found. Sir W. Logan states that their “united thickness may possibly
far surpass that of all the succeeding rocks, from the base of the
palæozoic series to the present time. We are thus carried back to a
period so remote, that the appearance of the so-called primordial fauna
(of Barrande) may by some be considered as a comparatively modern
event.” The Eozoon belongs to the most lowly organised of all classes
of animals, but is highly organised for its class; it existed in
countless numbers, and, as Dr. Dawson has remarked, certainly preyed on
other minute organic beings, which must have lived in great numbers.
Thus the words, which I wrote in 1859, about the existence of living
beings long before the Cambrian period, and which are almost the same
with those since used by Sir W. Logan, have proved true. Nevertheless,
the difficulty of assigning any good reason for the absence of vast
piles of strata rich in fossils beneath the Cambrian system is very
great. It does not seem probable that the most ancient beds have been
quite worn away by denudation, or that their fossils have been wholly
obliterated by metamorphic action, for if this had been the case we
should have found only small remnants of the formations next succeeding
them in age, and these would always have existed in a partially
metamorphosed condition. But the descriptions which we possess of the
Silurian deposits over immense territories in Russia and in North
America, do not support the view that the older a formation is the more
invariably it has suffered extreme denudation and metamorphism.

The case at present must remain inexplicable; and may be truly urged as
a valid argument against the views here entertained. To show that it
may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains which do not appear
to have inhabited profound depths, in the several formations of Europe
and of the United States; and from the amount of sediment, miles in
thickness, of which the formations are composed, we may infer that from
first to last large islands or tracts of land, whence the sediment was
derived, occurred in the neighbourhood of the now existing continents
of Europe and North America. This same view has since been maintained
by Agassiz and others. But we do not know what was the state of things
in the intervals between the several successive formations; whether
Europe and the United States during these intervals existed as dry
land, or as a submarine surface near land, on which sediment was not
deposited, or as the bed of an open and unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but hardly one truly
oceanic island (with the exception of New Zealand, if this can be
called a truly oceanic island) is as yet known to afford even a remnant
of any palæozoic or secondary formation. Hence, we may perhaps infer,
that during the palæozoic and secondary periods, neither continents nor
continental islands existed where our oceans now extend; for had they
existed, palæozoic and secondary formations would in all probability
have been accumulated from sediment derived from their wear and tear;
and would have been at least partially upheaved by the oscillations of
level, which must have intervened during these enormously long periods.
If, then, we may infer anything from these facts, we may infer that,
where our oceans now extend, oceans have extended from the remotest
period of which we have any record; and on the other hand, that where
continents now exist, large tracts of land have existed, subjected, no
doubt, to great oscillations of level, since the Cambrian period. The
coloured map appended to my volume on Coral Reefs, led me to conclude
that the great oceans are still mainly areas of subsidence, the great
archipelagoes still areas of oscillations of level, and the continents
areas of elevation. But we have no reason to assume that things have
thus remained from the beginning of the world. Our continents seem to
have been formed by a preponderance, during many oscillations of level,
of the force of elevation. But may not the areas of preponderant
movement have changed in the lapse of ages? At a period long antecedent
to the Cambrian epoch, continents may have existed where oceans are now
spread out, and clear and open oceans may have existed where our
continents now stand. Nor should we be justified in assuming that if,
for instance, the bed of the Pacific Ocean were now converted into a
continent we should there find sedimentary formations, in recognisable
condition, older than the Cambrian strata, supposing such to have been
formerly deposited; for it might well happen that strata which had
subsided some miles nearer to the centre of the earth, and which had
been pressed on by an enormous weight of superincumbent water, might
have undergone far more metamorphic action than strata which have
always remained nearer to the surface. The immense areas in some parts
of the world, for instance in South America, of naked metamorphic
rocks, which must have been heated under great pressure, have always
seemed to me to require some special explanation; and we may perhaps
believe that we see in these large areas the many formations long
anterior to the Cambrian epoch in a completely metamorphosed and
denuded condition.

The several difficulties here discussed, namely, that, though we find
in our geological formations many links between the species which now
exist and which formerly existed, we do not find infinitely numerous
fine transitional forms closely joining them all together. The sudden
manner in which several groups of species first appear in our European
formations, the almost entire absence, as at present known, of
formations rich in fossils beneath the Cambrian strata, are all
undoubtedly of the most serious nature. We see this in the fact that
the most eminent palæontologists, namely, Cuvier, Agassiz, Barrande,
Pictet, Falconer, E. Forbes, &c., and all our greatest geologists, as
Lyell, Murchison, Sedgwick, &c., have unanimously, often vehemently,
maintained the immutability of species. But Sir Charles Lyell now gives
the support of his high authority to the opposite side, and most
geologists and palæontologists are much shaken in their former belief.
Those who believe that the geological record is in any degree perfect,
will undoubtedly at once reject my theory. For my part, following out
Lyell’s metaphor, I look at the geological record as a history of the
world imperfectly kept and written in a changing dialect. Of this
history we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has been
preserved, and of each page, only here and there a few lines. Each word
of the slowly-changing language, more or less different in the
successive chapters, may represent the forms of life, which are
entombed in our consecutive formations, and which falsely appear to
have been abruptly introduced. On this view the difficulties above
discussed are greatly diminished or even disappear.




CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.


On the slow and successive appearance of new species—On their different
rates of change—Species once lost do not reappear—Groups of species
follow the same general rules in their appearance and disappearance as
do single species—On extinction—On simultaneous changes in the forms of
life throughout the world—On the affinities of extinct species to each
other and to living species—On the state of development of ancient
forms—On the succession of the same types within the same areas—Summary
of preceding and present chapters.


Let us now see whether the several facts and laws relating to the
geological succession of organic beings accord best with the common
view of the immutability of species, or with that of their slow and
gradual modification, through variation and natural selection.

New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between the stages,
and to make the proportion between the lost and existing forms more
gradual. In some of the most recent beds, though undoubtedly of high
antiquity if measured by years, only one or two species are extinct,
and only one or two are new, having appeared there for the first time,
either locally, or, as far as we know, on the face of the earth. The
secondary formations are more broken; but, as Bronn has remarked,
neither the appearance nor disappearance of the many species embedded
in each formation has been simultaneous.

Species belonging to different genera and classes have not changed at
the same rate, or in the same degree. In the older tertiary beds a few
living shells may still be found in the midst of a multitude of extinct
forms. Falconer has given a striking instance of a similar fact, for an
existing crocodile is associated with many lost mammals and reptiles in
the sub-Himalayan deposits. The Silurian Lingula differs but little
from the living species of this genus; whereas most of the other
Silurian Molluscs and all the Crustaceans have changed greatly. The
productions of the land seem to have changed at a quicker rate than
those of the sea, of which a striking instance has been observed in
Switzerland. There is some reason to believe that organisms high in the
scale, change more quickly than those that are low: though there are
exceptions to this rule. The amount of organic change, as Pictet has
remarked, is not the same in each successive so-called formation. Yet
if we compare any but the most closely related formations, all the
species will be found to have undergone some change. When a species has
once disappeared from the face of the earth, we have no reason to
believe that the same identical form ever reappears. The strongest
apparent exception to this latter rule is that of the so-called
“colonies” of M. Barrande, which intrude for a period in the midst of
an older formation, and then allow the pre-existing fauna to reappear;
but Lyell’s explanation, namely, that it is a case of temporary
migration from a distinct geographical province, seems satisfactory.

These several facts accord well with our theory, which includes no
fixed law of development, causing all the inhabitants of an area to
change abruptly, or simultaneously, or to an equal degree. The process
of modification must be slow, and will generally affect only a few
species at the same time; for the variability of each species is
independent of that of all others. Whether such variations or
individual differences as may arise will be accumulated through natural
selection in a greater or less degree, thus causing a greater or less
amount of permanent modification, will depend on many complex
contingencies—on the variations being of a beneficial nature, on the
freedom of intercrossing, on the slowly changing physical conditions of
the country, on the immigration of new colonists, and on the nature of
the other inhabitants with which the varying species come into
competition. Hence it is by no means surprising that one species should
retain the same identical form much longer than others; or, if
changing, should change in a less degree. We find similar relations
between the existing inhabitants of distinct countries; for instance,
the land-shells and coleopterous insects of Madeira have come to differ
considerably from their nearest allies on the continent of Europe,
whereas the marine shells and birds have remained unaltered. We can
perhaps understand the apparently quicker rate of change in terrestrial
and in more highly organised productions compared with marine and lower
productions, by the more complex relations of the higher beings to
their organic and inorganic conditions of life, as explained in a
former chapter. When many of the inhabitants of any area have become
modified and improved, we can understand, on the principle of
competition, and from the all-important relations of organism to
organism in the struggle for life, that any form which did not become
in some degree modified and improved, would be liable to extermination.
Hence, we see why all the species in the same region do at last, if we
look to long enough intervals of time, become modified; for otherwise
they would become extinct.

In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations
have been almost necessarily accumulated at wide and irregularly
intermittent intervals of time; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is
not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at
hazard, in an ever slowly changing drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances) to
fill the place of another species in the economy of nature, and thus
supplant it; yet the two forms—the old and the new—would not be
identically the same; for both would almost certainly inherit different
characters from their distinct progenitors; and organisms already
differing would vary in a different manner. For instance, it is
possible, if all our fantail-pigeons were destroyed, that fanciers
might make a new breed hardly distinguishable from the present breed;
but if the parent rock-pigeon were likewise destroyed, and under nature
we have every reason to believe that parent forms are generally
supplanted and exterminated by their improved offspring, it is
incredible that a fantail, identical with the existing breed, could be
raised from any other species of pigeon, or even from any other well
established race of the domestic pigeon, for the successive variations
would almost certainly be in some degree different, and the
newly-formed variety would probably inherit from its progenitor some
characteristic differences.

Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group, when it has once disappeared, never reappears; that
is, its existence, as long as it lasts, is continuous. I am aware that
there are some apparent exceptions to this rule, but the exceptions are
surprisingly few, so few that E. Forbes, Pictet, and Woodward (though
all strongly opposed to such views as I maintain) admit its truth; and
the rule strictly accords with the theory. For all the species of the
same group, however long it may have lasted, are the modified
descendants one from the other, and all from a common progenitor. In
the genus Lingula, for instance, the species which have successively
appeared at all ages must have been connected by an unbroken series of
generations, from the lowest Silurian stratum to the present day.

We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted to
give an explanation of this fact, which if true would be fatal to my
views. But such cases are certainly exceptional; the general rule being
a gradual increase in number, until the group reaches its maximum, and
then, sooner or later, a gradual decrease. If the number of the species
included within a genus, or the number of the genera within a family,
be represented by a vertical line of varying thickness, ascending
through the successive geological formations, in which the species are
found, the line will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly; it then gradually thickens
upwards, often keeping of equal thickness for a space, and ultimately
thins out in the upper beds, marking the decrease and final extinction
of the species. This gradual increase in number of the species of a
group is strictly conformable with the theory; for the species of the
same genus, and the genera of the same family, can increase only slowly
and progressively; the process of modification and the production of a
number of allied forms necessarily being a slow and gradual process,
one species first giving rise to two or three varieties, these being
slowly converted into species, which in their turn produce by equally
slow steps other varieties and species, and so on, like the branching
of a great tree from a single stem, till the group becomes large.

_On Extinction._


We have as yet only spoken incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the
extinction of old forms and the production of new and improved forms
are intimately connected together. The old notion of all the
inhabitants of the earth having been swept away by catastrophes at
successive periods is very generally given up, even by those
geologists, as Elie de Beaumont, Murchison, Barrande, &c., whose
general views would naturally lead them to this conclusion. On the
contrary, we have every reason to believe, from the study of the
tertiary formations, that species and groups of species gradually
disappear, one after another, first from one spot, then from another,
and finally from the world. In some few cases, however, as by the
breaking of an isthmus and the consequent irruption of a multitude of
new inhabitants into an adjoining sea, or by the final subsidence of an
island, the process of extinction may have been rapid. Both single
species and whole groups of species last for very unequal periods; some
groups, as we have seen, have endured from the earliest known dawn of
life to the present day; some have disappeared before the close of the
palæozoic period. No fixed law seems to determine the length of time
during which any single species or any single genus endures. There is
reason to believe that the extinction of a whole group of species is
generally a slower process than their production: if their appearance
and disappearance be represented, as before, by a vertical line of
varying thickness the line is found to taper more gradually at its
upper end, which marks the progress of extermination, than at its lower
end, which marks the first appearance and the early increase in number
of the species. In some cases, however, the extermination of whole
groups, as of ammonites, towards the close of the secondary period, has
been wonderfully sudden.

The extinction of species has been involved in the most gratuitous
mystery. Some authors have even supposed that, as the individual has a
definite length of life, so have species a definite duration. No one
can have marvelled more than I have done at the extinction of species.
When I found in La Plata the tooth of a horse embedded with the remains
of Mastodon, Megatherium, Toxodon and other extinct monsters, which all
co-existed with still living shells at a very late geological period, I
was filled with astonishment; for, seeing that the horse, since its
introduction by the Spaniards into South America, has run wild over the
whole country and has increased in numbers at an unparalleled rate, I
asked myself what could so recently have exterminated the former horse
under conditions of life apparently so favourable. But my astonishment
was groundless. Professor Owen soon perceived that the tooth, though so
like that of the existing horse, belonged to an extinct species. Had
this horse been still living, but in some degree rare, no naturalist
would have felt the least surprise at its rarity; for rarity is the
attribute of a vast number of species of all classes, in all countries.
If we ask ourselves why this or that species is rare, we answer that
something is unfavourable in its conditions of life; but what that
something is, we can hardly ever tell. On the supposition of the fossil
horse still existing as a rare species, we might have felt certain,
from the analogy of all other mammals, even of the slow-breeding
elephant, and from the history of the naturalisation of the domestic
horse in South America, that under more favourable conditions it would
in a very few years have stocked the whole continent. But we could not
have told what the unfavourable conditions were which checked its
increase, whether some one or several contingencies, and at what period
of the horse’s life, and in what degree they severally acted. If the
conditions had gone on, however slowly, becoming less and less
favourable, we assuredly should not have perceived the fact, yet the
fossil horse would certainly have become rarer and rarer, and finally
extinct—its place being seized on by some more successful competitor.

It is most difficult always to remember that the increase of every
living creature is constantly being checked by unperceived hostile
agencies; and that these same unperceived agencies are amply sufficient
to cause rarity, and finally extinction. So little is this subject
understood, that I have heard surprise repeatedly expressed at such
great monsters as the Mastodon and the more ancient Dinosaurians having
become extinct; as if mere bodily strength gave victory in the battle
of life. Mere size, on the contrary, would in some cases determine, as
has been remarked by Owen, quicker extermination, from the greater
amount of requisite food. Before man inhabited India or Africa, some
cause must have checked the continued increase of the existing
elephant. A highly capable judge, Dr. Falconer, believes that it is
chiefly insects which, from incessantly harassing and weakening the
elephant in India, check its increase; and this was Bruce’s conclusion
with respect to the African elephant in Abyssinia. It is certain that
insects and blood-sucking bats determine the existence of the larger
naturalised quadrupeds in several parts of South America.

We see in many cases in the more recent tertiary formations that rarity
precedes extinction; and we know that this has been the progress of
events with those animals which have been exterminated, either locally
or wholly, through man’s agency. I may repeat what I published in 1845,
namely, that to admit that species generally become rare before they
become extinct—to feel no surprise at the rarity of a species, and yet
to marvel greatly when the species ceases to exist, is much the same as
to admit that sickness in the individual is the forerunner of death—to
feel no surprise at sickness, but, when the sick man dies, to wonder
and to suspect that he died by some deed of violence.

The theory of natural selection is grounded on the belief that each new
variety and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition;
and the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the
less improved varieties in the same neighbourhood; when much improved
it is transported far and near, like our short-horn cattle, and takes
the place of other breeds in other countries. Thus the appearance of
new forms and the disappearance of old forms, both those naturally and
artificially produced, are bound together. In flourishing groups, the
number of new specific forms which have been produced within a given
time has at some periods probably been greater than the number of the
old specific forms which have been exterminated; but we know that
species have not gone on indefinitely increasing, at least during the
later geological epochs, so that, looking to later times, we may
believe that the production of new forms has caused the extinction of
about the same number of old forms.

The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like each
other in all respects. Hence the improved and modified descendants of a
species will generally cause the extermination of the parent-species;
and if many new forms have been developed from any one species, the
nearest allies of that species, _i.e._ the species of the same genus,
will be the most liable to extermination. Thus, as I believe, a number
of new species descended from one species, that is a new genus, comes
to supplant an old genus, belonging to the same family. But it must
often have happened that a new species belonging to some one group has
seized on the place occupied by a species belonging to a distinct
group, and thus have caused its extermination. If many allied forms be
developed from the successful intruder, many will have to yield their
places; and it will generally be the allied forms, which will suffer
from some inherited inferiority in common. But whether it be species
belonging to the same or to a distinct class, which have yielded their
places to other modified and improved species, a few of the sufferers
may often be preserved for a long time, from being fitted to some
peculiar line of life, or from inhabiting some distant and isolated
station, where they will have escaped severe competition. For instance,
some species of Trigonia, a great genus of shells in the secondary
formations, survive in the Australian seas; and a few members of the
great and almost extinct group of Ganoid fishes still inhabit our fresh
waters. Therefore, the utter extinction of a group is generally, as we
have seen, a slower process than its production.

With respect to the apparently sudden extermination of whole families
or orders, as of Trilobites at the close of the palæozoic period, and
of Ammonites at the close of the secondary period, we must remember
what has been already said on the probable wide intervals of time
between our consecutive formations; and in these intervals there may
have been much slow extermination. Moreover, when, by sudden
immigration or by unusually rapid development, many species of a new
group have taken possession of an area, many of the older species will
have been exterminated in a correspondingly rapid manner; and the forms
which thus yield their places will commonly be allied, for they will
partake of the same inferiority in common.

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct accords well with the theory of
natural selection. We need not marvel at extinction; if we must marvel,
let it be at our presumption in imagining for a moment that we
understand the many complex contingencies on which the existence of
each species depends. If we forget for an instant that each species
tends to increase inordinately, and that some check is always in
action, yet seldom perceived by us, the whole economy of nature will be
utterly obscured. Whenever we can precisely say why this species is
more abundant in individuals than that; why this species and not
another can be naturalised in a given country; then, and not until
then, we may justly feel surprise why we cannot account for the
extinction of any particular species or group of species.

_On the Forms of Life changing almost simultaneously throughout the
World._


Scarcely any palæontological discovery is more striking than the fact
that the forms of life change almost simultaneously throughout the
world. Thus our European Chalk formation can be recognised in many
distant regions, under the most different climates, where not a
fragment of the mineral chalk itself can be found; namely, in North
America, in equatorial South America, in Tierra del Fuego, at the Cape
of Good Hope, and in the peninsula of India. For at these distant
points, the organic remains in certain beds present an unmistakable
resemblance to those of the Chalk. It is not that the same species are
met with; for in some cases not one species is identically the same,
but they belong to the same families, genera, and sections of genera,
and sometimes are similarly characterised in such trifling points as
mere superficial sculpture. Moreover, other forms, which are not found
in the Chalk of Europe, but which occur in the formations either above
or below, occur in the same order at these distant points of the world.
In the several successive palæozoic formations of Russia, Western
Europe and North America, a similar parallelism in the forms of life
has been observed by several authors; so it is, according to Lyell,
with the European and North American tertiary deposits. Even if the few
fossil species which are common to the Old and New Worlds were kept
wholly out of view, the general parallelism in the successive forms of
life, in the palæozoic and tertiary stages, would still be manifest,
and the several formations could be easily correlated.

These observations, however, relate to the marine inhabitants of the
world: we have not sufficient data to judge whether the productions of
the land and of fresh water at distant points change in the same
parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to
Europe from La Plata, without any information in regard to their
geological position, no one would have suspected that they had
co-existed with sea-shells all still living; but as these anomalous
monsters co-existed with the Mastodon and Horse, it might at least have
been inferred that they had lived during one of the later tertiary
stages.

When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same year, or even to the same century, or
even that it has a very strict geological sense; for if all the marine
animals now living in Europe, and all those that lived in Europe during
the pleistocene period (a very remote period as measured by years,
including the whole glacial epoch) were compared with those now
existing in South America or in Australia, the most skilful naturalist
would hardly be able to say whether the present or the pleistocene
inhabitants of Europe resembled most closely those of the southern
hemisphere. So, again, several highly competent observers maintain that
the existing productions of the United States are more closely related
to those which lived in Europe during certain late tertiary stages,
than to the present inhabitants of Europe; and if this be so, it is
evident that fossiliferous beds now deposited on the shores of North
America would hereafter be liable to be classed with somewhat older
European beds. Nevertheless, looking to a remotely future epoch, there
can be little doubt that all the more modern _marine_ formations,
namely, the upper pliocene, the pleistocene and strictly modern beds of
Europe, North and South America, and Australia, from containing fossil
remains in some degree allied, and from not including those forms which
are found only in the older underlying deposits, would be correctly
ranked as simultaneous in a geological sense.

The fact of the forms of life changing simultaneously in the above
large sense, at distant parts of the world, has greatly struck those
admirable observers, MM. de Verneuil and d’Archiac. After referring to
the parallelism of the palæozoic forms of life in various parts of
Europe, they add, “If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom.” M. Barrande has made forcible remarks to precisely the
same effect. It is, indeed, quite futile to look to changes of
currents, climate, or other physical conditions, as the cause of these
great mutations in the forms of life throughout the world, under the
most different climates. We must, as Barrande has remarked, look to
some special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries and the
nature of their inhabitants.

This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selection.
New species are formed by having some advantage over older forms; and
the forms, which are already dominant, or have some advantage over the
other forms in their own country, give birth to the greatest number of
new varieties or incipient species. We have distinct evidence on this
head, in the plants which are dominant, that is, which are commonest
and most widely diffused, producing the greatest number of new
varieties. It is also natural that the dominant, varying and
far-spreading species, which have already invaded, to a certain extent,
the territories of other species, should be those which would have the
best chance of spreading still further, and of giving rise in new
countries to other new varieties and species. The process of diffusion
would often be very slow, depending on climatal and geographical
changes, on strange accidents, and on the gradual acclimatization of
new species to the various climates through which they might have to
pass, but in the course of time the dominant forms would generally
succeed in spreading and would ultimately prevail. The diffusion would,
it is probable, be slower with the terrestrial inhabitants of distinct
continents than with the marine inhabitants of the continuous sea. We
might therefore expect to find, as we do find, a less strict degree of
parallelism in the succession of the productions of the land than with
those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the
world, accords well with the principle of new species having been
formed by dominant species spreading widely and varying; the new
species thus produced being themselves dominant, owing to their having
had some advantage over their already dominant parents, as well as over
other species; and again spreading, varying, and producing new forms.
The old forms which are beaten and which yield their places to the new
and victorious forms, will generally be allied in groups, from
inheriting some inferiority in common; and, therefore, as new and
improved groups spread throughout the world, old groups disappear from
the world; and the succession of forms everywhere tends to correspond
both in their first appearance and final disappearance.

There is one other remark connected with this subject worth making. I
have given my reasons for believing that most of our great formations,
rich in fossils, were deposited during periods of subsidence; and that
blank intervals of vast duration, as far as fossils are concerned,
occurred during the periods when the bed of the sea was either
stationary or rising, and likewise when sediment was not thrown down
quickly enough to embed and preserve organic remains. During these long
and blank intervals I suppose that the inhabitants of each region
underwent a considerable amount of modification and extinction, and
that there was much migration from other parts of the world. As we have
reason to believe that large areas are affected by the same movement,
it is probable that strictly contemporaneous formations have often been
accumulated over very wide spaces in the same quarter of the world; but
we are very far from having any right to conclude that this has
invariably been the case, and that large areas have invariably been
affected by the same movements. When two formations have been deposited
in two regions during nearly, but not exactly, the same period, we
should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but the
species would not exactly correspond; for there will have been a little
more time in the one region than in the other for modification,
extinction, and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich, in
his admirable Memoirs on the eocene deposits of England and France, is
able to draw a close general parallelism between the successive stages
in the two countries; but when he compares certain stages in England
with those in France, although he finds in both a curious accordance in
the numbers of the species belonging to the same genera, yet the
species themselves differ in a manner very difficult to account for
considering the proximity of the two areas, unless, indeed, it be
assumed that an isthmus separated two seas inhabited by distinct, but
contemporaneous faunas. Lyell has made similar observations on some of
the later tertiary formations. Barrande, also, shows that there is a
striking general parallelism in the successive Silurian deposits of
Bohemia and Scandinavia; nevertheless he finds a surprising amount of
difference in the species. If the several formations in these regions
have not been deposited during the same exact periods—a formation in
one region often corresponding with a blank interval in the other—and
if in both regions the species have gone on slowly changing during the
accumulation of the several formations and during the long intervals of
time between them; in this case the several formations in the two
regions could be arranged in the same order, in accordance with the
general succession of the forms of life, and the order would falsely
appear to be strictly parallel; nevertheless the species would not all
be the same in the apparently corresponding stages in the two regions.

_On the Affinities of Extinct Species to each other, and to Living
Forms._


Let us now look to the mutual affinities of extinct and living species.
All fall into a few grand classes; and this fact is at once explained
on the principle of descent. The more ancient any form is, the more, as
a general rule, it differs from living forms. But, as Buckland long ago
remarked, extinct species can all be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the intervals between existing genera, families, and orders, is
certainly true; but as this statement has often been ignored or even
denied, it may be well to make some remarks on this subject, and to
give some instances. If we confine our attention either to the living
or to the extinct species of the same class, the series is far less
perfect than if we combine both into one general system. In the
writings of Professor Owen we continually meet with the expression of
generalised forms, as applied to extinct animals; and in the writings
of Agassiz, of prophetic or synthetic types; and these terms imply that
such forms are, in fact, intermediate or connecting links. Another
distinguished palæontologist, M. Gaudry, has shown in the most striking
manner that many of the fossil mammals discovered by him in Attica
serve to break down the intervals between existing genera. Cuvier
ranked the Ruminants and Pachyderms as two of the most distinct orders
of mammals; but so many fossil links have been disentombed that Owen
has had to alter the whole classification, and has placed certain
Pachyderms in the same sub-order with ruminants; for example, he
dissolves by gradations the apparently wide interval between the pig
and the camel. The Ungulata or hoofed quadrupeds are now divided into
the even-toed or odd-toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions. No one
will deny that the Hipparion is intermediate between the existing horse
and certain other ungulate forms. What a wonderful connecting link in
the chain of mammals is the Typotherium from South America, as the name
given to it by Professor Gervais expresses, and which cannot be placed
in any existing order. The Sirenia form a very distinct group of the
mammals, and one of the most remarkable peculiarities in existing
dugong and lamentin is the entire absence of hind limbs, without even a
rudiment being left; but the extinct Halitherium had, according to
Professor Flower, an ossified thigh-bone “articulated to a well-defined
acetabulum in the pelvis,” and it thus makes some approach to ordinary
hoofed quadrupeds, to which the Sirenia are in other respects allied.
The cetaceans or whales are widely different from all other mammals,
but the tertiary Zeuglodon and Squalodon, which have been placed by
some naturalists in an order by themselves, are considered by Professor
Huxley to be undoubtedly cetaceans, “and to constitute connecting links
with the aquatic carnivora.”

Even the wide interval between birds and reptiles has been shown by the
naturalist just quoted to be partially bridged over in the most
unexpected manner, on the one hand, by the ostrich and extinct
Archeopteryx, and on the other hand by the Compsognathus, one of the
Dinosaurians—that group which includes the most gigantic of all
terrestrial reptiles. Turning to the Invertebrata, Barrande asserts, a
higher authority could not be named, that he is every day taught that,
although palæozoic animals can certainly be classed under existing
groups, yet that at this ancient period the groups were not so
distinctly separated from each other as they now are.

Some writers have objected to any extinct species, or group of species,
being considered as intermediate between any two living species, or
groups of species. If by this term it is meant that an extinct form is
directly intermediate in all its characters between two living forms or
groups, the objection is probably valid. But in a natural
classification many fossil species certainly stand between living
species, and some extinct genera between living genera, even between
genera belonging to distinct families. The most common case, especially
with respect to very distinct groups, such as fish and reptiles, seems
to be that, supposing them to be distinguished at the present day by a
score of characters, the ancient members are separated by a somewhat
lesser number of characters, so that the two groups formerly made a
somewhat nearer approach to each other than they now do.

It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted to
those groups which have undergone much change in the course of
geological ages; and it would be difficult to prove the truth of the
proposition, for every now and then even a living animal, as the
Lepidosiren, is discovered having affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and Batrachians,
the older Fish, the older Cephalopods, and the eocene Mammals, with the
recent members of the same classes, we must admit that there is truth
in the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat
complex, I must request the reader to turn to the diagram in the fourth
chapter. We may suppose that the numbered letters in italics represent
genera, and the dotted lines diverging from them the species in each
genus. The diagram is much too simple, too few genera and too few
species being given, but this is unimportant for us. The horizontal
lines may represent successive geological formations, and all the forms
beneath the uppermost line may be considered as extinct. The three
existing genera, _a_14, _q_14, _p_14, will form a small family; _b_14
and _f_14, a closely allied family or subfamily; and _o_14, _e_14,
_m_14, a third family. These three families, together with the many
extinct genera on the several lines of descent diverging from the
parent form (A) will form an order; for all will have inherited
something in common from their ancient progenitor. On the principle of
the continued tendency to divergence of character, which was formerly
illustrated by this diagram, the more recent any form is the more it
will generally differ from its ancient progenitor. Hence, we can
understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of
character is a necessary contingency; it depends solely on the
descendants from a species being thus enabled to seize on many and
different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its slightly
altered conditions of life, and yet retain throughout a vast period the
same general characteristics. This is represented in the diagram by the
letter F14.

All the many forms, extinct and recent, descended from (A), make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to have
perished at different periods, and some to have endured to the present
day.

By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered
at several points low down in the series, the three existing families
on the uppermost line would be rendered less distinct from each other.
If, for instance, the genera _a_1, _a_5, _a_10, _f_8, _m_3, _m_6, _m_9,
were disinterred, these three families would be so closely linked
together that they probably would have to be united into one great
family, in nearly the same manner as has occurred with ruminants and
certain pachyderms. Yet he who objected to consider as intermediate the
extinct genera, which thus link together the living genera of three
families, would be partly justified, for they are intermediate, not
directly, but only by a long and circuitous course through many widely
different forms. If many extinct forms were to be discovered above one
of the middle horizontal lines or geological formations—for instance,
above No. VI.—but none from beneath this line, then only two of the
families (those on the left hand _a_14, &c., and _b_14, &c.) would have
to be united into one; and there would remain two families which would
be less distinct from each other than they were before the discovery of
the fossils. So again, if the three families formed of eight genera
(_a_14 to _m_14), on the uppermost line, be supposed to differ from
each other by half-a-dozen important characters, then the families
which existed at a period marked VI would certainly have differed from
each other by a less number of characters; for they would at this early
stage of descent have diverged in a less degree from their common
progenitor. Thus it comes that ancient and extinct genera are often in
a greater or less degree intermediate in character between their
modified descendants, or between their collateral relations.

Under nature the process will be far more complicated than is
represented in the diagram; for the groups will have been more
numerous; they will have endured for extremely unequal lengths of time,
and will have been modified in various degrees. As we possess only the
last volume of the geological record, and that in a very broken
condition, we have no right to expect, except in rare cases, to fill up
the wide intervals in the natural system, and thus to unite distinct
families or orders. All that we have a right to expect is, that those
groups which have, within known geological periods, undergone much
modification, should in the older formations make some slight approach
to each other; so that the older members should differ less from each
other in some of their characters than do the existing members of the
same groups; and this by the concurrent evidence of our best
palæontologists is frequently the case.

Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, are explained in a satisfactory manner. And
they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna during any one great
period in the earth’s history will be intermediate in general character
between that which preceded and that which succeeded it. Thus the
species which lived at the sixth great stage of descent in the diagram
are the modified offspring of those which lived at the fifth stage, and
are the parents of those which became still more modified at the
seventh stage; hence they could hardly fail to be nearly intermediate
in character between the forms of life above and below. We must,
however, allow for the entire extinction of some preceding forms, and
in any one region for the immigration of new forms from other regions,
and for a large amount of modification during the long and blank
intervals between the successive formations. Subject to these
allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas.
I need give only one instance, namely, the manner in which the fossils
of the Devonian system, when this system was first discovered, were at
once recognised by palæontologists as intermediate in character between
those of the overlying carboniferous and underlying Silurian systems.
But each fauna is not necessarily exactly intermediate, as unequal
intervals of time have elapsed between consecutive formations.

It is no real objection to the truth of the statement that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions
to the rule. For instance, the species of mastodons and elephants, when
arranged by Dr. Falconer in two series—in the first place according to
their mutual affinities, and in the second place according to their
periods of existence—do not accord in arrangement. The species extreme
in character are not the oldest or the most recent; nor are those which
are intermediate in character, intermediate in age. But supposing for
an instant, in this and other such cases, that the record of the first
appearance and disappearance of the species was complete, which is far
from the case, we have no reason to believe that forms successively
produced necessarily endure for corresponding lengths of time. A very
ancient form may occasionally have lasted much longer than a form
elsewhere subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things
with great; if the principal living and extinct races of the domestic
pigeon were arranged in serial affinity, this arrangement would not
closely accord with the order in time of their production, and even
less with the order of their disappearance; for the parent rock-pigeon
still lives; and many varieties between the rock-pigeon and the carrier
have become extinct; and carriers which are extreme in the important
character of length of beak originated earlier than short-beaked
tumblers, which are at the opposite end of the series in this respect.

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is
the fact, insisted on by all palæontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a
well-known instance, the general resemblance of the organic remains
from the several stages of the Chalk formation, though the species are
distinct in each stage. This fact alone, from its generality, seems to
have shaken Professor Pictet in his belief in the immutability of
species. He who is acquainted with the distribution of existing species
over the globe, will not attempt to account for the close resemblance
of distinct species in closely consecutive formations, by the physical
conditions of the ancient areas having remained nearly the same. Let it
be remembered that the forms of life, at least those inhabiting the
sea, have changed almost simultaneously throughout the world, and
therefore under the most different climates and conditions. Consider
the prodigious vicissitudes of climate during the pleistocene period,
which includes the whole glacial epoch, and note how little the
specific forms of the inhabitants of the sea have been affected.

On the theory of descent, the full meaning of the fossil remains from
closely consecutive formations, being closely related, though ranked as
distinct species, is obvious. As the accumulation of each formation has
often been interrupted, and as long blank intervals have intervened
between successive formations, we ought not to expect to find, as I
attempted to show in the last chapter, in any one or in any two
formations, all the intermediate varieties between the species which
appeared at the commencement and close of these periods: but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and
these assuredly we do find. We find, in short, such evidence of the
slow and scarcely sensible mutations of specific forms, as we have the
right to expect.

_On the State of Development of Ancient compared with Living Forms._


We have seen in the fourth chapter that the degree of differentiation
and specialisation of the parts in organic beings, when arrived at
maturity, is the best standard, as yet suggested, of their degree of
perfection or highness. We have also seen that, as the specialisation
of parts is an advantage to each being, so natural selection will tend
to render the organisation of each being more specialised and perfect,
and in this sense higher; not but that it may leave many creatures with
simple and unimproved structures fitted for simple conditions of life,
and in some cases will even degrade or simplify the organisation, yet
leaving such degraded beings better fitted for their new walks of life.
In another and more general manner, new species become superior to
their predecessors; for they have to beat in the struggle for life all
the older forms, with which they come into close competition. We may
therefore conclude that if under a nearly similar climate the eocene
inhabitants of the world could be put into competition with the
existing inhabitants, the former would be beaten and exterminated by
the latter, as would the secondary by the eocene, and the palæozoic by
the secondary forms. So that by this fundamental test of victory in the
battle for life, as well as by the standard of the specialisation of
organs, modern forms ought, on the theory of natural selection, to
stand higher than ancient forms. Is this the case? A large majority of
palæontologists would answer in the affirmative; and it seems that this
answer must be admitted as true, though difficult of proof.

It is no valid objection to this conclusion, that certain Brachiopods
have been but slightly modified from an extremely remote geological
epoch; and that certain land and fresh-water shells have remained
nearly the same, from the time when, as far as is known, they first
appeared. It is not an insuperable difficulty that Foraminifera have
not, as insisted on by Dr. Carpenter, progressed in organisation since
even the Laurentian epoch; for some organisms would have to remain
fitted for simple conditions of life, and what could be better fitted
for this end than these lowly organised Protozoa? Such objections as
the above would be fatal to my view, if it included advance in
organisation as a necessary contingent. They would likewise be fatal,
if the above Foraminifera, for instance, could be proved to have first
come into existence during the Laurentian epoch, or the above
Brachiopods during the Cambrian formation; for in this case, there
would not have been time sufficient for the development of these
organisms up to the standard which they had then reached. When advanced
up to any given point, there is no necessity, on the theory of natural
selection, for their further continued process; though they will,
during each successive age, have to be slightly modified, so as to hold
their places in relation to slight changes in their conditions. The
foregoing objections hinge on the question whether we really know how
old the world is, and at what period the various forms of life first
appeared; and this may well be disputed.

The problem whether organisation on the whole has advanced is in many
ways excessively intricate. The geological record, at all times
imperfect, does not extend far enough back to show with unmistakable
clearness that within the known history of the world organisation has
largely advanced. Even at the present day, looking to members of the
same class, naturalists are not unanimous which forms ought to be
ranked as highest: thus, some look at the selaceans or sharks, from
their approach in some important points of structure to reptiles, as
the highest fish; others look at the teleosteans as the highest. The
ganoids stand intermediate between the selaceans and teleosteans; the
latter at the present day are largely preponderant in number; but
formerly selaceans and ganoids alone existed; and in this case,
according to the standard of highness chosen, so will it be said that
fishes have advanced or retrograded in organisation. To attempt to
compare members of distinct types in the scale of highness seems
hopeless; who will decide whether a cuttle-fish be higher than a
bee—that insect which the great Von Baer believed to be “in fact more
highly organised than a fish, although upon another type?” In the
complex struggle for life it is quite credible that crustaceans, not
very high in their own class, might beat cephalopods, the highest
molluscs; and such crustaceans, though not highly developed, would
stand very high in the scale of invertebrate animals, if judged by the
most decisive of all trials—the law of battle. Beside these inherent
difficulties in deciding which forms are the most advanced in
organisation, we ought not solely to compare the highest members of a
class at any two periods—though undoubtedly this is one and perhaps the
most important element in striking a balance—but we ought to compare
all the members, high and low, at two periods. At an ancient epoch the
highest and lowest molluscoidal animals, namely, cephalopods and
brachiopods, swarmed in numbers; at the present time both groups are
greatly reduced, while others, intermediate in organisation, have
largely increased; consequently some naturalists maintain that molluscs
were formerly more highly developed than at present; but a stronger
case can be made out on the opposite side, by considering the vast
reduction of brachiopods, and the fact that our existing cephalopods,
though few in number, are more highly organised than their ancient
representatives. We ought also to compare the relative proportional
numbers, at any two periods, of the high and low classes throughout the
world: if, for instance, at the present day fifty thousand kinds of
vertebrate animals exist, and if we knew that at some former period
only ten thousand kinds existed, we ought to look at this increase in
number in the highest class, which implies a great displacement of
lower forms, as a decided advance in the organisation of the world. We
thus see how hopelessly difficult it is to compare with perfect
fairness, under such extremely complex relations, the standard of
organisation of the imperfectly-known faunas of successive periods.

We shall appreciate this difficulty more clearly by looking to certain
existing faunas and floras. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have
seized on places which must have been previously occupied by the
indigenes, we must believe, that if all the animals and plants of Great
Britain were set free in New Zealand, a multitude of British forms
would in the course of time become thoroughly naturalized there, and
would exterminate many of the natives. On the other hand, from the fact
that hardly a single inhabitant of the southern hemisphere has become
wild in any part of Europe, we may well doubt whether, if all the
productions of New Zealand were set free in Great Britain, any
considerable number would be enabled to seize on places now occupied by
our native plants and animals. Under this point of view, the
productions of Great Britain stand much higher in the scale than those
of New Zealand. Yet the most skilful naturalist, from an examination of
the species of the two countries, could not have foreseen this result.

Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of
successive generations, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by nature, of
the former and less modified condition of the species. This view may be
true, and yet may never be capable of proof. Seeing, for instance, that
the oldest known mammals, reptiles, and fishes strictly belong to their
proper classes, though some of these old forms are in a slight degree
less distinct from each other than are the typical members of the same
groups at the present day, it would be vain to look for animals having
the common embryological character of the Vertebrata, until beds rich
in fossils are discovered far beneath the lowest Cambrian strata—a
discovery of which the chance is small.

_On the Succession of the same Types within the same Areas, during the
later Tertiary periods._


Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even
to an uneducated eye, in the gigantic pieces of armour, like those of
the armadillo, found in several parts of La Plata; and Professor Owen
has shown in the most striking manner that most of the fossil mammals,
buried there in such numbers, are related to South American types. This
relationship is even more clearly seen in the wonderful collection of
fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was
so much impressed with these facts that I strongly insisted, in 1839
and 1845, on this “law of the succession of types,”—on “this wonderful
relationship in the same continent between the dead and the living.”
Professor Owen has subsequently extended the same generalisation to the
mammals of the Old World. We see the same law in this author’s
restorations of the extinct and gigantic birds of New Zealand. We see
it also in the birds of the caves of Brazil. Mr. Woodward has shown
that the same law holds good with sea-shells, but, from the wide
distribution of most molluscs, it is not well displayed by them. Other
cases could be added, as the relation between the extinct and living
land-shells of Madeira; and between the extinct and living brackish
water-shells of the Aralo-Caspian Sea.

Now, what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man who, after comparing
the present climate of Australia and of parts of South America, under
the same latitude, would attempt to account, on the one hand through
dissimilar physical conditions, for the dissimilarity of the
inhabitants of these two continents; and, on the other hand through
similarity of conditions, for the uniformity of the same types in each
continent during the later tertiary periods. Nor can it be pretended
that it is an immutable law that marsupials should have been chiefly or
solely produced in Australia; or that Edentata and other American types
should have been solely produced in South America. For we know that
Europe in ancient times was peopled by numerous marsupials; and I have
shown in the publications above alluded to, that in America the law of
distribution of terrestrial mammals was formerly different from what it
now is. North America formerly partook strongly of the present
character of the southern half of the continent; and the southern half
was formerly more closely allied, than it is at present, to the
northern half. In a similar manner we know, from Falconer and Cautley’s
discoveries, that Northern India was formerly more closely related in
its mammals to Africa than it is at the present time. Analogous facts
could be given in relation to the distribution of marine animals.

On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the
same areas, is at once explained; for the inhabitants of each quarter
of the world will obviously tend to leave in that quarter, during the
next succeeding period of time, closely allied though in some degree
modified descendants. If the inhabitants of one continent formerly
differed greatly from those of another continent, so will their
modified descendants still differ in nearly the same manner and degree.
But after very long intervals of time, and after great geographical
changes, permitting much intermigration, the feebler will yield to the
more dominant forms, and there will be nothing immutable in the
distribution of organic beings.

It may be asked in ridicule whether I suppose that the megatherium and
other allied huge monsters, which formerly lived in South America, have
left behind them the sloth, armadillo, and anteater, as their
degenerate descendants. This cannot for an instant be admitted. These
huge animals have become wholly extinct, and have left no progeny. But
in the caves of Brazil there are many extinct species which are closely
allied in size and in all other characters to the species still living
in South America; and some of these fossils may have been the actual
progenitors of the living species. It must not be forgotten that, on
our theory, all the species of the same genus are the descendants of
some one species; so that, if six genera, each having eight species, be
found in one geological formation, and in a succeeding formation there
be six other allied or representative genera, each with the same number
of species, then we may conclude that generally only one species of
each of the older genera has left modified descendants, which
constitute the new genera containing the several species; the other
seven species of each old genus having died out and left no progeny.
Or, and this will be a far commoner case, two or three species in two
or three alone of the six older genera will be the parents of the new
genera: the other species and the other old genera having become
utterly extinct. In failing orders, with the genera and species
decreasing in numbers as is the case with the Edentata of South
America, still fewer genera and species will leave modified
blood-descendants.

_Summary of the preceding and present Chapters._


I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been geologically
explored with care; that only certain classes of organic beings have
been largely preserved in a fossil state; that the number both of
specimens and of species, preserved in our museums, is absolutely as
nothing compared with the number of generations which must have passed
away even during a single formation; that, owing to subsidence being
almost necessary for the accumulation of deposits rich in fossil
species of many kinds, and thick enough to outlast future degradation,
great intervals of time must have elapsed between most of our
successive formations; that there has probably been more extinction
during the periods of subsidence, and more variation during the periods
of elevation, and during the latter the record will have been least
perfectly kept; that each single formation has not been continuously
deposited; that the duration of each formation is probably short
compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in
any one area and formation; that widely ranging species are those which
have varied most frequently, and have oftenest given rise to new
species; that varieties have at first been local; and lastly, although
each species must have passed through numerous transitional stages, it
is probable that the periods, during which each underwent modification,
though many and long as measured by years, have been short in
comparison with the periods during which each remained in an unchanged
condition. These causes, taken conjointly, will to a large extent
explain why—though we do find many links—we do not find interminable
varieties, connecting together all extinct and existing forms by the
finest graduated steps. It should also be constantly borne in mind that
any linking variety between two forms, which might be found, would be
ranked, unless the whole chain could be perfectly restored, as a new
and distinct species; for it is not pretended that we have any sure
criterion by which species and varieties can be discriminated.

He who rejects this view of the imperfection of the geological record,
will rightly reject the whole theory. For he may ask in vain where are
the numberless transitional links which must formerly have connected
the closely allied or representative species, found in the successive
stages of the same great formation? He may disbelieve in the immense
intervals of time which must have elapsed between our consecutive
formations; he may overlook how important a part migration has played,
when the formations of any one great region, as those of Europe, are
considered; he may urge the apparent, but often falsely apparent,
sudden coming in of whole groups of species. He may ask where are the
remains of those infinitely numerous organisms which must have existed
long before the Cambrian system was deposited? We now know that at
least one animal did then exist; but I can answer this last question
only by supposing that where our oceans now extend they have extended
for an enormous period, and where our oscillating continents now stand
they have stood since the commencement of the Cambrian system; but
that, long before that epoch, the world presented a widely different
aspect; and that the older continents, formed of formations older than
any known to us, exist now only as remnants in a metamorphosed
condition, or lie still buried under the ocean.

Passing from these difficulties, the other great leading facts in
palæontology agree admirably with the theory of descent with
modification through variation and natural selection. We can thus
understand how it is that new species come in slowly and successively;
how species of different classes do not necessarily change together, or
at the same rate, or in the same degree; yet in the long run that all
undergo modification to some extent. The extinction of old forms is the
almost inevitable consequence of the production of new forms. We can
understand why, when a species has once disappeared, it never
reappears. Groups of species increase in numbers slowly, and endure for
unequal periods of time; for the process of modification is necessarily
slow, and depends on many complex contingencies. The dominant species
belonging to large and dominant groups tend to leave many modified
descendants, which form new sub-groups and groups. As these are formed,
the species of the less vigorous groups, from their inferiority
inherited from a common progenitor, tend to become extinct together,
and to leave no modified offspring on the face of the earth. But the
utter extinction of a whole group of species has sometimes been a slow
process, from the survival of a few descendants, lingering in protected
and isolated situations. When a group has once wholly disappeared, it
does not reappear; for the link of generation has been broken.

We can understand how it is that dominant forms which spread widely and
yield the greatest number of varieties tend to people the world with
allied, but modified, descendants; and these will generally succeed in
displacing the groups which are their inferiors in the struggle for
existence. Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and
recent, make together a few grand classes. We can understand, from the
continued tendency to divergence of character, why the more ancient a
form is, the more it generally differs from those now living. Why
ancient and extinct forms often tend to fill up gaps between existing
forms, sometimes blending two groups, previously classed as distinct
into one; but more commonly bringing them only a little closer
together. The more ancient a form is, the more often it stands in some
degree intermediate between groups now distinct; for the more ancient a
form is, the more nearly it will be related to, and consequently
resemble, the common progenitor of groups, since become widely
divergent. Extinct forms are seldom directly intermediate between
existing forms; but are intermediate only by a long and circuitous
course through other extinct and different forms. We can clearly see
why the organic remains of closely consecutive formations are closely
allied; for they are closely linked together by generation. We can
clearly see why the remains of an intermediate formation are
intermediate in character.

The inhabitants of the world at each successive period in its history
have beaten their predecessors in the race for life, and are, in so
far, higher in the scale, and their structure has generally become more
specialised; and this may account for the common belief held by so many
palæontologists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos of the
more recent animals belonging to the same classes, and this wonderful
fact receives a simple explanation according to our views. The
succession of the same types of structure within the same areas during
the later geological periods ceases to be mysterious, and is
intelligible on the principle of inheritance.

If, then, the geological record be as imperfect as many believe, and it
may at least be asserted that the record cannot be proved to be much
more perfect, the main objections to the theory of natural selection
are greatly diminished or disappear. On the other hand, all the chief
laws of palæontology plainly proclaim, as it seems to me, that species
have been produced by ordinary generation: old forms having been
supplanted by new and improved forms of life, the products of variation
and the survival of the fittest.




CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.


Present distribution cannot be accounted for by differences in physical
conditions—Importance of barriers—Affinity of the productions of the
same continent—Centres of creation—Means of dispersal by changes of
climate and of the level of the land, and by occasional means—Dispersal
during the Glacial period—Alternate Glacial periods in the North and
South.


In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be wholly accounted for by climatal and other physical conditions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to
prove its truth; for if we exclude the arctic and northern temperate
parts, all authors agree that one of the most fundamental divisions in
geographical distribution is that between the New and Old Worlds; yet
if we travel over the vast American continent, from the central parts
of the United States to its extreme southern point, we meet with the
most diversified conditions; humid districts, arid deserts, lofty
mountains, grassy plains, forests, marshes, lakes and great rivers,
under almost every temperature. There is hardly a climate or condition
in the Old World which cannot be paralleled in the New—at least so
closely as the same species generally require. No doubt small areas can
be pointed out in the Old World hotter than any in the New World; but
these are not inhabited by a fauna different from that of the
surrounding districts; for it is rare to find a group of organisms
confined to a small area, of which the conditions are peculiar in only
a slight degree. Notwithstanding this general parallelism in the
conditions of Old and New Worlds, how widely different are their living
productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25° and 35°, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or, again, we may compare the
productions of South America south of latitude 35° with those north of
25°, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet
they are incomparably more closely related to each other than they are
to the productions of Australia or Africa under nearly the same
climate. Analogous facts could be given with respect to the inhabitants
of the sea.

A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions
of various regions. We see this in the great difference in nearly all
the terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a
slightly different climate, there might have been free migration for
the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same
latitude; for these countries are almost as much isolated from each
other as is possible. On each continent, also, we see the same fact;
for on the opposite sides of lofty and continuous mountain-ranges, and
of great deserts and even of large rivers, we find different
productions; though as mountain chains, deserts, &c., are not as
impassable, or likely to have endured so long, as the oceans separating
continents, the differences are very inferior in degree to those
characteristic of distinct continents.

Turning to the sea, we find the same law. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Günther has recently shown that about thirty per cent of the fishes are
the same on the opposite sides of the isthmus of Panama; and this fact
has led naturalists to believe that the isthmus was formerly open.
Westward of the shores of America, a wide space of open ocean extends,
with not an island as a halting-place for emigrants; here we have a
barrier of another kind, and as soon as this is passed we meet in the
eastern islands of the Pacific with another and totally distinct fauna.
So that three marine faunas range northward and southward in parallel
lines not far from each other, under corresponding climate; but from
being separated from each other by impassable barriers, either of land
or open sea, they are almost wholly distinct. On the other hand,
proceeding still farther westward from the eastern islands of the
tropical parts of the Pacific, we encounter no impassable barriers, and
we have innumerable islands as halting-places, or continuous coasts,
until, after travelling over a hemisphere, we come to the shores of
Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although so few marine animals are common to
the above-named three approximate faunas of Eastern and Western America
and the eastern Pacific islands, yet many fishes range from the Pacific
into the Indian Ocean, and many shells are common to the eastern
islands of the Pacific and the eastern shores of Africa on almost
exactly opposite meridians of longitude.

A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea,
though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent
offers innumerable instances. Nevertheless, the naturalist, in
travelling, for instance, from north to south, never fails to be struck
by the manner in which successive groups of beings, specifically
distinct, though nearly related, replace each other. He hears from
closely allied, yet distinct kinds of birds, notes nearly similar, and
sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of
Magellan are inhabited by one species of Rhea (American ostrich), and
northward the plains of La Plata by another species of the same genus;
and not by a true ostrich or emu, like those inhabiting Africa and
Australia under the same latitude. On these same plains of La Plata we
see the agouti and bizcacha, animals having nearly the same habits as
our hares and rabbits, and belonging to the same order of Rodents, but
they plainly display an American type of structure. We ascend the lofty
peaks of the Cordillera, and we find an alpine species of bizcacha; we
look to the waters, and we do not find the beaver or muskrat, but the
coypu and capybara, rodents of the South American type. Innumerable
other instances could be given. If we look to the islands off the
American shore, however much they may differ in geological structure,
the inhabitants are essentially American, though they may be all
peculiar species. We may look back to past ages, as shown in the last
chapter, and we find American types then prevailing on the American
continent and in the American seas. We see in these facts some deep
organic bond, throughout space and time, over the same areas of land
and water, independently of physical conditions. The naturalist must be
dull who is not led to inquire what this bond is.

The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we
see in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification
through variation and natural selection, and probably in a subordinate
degree to the definite influence of different physical conditions. The
degrees of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been more or
less effectually prevented, at periods more or less remote—on the
nature and number of the former immigrants—and on the action of the
inhabitants on each other in leading to the preservation of different
modifications; the relation of organism to organism in the struggle for
life being, as I have already often remarked, the most important of all
relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in
their own widely-extended homes, will have the best chance of seizing
on new places, when they spread out into new countries. In their new
homes they will be exposed to new conditions, and will frequently
undergo further modification and improvement; and thus they will become
still further victorious, and will produce groups of modified
descendants. On this principle of inheritance with modification we can
understand how it is that sections of genera, whole genera, and even
families, are confined to the same areas, as is so commonly and
notoriously the case.

There is no evidence, as was remarked in the last chapter, of the
existence of any law of necessary development. As the variability of
each species is an independent property, and will be taken advantage of
by natural selection, only so far as it profits each individual in its
complex struggle for life, so the amount of modification in different
species will be no uniform quantity. If a number of species, after
having long competed with each other in their old home, were to migrate
in a body into a new and afterwards isolated country, they would be
little liable to modification; for neither migration nor isolation in
themselves effect anything. These principles come into play only by
bringing organisms into new relations with each other and in a lesser
degree with the surrounding physical conditions. As we have seen in the
last chapter that some forms have retained nearly the same character
from an enormously remote geological period, so certain species have
migrated over vast spaces, and have not become greatly or at all
modified.

According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world,
must originally have proceeded from the same source, as they are
descended from the same progenitor. In the case of those species which
have undergone, during whole geological periods, little modification,
there is not much difficulty in believing that they have migrated from
the same region; for during the vast geographical and climatical
changes which have supervened since ancient times, almost any amount of
migration is possible. But in many other cases, in which we have reason
to believe that the species of a genus have been produced within
comparatively recent times, there is great difficulty on this head. It
is also obvious that the individuals of the same species, though now
inhabiting distant and isolated regions, must have proceeded from one
spot, where their parents were first produced: for, as has been
explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.

_Single Centres of supposed Creation._—We are thus brought to the
question which has been largely discussed by naturalists, namely,
whether species have been created at one or more points of the earth’s
surface. Undoubtedly there are many cases of extreme difficulty in
understanding how the same species could possibly have migrated from
some one point to the several distant and isolated points, where now
found. Nevertheless the simplicity of the view that each species was
first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with
subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a
species is continuous; and that when a plant or animal inhabits two
points so distant from each other, or with an interval of such a
nature, that the space could not have been easily passed over by
migration, the fact is given as something remarkable and exceptional.
The incapacity of migrating across a wide sea is more clear in the case
of terrestrial mammals than perhaps with any other organic beings; and,
accordingly, we find no inexplicable instances of the same mammals
inhabiting distant points of the world. No geologist feels any
difficulty in Great Britain possessing the same quadrupeds with the
rest of Europe, for they were no doubt once united. But if the same
species can be produced at two separate points, why do we not find a
single mammal common to Europe and Australia or South America? The
conditions of life are nearly the same, so that a multitude of European
animals and plants have become naturalised in America and Australia;
and some of the aboriginal plants are identically the same at these
distant points of the northern and southern hemispheres? The answer, as
I believe, is, that mammals have not been able to migrate, whereas some
plants, from their varied means of dispersal, have migrated across the
wide and broken interspaces. The great and striking influence of
barriers of all kinds, is intelligible only on the view that the great
majority of species have been produced on one side, and have not been
able to migrate to the opposite side. Some few families, many
subfamilies, very many genera, a still greater number of sections of
genera, are confined to a single region; and it has been observed by
several naturalists that the most natural genera, or those genera in
which the species are most closely related to each other, are generally
confined to the same country, or if they have a wide range that their
range is continuous. What a strange anomaly it would be if a directly
opposite rule were to prevail when we go down one step lower in the
series, namely to the individuals of the same species, and these had
not been, at least at first, confined to some one region!

Hence, it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration
and subsistence under past and present conditions permitted, is the
most probable. Undoubtedly many cases occur in which we cannot explain
how the same species could have passed from one point to the other. But
the geographical and climatical changes which have certainly occurred
within recent geological times, must have rendered discontinuous the
formerly continuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous,
and of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would
be hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a
moment pretend that any explanation could be offered of many instances.
But, after some preliminary remarks, I will discuss a few of the most
striking classes of facts, namely, the existence of the same species on
the summits of distant mountain ranges, and at distant points in the
Arctic and Antarctic regions; and secondly (in the following chapter),
the wide distribution of fresh water productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
nearest mainland, though separated by hundreds of miles of open sea. If
the existence of the same species at distant and isolated points of the
earth’s surface can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatical and geographical changes,
and to the various occasional means of transport, the belief that a
single birthplace is the law seems to me incomparably the safest.

In discussing this subject we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
species of a genus which must on our theory all be descended from a
common progenitor, can have migrated, undergoing modification during
their migration from some one area. If, when most of the species
inhabiting one region are different from those of another region,
though closely allied to them, it can be shown that migration from the
one region to the other has probably occurred at some former period,
our general view will be much strengthened; for the explanation is
obvious on the principle of descent with modification. A volcanic
island, for instance, upheaved and formed at the distance of a few
hundreds of miles from a continent, would probably receive from it in
the course of time a few colonists, and their descendants, though
modified, would still be related by inheritance to the inhabitants of
that continent. Cases of this nature are common, and are, as we shall
hereafter see, inexplicable on the theory of independent creation. This
view of the relation of the species of one region to those of another,
does not differ much from that advanced by Mr. Wallace, who concludes
that “every species has come into existence coincident both in space
and time with a pre-existing closely allied species.” And it is now
well known that he attributes this coincidence to descent with
modification.

The question of single or multiple centres of creation differs from
another though allied question, namely, whether all the individuals of
the same species are descended from a single pair, or single
hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With organic beings which never
intercross, if such exist, each species, must be descended from a
succession of modified varieties, that have supplanted each other, but
have never blended with other individuals or varieties of the same
species, so that, at each successive stage of modification, all the
individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which
habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be kept
nearly uniform by intercrossing; so that many individuals will go on
simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate
what I mean: our English race-horses differ from the horses of every
other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in the selecting
and training of many individuals during each generation.

Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of “single
centres of creation,” I must say a few words on the means of dispersal.

_Means of Dispersal._


Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts.
Change of climate must have had a powerful influence on migration. A
region now impassable to certain organisms from the nature of its
climate, might have been a high road for migration, when the climate
was different. I shall, however, presently have to discuss this branch
of the subject in some detail. Changes of level in the land must also
have been highly influential: a narrow isthmus now separates two marine
faunas; submerge it, or let it formerly have been submerged, and the
two faunas will now blend together, or may formerly have blended. Where
the sea now extends, land may at a former period have connected islands
or possibly even continents together, and thus have allowed terrestrial
productions to pass from one to the other. No geologist disputes that
great mutations of level have occurred within the period of existing
organisms. Edward Forbes insisted that all the islands in the Atlantic
must have been recently connected with Europe or Africa, and Europe
likewise with America. Other authors have thus hypothetically bridged
over every ocean, and united almost every island with some mainland.
If, indeed, the arguments used by Forbes are to be trusted, it must be
admitted that scarcely a single island exists which has not recently
been united to some continent. This view cuts the Gordian knot of the
dispersal of the same species to the most distant points, and removes
many a difficulty; but to the best of my judgment we are not authorized
in admitting such enormous geographical changes within the period of
existing species. It seems to me that we have abundant evidence of
great oscillations in the level of the land or sea; but not of such
vast changes in the position and extension of our continents, as to
have united them within the recent period to each other and to the
several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting-places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is
fully admitted, as it will some day be, that each species has proceeded
from a single birthplace, and when in the course of time we know
something definite about the means of distribution, we shall be enabled
to speculate with security on the former extension of the land. But I
do not believe that it will ever be proved that within the recent
period most of our continents which now stand quite separate, have been
continuously, or almost continuously united with each other, and with
the many existing oceanic islands. Several facts in distribution—such
as the great difference in the marine faunas on the opposite sides of
almost every continent—the close relation of the tertiary inhabitants
of several lands and even seas to their present inhabitants—the degree
of affinity between the mammals inhabiting islands with those of the
nearest continent, being in part determined (as we shall hereafter see)
by the depth of the intervening ocean—these and other such facts are
opposed to the admission of such prodigious geographical revolutions
within the recent period, as are necessary on the view advanced by
Forbes and admitted by his followers. The nature and relative
proportions of the inhabitants of oceanic islands are likewise opposed
to the belief of their former continuity of continents. Nor does the
almost universally volcanic composition of such islands favour the
admission that they are the wrecks of sunken continents; if they had
originally existed as continental mountain ranges, some at least of the
islands would have been formed, like other mountain summits, of
granite, metamorphic schists, old fossiliferous and other rocks,
instead of consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental means, but
which more properly should be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or that
plant is often stated to be ill adapted for wide dissemination; but the
greater or less facilities for transport across the sea may be said to
be almost wholly unknown. Until I tried, with Mr. Berkeley’s aid, a few
experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of
eighty-seven kinds, sixty-four germinated after an immersion of
twenty-eight days, and a few survived an immersion of 137 days. It
deserves notice that certain orders were far more injured than others:
nine Leguminosæ were tried, and, with one exception, they resisted the
salt-water badly; seven species of the allied orders, Hydrophyllaceæ
and Polemoniaceæ, were all killed by a month’s immersion. For
convenience’ sake I chiefly tried small seeds without the capsules or
fruit; and as all of these sank in a few days, they could not have been
floated across wide spaces of the sea, whether or not they were injured
by salt water. Afterwards I tried some larger fruits, capsules, &c.,
and some of these floated for a long time. It is well known what a
difference there is in the buoyancy of green and seasoned timber; and
it occurred to me that floods would often wash into the sea dried
plants or branches with seed-capsules or fruit attached to them. Hence
I was led to dry the stems and branches of ninety-four plants with ripe
fruit, and to place them on sea-water. The majority sank quickly, but
some which, whilst green, floated for a very short time, when dried
floated much longer; for instance, ripe hazel-nuts sank immediately,
but when dried they floated for ninety days, and afterwards when
planted germinated; an asparagus plant with ripe berries floated for
twenty-three days, when dried it floated for eighty-five days, and the
seeds afterwards germinated: the ripe seeds of Helosciadium sank in two
days, when dried they floated for above ninety days, and afterwards
germinated. Altogether, out of the ninety-four dried plants, eighteen
floated for above twenty-eight days; and some of the eighteen floated
for a very much longer period. So that as 64/87 kinds of seeds
germinated after an immersion of twenty-eight days; and as 18/94
distinct species with ripe fruit (but not all the same species as in
the foregoing experiment) floated, after being dried, for above
twenty-eight days, we may conclude, as far as anything can be inferred
from these scanty facts, that the seeds of 14/100 kinds of plants of
any country might be floated by sea-currents during twenty-eight days,
and would retain their power of germination. In Johnston’s Physical
Atlas, the average rate of the several Atlantic currents is
thirty-three miles per diem (some currents running at the rate of sixty
miles per diem); on this average, the seeds of 14/100 plants belonging
to one country might be floated across 924 miles of sea to another
country; and when stranded, if blown by an inland gale to a favourable
spot, would germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea,
so that they were alternately wet and exposed to the air like really
floating plants. He tried ninety-eight seeds, mostly different from
mine, but he chose many large fruits, and likewise seeds, from plants
which live near the sea; and this would have favoured both the average
length of their flotation and their resistance to the injurious action
of the salt-water. On the other hand, he did not previously dry the
plants or branches with the fruit; and this, as we have seen, would
have caused some of them to have floated much longer. The result was
that 18/98 of his seeds of different kinds floated for forty-two days,
and were then capable of germination. But I do not doubt that plants
exposed to the waves would float for a less time than those protected
from violent movement as in our experiments. Therefore, it would
perhaps be safer to assume that the seeds of about 10/100 plants of a
flora, after having been dried, could be floated across a space of sea
900 miles in width, and would then germinate. The fact of the larger
fruits often floating longer than the small, is interesting; as plants
with large seeds or fruit which, as Alph. de Candolle has shown,
generally have restricted ranges, could hardly be transported by any
other means.

Seeds may be occasionally transported in another manner. Drift timber
is thrown up on most islands, even on those in the midst of the widest
oceans; and the natives of the coral islands in the Pacific procure
stones for their tools, solely from the roots of drifted trees, these
stones being a valuable royal tax. I find that when irregularly shaped
stones are embedded in the roots of trees, small parcels of earth are
very frequently enclosed in their interstices and behind them, so
perfectly that not a particle could be washed away during the longest
transport: out of one small portion of earth thus _completely_ enclosed
by the roots of an oak about fifty years old, three dicotyledonous
plants germinated: I am certain of the accuracy of this observation.
Again, I can show that the carcasses of birds, when floating on the
sea, sometimes escape being immediately devoured; and many kinds of
seeds in the crops of floating birds long retain their vitality: peas
and vetches, for instance, are killed by even a few days’ immersion in
sea-water; but some taken out of the crop of a pigeon, which had
floated on artificial sea-water for thirty days, to my surprise nearly
all germinated.

Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the
ocean. We may safely assume that under such circumstances their rate of
flight would often be thirty-five miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of
nutritious seeds passing through the intestines of a bird; but hard
seeds of fruit pass uninjured through even the digestive organs of a
turkey. In the course of two months, I picked up in my garden twelve
kinds of seeds, out of the excrement of small birds, and these seemed
perfect, and some of them, which were tried, germinated. But the
following fact is more important: the crops of birds do not secrete
gastric juice, and do not, as I know by trial, injure in the least the
germination of seeds; now, after a bird has found and devoured a large
supply of food, it is positively asserted that all the grains do not
pass into the gizzard for twelve or even eighteen hours. A bird in this
interval might easily be blown to the distance of five hundred miles,
and hawks are known to look out for tired birds, and the contents of
their torn crops might thus readily get scattered. Some hawks and owls
bolt their prey whole, and after an interval of from twelve to twenty
hours, disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds of
the oat, wheat, millet, canary, hemp, clover, and beet germinated after
having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been
thus retained for two days and fourteen hours. Fresh-water fish, I
find, eat seeds of many land and water plants; fish are frequently
devoured by birds, and thus the seeds might be transported from place
to place. I forced many kinds of seeds into the stomachs of dead fish,
and then gave their bodies to fishing-eagles, storks, and pelicans;
these birds, after an interval of many hours, either rejected the seeds
in pellets or passed them in their excrement; and several of these
seeds retained the power of germination. Certain seeds, however, were
always killed by this process.

Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell
that in November, 1844, swarms of locusts visited the island of
Madeira. They were in countless numbers, as thick as the flakes of snow
in the heaviest snowstorm, and extended upward as far as could be seen
with a telescope. During two or three days they slowly careered round
and round in an immense ellipse, at least five or six miles in
diameter, and at night alighted on the taller trees, which were
completely coated with them. They then disappeared over the sea, as
suddenly as they had appeared, and have not since visited the island.
Now, in parts of Natal it is believed by some farmers, though on
insufficient evidence, that injurious seeds are introduced into their
grass-land in the dung left by the great flights of locusts which often
visit that country. In consequence of this belief Mr. Weale sent me in
a letter a small packet of the dried pellets, out of which I extracted
under the microscope several seeds, and raised from them seven grass
plants, belonging to two species, of two genera. Hence a swarm of
locusts, such as that which visited Madeira, might readily be the means
of introducing several kinds of plants into an island lying far from
the mainland.

Although the beaks and feet of birds are generally clean, earth
sometimes adheres to them: in one case I removed sixty-one grains, and
in another case twenty-two grains of dry argillaceous earth from the
foot of a partridge, and in the earth there was a pebble as large as
the seed of a vetch. Here is a better case: the leg of a woodcock was
sent to me by a friend, with a little cake of dry earth attached to the
shank, weighing only nine grains; and this contained a seed of the
toad-rush (Juncus bufonius) which germinated and flowered. Mr.
Swaysland, of Brighton, who during the last forty years has paid close
attention to our migratory birds, informs me that he has often shot
wagtails (Motacillæ), wheatears, and whinchats (Saxicolæ), on their
first arrival on our shores, before they had alighted; and he has
several times noticed little cakes of earth attached to their feet.
Many facts could be given showing how generally soil is charged with
seeds. For instance, Professor Newton sent me the leg of a red-legged
partridge (Caccabis rufa) which had been wounded and could not fly,
with a ball of hard earth adhering to it, and weighing six and a half
ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty-two plants
sprung from it: these consisted of twelve monocotyledons, including the
common oat, and at least one kind of grass, and of seventy
dicotyledons, which consisted, judging from the young leaves, of at
least three distinct species. With such facts before us, can we doubt
that the many birds which are annually blown by gales across great
spaces of ocean, and which annually migrate—for instance, the millions
of quails across the Mediterranean—must occasionally transport a few
seeds embedded in dirt adhering to their feet or beaks? But I shall
have to recur to this subject.

As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell,
have transported seeds from one part to another of the arctic and
antarctic regions; and during the Glacial period from one part of the
now temperate regions to another. In the Azores, from the large number
of plants common to Europe, in comparison with the species on the other
islands of the Atlantic, which stand nearer to the mainland, and (as
remarked by Mr. H.C. Watson) from their somewhat northern character, in
comparison with the latitude, I suspected that these islands had been
partly stocked by ice-borne seeds during the Glacial epoch. At my
request Sir C. Lyell wrote to M. Hartung to inquire whether he had
observed erratic boulders on these islands, and he answered that he had
found large fragments of granite and other rocks, which do not occur in
the archipelago. Hence we may safely infer that icebergs formerly
landed their rocky burdens on the shores of these mid-ocean islands,
and it is at least possible that they may have brought thither the
seeds of northern plants.

Considering that these several means of transport, and that other
means, which without doubt remain to be discovered, have been in action
year after year for tens of thousands of years, it would, I think, be a
marvellous fact if many plants had not thus become widely transported.
These means of transport are sometimes called accidental, but this is
not strictly correct: the currents of the sea are not accidental, nor
is the direction of prevalent gales of wind. It should be observed that
scarcely any means of transport would carry seeds for very great
distances; for seeds do not retain their vitality when exposed for a
great length of time to the action of sea water; nor could they be long
carried in the crops or intestines of birds. These means, however,
would suffice for occasional transport across tracts of sea some
hundred miles in breadth, or from island to island, or from a continent
to a neighbouring island, but not from one distant continent to
another. The floras of distant continents would not by such means
become mingled; but would remain as distinct as they now are. The
currents, from their course, would never bring seeds from North America
to Britain, though they might and do bring seeds from the West Indies
to our western shores, where, if not killed by their very long
immersion in salt water, they could not endure our climate. Almost
every year, one or two land-birds are blown across the whole Atlantic
Ocean, from North America to the western shores of Ireland and England;
but seeds could be transported by these rare wanderers only by one
means, namely, by dirt adhering to their feet or beaks, which is in
itself a rare accident. Even in this case, how small would be the
chance of a seed falling on favourable soil, and coming to maturity!
But it would be a great error to argue that because a well-stocked
island, like Great Britain, has not, as far as is known (and it would
be very difficult to prove this), received within the last few
centuries, through occasional means of transport, immigrants from
Europe or any other continent, that a poorly-stocked island, though
standing more remote from the mainland, would not receive colonists by
similar means. Out of a hundred kinds of seeds or animals transported
to an island, even if far less well-stocked than Britain, perhaps not
more than one would be so well fitted to its new home, as to become
naturalised. But this is no valid argument against what would be
effected by occasional means of transport, during the long lapse of
geological time, whilst the island was being upheaved, and before it
had become fully stocked with inhabitants. On almost bare land, with
few or no destructive insects or birds living there, nearly every seed
which chanced to arrive, if fitted for the climate, would germinate and
survive.

_Dispersal during the Glacial Period._


The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where Alpine species
could not possibly exist, is one of the most striking cases known of
the same species living at distant points, without the apparent
possibility of their having migrated from one point to the other. It is
indeed a remarkable fact to see so many plants of the same species
living on the snowy regions of the Alps or Pyrenees, and in the extreme
northern parts of Europe; but it is far more remarkable, that the
plants on the White Mountains, in the United States of America, are all
the same with those of Labrador, and nearly all the same, as we hear
from Asa Gray, with those on the loftiest mountains of Europe. Even as
long ago as 1747, such facts led Gmelin to conclude that the same
species must have been independently created at many distinct points;
and we might have remained in this same belief, had not Agassiz and
others called vivid attention to the Glacial period, which, as we shall
immediately see, affords a simple explanation of these facts. We have
evidence of almost every conceivable kind, organic and inorganic, that,
within a very recent geological period, central Europe and North
America suffered under an Arctic climate. The ruins of a house burnt by
fire do not tell their tale more plainly than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and
perched boulders, of the icy streams with which their valleys were
lately filled. So greatly has the climate of Europe changed, that in
Northern Italy, gigantic moraines, left by old glaciers, are now
clothed by the vine and maize. Throughout a large part of the United
States, erratic boulders and scored rocks plainly reveal a former cold
period.

The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially
as follows. But we shall follow the changes more readily, by supposing
a new glacial period slowly to come on, and then pass away, as formerly
occurred. As the cold came on, and as each more southern zone became
fitted for the inhabitants of the north, these would take the places of
the former inhabitants of the temperate regions. The latter, at the
same time would travel further and further southward, unless they were
stopped by barriers, in which case they would perish. The mountains
would become covered with snow and ice, and their former Alpine
inhabitants would descend to the plains. By the time that the cold had
reached its maximum, we should have an arctic fauna and flora, covering
the central parts of Europe, as far south as the Alps and Pyrenees, and
even stretching into Spain. The now temperate regions of the United
States would likewise be covered by arctic plants and animals and these
would be nearly the same with those of Europe; for the present
circumpolar inhabitants, which we suppose to have everywhere travelled
southward, are remarkably uniform round the world.

As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending, as the warmth increased and the snow still
further disappeared, higher and higher, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same species, which had lately lived together on the
European and North American lowlands, would again be found in the
arctic regions of the Old and New Worlds, and on many isolated
mountain-summits far distant from each other.

Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the first migration
when the cold came on, and the re-migration on the returning warmth,
would generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H.C. Watson, and those of the
Pyrenees, as remarked by Ramond, are more especially allied to the
plants of northern Scandinavia; those of the United States to Labrador;
those of the mountains of Siberia to the arctic regions of that
country. These views, grounded as they are on the perfectly
well-ascertained occurrence of a former Glacial period, seem to me to
explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in other
regions we find the same species on distant mountain-summits, we may
almost conclude, without other evidence, that a colder climate formerly
permitted their migration across the intervening lowlands, now become
too warm for their existence.

As the arctic forms moved first southward and afterwards backward to
the north, in unison with the changing climate, they will not have been
exposed during their long migrations to any great diversity of
temperature; and as they all migrated in a body together, their mutual
relations will not have been much disturbed. Hence, in accordance with
the principles inculcated in this volume, these forms will not have
been liable to much modification. But with the Alpine productions, left
isolated from the moment of the returning warmth, first at the bases
and ultimately on the summits of the mountains, the case will have been
somewhat different; for it is not likely that all the same arctic
species will have been left on mountain ranges far distant from each
other, and have survived there ever since; they will also, in all
probability, have become mingled with ancient Alpine species, which
must have existed on the mountains before the commencement of the
Glacial epoch, and which during the coldest period will have been
temporarily driven down to the plains; they will, also, have been
subsequently exposed to somewhat different climatical influences. Their
mutual relations will thus have been in some degree disturbed;
consequently they will have been liable to modification; and they have
been modified; for if we compare the present Alpine plants and animals
of the several great European mountain ranges, one with another, though
many of the species remain identically the same, some exist as
varieties, some as doubtful forms or sub-species and some as distinct
yet closely allied species representing each other on the several
ranges.

In the foregoing illustration, I have assumed that at the commencement
of our imaginary Glacial period, the arctic productions were as uniform
round the polar regions as they are at the present day. But it is also
necessary to assume that many sub-arctic and some few temperate forms
were the same round the world, for some of the species which now exist
on the lower mountain slopes and on the plains of North America and
Europe are the same; and it may be asked how I account for this degree
of uniformity of the sub-arctic and temperate forms round the world, at
the commencement of the real Glacial period. At the present day, the
sub-arctic and northern temperate productions of the Old and New Worlds
are separated from each other by the whole Atlantic Ocean and by the
northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than
they do at present, they must have been still more completely separated
from each other by wider spaces of ocean; so that it may well be asked
how the same species could then or previously have entered the two
continents. The explanation, I believe, lies in the nature of the
climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world
were specifically the same as now, and we have good reason to believe
that the climate was warmer than at the present day. Hence, we may
suppose that the organisms which now live under latitude 60°, lived
during the Pliocene period further north, under the Polar Circle, in
latitude 66°–67°; and that the present arctic productions then lived on
the broken land still nearer to the pole. Now, if we look at a
terrestrial globe, we see under the Polar Circle that there is almost
continuous land from western Europe through Siberia, to eastern
America. And this continuity of the circumpolar land, with the
consequent freedom under a more favourable climate for intermigration,
will account for the supposed uniformity of the sub-arctic and
temperate productions of the Old and New Worlds, at a period anterior
to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected to
great oscillations of level, I am strongly inclined to extend the above
view, and to infer that during some earlier and still warmer period,
such as the older Pliocene period, a large number of the same plants
and animals inhabited the almost continuous circumpolar land; and that
these plants and animals, both in the Old and New Worlds, began slowly
to migrate southwards as the climate became less warm, long before the
commencement of the Glacial period. We now see, as I believe, their
descendants, mostly in a modified condition, in the central parts of
Europe and the United States. On this view we can understand the
relationship with very little identity, between the productions of
North America and Europe—a relationship which is highly remarkable,
considering the distance of the two areas, and their separation by the
whole Atlantic Ocean. We can further understand the singular fact
remarked on by several observers that the productions of Europe and
America during the later tertiary stages were more closely related to
each other than they are at the present time; for during these warmer
periods the northern parts of the Old and New Worlds will have been
almost continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they will have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, must have taken place long ages ago. As the plants and
animals migrated southward, they will have become mingled in the one
great region with the native American productions, and would have had
to compete with them; and in the other great region, with those of the
Old World. Consequently we have here everything favourable for much
modification—for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain ranges and on the arctic lands of Europe and North
America. Hence, it has come, that when we compare the now living
productions of the temperate regions of the New and Old Worlds, we find
very few identical species (though Asa Gray has lately shown that more
plants are identical than was formerly supposed), but we find in every
great class many forms, which some naturalists rank as geographical
races, and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as
specifically distinct.

As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which, during the Pliocene or even a somewhat
earlier period, was nearly uniform along the continuous shores of the
Polar Circle, will account, on the theory of modification, for many
closely allied forms now living in marine areas completely sundered.
Thus, I think, we can understand the presence of some closely allied,
still existing and extinct tertiary forms, on the eastern and western
shores of temperate North America; and the still more striking fact of
many closely allied crustaceans (as described in Dana’s admirable
work), some fish and other marine animals, inhabiting the Mediterranean
and the seas of Japan—these two areas being now completely separated by
the breadth of a whole continent and by wide spaces of ocean.

These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America,
the Mediterranean and Japan, and the temperate lands of North America
and Europe, are inexplicable on the theory of creation. We cannot
maintain that such species have been created alike, in correspondence
with the nearly similar physical conditions of the areas; for if we
compare, for instance, certain parts of South America with parts of
South Africa or Australia, we see countries closely similar in all
their physical conditions, with their inhabitants utterly dissimilar.

_Alternate Glacial Periods in the North and South._


But we must return to our more immediate subject. I am convinced that
Forbes’s view may be largely extended. In Europe we meet with the
plainest evidence of the Glacial period, from the western shores of
Britain to the Ural range, and southward to the Pyrenees. We may infer
from the frozen mammals and nature of the mountain vegetation, that
Siberia was similarly affected. In the Lebanon, according to Dr.
Hooker, perpetual snow formerly covered the central axis, and fed
glaciers which rolled 4,000 feet down the valleys. The same observer
has recently found great moraines at a low level on the Atlas range in
North Africa. Along the Himalaya, at points 900 miles apart, glaciers
have left the marks of their former low descent; and in Sikkim, Dr.
Hooker saw maize growing on ancient and gigantic moraines. Southward of
the Asiatic continent, on the opposite side of the equator, we know,
from the excellent researches of Dr. J. Haast and Dr. Hector, that in
New Zealand immense glaciers formerly descended to a low level; and the
same plants, found by Dr. Hooker on widely separated mountains in this
island tell the same story of a former cold period. From facts
communicated to me by the Rev. W.B. Clarke, it appears also that there
are traces of former glacial action on the mountains of the
south-eastern corner of Australia.

Looking to America: in the northern half, ice-borne fragments of rock
have been observed on the eastern side of the continent, as far south
as latitude 36° and 37°, and on the shores of the Pacific, where the
climate is now so different, as far south as latitude 46°. Erratic
boulders have, also, been noticed on the Rocky Mountains. In the
Cordillera of South America, nearly under the equator, glaciers once
extended far below their present level. In central Chile I examined a
vast mound of detritus with great boulders, crossing the Portillo
valley, which, there can hardly be a doubt, once formed a huge moraine;
and Mr. D. Forbes informs me that he found in various parts of the
Cordillera, from latitude 13° to 30° south, at about the height of
12,000 feet, deeply-furrowed rocks, resembling those with which he was
familiar in Norway, and likewise great masses of detritus, including
grooved pebbles. Along this whole space of the Cordillera true glaciers
do not now exist even at much more considerable heights. Further south,
on both sides of the continent, from latitude 41° to the southernmost
extremity, we have the clearest evidence of former glacial action, in
numerous immense boulders transported far from their parent source.

From these several facts, namely, from the glacial action having
extended all round the northern and southern hemispheres—from the
period having been in a geological sense recent in both
hemispheres—from its having lasted in both during a great length of
time, as may be inferred from the amount of work effected—and lastly,
from glaciers having recently descended to a low level along the whole
line of the Cordillera, it at one time appeared to me that we could not
avoid the conclusion that the temperature of the whole world had been
simultaneously lowered during the Glacial period. But now, Mr. Croll,
in a series of admirable memoirs, has attempted to show that a glacial
condition of climate is the result of various physical causes, brought
into operation by an increase in the eccentricity of the earth’s orbit.
All these causes tend towards the same end; but the most powerful
appears to be the indirect influence of the eccentricity of the orbit
upon oceanic currents. According to Mr. Croll, cold periods regularly
recur every ten or fifteen thousand years; and these at long intervals
are extremely severe, owing to certain contingencies, of which the most
important, as Sir C. Lyell has shown, is the relative position of the
land and water. Mr. Croll believes that the last great glacial period
occurred about 240,000 years ago, and endured, with slight alterations
of climate, for about 160,000 years. With respect to more ancient
glacial periods, several geologists are convinced, from direct
evidence, that such occurred during the miocene and eocene formations,
not to mention still more ancient formations. But the most important
result for us, arrived at by Mr. Croll, is that whenever the northern
hemisphere passes through a cold period the temperature of the southern
hemisphere is actually raised, with the winters rendered much milder,
chiefly through changes in the direction of the ocean currents. So
conversely it will be with the northern hemisphere, while the southern
passes through a glacial period. This conclusion throws so much light
on geographical distribution that I am strongly inclined to trust in
it; but I will first give the facts which demand an explanation.

In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common
to North America and Europe, enormously remote as these areas in
opposite hemispheres are from each other. On the lofty mountains of
equatorial America a host of peculiar species belonging to European
genera occur. On the Organ Mountains of Brazil some few temperate
European, some Antarctic and some Andean genera were found by Gardner
which do not exist in the low intervening hot countries. On the Silla
of Caraccas the illustrious Humboldt long ago found species belonging
to genera characteristic of the Cordillera.

In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the
mountains several representative European forms are found which have
not been discovered in the intertropical parts of Africa. Dr. Hooker
has also lately shown that several of the plants living on the upper
parts of the lofty island of Fernando Po, and on the neighbouring
Cameroon Mountains, in the Gulf of Guinea, are closely related to those
on the mountains of Abyssinia, and likewise to those of temperate
Europe. It now also appears, as I hear from Dr. Hooker, that some of
these same temperate plants have been discovered by the Rev. R.T. Lowe
on the mountains of the Cape Verde Islands. This extension of the same
temperate forms, almost under the equator, across the whole continent
of Africa and to the mountains of the Cape Verde archipelago, is one of
the most astonishing facts ever recorded in the distribution of plants.

On the Himalaya, and on the isolated mountain ranges of the peninsula
of India, on the heights of Ceylon, and on the volcanic cones of Java,
many plants occur either identically the same or representing each
other, and at the same time representing plants of Europe not found in
the intervening hot lowlands. A list of the genera of plants collected
on the loftier peaks of Java, raises a picture of a collection made on
a hillock in Europe. Still more striking is the fact that peculiar
Australian forms are represented by certain plants growing on the
summits of the mountains of Borneo. Some of these Australian forms, as
I hear from Dr. Hooker, extend along the heights of the peninsula of
Malacca, and are thinly scattered on the one hand over India, and on
the other hand as far north as Japan.

On the southern mountains of Australia, Dr. F. Müller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable “Introduction to the
Flora of New Zealand,” by Dr. Hooker, analogous and striking facts are
given in regard to the plants of that large island. Hence, we see that
certain plants growing on the more lofty mountains of the tropics in
all parts of the world, and on the temperate plains of the north and
south, are either the same species or varieties of the same species. It
should, however, be observed that these plants are not strictly arctic
forms; for, as Mr. H.C. Watson has remarked, “in receding from polar
toward equatorial latitudes, the Alpine or mountain flora really become
less and less Arctic.” Besides these identical and closely allied
forms, many species inhabiting the same widely sundered areas, belong
to genera not now found in the intermediate tropical lowlands.

These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement by
the highest authority, Prof. Dana, that “it is certainly a wonderful
fact that New Zealand should have a closer resemblance in its crustacea
to Great Britain, its antipode, than to any other part of the world.”
Sir J. Richardson, also, speaks of the reappearance on the shores of
New Zealand, Tasmania, &c., of northern forms of fish. Dr. Hooker
informs me that twenty-five species of Algæ are common to New Zealand
and to Europe, but have not been found in the intermediate tropical
seas.

From the foregoing facts, namely, the presence of temperate forms on
the highlands across the whole of equatorial Africa, and along the
Peninsula of India, to Ceylon and the Malay Archipelago, and in a less
well-marked manner across the wide expanse of tropical South America,
it appears almost certain that at some former period, no doubt during
the most severe part of a Glacial period, the lowlands of these great
continents were everywhere tenanted under the equator by a considerable
number of temperate forms. At this period the equatorial climate at the
level of the sea was probably about the same with that now experienced
at the height of from five to six thousand feet under the same
latitude, or perhaps even rather cooler. During this, the coldest
period, the lowlands under the equator must have been clothed with a
mingled tropical and temperate vegetation, like that described by
Hooker as growing luxuriantly at the height of from four to five
thousand feet on the lower slopes of the Himalaya, but with perhaps a
still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann
found temperate European forms beginning to appear at the height of
about five thousand feet. On the mountains of Panama, at the height of
only two thousand feet, Dr. Seemann found the vegetation like that of
Mexico, “with forms of the torrid zone harmoniously blended with those
of the temperate.”

Now let us see whether Mr. Croll’s conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period,
the southern hemisphere was actually warmer, throws any clear light on
the present apparently inexplicable distribution of various organisms
in the temperate parts of both hemispheres, and on the mountains of the
tropics. The Glacial period, as measured by years, must have been very
long; and when we remember over what vast spaces some naturalised
plants and animals have spread within a few centuries, this period will
have been ample for any amount of migration. As the cold became more
and more intense, we know that Arctic forms invaded the temperate
regions; and from the facts just given, there can hardly be a doubt
that some of the more vigorous, dominant and widest-spreading temperate
forms invaded the equatorial lowlands. The inhabitants of these hot
lowlands would at the same time have migrated to the tropical and
subtropical regions of the south, for the southern hemisphere was at
this period warmer. On the decline of the Glacial period, as both
hemispheres gradually recovered their former temperature, the northern
temperate forms living on the lowlands under the equator, would have
been driven to their former homes or have been destroyed, being
replaced by the equatorial forms returning from the south. Some,
however, of the northern temperate forms would almost certainly have
ascended any adjoining high land, where, if sufficiently lofty, they
would have long survived like the Arctic forms on the mountains of
Europe. They might have survived, even if the climate was not perfectly
fitted for them, for the change of temperature must have been very
slow, and plants undoubtedly possess a certain capacity for
acclimatisation, as shown by their transmitting to their offspring
different constitutional powers of resisting heat and cold.

In the regular course of events the southern hemisphere would in its
turn be subjected to a severe Glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms would
invade the equatorial lowlands. The northern forms which had before
been left on the mountains would now descend and mingle with the
southern forms. These latter, when the warmth returned, would return to
their former homes, leaving some few species on the mountains, and
carrying southward with them some of the northern temperate forms which
had descended from their mountain fastnesses. Thus, we should have some
few species identically the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical regions. But
the species left during a long time on these mountains, or in opposite
hemispheres, would have to compete with many new forms and would be
exposed to somewhat different physical conditions; hence, they would be
eminently liable to modification, and would generally now exist as
varieties or as representative species; and this is the case. We must,
also, bear in mind the occurrence in both hemispheres of former Glacial
periods; for these will account, in accordance with the same
principles, for the many quite distinct species inhabiting the same
widely separated areas, and belonging to genera not now found in the
intermediate torrid zones.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical or slightly modified species have migrated from the
north to the south, than in a reversed direction. We see, however, a
few southern forms on the mountains of Borneo and Abyssinia. I suspect
that this preponderant migration from the north to the south is due to
the greater extent of land in the north, and to the northern forms
having existed in their own homes in greater numbers, and having
consequently been advanced through natural selection and competition to
a higher stage of perfection, or dominating power, than the southern
forms. And thus, when the two sets became commingled in the equatorial
regions, during the alternations of the Glacial periods, the northern
forms were the more powerful and were able to hold their places on the
mountains, and afterwards migrate southward with the southern forms;
but not so the southern in regard to the northern forms. In the same
manner, at the present day, we see that very many European productions
cover the ground in La Plata, New Zealand, and to a lesser degree in
Australia, and have beaten the natives; whereas extremely few southern
forms have become naturalised in any part of the northern hemisphere,
though hides, wool, and other objects likely to carry seeds have been
largely imported into Europe during the last two or three centuries
from La Plata and during the last forty or fifty years from Australia.
The Neilgherrie Mountains in India, however, offer a partial exception;
for here, as I hear from Dr. Hooker, Australian forms are rapidly
sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere yielded to the
more dominant forms generated in the larger areas and more efficient
workshops of the north. In many islands the native productions are
nearly equalled, or even outnumbered, by those which have become
naturalised; and this is the first stage towards their extinction.
Mountains are islands on the land; and their inhabitants have yielded
to those produced within the larger areas of the north, just in the
same way as the inhabitants of real islands have everywhere yielded and
are still yielding to continental forms naturalised through man’s
agency.

The same principles apply to the distribution of terrestrial animals
and of marine productions, in the northern and southern temperate
zones, and on the intertropical mountains. When, during the height of
the Glacial period, the ocean-currents were widely different to what
they now are, some of the inhabitants of the temperate seas might have
reached the equator; of these a few would perhaps at once be able to
migrate southwards, by keeping to the cooler currents, while others
might remain and survive in the colder depths until the southern
hemisphere was in its turn subjected to a glacial climate and permitted
their further progress; in nearly the same manner as, according to
Forbes, isolated spaces inhabited by Arctic productions exist to the
present day in the deeper parts of the northern temperate seas.

I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes on
the intermediate mountain ranges, are removed on the views above given.
The exact lines of migration cannot be indicated. We cannot say why
certain species and not others have migrated; why certain species have
been modified and have given rise to new forms, while others have
remained unaltered. We cannot hope to explain such facts, until we can
say why one species and not another becomes naturalised by man’s agency
in a foreign land; why one species ranges twice or thrice as far, and
is twice or thrice as common, as another species within their own
homes.

Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the
southern hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remarkable case.
Some of these species are so distinct, that we cannot suppose that
there has been time since the commencement of the last Glacial period
for their migration and subsequent modification to the necessary
degree. The facts seem to indicate that distinct species belonging to
the same genera have migrated in radiating lines from a common centre;
and I am inclined to look in the southern, as in the northern
hemisphere, to a former and warmer period, before the commencement of
the last Glacial period, when the Antarctic lands, now covered with
ice, supported a highly peculiar and isolated flora. It may be
suspected that before this flora was exterminated during the last
Glacial epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of transport, and
by the aid, as halting-places, of now sunken islands. Thus the southern
shores of America, Australia, and New Zealand may have become slightly
tinted by the same peculiar forms of life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll’s conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species,
explains a multitude of facts in the distribution of the same and of
the allied forms of life in all parts of the globe. The living waters
have flowed during one period from the north and during another from
the south, and in both cases have reached the equator; but the stream
of life has flowed with greater force from the north than in the
opposite direction, and has consequently more freely inundated the
south. As the tide leaves its drift in horizontal lines, rising higher
on the shores where the tide rises highest, so have the living waters
left their living drift on our mountain summits, in a line gently
rising from the Arctic lowlands to a great latitude under the equator.
The various beings thus left stranded may be compared with savage races
of man, driven up and surviving in the mountain fastnesses of almost
every land, which serves as a record, full of interest to us, of the
former inhabitants of the surrounding lowlands.




CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION—_continued_.


Distribution of fresh-water productions—On the inhabitants of oceanic
islands—Absence of Batrachians and of terrestrial Mammals—On the
relation of the inhabitants of islands to those of the nearest
mainland—On colonisation from the nearest source with subsequent
modification—Summary of the last and present chapters.


_Fresh-water Productions._


As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is
apparently a still more formidable barrier, that they would never have
extended to distant countries. But the case is exactly the reverse. Not
only have many fresh-water species, belonging to different classes, an
enormous range, but allied species prevail in a remarkable manner
throughout the world. When first collecting in the fresh waters of
Brazil, I well remember feeling much surprise at the similarity of the
fresh-water insects, shells, &c., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.

But the wide ranging power of fresh-water productions can, I think, in
most cases be explained by their having become fitted, in a manner
highly useful to them, for short and frequent migrations from pond to
pond, or from stream to stream, within their own countries; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases; of
these, some of the most difficult to explain are presented by fish. It
was formerly believed that the same fresh-water species never existed
on two continents distant from each other. But Dr. Günther has lately
shown that the Galaxias attenuatus inhabits Tasmania, New Zealand, the
Falkland Islands and the mainland of South America. This is a wonderful
case, and probably indicates dispersal from an Antarctic centre during
a former warm period. This case, however, is rendered in some degree
less surprising by the species of this genus having the power of
crossing by some unknown means considerable spaces of open ocean: thus
there is one species common to New Zealand and to the Auckland Islands,
though separated by a distance of about 230 miles. On the same
continent fresh-water fish often range widely, and as if capriciously;
for in two adjoining river systems some of the species may be the same
and some wholly different.

It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely
dropped at distant points by whirlwinds; and it is known that the ova
retain their vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attributed to changes in
the level of the land within the recent period, causing rivers to flow
into each other. Instances, also, could be given of this having
occurred during floods, without any change of level. The wide
differences of the fish on the opposite sides of most mountain-ranges,
which are continuous and consequently must, from an early period, have
completely prevented the inosculation of the river systems on the two
sides, leads to the same conclusion. Some fresh-water fish belong to
very ancient forms, and in such cases there will have been ample time
for great geographical changes, and consequently time and means for
much migration. Moreover, Dr. Günther has recently been led by several
considerations to infer that with fishes the same forms have a long
endurance. Salt-water fish can with care be slowly accustomed to live
in fresh water; and, according to Valenciennes, there is hardly a
single group of which all the members are confined to fresh water, so
that a marine species belonging to a fresh-water group might travel far
along the shores of the sea, and could, it is probable, become adapted
without much difficulty to the fresh waters of a distant land.

Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent, and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not
likely to be transported by birds; and the ova, as well as the adults,
are immediately killed by sea-water. I could not even understand how
some naturalised species have spread rapidly throughout the same
country. But two facts, which I have observed—and many others no doubt
will be discovered—throw some light on this subject. When ducks
suddenly emerge from a pond covered with duck-weed, I have twice seen
these little plants adhering to their backs; and it has happened to me,
in removing a little duck-weed from one aquarium to another, that I
have unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended the
feet of a duck in an aquarium, where many ova of fresh-water shells
were hatching; and I found that numbers of the extremely minute and
just-hatched shells crawled on the feet, and clung to them so firmly
that when taken out of the water they could not be jarred off, though
at a somewhat more advanced age they would voluntarily drop off. These
just-hatched molluscs, though aquatic in their nature, survived on the
duck’s feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and if blown across the sea to an oceanic island, or to any
other distant point, would be sure to alight on a pool or rivulet. Sir
Charles Lyell informs me that a Dyticus has been caught with an Ancylus
(a fresh-water shell like a limpet) firmly adhering to it; and a
water-beetle of the same family, a Colymbetes, once flew on board the
“Beagle,” when forty-five miles distant from the nearest land: how much
farther it might have been blown by a favouring gale no one can tell.

With respect to plants, it has long been known what enormous ranges
many fresh-water, and even marsh-species, have, both over continents
and to the most remote oceanic islands. This is strikingly illustrated,
according to Alph. de Candolle, in those large groups of terrestrial
plants, which have very few aquatic members; for the latter seem
immediately to acquire, as if in consequence, a wide range. I think
favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally adheres in some quantity to the feet
and beaks of birds. Wading birds, which frequent the muddy edges of
ponds, if suddenly flushed, would be the most likely to have muddy
feet. Birds of this order wander more than those of any other; and are
occasionally found on the most remote and barren islands of the open
ocean; they would not be likely to alight on the surface of the sea, so
that any dirt on their feet would not be washed off; and when gaining
the land, they would be sure to fly to their natural fresh-water
haunts. I do not believe that botanists are aware how charged the mud
of ponds is with seeds: I have tried several little experiments, but
will here give only the most striking case: I took in February three
tablespoonfuls of mud from three different points, beneath water, on
the edge of a little pond; this mud when dry weighed only 6 and 3/4
ounces; I kept it covered up in my study for six months, pulling up and
counting each plant as it grew; the plants were of many kinds, and were
altogether 537 in number; and yet the viscid mud was all contained in a
breakfast cup! Considering these facts, I think it would be an
inexplicable circumstance if water-birds did not transport the seeds of
fresh-water plants to unstocked ponds and streams, situated at very
distant points. The same agency may have come into play with the eggs
of some of the smaller fresh-water animals.

Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have gone
on daily devouring fish; they then take flight and go to other waters,
or are blown across the sea; and we have seen that seeds retain their
power of germination, when rejected many hours afterwards in pellets or
in the excrement. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle’s remarks
on the distribution of this plant, I thought that the means of its
dispersal must remain inexplicable; but Audubon states that he found
the seeds of the great southern water-lily (probably according to Dr.
Hooker, the Nelumbium luteum) in a heron’s stomach. Now this bird must
often have flown with its stomach thus well stocked to distant ponds,
and, then getting a hearty meal of fish, analogy makes me believe that
it would have rejected the seeds in the pellet in a fit state for
germination.

In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance on
a rising islet, it will be unoccupied; and a single seed or egg will
have a good chance of succeeding. Although there will always be a
struggle for life between the inhabitants of the same pond, however few
in kind, yet as the number even in a well-stocked pond is small in
comparison with the number of species inhabiting an equal area of land,
the competition between them will probably be less severe than between
terrestrial species; consequently an intruder from the waters of a
foreign country would have a better chance of seizing on a new place,
than in the case of terrestrial colonists. We should also remember that
many fresh-water productions are low in the scale of nature, and we
have reason to believe that such beings become modified more slowly
than the high; and this will give time for the migration of aquatic
species. We should not forget the probability of many fresh-water forms
having formerly ranged continuously over immense areas, and then having
become extinct at intermediate points. But the wide distribution of
fresh-water plants, and of the lower animals, whether retaining the
same identical form, or in some degree modified, apparently depends in
main part on the wide dispersal of their seeds and eggs by animals,
more especially by fresh-water birds, which have great powers of
flight, and naturally travel from one piece of water to another.

_On the Inhabitants of Oceanic Islands._


We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect
to distribution, on the view that not only all the individuals of the
same species have migrated from some one area, but that allied species,
although now inhabiting the most distant points, have proceeded from a
single area, the birthplace of their early progenitors. I have already
given my reasons for disbelieving in continental extensions within the
period of existing species on so enormous a scale that all the many
islands of the several oceans were thus stocked with their present
terrestrial inhabitants. This view removes many difficulties, but it
does not accord with all the facts in regard to the productions of
islands. In the following remarks I shall not confine myself to the
mere question of dispersal, but shall consider some other cases bearing
on the truth of the two theories of independent creation and of descent
with modification.

The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and Wollaston for insects. New
Zealand, for instance, with its lofty mountains and diversified
stations, extending over 780 miles of latitude, together with the
outlying islands of Auckland, Campbell and Chatham, contain altogether
only 960 kinds of flowering plants; if we compare this moderate number
with the species which swarm over equal areas in Southwestern Australia
or at the Cape of Good Hope, we must admit that some cause,
independently of different physical conditions, has given rise to so
great a difference in number. Even the uniform county of Cambridge has
847 plants, and the little island of Anglesea 764, but a few ferns and
a few introduced plants are included in these numbers, and the
comparison in some other respects is not quite fair. We have evidence
that the barren island of Ascension aboriginally possessed less than
half-a-dozen flowering plants; yet many species have now become
naturalised on it, as they have in New Zealand and on every other
oceanic island which can be named. In St. Helena there is reason to
believe that the naturalised plants and animals have nearly or quite
exterminated many native productions. He who admits the doctrine of the
creation of each separate species, will have to admit that a sufficient
number of the best adapted plants and animals were not created for
oceanic islands; for man has unintentionally stocked them far more
fully and perfectly than did nature.

Although in oceanic islands the species are few in number, the
proportion of endemic kinds (_i.e._ those found nowhere else in the
world) is often extremely large. If we compare, for instance, the
number of endemic land-shells in Madeira, or of endemic birds in the
Galapagos Archipelago, with the number found on any continent, and then
compare the area of the island with that of the continent, we shall see
that this is true. This fact might have been theoretically expected,
for, as already explained, species occasionally arriving, after long
intervals of time in the new and isolated district, and having to
compete with new associates, would be eminently liable to modification,
and would often produce groups of modified descendants. But it by no
means follows that, because in an island nearly all the species of one
class are peculiar, those of another class, or of another section of
the same class, are peculiar; and this difference seems to depend
partly on the species which are not modified having immigrated in a
body, so that their mutual relations have not been much disturbed; and
partly on the frequent arrival of unmodified immigrants from the
mother-country, with which the insular forms have intercrossed. It
should be borne in mind that the offspring of such crosses would
certainly gain in vigour; so that even an occasional cross would
produce more effect than might have been anticipated. I will give a few
illustrations of the foregoing remarks: in the Galapagos Islands there
are twenty-six land birds; of these twenty-one (or perhaps
twenty-three) are peculiar; whereas of the eleven marine birds only two
are peculiar; and it is obvious that marine birds could arrive at these
islands much more easily and frequently than land-birds. Bermuda, on
the other hand, which lies at about the same distance from North
America as the Galapagos Islands do from South America, and which has a
very peculiar soil, does not possess a single endemic land bird; and we
know from Mr. J.M. Jones’s admirable account of Bermuda, that very many
North American birds occasionally or even frequently visit this island.
Almost every year, as I am informed by Mr. E.V. Harcourt, many European
and African birds are blown to Madeira; this island is inhabited by
ninety-nine kinds, of which one alone is peculiar, though very closely
related to a European form; and three or four other species are
confined to this island and to the Canaries. So that the islands of
Bermuda and Madeira have been stocked from the neighbouring continents
with birds, which for long ages have there struggled together, and have
become mutually co-adapted. Hence, when settled in their new homes,
each kind will have been kept by the others to its proper place and
habits, and will consequently have been but little liable to
modification. Any tendency to modification will also have been checked
by intercrossing with the unmodified immigrants, often arriving from
the mother-country. Madeira again is inhabited by a wonderful number of
peculiar land-shells, whereas not one species of sea-shell is peculiar
to its shores: now, though we do not know how sea-shells are dispersed,
yet we can see that their eggs or larvæ, perhaps attached to seaweed or
floating timber, or to the feet of wading birds, might be transported
across three or four hundred miles of open sea far more easily than
land-shells. The different orders of insects inhabiting Madeira present
nearly parallel cases.

Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand is
here spoken of as an oceanic island, it is in some degree doubtful
whether it should be so ranked; it is of large size, and is not
separated from Australia by a profoundly deep sea; from its geological
character and the direction of its mountain ranges, the Rev. W.B.
Clarke has lately maintained that this island, as well as New
Caledonia, should be considered as appurtenances of Australia. Turning
to plants, Dr. Hooker has shown that in the Galapagos Islands the
proportional numbers of the different orders are very different from
what they are elsewhere. All such differences in number, and the
absence of certain whole groups of animals and plants, are generally
accounted for by supposed differences in the physical conditions of the
islands; but this explanation is not a little doubtful. Facility of
immigration seems to have been fully as important as the nature of the
conditions.

Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands not
tenanted by a single mammal, some of the endemic plants have
beautifully hooked seeds; yet few relations are more manifest than that
hooks serve for the transportal of seeds in the wool or fur of
quadrupeds. But a hooked seed might be carried to an island by other
means; and the plant then becoming modified would form an endemic
species, still retaining its hooks, which would form a useless
appendage, like the shrivelled wings under the soldered wing-covers of
many insular beetles. Again, islands often possess trees or bushes
belonging to orders which elsewhere include only herbaceous species;
now trees, as Alph. de Candolle has shown, generally have, whatever the
cause may be, confined ranges. Hence trees would be little likely to
reach distant oceanic islands; and an herbaceous plant, which had no
chance of successfully competing with the many fully developed trees
growing on a continent, might, when established on an island, gain an
advantage over other herbaceous plants by growing taller and taller and
overtopping them. In this case, natural selection would tend to add to
the stature of the plant, to whatever order it belonged, and thus first
convert it into a bush and then into a tree.

_Absence of Batrachians and Terrestrial mammals on Oceanic Islands._


With respect to the absence of whole orders of animals on oceanic
islands, Bory St. Vincent long ago remarked that Batrachians (frogs,
toads, newts) are never found on any of the many islands with which the
great oceans are studded. I have taken pains to verify this assertion,
and have found it true, with the exception of New Zealand, New
Caledonia, the Andaman Islands, and perhaps the Solomon Islands and the
Seychelles. But I have already remarked that it is doubtful whether New
Zealand and New Caledonia ought to be classed as oceanic islands; and
this is still more doubtful with respect to the Andaman and Solomon
groups and the Seychelles. This general absence of frogs, toads and
newts on so many true oceanic islands cannot be accounted for by their
physical conditions; indeed it seems that islands are peculiarly fitted
for these animals; for frogs have been introduced into Madeira, the
Azores, and Mauritius, and have multiplied so as to become a nuisance.
But as these animals and their spawn are immediately killed (with the
exception, as far as known, of one Indian species) by sea-water, there
would be great difficulty in their transportal across the sea, and
therefore we can see why they do not exist on strictly oceanic islands.
But why, on the theory of creation, they should not have been created
there, it would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt,
of a terrestrial mammal (excluding domesticated animals kept by the
natives) inhabiting an island situated above 300 miles from a continent
or great continental island; and many islands situated at a much less
distance are equally barren. The Falkland Islands, which are inhabited
by a wolf-like fox, come nearest to an exception; but this group cannot
be considered as oceanic, as it lies on a bank in connection with the
mainland at a distance of about 280 miles; moreover, icebergs formerly
brought boulders to its western shores, and they may have formerly
transported foxes, as now frequently happens in the arctic regions. Yet
it cannot be said that small islands will not support at least small
mammals, for they occur in many parts of the world on very small
islands, when lying close to a continent; and hardly an island can be
named on which our smaller quadrupeds have not become naturalised and
greatly multiplied. It cannot be said, on the ordinary view of
creation, that there has not been time for the creation of mammals;
many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered, and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is known that new
species of mammals appear and disappear at a quicker rate than other
and lower animals. Although terrestrial mammals do not occur on oceanic
islands, aërial mammals do occur on almost every island. New Zealand
possesses two bats found nowhere else in the world: Norfolk Island, the
Viti Archipelago, the Bonin Islands, the Caroline and Marianne
Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it
may be asked, has the supposed creative force produced bats and no
other mammals on remote islands? On my view this question can easily be
answered; for no terrestrial mammal can be transported across a wide
space of sea, but bats can fly across. Bats have been seen wandering by
day far over the Atlantic Ocean; and two North American species, either
regularly or occasionally, visit Bermuda, at the distance of 600 miles
from the mainland. I hear from Mr. Tomes, who has specially studied
this family, that many species have enormous ranges, and are found on
continents and on far distant islands. Hence, we have only to suppose
that such wandering species have been modified in their new homes in
relation to their new position, and we can understand the presence of
endemic bats on oceanic islands, with the absence of all other
terrestrial mammals.

Another interesting relation exists, namely, between the depth of the
sea separating islands from each other, or from the nearest continent,
and the degree of affinity of their mammalian inhabitants. Mr. Windsor
Earl has made some striking observations on this head, since greatly
extended by Mr. Wallace’s admirable researches, in regard to the great
Malay Archipelago, which is traversed near Celebes by a space of deep
ocean, and this separates two widely distinct mammalian faunas. On
either side, the islands stand on a moderately shallow submarine bank,
and these islands are inhabited by the same or by closely allied
quadrupeds. I have not as yet had time to follow up this subject in all
quarters of the world; but as far as I have gone, the relation holds
good. For instance, Britain is separated by a shallow channel from
Europe, and the mammals are the same on both sides; and so it is with
all the islands near the shores of Australia. The West Indian Islands,
on the other hand, stand on a deeply submerged bank, nearly one
thousand fathoms in depth, and here we find American forms, but the
species and even the genera are quite distinct. As the amount of
modification which animals of all kinds undergo partly depends on the
lapse of time, and as the islands which are separated from each other,
or from the mainland, by shallow channels, are more likely to have been
continuously united within a recent period than the islands separated
by deeper channels, we can understand how it is that a relation exists
between the depth of the sea separating two mammalian faunas, and the
degree of their affinity, a relation which is quite inexplicable on the
theory of independent acts of creation.

The foregoing statements in regard to the inhabitants of oceanic
islands, namely, the fewness of the species, with a large proportion
consisting of endemic forms—the members of certain groups, but not
those of other groups in the same class, having been modified—the
absence of certain whole orders, as of batrachians and of terrestrial
mammals, notwithstanding the presence of aërial bats, the singular
proportions of certain orders of plants, herbaceous forms having been
developed into trees, &c., seem to me to accord better with the belief
in the efficiency of occasional means of transport, carried on during a
long course of time, than with the belief in the former connection of
all oceanic islands with the nearest continent; for on this latter view
it is probable that the various classes would have immigrated more
uniformly, and from the species having entered in a body, their mutual
relations would not have been much disturbed, and consequently, they
would either have not been modified, or all the species in a more
equable manner.

I do not deny that there are many and serious difficulties in
understanding how many of the inhabitants of the more remote islands,
whether still retaining the same specific form or subsequently
modified, have reached their present homes. But the probability of
other islands having once existed as halting-places, of which not a
wreck now remains, must not be overlooked. I will specify one difficult
case. Almost all oceanic islands, even the most isolated and smallest,
are inhabited by land-shells, generally by endemic species, but
sometimes by species found elsewhere striking instances of which have
been given by Dr. A.A. Gould in relation to the Pacific. Now it is
notorious that land-shells are easily killed by sea-water; their eggs,
at least such as I have tried, sink in it and are killed. Yet there
must be some unknown, but occasionally efficient means for their
transportal. Would the just-hatched young sometimes adhere to the feet
of birds roosting on the ground and thus get transported? It occurred
to me that land-shells, when hybernating and having a membranous
diaphragm over the mouth of the shell, might be floated in chinks of
drifted timber across moderately wide arms of the sea. And I find that
several species in this state withstand uninjured an immersion in
sea-water during seven days. One shell, the Helix pomatia, after having
been thus treated, and again hybernating, was put into sea-water for
twenty days and perfectly recovered. During this length of time the
shell might have been carried by a marine country of average swiftness
to a distance of 660 geographical miles. As this Helix has a thick
calcareous operculum I removed it, and when it had formed a new
membranous one, I again immersed it for fourteen days in sea-water, and
again it recovered and crawled away. Baron Aucapitaine has since tried
similar experiments. He placed 100 land-shells, belonging to ten
species, in a box pierced with holes, and immersed it for a fortnight
in the sea. Out of the hundred shells twenty-seven recovered. The
presence of an operculum seems to have been of importance, as out of
twelve specimens of Cyclostoma elegans, which is thus furnished, eleven
revived. It is remarkable, seeing how well the Helix pomatia resisted
with me the salt-water, that not one of fifty-four specimens belonging
to four other species of Helix tried by Aucapitaine recovered. It is,
however, not at all probable that land-shells have often been thus
transported; the feet of birds offer a more probable method.

_On the Relations of the Inhabitants of Islands to those of the nearest
Mainland._


The most striking and important fact for us is the affinity of the
species which inhabit islands to those of the nearest mainland, without
being actually the same. Numerous instances could be given. The
Galapagos Archipelago, situated under the equator, lies at a distance
of between 500 and 600 miles from the shores of South America. Here
almost every product of the land and of the water bears the
unmistakable stamp of the American continent. There are twenty-six
land-birds. Of these twenty-one, or perhaps twenty-three, are ranked as
distinct species, and would commonly be assumed to have been here
created; yet the close affinity of most of these birds to American
species is manifest in every character in their habits, gestures, and
tones of voice. So it is with the other animals, and with a large
proportion of the plants, as shown by Dr. Hooker in his admirable Flora
of this archipelago. The naturalist, looking at the inhabitants of
these volcanic islands in the Pacific, distant several hundred miles
from the continent, feels that he is standing on American land. Why
should this be so? Why should the species which are supposed to have
been created in the Galapagos Archipelago, and nowhere else, bear so
plainly the stamp of affinity to those created in America? There is
nothing in the conditions of life, in the geological nature of the
islands, in their height or climate, or in the proportions in which the
several classes are associated together, which closely resembles the
conditions of the South American coast. In fact, there is a
considerable dissimilarity in all these respects. On the other hand,
there is a considerable degree of resemblance in the volcanic nature of
the soil, in the climate, height, and size of the islands, between the
Galapagos and Cape Verde Archipelagos: but what an entire and absolute
difference in their inhabitants! The inhabitants of the Cape Verde
Islands are related to those of Africa, like those of the Galapagos to
America. Facts, such as these, admit of no sort of explanation on the
ordinary view of independent creation; whereas, on the view here
maintained, it is obvious that the Galapagos Islands would be likely to
receive colonists from America, whether by occasional means of
transport or (though I do not believe in this doctrine) by formerly
continuous land, and the Cape Verde Islands from Africa; such colonists
would be liable to modification—the principle of inheritance still
betraying their original birthplace.

Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of
the nearest continent, or of the nearest large island. The exceptions
are few, and most of them can be explained. Thus, although Kerguelen
Land stands nearer to Africa than to America, the plants are related,
and that very closely, as we know from Dr. Hooker’s account, to those
of America: but on the view that this island has been mainly stocked by
seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its
endemic plants is much more closely related to Australia, the nearest
mainland, than to any other region: and this is what might have been
expected; but it is also plainly related to South America, which,
although the next nearest continent, is so enormously remote, that the
fact becomes an anomaly. But this difficulty partially disappears on
the view that New Zealand, South America, and the other southern lands,
have been stocked in part from a nearly intermediate though distant
point, namely, from the antarctic islands, when they were clothed with
vegetation, during a warmer tertiary period, before the commencement of
the last Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is real, between the flora of the south-western
corner of Australia and of the Cape of Good Hope, is a far more
remarkable case; but this affinity is confined to the plants, and will,
no doubt, some day be explained.

The same law which has determined the relationship between the
inhabitants of islands and the nearest mainland, is sometimes displayed
on a small scale, but in a most interesting manner, within the limits
of the same archipelago. Thus each separate island of the Galapagos
Archipelago is tenanted, and the fact is a marvellous one, by many
distinct species; but these species are related to each other in a very
much closer manner than to the inhabitants of the American continent,
or of any other quarter of the world. This is what might have been
expected, for islands situated so near to each other would almost
necessarily receive immigrants from the same original source, and from
each other. But how is it that many of the immigrants have been
differently modified, though only in a small degree, in islands
situated within sight of each other, having the same geological nature,
the same height, climate, etc? This long appeared to me a great
difficulty: but it arises in chief part from the deeply-seated error of
considering the physical conditions of a country as the most important;
whereas it cannot be disputed that the nature of the other species with
which each has to compete, is at least as important, and generally a
far more important element of success. Now if we look to the species
which inhabit the Galapagos Archipelago, and are likewise found in
other parts of the world, we find that they differ considerably in the
several islands. This difference might indeed have been expected if the
islands have been stocked by occasional means of transport—a seed, for
instance, of one plant having been brought to one island, and that of
another plant to another island, though all proceeding from the same
general source. Hence, when in former times an immigrant first settled
on one of the islands, or when it subsequently spread from one to
another, it would undoubtedly be exposed to different conditions in the
different islands, for it would have to compete with a different set of
organisms; a plant, for instance, would find the ground best-fitted for
it occupied by somewhat different species in the different islands, and
would be exposed to the attacks of somewhat different enemies. If,
then, it varied, natural selection would probably favour different
varieties in the different islands. Some species, however, might spread
and yet retain the same character throughout the group, just as we see
some species spreading widely throughout a continent and remaining the
same.

The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous cases, is that each new
species after being formed in any one island, did not spread quickly to
the other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases wider than the British
Channel, and there is no reason to suppose that they have at any former
period been continuously united. The currents of the sea are rapid and
deep between the islands, and gales of wind are extraordinarily rare;
so that the islands are far more effectually separated from each other
than they appear on a map. Nevertheless, some of the species, both of
those found in other parts of the world and of those confined to the
archipelago, are common to the several islands; and we may infer from
the present manner of distribution that they have spread from one
island to the others. But we often take, I think, an erroneous view of
the probability of closely allied species invading each other’s
territory, when put into free intercommunication. Undoubtedly, if one
species has any advantage over another, it will in a very brief time
wholly or in part supplant it; but if both are equally well fitted for
their own places, both will probably hold their separate places for
almost any length of time. Being familiar with the fact that many
species, naturalised through man’s agency, have spread with astonishing
rapidity over wide areas, we are apt to infer that most species would
thus spread; but we should remember that the species which become
naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct forms, belonging in a
large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, differ on the different
islands; thus there are three closely allied species of mocking-thrush,
each confined to its own island. Now let us suppose the mocking-thrush
of Chatham Island to be blown to Charles Island, which has its own
mocking-thrush; why should it succeed in establishing itself there? We
may safely infer that Charles Island is well stocked with its own
species, for annually more eggs are laid and young birds hatched than
can possibly be reared; and we may infer that the mocking-thrush
peculiar to Charles Island is at least as well fitted for its home as
is the species peculiar to Chatham Island. Sir C. Lyell and Mr.
Wollaston have communicated to me a remarkable fact bearing on this
subject; namely, that Madeira and the adjoining islet of Porto Santo
possess many distinct but representative species of land-shells, some
of which live in crevices of stone; and although large quantities of
stone are annually transported from Porto Santo to Madeira, yet this
latter island has not become colonised by the Porto Santo species:
nevertheless, both islands have been colonised by some European
land-shells, which no doubt had some advantage over the indigenous
species. From these considerations I think we need not greatly marvel
at the endemic species which inhabit the several islands of the
Galapagos Archipelago not having all spread from island to island. On
the same continent, also, pre-occupation has probably played an
important part in checking the commingling of the species which inhabit
different districts with nearly the same physical conditions. Thus, the
south-east and south-west corners of Australia have nearly the same
physical conditions, and are united by continuous land, yet they are
inhabited by a vast number of distinct mammals, birds, and plants; so
it is, according to Mr. Bates, with the butterflies and other animals
inhabiting the great, open, and continuous valley of the Amazons.

The same principle which governs the general character of the
inhabitants of oceanic islands, namely, the relation to the source
whence colonists could have been most easily derived, together with
their subsequent modification, is of the widest application throughout
nature. We see this on every mountain-summit, in every lake and marsh.
For Alpine species, excepting in as far as the same species have become
widely spread during the Glacial epoch, are related to those of the
surrounding lowlands; thus we have in South America, Alpine
humming-birds, Alpine rodents, Alpine plants, &c., all strictly
belonging to American forms; and it is obvious that a mountain, as it
became slowly upheaved, would be colonised from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes, excepting
in so far as great facility of transport has allowed the same forms to
prevail throughout large portions of the world. We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given.
It will, I believe, be found universally true, that wherever in two
regions, let them be ever so distant, many closely allied or
representative species occur, there will likewise be found some
identical species; and wherever many closely-allied species occur,
there will be found many forms which some naturalists rank as distinct
species, and others as mere varieties; these doubtful forms showing us
the steps in the process of modification.

The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the
existence at remote points of the world of closely allied species, is
shown in another and more general way. Mr. Gould remarked to me long
ago, that in those genera of birds which range over the world, many of
the species have very wide ranges. I can hardly doubt that this rule is
generally true, though difficult of proof. Among mammals, we see it
strikingly displayed in Bats, and in a lesser degree in the Felidæ and
Canidæ. We see the same rule in the distribution of butterflies and
beetles. So it is with most of the inhabitants of fresh water, for many
of the genera in the most distinct classes range over the world, and
many of the species have enormous ranges. It is not meant that all, but
that some of the species have very wide ranges in the genera which
range very widely. Nor is it meant that the species in such genera
have, on an average, a very wide range; for this will largely depend on
how far the process of modification has gone; for instance, two
varieties of the same species inhabit America and Europe, and thus the
species has an immense range; but, if variation were to be carried a
little further, the two varieties would be ranked as distinct species,
and their range would be greatly reduced. Still less is it meant, that
species which have the capacity of crossing barriers and ranging
widely, as in the case of certain powerfully-winged birds, will
necessarily range widely; for we should never forget that to range
widely implies not only the power of crossing barriers, but the more
important power of being victorious in distant lands in the struggle
for life with foreign associates. But according to the view that all
the species of a genus, though distributed to the most remote points of
the world, are descended from a single progenitor, we ought to find,
and I believe as a general rule we do find, that some at least of the
species range very widely.

We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for
dispersal and subsequent modification. There is also reason to believe,
from geological evidence, that within each great class the lower
organisms change at a slower rate than the higher; consequently they
will have had a better chance of ranging widely and of still retaining
the same specific character. This fact, together with that of the seeds
and eggs of most lowly organised forms being very minute and better
fitted for distant transportal, probably accounts for a law which has
long been observed, and which has lately been discussed by Alph. de
Candolle in regard to plants, namely, that the lower any group of
organisms stands the more widely it ranges.

The relations just discussed—namely, lower organisms ranging more
widely than the higher—some of the species of widely-ranging genera
themselves ranging widely—such facts, as alpine, lacustrine, and marsh
productions being generally related to those which live on the
surrounding low lands and dry lands—the striking relationship between
the inhabitants of islands and those of the nearest mainland—the still
closer relationship of the distinct inhabitants of the islands of the
same archipelago—are inexplicable on the ordinary view of the
independent creation of each species, but are explicable if we admit
colonisation from the nearest or readiest source, together with the
subsequent adaptation of the colonists to their new homes.

_Summary of the last and present Chapters._


In these chapters I have endeavoured to show that if we make due
allowance for our ignorance of the full effects of changes of climate
and of the level of the land, which have certainly occurred within the
recent period, and of other changes which have probably occurred—if we
remember how ignorant we are with respect to the many curious means of
occasional transport—if we bear in mind, and this is a very important
consideration, how often a species may have ranged continuously over a
wide area, and then have become extinct in the intermediate tracts—the
difficulty is not insuperable in believing that all the individuals of
the same species, wherever found, are descended from common parents.
And we are led to this conclusion, which has been arrived at by many
naturalists under the designation of single centres of creation, by
various general considerations, more especially from the importance of
barriers of all kinds, and from the analogical distribution of
subgenera, genera, and families.

With respect to distinct species belonging to the same genus, which on
our theory have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life have changed very slowly, enormous periods of time having been
thus granted for their migration, the difficulties are far from
insuperable; though in this case, as in that of the individuals of the
same species, they are often great.

As exemplifying the effects of climatical changes on distribution, I
have attempted to show how important a part the last Glacial period has
played, which affected even the equatorial regions, and which, during
the alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. As showing
how diversified are the means of occasional transport, I have discussed
at some little length the means of dispersal of fresh-water
productions.

If the difficulties be not insuperable in admitting that in the long
course of time all the individuals of the same species, and likewise of
the several species belonging to the same genus, have proceeded from
some one source; then all the grand leading facts of geographical
distribution are explicable on the theory of migration, together with
subsequent modification and the multiplication of new forms. We can
thus understand the high importance of barriers, whether of land or
water, in not only separating but in apparently forming the several
zoological and botanical provinces. We can thus understand the
concentration of related species within the same areas; and how it is
that under different latitudes, for instance, in South America, the
inhabitants of the plains and mountains, of the forests, marshes, and
deserts, are linked together in so mysterious a manner, and are
likewise linked to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relation of organism to
organism is of the highest importance, we can see why two areas, having
nearly the same physical conditions, should often be inhabited by very
different forms of life; for according to the length of time which has
elapsed since the colonists entered one of the regions, or both;
according to the nature of the communication which allowed certain
forms and not others to enter, either in greater or lesser numbers;
according or not as those which entered happened to come into more or
less direct competition with each other and with the aborigines; and
according as the immigrants were capable of varying more or less
rapidly, there would ensue in the to or more regions, independently of
their physical conditions, infinitely diversified conditions of life;
there would be an almost endless amount of organic action and reaction,
and we should find some groups of beings greatly, and some only
slightly modified; some developed in great force, some existing in
scanty numbers—and this we do find in the several great geographical
provinces of the world.

On these same principles we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but that of
these, a large proportion should be endemic or peculiar; and why, in
relation to the means of migration, one group of beings should have all
its species peculiar, and another group, even within the same class,
should have all its species the same with those in an adjoining quarter
of the world. We can see why whole groups of organisms, as batrachians
and terrestrial mammals, should be absent from oceanic islands, whilst
the most isolated islands should possess their own peculiar species of
aërial mammals or bats. We can see why, in islands, there should be
some relation between the presence of mammals, in a more or less
modified condition, and the depth of the sea between such islands and
the mainland. We can clearly see why all the inhabitants of an
archipelago, though specifically distinct on the several islets, should
be closely related to each other, and should likewise be related, but
less closely, to those of the nearest continent, or other source whence
immigrants might have been derived. We can see why, if there exist very
closely allied or representative species in two areas, however distant
from each other, some identical species will almost always there be
found.

As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space; the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the apparent exceptions to
the rule are so few that they may fairly be attributed to our not
having as yet discovered in an intermediate deposit certain forms which
are absent in it, but which occur above and below: so in space, it
certainly is the general rule that the area inhabited by a single
species, or by a group of species, is continuous, and the exceptions,
which are not rare, may, as I have attempted to show, be accounted for
by former migrations under different circumstances, or through
occasional means of transport, or by the species having become extinct
in the intermediate tracts. Both in time and space species and groups
of species have their points of maximum development. Groups of species,
living during the same period of time, or living within the same area,
are often characterised by trifling features in common, as of sculpture
or colour. In looking to the long succession of past ages, as in
looking to distant provinces throughout the world, we find that species
in certain classes differ little from each other, whilst those in
another class, or only in a different section of the same order, differ
greatly from each other. In both time and space the lowly organised
members of each class generally change less than the highly organised;
but there are in both cases marked exceptions to the rule. According to
our theory, these several relations throughout time and space are
intelligible; for whether we look to the allied forms of life which
have changed during successive ages, or to those which have changed
after having migrated into distant quarters, in both cases they are
connected by the same bond of ordinary generation; in both cases the
laws of variation have been the same, and modifications have been
accumulated by the same means of natural selection.




CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS.


Classification, groups subordinate to groups—Natural system—Rules and
difficulties in classification, explained on the theory of descent with
modification—Classification of varieties—Descent always used in
classification—Analogical or adaptive characters—Affinities, general,
complex and radiating—Extinction separates and defines
groups—Morphology, between members of the same class, between parts of
the same individual—Embryology, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding
age—Rudimentary organs; their origin explained—Summary.


_Classification._


From the most remote period in the history of the world organic beings
have been found to resemble each other in descending degrees, so that
they can be classed in groups under groups. This classification is not
arbitrary like the grouping of the stars in constellations. The
existence of groups would have been of simple significance, if one
group had been exclusively fitted to inhabit the land, and another the
water; one to feed on flesh, another on vegetable matter, and so on;
but the case is widely different, for it is notorious how commonly
members of even the same subgroup have different habits. In the second
and fourth chapters, on Variation and on Natural Selection, I have
attempted to show that within each country it is the widely ranging,
the much diffused and common, that is the dominant species, belonging
to the larger genera in each class, which vary most. The varieties, or
incipient species, thus produced, ultimately become converted into new
and distinct species; and these, on the principle of inheritance, tend
to produce other new and dominant species. Consequently the groups
which are now large, and which generally include many dominant species,
tend to go on increasing in size. I further attempted to show that from
the varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they constantly
tend to diverge in character. This latter conclusion is supported by
observing the great diversity of forms, which, in any small area, come
into the closest competition, and by certain facts in naturalisation.

I attempted also to show that there is a steady tendency in the forms
which are increasing in number and diverging in character, to supplant
and exterminate the preceding, less divergent and less improved forms.
I request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that
the inevitable result is, that the modified descendants proceeding from
one progenitor become broken up into groups subordinate to groups. In
the diagram each letter on the uppermost line may represent a genus
including several species; and the whole of the genera along this upper
line form together one class, for all are descended from one ancient
parent, and, consequently, have inherited something in common. But the
three genera on the left hand have, on this same principle, much in
common, and form a subfamily, distinct from that containing the next
two genera on the right hand, which diverged from a common parent at
the fifth stage of descent. These five genera have also much in common,
though less than when grouped in subfamilies; and they form a family
distinct from that containing the three genera still further to the
right hand, which diverged at an earlier period. And all these genera,
descended from (A), form an order distinct from the genera descended
from (I). So that we here have many species descended from a single
progenitor grouped into genera; and the genera into subfamilies,
families and orders, all under one great class. The grand fact of the
natural subordination of organic beings in groups under groups, which,
from its familiarity, does not always sufficiently strike us, is in my
judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus
arranged. In this case there is of course no relation to genealogical
succession, and no cause can at present be assigned for their falling
into groups. But with organic beings the case is different, and the
view above given accords with their natural arrangement in group under
group; and no other explanation has ever been attempted.

Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what
is meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions—that is, by
one sentence to give the characters common, for instance, to all
mammals, by another those common to all carnivora, by another those
common to the dog-genus, and then, by adding a single sentence, a full
description is given of each kind of dog. The ingenuity and utility of
this system are indisputable. But many naturalists think that something
more is meant by the Natural System; they believe that it reveals the
plan of the Creator; but unless it be specified whether order in time
or space, or both, or what else is meant by the plan of the Creator, it
seems to me that nothing is thus added to our knowledge. Expressions
such as that famous one by Linnæus, which we often meet with in a more
or less concealed form, namely, that the characters do not make the
genus, but that the genus gives the characters, seem to imply that some
deeper bond is included in our classifications than mere resemblance. I
believe that this is the case, and that community of descent—the one
known cause of close similarity in organic beings—is the bond, which,
though observed by various degrees of modification, is partially
revealed to us by our classifications.

Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms most
like each other. It might have been thought (and was in ancient times
thought) that those parts of the structure which determined the habits
of life, and the general place of each being in the economy of nature,
would be of very high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to a shrew, of
a dugong to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the
being, are ranked as merely “adaptive or analogical characters;” but to
the consideration of these resemblances we shall recur. It may even be
given as a general rule, that the less any part of the organisation is
concerned with special habits, the more important it becomes for
classification. As an instance: Owen, in speaking of the dugong, says,
“The generative organs, being those which are most remotely related to
the habits and food of an animal, I have always regarded as affording
very clear indications of its true affinities. We are least likely in
the modifications of these organs to mistake a merely adaptive for an
essential character.” With plants how remarkable it is that the organs
of vegetation, on which their nutrition and life depend, are of little
signification; whereas the organs of reproduction, with their product
the seed and embryo, are of paramount importance! So again, in formerly
discussing certain morphological characters which are not functionally
important, we have seen that they are often of the highest service in
classification. This depends on their constancy throughout many allied
groups; and their constancy chiefly depends on any slight deviations
not having been preserved and accumulated by natural selection, which
acts only on serviceable characters.

That the mere physiological importance of an organ does not determine
its classificatory value, is almost proved by the fact, that in allied
groups, in which the same organ, as we have every reason to suppose,
has nearly the same physiological value, its classificatory value is
widely different. No naturalist can have worked at any group without
being struck with this fact; and it has been fully acknowledged in the
writings of almost every author. It will suffice to quote the highest
authority, Robert Brown, who, in speaking of certain organs in the
Proteaceæ, says their generic importance, “like that of all their
parts, not only in this, but, as I apprehend in every natural family,
is very unequal, and in some cases seems to be entirely lost.” Again,
in another work he says, the genera of the Connaraceæ “differ in having
one or more ovaria, in the existence or absence of albumen, in the
imbricate or valvular æstivation. Any one of these characters singly
is frequently of more than generic importance, though here even, when
all taken together, they appear insufficient to separate Cnestis from
Connarus.” To give an example among insects: in one great division of
the Hymenoptera, the antennæ, as Westwood has remarked, are most
constant in structure; in another division they differ much, and the
differences are of quite subordinate value in classification; yet no
one will say that the antennæ in these two divisions of the same order
are of unequal physiological importance. Any number of instances could
be given of the varying importance for classification of the same
important organ within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of much value in classification. No one will
dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between Ruminants and
Pachyderms. Robert Brown has strongly insisted on the fact that the
position of the rudimentary florets is of the highest importance in the
classification of the Grasses.

Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance, but
which are universally admitted as highly serviceable in the definition
of whole groups. For instance, whether or not there is an open passage
from the nostrils to the mouth, the only character, according to Owen,
which absolutely distinguishes fishes and reptiles—the inflection of
the angle of the lower jaw in Marsupials—the manner in which the wings
of insects are folded—mere colour in certain Algæ—mere pubescence on
parts of the flower in grasses—the nature of the dermal covering, as
hair or feathers, in the Vertebrata. If the Ornithorhynchus had been
covered with feathers instead of hair, this external and trifling
character would have been considered by naturalists as an important aid
in determining the degree of affinity of this strange creature to
birds.

The importance, for classification, of trifling characters, mainly
depends on their being correlated with many other characters of more or
less importance. The value indeed of an aggregate of characters is very
evident in natural history. Hence, as has often been remarked, a
species may depart from its allies in several characters, both of high
physiological importance, and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence, also, it has
been found that a classification founded on any single character,
however important that may be, has always failed; for no part of the
organisation is invariably constant. The importance of an aggregate of
characters, even when none are important, alone explains the aphorism
enunciated by Linnæus, namely, that the characters do not give the
genus, but the genus gives the character; for this seems founded on the
appreciation of many trifling points of resemblance, too slight to be
defined. Certain plants, belonging to the Malpighiaceæ, bear perfect
and degraded flowers; in the latter, as A. de Jussieu has remarked,
“The greater number of the characters proper to the species, to the
genus, to the family, to the class, disappear, and thus laugh at our
classification.” When Aspicarpa produced in France, during several
years, only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper type
of the order, yet M. Richard sagaciously saw, as Jussieu observes, that
this genus should still be retained among the Malpighiaceæ. This case
well illustrates the spirit of our classifications.

Practically, when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which they
use in defining a group or in allocating any particular species. If
they find a character nearly uniform, and common to a great number of
forms, and not common to others, they use it as one of high value; if
common to some lesser number, they use it as of subordinate value. This
principle has been broadly confessed by some naturalists to be the true
one; and by none more clearly than by that excellent botanist, Aug. St.
Hilaire. If several trifling characters are always found in
combination, though no apparent bond of connexion can be discovered
between them, especial value is set on them. As in most groups of
animals, important organs, such as those for propelling the blood, or
for aerating it, or those for propagating the race, are found nearly
uniform, they are considered as highly serviceable in classification;
but in some groups all these, the most important vital organs, are
found to offer characters of quite subordinate value. Thus, as Fritz
Müller has lately remarked, in the same group of crustaceans, Cypridina
is furnished with a heart, while in two closely allied genera, namely
Cypris and Cytherea, there is no such organ; one species of Cypridina
has well-developed branchiæ, while another species is destitute of
them.

We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural
classification of course includes all ages. But it is by no means
obvious, on the ordinary view, why the structure of the embryo should
be more important for this purpose than that of the adult, which alone
plays its full part in the economy of nature. Yet it has been strongly
urged by those great naturalists, Milne Edwards and Agassiz, that
embryological characters are the most important of all; and this
doctrine has very generally been admitted as true. Nevertheless, their
importance has sometimes been exaggerated, owing to the adaptive
characters of larvæ not having been excluded; in order to show this,
Fritz Müller arranged, by the aid of such characters alone, the great
class of crustaceans, and the arrangement did not prove a natural one.
But there can be no doubt that embryonic, excluding larval characters,
are of the highest value for classification, not only with animals but
with plants. Thus the main divisions of flowering plants are founded on
differences in the embryo—on the number and position of the cotyledons,
and on the mode of development of the plumule and radicle. We shall
immediately see why these characters possess so high a value in
classification, namely, from the natural system being genealogical in
its arrangement.

Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of characters
common to all birds; but with crustaceans, any such definition has
hitherto been found impossible. There are crustaceans at the opposite
ends of the series, which have hardly a character in common; yet the
species at both ends, from being plainly allied to others, and these to
others, and so onwards, can be recognised as unequivocally belonging to
this, and to no other class of the Articulata.

Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity
of this practice in certain groups of birds; and it has been followed
by several entomologists and botanists.

Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera,
they seem to be, at least at present, almost arbitrary. Several of the
best botanists, such as Mr. Bentham and others, have strongly insisted
on their arbitrary value. Instances could be given among plants and
insects, of a group first ranked by practised naturalists as only a
genus, and then raised to the rank of a subfamily or family; and this
has been done, not because further research has detected important
structural differences, at first overlooked, but because numerous
allied species, with slightly different grades of difference, have been
subsequently discovered.

All the foregoing rules and aids and difficulties in classification may
be explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification—that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, all true classification being genealogical—that
community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike.

But I must explain my meaning more fully. I believe that the
_arrangement_ of the groups within each class, in due subordination and
relation to each other, must be strictly genealogical in order to be
natural; but that the _amount_ of difference in the several branches or
groups, though allied in the same degree in blood to their common
progenitor, may differ greatly, being due to the different degrees of
modification which they have undergone; and this is expressed by the
forms being ranked under different genera, families, sections or
orders. The reader will best understand what is meant, if he will take
the trouble to refer to the diagram in the fourth chapter. We will
suppose the letters A to L to represent allied genera existing during
the Silurian epoch, and descended from some still earlier form. In
three of these genera (A, F, and I) a species has transmitted modified
descendants to the present day, represented by the fifteen genera
(_a_14 to _z_14) on the uppermost horizontal line. Now, all these
modified descendants from a single species are related in blood or
descent in the same degree. They may metaphorically be called cousins
to the same millionth degree, yet they differ widely and in different
degrees from each other. The forms descended from A, now broken up into
two or three families, constitute a distinct order from those descended
from I, also broken up into two families. Nor can the existing species
descended from A be ranked in the same genus with the parent A, or
those from I with parent I. But the existing genus F14 may be supposed
to have been but slightly modified, and it will then rank with the
parent genus F; just as some few still living organisms belong to
Silurian genera. So that the comparative value of the differences
between these organic beings, which are all related to each other in
the same degree in blood, has come to be widely different.
Nevertheless, their genealogical _arrangement_ remains strictly true,
not only at the present time, but at each successive period of descent.
All the modified descendants from A will have inherited something in
common from their common parent, as will all the descendants from I; so
will it be with each subordinate branch of descendants at each
successive stage. If, however, we suppose any descendant of A or of I
to have become so much modified as to have lost all traces of its
parentage in this case, its place in the natural system will be lost,
as seems to have occurred with some few existing organisms. All the
descendants of the genus F, along its whole line of descent, are
supposed to have been but little modified, and they form a single
genus. But this genus, though much isolated, will still occupy its
proper intermediate position. The representation of the groups as here
given in the diagram on a flat surface, is much too simple. The
branches ought to have diverged in all directions. If the names of the
groups had been simply written down in a linear series the
representation would have been still less natural; and it is
notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature among the beings of the same
group. Thus, the natural system is genealogical in its arrangement,
like a pedigree. But the amount of modification which the different
groups have undergone has to be expressed by ranking them under
different so-called genera, subfamilies, families, sections, orders,
and classes.

It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken throughout
the world; and if all extinct languages, and all intermediate and
slowly changing dialects, were to be included, such an arrangement
would be the only possible one. Yet it might be that some ancient
languages had altered very little and had given rise to few new
languages, whilst others had altered much owing to the spreading,
isolation and state of civilisation of the several co-descended races,
and had thus given rise to many new dialects and languages. The various
degrees of difference between the languages of the same stock would
have to be expressed by groups subordinate to groups; but the proper or
even the only possible arrangement would still be genealogical; and
this would be strictly natural, as it would connect together all
languages, extinct and recent, by the closest affinities, and would
give the filiation and origin of each tongue.

In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the subvarieties
under the varieties; and in some cases, as with the domestic pigeon,
with several other grades of difference. Nearly the same rules are
followed as in classifying species. Authors have insisted on the
necessity of arranging varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the Swedish
and common turnip together, though the esculent and thickened stems are
so similar. Whatever part is found to be most constant, is used in
classing varieties: thus the great agriculturist Marshall says the
horns are very useful for this purpose with cattle, because they are
less variable than the shape or colour of the body, &c.; whereas with
sheep the horns are much less serviceable, because less constant. In
classing varieties, I apprehend that if we had a real pedigree, a
genealogical classification would be universally preferred; and it has
been attempted in some cases. For we might feel sure, whether there had
been more or less modification, that the principle of inheritance would
keep the forms together which were allied in the greatest number of
points. In tumbler pigeons, though some of the subvarieties differ in
the important character of the length of the beak, yet all are kept
together from having the common habit of tumbling; but the short-faced
breed has nearly or quite lost this habit; nevertheless, without any
thought on the subject, these tumblers are kept in the same group,
because allied in blood and alike in some other respects.

With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade,
that of species, the two sexes; and how enormously these sometimes
differ in the most important characters is known to every naturalist:
scarcely a single fact can be predicated in common of the adult males
and hermaphrodites of certain cirripedes, and yet no one dreams of
separating them. As soon as the three Orchidean forms, Monachanthus,
Myanthus, and Catasetum, which had previously been ranked as three
distinct genera, were known to be sometimes produced on the same plant,
they were immediately considered as varieties; and now I have been able
to show that they are the male, female, and hermaphrodite forms of the
same species. The naturalist includes as one species the various larval
stages of the same individual, however much they may differ from each
other and from the adult; as well as the so-called alternate
generations of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters and varieties,
not from their partial resemblance to the parent-form, but because they
are descended from it.

As descent has universally been used in classing together the
individuals of the same species, though the males and females and larvæ
are sometimes extremely different; and as it has been used in classing
varieties which have undergone a certain, and sometimes a considerable
amount of modification, may not this same element of descent have been
unconsciously used in grouping species under genera, and genera under
higher groups, all under the so-called natural system? I believe it has
been unconsciously used; and thus only can I understand the several
rules and guides which have been followed by our best systematists. As
we have no written pedigrees, we are forced to trace community of
descent by resemblances of any kind. Therefore, we choose those
characters which are the least likely to have been modified, in
relation to the conditions of life to which each species has been
recently exposed. Rudimentary structures on this view are as good as,
or even sometimes better than other parts of the organisation. We care
not how trifling a character may be—let it be the mere inflection of
the angle of the jaw, the manner in which an insect’s wing is folded,
whether the skin be covered by hair or feathers—if it prevail
throughout many and different species, especially those having very
different habits of life, it assumes high value; for we can account for
its presence in so many forms with such different habits, only by
inheritance from a common parent. We may err in this respect in regard
to single points of structure, but when several characters, let them be
ever so trifling, concur throughout a large group of beings having
different habits, we may feel almost sure, on the theory of descent,
that these characters have been inherited from a common ancestor; and
we know that such aggregated characters have especial value in
classification.

We can understand why a species or a group of species may depart from
its allies, in several of its most important characteristics, and yet
be safely classed with them. This may be safely done, and is often
done, as long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet, if these extreme
forms are connected together by a chain of intermediate groups, we may
at once infer their community of descent, and we put them all into the
same class. As we find organs of high physiological importance—those
which serve to preserve life under the most diverse conditions of
existence—are generally the most constant, we attach especial value to
them; but if these same organs, in another group or section of a group,
are found to differ much, we at once value them less in our
classification. We shall presently see why embryological characters are
of such high classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large genera,
because all the species of the same genus, inhabiting any distinct and
isolated region, are in all probability descended from the same
parents.

_Analogical Resemblances._—We can understand, on the above views, the
very important distinction between real affinities and analogical or
adaptive resemblances. Lamarck first called attention to this subject,
and he has been ably followed by Macleay and others. The resemblance in
the shape of the body and in the fin-like anterior limbs between
dugongs and whales, and between these two orders of mammals and fishes,
are analogical. So is the resemblance between a mouse and a shrew-mouse
(Sorex), which belong to different orders; and the still closer
resemblance, insisted on by Mr. Mivart, between the mouse and a small
marsupial animal (Antechinus) of Australia. These latter resemblances
may be accounted for, as it seems to me, by adaptation for similarly
active movements through thickets and herbage, together with
concealment from enemies.

Among insects there are innumerable instances; thus Linnæus, misled by
external appearances, actually classed an homopterous insect as a moth.
We see something of the same kind even with our domestic varieties, as
in the strikingly similar shape of the body in the improved breeds of
the Chinese and common pig, which are descended from distinct species;
and in the similarly thickened stems of the common and specifically
distinct Swedish turnip. The resemblance between the greyhound and
race-horse is hardly more fanciful than the analogies which have been
drawn by some authors between widely different animals.

On the view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why
analogical or adaptive characters, although of the utmost importance to
the welfare of the being, are almost valueless to the systematist. For
animals, belonging to two most distinct lines of descent, may have
become adapted to similar conditions, and thus have assumed a close
external resemblance; but such resemblances will not reveal—will rather
tend to conceal their blood-relationship. We can thus also understand
the apparent paradox, that the very same characters are analogical when
one group is compared with another, but give true affinities when the
members of the same group are compared together: thus the shape of the
body and fin-like limbs are only analogical when whales are compared
with fishes, being adaptations in both classes for swimming through the
water; but between the the several members of the whale family, the
shape of the body and the fin-like limbs offer characters exhibiting
true affinity; for as these parts are so nearly similar throughout the
whole family, we cannot doubt that they have been inherited from a
common ancestor. So it is with fishes.

Numerous cases could be given of striking resemblances in quite
distinct beings between single parts or organs, which have been adapted
for the same functions. A good instance is afforded by the close
resemblance of the jaws of the dog and Tasmanian wolf or
Thylacinus—animals which are widely sundered in the natural system. But
this resemblance is confined to general appearance, as in the
prominence of the canines, and in the cutting shape of the molar teeth.
For the teeth really differ much: thus the dog has on each side of the
upper jaw four pre-molars and only two molars; while the Thylacinus has
three pre-molars and four molars. The molars also differ much in the
two animals in relative size and structure. The adult dentition is
preceded by a widely different milk dentition. Any one may, of course,
deny that the teeth in either case have been adapted for tearing flesh,
through the natural selection of successive variations; but if this be
admitted in the one case, it is unintelligible to me that it should be
denied in the other. I am glad to find that so high an authority as
Professor Flower has come to this same conclusion.

The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs—of widely different insects
possessing luminous organs—and of orchids and asclepiads having
pollen-masses with viscid discs, come under this same head of
analogical resemblances. But these cases are so wonderful that they
were introduced as difficulties or objections to our theory. In all
such cases some fundamental difference in the growth or development of
the parts, and generally in their matured structure, can be detected.
The end gained is the same, but the means, though appearing
superficially to be the same, are essentially different. The principle
formerly alluded to under the term of _analogical variation_ has
probably in these cases often come into play; that is, the members of
the same class, although only distantly allied, have inherited so much
in common in their constitution, that they are apt to vary under
similar exciting causes in a similar manner; and this would obviously
aid in the acquirement through natural selection of parts or organs,
strikingly like each other, independently of their direct inheritance
from a common progenitor.

As species belonging to distinct classes have often been adapted by
successive slight modifications to live under nearly similar
circumstances—to inhabit, for instance, the three elements of land, air
and water—we can perhaps understand how it is that a numerical
parallelism has sometimes been observed between the subgroups of
distinct classes. A naturalist, struck with a parallelism of this
nature, by arbitrarily raising or sinking the value of the groups in
several classes (and all our experience shows that their valuation is
as yet arbitrary), could easily extend the parallelism over a wide
range; and thus the septenary, quinary, quaternary and ternary
classifications have probably arisen.

There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life,
but has been gained for the sake of protection. I allude to the
wonderful manner in which certain butterflies imitate, as first
described by Mr. Bates, other and quite distinct species. This
excellent observer has shown that in some districts of South America,
where, for instance, an Ithomia abounds in gaudy swarms, another
butterfly, namely, a Leptalis, is often found mingled in the same
flock; and the latter so closely resembles the Ithomia in every shade
and stripe of colour, and even in the shape of its wings, that Mr.
Bates, with his eyes sharpened by collecting during eleven years, was,
though always on his guard, continually deceived. When the mockers and
the mocked are caught and compared, they are found to be very different
in essential structure, and to belong not only to distinct genera, but
often to distinct families. Had this mimicry occurred in only one or
two instances, it might have been passed over as a strange coincidence.
But, if we proceed from a district where one Leptalis imitates an
Ithomia, another mocking and mocked species, belonging to the same two
genera, equally close in their resemblance, may be found. Altogether no
less than ten genera are enumerated, which include species that imitate
other butterflies. The mockers and mocked always inhabit the same
region; we never find an imitator living remote from the form which it
imitates. The mockers are almost invariably rare insects; the mocked in
almost every case abounds in swarms. In the same district in which a
species of Leptalis closely imitates an Ithomia, there are sometimes
other Lepidoptera mimicking the same Ithomia: so that in the same
place, species of three genera of butterflies and even a moth are found
all closely resembling a butterfly belonging to a fourth genus. It
deserves especial notice that many of the mimicking forms of the
Leptalis, as well as of the mimicked forms, can be shown by a graduated
series to be merely varieties of the same species; while others are
undoubtedly distinct species. But why, it may be asked, are certain
forms treated as the mimicked and others as the mimickers? Mr. Bates
satisfactorily answers this question by showing that the form which is
imitated keeps the usual dress of the group to which it belongs, while
the counterfeiters have changed their dress and do not resemble their
nearest allies.

We are next led to enquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and quite
distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt, hit
on the true explanation. The mocked forms, which always abound in
numbers, must habitually escape destruction to a large extent,
otherwise they could not exist in such swarms; and a large amount of
evidence has now been collected, showing that they are distasteful to
birds and other insect-devouring animals. The mocking forms, on the
other hand, that inhabit the same district, are comparatively rare, and
belong to rare groups; hence, they must suffer habitually from some
danger, for otherwise, from the number of eggs laid by all butterflies,
they would in three or four generations swarm over the whole country.
Now if a member of one of these persecuted and rare groups were to
assume a dress so like that of a well-protected species that it
continually deceived the practised eyes of an entomologist, it would
often deceive predaceous birds and insects, and thus often escape
destruction. Mr. Bates may almost be said to have actually witnessed
the process by which the mimickers have come so closely to resemble the
mimicked; for he found that some of the forms of Leptalis which mimic
so many other butterflies, varied in an extreme degree. In one district
several varieties occurred, and of these one alone resembled, to a
certain extent, the common Ithomia of the same district. In another
district there were two or three varieties, one of which was much
commoner than the others, and this closely mocked another form of
Ithomia. From facts of this nature, Mr. Bates concludes that the
Leptalis first varies; and when a variety happens to resemble in some
degree any common butterfly inhabiting the same district, this variety,
from its resemblance to a flourishing and little persecuted kind, has a
better chance of escaping destruction from predaceous birds and
insects, and is consequently oftener preserved; “the less perfect
degrees of resemblance being generation after generation eliminated,
and only the others left to propagate their kind.” So that here we have
an excellent illustration of natural selection.

Messrs. Wallace and Trimen have likewise described several equally
striking cases of imitation in the Lepidoptera of the Malay Archipelago
and Africa, and with some other insects. Mr. Wallace has also detected
one such case with birds, but we have none with the larger quadrupeds.
The much greater frequency of imitation with insects than with other
animals, is probably the consequence of their small size; insects
cannot defend themselves, excepting indeed the kinds furnished with a
sting, and I have never heard of an instance of such kinds mocking
other insects, though they are mocked; insects cannot easily escape by
flight from the larger animals which prey on them; therefore, speaking
metaphorically, they are reduced, like most weak creatures, to trickery
and dissimulation.

It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But, starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means, and if the
imitated form was subsequently and gradually modified through any
agency, the imitating form would be led along the same track, and thus
be altered to almost any extent, so that it might ultimately assume an
appearance or colouring wholly unlike that of the other members of the
family to which it belonged. There is, however, some difficulty on this
head, for it is necessary to suppose in some cases that ancient members
belonging to several distinct groups, before they had diverged to their
present extent, accidentally resembled a member of another and
protected group in a sufficient degree to afford some slight
protection, this having given the basis for the subsequent acquisition
of the most perfect resemblance.

_On the Nature of the Affinities connecting Organic Beings._—As the
modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to which
they belong large and their parents dominant, they are almost sure to
spread widely, and to seize on more and more places in the economy of
nature. The larger and more dominant groups within each class thus tend
to go on increasing in size, and they consequently supplant many
smaller and feebler groups. Thus, we can account for the fact that all
organisms, recent and extinct, are included under a few great orders
and under still fewer classes. As showing how few the higher groups are
in number, and how widely they are spread throughout the world, the
fact is striking that the discovery of Australia has not added an
insect belonging to a new class, and that in the vegetable kingdom, as
I learn from Dr. Hooker, it has added only two or three families of
small size.

In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some degree
intermediate between existing groups. As some few of the old and
intermediate forms having transmitted to the present day descendants
but little modified, these constitute our so-called osculant or
aberrant groups. The more aberrant any form is, the greater must be the
number of connecting forms which have been exterminated and utterly
lost. And we have evidence of aberrant groups having suffered severely
from extinction, for they are almost always represented by extremely
few species; and such species as do occur are generally very distinct
from each other, which again implies extinction. The genera
Ornithorhynchus and Lepidosiren, for example, would not have been less
aberrant had each been represented by a dozen species, instead of as at
present by a single one, or by two or three. We can, I think, account
for this fact only by looking at aberrant groups as forms which have
been conquered by more successful competitors, with a few members still
preserved under unusually favourable conditions.

Mr. Waterhouse has remarked that when a member belonging to one group
of animals exhibits an affinity to a quite distinct group, this
affinity in most cases is general and not special: thus, according to
Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this order, its
relations are general, that is, not to any one Marsupial species more
than to another. As these points of affinity are believed to be real
and not merely adaptive, they must be due in accordance with our view
to inheritance from a common progenitor. Therefore, we must suppose
either that all Rodents, including the bizcacha, branched off from some
ancient Marsupial, which will naturally have been more or less
intermediate in character with respect to all existing Marsupials; or
that both Rodents and Marsupials branched off from a common progenitor,
and that both groups have since undergone much modification in
divergent directions. On either view we must suppose that the bizcacha
has retained, by inheritance, more of the character of its ancient
progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all
or nearly all Marsupials, from having partially retained the character
of their common progenitor, or of some early member of the group. On
the other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the general
order of Rodents. In this case, however, it may be strongly suspected
that the resemblance is only analogical, owing to the Phascolomys
having become adapted to habits like those of a Rodent. The elder De
Candolle has made nearly similar observations on the general nature of
the affinities of distinct families of plants.

On the principle of the multiplication and gradual divergence in
character of the species descended from a common progenitor, together
with their retention by inheritance of some characters in common, we
can understand the excessively complex and radiating affinities by
which all the members of the same family or higher group are connected
together. For the common progenitor of a whole family, now broken up by
extinction into distinct groups and subgroups, will have transmitted
some of its characters, modified in various ways and degrees, to all
the species; and they will consequently be related to each other by
circuitous lines of affinity of various lengths (as may be seen in the
diagram so often referred to), mounting up through many predecessors.
As it is difficult to show the blood-relationship between the numerous
kindred of any ancient and noble family, even by the aid of a
genealogical tree, and almost impossible to do so without this aid, we
can understand the extraordinary difficulty which naturalists have
experienced in describing, without the aid of a diagram, the various
affinities which they perceive between the many living and extinct
members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account for the distinctness
of whole classes from each other—for instance, of birds from all other
vertebrate animals—by the belief that many ancient forms of life have
been utterly lost, through which the early progenitors of birds were
formerly connected with the early progenitors of the other and at that
time less differentiated vertebrate classes. There has been much less
extinction of the forms of life which once connected fishes with
Batrachians. There has been still less within some whole classes, for
instance the Crustacea, for here the most wonderfully diverse forms are
still linked together by a long and only partially broken chain of
affinities. Extinction has only defined the groups: it has by no means
made them; for if every form which has ever lived on this earth were
suddenly to reappear, though it would be quite impossible to give
definitions by which each group could be distinguished, still a natural
classification, or at least a natural arrangement, would be possible.
We shall see this by turning to the diagram: the letters, A to L, may
represent eleven Silurian genera, some of which have produced large
groups of modified descendants, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet
the arrangement in the diagram would still hold good and would be
natural; for, on the principle of inheritance, all the forms descended,
for instance from A, would have something in common. In a tree we can
distinguish this or that branch, though at the actual fork the two
unite and blend together. We could not, as I have said, define the
several groups; but we could pick out types, or forms, representing
most of the characters of each group, whether large or small, and thus
give a general idea of the value of the differences between them. This
is what we should be driven to, if we were ever to succeed in
collecting all the forms in any one class which have lived throughout
all time and space. Assuredly we shall never succeed in making so
perfect a collection: nevertheless, in certain classes, we are tending
toward this end; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we
can separate and define the groups to which such types belong.

Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction
and divergence of character in the descendants from any one
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages under one species, although they may have
but few characters in common; we use descent in classing acknowledged
varieties, however different they may be from their parents; and I
believe that this element of descent is the hidden bond of connexion
which naturalists have sought under the term of the Natural System. On
this idea of the natural system being, in so far as it has been
perfected, genealogical in its arrangement, with the grades of
difference expressed by the terms genera, families, orders, &c., we can
understand the rules which we are compelled to follow in our
classification. We can understand why we value certain resemblances far
more than others; why we use rudimentary and useless organs, or others
of trifling physiological importance; why, in finding the relations
between one group and another, we summarily reject analogical or
adaptive characters, and yet use these same characters within the
limits of the same group. We can clearly see how it is that all living
and extinct forms can be grouped together within a few great classes;
and how the several members of each class are connected together by the
most complex and radiating lines of affinities. We shall never,
probably, disentangle the inextricable web of the affinities between
the members of any one class; but when we have a distinct object in
view, and do not look to some unknown plan of creation, we may hope to
make sure but slow progress.

Professor Haeckel in his “Generelle Morphologie” and in another works,
has recently brought his great knowledge and abilities to bear on what
he calls phylogeny, or the lines of descent of all organic beings. In
drawing up the several series he trusts chiefly to embryological
characters, but receives aid from homologous and rudimentary organs, as
well as from the successive periods at which the various forms of life
are believed to have first appeared in our geological formations. He
has thus boldly made a great beginning, and shows us how classification
will in the future be treated.

_Morphology._


We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term “unity of
type;” or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included
under the general term of Morphology. This is one of the most
interesting departments of natural history, and may almost be said to
be its very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include similar bones, in
the same relative positions? How curious it is, to give a subordinate
though striking instance, that the hind feet of the kangaroo, which are
so well fitted for bounding over the open plains—those of the climbing,
leaf-eating koala, equally well fitted for grasping the branches of
trees—those of the ground-dwelling, insect or root-eating,
bandicoots—and those of some other Australian marsupials—should all be
constructed on the same extraordinary type, namely with the bones of
the second and third digits extremely slender and enveloped within the
same skin, so that they appear like a single toe furnished with two
claws. Notwithstanding this similarity of pattern, it is obvious that
the hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the
more striking by the American opossums, which follow nearly the same
habits of life as some of their Australian relatives, having feet
constructed on the ordinary plan. Professor Flower, from whom these
statements are taken, remarks in conclusion: “We may call this
conformity to type, without getting much nearer to an explanation of
the phenomenon;” and he then adds “but is it not powerfully suggestive
of true relationship, of inheritance from a common ancestor?”

Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together
in the same invariable order. We never find, for instance, the bones of
the arm and forearm, or of the thigh and leg, transposed. Hence the
same names can be given to the homologous bones in widely different
animals. We see the same great law in the construction of the mouths of
insects: what can be more different than the immensely long spiral
proboscis of a sphinx-moth, the curious folded one of a bee or bug, and
the great jaws of a beetle? Yet all these organs, serving for such
widely different purposes, are formed by infinitely numerous
modifications of an upper lip, mandibles, and two pairs of maxillæ. The
same law governs the construction of the mouths and limbs of
crustaceans. So it is with the flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity
of pattern in members of the same class, by utility or by the doctrine
of final causes. The hopelessness of the attempt has been expressly
admitted by Owen in his most interesting work on the “Nature of Limbs.”
On the ordinary view of the independent creation of each being, we can
only say that so it is; that it has pleased the Creator to construct
all the animals and plants in each great class on a uniform plan; but
this is not a scientific explanation.

The explanation is to a large extent simple, on the theory of the
selection of successive slight modifications, each being profitable in
some way to the modified form, but often affecting by correlation other
parts of the organisation. In changes of this nature, there will be
little or no tendency to alter the original pattern, or to transpose
the parts. The bones of a limb might be shortened and flattened to any
extent, becoming at the same time enveloped in thick membrane, so as to
serve as a fin; or a webbed hand might have all its bones, or certain
bones, lengthened to any extent, with the membrane connecting them
increased, so as to serve as a wing; yet all these modifications would
not tend to alter the framework of the bones or the relative connexion
of the parts. If we suppose that an early progenitor—the archetype, as
it may be called—of all mammals, birds and reptiles, had its limbs
constructed on the existing general pattern, for whatever purpose they
served, we can at once perceive the plain signification of the
homologous construction of the limbs throughout the class. So with the
mouths of insects, we have only to suppose that their common progenitor
had an upper lip, mandibles, and two pairs of maxillæ, these parts
being perhaps very simple in form; and then natural selection will
account for the infinite diversity in structure and function of the
mouths of insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be finally
lost, by the reduction and ultimately by the complete abortion of
certain parts, by the fusion of other parts, and by the doubling or
multiplication of others, variations which we know to be within the
limits of possibility. In the paddles of the gigantic extinct
sea-lizards, and in the mouths of certain suctorial crustaceans, the
general pattern seems thus to have become partially obscured.

There is another and equally curious branch of our subject; namely,
serial homologies, or the comparison of the different parts or organs
in the same individual, and not of the same parts or organs in
different members of the same class. Most physiologists believe that
the bones of the skull are homologous—that is, correspond in number and
in relative connexion—with the elemental parts of a certain number of
vertebræ. The anterior and posterior limbs in all the higher vertebrate
classes are plainly homologous. So it is with the wonderfully complex
jaws and legs of crustaceans. It is familiar to almost every one, that
in a flower the relative position of the sepals, petals, stamens, and
pistils, as well as their intimate structure, are intelligible on the
view that they consist of metamorphosed leaves, arranged in a spire. In
monstrous plants, we often get direct evidence of the possibility of
one organ being transformed into another; and we can actually see,
during the early or embryonic stages of development in flowers, as well
as in crustaceans and many other animals, that organs, which when
mature become extremely different are at first exactly alike.

How inexplicable are the cases of serial homologies on the ordinary
view of creation! Why should the brain be enclosed in a box composed of
such numerous and such extraordinarily shaped pieces of bone apparently
representing vertebræ? As Owen has remarked, the benefit derived from
the yielding of the separate pieces in the act of parturition by
mammals, will by no means explain the same construction in the skulls
of birds and reptiles. Why should similar bones have been created to
form the wing and the leg of a bat, used as they are for such totally
different purposes, namely flying and walking? Why should one
crustacean, which has an extremely complex mouth formed of many parts,
consequently always have fewer legs; or conversely, those with many
legs have simpler mouths? Why should the sepals, petals, stamens, and
pistils, in each flower, though fitted for such distinct purposes, be
all constructed on the same pattern?

On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some
animals first became divided into a series of segments, or how they
became divided into right and left sides, with corresponding organs,
for such questions are almost beyond investigation. It is, however,
probable that some serial structures are the result of cells
multiplying by division, entailing the multiplication of the parts
developed from such cells. It must suffice for our purpose to bear in
mind that an indefinite repetition of the same part or organ is the
common characteristic, as Owen has remarked, of all low or little
specialised forms; therefore the unknown progenitor of the Vertebrata
probably possessed many vertebræ; the unknown progenitor of the
Articulata, many segments; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We have also
formerly seen that parts many times repeated are eminently liable to
vary, not only in number, but in form. Consequently such parts, being
already present in considerable numbers, and being highly variable,
would naturally afford the materials for adaptation to the most
different purposes; yet they would generally retain, through the force
of inheritance, plain traces of their original or fundamental
resemblance. They would retain this resemblance all the more, as the
variations, which afforded the basis for their subsequent modification
through natural selection, would tend from the first to be similar; the
parts being at an early stage of growth alike, and being subjected to
nearly the same conditions. Such parts, whether more or less modified,
unless their common origin became wholly obscured, would be serially
homologous.

In the great class of molluscs, though the parts in distinct species
can be shown to be homologous, only a few serial homologies; such as
the valves of Chitons, can be indicated; that is, we are seldom enabled
to say that one part is homologous with another part in the same
individual. And we can understand this fact; for in molluscs, even in
the lowest members of the class, we do not find nearly so much
indefinite repetition of any one part as we find in the other great
classes of the animal and vegetable kingdoms.

But morphology is a much more complex subject than it at first appears,
as has lately been well shown in a remarkable paper by Mr. E. Ray
Lankester, who has drawn an important distinction between certain
classes of cases which have all been equally ranked by naturalists as
homologous. He proposes to call the structures which resemble each
other in distinct animals, owing to their descent from a common
progenitor with subsequent modification, _homogenous;_ and the
resemblances which cannot thus be accounted for, he proposes to call
_homoplastic_. For instance, he believes that the hearts of birds and
mammals are as a whole homogenous—that is, have been derived from a
common progenitor; but that the four cavities of the heart in the two
classes are homoplastic—that is, have been independently developed. Mr.
Lankester also adduces the close resemblance of the parts on the right
and left sides of the body, and in the successive segments of the same
individual animal; and here we have parts commonly called homologous
which bear no relation to the descent of distinct species from a common
progenitor. Homoplastic structures are the same with those which I have
classed, though in a very imperfect manner, as analogous modifications
or resemblances. Their formation may be attributed in part to distinct
organisms, or to distinct parts of the same organism, having varied in
an analogous manner; and in part to similar modifications, having been
preserved for the same general purpose or function, of which many
instances have been given.

Naturalists frequently speak of the skull as formed of metamorphosed
vertebræ; the jaws of crabs as metamorphosed legs; the stamens and
pistils in flowers as metamorphosed leaves; but it would in most cases
be more correct, as Professor Huxley has remarked, to speak of both
skull and vertebræ, jaws and legs, &c., as having been metamorphosed,
not one from the other, as they now exist, but from some common and
simpler element. Most naturalists, however, use such language only in a
metaphorical sense: they are far from meaning that during a long course
of descent, primordial organs of any kind—vertebræ in the one case and
legs in the other—have actually been converted into skulls or jaws. Yet
so strong is the appearance of this having occurred that naturalists
can hardly avoid employing language having this plain signification.
According to the views here maintained, such language may be used
literally; and the wonderful fact of the jaws, for instance, of a crab
retaining numerous characters, which they probably would have retained
through inheritance, if they had really been metamorphosed from true
though extremely simple legs, is in part explained.

_Development and Embryology._


This is one of the most important subjects in the whole round of
natural history. The metamorphoses of insects, with which every one is
familiar, are generally effected abruptly by a few stages; but the
transformations are in reality numerous and gradual, though concealed.
A certain ephemerous insect (Chlöeon) during its development, moults,
as shown by Sir J. Lubbock, above twenty times, and each time undergoes
a certain amount of change; and in this case we see the act of
metamorphosis performed in a primary and gradual manner. Many insects,
and especially certain crustaceans, show us what wonderful changes of
structure can be effected during development. Such changes, however,
reach their acme in the so-called alternate generations of some of the
lower animals. It is, for instance, an astonishing fact that a delicate
branching coralline, studded with polypi, and attached to a submarine
rock, should produce, first by budding and then by transverse division,
a host of huge floating jelly-fishes; and that these should produce
eggs, from which are hatched swimming animalcules, which attach
themselves to rocks and become developed into branching corallines; and
so on in an endless cycle. The belief in the essential identity of the
process of alternate generation and of ordinary metamorphosis has been
greatly strengthened by Wagner’s discovery of the larva or maggot of a
fly, namely the Cecidomyia, producing asexually other larvæ, and these
others, which finally are developed into mature males and females,
propagating their kind in the ordinary manner by eggs.

It may be worth notice that when Wagner’s remarkable discovery was
first announced, I was asked how was it possible to account for the
larvæ of this fly having acquired the power of a sexual reproduction.
As long as the case remained unique no answer could be given. But
already Grimm has shown that another fly, a Chironomus, reproduces
itself in nearly the same manner, and he believes that this occurs
frequently in the order. It is the pupa, and not the larva, of the
Chironomus which has this power; and Grimm further shows that this
case, to a certain extent, “unites that of the Cecidomyia with the
parthenogenesis of the Coccidæ;” the term parthenogenesis implying that
the mature females of the Coccidæ are capable of producing fertile eggs
without the concourse of the male. Certain animals belonging to several
classes are now known to have the power of ordinary reproduction at an
unusually early age; and we have only to accelerate parthenogenetic
reproduction by gradual steps to an earlier and earlier age—Chironomus
showing us an almost exactly intermediate stage, viz., that of the
pupa—and we can perhaps account for the marvellous case of the
Cecidomyia.

It has already been stated that various parts in the same individual,
which are exactly alike during an early embryonic period, become widely
different and serve for widely different purposes in the adult state.
So again it has been shown that generally the embryos of the most
distinct species belonging to the same class are closely similar, but
become, when fully developed, widely dissimilar. A better proof of this
latter fact cannot be given than the statement by Von Baer that “the
embryos of mammalia, of birds, lizards and snakes, probably also of
chelonia, are in the earliest states exceedingly like one another, both
as a whole and in the mode of development of their parts; so much so,
in fact, that we can often distinguish the embryos only by their size.
In my possession are two little embryos in spirit, whose names I have
omitted to attach, and at present I am quite unable to say to what
class they belong. They may be lizards or small birds, or very young
mammalia, so complete is the similarity in the mode of formation of the
head and trunk in these animals. The extremities, however, are still
absent in these embryos. But even if they had existed in the earliest
stage of their development we should learn nothing, for the feet of
lizards and mammals, the wings and feet of birds, no less than the
hands and feet of man, all arise from the same fundamental form.” The
larvæ of most crustaceans, at corresponding stages of development,
closely resemble each other, however different the adults may become;
and so it is with very many other animals. A trace of the law of
embryonic resemblance occasionally lasts till a rather late age: thus
birds of the same genus, and of allied genera, often resemble each
other in their immature plumage; as we see in the spotted feathers in
the young of the thrush group. In the cat tribe, most of the species
when adult are striped or spotted in lines; and stripes or spots can be
plainly distinguished in the whelp of the lion and the puma. We
occasionally, though rarely, see something of the same kind in plants;
thus the first leaves of the ulex or furze, and the first leaves of the
phyllodineous acacias, are pinnate or divided like the ordinary leaves
of the leguminosæ.

The points of structure, in which the embryos of widely different
animals within the same class resemble each other, often have no direct
relation to their conditions of existence. We cannot, for instance,
suppose that in the embryos of the vertebrata the peculiar loop-like
courses of the arteries near the branchial slits are related to similar
conditions—in the young mammal which is nourished in the womb of its
mother, in the egg of the bird which is hatched in a nest, and in the
spawn of a frog under water. We have no more reason to believe in such
a relation than we have to believe that the similar bones in the hand
of a man, wing of a bat, and fin of a porpoise, are related to similar
conditions of life. No one supposes that the stripes on the whelp of a
lion, or the spots on the young blackbird, are of any use to these
animals.

The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period
of activity may come on earlier or later in life; but whenever it comes
on, the adaptation of the larva to its conditions of life is just as
perfect and as beautiful as in the adult animal. In how important a
manner this has acted, has recently been well shown by Sir J. Lubbock
in his remarks on the close similarity of the larvæ of some insects
belonging to very different orders, and on the dissimilarity of the
larvæ of other insects within the same order, according to their habits
of life. Owing to such adaptations the similarity of the larvæ of
allied animals is sometimes greatly obscured; especially when there is
a division of labour during the different stages of development, as
when the same larva has during one stage to search for food, and during
another stage has to search for a place of attachment. Cases can even
be given of the larvæ of allied species, or groups of species,
differing more from each other than do the adults. In most cases,
however, the larvæ, though active, still obey, more or less closely,
the law of common embryonic resemblance. Cirripedes afford a good
instance of this: even the illustrious Cuvier did not perceive that a
barnacle was a crustacean: but a glance at the larva shows this in an
unmistakable manner. So again the two main divisions of cirripedes, the
pedunculated and sessile, though differing widely in external
appearance, have larvæ in all their stages barely distinguishable.

The embryo in the course of development generally rises in
organisation. I use this expression, though I am aware that it is
hardly possible to define clearly what is meant by organisation being
higher or lower. But no one probably will dispute that the butterfly is
higher than the caterpillar. In some cases, however, the mature animal
must be considered as lower in the scale than the larva, as with
certain parasitic crustaceans. To refer once again to cirripedes: the
larvæ in the first stage have three pairs of locomotive organs, a
simple single eye, and a probosciformed mouth, with which they feed
largely, for they increase much in size. In the second stage, answering
to the chrysalis stage of butterflies, they have six pairs of
beautifully constructed natatory legs, a pair of magnificent compound
eyes, and extremely complex antennæ; but they have a closed and
imperfect mouth, and cannot feed: their function at this stage is, to
search out by their well-developed organs of sense, and to reach by
their active powers of swimming, a proper place on which to become
attached and to undergo their final metamorphosis. When this is
completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but they
have no antennæ, and their two eyes are now reconverted into a minute,
single, simple eye-spot. In this last and complete state, cirripedes
may be considered as either more highly or more lowly organised than
they were in the larval condition. But in some genera the larvæ become
developed into hermaphrodites having the ordinary structure, or into
what I have called complemental males; and in the latter the
development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time and is destitute of mouth, stomach, and
every other organ of importance, excepting those for reproduction.

We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as
in some necessary manner contingent on growth. But there is no reason
why, for instance, the wing of a bat, or the fin of a porpoise, should
not have been sketched out with all their parts in proper proportion,
as soon as any part became visible. In some whole groups of animals and
in certain members of other groups this is the case, and the embryo
does not at any period differ widely from the adult: thus Owen has
remarked in regard to cuttle-fish, “there is no metamorphosis; the
cephalopodic character is manifested long before the parts of the
embryo are completed.” Land-shells and fresh-water crustaceans are born
having their proper forms, while the marine members of the same two
great classes pass through considerable and often great changes during
their development. Spiders, again, barely undergo any metamorphosis.
The larvæ of most insects pass through a worm-like stage, whether they
are active and adapted to diversified habits, or are inactive from
being placed in the midst of proper nutriment, or from being fed by
their parents; but in some few cases, as in that of Aphis, if we look
to the admirable drawings of the development of this insect, by
Professor Huxley, we see hardly any trace of the vermiform stage.

Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Müller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple
nauplius-form, and after passing through two or more zoëa-stages, and
then through the mysis-stage, finally acquire their mature structure:
now in the whole great malacostracan order, to which these crustaceans
belong, no other member is as yet known to be first developed under the
nauplius-form, though many appear as zoëas; nevertheless Müller assigns
reasons for his belief, that if there had been no suppression of
development, all these crustaceans would have appeared as nauplii.

How, then, can we explain these several facts in embryology—namely, the
very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes,
being at an early period of growth alike; the common, but not
invariable, resemblance between the embryos or larvæ of the most
distinct species in the same class; the embryo often retaining, while
within the egg or womb, structures which are of no service to it,
either at that or at a later period of life; on the other hand, larvæ
which have to provide for their own wants, being perfectly adapted to
the surrounding conditions; and lastly, the fact of certain larvæ
standing higher in the scale of organisation than the mature animal
into which they are developed? I believe that all these facts can be
explained as follows.

It is commonly assumed, perhaps from monstrosities affecting the embryo
at a very early period, that slight variations or individual
differences necessarily appear at an equally early period. We have
little evidence on this head, but what we have certainly points the
other way; for it is notorious that breeders of cattle, horses and
various fancy animals, cannot positively tell, until some time after
birth, what will be the merits and demerits of their young animals. We
see this plainly in our own children; we cannot tell whether a child
will be tall or short, or what its precise features will be. The
question is not, at what period of life any variation may have been
caused, but at what period the effects are displayed. The cause may
have acted, and I believe often has acted, on one or both parents
before the act of generation. It deserves notice that it is of no
importance to a very young animal, as long as it is nourished and
protected by its parent, whether most of its characters are acquired a
little earlier or later in life. It would not signify, for instance, to
a bird which obtained its food by having a much-curved beak whether or
not while young it possessed a beak of this shape, as long as it was
fed by its parents.

I have stated in the first chapter, that at whatever age any variation
first appears in the parent, it tends to reappear at a corresponding
age in the offspring. Certain variations can only appear at
corresponding ages; for instance, peculiarities in the caterpillar,
cocoon, or imago states of the silk-moth; or, again, in the full-grown
horns of cattle. But variations which, for all that we can see might
have appeared either earlier or later in life, likewise tend to
reappear at a corresponding age in the offspring and parent. I am far
from meaning that this is invariably the case, and I could give several
exceptional cases of variations (taking the word in the largest sense)
which have supervened at an earlier age in the child than in the
parent.

These two principles, namely, that slight variations generally appear
at a not very early period of life, and are inherited at a
corresponding not early period, explain, as I believe, all the above
specified leading facts in embryology. But first let us look to a few
analogous cases in our domestic varieties. Some authors who have
written on Dogs maintain that the greyhound and bull-dog, though so
different, are really closely allied varieties, descended from the same
wild stock, hence I was curious to see how far their puppies differed
from each other. I was told by breeders that they differed just as much
as their parents, and this, judging by the eye, seemed almost to be the
case; but on actually measuring the old dogs and their six-days-old
puppies, I found that the puppies had not acquired nearly their full
amount of proportional difference. So, again, I was told that the foals
of cart and race-horses—breeds which have been almost wholly formed by
selection under domestication—differed as much as the full-grown
animals; but having had careful measurements made of the dams and of
three-days-old colts of race and heavy cart-horses, I find that this is
by no means the case.

As we have conclusive evidence that the breeds of the Pigeon are
descended from a single wild species, I compared the young pigeons
within twelve hours after being hatched. I carefully measured the
proportions (but will not here give the details) of the beak, width of
mouth, length of nostril and of eyelid, size of feet and length of leg,
in the wild parent species, in pouters, fantails, runts, barbs,
dragons, carriers, and tumblers. Now, some of these birds, when mature,
differ in so extraordinary a manner in the length and form of beak, and
in other characters, that they would certainly have been ranked as
distinct genera if found in a state of nature. But when the nestling
birds of these several breeds were placed in a row, though most of them
could just be distinguished, the proportional differences in the above
specified points were incomparably less than in the full-grown birds.
Some characteristic points of difference—for instance, that of the
width of mouth—could hardly be detected in the young. But there was one
remarkable exception to this rule, for the young of the short-faced
tumbler differed from the young of the wild rock-pigeon, and of the
other breeds, in almost exactly the same proportions as in the adult
stage.

These facts are explained by the above two principles. Fanciers select
their dogs, horses, pigeons, &c., for breeding, when nearly grown up.
They are indifferent whether the desired qualities are acquired earlier
or later in life, if the full-grown animal possesses them. And the
cases just given, more especially that of the pigeons, show that the
characteristic differences which have been accumulated by man’s
selection, and which give value to his breeds, do not generally appear
at a very early period of life, and are inherited at a corresponding
not early period. But the case of the short-faced tumbler, which when
twelve hours old possessed its proper characters, proves that this is
not the universal rule; for here the characteristic differences must
either have appeared at an earlier period than usual, or, if not so,
the differences must have been inherited, not at a corresponding, but
at an earlier age.

Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and
modified through natural selection for different habits. Then, from the
many slight successive variations having supervened in the several
species at a not early age, and having been inherited at a
corresponding age, the young will have been but little modified, and
they will still resemble each other much more closely than do the
adults, just as we have seen with the breeds of the pigeon. We may
extend this view to widely distinct structures and to whole classes.
The fore-limbs, for instance, which once served as legs to a remote
progenitor, may have become, through a long course of modification,
adapted in one descendant to act as hands, in another as paddles, in
another as wings; but on the above two principles the fore-limbs will
not have been much modified in the embryos of these several forms;
although in each form the fore-limb will differ greatly in the adult
state. Whatever influence long continued use or disuse may have had in
modifying the limbs or other parts of any species, this will chiefly or
solely have affected it when nearly mature, when it was compelled to
use its full powers to gain its own living; and the effects thus
produced will have been transmitted to the offspring at a corresponding
nearly mature age. Thus the young will not be modified, or will be
modified only in a slight degree, through the effects of the increased
use or disuse of parts.

With some animals the successive variations may have supervened at a
very early period of life, or the steps may have been inherited at an
earlier age than that at which they first occurred. In either of these
cases the young or embryo will closely resemble the mature parent-form,
as we have seen with the short-faced tumbler. And this is the rule of
development in certain whole groups, or in certain sub-groups alone, as
with cuttle-fish, land-shells, fresh-water crustaceans, spiders, and
some members of the great class of insects. With respect to the final
cause of the young in such groups not passing through any
metamorphosis, we can see that this would follow from the following
contingencies: namely, from the young having to provide at a very early
age for their own wants, and from their following the same habits of
life with their parents; for in this case it would be indispensable for
their existence that they should be modified in the same manner as
their parents. Again, with respect to the singular fact that many
terrestrial and fresh-water animals do not undergo any metamorphosis,
while marine members of the same groups pass through various
transformations, Fritz Müller has suggested that the process of slowly
modifying and adapting an animal to live on the land or in fresh water,
instead of in the sea, would be greatly simplified by its not passing
through any larval stage; for it is not probable that places well
adapted for both the larval and mature stages, under such new and
greatly changed habits of life, would commonly be found unoccupied or
ill-occupied by other organisms. In this case the gradual acquirement
at an earlier and earlier age of the adult structure would be favoured
by natural selection; and all traces of former metamorphoses would
finally be lost.

If, on the other hand, it profited the young of an animal to follow
habits of life slightly different from those of the parent-form, and
consequently to be constructed on a slightly different plan, or if it
profited a larva already different from its parent to change still
further, then, on the principle of inheritance at corresponding ages,
the young or the larvæ might be rendered by natural selection more and
more different from their parents to any conceivable extent.
Differences in the larva might, also, become correlated with successive
stages of its development; so that the larva, in the first stage, might
come to differ greatly from the larva in the second stage, as is the
case with many animals. The adult might also become fitted for sites or
habits, in which organs of locomotion or of the senses, &c., would be
useless; and in this case the metamorphosis would be retrograde.

From the remarks just made we can see how by changes of structure in
the young, in conformity with changed habits of life, together with
inheritance at corresponding ages, animals might come to pass through
stages of development, perfectly distinct from the primordial condition
of their adult progenitors. Most of our best authorities are now
convinced that the various larval and pupal stages of insects have thus
been acquired through adaptation, and not through inheritance from some
ancient form. The curious case of Sitaris—a beetle which passes through
certain unusual stages of development—will illustrate how this might
occur. The first larval form is described by M. Fabre, as an active,
minute insect, furnished with six legs, two long antennæ, and four
eyes. These larvæ are hatched in the nests of bees; and when the male
bees emerge from their burrows, in the spring, which they do before the
females, the larvæ spring on them, and afterwards crawl on to the
females while paired with the males. As soon as the female bee deposits
her eggs on the surface of the honey stored in the cells, the larvæ of
the Sitaris leap on the eggs and devour them. Afterwards they undergo a
complete change; their eyes disappear; their legs and antennæ become
rudimentary, and they feed on honey; so that they now more closely
resemble the ordinary larvæ of insects; ultimately they undergo a
further transformation, and finally emerge as the perfect beetle. Now,
if an insect, undergoing transformations like those of the Sitaris,
were to become the progenitor of a whole new class of insects, the
course of development of the new class would be widely different from
that of our existing insects; and the first larval stage certainly
would not represent the former condition of any adult and ancient form.

On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the
condition of the progenitor of the whole group in its adult state. In
the great class of the Crustacea, forms wonderfully distinct from each
other, namely, suctorial parasites, cirripedes, entomostraca, and even
the malacostraca, appear at first as larvæ under the nauplius-form; and
as these larvæ live and feed in the open sea, and are not adapted for
any peculiar habits of life, and from other reasons assigned by Fritz
Müller, it is probable that at some very remote period an independent
adult animal, resembling the Nauplius, existed, and subsequently
produced, along several divergent lines of descent, the above-named
great Crustacean groups. So again, it is probable, from what we know of
the embryos of mammals, birds, fishes and reptiles, that these animals
are the modified descendants of some ancient progenitor, which was
furnished in its adult state with branchiæ, a swim-bladder, four
fin-like limbs, and a long tail, all fitted for an aquatic life.

As all the organic beings, extinct and recent, which have ever lived,
can be arranged within a few great classes; and as all within each
class have, according to our theory, been connected together by fine
gradations, the best, and, if our collections were nearly perfect, the
only possible arrangement, would be genealogical; descent being the
hidden bond of connexion which naturalists have been seeking under the
term of the Natural System. On this view we can understand how it is
that, in the eyes of most naturalists, the structure of the embryo is
even more important for classification than that of the adult. In two
or more groups of animals, however much they may differ from each other
in structure and habits in their adult condition, if they pass through
closely similar embryonic stages, we may feel assured that they are all
descended from one parent-form, and are therefore closely related.
Thus, community in embryonic structure reveals community of descent;
but dissimilarity in embryonic development does not prove discommunity
of descent, for in one of two groups the developmental stages may have
been suppressed, or may have been so greatly modified through
adaptation to new habits of life as to be no longer recognisable. Even
in groups, in which the adults have been modified to an extreme degree,
community of origin is often revealed by the structure of the larvæ; we
have seen, for instance, that cirripedes, though externally so like
shell-fish, are at once known by their larvæ to belong to the great
class of crustaceans. As the embryo often shows us more or less plainly
the structure of the less modified and ancient progenitor of the group,
we can see why ancient and extinct forms so often resemble in their
adult state the embryos of existing species of the same class. Agassiz
believes this to be a universal law of nature; and we may hope
hereafter to see the law proved true. It can, however, be proved true
only in those cases in which the ancient state of the progenitor of the
group has not been wholly obliterated, either by successive variations
having supervened at a very early period of growth, or by such
variations having been inherited at an earlier age than that at which
they first appeared. It should also be borne in mind, that the law may
be true, but yet, owing to the geological record not extending far
enough back in time, may remain for a long period, or for ever,
incapable of demonstration. The law will not strictly hold good in
those cases in which an ancient form became adapted in its larval state
to some special line of life, and transmitted the same larval state to
a whole group of descendants; for such larval state will not resemble
any still more ancient form in its adult state.

Thus, as it seems to me, the leading facts in embryology, which are
second to none in importance, are explained on the principle of
variations in the many descendants from some one ancient progenitor,
having appeared at a not very early period of life, and having been
inherited at a corresponding period. Embryology rises greatly in
interest, when we look at the embryo as a picture, more or less
obscured, of the progenitor, either in its adult or larval state, of
all the members of the same great class.

_Rudimentary, Atrophied, and Aborted Organs._


Organs or parts in this strange condition, bearing the plain stamp of
inutility, are extremely common, or even general, throughout nature. It
would be impossible to name one of the higher animals in which some
part or other is not in a rudimentary condition. In the mammalia, for
instance, the males possess rudimentary mammæ; in snakes one lobe of
the lungs is rudimentary; in birds the “bastard-wing” may safely be
considered as a rudimentary digit, and in some species the whole wing
is so far rudimentary that it cannot be used for flight. What can be
more curious than the presence of teeth in foetal whales, which when
grown up have not a tooth in their heads; or the teeth, which never cut
through the gums, in the upper jaws of unborn calves?

Rudimentary organs plainly declare their origin and meaning in various
ways. There are beetles belonging to closely allied species, or even to
the same identical species, which have either full-sized and perfect
wings, or mere rudiments of membrane, which not rarely lie under
wing-covers firmly soldered together; and in these cases it is
impossible to doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality: this occasionally occurs
with the mammæ of male mammals, which have been known to become well
developed and to secrete milk. So again in the udders of the genus Bos,
there are normally four developed and two rudimentary teats; but the
latter in our domestic cows sometimes become well developed and yield
milk. In regard to plants, the petals are sometimes rudimentary, and
sometimes well developed in the individuals of the same species. In
certain plants having separated sexes Kölreuter found that by crossing
a species, in which the male flowers included a rudiment of a pistil,
with an hermaphrodite species, having of course a well-developed
pistil, the rudiment in the hybrid offspring was much increased in
size; and this clearly shows that the rudimentary and perfect pistils
are essentially alike in nature. An animal may possess various parts in
a perfect state, and yet they may in one sense be rudimentary, for they
are useless: thus the tadpole of the common salamander or water-newt,
as Mr. G.H. Lewes remarks, “has gills, and passes its existence in the
water; but the Salamandra atra, which lives high up among the
mountains, brings forth its young full-formed. This animal never lives
in the water. Yet if we open a gravid female, we find tadpoles inside
her with exquisitely feathered gills; and when placed in water they
swim about like the tadpoles of the water-newt. Obviously this aquatic
organisation has no reference to the future life of the animal, nor has
it any adaptation to its embryonic condition; it has solely reference
to ancestral adaptations, it repeats a phase in the development of its
progenitors.”

An organ, serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is
to allow the pollen-tubes to reach the ovules within the ovarium. The
pistil consists of a stigma supported on the style; but in some
Compositæ, the male florets, which of course cannot be fecundated, have
a rudimentary pistil, for it is not crowned with a stigma; but the
style remains well developed and is clothed in the usual manner with
hairs, which serve to brush the pollen out of the surrounding and
conjoined anthers. Again, an organ may become rudimentary for its
proper purpose, and be used for a distinct one: in certain fishes the
swim-bladder seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent breathing organ or
lung. Many similar instances could be given.

Useful organs, however little they may be developed, unless we have
reason to suppose that they were formerly more highly developed, ought
not to be considered as rudimentary. They may be in a nascent
condition, and in progress towards further development. Rudimentary
organs, on the other hand, are either quite useless, such as teeth
which never cut through the gums, or almost useless, such as the wings
of an ostrich, which serve merely as sails. As organs in this condition
would formerly, when still less developed, have been of even less use
than at present, they cannot formerly have been produced through
variation and natural selection, which acts solely by the preservation
of useful modifications. They have been partially retained by the power
of inheritance, and relate to a former state of things. It is, however,
often difficult to distinguish between rudimentary and nascent organs;
for we can judge only by analogy whether a part is capable of further
development, in which case alone it deserves to be called nascent.
Organs in this condition will always be somewhat rare; for beings thus
provided will commonly have been supplanted by their successors with
the same organ in a more perfect state, and consequently will have
become long ago extinct. The wing of the penguin is of high service,
acting as a fin; it may, therefore, represent the nascent state of the
wing: not that I believe this to be the case; it is more probably a
reduced organ, modified for a new function: the wing of the Apteryx, on
the other hand, is quite useless, and is truly rudimentary. Owen
considers the simple filamentary limbs of the Lepidosiren as the
“beginnings of organs which attain full functional development in
higher vertebrates;” but, according to the view lately advocated by Dr.
Günther, they are probably remnants, consisting of the persistent axis
of a fin, with the lateral rays or branches aborted. The mammary glands
of the Ornithorhynchus may be considered, in comparison with the udders
of a cow, as in a nascent condition. The ovigerous frena of certain
cirripedes, which have ceased to give attachment to the ova and are
feebly developed, are nascent branchiæ.

Rudimentary organs in the individuals of the same species are very
liable to vary in the degree of their development and in other
respects. In closely allied species, also, the extent to which the same
organ has been reduced occasionally differs much. This latter fact is
well exemplified in the state of the wings of female moths belonging to
the same family. Rudimentary organs may be utterly aborted; and this
implies, that in certain animals or plants, parts are entirely absent
which analogy would lead us to expect to find in them, and which are
occasionally found in monstrous individuals. Thus in most of the
Scrophulariaceæ the fifth stamen is utterly aborted; yet we may
conclude that a fifth stamen once existed, for a rudiment of it is
found in many species of the family, and this rudiment occasionally
becomes perfectly developed, as may sometimes be seen in the common
snap-dragon. In tracing the homologies of any part in different members
of the same class, nothing is more common, or, in order fully to
understand the relations of the parts, more useful than the discovery
of rudiments. This is well shown in the drawings given by Owen of the
leg bones of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the
embryo, but afterwards wholly disappear. It is also, I believe, a
universal rule, that a rudimentary part is of greater size in the
embryo relatively to the adjoining parts, than in the adult; so that
the organ at this early age is less rudimentary, or even cannot be said
to be in any degree rudimentary. Hence rudimentary organs in the adult
are often said to have retained their embryonic condition.

I have now given the leading facts with respect to rudimentary organs.
In reflecting on them, every one must be struck with astonishment; for
the same reasoning power which tells us that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs are imperfect and useless.
In works on natural history, rudimentary organs are generally said to
have been created “for the sake of symmetry,” or in order “to complete
the scheme of nature.” But this is not an explanation, merely a
restatement of the fact. Nor is it consistent with itself: thus the
boa-constrictor has rudiments of hind limbs and of a pelvis, and if it
be said that these bones have been retained “to complete the scheme of
nature,” why, as Professor Weismann asks, have they not been retained
by other snakes, which do not possess even a vestige of these same
bones? What would be thought of an astronomer who maintained that the
satellites revolve in elliptic courses round their planets “for the
sake of symmetry,” because the planets thus revolve round the sun? An
eminent physiologist accounts for the presence of rudimentary organs,
by supposing that they serve to excrete matter in excess, or matter
injurious to the system; but can we suppose that the minute papilla,
which often represents the pistil in male flowers, and which is formed
of mere cellular tissue, can thus act? Can we suppose that rudimentary
teeth, which are subsequently absorbed, are beneficial to the rapidly
growing embryonic calf by removing matter so precious as phosphate of
lime? When a man’s fingers have been amputated, imperfect nails have
been known to appear on the stumps, and I could as soon believe that
these vestiges of nails are developed in order to excrete horny matter,
as that the rudimentary nails on the fin of the manatee have been
developed for this same purpose.

On the view of descent with modification, the origin of rudimentary
organs is comparatively simple; and we can understand to a large extent
the laws governing their imperfect development. We have plenty of cases
of rudimentary organs in our domestic productions, as the stump of a
tail in tailless breeds, the vestige of an ear in earless breeds of
sheep—the reappearance of minute dangling horns in hornless breeds of
cattle, more especially, according to Youatt, in young animals—and the
state of the whole flower in the cauliflower. We often see rudiments of
various parts in monsters; but I doubt whether any of these cases throw
light on the origin of rudimentary organs in a state of nature, further
than by showing that rudiments can be produced; for the balance of
evidence clearly indicates that species under nature do not undergo
great and abrupt changes. But we learn from the study of our domestic
productions that the disuse of parts leads to their reduced size; and
that the result is inherited.

It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more
and more complete reduction of a part, until at last it became
rudimentary—as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which
have seldom been forced by beasts of prey to take flight, and have
ultimately lost the power of flying. Again, an organ, useful under
certain conditions, might become injurious under others, as with the
wings of beetles living on small and exposed islands; and in this case
natural selection will have aided in reducing the organ, until it was
rendered harmless and rudimentary.

Any change in structure and function, which can be effected by small
stages, is within the power of natural selection; so that an organ
rendered, through changed habits of life, useless or injurious for one
purpose, might be modified and used for another purpose. An organ
might, also, be retained for one alone of its former functions. Organs,
originally formed by the aid of natural selection, when rendered
useless may well be variable, for their variations can no longer be
checked by natural selection. All this agrees well with what we see
under nature. Moreover, at whatever period of life either disuse or
selection reduces an organ, and this will generally be when the being
has come to maturity and to exert its full powers of action, the
principle of inheritance at corresponding ages will tend to reproduce
the organ in its reduced state at the same mature age, but will seldom
affect it in the embryo. Thus we can understand the greater size of
rudimentary organs in the embryo relatively to the adjoining parts, and
their lesser relative size in the adult. If, for instance, the digit of
an adult animal was used less and less during many generations, owing
to some change of habits, or if an organ or gland was less and less
functionally exercised, we may infer that it would become reduced in
size in the adult descendants of this animal, but would retain nearly
its original standard of development in the embryo.

There remains, however, this difficulty. After an organ has ceased
being used, and has become in consequence much reduced, how can it be
still further reduced in size until the merest vestige is left; and how
can it be finally quite obliterated? It is scarcely possible that
disuse can go on producing any further effect after the organ has once
been rendered functionless. Some additional explanation is here
requisite which I cannot give. If, for instance, it could be proved
that every part of the organisation tends to vary in a greater degree
towards diminution than toward augmentation of size, then we should be
able to understand how an organ which has become useless would be
rendered, independently of the effects of disuse, rudimentary and would
at last be wholly suppressed; for the variations towards diminished
size would no longer be checked by natural selection. The principle of
the economy of growth, explained in a former chapter, by which the
materials forming any part, if not useful to the possessor, are saved
as far as is possible, will perhaps come into play in rendering a
useless part rudimentary. But this principle will almost necessarily be
confined to the earlier stages of the process of reduction; for we
cannot suppose that a minute papilla, for instance, representing in a
male flower the pistil of the female flower, and formed merely of
cellular tissue, could be further reduced or absorbed for the sake of
economising nutriment.

Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a
former state of things, and have been retained solely through the power
of inheritance—we can understand, on the genealogical view of
classification, how it is that systematists, in placing organisms in
their proper places in the natural system, have often found rudimentary
parts as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless
in the pronunciation, but which serve as a clue for its derivation. On
the view of descent with modification, we may conclude that the
existence of organs in a rudimentary, imperfect, and useless condition,
or quite aborted, far from presenting a strange difficulty, as they
assuredly do on the old doctrine of creation, might even have been
anticipated in accordance with the views here explained.

_Summary._


In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups—that the
nature of the relationships by which all living and extinct organisms
are united by complex, radiating, and circuitous lines of affinities
into a few grand classes—the rules followed and the difficulties
encountered by naturalists in their classifications—the value set upon
characters, if constant and prevalent, whether of high or of the most
trifling importance, or, as with rudimentary organs of no
importance—the wide opposition in value between analogical or adaptive
characters, and characters of true affinity; and other such rules—all
naturally follow if we admit the common parentage of allied forms,
together with their modification through variation and natural
selection, with the contingencies of extinction and divergence of
character. In considering this view of classification, it should be
borne in mind that the element of descent has been universally used in
ranking together the sexes, ages, dimorphic forms, and acknowledged
varieties of the same species, however much they may differ from each
other in structure. If we extend the use of this element of descent—the
one certainly known cause of similarity in organic beings—we shall
understand what is meant by the Natural System: it is genealogical in
its attempted arrangement, with the grades of acquired difference
marked by the terms, varieties, species, genera, families, orders, and
classes.

On this same view of descent with modification, most of the great facts
in Morphology become intelligible—whether we look to the same pattern
displayed by the different species of the same class in their
homologous organs, to whatever purpose applied, or to the serial and
lateral homologies in each individual animal and plant.

On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the leading
facts in embryology; namely, the close resemblance in the individual
embryo of the parts which are homologous, and which when matured become
widely different in structure and function; and the resemblance of the
homologous parts or organs in allied though distinct species, though
fitted in the adult state for habits as different as is possible. Larvæ
are active embryos, which have become specially modified in a greater
or less degree in relation to their habits of life, with their
modifications inherited at a corresponding early age. On these same
principles, and bearing in mind that when organs are reduced in size,
either from disuse or through natural selection, it will generally be
at that period of life when the being has to provide for its own wants,
and bearing in mind how strong is the force of inheritance—the
occurrence of rudimentary organs might even have been anticipated. The
importance of embryological characters and of rudimentary organs in
classification is intelligible, on the view that a natural arrangement
must be genealogical.

Finally, the several classes of facts which have been considered in
this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera and families, with which this world is peopled, are all
descended, each within its own class or group, from common parents, and
have all been modified in the course of descent, that I should without
hesitation adopt this view, even if it were unsupported by other facts
or arguments.




CHAPTER XV.
RECAPITULATION AND CONCLUSION.


Recapitulation of the objections to the theory of Natural
Selection—Recapitulation of the general and special circumstances in
its favour—Causes of the general belief in the immutability of
species—How far the theory of Natural Selection may be extended—Effects
of its adoption on the study of Natural History—Concluding remarks.


As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.

That many and serious objections may be advanced against the theory of
descent with modification through variation and natural selection, I do
not deny. I have endeavoured to give to them their full force. Nothing
at first can appear more difficult to believe than that the more
complex organs and instincts have been perfected, not by means superior
to, though analogous with, human reason, but by the accumulation of
innumerable slight variations, each good for the individual possessor.
Nevertheless, this difficulty, though appearing to our imagination
insuperably great, cannot be considered real if we admit the following
propositions, namely, that all parts of the organisation and instincts
offer, at least individual differences—that there is a struggle for
existence leading to the preservation of profitable deviations of
structure or instinct—and, lastly, that gradations in the state of
perfection of each organ may have existed, each good of its kind. The
truth of these propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially among
broken and failing groups of organic beings, which have suffered much
extinction; but we see so many strange gradations in nature, that we
ought to be extremely cautious in saying that any organ or instinct, or
any whole structure, could not have arrived at its present state by
many graduated steps. There are, it must be admitted, cases of special
difficulty opposed to the theory of natural selection; and one of the
most curious of these is the existence in the same community of two or
three defined castes of workers or sterile female ants; but I have
attempted to show how these difficulties can be mastered.

With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the ninth chapter,
which seem to me conclusively to show that this sterility is no more a
special endowment than is the incapacity of two distinct kinds of trees
to be grafted together; but that it is incidental on differences
confined to the reproductive systems of the intercrossed species. We
see the truth of this conclusion in the vast difference in the results
of crossing the same two species reciprocally—that is, when one species
is first used as the father and then as the mother. Analogy from the
consideration of dimorphic and trimorphic plants clearly leads to the
same conclusion, for when the forms are illegitimately united, they
yield few or no seed, and their offspring are more or less sterile; and
these forms belong to the same undoubted species, and differ from each
other in no respect except in their reproductive organs and functions.

Although the fertility of varieties when intercrossed, and of their
mongrel offspring, has been asserted by so many authors to be
universal, this cannot be considered as quite correct after the facts
given on the high authority of Gärtner and Kölreuter. Most of the
varieties which have been experimented on have been produced under
domestication; and as domestication (I do not mean mere confinement)
almost certainly tends to eliminate that sterility which, judging from
analogy, would have affected the parent-species if intercrossed, we
ought not to expect that domestication would likewise induce sterility
in their modified descendants when crossed. This elimination of
sterility apparently follows from the same cause which allows our
domestic animals to breed freely under diversified circumstances; and
this again apparently follows from their having been gradually
accustomed to frequent changes in their conditions of life.

A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid
offspring. On the one side, there is good reason to believe that slight
changes in the conditions of life give vigour and fertility to all
organic beings. We know also that a cross between the distinct
individuals of the same variety, and between distinct varieties,
increases the number of their offspring, and certainly gives to them
increased size and vigour. This is chiefly owing to the forms which are
crossed having been exposed to somewhat different conditions of life;
for I have ascertained by a labourious series of experiments that if
all the individuals of the same variety be subjected during several
generations to the same conditions, the good derived from crossing is
often much diminished or wholly disappears. This is one side of the
case. On the other side, we know that species which have long been
exposed to nearly uniform conditions, when they are subjected under
confinement to new and greatly changed conditions, either perish, or if
they survive, are rendered sterile, though retaining perfect health.
This does not occur, or only in a very slight degree, with our
domesticated productions, which have long been exposed to fluctuating
conditions. Hence when we find that hybrids produced by a cross between
two distinct species are few in number, owing to their perishing soon
after conception or at a very early age, or if surviving that they are
rendered more or less sterile, it seems highly probable that this
result is due to their having been in fact subjected to a great change
in their conditions of life, from being compounded of two distinct
organisations. He who will explain in a definite manner why, for
instance, an elephant or a fox will not breed under confinement in its
native country, whilst the domestic pig or dog will breed freely under
the most diversified conditions, will at the same time be able to give
a definite answer to the question why two distinct species, when
crossed, as well as their hybrid offspring, are generally rendered more
or less sterile, while two domesticated varieties when crossed and
their mongrel offspring are perfectly fertile.

Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are serious enough. All the
individuals of the same species, and all the species of the same genus,
or even higher group, are descended from common parents; and therefore,
in however distant and isolated parts of the world they may now be
found, they must in the course of successive generations have travelled
from some one point to all the others. We are often wholly unable even
to conjecture how this could have been effected. Yet, as we have reason
to believe that some species have retained the same specific form for
very long periods of time, immensely long as measured by years, too
much stress ought not to be laid on the occasional wide diffusion of
the same species; for during very long periods there will always have
been a good chance for wide migration by many means. A broken or
interrupted range may often be accounted for by the extinction of the
species in the intermediate regions. It cannot be denied that we are as
yet very ignorant as to the full extent of the various climatical and
geographical changes which have affected the earth during modern
periods; and such changes will often have facilitated migration. As an
example, I have attempted to show how potent has been the influence of
the Glacial period on the distribution of the same and of allied
species throughout the world. We are as yet profoundly ignorant of the
many occasional means of transport. With respect to distinct species of
the same genus, inhabiting distant and isolated regions, as the process
of modification has necessarily been slow, all the means of migration
will have been possible during a very long period; and consequently the
difficulty of the wide diffusion of the species of the same genus is in
some degree lessened.

As according to the theory of natural selection an interminable number
of intermediate forms must have existed, linking together all the
species in each group by gradations as fine as our existing varieties,
it may be asked, Why do we not see these linking forms all around us?
Why are not all organic beings blended together in an inextricable
chaos? With respect to existing forms, we should remember that we have
no right to expect (excepting in rare cases) to discover _directly_
connecting links between them, but only between each and some extinct
and supplanted form. Even on a wide area, which has during a long
period remained continuous, and of which the climatic and other
conditions of life change insensibly in proceeding from a district
occupied by one species into another district occupied by a closely
allied species, we have no just right to expect often to find
intermediate varieties in the intermediate zones. For we have reason to
believe that only a few species of a genus ever undergo change; the
other species becoming utterly extinct and leaving no modified progeny.
Of the species which do change, only a few within the same country
change at the same time; and all modifications are slowly effected. I
have also shown that the intermediate varieties which probably at first
existed in the intermediate zones, would be liable to be supplanted by
the allied forms on either hand; for the latter, from existing in
greater numbers, would generally be modified and improved at a quicker
rate than the intermediate varieties, which existed in lesser numbers;
so that the intermediate varieties would, in the long run, be
supplanted and exterminated.

On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why
is not every geological formation charged with such links? Why does not
every collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life? Although geological
research has undoubtedly revealed the former existence of many links,
bringing numerous forms of life much closer together, it does not yield
the infinitely many fine gradations between past and present species
required on the theory, and this is the most obvious of the many
objections which may be urged against it. Why, again, do whole groups
of allied species appear, though this appearance is often false, to
have come in suddenly on the successive geological stages? Although we
now know that organic beings appeared on this globe, at a period
incalculably remote, long before the lowest bed of the Cambrian system
was deposited, why do we not find beneath this system great piles of
strata stored with the remains of the progenitors of the Cambrian
fossils? For on the theory, such strata must somewhere have been
deposited at these ancient and utterly unknown epochs of the world’s
history.

I can answer these questions and objections only on the supposition
that the geological record is far more imperfect than most geologists
believe. The number of specimens in all our museums is absolutely as
nothing compared with the countless generations of countless species
which have certainly existed. The parent form of any two or more
species would not be in all its characters directly intermediate
between its modified offspring, any more than the rock-pigeon is
directly intermediate in crop and tail between its descendants, the
pouter and fantail pigeons. We should not be able to recognise a
species as the parent of another and modified species, if we were to
examine the two ever so closely, unless we possessed most of the
intermediate links; and owing to the imperfection of the geological
record, we have no just right to expect to find so many links. If two
or three, or even more linking forms were discovered, they would simply
be ranked by many naturalists as so many new species, more especially
if found in different geological substages, let their differences be
ever so slight. Numerous existing doubtful forms could be named which
are probably varieties; but who will pretend that in future ages so
many fossil links will be discovered, that naturalists will be able to
decide whether or not these doubtful forms ought to be called
varieties? Only a small portion of the world has been geologically
explored. Only organic beings of certain classes can be preserved in a
fossil condition, at least in any great number. Many species when once
formed never undergo any further change but become extinct without
leaving modified descendants; and the periods during which species have
undergone modification, though long as measured by years, have probably
been short in comparison with the periods during which they retained
the same form. It is the dominant and widely ranging species which vary
most frequently and vary most, and varieties are often at first
local—both causes rendering the discovery of intermediate links in any
one formation less likely. Local varieties will not spread into other
and distant regions until they are considerably modified and improved;
and when they have spread, and are discovered in a geological
formation, they appear as if suddenly created there, and will be simply
classed as new species. Most formations have been intermittent in their
accumulation; and their duration has probably been shorter than the
average duration of specific forms. Successive formations are in most
cases separated from each other by blank intervals of time of great
length, for fossiliferous formations thick enough to resist future
degradation can, as a general rule, be accumulated only where much
sediment is deposited on the subsiding bed of the sea. During the
alternate periods of elevation and of stationary level the record will
generally be blank. During these latter periods there will probably be
more variability in the forms of life; during periods of subsidence,
more extinction.

With respect to the absence of strata rich in fossils beneath the
Cambrian formation, I can recur only to the hypothesis given in the
tenth chapter; namely, that though our continents and oceans have
endured for an enormous period in nearly their present relative
positions, we have no reason to assume that this has always been the
case; consequently formations much older than any now known may lie
buried beneath the great oceans. With respect to the lapse of time not
having been sufficient since our planet was consolidated for the
assumed amount of organic change, and this objection, as urged by Sir
William Thompson, is probably one of the gravest as yet advanced, I can
only say, firstly, that we do not know at what rate species change, as
measured by years, and secondly, that many philosophers are not as yet
willing to admit that we know enough of the constitution of the
universe and of the interior of our globe to speculate with safety on
its past duration.

That the geological record is imperfect all will admit; but that it is
imperfect to the degree required by our theory, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly
declares that species have all changed; and they have changed in the
manner required by the theory, for they have changed slowly and in a
graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to
each other than are the fossils from widely separated formations.

Such is the sum of the several chief objections and difficulties which
may justly be urged against the theory; and I have now briefly
recapitulated the answers and explanations which, as far as I can see,
may be given. I have felt these difficulties far too heavily during
many years to doubt their weight. But it deserves especial notice that
the more important objections relate to questions on which we are
confessedly ignorant; nor do we know how ignorant we are. We do not
know all the possible transitional gradations between the simplest and
the most perfect organs; it cannot be pretended that we know all the
varied means of Distribution during the long lapse of years, or that we
know how imperfect is the Geological Record. Serious as these several
objections are, in my judgment they are by no means sufficient to
overthrow the theory of descent with subsequent modification.

Now let us turn to the other side of the argument. Under domestication
we see much variability, caused, or at least excited, by changed
conditions of life; but often in so obscure a manner, that we are
tempted to consider the variations as spontaneous. Variability is
governed by many complex laws, by correlated growth, compensation, the
increased use and disuse of parts, and the definite action of the
surrounding conditions. There is much difficulty in ascertaining how
largely our domestic productions have been modified; but we may safely
infer that the amount has been large, and that modifications can be
inherited for long periods. As long as the conditions of life remain
the same, we have reason to believe that a modification, which has
already been inherited for many generations, may continue to be
inherited for an almost infinite number of generations. On the other
hand we have evidence that variability, when it has once come into
play, does not cease under domestication for a very long period; nor do
we know that it ever ceases, for new varieties are still occasionally
produced by our oldest domesticated productions.

Variability is not actually caused by man; he only unintentionally
exposes organic beings to new conditions of life and then nature acts
on the organisation and causes it to vary. But man can and does select
the variations given to him by nature, and thus accumulates them in any
desired manner. He thus adapts animals and plants for his own benefit
or pleasure. He may do this methodically, or he may do it unconsciously
by preserving the individuals most useful or pleasing to him without
any intention of altering the breed. It is certain that he can largely
influence the character of a breed by selecting, in each successive
generation, individual differences so slight as to be inappreciable
except by an educated eye. This unconscious process of selection has
been the great agency in the formation of the most distinct and useful
domestic breeds. That many breeds produced by man have to a large
extent the character of natural species, is shown by the inextricable
doubts whether many of them are varieties or aboriginally distinct
species.

There is no reason why the principles which have acted so efficiently
under domestication should not have acted under nature. In the survival
of favoured individuals and races, during the constantly recurrent
Struggle for Existence, we see a powerful and ever-acting form of
Selection. The struggle for existence inevitably follows from the high
geometrical ratio of increase which is common to all organic beings.
This high rate of increase is proved by calculation—by the rapid
increase of many animals and plants during a succession of peculiar
seasons, and when naturalised in new countries. More individuals are
born than can possibly survive. A grain in the balance may determine
which individuals shall live and which shall die—which variety or
species shall increase in number, and which shall decrease, or finally
become extinct. As the individuals of the same species come in all
respects into the closest competition with each other, the struggle
will generally be most severe between them; it will be almost equally
severe between the varieties of the same species, and next in severity
between the species of the same genus. On the other hand the struggle
will often be severe between beings remote in the scale of nature. The
slightest advantage in certain individuals, at any age or during any
season, over those with which they come into competition, or better
adaptation in however slight a degree to the surrounding physical
conditions, will, in the long run, turn the balance.

With animals having separated sexes, there will be in most cases a
struggle between the males for the possession of the females. The most
vigorous males, or those which have most successfully struggled with
their conditions of life, will generally leave most progeny. But
success will often depend on the males having special weapons or means
of defence or charms; and a slight advantage will lead to victory.

As geology plainly proclaims that each land has undergone great
physical changes, we might have expected to find that organic beings
have varied under nature, in the same way as they have varied under
domestication. And if there has been any variability under nature, it
would be an unaccountable fact if natural selection had not come into
play. It has often been asserted, but the assertion is incapable of
proof, that the amount of variation under nature is a strictly limited
quantity. Man, though acting on external characters alone and often
capriciously, can produce within a short period a great result by
adding up mere individual differences in his domestic productions; and
every one admits that species present individual differences. But,
besides such differences, all naturalists admit that natural varieties
exist, which are considered sufficiently distinct to be worthy of
record in systematic works. No one has drawn any clear distinction
between individual differences and slight varieties; or between more
plainly marked varieties and subspecies and species. On separate
continents, and on different parts of the same continent, when divided
by barriers of any kind, and on outlying islands, what a multitude of
forms exist, which some experienced naturalists rank as varieties,
others as geographical races or sub species, and others as distinct,
though closely allied species!

If, then, animals and plants do vary, let it be ever so slightly or
slowly, why should not variations or individual differences, which are
in any way beneficial, be preserved and accumulated through natural
selection, or the survival of the fittest? If man can by patience
select variations useful to him, why, under changing and complex
conditions of life, should not variations useful to nature’s living
products often arise, and be preserved or selected? What limit can be
put to this power, acting during long ages and rigidly scrutinising the
whole constitution, structure, and habits of each creature, favouring
the good and rejecting the bad? I can see no limit to this power, in
slowly and beautifully adapting each form to the most complex relations
of life. The theory of natural selection, even if we look no further
than this, seems to be in the highest degree probable. I have already
recapitulated, as fairly as I could, the opposed difficulties and
objections: now let us turn to the special facts and arguments in
favour of the theory.

On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see
why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation,
and varieties which are acknowledged to have been produced by secondary
laws. On this same view we can understand how it is that in a region
where many species of a genus have been produced, and where they now
flourish, these same species should present many varieties; for where
the manufactory of species has been active, we might expect, as a
general rule, to find it still in action; and this is the case if
varieties be incipient species. Moreover, the species of the larger
genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for
they differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of a larger
genera apparently have restricted ranges, and in their affinities they
are clustered in little groups round other species—in both respects
resembling varieties. These are strange relations on the view that each
species was independently created, but are intelligible if each existed
first as a variety.

As each species tends by its geometrical rate of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by as much as they become more
diversified in habits and structure, so as to be able to seize on many
and widely different places in the economy of nature, there will be a
constant tendency in natural selection to preserve the most divergent
offspring of any one species. Hence during a long-continued course of
modification, the slight differences characteristic of varieties of the
same species, tend to be augmented into the greater differences
characteristic of the species of the same genus. New and improved
varieties will inevitably supplant and exterminate the older, less
improved and intermediate varieties; and thus species are rendered to a
large extent defined and distinct objects. Dominant species belonging
to the larger groups within each class tend to give birth to new and
dominant forms; so that each large group tends to become still larger,
and at the same time more divergent in character. But as all groups
cannot thus go on increasing in size, for the world would not hold
them, the more dominant groups beat the less dominant. This tendency in
the large groups to go on increasing in size and diverging in
character, together with the inevitable contingency of much extinction,
explains the arrangement of all the forms of life in groups subordinate
to groups, all within a few great classes, which has prevailed
throughout all time. This grand fact of the grouping of all organic
beings under what is called the Natural System, is utterly inexplicable
on the theory of creation.

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modifications;
it can act only by short and slow steps. Hence, the canon of “Natura
non facit saltum,” which every fresh addition to our knowledge tends to
confirm, is on this theory intelligible. We can see why throughout
nature the same general end is gained by an almost infinite diversity
of means, for every peculiarity when once acquired is long inherited,
and structures already modified in many different ways have to be
adapted for the same general purpose. We can, in short, see why nature
is prodigal in variety, though niggard in innovation. But why this
should be a law of nature if each species has been independently
created no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of a woodpecker, should prey
on insects on the ground; that upland geese, which rarely or never
swim, would possess webbed feet; that a thrush-like bird should dive
and feed on sub-aquatic insects; and that a petrel should have the
habits and structure fitting it for the life of an auk! and so in
endless other cases. But on the view of each species constantly trying
to increase in number, with natural selection always ready to adapt the
slowly varying descendants of each to any unoccupied or ill-occupied
place in nature, these facts cease to be strange, or might even have
been anticipated.

We can to a certain extent understand how it is that there is so much
beauty throughout nature; for this may be largely attributed to the
agency of selection. That beauty, according to our sense of it, is not
universal, must be admitted by every one who will look at some venomous
snakes, at some fishes, and at certain hideous bats with a distorted
resemblance to the human face. Sexual selection has given the most
brilliant colours, elegant patterns, and other ornaments to the males,
and sometimes to both sexes of many birds, butterflies and other
animals. With birds it has often rendered the voice of the male musical
to the female, as well as to our ears. Flowers and fruit have been
rendered conspicuous by brilliant colours in contrast with the green
foliage, in order that the flowers may be easily seen, visited and
fertilised by insects, and the seeds disseminated by birds. How it
comes that certain colours, sounds and forms should give pleasure to
man and the lower animals, that is, how the sense of beauty in its
simplest form was first acquired, we do not know any more than how
certain odours and flavours were first rendered agreeable.

As natural selection acts by competition, it adapts and improves the
inhabitants of each country only in relation to their co-inhabitants;
so that we need feel no surprise at the species of any one country,
although on the ordinary view supposed to have been created and
specially adapted for that country, being beaten and supplanted by the
naturalised productions from another land. Nor ought we to marvel if
all the contrivances in nature be not, as far as we can judge,
absolutely perfect; as in the case even of the human eye; or if some of
them be abhorrent to our ideas of fitness. We need not marvel at the
sting of the bee, when used against the enemy, causing the bee’s own
death; at drones being produced in such great numbers for one single
act, and being then slaughtered by their sterile sisters; at the
astonishing waste of pollen by our fir-trees; at the instinctive hatred
of the queen-bee for her own fertile daughters; at ichneumonidæ feeding
within the living bodies of caterpillars; and at other such cases. The
wonder, indeed, is, on the theory of natural selection, that more cases
of the want of absolute perfection have not been detected.

The complex and little known laws governing the production of varieties
are the same, as far as we can judge, with the laws which have governed
the production of distinct species. In both cases physical conditions
seem to have produced some direct and definite effect, but how much we
cannot say. Thus, when varieties enter any new station, they
occasionally assume some of the characters proper to the species of
that station. With both varieties and species, use and disuse seem to
have produced a considerable effect; for it is impossible to resist
this conclusion when we look, for instance, at the logger-headed duck,
which has wings incapable of flight, in nearly the same condition as in
the domestic duck; or when we look at the burrowing tucu-tucu, which is
occasionally blind, and then at certain moles, which are habitually
blind and have their eyes covered with skin; or when we look at the
blind animals inhabiting the dark caves of America and Europe. With
varieties and species, correlated variation seems to have played an
important part, so that when one part has been modified other parts
have been necessarily modified. With both varieties and species,
reversions to long-lost characters occasionally occur. How inexplicable
on the theory of creation is the occasional appearance of stripes on
the shoulders and legs of the several species of the horse-genus and of
their hybrids! How simply is this fact explained if we believe that
these species are all descended from a striped progenitor, in the same
manner as the several domestic breeds of the pigeon are descended from
the blue and barred rock-pigeon!

On the ordinary view of each species having been independently created,
why should specific characters, or those by which the species of the
same genus differ from each other, be more variable than the generic
characters in which they all agree? Why, for instance, should the
colour of a flower be more likely to vary in any one species of a
genus, if the other species possess differently coloured flowers, than
if all possessed the same coloured flowers? If species are only
well-marked varieties, of which the characters have become in a high
degree permanent, we can understand this fact; for they have already
varied since they branched off from a common progenitor in certain
characters, by which they have come to be specifically distinct from
each other; therefore these same characters would be more likely again
to vary than the generic characters which have been inherited without
change for an immense period. It is inexplicable on the theory of
creation why a part developed in a very unusual manner in one species
alone of a genus, and therefore, as we may naturally infer, of great
importance to that species, should be eminently liable to variation;
but, on our view, this part has undergone, since the several species
branched off from a common progenitor, an unusual amount of variability
and modification, and therefore we might expect the part generally to
be still variable. But a part may be developed in the most unusual
manner, like the wing of a bat, and yet not be more variable than any
other structure, if the part be common to many subordinate forms, that
is, if it has been inherited for a very long period; for in this case
it will have been rendered constant by long-continued natural
selection.

Glancing at instincts, marvellous as some are, they offer no greater
difficulty than do corporeal structures on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I
have attempted to show how much light the principle of gradation throws
on the admirable architectural powers of the hive-bee. Habit no doubt
often comes into play in modifying instincts; but it certainly is not
indispensable, as we see in the case of neuter insects, which leave no
progeny to inherit the effects of long-continued habit. On the view of
all the species of the same genus having descended from a common
parent, and having inherited much in common, we can understand how it
is that allied species, when placed under widely different conditions
of life, yet follow nearly the same instincts; why the thrushes of
tropical and temperate South America, for instance, line their nests
with mud like our British species. On the view of instincts having been
slowly acquired through natural selection, we need not marvel at some
instincts being not perfect and liable to mistakes, and at many
instincts causing other animals to suffer.

If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents—in being
absorbed into each other by successive crosses, and in other such
points—as do the crossed offspring of acknowledged varieties. This
similarity would be a strange fact, if species had been independently
created and varieties had been produced through secondary laws.

If we admit that the geological record is imperfect to an extreme
degree, then the facts, which the record does give, strongly support
the theory of descent with modification. New species have come on the
stage slowly and at successive intervals; and the amount of change
after equal intervals of time, is widely different in different groups.
The extinction of species and of whole groups of species, which has
played so conspicuous a part in the history of the organic world,
almost inevitably follows from the principle of natural selection; for
old forms are supplanted by new and improved forms. Neither single
species nor groups of species reappear when the chain of ordinary
generation is once broken. The gradual diffusion of dominant forms,
with the slow modification of their descendants, causes the forms of
life, after long intervals of time, to appear as if they had changed
simultaneously throughout the world. The fact of the fossil remains of
each formation being in some degree intermediate in character between
the fossils in the formations above and below, is simply explained by
their intermediate position in the chain of descent. The grand fact
that all extinct beings can be classed with all recent beings,
naturally follows from the living and the extinct being the offspring
of common parents. As species have generally diverged in character
during their long course of descent and modification, we can understand
why it is that the more ancient forms, or early progenitors of each
group, so often occupy a position in some degree intermediate between
existing groups. Recent forms are generally looked upon as being, on
the whole, higher in the scale of organisation than ancient forms; and
they must be higher, in so far as the later and more improved forms
have conquered the older and less improved forms in the struggle for
life; they have also generally had their organs more specialised for
different functions. This fact is perfectly compatible with numerous
beings still retaining simple and but little improved structures,
fitted for simple conditions of life; it is likewise compatible with
some forms having retrograded in organisation, by having become at each
stage of descent better fitted for new and degraded habits of life.
Lastly, the wonderful law of the long endurance of allied forms on the
same continent—of marsupials in Australia, of edentata in America, and
other such cases—is intelligible, for within the same country the
existing and the extinct will be closely allied by descent.

Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the
world to another, owing to former climatical and geographical changes
and to the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the
great leading facts in Distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings throughout
space, and in their geological succession throughout time; for in both
cases the beings have been connected by the bond of ordinary
generation, and the means of modification have been the same. We see
the full meaning of the wonderful fact, which has struck every
traveller, namely, that on the same continent, under the most diverse
conditions, under heat and cold, on mountain and lowland, on deserts
and marshes, most of the inhabitants within each great class are
plainly related; for they are the descendants of the same progenitors
and early colonists. On this same principle of former migration,
combined in most cases with modification, we can understand, by the aid
of the Glacial period, the identity of some few plants, and the close
alliance of many others, on the most distant mountains, and in the
northern and southern temperate zones; and likewise the close alliance
of some of the inhabitants of the sea in the northern and southern
temperate latitudes, though separated by the whole intertropical ocean.
Although two countries may present physical conditions as closely
similar as the same species ever require, we need feel no surprise at
their inhabitants being widely different, if they have been for a long
period completely sundered from each other; for as the relation of
organism to organism is the most important of all relations, and as the
two countries will have received colonists at various periods and in
different proportions, from some other country or from each other, the
course of modification in the two areas will inevitably have been
different.

On this view of migration, with subsequent modification, we see why
oceanic islands are inhabited by only few species, but of these, why
many are peculiar or endemic forms. We clearly see why species
belonging to those groups of animals which cannot cross wide spaces of
the ocean, as frogs and terrestrial mammals, do not inhabit oceanic
islands; and why, on the other hand, new and peculiar species of bats,
animals which can traverse the ocean, are often found on islands far
distant from any continent. Such cases as the presence of peculiar
species of bats on oceanic islands and the absence of all other
terrestrial mammals, are facts utterly inexplicable on the theory of
independent acts of creation.

The existence of closely allied representative species in any two
areas, implies, on the theory of descent with modification, that the
same parent-forms formerly inhabited both areas; and we almost
invariably find that wherever many closely allied species inhabit two
areas, some identical species are still common to both. Wherever many
closely allied yet distinct species occur, doubtful forms and varieties
belonging to the same groups likewise occur. It is a rule of high
generality that the inhabitants of each area are related to the
inhabitants of the nearest source whence immigrants might have been
derived. We see this in the striking relation of nearly all the plants
and animals of the Galapagos Archipelago, of Juan Fernandez, and of the
other American islands, to the plants and animals of the neighbouring
American mainland; and of those of the Cape de Verde Archipelago, and
of the other African islands to the African mainland. It must be
admitted that these facts receive no explanation on the theory of
creation.

The fact, as we have seen, that all past and present organic beings can
be arranged within a few great classes, in groups subordinate to
groups, and with the extinct groups often falling in between the recent
groups, is intelligible on the theory of natural selection with its
contingencies of extinction and divergence of character. On these same
principles we see how it is that the mutual affinities of the forms
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification; why
adaptive characters, though of paramount importance to the beings, are
of hardly any importance in classification; why characters derived from
rudimentary parts, though of no service to the beings, are often of
high classificatory value; and why embryological characters are often
the most valuable of all. The real affinities of all organic beings, in
contradistinction to their adaptive resemblances, are due to
inheritance or community of descent. The Natural System is a
genealogical arrangement, with the acquired grades of difference,
marked by the terms, varieties, species, genera, families, &c.; and we
have to discover the lines of descent by the most permanent characters,
whatever they may be, and of however slight vital importance.

The similar framework of bones in the hand of a man, wing of a bat, fin
of the porpoise, and leg of the horse—the same number of vertebræ
forming the neck of the giraffe and of the elephant—and innumerable
other such facts, at once explain themselves on the theory of descent
with slow and slight successive modifications. The similarity of
pattern in the wing and in the leg of a bat, though used for such
different purpose—in the jaws and legs of a crab—in the petals,
stamens, and pistils of a flower, is likewise, to a large extent,
intelligible on the view of the gradual modification of parts or
organs, which were aboriginally alike in an early progenitor in each of
these classes. On the principle of successive variations not always
supervening at an early age, and being inherited at a corresponding not
early period of life, we clearly see why the embryos of mammals, birds,
reptiles, and fishes should be so closely similar, and so unlike the
adult forms. We may cease marvelling at the embryo of an air-breathing
mammal or bird having branchial slits and arteries running in loops,
like those of a fish which has to breathe the air dissolved in water by
the aid of well-developed branchiæ.

Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of
life; and we can understand on this view the meaning of rudimentary
organs. But disuse and selection will generally act on each creature,
when it has come to maturity and has to play its full part in the
struggle for existence, and will thus have little power on an organ
during early life; hence the organ will not be reduced or rendered
rudimentary at this early age. The calf, for instance, has inherited
teeth, which never cut through the gums of the upper jaw, from an early
progenitor having well-developed teeth; and we may believe, that the
teeth in the mature animal were formerly reduced by disuse owing to the
tongue and palate, or lips, having become excellently fitted through
natural selection to browse without their aid; whereas in the calf, the
teeth have been left unaffected, and on the principle of inheritance at
corresponding ages have been inherited from a remote period to the
present day. On the view of each organism with all its separate parts
having been specially created, how utterly inexplicable is it that
organs bearing the plain stamp of inutility, such as the teeth in the
embryonic calf or the shrivelled wings under the soldered wing-covers
of many beetles, should so frequently occur. Nature may be said to have
taken pains to reveal her scheme of modification, by means of
rudimentary organs, of embryological and homologous structures, but we
are too blind to understand her meaning.

I have now recapitulated the facts and considerations which have
thoroughly convinced me that species have been modified, during a long
course of descent. This has been effected chiefly through the natural
selection of numerous successive, slight, favourable variations; aided
in an important manner by the inherited effects of the use and disuse
of parts; and in an unimportant manner, that is, in relation to
adaptive structures, whether past or present, by the direct action of
external conditions, and by variations which seem to us in our
ignorance to arise spontaneously. It appears that I formerly underrated
the frequency and value of these latter forms of variation, as leading
to permanent modifications of structure independently of natural
selection. But as my conclusions have lately been much misrepresented,
and it has been stated that I attribute the modification of species
exclusively to natural selection, I may be permitted to remark that in
the first edition of this work, and subsequently, I placed in a most
conspicuous position—namely, at the close of the Introduction—the
following words: “I am convinced that natural selection has been the
main but not the exclusive means of modification.” This has been of no
avail. Great is the power of steady misrepresentation; but the history
of science shows that fortunately this power does not long endure.

It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the
several large classes of facts above specified. It has recently been
objected that this is an unsafe method of arguing; but it is a method
used in judging of the common events of life, and has often been used
by the greatest natural philosophers. The undulatory theory of light
has thus been arrived at; and the belief in the revolution of the earth
on its own axis was until lately supported by hardly any direct
evidence. It is no valid objection that science as yet throws no light
on the far higher problem of the essence or origin of life. Who can
explain what is the essence of the attraction of gravity? No one now
objects to following out the results consequent on this unknown element
of attraction; notwithstanding that Leibnitz formerly accused Newton of
introducing “occult qualities and miracles into philosophy.”

I see no good reasons why the views given in this volume should shock
the religious feelings of any one. It is satisfactory, as showing how
transient such impressions are, to remember that the greatest discovery
ever made by man, namely, the law of the attraction of gravity, was
also attacked by Leibnitz, “as subversive of natural, and inferentially
of revealed, religion.” A celebrated author and divine has written to
me that “he has gradually learned to see that it is just as noble a
conception of the Deity to believe that He created a few original forms
capable of self-development into other and needful forms, as to believe
that He required a fresh act of creation to supply the voids caused by
the action of His laws.”

Why, it may be asked, until recently did nearly all the most eminent
living naturalists and geologists disbelieve in the mutability of
species? It cannot be asserted that organic beings in a state of nature
are subject to no variation; it cannot be proved that the amount of
variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile and varieties invariably fertile; or that sterility
is a special endowment and sign of creation. The belief that species
were immutable productions was almost unavoidable as long as the
history of the world was thought to be of short duration; and now that
we have acquired some idea of the lapse of time, we are too apt to
assume, without proof, that the geological record is so perfect that it
would have afforded us plain evidence of the mutation of species, if
they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great changes of which we do not see the
steps. The difficulty is the same as that felt by so many geologists,
when Lyell first insisted that long lines of inland cliffs had been
formed, and great valleys excavated, by the agencies which we still see
at work. The mind cannot possibly grasp the full meaning of the term of
even a million years; it cannot add up and perceive the full effects of
many slight variations, accumulated during an almost infinite number of
generations.

Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of
facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy to hide our ignorance under
such expressions as the “plan of creation,” “unity of design,” &c., and
to think that we give an explanation when we only restate a fact. Any
one whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts will
certainly reject the theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt the
immutability of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists, who
will be able to view both sides of the question with impartiality.
Whoever is led to believe that species are mutable will do good service
by conscientiously expressing his conviction; for thus only can the
load of prejudice by which this subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at. They admit that a
multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority
of naturalists, and which consequently have all the external
characteristic features of true species—they admit that these have been
produced by variation, but they refuse to extend the same view to other
and slightly different forms. Nevertheless, they do not pretend that
they can define, or even conjecture, which are the created forms of
life, and which are those produced by secondary laws. They admit
variation as a vera causa in one case, they arbitrarily reject it in
another, without assigning any distinction in the two cases. The day
will come when this will be given as a curious illustration of the
blindness of preconceived opinion. These authors seem no more startled
at a miraculous act of creation than at an ordinary birth. But do they
really believe that at innumerable periods in the earth’s history
certain elemental atoms have been commanded suddenly to flash into
living tissues? Do they believe that at each supposed act of creation
one individual or many were produced? Were all the infinitely numerous
kinds of animals and plants created as eggs or seed, or as full grown?
and in the case of mammals, were they created bearing the false marks
of nourishment from the mother’s womb? Undoubtedly some of these same
questions cannot be answered by those who believe in the appearance or
creation of only a few forms of life or of some one form alone. It has
been maintained by several authors that it is as easy to believe in the
creation of a million beings as of one; but Maupertuis’ philosophical
axiom “of least action” leads the mind more willingly to admit the
smaller number; and certainly we ought not to believe that innumerable
beings within each great class have been created with plain, but
deceptive, marks of descent from a single parent.

As a record of a former state of things, I have retained in the
foregoing paragraphs, and elsewhere, several sentences which imply that
naturalists believe in the separate creation of each species; and I
have been much censured for having thus expressed myself. But
undoubtedly this was the general belief when the first edition of the
present work appeared. I formerly spoke to very many naturalists on the
subject of evolution, and never once met with any sympathetic
agreement. It is probable that some did then believe in evolution, but
they were either silent or expressed themselves so ambiguously that it
was not easy to understand their meaning. Now, things are wholly
changed, and almost every naturalist admits the great principle of
evolution. There are, however, some who still think that species have
suddenly given birth, through quite unexplained means, to new and
totally different forms. But, as I have attempted to show, weighty
evidence can be opposed to the admission of great and abrupt
modifications. Under a scientific point of view, and as leading to
further investigation, but little advantage is gained by believing that
new forms are suddenly developed in an inexplicable manner from old and
widely different forms, over the old belief in the creation of species
from the dust of the earth.

It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we consider, by so much the arguments in favour of
community of descent become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All the members of
whole classes are connected together by a chain of affinities, and all
can be classed on the same principle, in groups subordinate to groups.
Fossil remains sometimes tend to fill up very wide intervals between
existing orders.

Organs in a rudimentary condition plainly show that an early progenitor
had the organ in a fully developed condition, and this in some cases
implies an enormous amount of modification in the descendants.
Throughout whole classes various structures are formed on the same
pattern, and at a very early age the embryos closely resemble each
other. Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same great class or
kingdom. I believe that animals are descended from at most only four or
five progenitors, and plants from an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all
animals and plants are descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their cellular structure, their
laws of growth, and their liability to injurious influences. We see
this even in so trifling a fact as that the same poison often similarly
affects plants and animals; or that the poison secreted by the gall-fly
produces monstrous growths on the wild rose or oak-tree. With all
organic beings, excepting perhaps some of the very lowest, sexual
reproduction seems to be essentially similar. With all, as far as is at
present known, the germinal vesicle is the same; so that all organisms
start from a common origin. If we look even to the two main
divisions—namely, to the animal and vegetable kingdoms—certain low
forms are so far intermediate in character that naturalists have
disputed to which kingdom they should be referred. As Professor Asa
Gray has remarked, “the spores and other reproductive bodies of many of
the lower algæ may claim to have first a characteristically animal, and
then an unequivocally vegetable existence.” Therefore, on the principle
of natural selection with divergence of character, it does not seem
incredible that, from some such low and intermediate form, both animals
and plants may have been developed; and, if we admit this, we must
likewise admit that all the organic beings which have ever lived on
this earth may be descended from some one primordial form. But this
inference is chiefly grounded on analogy, and it is immaterial whether
or not it be accepted. No doubt it is possible, as Mr. G.H. Lewes has
urged, that at the first commencement of life many different forms were
evolved; but if so, we may conclude that only a very few have left
modified descendants. For, as I have recently remarked in regard to the
members of each great kingdom, such as the Vertebrata, Articulata, &c.,
we have distinct evidence in their embryological, homologous, and
rudimentary structures, that within each kingdom all the members are
descended from a single progenitor.

When the views advanced by me in this volume, and by Mr. Wallace or
when analogous views on the origin of species are generally admitted,
we can dimly foresee that there will be a considerable revolution in
natural history. Systematists will be able to pursue their labours as
at present; but they will not be incessantly haunted by the shadowy
doubt whether this or that form be a true species. This, I feel sure
and I speak after experience, will be no slight relief. The endless
disputes whether or not some fifty species of British brambles are good
species will cease. Systematists will have only to decide (not that
this will be easy) whether any form be sufficiently constant and
distinct from other forms, to be capable of definition; and if
definable, whether the differences be sufficiently important to deserve
a specific name. This latter point will become a far more essential
consideration than it is at present; for differences, however slight,
between any two forms, if not blended by intermediate gradations, are
looked at by most naturalists as sufficient to raise both forms to the
rank of species.

Hereafter we shall be compelled to acknowledge that the only
distinction between species and well-marked varieties is, that the
latter are known, or believed to be connected at the present day by
intermediate gradations, whereas species were formerly thus connected.
Hence, without rejecting the consideration of the present existence of
intermediate gradations between any two forms, we shall be led to weigh
more carefully and to value higher the actual amount of difference
between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be thought worthy of
specific names; and in this case scientific and common language will
come into accordance. In short, we shall have to treat species in the
same manner as those naturalists treat genera, who admit that genera
are merely artificial combinations made for convenience. This may not
be a cheering prospect; but we shall at least be freed from the vain
search for the undiscovered and undiscoverable essence of the term
species.

The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists, of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, &c., will cease to be
metaphorical and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as something
wholly beyond his comprehension; when we regard every production of
nature as one which has had a long history; when we contemplate every
complex structure and instinct as the summing up of many contrivances,
each useful to the possessor, in the same way as any great mechanical
invention is the summing up of the labour, the experience, the reason,
and even the blunders of numerous workmen; when we thus view each
organic being, how far more interesting—I speak from experience—does
the study of natural history become!

A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation, on the effects of use and
disuse, on the direct action of external conditions, and so forth. The
study of domestic productions will rise immensely in value. A new
variety raised by man will be a far more important and interesting
subject for study than one more species added to the infinitude of
already recorded species. Our classifications will come to be, as far
as they can be so made, genealogies; and will then truly give what may
be called the plan of creation. The rules for classifying will no doubt
become simpler when we have a definite object in view. We possess no
pedigree or armorial bearings; and we have to discover and trace the
many diverging lines of descent in our natural genealogies, by
characters of any kind which have long been inherited. Rudimentary
organs will speak infallibly with respect to the nature of long-lost
structures. Species and groups of species which are called aberrant,
and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will often
reveal to us the structure, in some degree obscured, of the prototypes
of each great class.

When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have, within a not
very remote period descended from one parent, and have migrated from
some one birth-place; and when we better know the many means of
migration, then, by the light which geology now throws, and will
continue to throw, on former changes of climate and of the level of the
land, we shall surely be enabled to trace in an admirable manner the
former migrations of the inhabitants of the whole world. Even at
present, by comparing the differences between the inhabitants of the
sea on the opposite sides of a continent, and the nature of the various
inhabitants of that continent in relation to their apparent means of
immigration, some light can be thrown on ancient geography.

The noble science of geology loses glory from the extreme imperfection
of the record. The crust of the earth, with its embedded remains, must
not be looked at as a well-filled museum, but as a poor collection made
at hazard and at rare intervals. The accumulation of each great
fossiliferous formation will be recognised as having depended on an
unusual occurrence of favourable circumstances, and the blank intervals
between the successive stages as having been of vast duration. But we
shall be able to gauge with some security the duration of these
intervals by a comparison of the preceding and succeeding organic
forms. We must be cautious in attempting to correlate as strictly
contemporaneous two formations, which do not include many identical
species, by the general succession of the forms of life. As species are
produced and exterminated by slowly acting and still existing causes,
and not by miraculous acts of creation; and as the most important of
all causes of organic change is one which is almost independent of
altered and perhaps suddenly altered physical conditions, namely, the
mutual relation of organism to organism—the improvement of one organism
entailing the improvement or the extermination of others; it follows,
that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the relative, though
not actual lapse of time. A number of species, however, keeping in a
body might remain for a long period unchanged, whilst within the same
period, several of these species, by migrating into new countries and
coming into competition with foreign associates, might become modified;
so that we must not overrate the accuracy of organic change as a
measure of time.

In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well laid
by Mr. Herbert Spencer, that of the necessary acquirement of each
mental power and capacity by gradation. Much light will be thrown on
the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with the
view that each species has been independently created. To my mind it
accords better with what we know of the laws impressed on matter by the
Creator, that the production and extinction of the past and present
inhabitants of the world should have been due to secondary causes, like
those determining the birth and death of the individual. When I view
all beings not as special creations, but as the lineal descendants of
some few beings which lived long before the first bed of the Cambrian
system was deposited, they seem to me to become ennobled. Judging from
the past, we may safely infer that not one living species will transmit
its unaltered likeness to a distinct futurity. And of the species now
living very few will transmit progeny of any kind to a far distant
futurity; for the manner in which all organic beings are grouped, shows
that the greater number of species in each genus, and all the species
in many genera, have left no descendants, but have become utterly
extinct. We can so far take a prophetic glance into futurity as to
foretell that it will be the common and widely spread species,
belonging to the larger and dominant groups within each class, which
will ultimately prevail and procreate new and dominant species. As all
the living forms of life are the lineal descendants of those which
lived long before the Cambrian epoch, we may feel certain that the
ordinary succession by generation has never once been broken, and that
no cataclysm has desolated the whole world. Hence, we may look with
some confidence to a secure future of great length. And as natural
selection works solely by and for the good of each being, all corporeal
and mental endowments will tend to progress towards perfection.

It is interesting to contemplate a tangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp earth,
and to reflect that these elaborately constructed forms, so different
from each other, and dependent upon each other in so complex a manner,
have all been produced by laws acting around us. These laws, taken in
the largest sense, being Growth with reproduction; Inheritance which is
almost implied by reproduction; Variability from the indirect and
direct action of the conditions of life, and from use and disuse; a
Ratio of Increase so high as to lead to a Struggle for Life, and as a
consequence to Natural Selection, entailing Divergence of Character and
the Extinction of less improved forms. Thus, from the war of nature,
from famine and death, the most exalted object which we are capable of
conceiving, namely, the production of the higher animals, directly
follows. There is grandeur in this view of life, with its several
powers, having been originally breathed by the Creator into a few forms
or into one; and that, whilst this planet has gone circling on
according to the fixed law of gravity, from so simple a beginning
endless forms most beautiful and most wonderful have been, and are
being evolved.




GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.*


   * I am indebted to the kindness of Mr. W.S. Dallas for this
   Glossary, which has been given because several readers have
   complained to me that some of the terms used were unintelligible to
   them. Mr. Dallas has endeavoured to give the explanations of the
   terms in as popular a form as possible.

ABERRANT.—Forms or groups of animals or plants which deviate in
important characters from their nearest allies, so as not to be easily
included in the same group with them, are said to be aberrant.

ABERRATION (in Optics).—In the refraction of light by a convex lens the
rays passing through different parts of the lens are brought to a focus
at slightly different distances—this is called _spherical aberration;_
at the same time the coloured rays are separated by the prismatic
action of the lens and likewise brought to a focus at different
distances—this is _chromatic aberration_.

ABNORMAL.—Contrary to the general rule.

ABORTED.—An organ is said to be aborted, when its development has been
arrested at a very early stage.

ALBINISM.—Albinos are animals in which the usual colouring matters
characteristic of the species have not been produced in the skin and
its appendages. Albinism is the state of being an albino.

ALGÆ.—A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.

ALTERNATION OF GENERATIONS.—This term is applied to a peculiar mode of
reproduction which prevails among many of the lower animals, in which
the egg produces a living form quite different from its parent, but
from which the parent-form is reproduced by a process of budding, or by
the division of the substance of the first product of the egg.

AMMONITES.—A group of fossil, spiral, chambered shells, allied to the
existing pearly Nautilus, but having the partitions between the
chambers waved in complicated patterns at their junction with the outer
wall of the shell.

ANALOGY.—That resemblance of structures which depends upon similarity
of function, as in the wings of insects and birds. Such structures are
said to be _analogous_, and to be _analogues_ of each other.

ANIMALCULE.—A minute animal: generally applied to those visible only by
the microscope.

ANNELIDS.—A class of worms in which the surface of the body exhibits a
more or less distinct division into rings or segments, generally
provided with appendages for locomotion and with gills. It includes the
ordinary marine worms, the earth-worms, and the leeches.

ANTENNÆ.—Jointed organs appended to the head in Insects, Crustacea and
Centipedes, and not belonging to the mouth.

ANTHERS.—The summits of the stamens of flowers, in which the pollen or
fertilising dust is produced.

APLACENTALIA, APLACENTATA or Aplacental Mammals.—See _mammalia_.

ARCHETYPAL.—Of or belonging to the Archetype, or ideal primitive form
upon which all the beings of a group seem to be organised.

ARTICULATA.—A great division of the Animal Kingdom characterised
generally by having the surface of the body divided into rings called
segments, a greater or less number of which are furnished with jointed
legs (such as Insects, Crustaceans and Centipedes).

ASYMMETRICAL.—Having the two sides unlike.

ATROPHIED.—Arrested in development at a very early stage.

BALANUS.—The genus including the common Acorn-shells which live in
abundance on the rocks of the sea-coast.

BATRACHIANS.—A class of animals allied to the Reptiles, but undergoing
a peculiar metamorphosis, in which the young animal is generally
aquatic and breathes by gills. (_Examples_, Frogs, Toads, and Newts.)

BOULDERS.—Large transported blocks of stone generally embedded in clays
or gravels.

BRACHIOPODA.—A class of marine Mollusca, or soft-bodied animals,
furnished with a bivalve shell, attached to submarine objects by a
stalk which passes through an aperture in one of the valves, and
furnished with fringed arms, by the action of which food is carried to
the mouth.

BRANCHIÆ.—Gills or organs for respiration in water.

BRANCHIAL.—Pertaining to gills or branchiæ.

CAMBRIAN SYSTEM.—A series of very ancient Palæozoic rocks, between the
Laurentian and the Silurian. Until recently these were regarded as the
oldest fossiliferous rocks.

CANIDÆ.—The Dog-family, including the Dog, Wolf, Fox, Jackal, &c.

CARAPACE.—The shell enveloping the anterior part of the body in
Crustaceans generally; applied also to the hard shelly pieces of the
Cirripedes.

CARBONIFEROUS.—This term is applied to the great formation which
includes, among other rocks, the coal-measures. It belongs to the
oldest, or Palæozoic, system of formations.

CAUDAL.—Of or belonging to the tail.

CEPHALOPODS.—The highest class of the Mollusca, or soft-bodied animals,
characterised by having the mouth surrounded by a greater or less
number of fleshy arms or tentacles, which, in most living species, are
furnished with sucking-cups. (_Examples_, Cuttle-fish, Nautilus.)

CETACEA.—An order of Mammalia, including the Whales, Dolphins, &c.,
having the form of the body fish-like, the skin naked, and only the
fore limbs developed.

CHELONIA.—An order of Reptiles including the Turtles, Tortoises, &c.

CIRRIPEDES.—An order of Crustaceans including the Barnacles and
Acorn-shells. Their young resemble those of many other Crustaceans in
form; but when mature they are always attached to other objects, either
directly or by means of a stalk, and their bodies are enclosed by a
calcareous shell composed of several pieces, two of which can open to
give issue to a bunch of curled, jointed tentacles, which represent the
limbs.

COCCUS.—The genus of Insects including the Cochineal. In these the male
is a minute, winged fly, and the female generally a motionless,
berry-like mass.

COCOON.—A case usually of silky material, in which insects are
frequently enveloped during the second or resting-stage (pupa) of their
existence. The term “cocoon-stage” is here used as equivalent to
“pupa-stage.”

CŒLOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which
have the seed hollowed on the inner face.

COLEOPTERA.—Beetles, an order of Insects, having a biting mouth and the
first pair of wings more or less horny, forming sheaths for the second
pair, and usually meeting in a straight line down the middle of the
back.

COLUMN.—A peculiar organ in the flowers of Orchids, in which the
stamens, style and stigma (or the reproductive parts) are united.

COMPOSITÆ or COMPOSITOUS PLANTS.—Plants in which the inflorescence
consists of numerous small flowers (florets) brought together into a
dense head, the base of which is enclosed by a common envelope.
(_Examples_, the Daisy, Dandelion, &c.)

CONFERVÆ.—The filamentous weeds of fresh water.

CONGLOMERATE.—A rock made up of fragments of rock or pebbles, cemented
together by some other material.

COROLLA.—The second envelope of a flower usually composed of coloured,
leaf-like organs (petals), which may be united by their edges either in
the basal part or throughout.

CORRELATION.—The normal coincidence of one phenomenon, character, &c.,
with another.

CORYMB.—A bunch of flowers in which those springing from the lower part
of the flower stalks are supported on long stalks so as to be nearly on
a level with the upper ones.

COTYLEDONS.—The first or seed-leaves of plants.

CRUSTACEANS.—A class of articulated animals, having the skin of the
body generally more or less hardened by the deposition of calcareous
matter, breathing by means of gills. (_Examples_, Crab, Lobster,
Shrimp, &c.)

CURCULIO.—The old generic term for the Beetles known as Weevils,
characterised by their four-jointed feet, and by the head being
produced into a sort of beak, upon the sides of which the antennæ are
inserted.

CUTANEOUS.—Of or belonging to the skin.

DEGRADATION.—The wearing down of land by the action of the sea or of
meteoric agencies.

DENUDATION.—The wearing away of the surface of the land by water.

DEVONIAN SYSTEM or FORMATION.—A series of Palæozoic rocks, including
the Old Red Sandstone.

DICOTYLEDONS, or DICOTYLEDONOUS PLANTS.—A class of plants characterised
by having two seed-leaves, by the formation of new wood between the
bark and the old wood (exogenous growth) and by the reticulation of the
veins of the leaves. The parts of the flowers are generally in
multiples of five.

DIFFERENTATION.—The separation or discrimination of parts or organs
which in simpler forms of life are more or less united.

DIMORPHIC.—Having two distinct forms.—DIMORPHISM is the condition of
the appearance of the same species under two dissimilar forms.

DIOECIOUS.—Having the organs of the sexes upon distinct individuals.

DIORITE.—A peculiar form of Greenstone.

DORSAL.—Of or belonging to the back.

EDENTATA.—A peculiar order of Quadrupeds, characterised by the absence
of at least the middle incisor (front) teeth in both jaws. (_Examples_,
the Sloths and Armadillos.)

ELYTRA.—The hardened fore-wings of Beetles, serving as sheaths for the
membranous hind-wings, which constitute the true organs of flight.

EMBRYO.—The young animal undergoing development within the egg or womb.

EMBRYOLOGY.—The study of the development of the embryo.

ENDEMIC.—Peculiar to a given locality.

ENTOMOSTRACA.—A division of the class Crustacea, having all the
segments of the body usually distinct, gills attached to the feet or
organs of the mouth, and the feet fringed with fine hairs. They are
generally of small size.

EOCENE.—The earliest of the three divisions of the Tertiary epoch of
geologists. Rocks of this age contain a small proportion of shells
identical with species now living.

EPHEMEROUS INSECTS.—Insects allied to the May-fly.

FAUNA.—The totality of the animals naturally inhabiting a certain
country or region, or which have lived during a given geological
period.

FELIDÆ.—The Cat-family.

FERAL.—Having become wild from a state of cultivation or domestication.

FLORA.—The totality of the plants growing naturally in a country, or
during a given geological period.

FLORETS.—Flowers imperfectly developed in some respects, and collected
into a dense spike or head, as in the Grasses, the Dandelion, &c.

FOETAL.—Of or belonging to the foetus, or embryo in course of
development.

FORAMINIFERA.—A class of animals of very low organisation and generally
of small size, having a jelly-like body, from the surface of which
delicate filaments can be given off and retracted for the prehension of
external objects, and having a calcareous or sandy shell, usually
divided into chambers and perforated with small apertures.

FOSSILIFEROUS.—Containing fossils.

FOSSORIAL.—Having a faculty of digging. The Fossorial Hymenoptera are a
group of Wasp-like Insects, which burrow in sandy soil to make nests
for their young.

FRENUM (pl. FRENA).—A small band or fold of skin.

FUNGI (sing. FUNGUS).—A class of cellular plants, of which Mushrooms,
Toadstools, and Moulds, are familiar examples.

FURCULA.—The forked bone formed by the union of the collar-bones in
many birds, such as the common Fowl.

GALLINACEOUS BIRDS.—An order of birds of which the common Fowl, Turkey,
and Pheasant, are well-known examples.

GALLUS.—The genus of birds which includes the common Fowl.

GANGLION.—A swelling or knot from which nerves are given off as from a
centre.

GANOID FISHES.—Fishes covered with peculiar enamelled bony scales. Most
of them are extinct.

GERMINAL VESICLE.—A minute vesicle in the eggs of animals, from which
the development of the embryo proceeds.

GLACIAL PERIOD.—A period of great cold and of enormous extension of ice
upon the surface of the earth. It is believed that glacial periods have
occurred repeatedly during the geological history of the earth, but the
term is generally applied to the close of the Tertiary epoch, when
nearly the whole of Europe was subjected to an arctic climate.

GLAND.—An organ which secretes or separates some peculiar product from
the blood or sap of animals or plants.

GLOTTIS.—The opening of the windpipe into the œsophagus or gullet.

GNEISS.—A rock approaching granite in composition, but more or less
laminated, and really produced by the alteration of a sedimentary
deposit after its consolidation.

GRALLATORES.—The so-called wading-birds (storks, cranes, snipes, &c.),
which are generally furnished with long legs, bare of feathers above
the heel, and have no membranes between the toes.

GRANITE.—A rock consisting essentially of crystals of felspar and mica
in a mass of quartz.

HABITAT.—The locality in which a plant or animal naturally lives.

HEMIPTERA.—An order or sub-order of insects, characterised by the
possession of a jointed beak or rostrum, and by having the fore-wings
horny in the basal portion and membranous at the extremity, where they
cross each other. This group includes the various species of bugs.

HERMAPHRODITE.—Possessing the organs of both sexes.

HOMOLOGY.—That relation between parts which results from their
development from corresponding embryonic parts, either in different
animals, as in the case of the arm of man, the fore-leg of a quadruped,
and the wing of a bird; or in the same individual, as in the case of
the fore and hind legs in quadrupeds, and the segments or rings and
their appendages of which the body of a worm, a centipede, &c., is
composed. The latter is called _serial homology_. The parts which stand
in such a relation to each other are said to be _homologous_, and one
such part or organ is called the _homologue_ of the other. In different
plants the parts of the flower are homologous, and in general these
parts are regarded as homologous with leaves.

HOMOPTERA.—An order or sub-order of insects having (like the Hemiptera)
a jointed beak, but in which the fore-wings are either wholly
membranous or wholly leathery, The _Cicadæ_, frog-hoppers, and
_Aphides_, are well-known examples.

HYBRID.—The offspring of the union of two distinct species.

HYMENOPTERA.—An order of insects possessing biting jaws and usually
four membranous wings in which there are a few veins. Bees and wasps
are familiar examples of this group.

HYPERTROPHIED.—Excessively developed.

ICHNEUMONIDÆ.—A family of hymenopterous insects, the members of which
lay their eggs in the bodies or eggs of other insects.

IMAGO.—The perfect (generally winged) reproductive state of an insect.

INDIGENES.—The aboriginal animal or vegetable inhabitants of a country
or region.

INFLORESCENCE.—The mode of arrangement of the flowers of plants.

INFUSORIA.—A class of microscopic animalcules, so called from their
having originally been observed in infusions of vegetable matters. They
consist of a gelatinous material enclosed in a delicate membrane, the
whole or part of which is furnished with short vibrating hairs (called
cilia), by means of which the animalcules swim through the water or
convey the minute particles of their food to the orifice of the mouth.

INSECTIVOROUS.—Feeding on insects.

INVERTEBRATA, or INVERTEBRATE ANIMALS.—Those animals which do not
possess a backbone or spinal column.

LACUNÆ.—Spaces left among the tissues in some of the lower animals and
serving in place of vessels for the circulation of the fluids of the
body.

LAMELLATED.—Furnished with lamellæ or little plates.

LARVA (pl. LARVÆ).—The first condition of an insect at its issuing from
the egg, when it is usually in the form of a grub, caterpillar, or
maggot.

LARYNX.—The upper part of the windpipe opening into the gullet.

LAURENTIAN.—A group of greatly altered and very ancient rocks, which is
greatly developed along the course of the St. Laurence, whence the
name. It is in these that the earliest known traces of organic bodies
have been found.

LEGUMINOSÆ.—An order of plants represented by the common peas and
beans, having an irregular flower in which one petal stands up like a
wing, and the stamens and pistil are enclosed in a sheath formed by two
other petals. The fruit is a pod (or legume).

LEMURIDÆ.—A group of four-handed animals, distinct from the monkeys and
approaching the insectivorous quadrupeds in some of their characters
and habits. Its members have the nostrils curved or twisted, and a claw
instead of a nail upon the first finger of the hind hands.

LEPIDOPTERA.—An order of insects, characterised by the possession of a
spiral proboscis, and of four large more or less scaly wings. It
includes the well-known butterflies and moths.

LITTORAL.—Inhabiting the seashore.

LOESS.—A marly deposit of recent (Post-Tertiary) date, which occupies a
great part of the valley of the Rhine.

MALACOSTRACA.—The higher division of the Crustacea, including the
ordinary crabs, lobsters, shrimps, &c., together with the woodlice and
sand-hoppers.

MAMMALIA.—The highest class of animals, including the ordinary hairy
quadrupeds, the whales and man, and characterised by the production of
living young which are nourished after birth by milk from the teats
(_Mammæ_, _Mammary glands_) of the mother. A striking difference in
embryonic development has led to the division of this class into two
great groups; in one of these, when the embryo has attained a certain
stage, a vascular connection, called the _placenta_, is formed between
the embryo and the mother; in the other this is wanting, and the young
are produced in a very incomplete state. The former, including the
greater part of the class, are called _Placental Mammals;_ the latter,
or _Aplacental Mammals_, include the Marsupials and Monotremes
(_Ornithorhynchus_).

MAMMIFEROUS.—Having mammæ or teats (see MAMMALIA).

MANDIBLES.—in insects, the first or uppermost pair of jaws, which are
generally solid, horny, biting organs. In birds the term is applied to
both jaws with their horny coverings. In quadrupeds the mandible is
properly the lower jaw.

MARSUPIALS.—An order of Mammalia in which the young are born in a very
incomplete state of development, and carried by the mother, while
sucking, in a ventral pouch (marsupium), such as the kangaroos,
opossums, &c. (see MAMMALIA).

MAXILLÆ.—in insects, the second or lower pair of jaws, which are
composed of several joints and furnished with peculiar jointed
appendages called palpi, or feelers.

MELANISM.—The opposite of albinism; an undue development of colouring
material in the skin and its appendages.

METAMORPHIC ROCKS.—Sedimentary rocks which have undergone alteration,
generally by the action of heat, subsequently to their deposition and
consolidation.

MOLLUSCA.—One of the great divisions of the animal kingdom, including
those animals which have a soft body, usually furnished with a shell,
and in which the nervous ganglia, or centres, present no definite
general arrangement. They are generally known under the denomination of
“shellfish”; the cuttle-fish, and the common snails, whelks, oysters,
mussels, and cockles, may serve as examples of them.

MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.—Plants in which the seed
sends up only a single seed-leaf (or cotyledon); characterised by the
absence of consecutive layers of wood in the stem (endogenous growth),
by the veins of the leaves being generally straight, and by the parts
of the flowers being generally in multiples of three. (_Examples_,
Grasses, Lilies, Orchids, Palms, &c.)

MORAINES.—The accumulations of fragments of rock brought down by
glaciers.

MORPHOLOGY.—The law of form or structure independent of function.

MYSIS-STAGE.—A stage in the development of certain crustaceans
(prawns), in which they closely resemble the adults of a genus
(_Mysis_) belonging to a slightly lower group.

NASCENT.—Commencing development.

NATATORY.—Adapted for the purpose of swimming.

NAUPLIUS-FORM.—The earliest stage in the development of many Crustacea,
especially belonging to the lower groups. In this stage the animal has
a short body, with indistinct indications of a division into segments,
and three pairs of fringed limbs. This form of the common fresh-water
_Cyclops_ was described as a distinct genus under the name of
_Nauplius_.

NEURATION.—The arrangement of the veins or nervures in the wings of
insects.

NEUTERS.—Imperfectly developed females of certain social insects (such
as ants and bees), which perform all the labours of the community.
Hence, they are also called _workers_.

NICTITATING MEMBRANE.—A semi-transparent membrane, which can be drawn
across the eye in birds and reptiles, either to moderate the effects of
a strong light or to sweep particles of dust, &c., from the surface of
the eye.

OCELLI.—The simple eyes or stemmata of insects, usually situated on the
crown of the head between the great compound eyes.

ŒSOPHAGUS.—The gullet.

OOLITIC.—A great series of secondary rocks, so called from the texture
of some of its members, which appear to be made up of a mass of small
EGG-LIKE calcareous bodies.

OPERCULUM.—A calcareous plate employed by many Molluscæ to close the
aperture of their shell. The OPERCULAR VALVES of Cirripedes are those
which close the aperture of the shell.

ORBIT.—The bony cavity for the reception of the eye.

ORGANISM.—An organised being, whether plant or animal.

ORTHOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which
have the seed straight.

OSCULANT.—Forms or groups apparently intermediate between and
connecting other groups are said to be osculant.

OVA.—Eggs.

OVARIUM or OVARY (in plants).—The lower part of the pistil or female
organ of the flower, containing the ovules or incipient seeds; by
growth after the other organs of the flower have fallen, it usually
becomes converted into the fruit.

OVIGEROUS.—Egg-bearing.

OVULES (of plants).—The seeds in the earliest condition.

PACHYDERMS.—A group of Mammalia, so called from their thick skins, and
including the elephant, rhinoceros, hippopotamus, &c.

PALÆOZOIC.—The oldest system of fossiliferous rocks.

PALPI.—Jointed appendages to some of the organs of the mouth in insects
and Crustacea.

PAPILIONACEÆ.—An order of plants (see LEGUMINOSÆ), The flowers of these
plants are called _papilionaceous_, or butterfly-like, from the fancied
resemblance of the expanded superior petals to the wings of a
butterfly.

PARASITE.—An animal or plant living upon or in, and at the expense of,
another organism.

PARTHENOGENESIS.—The production of living organisms from unimpregnated
eggs or seeds.

PEDUNCULATED.—Supported upon a stem or stalk. The pedunculated oak has
its acorns borne upon a footstool.

PELORIA or PELORISM.—The appearance of regularity of structure in the
flowers of plants which normally bear irregular flowers.

PELVIS.—The bony arch to which the hind limbs of vertebrate animals are
articulated.

PETALS.—The leaves of the corolla, or second circle of organs in a
flower. They are usually of delicate texture and brightly coloured.

PHYLLODINEOUS.—Having flattened, leaf-like twigs or leafstalks instead
of true leaves.

PIGMENT.—The colouring material produced generally in the superficial
parts of animals. The cells secreting it are called _pigment-cells_.

PINNATE.—Bearing leaflets on each side of a central stalk.

PISTILS.—The female organs of a flower, which occupy a position in the
centre of the other floral organs. The pistil is generally divisible
into the ovary or germen, the style and the stigma.

PLACENTALIA, PLACENTATA.—or PLACENTAL MAMMALS, See MAMMALIA.

PLANTIGRADES.—Quadrupeds which walk upon the whole sole of the foot,
like the bears.

PLASTIC.—Readily capable of change.

PLEISTOCENE PERIOD.—The latest portion of the Tertiary epoch.

PLUMULE (in plants).—The minute bud between the seed-leaves of
newly-germinated plants.

PLUTONIC ROCKS.—Rocks supposed to have been produced by igneous action
in the depths of the earth.

POLLEN.—The male element in flowering plants; usually a fine dust
produced by the anthers, which, by contact with the stigma effects the
fecundation of the seeds. This impregnation is brought about by means
of tubes (_pollen-tubes_) which issue from the pollen-grains adhering
to the stigma, and penetrate through the tissues until they reach the
ovary.

POLYANDROUS (flowers).—Flowers having many stamens.

POLYGAMOUS PLANTS.—Plants in which some flowers are unisexual and
others hermaphrodite. The unisexual (male and female) flowers, may be
on the same or on different plants.

POLYMORPHIC.—Presenting many forms.

POLYZOARY.—The common structure formed by the cells of the Polyzoa,
such as the well-known seamats.

PREHENSILE.—Capable of grasping.

PREPOTENT.—Having a superiority of power.

PRIMARIES.—The feathers forming the tip of the wing of a bird, and
inserted upon that part which represents the hand of man.

PROCESSES.—Projecting portions of bones, usually for the attachment of
muscles, ligaments, &c.

PROPOLIS.—A resinous material collected by the hivebees from the
opening buds of various trees.

PROTEAN.—Exceedingly variable.

PROTOZOA.—The lowest great division of the animal kingdom. These
animals are composed of a gelatinous material, and show scarcely any
trace of distinct organs. The Infusoria, Foraminifera, and sponges,
with some other forms, belong to this division.

PUPA (pl. PUPÆ).—The second stage in the development of an insect, from
which it emerges in the perfect (winged) reproductive form. In most
insects the _pupal stage_ is passed in perfect repose. The _chrysalis_
is the pupal state of butterflies.

RADICLE.—The minute root of an embryo plant.

RAMUS.—One half of the lower jaw in the Mammalia. The portion which
rises to articulate with the skull is called the _ascending ramus_.

RANGE.—The extent of country over which a plant or animal is naturally
spread. _Range in time_ expresses the distribution of a species or
group through the fossiliferous beds of the earth’s crust.

RETINA.—The delicate inner coat of the eye, formed by nervous filaments
spreading from the optic nerve, and serving for the perception of the
impressions produced by light.

RETROGRESSION.—Backward development. When an animal, as it approaches
maturity, becomes less perfectly organised than might be expected from
its early stages and known relationships, it is said to undergo a
_retrogade development_ or _metamorphosis_.

RHIZOPODS.—A class of lowly organised animals (Protozoa), having a
gelatinous body, the surface of which can be protruded in the form of
root-like processes or filaments, which serve for locomotion and the
prehension of food. The most important order is that of the
Foraminifera.

RODENTS.—The gnawing Mammalia, such as the rats, rabbits, and
squirrels. They are especially characterised by the possession of a
single pair of chisel-like cutting teeth in each jaw, between which and
the grinding teeth there is a great gap.

RUBUS.—The bramble genus.

RUDIMENTARY.—Very imperfectly developed.

RUMINANTS.—The group of quadrupeds which ruminate or chew the cud, such
as oxen, sheep, and deer. They have divided hoofs, and are destitute of
front teeth in the upper jaw.

SACRAL.—Belonging to the sacrum, or the bone composed usually of two or
more united vertebræ to which the sides of the pelvis in vertebrate
animals are attached.

SARCODE.—The gelatinous material of which the bodies of the lowest
animals (Protozoa) are composed.

SCUTELLÆ.—The horny plates with which the feet of birds are generally
more or less covered, especially in front.

SEDIMENTARY FORMATIONS.—Rocks deposited as sediments from water.

SEGMENTS.—The transverse rings of which the body of an articulate
animal or annelid is composed.

SEPALS.—The leaves or segments of the calyx, or outermost envelope of
an ordinary flower. They are usually green, but sometimes brightly
coloured.

SERRATURES.—Teeth like those of a saw.

SESSILE.—Not supported on a stem or footstalk.

SILURIAN SYSTEM.—A very ancient system of fossiliferous rocks belonging
to the earlier part of the Palæozoic series.

SPECIALISATION.—The setting apart of a particular organ for the
performance of a particular function.

SPINAL CORD.—The central portion of the nervous system in the
Vertebrata, which descends from the brain through the arches of the
vertebræ, and gives off nearly all the nerves to the various organs of
the body.

STAMENS.—The male organs of flowering plants, standing in a circle
within the petals. They usually consist of a filament and an anther,
the anther being the essential part in which the pollen, or fecundating
dust, is formed.

STERNUM.—The breast-bone.

STIGMA.—The apical portion of the pistil in flowering plants.

STIPULES.—Small leafy organs placed at the base of the footstalks of
the leaves in many plants.

STYLE.—The middle portion of the perfect pistil, which rises like a
column from the ovary and supports the stigma at its summit.

SUBCUTANEOUS.—Situated beneath the skin.

SUCTORIAL.—Adapted for sucking.

SUTURES (in the skull).—The lines of junction of the bones of which the
skull is composed.

TARSUS (pl. TARSI).—The jointed feet of articulate animals, such as
insects.

TELEOSTEAN FISHES.—Fishes of the kind familiar to us in the present
day, having the skeleton usually completely ossified and the scales
horny.

TENTACULA or TENTACLES.—Delicate fleshy organs of prehension or touch
possessed by many of the lower animals.

TERTIARY.—The latest geological epoch, immediately preceding the
establishment of the present order of things.

TRACHEA.—The windpipe or passage for the admission of air to the lungs.

TRIDACTYLE.—Three-fingered, or composed of three movable parts attached
to a common base.

TRILOBITES.—A peculiar group of extinct crustaceans, somewhat
resembling the woodlice in external form, and, like some of them,
capable of rolling themselves up into a ball. Their remains are found
only in the Palæozoic rocks, and most abundantly in those of Silurian
age.

TRIMORPHIC.—Presenting three distinct forms.

UMBELLIFERÆ.—An order of plants in which the flowers, which contain
five stamens and a pistil with two styles, are supported upon
footstalks which spring from the top of the flower stem and spread out
like the wires of an umbrella, so as to bring all the flowers in the
same head (_umbel_) nearly to the same level. (_Examples_, Parsley and
Carrot.)

UNGULATA.—Hoofed quadrupeds.

UNICELLULAR.—Consisting of a single cell.

VASCULAR.—Containing blood-vessels.

VERMIFORM.—Like a worm.

VERTEBRATA or VERTEBRATE ANIMALS.—The highest division of the animal
kingdom, so called from the presence in most cases of a backbone
composed of numerous joints or _vertebræ_, which constitutes the centre
of the skeleton and at the same time supports and protects the central
parts of the nervous system.

WHORLS.—The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.

WORKERS.—See neuters.

ZOËA-STAGE.—The earliest stage in the development of many of the higher
Crustacea, so called from the name of _Zoëa_ applied to these young
animals when they were supposed to constitute a peculiar genus.

ZOOIDS.—In many of the lower animals (such as the Corals, Medusæ, &c.)
reproduction takes place in two ways, namely, by means of eggs and by a
process of budding with or without separation from the parent of the
product of the latter, which is often very different from that of the
egg. The individuality of the species is represented by the whole of
the form produced between two sexual reproductions; and these forms,
which are apparently individual animals, have been called _zooids_.




INDEX.


Aberrant groups, 379.

Abyssinia, plants of, 340.

Acclimatisation, 112.

Adoxa, 173.

Affinities of extinct species, 301.
—, of organic beings, 378.

Agassiz on Amblyopsis, 112.
—, on groups of species suddenly appearing, 289.
—, on prophetic forms, 301.
—, on embryological succession, 310.
—, on the Glacial period, 330.
—, on embryological characters, 368.
—, on the latest tertiary forms, 278.
—, on parallelism of embryological development and geological
succession, 396.
—, Alex., on pedicellariæ, 191.

Algæ of New Zealand, 338.

Alligators, males, fighting, 69.

Alternate generations, 387.

Amblyopsis, blind fish, 112.

America, North, productions allied to those of Europe, 333.
—, boulders and glaciers of, 335.
—, South, no modern formations on west coast, 272.

Ammonites, sudden extinction of, 297.

Anagallis, sterility of, 236.

Analogy of variations, 127.

Andaman Islands inhabited by a toad, 350.

Ancylus, 345.

Animals, not domesticated from being variable, 13.
—, domestic; descended from several stocks, 14.
—, acclimatisation of, 112.

Animals of Australia, 90.
—, with thicker fur in cold climates, 107.
—, blind, in caves, 110.
—, extinct, of Australia, 310.

Anomma, 232.

Antarctic islands, ancient flora of, 355.

Antechinus, 373.

Ants attending aphides, 207.
—, slave-making instinct, 217.
—, neuters, structure of, 230.

Apes, not having acquired intellectual powers, 181.

Aphides attended by ants, 207.

Aphis, development of, 390.

Apteryx, 140.

Arab horses, 26.

Aralo-Caspian Sea, 311.

Archeopteryx, 284.

Archiac, M. de, on the succession of species, 299.

Artichoke, Jerusalem, 114.

Ascension, plants of, 347.

Asclepias, pollen of, 151.

Asparagus, 325.

Aspicarpa, 367.

Asses, striped, 127.
—, improved by selection, 30.

Ateuchus, 109.

Aucapitaine, on land-shells, 353.

Audubon, on habits of frigate-bird, 142.
—, on variation in birds’ nests, 208.
—, on heron eating seeds, 346.

Australia, animals of, 90.
—, dogs of, 211.
—, extinct animals of, 310.
—, European plants in, 337.
—, glaciers of, 335.

Azara, on flies destroying cattle, 56.

Azores, flora of, 328.

Babington, Mr., on British plants, 37.

Baer, Von, standard of Highness, 97.
—, comparison of bee and fish, 308.
—, embryonic similarity of the Vertebrata, 387.

Baker, Sir S., on the giraffe, 178.

Balancement of growth, 117.

Baleen, 182.

Barberry, flowers of, 77.

Barrande, M., on Silurian colonies, 291.
—, on the succession of species, 299.
—, on parallelism of palæozoic formations, 301.
—, on affinities of ancient species, 302.

Barriers, importance of, 317.

Bates, Mr., on mimetic butterflies, 375, 376.

Batrachians on islands, 350.

Bats, how structure acquired, 140.
—, distribution of, 351.

Bear, catching water-insects, 141.

Beauty, how acquired, 159, 414.

Bee, sting of, 163.
—, queen, killing rivals, 164.
—, Australian, extermination of, 59.

Bees, fertilizing flowers, 57.
—, hive, not sucking the red clover, 75.
—, Ligurian, 75.
—, hive, cell-making instinct, 220.
—, variation in habits, 208.
—, parasitic, 216.
—, humble, cells of, 220.

Beetles, wingless, in Madeira, 109.
—, with deficient tarsi, 109.

Bentham, Mr., on British plants, 37.
—, on classification, 369.

Berkeley, Mr., on seeds in salt-water, 324.

Bermuda, birds of, 348.

Birds acquiring fear, 208.
—, beauty of, 161.
—, annually cross the Atlantic, 329.
—, colour of, on continents, 107.
—, footsteps, and remains of, in secondary rocks, 284.
—, fossil, in caves of Brazil, 310.
—, of Madeira, Bermuda, and Galapagos, 349, 349.
—, song of males, 70.
—, transporting seeds, 328.
—, waders, 345.
—, wingless, 108, 140.

Bizcacha, 318.
—, , affinities of, 379.

Bladder for swimming, in fish, 147.

Blindness of cave animals, 110.

Blyth, Mr., on distinctness of Indian cattle, 14.
—, on striped Hemionus, 128.
—, on crossed geese, 240.

Borrow, Mr., on the Spanish pointer, 26.

Bory St. Vincent, on Batrachians, 350.

Bosquet, M., on fossil Chthamalus, 284.

Boulders, erratic, on the Azores, 328.

Branchiæ, 148, 149.
—, of crustaceans, 152.

Braun, Prof., on the seeds of Fumariaceæ, 174.

Brent, Mr., on house-tumblers, 210.

Britain, mammals of, 352.

Broca, Prof., on Natural Selection, 170.

Bronn, Prof., on duration of specific forms, 275.
—, various objections by, 170.

Brown, Robert, on classification, 366.

Brown-Sequard, on inherited mutilations, 108.

Busk, Mr., on the Polyzoa, 193.

Butterflies, mimetic, 375, 376.

Buzareingues, on sterility of varieties, 258.

Cabbage, varieties of, crossed, 78.

Calceolaria, 239.

Canary-birds, sterility of hybrids, 240.

Cape de Verde Islands, productions of, 354.
—, plants of, on mountains, 337.

Cape of Good Hope, plants of, 101, 347.

Carpenter, Dr., on foraminifera, 308.

Carthemus, 173.

Catasetum, 155, 372.

Cats, with blue eyes, deaf, 9.
—, variation in habits of, 209.
—, curling tail when going to spring, 162.

Cattle destroying fir-trees, 56.
—, destroyed by flies in Paraguay, 56.
—, breeds of, locally extinct, 86.
—, fertility of Indian and European breeds, 241.
—, Indian, 14, 241.

Cave, inhabitants of, blind, 110.

Cecidomyia, 387.

Celts, proving antiquity of man, 13.

Centres of creation, 320.

Cephalopodæ, structures of eyes, 151.
—, development of, 390.

Cercopithecus, tail of, 189.

Ceroxylus laceratus, 182.

Cervulus, 240.

Cetacea, teeth and hair, 115.
—, development of the whalebone, 182.

Cetaceans, 182.

Ceylon, plants of, 338.

Chalk formation, 297.

Characters, divergence of, 86.
—, sexual, variable, 119, 123.
—, adaptive or analogical, 373.

Charlock, 59.

Checks to increase, 53.
—, mutual, 55.

Chelæ of Crustaceans, 193.

Chickens, instinctive tameness of, 211.

Chironomus, its asexual reproduction, 387.

Chthamalinæ, 271.

Chthamalus, cretacean species of, 384.

Circumstances favourable to selection of domestic products, 29.
—, to natural selection, 80.

Cirripedes capable of crossing, 79.
—, carapace aborted, 118.
—, their ovigerous frena, 148.
—, fossil, 284.
—, larvæ of, 389.

Claparède, Prof., on the hair-claspers of the Acaridæ, 153.

Clarke, Rev. W.B., on old glaciers in Australia, 335.

Classification, 363.

Clift, Mr., on the succession of types, 310.

Climate, effects of, in checking increase of beings, 54.
—, adaptation of, to organisms, 112.

Climbing plants, 147.
—, development of, 96.

Clover visited by bees, 75.

Cobites, intestine of, 147.

Cockroach, 59.

Collections, palæontological, poor, 270.

Colour, influenced by climate, 107.
—, in relation to attacks by flies, 159.

Columba livia, parent of domestic pigeons, 17.

Colymbetes, 345.

Compensation of growth, 117.

Compositæ, flowers and seeds of, 116.
—, outer and inner florets of, 173.
—, male flowers of, 398.

Conclusion, general, 421.

Conditions, slight changes in, favourable to fertility, 251.

Convergence of genera, 100.

Coot, 142.

Cope, Prof., on the acceleration or retardation of the period of
reproduction, 149.

Coral-islands, seeds drifted to, 326.
—, reefs, indicating movements of earth, 326.

Corn-crake, 143.

Correlated variation in domestic productions, 9.

Coryanthes, 154.

Creation, single centres of, 320.

Crinum, 238.

Croll, Mr., on subaërial denudation, 267, 269.
—, on the age of our oldest formations, 286.
—, on alternate Glacial periods in the North and South, 336.

Crosses, reciprocal, 244.

Crossing of domestic animals, importance in altering breeds, 15.
—, advantages of, 76, 77.
—, unfavourable to selection, 80.

Crüger, Dr., on Coryanthes, 154.

Crustacea of New Zealand, 338.

Crustacean, blind, 110.
air-breathers, 152.

Crustaceans, their chelæ, 193.

Cryptocerus, 231.

Ctenomys, blind, 110.

Cuckoo, instinct of, 205, 212.

Cunningham, Mr., on the flight of the logger-headed duck, 108.

Currants, grafts of, 246.

Currents of sea, rate of, 325.

Cuvier on conditions of existence, 205.
—, on fossil monkeys, 283, 284.

Cuvier, Fred., on instinct, 205.

Cyclostoma, resisting salt water, 353.

Dana, Prof., on blind cave-animals, 111.
—, on relations of crustaceans of Japan, 334.
—, on crustaceans of New Zealand, 338.

Dawson, Dr., on eozoon, 287.

De Candolle, Aug. Pyr., on struggle for existence, 49.
—, on umbelliferæ, 116.
—, on general affinities, 379.

De Candolle, Alph., on the variability of oaks, 40.
—, on low plants, widely dispersed, 359.
—, on widely-ranging plants being variable, 43.
—, on naturalisation, 89.
—, on winged seeds, 117.
—, on Alpine species suddenly becoming rare, 135.
—, on distribution of plants with large seeds, 326.
—, on vegetation of Australia, 340.
—, on fresh-water plants, 345.
—, on insular plants, 347.

Degradation of rocks, 266.

Denudation, rate of, 268.
—, of oldest rocks, 287.
—, of granitic areas, 274.

Development of ancient forms, 307.

Devonian system, 305.

Dianthus, fertility of crosses, 243.

Dimorphism in plants, 35, 252.

Dirt on feet of birds, 328.

Dispersal, means of, 323.
—, during Glacial period, 330.

Distribution, geographical, 316.
—, means of, 323.

Disuse, effect of, under nature, 108.

Diversification of means for same general purpose, 153.

Division, physiological, of labour, 89.

Divergence of character, 86.

Dog, resemblance of jaw to that of the Thylacinus, 374.

Dogs, hairless, with imperfect teeth, 9.
—, descended from several wild stocks, 15.
—, domestic instincts of, 210.
—, inherited civilisation of, 210.
—, fertility of breeds together, 241.
—, of crosses, 256.
—, proportions of body in different breeds, when young, 392.

Domestication, variation under, 5.

Double flowers, 230.

Downing, Mr., on fruit-trees in America, 66.

Dragon-flies, intestines of, 147.

Drift-timber, 326.

Driver-ant, 232.

Drones killed by other bees, 164.

Duck, domestic, wings of, reduced, 8.
—, beak of, 183.
—, logger-headed, 140.

Duckweed, 344.

Dugong, affinities of, 365.

Dung-beetles with deficient tarsi, 108.

Dyticus, 345.

Earl, Mr., W., on the Malay Archipelago, 351.

Ears, drooping, in domestic animals, 8.
—, rudimentary, 400.

Earth, seeds in roots of trees, 326.
—, charged with seeds, 328.

Echinodermata, their pedicellariæ, 191.

Eciton, 230.

Economy of organisation, 117.

Edentata, teeth and hair, 115.
—, fossil species of, 417.

Edwards, Milne, on physiological division of labour, 89.
—, on gradations of structure, 156.

Edwards, on embryological characters, 368.

Eggs, young birds escaping from, 68.

Egypt, productions of, not modified, 169.

Electric organs, 150.

Elephant, rate of increase, 51.
—, of Glacial period, 113.

Embryology, 386.

Eozoon Canadense, 287.

Epilipsy inherited, 108.

Existence, struggle for, 48.
—, condition of, 167.

Extinction, as bearing on natural selection, 96.
—, of domestic varieties, 93.
—, , 293.

Eye, structure of, 144.
—, correction for aberration, 163.

Eyes, reduced, in moles, 110.

Fabre, M., on hymenoptera fighting, 69.
—, on parasitic sphex, 216.
—, on Sitaris, 394.

Falconer, Dr., on naturalisation of plants in India, 51.
—, on elephants and mastodons, 306.
—, and Cautley on mammals of sub-Himalayan beds, 311.

Falkland Islands, wolf of, 351.

Faults, 268.

Faunas, marine, 317.

Fear, instinctive, in birds, 211.

Feet of birds, young molluscs adhering to, 345.

Fertilisation variously effected, 154, 161.

Fertility of hybrids, 238.
—, from slight changes in conditions, 252.
—, of crossed varieties, 255.

Fir-trees destroyed by cattle, 56.
—, pollen of, 164.

Fish, flying, 140.
—, teleostean, sudden appearance of, 285.
—, eating seeds, 327, 346.
—, fresh-water, distribution of, 343.

Fishes, ganoid, now confined to fresh water, 83.
—, electric organs of, 150.
—, ganoid, living in fresh water, 296.
—, of southern hemisphere, 338.

Flat-fish, their structure, 186.

Flight, powers of, how acquired, 140.

Flint-tools, proving antiquity of man, 13.

Flower, Prof., on the larynx, 190.
—, on Halitherium, 302.
—, on the resemblance between the jaws of the dog and Thylacinus, 375.
—, on the homology of the feet of certain marsupials, 382.

Flowers, structure of in relation to crossing, 73.
—, of compositæ and umbelliferæ, 116, 173.
—, beauty of, 161.
—, double, 230.

Flysch formation, destitute of organic remains, 271.

Forbes, Mr. D., on glacial action in the Andes, 335.

Forbes, E., on colours of shells, 107.
—, on abrupt range of shells in depth, 135.
—, on poorness of palæontological collections, 270.
—, on continuous succession of genera, 293.
—, on continental extensions, 323.
—, on distribution during Glacial period, 330.
—, on parallelism in time and space, 361.

Forests, changes in, in America, 58.

Formation, Devonian, 305.
—, Cambrian, 287.
Formations, thickness of, in Britain, 268.
—, intermittent, 277.

Formica rufescens, 216.
—, sanguinea, 217.
—, flava, neuter of, 231.

Forms, lowly organised, long enduring, 99.

Frena, ovigerous, of cirripedes, 148.

Fresh-water productions, dispersal of, 343.

Fries on species in large genera being closely allied to other species,
45.

Frigate-bird, 142.

Frogs on islands, 350.

Fruit-trees, gradual improvement of, 27.
—, in United States, 66.
—, varieties of, acclimatised in United States, 114.

Fuci, crossed, 249, 343.

Fur, thicker in cold climates, 107.

Furze, 388.

Galapagos Archipelago, birds of, 348.
—, productions of, 353, 355.

Galaxias, its wide range, 343.

Galeopithecus, 139.

Game, increase of, checked by vermin, 55.

Gärtner on sterility of hybrids, 237, 241.
—, on reciprocal crosses, 243.
—, on crossed maize and verbascum, 257, 258.
—, on comparison of hybrids and mongrels, 259, 260.

Gaudry, Prof., on intermediate genera of fossil mammals in Attica, 301.

Geese, fertility when crossed, 307.
—, upland, 142.

Geikie, Mr., on subaërial denudation, 267.

Genealogy, important in classification, 369.

Generations, alternate, 387.

Geoffroy St. Hilaire, on balancement, 117.
—, on homologous organs, 382.
—, , Isidore, on variability of repeated parts, 118.
—, on correlation, in monstrosities, 9.
—, on correlation, 115.
—, on variable parts being often monstrous, 122.

Geographical distribution, 316.

Geography, ancient, 427.

Geology, future progress of, 427.
—, imperfection of the record, 427.

Gervais, Prof., on Typotherium, 302.

Giraffe, tail of, 157.
—, structure of, 177.

Glacial period, 330.
—, affecting the North and South, 335.

Glands, mammary, 189.

Gmelin, on distribution, 330.

Godwin-Austin, Mr., on the Malay Archipelago, 280.

Goethe, on compensation of growth, 117.

Gomphia, 174.

Gooseberry, grafts of, 246.

Gould, Dr. Aug. A., on land-shells, 353.

Gould, Mr., on colours of birds, 107.
—, on instincts of cuckoo, 214.
—, on distribution of genera of birds, 358.

Gourds, crossed, 258.

Graba, on the Uria lacrymans, 72.

Grafting, capacity of, 245, 246.

Granite, areas of denuded, 274.

Grasses, varieties of, 88.

Gray, Dr. Asa, on the variability of oaks, 40.
—, on man not causing variability, 62.
—, on sexes of the holly, 74.
—, on trees of the United States, 79.
—, on naturalised plants in the United States, 89.
—, on æstivation, 174.
—, on Alpine plants, 330.
—, on rarity of intermediate varieties, 136.

Gray, Dr. J.E., on striped mule, 128.

Grebe, 142.

Grimm, on asexual reproduction, 387.

Groups, aberrant, 378.

Grouse, colours of, 66.
—, red, a doubtful species, 38.

Growth, compensation of, 117.

Günther, Dr., on flat-fish, 187.
—, on prehensile tails, 189.
—, on the fishes of Panama, 317.
—, on the range of fresh-water fishes, 343.
—, on the limbs of Lepidosiren, 399.

Haast, Dr., on glaciers of New Zealand, 335.

Habit, effect of, under domestication, 8.
—, effect of, under nature, 108.
—, diversified, of same species, 141.

Häckel, Prof., on classification and the lines of descent, 381.

Hair and teeth, correlated, 115.

Halitherium, 302.

Harcourt, Mr. E.V., on the birds of Madeira, 348.

Hartung, M., on boulders in the Azores, 328.

Hazel-nuts, 325.

Hearne, on habits of bears, 141.

Heath, changes in vegetation, 55.

Hector, Dr., on glaciers of New Zealand, 335.

Heer, Oswald, on ancient cultivated plants, 13.
—, on plants of Madeira, 83.

Helianthemum, 174.

Helix, resisting salt water, 353.

Helix pomatia, 353.

Helmholtz, M., on the imperfection of the human eye, 163.

Helosciadium, 325.

Hemionus, striped, 128.

Hensen, Dr., on the eyes of Cephalopods, 152.

Herbert, W., on struggle for existence, 49.
—, on sterility of hybrids, 238.

Hermaphrodites crossing, 76.

Heron eating seed, 346.

Heron, Sir R., on peacocks, 70.

Heusinger, on white animals poisoned by certain plants, 9.

Hewitt, Mr., on sterility of first crosses, 249.

Hildebrand, Prof., on the self-sterility of Corydalis, 238.

Hilgendorf, on intermediate varieties, 275.

Himalaya, glaciers of, 335.
—, plants of, 337.

Hippeastrum, 238.

Hippocampus, 189.

Hofmeister, Prof., on the movements of plants, 197.

Holly-trees, sexes of, 73.

Hooker, Dr., on trees of New Zealand, 78.
—, on acclimatisation of Himalayan trees, 112.
—, on flowers of umbelliferæ, 116.
—, on the position of ovules, 172.
—, on glaciers of Himalaya, 335.
—, on algæ of New Zealand, 338.
—, on vegetation at the base of the Himalaya, 338.
—, on plants of Tierra del Fuego, 336.
—, on Australian plants, 337, 355.
—, on relations of flora of America, 340.
—, on flora of the Antarctic lands, 341, 354.
—, on the plants of the Galapagos, 349, 354.
—, on glaciers of the Lebanon, 335.
—, on man not causing variability, 62.
—, on plants of mountains of Fernando Po, 337.

Hooks on palms, 158.
—, on seeds, on islands, 349.

Hopkins, Mr., on denudation, 274.

Hornbill, remarkable instinct of, 234.

Horns, rudimentary, 400.

Horse, fossil in La Plata, 294.
—, proportions of, when young, 392.

Horses destroyed by flies in Paraguay, 56.
—, striped, 128.

Horticulturists, selection applied by, 23.

Huber on cells of bees, 224.

Huber, P., on reason blended with instinct, 205.
—, on habitual nature of instincts, 206.
—, on slave-making ants, 216.
—, on Melipona domestica, 220.

Hudson, Mr., on the Ground-woodpecker of La Plata, 142.
—, on the Molothrus, 215.

Humble-bees, cells of, 221.

Hunter, J., on secondary sexual characters, 119.

Hutton, Captain, on crossed geese, 240.

Huxley, Prof., on structure of hermaphrodites, 79.
—, on the affinities of the Sirenia, 302.
—, on forms connecting birds and reptiles, 302.
—, on homologous organs, 386.
—, on the development of aphis, 390.

Hybrids and mongrels compared, 259.

Hybridism, 235.

Hydra, structure of, 147.

Hymenoptera, fighting, 69.

Hymenopterous insect, diving, 142.

Hyoseris, 173.

Ibla, 118.

Icebergs transporting seeds, 329.

Increase, rate of, 50.

Individuals, numbers favourable to selection, 80.
—, many, whether simultaneously created, 322.

Inheritance, laws of, 10.
—, at corresponding ages, 10, 67.

Insects, colour of, fitted for their stations, 66.
—, sea-side, colours of, 107.
—, blind, in caves, 110.
—, luminous, 151.
—, their resemblance to various objects, 181.
—, neuter, 2320.

Instinct, 205.
—, , not varying simultaneously with structure, 229.

Instincts, domestic, 209.

Intercrossing, advantages of, 76, 251.

Islands, oceanic, 347.

Isolation favourable to selection, 81.

Japan, productions of, 334.

Java, plants of, 337.

Jones, Mr. J.M., on the birds of Bermuda, 348.

Jordain, M., on the eye-spots of star fishes, 144.

Jukes, Prof., on subaërial denudation, 267.

Jussieu on classification, 367.

Kentucky, caves of, 111.

Kerguelen-land, flora of, 341, 354.

Kidney-bean, acclimatisation of, 114.

Kidneys of birds, 115.

Kirby, on tarsi deficient in beetles, 108.

Knight, Andrew, on cause of variation, 5.

Kölreuter, on intercrossing, 76.
—, on the barberry, 77.
—, on sterility of hybrids, 237.
—, on reciprocal crosses, 243.
—, on crossed varieties of nicotiana, 258.
—, on crossing male and hermaphrodite flowers, 397.

Lamarck, on adaptive characters, 373.

Lancelet, 99.
—, , eyes of, 145.

Landois, on the development of the wings of insects, 148.

Land-shells, distribution of, 353.
—, of Madeira, naturalised, 357.
—, resisting salt water, 353.

Languages, classification of, 371.

Lankester, Mr. E. Ray, on longevity, 169.
—, on homologies, 385.

Lapse, great, of time, 266.

Larvæ, 388, 389.

Laurel, nectar secreted by the leaves, 73.

Laurentian formation, 287.

Laws of variation, 106.

Leech, varieties of, 59.

Leguminosæ, nectar secreted by glands, 73.

Leibnitz’ attack on Newton, 421.

Lepidosiren, 83, 303.
—, , limbs in a nascent condition, 398, 399.

Lewes, Mr. G.H., on species not having changed in Egypt, 169.
—, on the Salamandra atra, 397.
—, on many forms of life having been at first evolved, 425.

Life, struggle for, 49.

Lingula, Silurian, 286.

Linnæus, aphorism of, 365.

Lion, mane of, 69.
—, young of, striped, 388.

Lobelia fulgens, 57, 77.

Lobelia, sterility of crosses, 238.

Lockwood, Mr., on the ova of the Hippocampus, 189.

Locusts transporting seeds, 327.

Logan, Sir W., on Laurentian formation, 287.

Lowe, Rev. R.T., on locusts visiting Madeira, 327.

Lowness, of structure connected with variability, 118.
—, related to wide distribution, 359.

Lubbock, Sir J., on the nerves of coccus, 35.
—, on secondary sexual characters, 124.
—, on a diving hymenopterous insect, 142.
—, on affinities, 280.
—, on metamorphoses, 386, 389.

Lucas, Dr. P., on inheritance, 9.
—, on resemblance of child to parent, 261.

Lund and Clausen, on fossils of Brazil, 310.

Lyell, Sir C., on the struggle for existence, 49.
—, on modern changes of the earth, 75.
—, on terrestrial animals not having been developed on islands, 180.
—, on a carboniferous land-shell, 271.
—, on strata beneath Silurian system, 287.
—, on the imperfection of the geological record, 289.
—, on the appearance of species, 289.
—, on Barrande’s colonies, 291.
—, on tertiary formations of Europe and North America, 298.
—, on parallelism of tertiary formations, 301.
—, on transport of seeds by icebergs, 328.
—, on great alternations of climate, 342.
—, on the distribution of fresh-water shells, 345.
—, on land-shells of Madeira, 357.

Lyell and Dawson, on fossilized trees in Nova Scotia, 278.

Lythrum salicaria, trimorphic, 254.

Macleay, on analogical characters, 373.

Macrauchenia, 302.

McDonnell, Dr., on electric organs, 150.

Madeira, plants of, 83.
—, beetles of, wingless, 109.
—, fossil land-shells of, 311.
—, birds of, 348.

Magpie tame in Norway, 209.

Males, fighting, 69.

Maize, crossed, 257.

Malay Archipelago, compared with Europe, 280.
—, mammals of, 352.

Malm, on flat-fish, 186.

Malpighiaceæ, small imperfect flowers of, 173.

Malpighiaceæ, 367.

Mammæ, their development, 189.
—, rudimentary, 397.

Mammals, fossil, in secondary formation, 283.
—, insular, 351.

Man, origin of, 428.

Manatee, rudimentary nails of, 400.

Marsupials of Australia, 90.
—, , fossil species of, 382.
—, , structure of their feet, 310.

Martens, M., experiment on seeds, 325.

Martin, Mr. W.C., on striped mules, 129.

Masters, Dr., on Saponaria, 174.

Matteucci, on the electric organs of rays, 150.

Matthiola, reciprocal crosses of, 244.

Maurandia, 197.

Means of dispersal, 323.

Melipona domestica, 220.

Merrill, Dr., on the American cuckoo, 212.

Metamorphism of oldest rocks, 287.

Mice destroying bees, 56.
—, acclimatisation of, 113.
—, tails of, 189.

Miller, Prof., on the cells of bees, 221, 224.

Mirabilis, crosses of, 243.

Missel-thrush, 59.

Mistletoe, complex relations of, 2.

Mivart, Mr., on the relation of hair and teeth, 115.
—, on the eyes of cephalopods, 151.
—, various objections to Natural Selection, 174.
—, on abrupt modifications, 201.
—, on the resemblance of the mouse and antechinus, 373.

Mocking-thrush of the Galapagos, 357.

Modification of species, not abrupt, 424.

Moles, blind, 110.

Molothrus, habits of, 215.

Mongrels, fertility and sterility of, 255.
—, and hybrids compared, 259.

Monkeys, fossil, 284, 285.

Monachanthus, 372.

Mons, Van, on the origin of fruit-trees, 21.

Monstrosities, 33.

Moquin-Tandon, on sea-side plants, 107.

Morphology, 382.

Morren, on the leaves of Oxalis, 197.

Moths, hybrid, 240.

Mozart, musical powers of, 206.

Mud, seeds in, 345.

Mules, striped, 129.

Müller, Adolph, on the instincts of the cuckoo, 213.

Müller, Dr. Ferdinand, on Alpine Australian plants, 337.

Müller, Fritz, on dimorphic crustaceans, 35, 233.
—, on the lancelet, 99.
—, on air-breathing crustaceans, 152.
—, on climbing plants, 197.
—, on the self-sterility of orchids, 238.
—, on embryology in relation to classification, 368.
—, on the metamorphoses of crustaceans, 390, 395.
—, on terrestrial and fresh-water organisms not undergoing any
metamorphosis, 394.

Multiplication of species not indefinite, 101.

Murchison, Sir, R., on the formations of Russia, 272.
—, on azoic formations, 286.
—, on extinction, 293.

Murie, Dr., on the modification of the skull in old age, 149.

Murray, Mr. A., on cave-insects, 111.

Mustela vison, 138.

Myanthus, 372.

Myrmecocystus, 231.

Myrmica, eyes of, 232.

Nägeli, on morphological characters, 170.

Nails, rudimentary, 400.

Nathusius, Von, on pigs, 159.

Natural history, future progress of, 426.
—, selection, 62.
—, system, 364.

Naturalisation of forms distinct from the indigenous species, 89.
—, in New Zealand, 163.

Naudin, on analagous variations in gourds, 125.
—, on hybrid gourds, 258.
—, on reversion, 260.

Nautilus, Silurian, 286.

Nectar of plants, 73.

Nectaries, how formed, 73.

Nelumbium luteum, 346.

Nests, variation in, 208, 228, 234.

Neuter insects, 230, 231.

New Zealand, productions of, not perfect, 163.
—, naturalised products of, 309.
—, fossil birds of, 310.
—, glaciers of, 335.
—, crustaceans of, 338.
—, algæ of, 338.
—, flora of, 354.
—, number of plants of, 374.

Newman, Col., on humble-bees, 57.

Newton, Prof., on earth attached to a partridge’s foot, 328.

Newton, Sir I., attacked for irreligion, 421.

Nicotiana, crossed varieties of, 258.
—, certain species very sterile, 243.

Nitsche, Dr., on the Polyzoa, 193.

Noble, Mr., on fertility of Rhododendron, 239.

Nodules, phosphatic, in azoic rocks, 287.

Oaks, variability of, 40.

Œnonis, small imperfect flowers of, 173.

Onites apelles, 108.

Orchids, fertilisation of, 154.
—, the development of their flowers, 195.
—, forms of, 372.

Orchis, pollen of, 151.

Organisation, tendency to advance, 97.

Organs of extreme perfection, 143.
—, electric, of fishes, 150.
—, of little importance, 156.
—, homologous, 382.
—, rudiments of, and nascent, 397.

Ornithorhynchus, 83, 367.
—, mammæ of, 190.

Ostrich not capable of flight, 180.
—, habit of laying eggs together, 215.
—, American, two species of, 318.

Otter, habits of, how acquired, 138.

Ouzel, water, 142.

Owen, Prof., on birds not flying, 108.
—, on vegetative repetition, 118.
—, on variability of unusually developed parts, 119.
—, on the eyes of fishes, 145.
—, on the swim-bladder of fishes, 148.
—, on fossil horse of La Plata, 294.
—, on generalised form, 301.
—, on relation of ruminants and pachyderms, 303.
—, on fossil birds of New Zealand, 310.
—, on succession of types, 310.
—, on affinities of the dugong, 365.
—, on homologous organs, 383.
—, on the metamorphosis of cephalopods, 390.

Pacific Ocean, faunas of, 317.

Pacini, on electric organs, 151.

Paley, on no organ formed to give pain, 163.

Pallas, on the fertility of the domesticated descendants of wild
stocks, 241.

Palm with hooks, 158.

Papaver bracteatum, 174.

Paraguay, cattle destroyed by flies, 56.

Parasites, 215.

Partridge, with ball of dirt attached to foot, 328.

Parts greatly developed, variable, 119.

Parus major, 141.

Passiflora, 238.

Peaches in United States, 66.

Pear, grafts of, 246.

Pedicellariæ, 191.

Pelargonium, flowers of, 166.
—, sterility of, 239.

Peloria, 116.

Pelvis of women, 115.

Period, glacial, 330.

Petrels, habits of, 142.

Phasianus, fertility of hybrids, 240.

Pheasant, young, wild, 211.

Pictet, Prof., on groups of species suddenly appearing, 282.
—, on rate of organic change, 291.
—, on continuous succession of genera, 293.
—, on change in latest tertiary forms, 278.
—, on close alliance of fossils in consecutive formations, 306.
—, on early transitional links, 283.

Pierce, Mr., on varieties of wolves, 71.

Pigeons with feathered feet and skin between toes, 9.
—, breeds described, and origin of, 15.
—, breeds of, how produced, 28, 30.
—, tumbler, not being able to get out of egg, 68.
—, reverting to blue colour, 127.
—, instinct of tumbling, 210.
—, young of, 392.

Pigs, black, not affected by the paint-root, 9.
—, modified by want of exercise, 159.

Pistil, rudimentary, 397.

Plants, poisonous, not affecting certain coloured animals, 9.
—, selection, applied to, 27.
—, gradual improvement of, 27.
—, not improved in barbarous countries, 27.
—, dimorphic, 35, 253.
—, destroyed by insects, 53.
—, in midst of range, have to struggle with other plants, 60.
—, nectar of, 73.
—, fleshy, on sea-shores, 107.
—, climbing, 147, 196.
—, fresh-water, distribution of, 345.
—, low in scale, widely distributed, 359.

Pleuronectidæ, their structure, 186.

Plumage, laws of change in sexes of birds, 70.

Plums in the United States, 66.

Pointer dog, origin of, 25.
—, habits of, 210.

Poison not affecting certain coloured animals, 9.

Poison, similar effect of, on animals and plants, 425.

Pollen of fir-trees, 164.
—, transported by various means, 154, 161.

Pollinia, their development, 195.

Polyzoa, their avicularia, 193.

Poole, Col., on striped hemionus, 128.

Potemogeton, 346.

Pouchet, on the colours of flat-fish, 188.

Prestwich, Mr., on English and French eocene formations, 300.

Proctotrupes, 142.

Proteolepas, 118.

Proteus, 112.

Psychology, future progress of, 428.

Pyrgoma, found in the chalk, 284.

Quagga, striped, 129.

Quatrefages, M., on hybrid moths, 240.

Quercus, variability of, 40.

Quince, grafts of, 246.

Rabbit, disposition of young, 211.

Races, domestic, characters of, 12.

Race-horses, Arab, 26.
—, English, 323.

Radcliffe, Dr., the electrical organs of the torpedo, 150.

Ramond, on plants of Pyrenees, 331.

Ramsay, Prof., on subaërial denudation, 267.
—, on thickness of the British formations, 268, 269.
—, on faults, .

Ramsay, Mr., on instincts of cuckoo, 213.

Ratio of increase, 50.

Rats, supplanting each other, 59.
—, acclimatisation of, 113.
—, blind, in cave, 110.

Rattle-snake, 162.

Reason and instinct, 205.

Recapitulation, general, 404.

Reciprocity of crosses, 243.

Record, geological, imperfect, 264.

Rengger, on flies destroying cattle, 56.

Reproduction, rate of, 50.

Resemblance, protective, of insects, 181.
—, to parents in mongrels and hybrids, 260.

Reversion, law of inheritance, 11.
—, in pigeons, to blue colour, 127.

Rhododendron, sterility of, 239.

Richard, Prof., on Aspicarpa, 367.

Richardson, Sir J., on structure of squirrels, 139.
—, on fishes of the southern hemisphere, 338.

Robinia, grafts of, 246.

Rodents, blind, 110.

Rogers, Prof., Map of N. America, 274.

Rudimentary organs, 397.

Rudiments important for classification, 367.

Rütimeyer, on Indian cattle, 14, 241.

Sageret, on grafts, 246.

Salamandra atra, 397.

Saliva used in nests, 228.

Salmons, males fighting, and hooked jaws of, 69.

Salt-water, how far injurious to seeds, 325.
—, not destructive to land-shells, 353.

Salter, Mr., on early death of hybrid embryos, 249.

Salvin, Mr., on the beaks of ducks, 184.

Saurophagus sulphuratus, 141.

Schacht, Prof., on Phyllotaxy, 173.

Schiödte, on blind insects, 110.
—, on flat-fish, 186.

Schlegel, on snakes, 115.

Schöbl, Dr., on the ears of mice, 172.

Scott, Mr. J., on the self-sterility of orchids, 238.
—, on the crossing of varieties of verbascum, 258.

Sea-water, how far injurious to seeds, 325.
—, not destructive to land-shells, 325.

Sebright, Sir J., on crossed animals, 15.

Sedgwick, Prof., on groups of species suddenly appearing, 282.

Seedlings destroyed by insects, 53.

Seeds, nutriment in, 60.
—, winged, 117.
—, means of dissemination, 154, 161, 327, 328.
—, power of resisting salt-water, 325.
—, in crops and intestines of birds, 326, 327.
—, eaten by fish, 327, 346.
—, in mud, 345.
—, hooked, on islands, 349.

Selection of domestic products, 22.
—, principle not of recent origin, 27.
—, unconscious, 27.
—, natural, 62.
—, sexual, 69.
—, objections to term, 63.
—, natural, has not induced sterility, 247.

Sexes, relations of, 69.

Sexual characters variable, 123.
—, selection, 69.

Sheep, Merino, their selection, 23.
—, two sub-breeds, unintentionally produced, 26.
—, mountain, varieties of, 59.

Shells, colours of, 107.
—, hinges of, 154.
—, littoral, seldom embedded, 270.
—, fresh-water, long retain the same forms, 308.
—, fresh-water, dispersal of, 344.
—, of Madeira, 349.
—, land, distribution of, 349.
—, land, resisting salt water, 325.

Shrew-mouse, 373.

Silene, infertility of crosses, 243.

Silliman, Prof., on blind rat, 110.

Sirenia, their affinities, 302.

Sitaris, metamorphosis of, 394.

Skulls of young mammals, 159, 384.

Slave-making instinct, 216.

Smith, Col. Hamilton, on striped horses, 129.

Smith, Dr., on the Polyzoa, 193.

Smith, Mr. Fred., on slave-making ants, 217.
—, on neuter ants, 231.

Snake with tooth for cutting through egg-shell, 214.

Somerville, Lord, on selection of sheep, 23.

Sorbus, grafts of, 246.

Sorex, 373.

Spaniel, King Charles’ breed, 25.

Specialisation of organs, 98.

Species, polymorphic, 35.
—, dominant, 43.
—, common, variable, 42.
—, in large genera variable, 44.
—, groups of, suddenly appearing, 282, 285.
—, beneath Silurian formations, 287.
—, successively appearing, 290.
—, changing simultaneously throughout the world, 297.

Spencer, Lord, on increase in size of cattle, 26.

Spencer, Mr. Herbert, on the first steps in differentiation, 100.
—, on the tendency to an equilibrium in all forces, 252.

Sphex, parasitic, 216.

Spiders, development of, 390.

Sports in plants, 8.

Sprengel, C.C., on crossing, 76.
—, on ray-florets, 116.

Squalodon, 302.

Squirrels, gradations in structure, 139.

Staffordshire, heath, changes in, 55.

Stag-beetles, fighting, 69.

Star fishes, eyes of, 144.
—, their pedicellariæ, 192.

Sterility from changed conditions of life, 7.
—, of hybrids, 236.
—, laws of, 241.
—, causes of, 247.
—, from unfavourable conditions, 250.
—, not induced through natural selection, 247.

St. Helena, productions of, 347.

St. Hilaire, Aug., on variability of certain plants, 174.
—, on classification, 368.

St. John, Mr., on habits of cats, 209.

Sting of bee, 163.

Stocks, aboriginal, of domestic animals, 14.

Strata, thickness of, in Britain, 268, 269.

Stripes on horses, 128.

Structure, degrees of utility of, 159.

Struggle for existence, 48.

Succession, geological, 290.
—, of types in same areas, 310.

Swallow, one species supplanting another, 59.

Swaysland, Mr., on earth adhering to the feet of migratory birds, 328.

Swifts, nests of, 228.

Swim-bladder, 148.

Switzerland, lake habitations of, 13.

System, natural, 364.

Tail of giraffe, 157.
—, of aquatic animals, 157.
—, prehensile, 188.
—, rudimentary, 400.

Tanais, dimorphic, 36.

Tarsi deficient, 108.

Tausch, Dr., on umbelliferæ, 173.

Teeth and hair correlated, 115.
—, rudimentary, in embryonic calf, 397, 420.

Tegetmeier, Mr., on cells of bees, 222, 226.

Temminck, on distribution aiding classification, 369.

Tendrils, their development, 196.

Thompson, Sir W., on the age of the habitable world, 286.
—, on the consolidation of the crust of the earth, 409.

Thouin, on grafts, 246.

Thrush, aquatic species of, 142.
—, mocking, of the Galapagos, 356.
—, young of, spotted, 388.
—, nest of, 234.

Thuret, M., on crossed fuci, 243.

Thwaites, Mr., on acclimatisation, 112.

Thylacinus, 374.

Tierra del Fuego, dogs of, 211.
—, plants of, 341.

Timber-drift, 326.

Time, lapse of, 266.
—, by itself not causing modification, 81.

Titmouse, 141.

Toads on islands, 350.

Tobacco, crossed varieties of, 258.

Tomes, Mr., on the distribution of bats, 351.

Transitions in varieties rare, 134.

Traquair, Dr., on flat-fish, 188.

Trautschold, on intermediate varieties, 275.

Trees on islands belong to peculiar orders, 350.
—, with separated sexes, 78.

Trifolium pratense, 57, 75.
—, incarnatum, 75.

Trigonia, 296.

Trilobites, 286.
—, sudden extinction of, 297.

Trimen, Mr., on imitating-insects, 377.

Trimorphism in plants, 35, 252.

Troglodytes, 234.

Tuco-tuco, blind, 110.

Tumbler pigeons, habits of, hereditary, 210.
—, young of, 392.

Turkey-cock, tuft of hair on breast, 70.

—, naked skin on head, 158.
—, young of, instinctively wild, 265.

Turnip and cabbage, analogous variations of, 125.

Type, unity of, 166, 167.

Types, succession of, in same areas, 310.

Typotherium, 302.

Udders enlarged by use, 8.
—, rudimentary, 397.

Ulex, young leaves of, 388.

Umbelliferæ, flowers and seeds of, 116.
—, outer and inner florets of, 173.

Unity of type, 166, 167.

Uria lacrymans, 72.

Use, effects of, under domestication, 8.
—, effects of, in a state of nature, 108.

Utility, how far important in the construction of each part, 159.

Valenciennes, on fresh-water fish, 344.

Variability of mongrels and hybrids, 259.

Variation, under domestication, 5.
—, caused by reproductive system being affected by conditions of life,
7.
—, under nature, 33.
—, laws of, 106.
—, correlated, 9, 114, 159.

Variations appear at corresponding ages, 10, 67.
—, analogous in distinct species, 124.

Varieties, natural, 32.
—, struggle between, 59.
—, domestic, extinction of, 86.
—, transitional, rarity of, 134.
—, when crossed, fertile, 257.
—, when crossed, sterile, 256.
—, classification of, 371.

Verbascum, sterility of, 238.
—, varieties of, crossed, 258.

Verlot, M., on double stocks, 230.

Verneuil, M. de, on the succession of species, 299.

Vibracula of the Polyzoa, 193.

Viola, small imperfect flowers of, 173.
—, tricolor, 57.

Virchow, on the structure of the crystalline lens, 145.

Virginia, pigs of, 66.

Volcanic islands, denudation of, 268.

Vulture, naked skin on head, 158.

Wading-birds, 375.

Wagner, Dr., on Cecidomyia, 387.

Wagner, Moritz, on the importance of isolation, 81.

Wallace, Mr., on origin of species, 1.
—, on the limit of variation under domestication, 31.
—, on dimorphic lepidoptera, 36, 232.
—, on races in the Malay Archipelago, 37.
—, on the improvement of the eye, 145.
—, on the walking-stick insect, 182.
—, on laws of geographical distribution, 322.
—, on the Malay Archipelago, 351.
—, on mimetic animals, 377.

Walsh, Mr. B.D., on phytophagic forms, 38.
—, on equal variability, 125.

Water, fresh, productions of, 343.

Water-hen, 143.

Waterhouse, Mr., on Australian marsupials, 90.
—, on greatly developed parts being variable, 119.
—, on the cells of bees, 220.
—, on general affinities, 379.

Water-ouzel, 142.

Watson, Mr. H.C., on range of varieties of British plants, 37, 46.
—, on acclimatisation, 112.
—, on flora of Azores, 328.
—, on rarity of intermediate varieties, 136.
—, on Alpine plants, 331.
—, on convergence, 100.
—, on the indefinite multiplication of species, 101.

Weale, Mr., on locusts transporting seeds, 327.

Web of feet in water-birds, 142.

Weismann, Prof., on the causes of variability, 6.
—, on rudimentary organs, 400.

West Indian islands, mammals of, 352.

Westwood, on species in large genera being closely allied to others,
45.
—, on the tarsi of Engidæ, 124.
—, on the antennæ of hymenopterous insects, 366.

Whales, 182.

Wheat, varieties of, 88.

White Mountains, flora of, 330.

Whittaker, Mr., on lines of escarpment, 267.

Wichura, Max, on hybrids, 249, 251, 260.

Wings, reduction of size, 109.
—, of insects homologous with branchiæ, 148.
—, rudimentary, in insects, 397.

Wolf crossed with dog, 210.
—, of Falkland Isles, 351.

Wollaston, Mr., on varieties of insects, 38.
—, on fossil varieties of shells in Madeira, 42.
—, on colours of insects on sea-shore, 107.
—, on wingless beetles, 109.
—, on rarity of intermediate varieties, 136.
—, on insular insects, 347.
—, on land-shells of Madeira naturalised, 357.

Wolves, varieties of, 71.

Woodcock with earth attached to leg, 328.

Woodpecker, habits of, 141.
—, green colour of, 158.

Woodward, Mr., on the duration of specific forms, 276.
—, on Pyrgoma, 284.
—, on the continuous succession of genera, 293.
—, on the succession of types, 311.

World, species changing simultaneously throughout, 297.

Wrens, nest of, 234.

Wright, Mr. Chauncey, on the giraffe, 178.
—, on abrupt modifications, 203.

Wyman, Prof., on correlation of colour and effects of poison, 9.
—, on the cells of the bee, 22.

Youatt, Mr., on selection, 23.
—, on sub-breeds of sheep, 26.
—, on rudimentary horns in young cattle, 400.

Zanthoxylon, 174.

Zebra, stripes on, 128.

Zeuglodon, 302.