THE
                           EVOLUTION THEORY

                               VOLUME I




                                  THE
                           EVOLUTION THEORY

                                  BY

                          DR. AUGUST WEISMANN
    PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU

               TRANSLATED WITH THE AUTHOR'S CO-OPERATION

                                  BY

                           J. ARTHUR THOMSON
   REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN

                                  AND

                          MARGARET R. THOMSON

                              ILLUSTRATED

                            IN TWO VOLUMES

                                VOL. I

                                LONDON
                             EDWARD ARNOLD
                41 & 43 MADDOX STREET, BOND STREET, W.

                                 1904

                         _All rights reserved_




AUTHOR'S PREFACE


WHEN a life of pleasant labour is drawing towards a close, the wish
naturally asserts itself to gather together the main results, and to
combine them in a well-defined and harmonious picture which may be left
as a legacy to succeeding generations.

This wish has been my main motive in the publication of these lectures,
which I delivered in the University of Freiburg in Breisgau. But
there has been an additional motive in the fact that the theory of
heredity published by me a decade ago has given rise not only to many
investigations prompted by it, but also to a whole literature of
'refutations,' and, what is much better, has brought to light a mass of
new facts which, at first sight at least, seem to contradict my main
theory. As I remain as convinced that the essential part of my theory
is well grounded as I was when I first sketched it, I naturally wish to
show how the new facts may be brought into harmony with it.

It is by no means only with the theory of heredity by itself that
I am concerned, for that has served, so to speak, as a means to a
higher end, as a groundwork on which to base an interpretation of
the transformations of life through the course of the ages. For the
phenomena of heredity, like all the functions of individual life, stand
in the closest association with the whole evolution of life upon our
earth; indeed, they form its roots, the nutritive basis from which
all its innumerable branches and twigs are, in the long run, derived.
Thus the phenomena of the individual life, and especially those of
reproduction and inheritance, must be considered in connexion with the
Theory of Descent, that the latter may be illumined by them, and so
brought nearer our understanding.

I make this attempt to sum up and present as a harmonious whole the
theories which for forty years I have been gradually building up on
the basis of the legacy of the great workers of the past, and on the
results of my own investigations and those of many fellow workers, not
because I regard the picture as complete or incapable of improvement,
but because I believe its essential features to be correct, and because
an eye-trouble which has hindered my work for many years makes it
uncertain whether I shall have much more time and strength granted
to me for its further elaboration. We are standing in the midst of
a flood-tide of investigation, which is ceaselessly heaping up new
facts bearing upon the problem of evolution. Every theory formulated
at this time must be prepared shortly to find itself face to face with
a mass of new facts which may necessitate its more or less complete
reconstruction. How much or how little of it may remain, in face of
the facts of the future, it is impossible to predict. But this will
be so for a long time, and it seems to me we must not on that account
refrain from following out our convictions to the best of our ability
and presenting them sharply and definitely, for it is only well-defined
arguments which can be satisfactorily criticized, and can be improved
if they are imperfect, or rejected if they are erroneous. In both these
processes progress lies.

This book consists of 'Lectures' which were given publicly at the
university here. In my introductory lecture in 1867 I championed the
Theory of Descent, which was then the subject of lively controversy,
but it was not till seven years later that I gave, by way of
experiment, a short summer course with a view to aiding in the
dissemination of Darwin's views. Then very gradually my own studies and
researches and those of others led me to add to the Darwinian edifice,
and to attempt a further elaboration of it, and accordingly these
'Lectures,' which were delivered almost regularly every year from 1880
onwards, were gradually modified in accordance with the state of my
knowledge at the time, so that they have been, I may say, a mirror of
the course of my own intellectual evolution.

In the last two decades of the nineteenth century much that is
new has been introduced into biological science; Nägeli's idea of
'idioplasm'--the substance which determines form; Roux's _Struggle of
the Parts_, the recognition of a special hereditary substance, 'the
germ-plasm,' its analysis into chromosomes, and its continuity from
generation to generation; the potential immortality of unicellular
organisms and of the germ-cells in contrast to the natural death
of higher forms and 'bodies'; a deeper interpretation of mitotic
nuclear division, the discovery of the centrosphere--the marvellous
dividing apparatus of the cell--which at once allowed us to penetrate
a whole stratum deeper into the unfathomable mine of microscopic
vital structure; then the clearing up of our ideas in regard to
fertilization, and the analysis of this into the two processes combined
in it, reproduction and the mingling of the germ-plasms (Amphimixis);
in connexion with this, the phenomena of maturation, first in the
female and then in the male cell, and their significance as a reduction
of the hereditary units:--all this and much more we have gained during
this period. Finally, there is the refutation of the Lamarckian
principle, and the consequent elaboration of the principle of selection
by applying it to the hitherto closed region of the ultimate vital
elements of the germ-plasm.

The actual form of these lectures has developed as they were
transcribed. But although the form is thus to some extent new, I have
followed in the main the same train of thought as in the lectures
of recent years. The lecture-form has been adhered to in the book,
not merely because of the greater vividness of presentation which it
implies, but for many other reasons, of which the greater freedom in
the choice of material and the limiting of quotation to a minimum
are not the least. That all polemics of a personal kind have thus
been excluded will not injure the book, but it is by no means lacking
in discussions of opinion, and will, therefore, I trust, contribute
something towards the clearing up of disputed points.

I have endeavoured to introduce as much of the researches and writings
of others as possible without making the book heavy; but my aim has
been to write a book to be read, not merely one to be referred to.

If it be asked, finally, for whom the book is intended, I can hardly
answer otherwise than 'For him whom it interests.' The lectures were
delivered to an audience consisting for the most part of students of
medicine and natural science, but including some from other faculties,
and sometimes even some of my colleagues in other departments. In
writing the book I have presupposed as little special knowledge as
possible, and I venture to hope that any one who _reads_ the book and
does not merely skim it, will be able without difficulty to enter into
the abstruse questions treated of in the later lectures.

It would be a great satisfaction to me if this book were to be
the means of introducing my theoretical views more freely among
investigators, and to this end I have elaborated special sections more
fully than in the lectures. Notwithstanding much controversy, I still
regard its fundamental features as correct, especially the assumption
of 'controlling' vital units, the determinants, and their aggregation
into 'ids'; but the determinant theory also implies germinal selection,
and without it the whole idea of the guiding of the course of
transformation of the forms of life, through selection which rejects
the unfit and favours the more fit, is, to my mind, a mere torso, or a
tree without roots.

I only know of two prominent workers of our day who have given
thorough-going adherence to my views: Emery in Bologna and J. Arthur
Thomson in Aberdeen. But I still hope to be able to convince many
others when the consistency and the far-reachingness of these ideas
are better understood. In many details I may have made mistakes which
the investigations of the future will correct, but as far as the basis
of my theory is concerned I am confident: _the principle of selection
does rule over all the categories of vital units_. It does not, indeed,
create primary variations, but it determines the paths of evolution
which these are to follow, and thus controls all differentiation, all
ascent of organization, and ultimately the whole course of organic
evolution on the earth, for everything about living beings depends upon
adaptation, though not on adaptation in the sense in which Darwin used
the word.

The great prominence thus given to the idea of selection has been
condemned as one-sided and exaggerated, but the physicist is quite as
open to the same reproach when he thinks of gravity as operative not on
our earth alone, but as dominating the whole cosmos, whether visible
to us or not. If there is gravity at all it must prevail everywhere,
that is, wherever material masses exist; and in the same way the
co-operation of certain conditions with certain primary vital forces
must call forth the same process of selection wherever living beings
exist; thus not only are the vital units which we can perceive, such
as individuals and cells, subject to selection, but those units the
existence of which we can only deduce theoretically, because they are
too minute for our microscopes, are subject to it likewise.

This extension of the principle of selection to all grades of vital
units is the characteristic feature of my theories; it is to this idea
that these lectures lead, and it is this--in my own opinion--which
gives this book its importance. This idea will endure even if
everything else in the book should prove transient.

Many may wonder, perhaps, why in the earlier lectures much that
has long been known should be presented afresh, but I regard it as
indispensable that the student who wishes to make up his own mind in
regard to the selection-idea should not only be clear as to what it
means theoretically, but should also form for himself a conception
of its sphere of influence. Many prejudiced utterances in regard
to 'Natural Selection' would never have been published if those
responsible for them had known more of the facts; if they had had
any idea of the inexhaustible wealth of phenomena which can only be
interpreted in the light of this principle, in as far, that is, as
we are able to give explanations of life at all. For this reason I
have gone into the subject of colour-adaptations, and especially into
that of mimicry, in great detail; I wished to give the reader a firm
foundation of fact from which he could select what suited him when
he wished to test by the light of facts the more difficult problems
discussed in the book.

In conclusion, I wish to thank all those who have given me assistance
in one way or other in this work: my former assistant and friend
Professor V. Häcker in Stuttgart, my pupils and fellow workers Dr.
Gunther and Dr. Petrunkewitsch, and the publisher, who has met my
wishes in the most amiable manner.

  FREIBURG-I-BR.,
  _February 20, 1902_.




PREFATORY NOTE TO ENGLISH EDITION


PROFESSOR WEISMANN'S _Evolution Theory_, here translated from the
second German edition (1904), is a work of compelling interest, the
fruit of a lifetime of observation and reflection, a veteran's judicial
summing up of his results, and certainly one of the most important
contributions to Evolution literature since Darwin's day.

As the author's preface indicates, the salient features of his crowning
work are (1) the illumination of the Evolution process with a wealth
of fresh illustrations; (2) the vindication of the 'Germ-plasm'
concept as a valuable working hypothesis; (3) the final abandonment
of any assumption of transmissible acquired characters; (4) a further
analysis of the nature and origin of variations; and (5), above all, an
extension of the Selection principle of Darwin and Wallace, which finds
its logical outcome in the suggestive theory of Germinal Selection.

The translation will be welcomed, we believe, not only by biological
experts who have followed the development of 'Weismannism' during the
last twenty years, and will here find its full expression for the time
being, but also by those who, while acquainted with individual essays,
have not hitherto realized the author's complete system. Apart from
the theoretical conceptions which unify the book and mark it as an
original contribution of great value, there is a lucid exposition of
recent biological advances which will appeal to those who care more
for facts than theories. To critics of evolutionism, who are still
happily with us, the book ought to be indispensable; it will afford
them much material for argumentation, and should save them many tilts
against windmills. But, above all, the book will be valued by workers
in many departments of Biology, who are trying to help in the evolution
of Evolution Theory, for it is characteristic of the author, as the
history of recent research shows, to be suggestive and stimulating,
claiming no finality for his conclusions, but urging us to test them in
a mood of 'thätige Skepsis.'

The translation of this book--the burden of which has been borne
by my wife--has been a pleasure, but it has also been a serious
responsibility. We have had fine examples set us by previous
translators of some of Weismann's works, Meldola, Poulton, Shipley,
Parker, and others; and if we have fallen short of their achievements,
it has not been for lack of endeavour to follow the original with
fidelity, nor for lack of encouragement on the part of the author, who
revised every page and suggested many emendations.

  J. ARTHUR THOMSON.

  UNIVERSITY OF ABERDEEN,
  _October, 1904_.




CONTENTS


         LECTURE                                                   PAGE

      I. INTRODUCTORY                                                 1

     II. THE DARWINIAN THEORY                                        25

    III. THE DARWINIAN THEORY (_continued_)                          42

     IV. THE COLORATION OF ANIMALS AND ITS RELATION TO THE
         PROCESSES OF SELECTION                                      57

      V. TRUE MIMICRY                                                91

     VI. PROTECTIVE ADAPTATIONS IN PLANTS                           119

    VII. CARNIVOROUS PLANTS                                         132

   VIII. THE INSTINCTS OF ANIMALS                                   141

     IX. ORGANIC PARTNERSHIPS OR SYMBIOSIS                          161

      X. THE ORIGIN OF FLOWERS                                      179

     XI. SEXUAL SELECTION                                           210

    XII. INTRA-SELECTION OR SELECTION AMONG TISSUES                 240

   XIII. REPRODUCTION IN UNICELLULAR ORGANISMS                      253

    XIV. REPRODUCTION BY GERM-CELLS                                 266

     XV. THE PROCESS OF FERTILIZATION                               286

    XVI. FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS
         AND ITS IMMEDIATE SIGNIFICANCE                             312

   XVII. THE GERM-PLASM THEORY                                      345

  XVIII. THE GERM-PLASM THEORY (_continued_)                        373

    XIX. THE GERM-PLASM THEORY (_continued_)                        392




LIST OF ILLUSTRATIONS


   FIGURE                                                          PAGE

   1. Group of various races of domestic pigeons                     35

   2. Longitudinally striped caterpillar of a Satyrid                67

   3. Full-grown caterpillar of the Eyed Hawk-moth
      (_Smerinthus ocellatus_)                                       67

   4. Full-grown caterpillar of the Elephant Hawk-moth (_Chærocampa
      elpenor_)                                                      68

   5. The Eyed Hawk-moth in its 'terrifying attitude'                69

   6. Under surface of the wings of _Caligo_                         70

   7. Caterpillar of a North American _Darapsa_                      71

   8. Caterpillar of the Buckthorn Hawk-moth
      (_Deilephila hippophaës_)                                      73

   9. _Hebomoja glaucippe_, from India; under surface                76

  10. _Xylina vetusta_, in flight and at rest                        77

  11. _Tropidoderus childreni_, in flying pose                       79

  12. _Notodonta camelina_, in flight and at rest                    80

  13. _Kallima paralecta_, from India, right under side of the
       butterfly at rest                                        83, 357

  14. _Cœnophlebia archidona_, from Bolivia, in its resting attitude 85

  15. _Cærois chorinæus_, from the lower Amazon, in its resting
       attitude                                                      86

  16. _Phyllodes ornata_, from Assam                                 87

  17. Caterpillar of _Selenia tetralunaria_, seated on a birch
      twig                                                      90, 360

  18. Upper surfaces of _Acræa egina_, _Papilio ridleyanus_, and
      _Pseudacræa boisduvalii_                                      102

  19. Barbed bristles of _Opuntia rafinesquii_                      123

  20. Vertical section through a piece of a leaf of the
      Stinging-nettle (_Urtica dioica_)                             123

  21. A piece of a twig of Barberry (_Berberis vulgaris_)           124

  22. Tragacanth (_Astragalus tragacantha_)                         125

  23. Bladderwort (_Utricularia grafiana_)                          133

  24. Pitcher of _Nepenthes villosa_                                134

  25. Butterwort (_Pinguicula vulgaris_)                            136

  26. The Sundew (_Drosera rotundifolia_)                           137

  27. A leaf of the Sundew                                          137

  28. Leaf of Venus Fly-trap                                        138

  29. _Aldrovandia vesiculosa_                                      138

  30. _Aldrovandia_, its trap apparatus                             139

  31. Sea-cucumber (_Cucumaria_)                                    148

  32. Metamorphosis of _Sitaris humeralis_, an oil-beetle           150

  33. Cocoon of the Emperor Moth (_Saturnia carpini_)               158

  34. Hermit-crab                                                   163

  35. _Hydra viridis_, the Green Freshwater Polyp                   169

  36. _Amœba viridis_                                               170

  37. Twig of an Imbauba-tree, showing hair cushions                172

  38. A fragment of a Lichen                                        173

  39. A fragment of a Silver Poplar root                            176

  40. _Potentilla verna_                                            181

  41. Flower of Meadow Sage                                         183

  42. Alpine Lousewort (_Pedicularis asplenifolia_)                 184

  43. Flower of Birthwort (_Aristolochia clematitis_)               185

  44. Alpine Butterwort (_Pinguicula alpina_)                       185

  45. _Daphne mezereum_ and _Daphne striata_                        187

  46. Common Orchis (_Orchis mascula_)                              188

  47. Head of a Butterfly                                           190

  48. Mouth-parts of the Cockroach                                  191

  49. Head of the Bee                                               192

  50. Flowers of the Willow                                         194

  51. The Yucca-moth (_Pronuba yuccasella_)                         201

  52. The fertilization of the Yucca                                202

  53. Scent-scales of diurnal Butterflies                           217

  54. A portion of the upper surface of the wing of a male 'blue'
    (_Lycæna menalcas_)                                             218

  55. _Zeuxidia wallacei_, male                                     218

  56. _Leptodora hyalina_                                           224

  57. _Moina paradoxa_, male                                        225

  58. _Moina paradoxa_, female                                      226

  59. An Amœba: the process of division                             253

  60. _Stentor rœselii_, trumpet-animalcule                         254

  61. _Holophrya multifiliis_                                       256

  62. _Pandorina morum_                                             257

  63. _Volvox aureus_                                               270

  64. _Fucus platycarpus_, brown sea-wrack                          272

  65. Copulation in a Daphnid (Lyncæid)                             276

  66. Spermatozoa of various Daphnids                               277

  67. Spermatozoa of various animals                                278

  68. Diagram of a spermatozoon                                279, 338

  69. Ovum of the Sea-urchin                                   281, 338

  70. _Daphnella_                                                   283

  71. _Bythotrephes longimanus_                                     283

  72. _Sida crystallina_, a Daphnid                                 284

  73. Diagrammatic longitudinal section of a hen's egg before
      incubation                                                    285

  74. Diagram of nuclear division                                   288

  75. Process of fertilization in _Ascaris megalocephala_           296

  76. Diagram of the maturation divisions of the ovum               299

  77. Diagram of the maturation divisions of the sperm-cell         301

  78. Diagram of the maturation of a parthenogenetic ovum           305

  79. The two maturation divisions of the 'drone eggs'
      of the Bee                                               307, 337

  80. Fertilization of the ovum of a Gasteropod                     310

  81. Formation of polar bodies in a Lichen                         313

  82. Fertilization in the Lily                                     314

  83. Conjugation of Noctiluca                                      317

  84. Conjugation and polar body formation in the Sun-animalcule    319

  85. Diagram of the conjugation of an Infusorian                   321

  86. Conjugation of an Infusorian                                  323

  87. Diagram to illustrate the operation of amphimixis             348

  88. Sperm-mother-cells (spermatocytes) of the Salamander          350

  89. Anterior region of the larva of a Midge                  364, 393

  90. The Common Shore-Crab, seen from below                        367

  91. Hind leg of a Locustid                                        371

  92. Echinoderm-larvæ                                              387

  93. Development of a limb in the pupa of a Fly                    395

  94. Diagram to illustrate the phylogenetic shifting back of
      the origins of the germ-cells in medusoids and hydroids       412

  95. Diagram to illustrate the migration of the germ-cells
      in Hydromedusæ                                                414


  COLOURED PLATES

  SOME MIMETIC BUTTERFLIES AND THEIR IMMUNE MODELS

  PLATE I       _to face page_      112

  PLATE II            "     "       114

  PLATE III           "     "       116




LECTURE I

INTRODUCTORY


EVERY one knows in a general way what is meant by the doctrine of
descent--that it is the theory which maintains that the forms of
life, animals and plants, which we see on our earth to-day, have not
been the same from all time, but have been developed, by a process
of transformation, from others of an earlier age, and are in fact
descended from ancestors specifically different. According to this
doctrine of descent, the whole diversity of animals and plants owes
its origin to a transformation process, in the course of which the
earliest inhabitants of our earth, extremely simple forms of life,
were in part evolved in the course of time into forms of continually
increasing complexity of structure and efficiency of function, somewhat
in the same way as we can see every day, when any higher animal is
developed from a single cell, the egg-cell, not suddenly or directly,
but connected with its origin by a long series of ever more complex
transformation stages, each of which is the preparation for, and leads
on to the succeeding one. The theory of descent is thus a theory of
development or evolution. It does not merely, as earlier science did,
take for granted and describe existing forms of life, but regards them
as having become what they are through a process of evolution, and it
seeks to investigate the stages of this process, and to discover the
impelling forces that lie behind it. Briefly, the theory of descent is
an attempt at a scientific interpretation of the origin and diversity
of the animate world.

In these lectures, therefore, we have not merely to show on what
grounds we make this postulate of an evolution process, and to marshall
the facts which necessitate it; we must also try to penetrate as far
as possible towards the causes which bring about such transformations.
For this reason we are forced to go beyond the limits of the theory of
descent in the narrow sense, and to deal with the general processes of
life itself, especially with reproduction and the closely associated
problem of heredity. The transformation of species can only be
interpreted in one of two ways; either it depends on a peculiar
internal force, which is usually only latent in the organism, but from
time to time becomes active, and then, to a certain extent, moulds it
into new forms; or it depends on the continually operating forces which
make up life, and on the way in which these are influenced by changing
external conditions. Which of these alternatives is correct we can only
undertake to determine when we know the phenomena of life, and as far
as possible their causes, so that it is indispensable to make ourselves
acquainted with these as far as we can.

When we look at one of the lowest forms of life, such as an Amœba or
a single-celled Alga, and reflect that, according to the theory of
evolution, the whole realm of creation as we see it now, with Man at
its head, has evolved from similar or perhaps even smaller and simpler
organisms, it seems at first sight a monstrous assumption, and one
which quite contradicts our simplest and most certain observations. For
what is more certain than that the animals and plants around us remain
the same, as long as we can observe them, not through the lifetime of
an individual only, but through centuries, and in the case of many
species, for several thousand years?

This being so, it is intelligible enough that the doctrine of
evolution, on its first emergence at the end of the eighteenth century,
was received with violent opposition, not on the part of the laity
only, but by the majority of scientific minds, and instead of being
followed up, was at first opposed, then neglected, and finally totally
forgotten, to spring up anew in our own day. But even then a host of
antagonists ranged themselves against the doctrine, and, not content
with loftily ignoring it, made it the subject of the most violent and
varied attacks.

This was the state of affairs when, in 1858, Darwin's book on _The
Origin of Species_ appeared, and hoisted the flag of evolution afresh.
The struggle that ensued may now be regarded as at an end, at least
as far as we are concerned--that is, in the domain of science. The
doctrine of descent has gained the day, and we can confidently say
that the Evolution theory has become a permanent possession of science
that can never again be taken away. It forms the foundation of all our
theories of the organic world, and all further progress must start from
this basis.

In the course of these lectures, we shall find at every step fresh
evidence of the truth of this assertion, which may at first seem all
too bold. It is not by any means to be supposed that the whole question
in regard to the transformation of organisms and the succession of
new forms of life has been answered in full, or that we have now been
fortunate enough to solve the riddle of life itself. No! whether we
ever reach that goal or not, we are a long way from it as yet, and
even the much easier problem, how and by what forces the evolution of
the living world has proceeded from a given beginning, is far from
being finally settled; antagonistic views are still in conflict, and
there is no arbitrator whose authoritative word can decide which is
right. The _How?_ of evolution is still doubtful, but not the _fact_,
and this is the secure foundation on which we stand to-day: The world
of life, as we know it, has been evolved, and did not originate all at
once.

Were I to try to give, in advance, even an approximate idea of the
confidence with which we can take our stand on this foundation, I
should be almost embarrassed by the wealth of facts on which I might
draw. It is hardly possible nowadays to open a book on the minute or
general structural relations, or on the development of any animal
whatever, without finding in it evidences in favour of the Evolution
theory, that is to say, facts which can only be understood on the
assumption of the evolution of the organic world. This, too, without
taking into account at all the continually increasing number of facts
Palæontology is bringing to light, placing before our eyes the forms
which the Evolution theory postulates as the ancestors of the organic
world of to-day: birds with teeth in their bills, reptile-like forms
clothed with feathers, and numerous other long-extinct forms of life,
which, covered up by the mud of earlier waters, and preserved as
'fossils' in the later sedimentary rocks, tell us plainly how the
earlier world of animals and plants was constituted. Later, we shall
see that the geographical distribution of plant and animal species of
the present day can only be understood in the light of the Evolution
theory. But meantime, before we go into details, what may justify my
assumption is the fact that the Evolution theory enables us to predict.

Let us take only a few examples. The skeleton of the wrist in all
vertebrate animals above Fishes consists of two rows of small bones, on
the outer of which are placed the five bones of the palm, corresponding
to the five fingers. The outer row is curved, and there is thus a space
between the two rows, which, in Amphibians and Reptiles, is filled by
a special small bone. This 'os centrale' is absent in many Mammals,
notably, for instance, in Man, and the space between the two rows is
filled up by an enlargement of one of the other bones. Now if Mammals
be descended from the lower vertebrates, as the theory of descent
assumes, we should expect to find the 'os centrale' even in Man in
young stages, and, after many unsuccessful attempts, Rosenberg has at
last been able to demonstrate it at a very early stage of embryonic
development.

This prediction, with another to be explained later, is based upon the
experience that the development of an individual animal follows, in a
general way, the same course as the racial evolution of the species,
so that structures of the ancestors of a species, even if they are
not found in the fully developed animal, may occur in one of its
earlier embryonic stages. Further on, we shall come to know this fact
more intimately as a 'biogenetic law,' and it alone would be almost
enough to justify the theory of evolution. Thus, for instance, the
lowest vertebrates, the Fishes, breathe by means of gills, and these
breathing organs are supported by four or more gill-arches, between
which spaces, the gill-slits, remain open for the passage of water.
Although Reptiles, Birds, and Mammals breathe by lungs, and at no time
of their life by gills, yet, in their earliest youth, that is, during
their early development in the egg, they possess these gill-arches and
gill-slits, which subsequently disappear, or are transformed into other
structures.

On the strength of this 'biogenetic law' it could also be predicted
that Man, in whom, as is well known, there are twelve pairs of ribs,
would, in his earliest youth, possess a thirteenth pair, for the lower
Mammals have more numerous ribs, and even our nearest relatives, the
anthropoid Apes, the gorilla and chimpanzee, have a thirteenth rib,
though a very small one, and the siamang has even a fourteenth. This
prediction also has been verified by the examination of young human
embryos, in which a small thirteenth rib is present, though it rapidly
disappears.

During the seventies I was engaged in investigating the development
of the curious marking which adorns the long body of many of our
caterpillars. I studied in particular the caterpillars of our Sphingidæ
or hawk-moths, and found, by a comparison of the various stages of
development from the emergence of the caterpillar from the egg on to
its full growth, that there is a definite succession of different
kinds of markings following each other, in a whole range of species,
in a similar manner. From the standpoint of the Evolution theory,
I concluded that the markings of the youngest caterpillars, simple
longitudinal stripes, must have been those of the most remote ancestors
of our present species, while those of the later stages, oblique
stripes, were those of ancestors of a later date.

If this were the case, then all the species of caterpillar which
now exhibit oblique stripes in their full-grown stage must have had
longitudinal stripes in their youngest stages, and because of this
succession of markings in the individual development, I was able to
predict that the then unknown young form of the caterpillar of our
privet hawk-moth (_Sphinx ligustri_) must have a white line along each
side of the back. Ten years later, the English zoologist, Poulton,
succeeded in rearing the eggs of _Sphinx ligustri_, and it was then
demonstrated that the young caterpillar actually possessed the
postulated white lines.

Such predictions undoubtedly give the hypothesis on which they are
based, the Evolution theory, a high degree of certainty, and are almost
comparable to the prediction of the discovery of the planet Neptune
by Leverrier. As is well known, this, the most distant of all the
planets, whose period of revolution round the sun is almost 165 of
our years, would probably never have been recognized as a planet, had
not Adams, an astronomer at the Greenwich Observatory, and afterwards
Leverrier, deduced its presence from slight disturbances in the path of
Jupiter's moons, and indicated the spot where an unknown planet must be
looked for. Immediately all telescopes were directed towards the spot
indicated, and Galle, at the Berlin Observatory, found the sought-for
planet.

We might with justice regard as lacking in discernment those who, in
the face of such experiences, still doubt that the earth revolves round
the sun, and we might fairly say the same of any one who, in the face
of the known facts, would dispute the truth of the Evolution theory. It
is the only basis on which an understanding of these facts is possible,
just as the Kant-Laplace theory of the solar system is the only basis
on which an adequate interpretation of the facts of the heavens can be
arrived at.

To this comparison of the two theories it has been objected that the
Evolution theory has far less validity than the other, first, because
it can never be mathematically demonstrated, and secondly, because
at the best it can only interpret the transformations of the animate
world, and not its origin. Both objections are just: the phenomena
of life are in their nature much too intricate for mathematics to
deal with, except with extreme diffidence; and the question of the
origin of life is a problem which will probably have to wait long for
solution. So, if it gives pleasure to any one to regard the one theory
as having more validity than the other, no one can object; but there
is no particular advantage to be gained by doing so. In any case, the
Evolution theory shares the disadvantage of not being able to explain
everything in its own province with the Kant-Laplace cosmogony, for
that, too, must presuppose the first beginning, the rotating nebula.

Although I regard the doctrine of descent as proved, and hold it to be
one of the greatest acquisitions of human knowledge, I must repeat
that I do not mean to say that everything is clear in regard to the
evolution of the living world. On the contrary, I believe that we
still stand merely on the threshold of investigation, and that our
insight into the mighty process of evolution, which has brought about
the endless diversity of life upon our earth, is still very incomplete
in relation to what may yet be found out, and that, instead of being
vainglorious, our attitude should be one of modesty. We may well
rejoice over the great step forward which the dominant recognition of
the Evolution theory implies, but we must confess that the beginnings
of life are as little clear to us as those of the solar system. But
we can do this at least: we can refer the innumerable and wonderful
inter-relations of the organic cosmos to their causes--common descent
and adaptation--and we can try to discover the ways and means which
have co-operated to bring the organic world to the state in which we
know it.

When I say that the theory of descent is the most progressive step that
has yet been taken in the development of human knowledge, I am bound
to give my reasons for this opinion. It is justified, it seems to me,
even by this fact alone, that the Evolution idea is not merely a new
light on the special region of biological science, zoology and botany,
but is of quite general importance. The conception of an evolution of
the world of life upon the earth reaches far beyond the bounds of any
single science, and influences our whole realm of thought. It means
nothing less than the elimination of the miraculous from our knowledge
of nature, and the placing of the phenomena of life on the same plane
as the other natural processes, that is, as having been brought about
by the same forces, and being subject to the same laws. In the domain
of the inorganic, no one now doubts that out of nothing nothing can
come: energy and matter are from everlasting to everlasting, they can
neither be increased or decreased, they can only be transformed--heat
into mechanical energy, into light, into electricity, and so on. For
us moderns, the lightning is no longer hurled by the Thunderer Zeus
on the head of the wicked, but, careless alike of merit or guilt, it
strikes where the electric tension finds the easiest and shortest line
of discharge. Thus to our mode of thought it now seems clear that no
event in the world of the living depends upon caprice, that at no
time have organisms been called forth out of nothing by the mighty
word of a Creator, but they have been produced at all times by the
co-operation of the existing forces of nature, and every species must
have arisen just where, and when, and in the form in which it actually
did arise, as the necessary outcome of the existing conditions of
energy and matter, and of their interactions upon each other. It is
this correlation of animate nature with natural forces and natural laws
which gives to the doctrine of evolution its most general importance.
For it thus supplies the keystone in the arch of our interpretation
of nature and gives it unity; for the first time it makes it possible
to form a conception of a world-mechanism, in which each stage is the
result of the one before it, and the cause of the succeeding one.

How deeply all our earlier opinions are affected by this doctrine will
become clear if we fix our attention on a single point, the derivation
of the human understanding from that of animal ancestors. What of the
reason of Man, of his morals, of his freedom of will? may be asked,
as it has been, and still is often asked. What has been regarded as
absolutely distinct from the nature of animals is said to differ from
their mental activities only in degree, and to have evolved from them.
The mind of a Kant, of a Laplace, of a Darwin--or to ascend into
the plane of the highest and finest emotional life, the genius of a
Raphael or a Mozart--to have any real connexion, however far back,
with the lowly psychical life of an animal! That is contrary to all
our traditionary, we might say our inborn, ideas, and it is not to be
wondered at that the laity, and especially the more cultured among
them, should have opposed such a doctrine whose dominating power was
unintelligible to them, because they were ignorant of the facts on
which it rests. That a man should feel his dignity lowered by the
idea of descent from animals is almost comical to the naturalist,
for he knows that every one of us, in his first beginning, occupied
a much lowlier position than that of our mammalian ancestors--was,
in fact, as regards visible structure, on a level with the Amœba,
that microscopically minute unicellular animal, which can hardly be
said to possess organs, and whose psychical activities are limited to
recognizing and engulfing its food. Very gradually at first, and step
by step, there develop from this single cell, the ovum, more and more
numerous cells; this mass of cells segregates into different groups,
which differentiate further and further, until at last they form the
perfect man. This occurs in the development of every human being, and
we are merely unaccustomed to the thought that it means nothing else
than an incredibly rapid ascent of the organism from a very low level
of life to the highest.

Still less is it to be wondered at that the Evolution doctrine met with
violent opposition on the part of the representatives of religion,
for it stood in open contradiction to that remarkable and venerable
cosmogony, the Mosaic story of Creation, which people had been
accustomed to regard, not as what it is--a conception of nature at an
early stage of human culture--but as an inalienable part of our own
religion. But investigation shows us that the doctrine of evolution is
true, and it is only a weak religion which is incapable of adapting
itself to the truth, retaining what is essential, and letting go what
is unessential and subject to change with the development of the
human mind. Even the heliocentric hypothesis was in its day declared
false by the Church, and Galilei was forced to retract; but the earth
continued to revolve round the sun, and nowadays any one who doubted
it would be considered mentally weak or warped. So in all likelihood
the time is not far distant when the champions of religion will abandon
their profitless struggle against the new truth, and will see that the
recognition of a law-governed evolution of the organic world is no more
prejudicial to true religion than is the revolution of the earth round
the sun.

       *       *       *       *       *

Having given this very general orientation of the Evolution problem,
which is to engage our attention in detail, I shall approach the
problem itself by the historical method, for I do not wish to bring
the views of present-day science quite suddenly and directly into
prominence. I would rather seek first to illustrate how earlier
generations have tried to solve the question of the origin of the
living world. We shall see that few attempts at solution were made
until quite recently, that is, until the end of the eighteenth and the
beginning of the nineteenth century. Only then there appeared a few
gifted naturalists with evolutionist ideas, but these ideas did not
penetrate far; and it was not till after the middle of the nineteenth
century that they found a new champion, who was to make them common
property and a permanent possession of science. It was the teaching of
Charles Darwin that brought about this thorough awakening, and laid
the foundations of our present interpretations, and his work will
therefore engross our attention for a number of lectures. Only after
we have made ourselves acquainted with his teaching shall we try to
test its foundations, and to see how far this splendid structure stands
on a secure basis of fact, and how deeply its power of interpretation
penetrates towards the roots of phenomena. We shall examine the forces
by which organisms are dominated, and the phenomena produced, and
thereby test Darwin's principles of interpretation, in part rejecting
them, in part accepting them, though in a much extended form, and thus
try to give the whole theoretic structure a more secure foundation.
I hope to be able to show that we have made some real progress since
Darwin's day, that deductions have been drawn from his theory which
even he did not dream of, which have thrown fresh light on a vast range
of phenomena, and, finally, that through the more extended use of his
own principles, the Evolution theory has gained a completeness, and an
intrinsic harmony which it previously lacked.

This at least is my own opinion, but I cannot ignore the fact that it
is by no means shared by all living naturalists. The obvious gaps and
insufficiencies of the Darwinian theory have in the last few decennia
prompted all sorts of attempts at improving it. Some of these were lost
sight of almost as soon as they were suggested, but others have held
their own, and can still claim numerous supporters. It would only tend
to bewilder if I gave an account of those of the former class, but
those which still hold their own must be noticed in these lectures,
though it is by no means my intention to expound the confused mass of
opinions which has gathered round the doctrine of evolution, but rather
to give a presentation of the theory as it has gradually grown up in my
own mind in the course of the last four decades. Even this will not be
the last of which science will take knowledge, but it will, I hope, at
least be one which can be further built upon.

Let us, then, begin at once with that earliest forerunner of the modern
theory of descent, the gifted Greek philosopher Empedocles, who,
equally important as a leader of the state of Agrigentum, and as a
thinker in purely theoretical regions of thought, advanced very notable
views regarding the origin of organisms. We must, however, be prepared
to hear something that is hardly a theory in the modern scientific
acceptation of that term; and we must not be repelled by the unbridled
poetical fancy of the speculative philosopher; we have to recognize
that there is a sound kernel contained in his amusing pictures--a
thought which we meet with later, in much more concrete form, in the
Darwinian theory, and which, if I mistake not, we shall keep firm hold
of in all time to come.

According to Empedocles the world was formed by the four elements of
the ancients, Earth, Water, Fire, and Air, moved and guided by two
fundamental forces, Hate and Love, or, as we should now say, Repulsion
and Attraction. Through the chance play of these two forces with the
elements, there arose first the plants, then the animals, in such a
manner that at first only parts and organs of animals were formed:
single eyes without faces, arms without bodies, and so on. Then, in
wild play, Nature attempted to put together these separate parts, and
so created all manner of combinations, for the most part inept monsters
unfit for life, but in a few cases, where the parts fitted, there
resulted a creature capable not only of life, but, if the juxtaposition
was perfect, even of reproduction.

This phantastic picture of creation seems to us mad enough, but there
slumbers in it, all unsuspected though it may have been by the author,
the true idea of selection, the idea that much that is unfit certainly
arises, but that only the fit endures. The mechanical coming-to-be of
the fit is the sound kernel in this wondersome doctrine.

The natural science of the ancients, in regard to life and its forms,
reached its climax in Aristotle (died 322 B. C.). A true polyhistorian,
his writings comprehended all the knowledge of his time, but he also
added much to it from his own observation. In his writings we find many
good observations on the structure and habits of a number of organisms,
and he also had the merit of being the first to attempt a systematic
grouping of animals. With true insight, he grouped all the vertebrates
together as Enaimata or animals with blood, and classed all the rest
together as Anaimata or bloodless animals. That he denied to the latter
group the possession of blood is not to be wondered at, when we take
into account the extremely imperfect means of investigation available
in his time, nor is it surprising that he should have ranked this
motley company, in antithesis to the blood-possessing animals, as a
unified and equivalent group. Two thousand years later, Lamarck did
exactly the same thing, when he divided the animals into backboned and
backboneless, and we reckon this nowadays as a merit only in so far
that he was the first, after Aristotle, to re-express the solidarity of
the classes of animals which we now call vertebrates.

Aristotle was, however, not a systematic zoologist in our sense of the
term, as indeed was hardly possible, considering the very small number
of animal forms that were known in his time. In our day we have before
us descriptions of nearly 300,000 named species wherefrom to construct
our classification, while Aristotle knew hardly more than 200. Of the
whole world of microscopic animals he could, of course, have no idea,
any more than of the remains of prehistoric animals, of which we now
know about 40,000 named and adequately described species. One would
have thought that it would have occurred to a quick-witted people like
the Greeks to pause and ponder when they found mussel-shells and marine
snail-shells on the hills far above the sea; but they explained these
by the great flood in the time of Deucalion and Pyrrha, and they did
not observe that the fossil molluscs were of different species from the
similar animals living in the sea in their own day.

Thus there was nothing to suggest to Aristotle and others of his time
the idea that a transformation of species had been going on through the
ages, and even the centuries after him evoked no such idea, nor did
there arise new speculations, after the manner of Empedocles, in regard
to the origin of the organic world. On the whole, the knowledge of the
living world retrograded rather than advanced until the beginning of
the Roman Empire. What Aristotle had known was forgotten, and Pliny's
work on animals is a catalogue embellished with numerous fables,
arranged according to a purely external principle of division. Pliny
divided animals into those belonging to earth, water, and air, which is
not very much more scientific than if he had arranged them according to
the letters of the alphabet.

During the time of the Roman Empire, as is well known, the knowledge of
natural history sank lower and lower; there was no more investigation
of nature, and even the physicians lost all scientific basis, and
practised only in accordance with their traditional esoteric secrets.
As the whole culture of the West gradually disappeared, the knowledge
of nature possessed by earlier centuries was also completely lost,
and in the first half of the Middle Ages Europeans revealed a depth
of ignorance of the natural objects lying about them, which it is
difficult for us now to form any conception of.

Christianity was in part responsible for this, because it regarded
natural science as a product of heathendom, and therefore felt bound
to look coldly on it, if not even to oppose it. Later, however, even
the Christian Church felt itself forced to give the people some mental
nourishment in the form of natural history, and under its influence,
perhaps actually composed by teachers of the Church, there appeared a
little book, the so-called _Physiologus_, which was meant to instruct
the people in regard to the animal world. This remarkable work, which
has been preserved, must have had a very wide distribution in the
earlier Middle Ages, for it was translated into no fewer than twelve
languages, Greek, Armenian, Syriac, Arabic, Ethiopic, and so on. The
contents are very remarkable, and come from the most diverse sources,
that is, from the most different writers of antiquity, from Herodotus,
from the Bible, and so forth, but never from original observation. The
compilation does not really give descriptions of animals or of their
habits, but, of each of the forty-one animals which the _Physiologus_
recognizes, something remarkable is briefly related in true lapidary
style, sometimes a mere curiosity without further import, or sometimes
a symbolical interpretation. Thus the book says of the panther: 'he
is gaily coloured; after satiating himself he sleeps three days, and
awakes roaring, giving forth such an agreeable odour that all animals
come to him.' Of the pelican the well-known legend is related, that
it tears open its own breast to feed its young with its blood, thus
standing as a symbol of mother-love. Fabulous creatures, too, appear
in these pages. Of the Phœnix, that bird whose plumage glitters with
gold and precious stones, which was known even to Herodotus, and which
has survived through Eastern fairy-tales on to the time of our own
romanticists (Tieck), we read: 'it lives a thousand years, because it
has not eaten of the tree of knowledge'; then 'it sets fire to itself
and arises anew from its own ashes,' a symbol of nature's infinite
power of renewing its youth.

But while among the peoples of Europe all the science of the ancients
was lost, except a few barely recognizable fragments, the old lore was
preserved, both as regards organic nature and other orders of facts,
among the Arabs, through whom so many treasures of antiquity have
eventually been handed down to us, coming in the track of the Arabian
conquests across North Africa and Spain to the nations of Europe.

It was in this way, too, that the writings of Aristotle again found
recognition, after having been translated into Latin at Palermo at
the order of that enthusiast for Science and Art, the Hohenstaufen
Emperor, Frederick the Second. Our Emperor presented one copy of
Aristotle's writings to the University of Bologna, and thus the
wisdom of the ancient Greeks again became the common property of
European culture. From the thirteenth century to the eighteenth, the
study of natural science was limited to repeating and extending the
work of Aristotle. Nothing new, depending upon personal observation,
was added, and it does not even seem to have occurred to any one to
subject the statements of the Stagirite to any test, even when they
concerned the most familiar objects. No one noticed the error which
ascribed to the fly eight legs instead of six; there was in fact as
yet no investigation, and all knowledge of natural history was purely
scholastic, and gave absolute credence to the authority of the ancients.

A revulsion, however, occurred in the century of the Reformation, with
the breaking down of the blind belief in authority which had till
then prevailed in all provinces of human knowledge and thought. After
a long and severe struggle, dry scholasticism was finally overcome,
and natural science, with the rest, turned from a mere reliance on
books to original thinking and personal observation. Thenceforward
interpretations of natural processes were sought for no longer in the
writings of the ancients, but in Nature herself. Of the magnitude
of this emancipation, and of the severity of the struggle against
deep-rooted authority, one could form a faint idea from experience
even in my own youth. Our young minds were so deeply imbued with the
involuntary feeling that the ancients were superior to us moderns
in each and every respect, that not only the hardly re-attainable
plastic art of the Greeks and the immortal songs of Homer, but all the
mental products of antiquity seemed to us models which could never be
equalled; the tragedies of Sophocles were for us the greatest tragedies
that the world had ever seen, the odes of Horace the most beautiful
poems of all time!

In the domain of natural science the new era began with the overthrow
of the Ptolemaic cosmogony, which, for more than a thousand years,
had served as a basis for astronomy. When the German canon, Nicolas
Copernicus (born at Thorn, 1473, died 1543), reversed the old
theory, and showed that the sun did not revolve round the earth, but
the earth round the sun, the ice was broken and the way paved for
further progress. Galilei uttered his famous 'e pur si muove,' Kepler
established his three laws of the movements of the planets, and Newton,
a century later, interpreted their courses in terms of the law of
gravitation.

But we have not here to do with a history of physics or astronomy, and
I only wish to recall these well-known facts, in order that we may
see how increased knowledge in this domain was always accompanied by
advances in that of biology.

Here, however, we cannot yet chronicle any such thoroughgoing
revolution of general conceptions; the basis of detailed empirical
knowledge was not nearly broad enough for that, and it was in the
acquiring of such a foundation that the next three centuries, from the
sixteenth to the end of the eighteenth, were eagerly occupied.

The first step necessary was to collate the items of individual
knowledge in regard to the various forms of life, and to bring the
whole in unified form into general notice. This need was met for the
first time by Conrad Gessner's _Thierbuch_, a handsome folio volume,
printed at Zurich in 1551, and embellished with numerous woodcuts,
some of them very good. This was followed, in 1600, by a great work in
many volumes, written in Latin, by a professor of Bologna, Aldrovandi.
Not native animals alone but foreign ones also were described in these
works, for, after the discovery of America and the opening up of
communication with the East Indies, many new animal and plant forms
came to the knowledge of European nations by way of the sea. Thus
Francesco Hernandez (died 1600), physician in ordinary to Philip II,
described no fewer than forty new Mammals, more than two hundred
Birds, and many other American animals.

Again, in a quite different way, the naturalist's field of vision
was widened, namely, by the invention of the simple microscope, with
which Leeuwenhoek first discovered the new world of Infusorians,
and Swammerdam made his notable observations on the structure and
development of the very varied minute animal inhabitants of fresh
water. In the same century, the seventeenth, anatomists like Tulpius,
Malpighi, and many others extended the knowledge of the internal
structure of the higher animals and of Man, and a foundation was
laid for a deeper insight into the nature of vital functions by the
discovery of the circulation of the blood in Man and the higher
animals. In the following century, the eighteenth, this path of active
research was eagerly followed, and we need only mention such names as
Réaumur, Rösel von Rosenhof, De Geer, Bonnet, J. Chr. Schäfer, and
Ledermüller, to be immediately reminded of the wealth of facts about
the structure, life, and especially the development of our indigenous
animals, which we owe to the labours of these men.

       *       *       *       *       *

All these advances, great and many-sided as they were, did not at once
lead to a renewal of the attempt of Empedocles to explain the origin
of the organic world. This was as yet not even recognized as a problem
requiring investigation, for men were content to take the world of
life simply as a fact. The idea of getting beyond the naïve, poetic
standpoint of the Mosaic story of Creation was as yet remote from the
minds of naturalists, partly because they were wholly fascinated by
the observation of masses of details, but chiefly because, first by
the English physician, John Ray (died 1678), then by the great Swede,
Carl Linné, the conception of organic 'species' had been formulated and
sharply defined. It is true enough that before the works of these two
men 'species' had been spoken of, but without being connected with any
definite idea; the word was used rather in the same vague sense as the
word 'genus,' to designate one of the smaller groups of organic forms,
but without implying any clear idea of its scope or of its limitations.
Now, however, for the first time, the term 'species' came to be used
strictly to mean the smallest homogeneous group of individual forms
of life upon the earth. John Ray held that the surest indication of a
'species' was that its members had been produced from the same seed;
that is, 'forms which are of different species maintain this specific
nature constantly, and one species does not arise from the seed of
another.' Here we have the germ of the doctrine of the absolute nature
and the immutability of species which Linné briefly characterized in
these words: 'Species tot sunt, quot formæ ab initio creatæ sunt,'
'there are just so many species as there were forms created in the
beginning.' It is here clearly implied, that species as we know them
have been as they are from all time, that, therefore, they exist in
nature as such and unchangeably, and have not been merely read into
nature by man.

This view, though we cannot now regard it as correct, was undoubtedly
reasonable, and thoroughly in accordance with the spirit of the time;
it was congruent with the knowledge, and above all with the scientific
endeavours of the age. In the eighteenth century there was danger that
all outlook on nature as a whole would be lost--smothered under the
enormous mass of isolated facts, and especially under the inundation of
diverse animal and plant forms which were continually being recognized.
It must therefore have been regarded as a real deliverance, when Linné
reduced this chaos of forms to a clearly ordered system, and relegated
each form to its proper place and value in relation to the whole. How,
indeed, could the great systematist have performed his task at all, if
he had not been able to work with definite and sharply circumscribed
groups of forms, if he had not been able to regard at least the lowest
elements of his system, the species, as fixed and definite types?
On the other hand, Linné was much too shrewd an observer not to
entertain, in the course of his long life, and under the influence of
the continually accumulating material, doubts as to the correctness
of his assumption of the fixity and absoluteness of his species. He
discovered from his own experience, what is fully borne out by ours,
that it is easy enough to define a species when there are only a few
specimens of a form to deal with, but that the difficulty increases
in proportion to the number and to the diversity of habitat of those
that are to be brought under one category. In the last edition of
the _Systema Naturæ_ we find very noteworthy passages in which Linné
wonders whether, after all, a species may not change, and in the course
of time diverge into varieties, and so forth. Of these doubts no notice
was taken at the time; the accepted doctrine of the fixity of species
was held to and even raised to the rank of a scientific dogma. Georges
Cuvier, the great disciple of the Stuttgart 'Karlschule,' accentuated
the doctrine still further by his establishment of animal-types, the
largest groups of forms in the animal kingdom within which a definite
and fundamentally distinct plan of architecture prevails. His four
types, Vertebrates, Molluscs, Articulate and Radiate animals, furnished
a further corroboration of the absolute nature of species, since they
seemed to show that even the highest and most comprehensive groups are
sharply defined off from one another.

Let me add that this doctrine of the absolute nature of species was not
fully elaborated till our own day, when the Swiss (afterwards American)
naturalist, Louis Agassiz, went so far as to maintain that not only
the highest and the lowest categories, but all those coming between
them, were categories established and sharply separated by Nature
herself. But in spite of much ingenuity and his wide and comprehensive
outlook he exerted himself in vain to find satisfactory and really
characteristic definitions of what was to be considered a class, an
order, a family, or a genus. He did not succeed in finding a rational
definition of these systematic concepts, and his endeavour may be
regarded as the last important attempt to prop up an interpretation of
nature already doomed to fall. But in referring to Louis Agassiz I have
anticipated the historical course of scientific development, and must
therefore go back to the last quarter of the eighteenth century.

The first unmistakable pioneer of the theory of descent, which now
emerged for the first time as a scientific doctrine, was our great
poet Goethe. He has indeed been often named as the founder of the
theory, but that seems to me saying too much. It is true, however, that
the inquiring mind of the poet certainly recognized in the structure
of 'related' animals the marvellous general resemblances amid all
the differences in detail, and he probed for the reason of these
form-relations. Through the science of 'comparative anatomy,' as it
was taught at the close of the century by Kielmeyer, Cuvier's teacher,
and later by Cuvier himself, Blumenbach, and others, numerous facts
had become known, which paved the way for such questions. It had, for
instance, been recognized that the arm of man, the wing of the bird,
the paddle of the seal, and even the foreleg of the horse, contain
essentially the same chain of bones, and Goethe had already expressed
these relations in his well-known verse,

  'Alle Gestalten sind ähnlich, doch keine gleichet der andern,
  Und so deutet der Chor auf ein geheimes Gesetz.'

As to what this law was he did not at that time pronounce an opinion,
though he may even then have thought of the transformation of species.
At first he contented himself with seeking for an ideal archetype or
'Urtypus' which was supposed to lie at the foundation of a larger or
smaller group. He discovered the archetypal plant or 'Urpflanze,'
when he rightly recognized that the parts of the flower are nothing
more than modified leaves. He spoke plainly of the 'metamorphosis of
plants,' meaning by that the transformation of his 'archetype' into
the endless diversity of actual plant forms. But at first he certainly
thought of this transformation only in the ideal sense, and not as a
factual evolutionary process.

The first who definitely maintained the latter view was, remarkably
enough, the grandfather of the man who, in our own day, made the theory
of descent finally triumphant, the English physician Erasmus Darwin,
born 1731. This quiet thinker published, in 1794, a book entitled
_Zoonomia_, and in it he takes the important step of substituting for
Goethe's 'secret law' a real relationship of species. He proclaims the
gradual establishment and ennobling of the animal world, and bases
his view mainly on the numerous obvious adaptations of the structure
of an organ to its use. I have not been able to find any passage in
the book in which he has expressly indicated that, because many of
the conditions of life could not have existed from the beginning,
these adaptations are therefore, as such, an argument for the gradual
transformation of species. But he assumed that such exact adaptations
to the functions of an organ could only arise through the exercise of
that function, and in this he saw a proof of transformation. Goethe had
expressed the same idea when he said, 'Thus the eagle has conformed
itself through the air to the air, the mole through the earth to the
earth, and the seal through the water to the water,' and this shows
that he too at one time thought of an actual transformation. But
neither he nor Erasmus Darwin were at all clear as to _how_ the use
of an organ could bring about its variation and transformation. The
latter only says that, for instance, the snout of the pig has become
hard through its constant grubbing in the ground; the trunk of the
elephant has acquired its great mobility through the perpetual use of
it for all sorts of purposes; the tongue of the herbivore owes its
hard, grater-like condition to the rubbing to and fro of the hard grass
in the mouth, and so on. How acute and thoughtful an observer Erasmus
Darwin was, is shown by the fact that he had correctly appreciated the
biological significance of many of the colour-adaptations of animals
to their surroundings, though it was reserved for his grandson to make
this fully clear at a much later date. Thus he regarded the varied
colouring of the python, of the leopard, and of the wild cat as the
best adapted for concealing them from their prey amid the play of light
and shadow in a leafy thicket. The black spot in front of the eye of
the swan he considered an arrangement to prevent the bird from being
dazzled, as would happen if that spot were as snow-white as the rest of
the plumage.

At the end of the book he sums up his views in the following
sentences: 'The world has been evolved, not created; it has arisen
little by little from a small beginning, and has increased through
the activity of the elemental forces embodied in itself, and so has
rather grown than suddenly come into being at an almighty word.' 'What
a sublime idea of the infinite might of the great Architect! the Cause
of all causes, the Father of all fathers, the Ens entium! For if we
could compare the Infinite it would surely require a greater Infinite
to cause the causes of effects than to produce the effects themselves.'

In these words he sets forth his position in regard to religion, and
does so in precisely the same terms as we may use to-day when we say:
'All that happens in the world depends on the forces that prevail in
it, and results according to law; but where these forces and their
substratum, Matter, come from, we know not, and here we have room for
faith.'

I have not been able to discover whether the _Zoonomia_, with its
revolutionary ideas, attracted much attention at the time when it
appeared, but it would seem not. In any case, it was afterwards so
absolutely forgotten, that in an otherwise very complete _History of
Zoology_, published in 1872 by Victor Carus, it was not even mentioned.
About a year after the appearance of _Zoonomia_, Isidore Geoffrey
St.-Hilaire in Paris expounded the view that what are called species
are really only 'degenerations,' deteriorations from one and the same
type, which shows that he too had begun to have doubts as to the fixity
of species. Yet it was not till the third decade of the nineteenth
century that he clearly and definitely took up the position of the
doctrine of transformation, and to this we shall have to return later
on.

But as early as the first decade of the century this position was taken
up by two noteworthy naturalists, a German and a Frenchman, Treviranus
and Lamarck.

Gottfried Reinhold Treviranus, born at Bremen in 1776, an excellent
observer and an ingenious investigator, published, in 1802, a book
entitled _Biologie, oder Philosophie der lebenden Natur_ [_Biology, or
Philosophy of Animate Nature_], in which he expresses and elaborates
the idea of the Evolution theory with perfect clearness. We read
there, for instance: 'In every living being there exists a capacity
for endless diversity of form; each possesses the power of adapting
its organization to the variations of the external world, and it is
this power, called into activity by cosmic changes, which has enabled
the simple zoophytes of the primitive world to climb to higher and
higher stages of organization, and has brought endless variety into
nature.' But where the motive power lies, which brings about these
transformations from the lowliest to ever higher forms of life, was a
question which Treviranus apparently did not venture to discuss. To do
this, and thus to take the first step towards a causal explanation of
the assumed transformations, was left for his successor.

Jean Baptiste de Lamarck, born in 1744 in a village of Picardy, was
first a soldier, then a botanist, and finally a zoologist. He won his
scientific spurs first by his _Flora of France_, and zoology holds him
in honour as the founder of the category of 'vertebrates.' Not that he
occupied himself in particular detail with these, but he recognized the
close alliance of the classes of animals in question--an alliance which
was subsequently expressed by Cuvier by the systematic term 'type' or
'embranchement.'

In his _Philosophie zoologique_, published in 1809, Lamarck set
forth a theory of evolution whose truth he attempted to vindicate by
showing--as Treviranus had done before him--that the conception of
species, on the immutability of which the whole hypothesis of creation
had been based, was an artificial one, read into nature by us; that
sharply circumscribed groups do not exist in nature at all; and that
it is often very difficult, and not infrequently quite impossible, to
define one species precisely from allied forms, because it is connected
with these on all sides by transition stages. Groups of forms which
thus melted into one another indicated that the doctrine of the fixity
of species could not be correct, any more than that of their absolute
nature. Species, he maintained, are not immutable, and are not so old
as nature; they are fixed only for a certain time. The shortness of
our life prevents our directly recognizing this. 'If we lived a much
shorter time, say about a second, the hour-hand of the clock would
appear to us to stand still, and even the combined observations of
thirty generations would afford no decisive evidence as to the hand's
movement, and yet it had been moving.'

The causes on which, according to Lamarck, the transformation of
species, their modification into new species, depends, lie in the
changes in the conditions of life which must have occurred ceaselessly
from the earliest period of the earth's history till our own day, now
here, now there, due in part to changes in climate and in food-supply,
in part to changes in the earth's crust by the rising or sinking of
land-masses, and so forth. These external changes have sometimes been
the _direct_ cause of changes in bodily structure, as in the case
of heat or cold; but they have sometimes and much more effectively
operated _indirectly_. Thus changed conditions may have prompted
an animal of a given species to use certain parts of its body in a
new way, more vigorously, or less actively, or even not at all, and
the more vigorous use, or, conversely, the disuse, has brought about
variations in the organ in question.

Thus the whales lost their teeth when they abandoned their fish diet,
and acquired the habit of feeding on minute and delicate molluscs,
which they swallowed whole without seizure or mastication. Thus, too,
the eyes of the mole degenerated through its life in the dark, and a
still greater degeneration of the eyes has taken place in animals,
like the proteus-salamander, which always inhabit lightless caves.
In mussels both head and eyes degenerated because the animals could
no longer use them after they became enclosed in opaque mantles and
shells. In the same way snakes lost their legs _pari passu_ with the
acquisition of the habit of moving along by wriggling their long
bodies, and of creeping through narrow fissures and holes. On the
other hand, Lamarck interpreted the evolution of the web-feet of
swimming birds by supposing that some land-bird or other had formed the
habit of going into the water to seek for food, and consequently of
spreading out its toes as widely as possible so as to strike the water
more vigorously. In this way the fold of skin between the toes was
stretched, and as the extension of the toes was very frequent and was
continued through many generations, the web expanded and grew larger,
and thus formed the web-foot.

In the same way the long legs of the wading birds have been, according
to Lamarck, gradually evolved by the continual stretching of the limbs
by wading in deeper and deeper water, and similarly for the long necks
and bills of the waders, the herons and the storks. Finally we may
mention the case of the giraffe, whose enormously long neck and tall
forelegs are interpreted as due to the fact that the animal feeds on
the foliage of trees, and was always stretching as far as possible, in
order to reach the higher leaves.

We shall see later in what a different way Charles Darwin explained
this case of the giraffe. Lamarck's idea is at once clear; it is true
that exercising an organ strengthens it, that disuse makes it weaker.
Through much gymnastic exercise the muscles of the arm become thicker
and more capable, and memory too may be improved, that is to say,
even a definite part of the brain may be considerably strengthened
by use. Indeed, we may now go so far as to admit that every organ is
strengthened by use and weakened by disuse, and so far the foundations
of Lamarck's interpretations are sound. But he presupposes something
that cannot be admitted so readily, namely, that such 'functional'
improvement or diminution in the strength of an organ can be
transmitted by inheritance to the succeeding generation. We shall have
to discuss this question in detail at a later stage, and I shall only
say now that opinions as to whether this is possible or not are very
much divided. I myself doubt this possibility, and therefore cannot
admit the validity of the Lamarckian evolutionary principle in so far
as it implies the directly transforming effect of the functioning of
an organ. But even if we recognize the Lamarckian factor as a _vera
causa_, it is easy to show that there are a great many characters
which it is not in a position to interpret. Many insects which live
upon green leaves are green, and not a few of them possess exactly the
shade of green which marks the plant on which they feed; they are thus
protected in a certain measure from injuries. But how could this green
colour of the skin have been brought about by the activity of the skin,
since the colour of the surroundings does not usually stimulate the
skin to activity at all? Or how should a grasshopper, which is in the
habit of sitting on dry branches of herbs, have thereby been incited
to an activity which imparts to it the colour and shape of a dry twig?
Just as little, or perhaps still less, can the protective green colour
of a bird's or insect's eggs be explained through the direct influence
of their usually green surroundings, even if we disregard the fact that
the eggs are green when they are laid--that is, before the environment
can have had any influence on them.

The Lamarckian principle of modification through use does not, in
any case, nearly suffice as an interpretation of the transformations
of the organic world. It must be allowed that Lamarck's theory of
transformation was well founded at the time when it was advanced; it
not only attacked the doctrine of the immutability of species, but
sought for the first time to indicate the forces and influences which
must be operative in the transformations of species; it was therefore
well worth careful testing. Nevertheless it did not divert science from
its chosen path; very little notice was taken of it, and in the great
Cuvier's chronicle of scientific publications for 1809, not a syllable
is devoted to Lamarck's book, so strong was the power of prejudice.

But, although the new doctrine was thus ignored, it did not altogether
fall to the ground; it glimmered for a while in Germany, where it found
its champions in the 'Naturphilosophie' of the time, and especially in
Lorenz Oken, a peasant's son, born at Ortenau, near Offenburg, in 1783.

Oken professed views similar to those of Erasmus Darwin, Treviranus,
and Lamarck, though they were not clothed in such purely scientific
garb, being, in fact, bound up with the general philosophical
speculations which came increasingly into favour at that time, chiefly
through the writings of Schelling. In the same year, 1809, in which
Lamarck published his _Philosophie zoologique_, Oken's _Lehrbuch der
Naturphilosophie_ appeared.

This book is by no means simply a theory of descent; its scope is much
wider, including the phenomena of the whole cosmos; on the other hand,
it goes too little into details and is too indefinite to deserve its
title. Its way of playing with ideas, its conjectures and inferences
from a fanciful basis, make it difficult for us now to think ourselves
into its mode of speculation, but I should like to give some indication
of it, for it was just these speculative encroachments of the
'categories' of the so-called 'Naturphilosophie' which played a fatal
part in causing the temporary disappearance of the Evolution-theory
from science, so that, later on, it had to be established anew.

Oken defines natural science as 'the science of the everlasting
transmutations of God (the Spirit) in the world': Every thing,
considered in the light of the genetic process of the whole, includes,
besides the idea of being, that of not-being, in that it is involved
in a higher form. 'In these antitheses the category of polarity is
included. The simpler elementary bodies unite into higher forms,
which are thus merely repetitions at a potential higher than that of
their causes. Thus the different genera of bodies form parallel and
corresponding series, the reasonable arrangement of which results as an
intrinsic necessity from their genetic connexion. In individuals these
lowlier series make their appearance again during development. The
contrasts in the solar system between planets and sun are repeated in
plants and animals, and, as light is the principle of movement, animals
have the power of independent movement in advance of the plants which
belong to the earth.'

Obviously enough, this is no longer the study of nature; it is
nature-construction from a basis of guesses and analogies rather than
of knowledge and facts. Light is the principle of motion, and as
animals move, they correspond to the sun, and plants to the planets!
Here there is not even a hint of a deepening of knowledge, and all
these deductions now seem to us quite worthless.

On the other hand, it must be allowed that good ideas are by no means
absent from this 'philosophy,' nor can we deny to this restlessly
industrious man a great mind always bent on discovering what was
general and essential. Much of what we now _know_ he even then guessed
at and taught, as, for instance, that the basis of all forms of life
in this infinitely diverse world of organisms was one and the same
substance--'primitive slime,' 'Urschleim' as he called it, or, as we
should now say, 'protoplasm.' We can therefore, _mutatis mutandis_,
agree with Oken when he says,'Everything organic has come from slime,
and is nothing but diversely organized slime.' Many naturalists of the
present day would go further, and agree with Oken when he suggests
that 'this primitive slime has arisen in the sea, in the course of the
planet's (the earth's) evolution out of inorganic material.'

Thus Oken postulated, as the specific vehicle of life, a primitive
substance, in essence at least homogeneous. But he went further, and
maintained that his 'Urschleim' assumed _the form of vesicles_, of
which the various organisms were composed. 'The organic world has as
its basis an infinitude of such vesicles.' Who is not at once reminded
of the now dominant _Cell-theory_? And, in fact, thirty years later,
when the cell was discovered, Oken did claim priority for himself. In
so doing, he obviously confused the formulating of a problem with the
solving of it; he had, quite rightly, divined that organisms must be
built up of very minute concentrations of the primitive substance, but
he had never seen a cell, or proved the necessity for its existence, or
even attempted to prove it. His vesicle-theory was a pure divination, a
prevision of genius, but one which could not directly deepen knowledge;
it did not prompt, or even hasten, the discovery of the cell. Here,
as throughout in his natural philosophy, Oken built, not from beneath
upwards, by first establishing facts and then drawing conclusions from
them, but, inversely, he invented ideas and principles, and out of
them reconstructed the world. In this he differs essentially from his
predecessors Erasmus Darwin, Treviranus, and Lamarck, who all reasoned
inductively, that is, from observed data.

Thus the whole evolutionary movement was lost in indefiniteness;
because men wanted to find a reason for everything, they missed
even what might then have been explained. Moreover, the theory of
evolution still lacked a sufficiently broad basis of facts; the
'Naturphilosophie,' by its want of moderation, robbed it of all
credit; and it is not to be wondered at that men soon ceased to occupy
themselves with the problem of the evolution of the living world. A few
indeed held fast to the doctrine of evolution during the first third
of the century, but then it disappeared completely from the realm of
science.

Its last flicker of life was seen in France, in 1830, at the time of
the July revolution, when the legitimate sovereignty of Charles X was
overthrown. It is interesting to note the lively interest that Goethe,
the first forerunner of the theory, and then aged eighty-one, had in
the intellectual combat that took place in the French Academy between
Cuvier and Isidore Geoffroy St.-Hilaire. A friend of Goethe's, Soret,
relates that on August 2, 1830, he went into the poet's room, and was
greeted with the words: 'Well, what do you think of this great event?
The volcano is in eruption, and all is in flames. There can no longer
be discussion with closed doors.' Soret replied: 'It is a terrible
business! But what else was to be expected with things as they are,
and with such a ministry, than that it should end in the expulsion
of the reigning family?' To which Goethe answered: 'We don't seem to
understand each other, my dear friend. I am not talking of these people
at all; I am thinking of quite different affairs. I refer to the open
rupture in the Academy between Cuvier and Geoffroy St.-Hilaire; it is
of the utmost importance to science.'

In this conflict of opinions, Cuvier opposed Geoffroy's conception of
the unity of the plan of structure in all animals, confronting him with
the four Cuvierian types, in each of which the plan of structure was
altogether different, and strongly insisting on the doctrine of the
fixity of species, which he maintained to be the necessary postulate of
a scientific natural history.

The victory fell to Cuvier, and it cannot be denied that there was much
justification for his opinions at the time, for the knowledge of facts
at that stage was not nearly comprehensive enough to give security to
the Evolution theory, and moreover the quiet progress of science might
have been hindered rather than furthered by premature generalization
and theorizing. It had now been seen how far the interpretation of
general biological problems could be carried with the available
material; the 'Naturphilosophie' had not merely exploited it as far as
possible, but had burdened it much beyond its carrying power, and the
world was weary of insecure speculations. The 'Naturphilosophie' was
for the time quite worked out, and a long period set in, during which
all energies were devoted to detailed research.




LECTURE II

THE DARWINIAN THEORY

 Period of detailed research--Appearance of Darwin's _Origin
 of Species_--Darwin's life--Voyage round the world--His
 teaching--Domesticated animals, dog, horse--Pigeons--Artificial
 selection--Unconscious selection--Correlated variations.


THE period of wholly unphilosophical, purely detailed research may be
reckoned as from about 1830 to 1860, though, of course, many of the
labours of the earlier part of the century must be counted among the
investigations which were carried out without any reference to general
questions, and even after 1860 numerous such works appeared. Nor could
it be otherwise, for the basis of all science must be found in facts,
and the thorough working up of the fact-material will always remain
the first and most indispensable condition of our scientific progress.
During the period referred to, however, it had become the sole end to
be striven for; and all energies were concentrated exclusively on the
accumulation of facts.

The previous century had added much to the knowledge of the inner
structure of animals, the so-called 'comparative anatomy,' and in
the nineteenth century this line of investigation was pursued even
more extensively and energetically, so that the knowledge increased
enormously. Up till this time it was chiefly the structure of the
backboned animals and of a few 'backboneless' animals, so called, that
had been studied, but now all the lower groups of the animal kingdom
were also investigated, and became known better and in more detail as
the methods of research improved.

Not content, however, with a knowledge of the adult animal, naturalists
began to investigate its development. In the year 1814 the first
great work on development appeared, on the development of the chick,
by Pander and Von Baer. It was there shown for the first time, how
the chick begins as a little disk-shaped membrane on the surface of
the yolk of the egg, at first simply as a pale streak, the 'primitive
streak,' then as a groove, the 'primitive groove,' by the side of which
arise two folds, the 'medullary folds,' and further how a system of
blood-vessels is developed around this primitive rudiment on the upper
surface of the yolk, how a heart arises before the rest of the body is
complete, and how the blood begins to circulate; in short, there was
disclosed all the marvel of development to which we are now so much
accustomed, that we can hardly understand the sensation it made at that
time.

Later on, attention was turned to the development of Fishes and
Amphibians (Agassiz and Vogt, later Remak), then to that of the Worms
(Bagge), of Insects (Kölliker), and gradually the development of
all the groups of the animal-kingdom--from Sponges to Man--was so
thoroughly investigated that it almost seems to-day as if there could
not be much that is new to discover in this department. This impression
may indeed be true as far as the less complex processes and the more
obvious questions are concerned, but it is impossible to predict what
new problems may confront us, whose solution will depend on a still
more detailed study of development.

As embryology is a science of the nineteenth century, so also is
histology, the science of tissues. Its pioneer was Bichat, but its
real foundations were not laid till Schwann and Schleiden formulated
the conception of the 'cell,' and proved that all animals and plants
were composed of cells. What Oken had only guessed at they now proved,
that there are very minute form-elements of life which build up all the
parts of animals and plants or produce them by processes of secretion.
New light was thus shed on embryonic development, and this gradually
led to the recognition of the fact that the egg, too, is a cell, and
that development depends on a cell-division process in this egg-cell.
This led further to the conception of many-celled and single-celled
organisms, and so on to many items of knowledge to speak of which here
would carry us too far.

For it is not my intention to attempt a complete review of the
development of biology in the nineteenth century, or even in the
period which we have mentioned as devoted to detailed research; it
is rather my desire to convey a general impression of the enormous
extent and many-sidedness of the progress that was made in this time.
Let us therefore briefly recall the entirely new facts which were
brought to light in this period with regard to the reproduction of
animals. Asexual reproduction by budding and division was already
known, but parthenogenesis is a discovery of this period, and so also
is alternation of generations, so far-reaching in its bearing on
general problems. It was first observed (1819) by Chamisso in Salpa,
then by Steenstrup in Medusæ and trematodes, and was later made fully
clear in its most diverse forms and relations by the researches of
Leuckart, Vogt, Kölliker, Gegenbaur, Agassiz, and other illustrious
investigators. Reproduction by heterogony, too, which occurs in
many crustaceans, and in aphides and certain worms, was recognized
at that time, and in the sixties Carl Ernst von Baer added to the
list precocious reproduction, or pædogenesis, which is illustrated in
certain insects which reproduce in the larval state.

This may suffice to convey some idea of the great mass of new, and in
some cases startling facts previously unguessed at, which were then
brought to light in the department of animal biology alone. To this
must be added the vast increase in the number of known species and
varieties, their distribution on the earth, and all this, _mutatis
mutandis_, for plants also. Nor can we omit to mention the rapidly
growing number of fossil species of animals and plants.

Thus there gradually accumulated a new mass of material; investigation
became more and more specialized, and the danger became imminent that
workers in the various departments would be unable to understand
each other, so completely were they independent of one another in
their specialist researches. There was lack of any unifying bond, for
workers had lost sight of the general problem in which all branches
of the science meet, and through which alone they can be united into
a general science of biology. The time had come for again combining
and correlating the details, lest they should grow into an unconnected
chaos, through which it would be impossible to find one's way, because
no one could overlook it and grasp it as a whole. In a word, it was
high time to return to general questions.

       *       *       *       *       *

Though I have called the period from 1830 to 1860 that of purely
detailed research, I do not mean to ignore the fact that, during that
time, there were a few feeble attempts to return to the great questions
which had been raised at the beginning of the century. But the point
is, that all such attempts remained unnoticed. Thus there appeared, in
1844, a book entitled _Vestiges of the Natural History of Creation_,
the anonymous author of which revealed himself much later as Robert
Chambers, an Edinburgh publisher. In this book the evolution of species
was ascribed to two powers, a power of transformation and a power of
adaptation. Two Frenchmen, Naudin and Lecoq, also published a work in
which the theory of evolution was set forth, and from 1852 to 1854 the
well-known German anthropologist Schaafhausen was writing on similar
lines. But all these calls sounded unheard, so deeply were naturalists
plunged in detailed investigations, and it required a much mightier
voice to command the ear of the scientific world.

It is impossible to estimate the effect of Darwin's book on _The
Origin of Species_, published in English in 1858, in German in 1859
unless we fully realize how completely the biologists of that time
had turned away from general problems. I can only say that we, who
were then the younger men, studying in the fifties, had no idea that
a theory of evolution had ever been put forward, for no one spoke of
it to us, and it was never mentioned in a lecture. It seemed as if all
the teachers in our universities had drunk of the waters of Lethe,
and had utterly forgotten that such a theory had ever been discussed,
or as if they were ashamed of these philosophical flights on the part
of natural science, and wished to guard their students from similar
deviations. The over-speculation of the 'Naturphilosophie' had left in
their minds a deep antipathy to all far-reaching deductions, and, in
their legitimate striving after purely inductive investigation, they
forgot that the mere gathering of facts is not enough, that the drawing
of conclusions is an essential part of the induction, and that a mass
of bare facts, however enormous, does not constitute a science.

One of my most stimulating teachers at that time, the gifted anatomist,
Jacob Henle, had written as a motto under his picture, 'There is a
virtue of renunciation, not in the province of morality alone, but
in that of intellect as well,' a sentence which expressly indicated
the desirability of refraining from all attempts to probe into the
more general problems of life. Thus the young students of that time
were nourished only on the results of detailed research, in part
indeed interesting enough, but in part dry and, because uncorrelated,
unintelligible in the higher sense, and only here and there awakening a
deeper interest, when, as in physiology and in embryology, they formed
a connected system in themselves. Without being fully clear as to what
was lacking, we certainly missed the deeper correlation of the many
separate disciplines.

It is therefore not to be wondered that Darwin's book fell like a
bolt from the blue; it was eagerly devoured, and while it excited in
the minds of the younger students delight and enthusiasm, it aroused
among the older naturalists anything from cool aversion to violent
opposition. The world was as though thunderstruck, as we can readily
see from the preface with which the excellent zoologist of Heidelberg,
Bronn, introduced his translation of Darwin's book, where he asks this
question among others, 'How will it be with you, dear reader, after you
have read this book?' and so forth.

But before I enter on a detailed examination of the contents of this
epoch-making book, I should like to say a few words about the man
himself, who thus revolutionized our thinking.

Charles Darwin was born in 1809, the year of the publication of
Lamarck's _Philosophie zoologique_, and of Oken's _Lehrbuch der
Naturphilosophie_. There was thus a whole generation between the first
emergence of the Evolution theory and its later revival. Darwin's
father was a physician, and his education was not a regular one. In his
youth he seems to have devoted much time and enthusiasm to hunting, and
only very slowly to have taken up regular studies towards a definite
end. In accordance with his father's wishes, he studied medicine for a
time, but soon abandoned it to devote himself to botany and zoology.
Before he had had time to distinguish himself in any special way in
these subjects, he was offered, in his twenty-first year, the post of
naturalist on an English war-ship which was to make a voyage round the
world, and that at a leisurely rate.

This was decisive not only for Darwin's immediate studies, but for
the work of his life, for, as he tells us himself, it was during this
voyage on the _Beagle_ that the idea of the Evolution theory first came
to him. While the vessel made a stay at the Galapagos Islands, west
of South America, he noticed that quite a number of little land-birds
occurred there which closely resembled those of the neighbouring
mainland, but yet were different from them. Almost every little island
had its own species, and so he concluded that all these might be
descended from representatives of a few species which had long before
drifted over from the mainland to these volcanic islands, become
established there, and in the course of time taken on the character of
new species. The problem of the transformation of species opened up
before him, and he made up his mind to follow up the idea after his
return, in the hope that by a patient collecting of facts, he would by
and by arrive at some security with regard to this great question.

I need not linger over any detailed account of his travels; one can
readily understand how a voyage round the world, lasting for five
years, would offer to the inquiring mind of a Darwin rich opportunities
for the most varied observations. That he did not fail to make use of
these is evidenced not only by his book on _The Origin of Species_, but
by several more special works, published shortly after his return--his
natural history of those remarkable sessile crustaceans, the barnacles
or Cirripedia, and his studies on the origin of coral reefs. The
first-named book still holds its own as a classic monograph on this
animal group, with its wealth of forms; and the theory of the origin of
coral reefs which Darwin elaborated has still many adherents, in spite
of various rival interpretations.

But Darwin would hardly have achieved what he did if he had been
compelled to secure for himself a professional position in order to
obtain bread and butter. Such great problems demand not only the whole
of a man's mental energy, they monopolize his time. Studies of detail
may well be taken up in leisure hours, but big problems absorb all the
thoughts and must always be present to the mind, lest the connexion
between the many individual inquiries, which make up the whole task, be
lost sight of. Darwin had the good fortune to be a free investigator,
and to be able to retire, on his return from his travels, to a small
property at Down in Kent, there to live for his family and his work.
Here he followed up the idea of evolution which he had already
formulated, and it has always seemed to me the most remarkable thing
about him, that he was able to keep in mind and work up the hundreds
of isolated inquiries that were eventually to be brought together to
form the main fabric of his theory. When one studies his many later
writings, one cannot but be surprised afresh by the number of different
sets of facts he collected at the same time, partly from others,
partly from personal observation, and continually also from his own
experiments. He made experiments on plants and on animals, and the
number of people with whom he carried on a scientific correspondence
is simply astounding. In this way he brought together, in the course
of twenty years, an extraordinarily rich material of facts, from the
fullness of which he was able later to write his book on _The Origin
of Species_. Never before had a theory of evolution been so thoroughly
prepared for, and it is undoubtedly to this that it owed a great part
of its success; not to this alone, however, but still more, if not
mainly, to the fact that it presented a principle of interpretation
that had never before been thought of, but whose importance was
apparent as soon as attention was called to it--the principle of
selection.

Charles Darwin championed, in the main, the same fundamental ideas as
had been promulgated by his grandfather, Erasmus Darwin, by Treviranus,
and by Lamarck: species only seem to us immutable; in reality they can
vary, and become transformed into other species, and the living world
of our day has arisen through such transformations, through a sublime
process of evolution which began with the lowest forms of life, but
by degrees, in the course of unthinkably long ages, progressed to
organisms more and more complex in structure, more and more effective
in function.

It is interesting to note at what point Darwin first put in his lever
to attempt the solution of the problem of evolution. He started from
quite a different point from the investigators of the early part of
the century, for he began with forms of life which had previously
been markedly neglected by science, the varieties of our domesticated
animals and cultivated plants.

Previously these had been in a sense mere step-children of biology,
inconvenient existences which would not fit properly into the system,
which were therefore as far as possible ignored or dismissed as outside
the scope of 'the natural,' because it was difficult to know what else
to do with them. I can quite well remember that, even as a boy, I was
struck by the fact that one could find nothing in the systematic books
about the many well-established garden forms of plants, or about our
domestic animals, which seemed to be regarded as in a sense artificial
products, and as such not worthy of scientific consideration. But it
was in these that Darwin particularly interested himself, making them
virtually the basis of his theory, for he led up from them to the very
principle of transformation, which was his most important addition to
the earlier presentations of the Evolution theory.

He started from the existence of varieties which may be observed in
so many wild species. His line of thought was somewhat as follows: If
species have really arisen through a gradual process of transformation,
then varieties must be regarded as possible first steps towards new
species; if, therefore, we can only succeed in finding out the causes
which underlie the formation of any varieties whatever, we shall have
discovered the causes of the transformation of species. Now we find by
far the greatest number of varieties, and the most marked ones, among
our domesticated animals and plants, and unless we are to assume that
each of these is descended from a special wild species, the reason why
there has been such a wealth of variety-formation among them must lie
in the conditions which influence the relevant species in the course of
domestication; and it remains for us to analyse these conditions till
we come upon the track of the operative factors. With this conviction,
Darwin devoted himself to the study of domesticated animals and plants.

The first essential was to prove that every variety had not a separate
wild species as ancestor, but that the whole wealth of our domesticated
breeds originated, in each case, from one, or at least from a few wild
species. Of course I cannot here recapitulate the multitudinous facts
which were marshalled by Darwin, especially in his later works, notably
his _Animals and Plants under Domestication_, but this is not necessary
to an understanding of his conclusions, and I shall therefore restrict
myself to a few examples.

Let us take first the domestic dog, _Canis familiaris_, Linné. We
have at the present day no fewer than seven main breeds, each of
which has its sub-breeds, often numerous. Thus there are forty-eight
sub-breeds which are used as guardians of our houses, 'house-dogs'
in the restricted sense, thirty sub-breeds of dogs with silk-like
hair (King Charles dogs, Newfoundland dogs, &c.), twelve of terriers,
and thirty-five of sporting dogs, among them such different forms as
the deerhound and the pointer. We have further nineteen sub-breeds
of bulldogs, thirty-five of greyhounds, and six of naked or hairless
dogs. Not only the main breeds, but even the sub-breeds often differ
as markedly from one another as wild species do, and the question must
first be decided whether each of the very distinct breeds has not a
special wild species as ancestor.

Obviously, however, this cannot be maintained, for so many species of
wild dog have never existed on the earth at any time. We know, too,
that 4,000 or 5,000 years ago a large number of breeds of dogs were
in existence in India and Egypt. There were Pariah dogs, coursers,
greyhounds, mastiffs, house-dogs, lapdogs and terriers. It is not
possible that the products of all lands could, at that time, have been
gathered into one, and it is inconceivable that so many wild species
could have existed in the one country of India.

On the other hand, however, it cannot be maintained that all our
present breeds have descended from _a single_ wild species; it is much
more probable that several wild species were domesticated in different
countries.

It has often been supposed that the manifold diversity of our present
breeds has been brought about by crossing the various tamed species.
That cannot be the case, however, because crossing gives rise only to
hybrid mongrel forms, not to distinct breeds with quite new characters.
It is true that all breeds of dogs can be very readily crossed with
each other, but the result is not new breeds, but those numberless
and transient intermediate forms which the dog-breeder despises as
worthless for his purpose. It must therefore have been through the
influence of domestication, combined with crossing, that a few wild
species gave rise to the various breeds of dogs.

The pedigree of the horse is rather more clear than that of the dog.
Even in this case, indeed, one cannot definitely name the ancestral
wild form, but it is very probable that it was of a grey-brown colour,
and similar to the wild horses of our own day. Darwin supposes that
it must also have had the black stripe on the back which is exhibited
by the domestic ass, and by several wild species of ass, basing his
opinion on the fact that the spinal stripe often occurs in foals,
especially in those of a grey-brown colour.

But though there can be no doubt that this is to be interpreted as a
reversion to a character of a remote ancestor, it by no means follows
that the _direct_ ancestral form must have had this stripe. I am
more inclined to believe that the ancestor which bore this mark was
considerably more remote, and lived before the differentiation of the
horse from the ass. Darwin himself noted the remarkable fact that in
rare cases, especially in foals, not only may the stripe on the back be
present, but there may be more or less distinct zebra-striping on the
legs and withers: this, however, must be interpreted as a reversion to
the character of a very much more remote ancestor, to a common ancestor
of all our present-day horses and asses, which must have been striped
over its whole body, like the zebra living in Africa now.

It cannot be proved of any of the wild horses of to-day that they are
not descended from domesticated ancestors; indeed, we can say with
certainty that the thousands of wild horses which roam the plains of
North and South America are descended from domestic horses, for there
was no horse in America at the time it was discovered by the Europeans.
In all probability our horse originated in Middle Asia, was there
first domesticated, and has thence been gradually introduced into
other countries. In Egypt it appears first on the monuments in the
seventeenth century B.C., and it seems to have been introduced by the
conquering Hyksos. On the ancient Assyrian monuments the chase after
wild horses is depicted, and they were not caught, but killed with
arrow and lance, like the lion and the gazelle.

But even if two wild species of horse had been tamed in different parts
of the great continent of Asia, these two domesticated animals would
have varied much and in the most diverse manner, as we may infer from
our different breeds of horses at the present day. There are a great
many of these, and many of them differ very considerably from each
other. If we think of the lightly built Arab horse, and place beside
it the little pony, or the enormous Percheron, the powerful cart-horse
from the old French province of La Perche, which easily draws a load of
fifty kilograms, we are face to face with differences as great as those
between natural species. And we may realize how many breeds of horses
there are now upon the earth if we remember that nearly every oceanic
island has its special breed of ponies. Not only in the cold Shetland
Islands, England, Sardinia and Corsica, but in almost every one of the
larger islands of the extensive Indian Archipelago there is one, and
Borneo and Sumatra have several.

But the most conclusive proof of descent from a single wild species
is afforded by the pigeons, and as the production of new breeds among
them has been, and will continue to be, carried on with particular
enthusiasm and deliberateness, I propose to deal with them somewhat
more in detail.

Darwin's work proves beyond a doubt that all our present-day breeds
of pigeons are descended from a single wild species, the rock-dove,
_Columba livia_. In appearance, this form, which still lives in a wild
state, differs little from our half-wild blue-grey field-pigeon. It has
the same metallic shimmer on the feathers of the neck, the same two
black cross-bars on the wings as well as the band over the tail, and it
has also the same slate-blue general colour. Now, all breeds of pigeons
are without restriction fertile _inter se_, so that any breed can be
crossed with any other, and it often happens that, in the products
of such crossing, characters appear which the parents, that is, the
two or more crossed breeds, did not possess, but which are among the
characters of the rock-dove. Thus Darwin obtained, by crossing a pure
white fantail with a black barb, hybrids which were partly blackish
brown, partly mixed with white, but when he crossed these hybrids with
others from two breeds which were likewise not blue, and had no bars,
he obtained a slate-blue rock-pigeon, with bars on the wings and tail.
We shall inquire later on how far it is correct to regard such cases
as reversions to remote ancestors, but if we take it for granted in
the meantime, we have here a proof of the descent of our breeds from a
single wild species. This is corroborated, too, by everything that we
know about the distribution of the rock-pigeon and the place and time
of its domestication. It still lives on the cliff-guarded shores of
England, Brittany, Portugal, and Spain, and both in India and in Egypt
there were tame pigeons at a very early period. Pigeons appear on the
menu of a Pharaoh of the fourth dynasty (3000 B.C.), and of India we
know at least that in 1600 A.D. there were 20,000 pigeons belonging to
the court of one of the princes.

The beauty of this bird, and the ease with which it can be tamed,
obviously called man's attention to it at a very early date, and it has
been one of man's domestic companions for several thousands of years.
Now we can distinguish at least twenty main races (Fig. 1), which
differ from each other as markedly as, if not more markedly than, the
most nearly allied of the 288 wild species of pigeons which inhabit the
earth. We have carriers and tumblers, runts and barbs, pouters, turbits
and Jacobins, trumpeters and laughers, fantails, swallows, Indian
pigeons, &c.

[Illustration: FIG. 1. Group of various races of domestic pigeons
(after Prütz). 1. Pouter. 2. Indian barb. 3. Bucharest trumpeter with
a whorl of feathers (_Nelke_) on its forehead. 4. Nürnberger swallow.
5. Nürnberger bagadotte. 6. English carrier. 7. Fantail. 8. Eastern
turbit. 9. Schmalkaldener Jacobin. 10. Chinese owl. 11. German turbit.]

Each of these races falls into sub-races; thus there is a German, an
English, and a Dutch pouter-pigeon. The books on pigeons mention over
150 kinds which are quite distinct from one another, and breed true,
that is, always produce young similar to themselves.

Without entering upon a detailed description of any of these, I should
like to call attention to the way in which certain characters have
varied among them. Colour is a subordinate race-character, in so far
that colour alone does not constitute a race, yet the colouring within
a particular sub-race is usually very sharply defined, and in every
breed there are sub-races of different colours. Thus there are white,
black, and blue fantails, there are white turbits with red-brown wings,
but also red ones with white heads, and white tumblers with black
heads, &c. Very unusual colours and colour-markings sometimes occur.
Thus one sub-race of tumblers exhibits a peculiar clayey-yellow colour
splashed with black markings, otherwise rare among pigeons, and almost
suggestive of a prairie-hen; there is also a copper-red spot-pigeon, a
cherry-red 'Gimpel'-pigeon, lark-coloured pigeons, &c. Then we find all
possible juxtapositions of colours, limited to quite definite regions
of the body; thus we have white tumblers with a red head, red tail,
and red wing-tips, or white tumblers with a black head, red turbits
with white head, Indian pigeons quite black except for white wing-tips,
and so on. The distribution of colour is often very complicated, but
nevertheless, all the individuals of the breed show it in exactly the
same manner. Thus there are the so-called blondinettes in which almost
the whole body is copper-red, but the wings white, save that each quill
bears at the rounded end of its vane a black and red fringe. I should
never come to an end, if I were to try to give anything like a complete
idea of the diversity of colouring among the various breeds of pigeons.

Even such an important and, among wild species, unusually constant
organ as the bill has varied among pigeons to an astonishing degree.
Carrier-pigeons (Fig. 1, No. 6) have an enormously long and strong
bill, which is moreover covered with a thick red growth of the cere,
while in the turbits and owls (Fig. 1, Nos. 8 and 10) the bill is
shorter than any we find among wild birds. In many breeds even the
_form_ of the bill deviates far from the normal, as in the bagadottes
(No. 5) with crooked bill.

Like the bill, the legs vary in regard to their length. The pouters
(No. 1) stand on their long legs as on stilts, while the legs of the
'Nürnberger swallow' are strikingly small. Remarkable, too, and very
different from the wild species, is the thick growth of feathers on the
feet and toes of the pouters and trumpeters (Fig. 1, No. 1), as well
as of some other breeds, which suggests the arrangement of feathers on
a wing.

Furthermore, the number and size of wing and tail-feathers in the
different breeds often deviate considerably from the normal. The
fantail (No. 7) in its most perfect form possesses forty tail-feathers,
instead of the twelve usual in the wild rock-pigeon, and they are
carried upright like a fan, while the head and neck of the bird are
bent sharply backwards. In the hen-like pigeons the tail-feathers are
few and short, so that they show an upright tail like that of a hen. I
have already referred to the extraordinary carunculated skin-growth on
the bill of many breeds; such folds also often surround the eye, and,
as in the Indian barb (No. 3), are developed into well-formed thick
circular ridges, while in the English carrier (No. 6) they lie about
the bill as a formless mass of flesh.

Even the skull has undergone many variations, as can be observed
even in the living bird in many of the breeds with short forehead.
Differences are to be found, too, in the number and breadth of the
ribs, the length of the breast-bone, the number and size of the
tail-vertebræ in different breeds. Of the internal organs, the crop in
many breeds, but particularly in the pouters (No. 1), has attained an
enormous size, and with this size is usually associated the habit of
blowing it out with air, and assuming the characteristically upright
position.

That variations have taken place, too, in the most delicate structure
of the brain, is shown by certain new instincts, such as the trumpeting
of the trumpeters, the cooing of others, and the silence of yet other
breeds, as well as by the curious habit of the tumblers of ascending
quickly and vertically to a considerable height, and then turning
over once, or even several times, in the course of their descent. In
contrast to this, other breeds like the fantails have altogether given
up the habit of flying high, and usually remain close to the dove-cot.

Lastly, let me mention that the unusual development of individual
feathers, or of groups of feathers, has become a race-character, upon
which depend such remarkable structures as the feather-mantle turned
over the head in the Jacobins (No. 9), the cap or plume on the head of
various breeds, the white beard in the bearded tumbler, the collars
which lie like a shirt-collar on the breast, or run down the sides of
the neck (Nos. 8 and 10), and the circle of feathers which marks the
root of the bill in the Bucharest trumpeter (No. 3).

After what has been said, it is hardly necessary to add that the size
of the whole body differs in different races. But the differences
are very considerable, for, according to Darwin, one of the largest
runt-pigeons weighed exactly five times as much as one of the smallest
tumblers with short forehead, and in the illustration (Fig. 1) the
pouter looks a giant beside the little barb to its left.

Thus we see that nearly every part of the body of the pigeon has varied
under domestication in the most diverse ways, and to a high degree;
and the same is true of several other domesticated animals, poultry,
horses, sheep, cattle, pigs, and so on, though the matter is not
altogether so clear in their case, since descent from a single wild
species cannot be proved, and is in many cases improbable. But in the
case of pigeons this common descent is certain, and we have now to
inquire in what manner all these variations from the parent form have
been brought about.

The answering of this question is rendered easier by the fact that new
breeds arise even now, and that, to some extent at least, they can be
caused to arise, consciously and intentionally. In England, as well
as in Germany and France, there are associations for the breeding of
birds, and in England especially pigeon and poultry clubs are numerous
and highly developed. These by no means confine themselves to simply
preserving the purity of existing breeds, they are continually striving
to improve them, by increasing and accentuating their characters, or
even by introducing quite new qualities, and in many cases they succeed
even in this last. Prizes are offered for particular new variations,
and thus a spirit of rivalry is fostered among the breeders, and each
strives to produce the desired character as quickly as possible. Darwin
says: 'The English judges decided that the comb of the Spanish cock,
which had previously hung limply down, should stand erect, and in
five years this end was achieved; they ordained that hens should have
beards, and six years later fifty-seven of the groups of hens exhibited
at the Crystal Palace in London were bearded.' The transformation
does not always come about so quickly, however; thus, for instance,
it required thirteen years before a certain breed of tumblers was
furnished with a white head. But the breeders cause every visible part
of the body to vary as seems good to them, and within the last fifty
years they have really brought about very considerable changes in many
breeds. Their method of procedure is carefully to select for breeding
those birds which already possess a faint beginning of the desired
character. Domesticated animals have on the whole a higher degree of
variability than wild species, and the breeder takes advantage of
this. Suppose it is a question of adding a crown of feathers to a
smooth-headed breed, a bird is chosen which has the feathers on the
back of the head a little longer than usual, and mated for breeding.
Among its descendants there will probably be some which also exhibit
these slightly prominent feathers, and possibly there may be one or
other of them which has these feathers considerably lengthened. This
one is then used for breeding, and by continually proceeding thus,
and selecting for breeding, from generation to generation, only
the individuals which approach most nearly to the desired end, the
wished-for character is at last secured.

Thus it is not by crossing of different breeds, but by a patient
accumulating of insignificant little variations through many
generations, that the desired transformations are brought about. That
is the magic wand by means of which the expert breeder produces his
different breeds, we might almost say, as the sculptor moulds and
remoulds his clay model according to his fancy. Quite according to his
fancy the breeder has brought about all the fantastic forms we are
familiar with among pigeons, mere variations which are of no use either
to the bird itself or to man, which simply gratify man's whim without
in many cases even satisfying his sense of beauty. For many of the
existing breeds of pigeons, hens, and other domesticated animals, are
anything but beautiful, the body being often unharmonious in structure
and sometimes actually monstrous.

Among pigeons, as well as among other domesticated animals, some
changes have been brought about, which are not only of no use to their
possessors, but would be actually disadvantageous if they were living
under natural conditions. Some of the very short-billed breeds of
pigeons have the bill so short and soft that the young can no longer
use it to scratch and break the egg-shell, and would perish miserably
if human aid were not at hand. The Yorkshire pig has become such a
colossus of fat on weak, short legs, that if it were dependent on
its own resources, it could not secure its food, much less escape
from a beast of prey; and among horses the heavy cart-horse and the
racer are alike unfit to cope with the dangers of a wild life, or the
vicissitudes of weather.

Breeding has done much to bring about variations useful to man. Thus we
have breeds of cattle which excel in flesh, or in milk, or as draught
animals, and sheep which excel in flesh or in wool, and to what a
height the perfecting of a useful quality can be brought is shown, in
regard to fineness of wool, by that finest breed of sheep, the merino,
which instead of the 5,500 hairs borne by the old German sheep on a
square inch, possesses 48,000.

Not infrequently it is a particular stage of a species that has
been bred by man, and the other stages have remained more or less
unaltered. Thus it is with one of the few domesticated insects, the
silk-moth. Only the cocoon is of use to man, and according to the
cocoon different breeds are distinguished, differing in fineness,
colour, &c.; but no breeds can be distinguished in reference to the
larvæ, or the perfect insects. Among gooseberries there are about a
hundred varieties distinguished according to the form, colour, size,
thickness of skin, hairiness, &c., of the fruits, but the little,
inconspicuous, green blossoms, of which the breeders take no account,
are alike in them all. In the pansies (_Viola tricolor_), on the other
hand, it is only by the flowers that the varieties are distinguished,
while the seeds have remained alike in all.

It may be asked how it could have occurred to any one, when pigeons,
for instance, first began to be domesticated, to wish to produce
fantails or pouters, since he could have no mental picture of them
in advance. Darwin replies to this objection, that it was not always
conscious and methodical artificial selection, such as is now
practised, that brought about the origin of breeds, but that they have
very often resulted, and at first perhaps always, from unconscious
selection. When savages tamed a dog, they used the 'best' of their dogs
for breeding, that is, they chose those which had in the highest degree
the qualities they valued, watchfulness, for instance, or if the dog
were intended for the chase, keen scent and swiftness. In this way the
body of the animal would be changed in a definite direction, especially
if rivalry helped, and if it was the ambition of each to possess a
dog as good as, or better than those of his tribal companions. That
perfectly definite changes in bodily form can thus be brought about
unconsciously is well illustrated by the case of a racehorse. This has
arisen within the last two hundred years simply because the fleetest
of the products of crossing between the Arab and the English horse
were always chosen for breeding. It could not have been predicted that
horses with thin neck, small head, long rump, and slender legs would
necessarily be the swiftest runners; but this is the form which has
resulted from the selection,--a very ugly, but very swift horse. This
unconscious selection must undoubtedly have played a large part in the
early stages of the evolution of the breeds of our domestic animals.

But even in the fully conscious and methodical selective breeding of
particular characters, the breeder rarely alters only the one his
attention is fixed on; generally quite a number of other characters
alter apart from his intention as an inevitable accompaniment of the
desired variation on which attention was riveted. There are breeds
of rabbits whose ears hang limply down instead of standing erect,
and in these so-called lop-eared rabbits the ear-muscles are partly
degenerated, and as a consequence of this lack of muscular strain the
skull has assumed another form. Thus the variation of one part may
influence the development of a second and a third organ, and may even
not stop there, for very often the influence has penetrated much deeper
and affected quite remote parts of the body.

If any one were to succeed in adding a heavy pair of horns to a breed
of hornless sheep, there would run parallel with the course of this
variation, which was directly aimed at, a long series of secondary
changes which would affect at least the whole of the anterior half of
the body; the skull would become thicker and stronger to support the
weight of the heavy horns; the neck-tendon (_ligamentum nuchæ_) would
have to become thicker to hold up the heavy head, and so also with the
muscles of the neck; the spinous processes of the cervical and dorsal
vertebrae would become longer and stronger, and the forelegs, too,
would need to adapt themselves to the heavier burden. Every organism
thus resembles, as it were, a mosaic, out of which no individual group
of pieces can be taken and replaced by another without in some measure
disturbing the correlation and harmony of the whole: in order to
restore this, the pieces all round about the changed part must be moved
or replaced by others.

According to Darwin, it is to this correlation of parts that we must
refer the variation of other parts besides the one intentionally
altered in the course of breeding. It must be admitted that the mutual
dependence of the parts plays a very important rôle in the economy
and development of the animal body, as we shall see later, and these
connexions still remain very mysterious to us. Especially is this
the case with the connexion between the reproductive organs and the
so-called secondary sexual characters. Removal of the reproductive
organs or gonads induces, in Man, for instance, if it be effected in
youth, the persistence of the childish voice and the non-development
of the beard; in the stag the antlers do not appear, and in the cock
the comb does not develop perfectly, &c., but we are not yet able to
understand clearly why this should be so.




LECTURE III

THE DARWINIAN THEORY (_continued_)

 Natural selection--Variation--Struggle for existence--Geometric ratio
 of rate of increase--Normal number and ratio of elimination in a
 species--Accidental causes of extinction--Dependence of the strength
 of a species on enemies--Struggle for existence between individuals
 of the same species--Natural selection affects all organs and
 stages--Summary.


IN artificial selection, through which, with or without conscious
intention, our domesticated animals and cultivated plants have arisen,
there must obviously be three kinds of co-operative factors: first, the
_variability_ of the species; second, the capacity of the organism for
_transmitting_ its particular characters to its progeny; and third, the
_breeder_ who selects particular qualities for breeding. No one of the
factors can be dispensed with; the breeder could effect nothing, were
there not presented to him the variations of parts in the particular
direction in which he wishes them to vary; an indefinite variation,
that is, a variation not guided by selection, would never lead to the
formation of new breeds; the species would probably become in time a
motley mixture of all sorts of variations, but a breed with definite
characters, transmissible in their purity to its descendants, could
never be formed. Finally, every process of selective breeding would be
futile, if the variations which appeared could not be transmitted.

Darwin assumes that processes of transformation quite similar to those
which take place under the guidance of Man occur also in nature,
and that it is mainly these which have brought about and guided the
transformations of species which have taken place in the course of the
earth's history. This process he calls _natural selection_.

It will readily be admitted that two out of the three factors necessary
to a process of selective breeding are present also in the natural
conditions of the life of species. Variability in some degree or
other is absent from no species of animal or plant, though it may be
greater in one than in another, and it cannot be doubted that the
inborn differences which distinguish one individual from another are
capable of transmission. It is only to untrained observers that all the
individuals of a species appear alike; for instance, all garden whites,
or all the individuals of the small tortoiseshell butterfly (_Vanessa
urticæ_), or all the chaffinches. If the individuals are carefully
compared it will be recognized that, even in these relatively constant
species, no individual exactly resembles another; that even among
butterflies twenty black scales may go to form a particular spot on
the wings in one individual and thirty or twenty-five in others; that
the length of the body, the legs, the antennæ, the proboscis exhibit
minute differences; and it is probable that the same combination of
quite similar parts never occurs twice. In many animals this cannot, of
course, be proved, because our power of diagnosis is not fine enough to
be able to estimate the differences directly, and because a comparison
of measurements of all the parts in detail is not practicable. So we
may here confine ourselves to the differences in the human race, which
we can recognize with ease and certainty. Even as regards the face
alone, all men differ from one another, and, numerous and complete as
likenesses may be, it is impossible to find two human beings in which
even the characters of the face are exactly similar. Even so-called
'identical twins' can always be distinguished if they are directly
compared either in person or in a photograph, and if the rest of the
body be also taken into consideration we find numerous small, sometimes
even measurable differences.

The same is true of animals, and it is only our lack of practice
that is at fault if we frequently fail to detect their individual
differences. The Bohemian shepherds are said to know personally, and be
able to distinguish from all the rest, every sheep in their herds of
many thousands. Thus the factors of variability and transmissibility
must be granted, and it remains only to ask: Who plays the part of
selecting breeder in wild nature? The answer to this question forms
the kernel to the whole Darwinian theory, which ascribes this rôle to
the conditions of life, to definite relations of individuals to the
external influences which they meet with during the course of their
lives, and which together make up their 'struggle for existence.'

To make this idea clear I must to some extent diverge.

It is a generally observed fact that, in every species of animals
or of plants, more germs and more individuals are produced than
grow to maturity, or become capable of reproduction. Numerous young
individuals perish at an early stage, often because of unfavourable
circumstances--cold, drought, damp, or through hunger, or at the hands
of their enemies. When we ask which of the progeny perish early, and
which survive to carry on the species, we are at first sight inclined
to suppose that this is entirely a matter of chance; but this is just
what Darwin disputed. It is not chance alone, it is, above all, the
differences between individuals, which enable them to withstand adverse
circumstances better or worse, and thus decide, according to his view,
which shall perish and which shall survive. If this be so, then we
have a veritable process of selection, and one which secures that the
'best,' that is, the most capable of resistance, survive to breed,
being thus, so to speak, 'selected.'

It may be asked, however, why so many individuals must perish in youth,
and whether it could not have been arranged that all, or at least most,
should survive till they had reproduced. But this is an impossibility,
unrealizable for this among other reasons, that organisms multiply in
geometrical progression, and that their progeny would very soon exceed
the limits of computability. This does not occur, for there is a limit
set which they can in no case overstep,--which, indeed, as we shall
see, they never reach--I mean the limits of space and food-supply.
Every species, by the natural requirements of its life, is restricted
to a particular habitat, to land or to water, but most are still more
strictly limited to a definite area of the earth's surface, which alone
affords the climate suited to them, or where alone the still more
specialized conditions of their existence can be realized. Thus, for
instance, the occurrence of a particular species of plant determines
that of the animal which is dependent on it for its food-supply. If
they could multiply unchecked, that is, without the loss of many of
their progeny, every species would fill up its area of occurrence and
exhaust the whole of its food-supply, and thus bring about its own
extermination. This seems to be prevented in some way, for as a matter
of fact it does not happen.

It may, perhaps, be imagined that this might be prevented by a
regulation of the productivity of the species, and that those which
have not a large area of distribution, or can only count on a
relatively limited food-supply, have also a low rate of multiplication,
but this is not the case; even the lowest rate of multiplication would
very soon suffice to make any species fill up its whole available space
and completely exhaust its food-supply. Darwin takes as an example
the elephant, which only begins to breed at thirty years of age, and
continues to do so till about ninety, but so slowly that in these
sixty years only three pairs of young are produced. Nevertheless, in
500 years an elephant pair would be represented by fifteen millions
of descendants, if all the young survived till they were capable of
reproduction. A species of bird with a duration of life of five years,
during which it breeds four times, producing and rearing four young
each time, would in the course of fifteen years have 2,000 millions of
descendants.

Thus, although the fertility of each species is, as a matter of fact,
precisely regulated, a low rate of multiplication is not in itself
sufficient to prevent the excessive increase of any species, nor is
the quantity of the relevant food-supply. Whether this be very large
or very small, we see that in reality it is never entirely used up,
that, as a matter of fact, a much greater quantity is always left over
than has been consumed. If increase depended only on food-supply, there
would, for instance, be food enough in their tropical home for many
thousand times more elephants than actually occur; and among ourselves
the cockchafers might appear in much greater numbers than they do even
in the worst cockchafer year, for all the leaves of all the trees are
never eaten up; a great many leaves and a great many trees are left
untouched even in the years when the voracious insects are the most
numerous. Nor do the rose-aphides, notwithstanding their enormously
rapid multiplication, ever destroy all the young shoots of a rose-bush,
or all the rose-bushes of a garden, or of the whole area in which roses
grow.

At the same time it must be noted, that the number of individuals in
a species undoubtedly does bear some relation to the amount of the
food-supply available; for instance, it is very low among the large
carnivores, the lion, the eagle, and the like. In our Alps the eagles
have become rarer with the decrease of game, and where one eagle pair
make their eyrie they rule alone over a hunting territory of more than
sixty miles, a preserve on which no others of the same species are
allowed to intrude. If there were several pairs of eagles in such a
preserve, they would soon have so decimated the food-supply that they
would starve. On the other hand, numerous herbivores, e.g. chamois and
marmots, live within the bounds of the pair of eagles' hunting grounds,
since the food they require is present in enormously greater quantity.

While it is true that the number of individuals of a given species
which live in a particular area is not exactly the same year in year
out, being subject to small, and sometimes, as in the case of the
aphides and cockchafers, to very great fluctuations, nevertheless we
may assume that the _average number_ remains the same, that in the
course of a century, or, let us say, of a thousand years, the number
of mature individuals inhabiting the particular area remains the
same. This, of course, only holds true on the supposition that there
has been no great change in the external conditions of life during
this period. But before Man began to interfere with nature, these
external conditions would remain uniform for much longer periods
than we have assumed. Let us call the average number of individuals
occurring on such a uniform area, _the normal number_ of the species;
this number will be determined in the first instance by the number of
offspring that are annually brought forth, and secondly by the number
that annually perish before reaching maturity. As the fertility of
a species is a definite quantity, so also will its elimination be
definite, or, as we may say, when the normal number under uniform
conditions of life remains constant, the ratio of elimination will
also remain constant. Each species is therefore subject to a perfectly
definite ratio of elimination which remains on the average constant,
and this is the reason why a species does not multiply beyond its
normal number notwithstanding the great excess of the food-supply, and
notwithstanding the fertility which, in all species, is sufficient to
lead to boundless multiplication.

It is not difficult to calculate the ratio of elimination for a
particular species, if one knows its rate of multiplication; for if
the normal number remains constant, it follows that only two of all
the offspring which a pair brings forth in the course of its life can
attain to reproductive maturity, and that all the rest must perish.

Suppose, for instance, a pair of storks produced four young ones
annually for twenty years, of these eighty young ones which are born
within this period, on an average seventy-eight must perish, and only
two can become mature animals. If more than two attained maturity
the total number of storks would increase, and this is against the
presupposition of constancy in the normal number. It is important, in
reference to the fact on which we are now focusing our attention, that
we should consider some other illustrations from the same point of
view. The female trout yearly produces about 600 eggs; let us assume
that it remains capable of reproduction for only ten years, then the
elimination-number of the species will be 6,000 less two, that is,
5,998, for of the 6,000 eggs only two can become mature animals. But
in the majority of fishes the ratio of extermination is enormously
greater than this. Thus a female herring brings forth 40,000 eggs
annually, the duration of life is estimated at ten years, and this
means an elimination number of 400,000 less two, that is, 399,998. The
carp produces 200,000 eggs a year, and the sturgeon two millions, and
both species live long, and remain capable of reproduction for at least
fifty years. But of all the 100 million eggs which are produced by the
sturgeon, only two reach their full development and reproduce; all
others perish prematurely.

But even with these examples we have not reached the highest
elimination number, for many of the lower animals--not to speak of
many plants--produce an even greater number of offspring. Leuwenhoek
calculated the fertility of a thread-worm at sixty million eggs, and a
tape-worm produces hardly less than 100 millions.

There exists, therefore, a constant relation between fertility and the
ratio of elimination; the higher the latter is, the greater must the
former be, if the species is to survive at all. The example of the
tape-worm makes this very obvious, for here we can readily understand
why the fertility must be so enormous, as we are aware of the long
chain of chances on which the successful development of this animal
depends. The common tape-worm of Man, _Tænia solium_, does not lay
its eggs, they remain enclosed within one of the liberated joints or
'proglottides.' Only if this liberated joint or one of the embryos
within it happens to be fortuitously eaten by a pig or other mammal can
there be successful development, and even then under difficulties and
possible failures, and not right away into adult animals, but first
into microscopically minute larvæ which may bore their way into the
walls of the intestine, or, if they are fortunate enough, may get into
the blood-stream and be carried by it to a remote part of the body.
There they develop into 'measles,' the so-called bladder-worms, within
which the head of the tape-worm arises. But in order that this may
become a complete and reproductive adult worm the pig must die, and the
next step necessary is that a piece of the flesh of the infected first
host must happen to be swallowed raw by a man or other mammal! Only
then does the fortunate bladder-worm--swallowed with the flesh--attain
the goal of its life, that is, a suitable place to mature in, the
food-canal of a human being. It is obvious that countless eggs must
be lost for one that succeeds in getting through the whole course of
a development depending so greatly on chance. Hence the necessity for
such enormous productivity of eggs.

In many cases the causes of elimination, which keep a species within
due bounds, are very difficult to determine. Enemies, that is to say,
other species which use the species in question as food, play an
important rôle; often, however, the cause lies in the unfavourableness
of external conditions, in chance, which is favourable only to one
of a thousand. The oak would only require to produce one seed in
the 500 years of its life, if it were certain that that one would
grow into an oak-tree; but most of the little acorns are eaten up by
pigs, squirrels, insects, &c., before they have had time to sprout,
thousands fall on ground already thickly covered with growth where
they cannot take root, and even if they do succeed in finding an
unoccupied space in which to germinate, the young plants are still
surrounded by a thousand dangers--the possibility of being devoured by
many animals large and small, of being suffocated by the surrounding
vegetation, and so on. We can thus understand, to some extent, though
only approximately, why it is that the oak must year by year produce
thousands of seeds in order that the species may maintain its normal
number, and not be exterminated; for it is obvious that a constant,
even though slow diminution of the normal number, a regular deficit, so
to speak, can end in nothing else than the gradual extinction of the
species.

But even this prodigality of seeds is not the greatest reach of
fertility that we meet with in nature; it is, perhaps, amongst the
simpler flowerless plants that we find the climax. It has been
calculated that a single frond of the beautiful fern so common in our
woods, _Aspidium filix mas_, produces about fourteen million spores.
They serve to distribute the species, and are carried as motes by the
wind, but comparatively few of the millions ever get the length of
germinating at all, much less of attaining to full development into
adult plants. Thus we see that the apparent prodigality of nature is a
real necessity, an indispensable condition of the maintenance of the
species; the fertility of each species is related to the actualities of
elimination to which it is exposed. This is clearly seen when a species
is placed under new and more favourable conditions of life, in which
it has an abundant food-supply and few enemies. This was the case, for
instance, with the horses introduced from Europe into South America,
where they reverted to a feral state, and are now represented by
herds of many thousands roaming the great grassy plains. If the small
singing-birds of a region diminish in number, there is a great increase
of caterpillars and other injurious insects which form part of their
food-supply. The colossal destruction which the much-dreaded nun-moth
from time to time brings about in our woods probably depends in part
on the diminution of one or another of the many animals inimical to
insects; but the occurrence of several years of weather-conditions
favourable to the larvæ must also be taken into account. How
enormously, indeed almost inconceivably, the number of larvæ may
increase under favourable conditions is shown by such devastations
as that in Prussia in 1856, when many square miles of forest were
absolutely eaten up. The caterpillars were so numerous that even from
some distance the falling excrement could be heard rustling like rain,
and ten hundredweights of the eggs were collected, with an average of
20,000 eggs to the half-ounce!

But it would be a great mistake to conclude, from this enormous and
sudden increase in the number of individuals, that the normal number
of individuals is determined by the number of enemies alone. The
average number of individuals in a species depends on many other
conditions, especially on the extent of the available area, and on
the amount of the food-supply in relation to the size of body in the
species. I cannot dwell on this now, but I wish to point out that,
for the continuance of a species, it is indifferent whether it is
'frequent' or 'rare,' if we presuppose that its normal number remains
on an average constant for centuries, that is, that its fertility
suffices to make good the continual losses through enemies and other
causes of elimination. One would be inclined to conclude from such
cases of sudden and enormous increase in the number of individuals as
these caterpillar-blights, that enemies and other causes of destruction
played the major part in the regulation of the normal number of the
species. But this is only apparently the case. Enemies necessitate
a certain fertility in the species on which they prey, so that the
elimination in each generation may be made good; but the number of
pairs capable of reproduction is not thereby decisively determined.
We must not forget that the number of enemies is also, on the other
hand, dependent on the number of victims, and that the normal number of
enemies must rise and fall with that of the species preyed upon.

For this reason, such an enormous increase as that of the caterpillars
cannot last long; it carries its corrective in itself. The appearance
of the caterpillars in such enormous numbers in itself increases the
host of their enemies; singing-birds, ichneumon-flies, beetle-grubs,
and predaceous beetles find abundant and available food, and therefore
reproduce and multiply so rapidly, that, with the help of the
caterpillar's plant-enemies, especially the insect-destroying fungi,
they soon reduce the caterpillars to their normal number, or even
below it. But then the reverse process begins; the enemies of the
caterpillars diminish because their food has become scarce, and their
normal number is lowered, while that of the caterpillars gradually
rises again.

When the number of foxes in a hunting district increases, the number of
the hares that they prey upon diminishes, and, on the other hand, the
decimating of the foxes by Man brings about an increase in the number
of hares in the district. Under natural conditions, that is, without
the intervention of Man, there would be a constant balancing of the
numbers of hares and foxes, for every noteworthy increase of the hares
would be followed by a similar increase of foxes, and this, in its
turn, would diminish the number of hares, so that they would no longer
suffice for the support of so many foxes, and these would decrease in
number again, until the number of hares had again increased because
of the lessened persecution and elimination. In nature the case is not
quite so simple, because the fox does not live on hares alone, and the
hare is not preyed upon only by the fox; but the illustration may serve
to elucidate the point that a moving equilibrium is maintained between
the species of a district, between persecutors and persecuted, in such
a way that the number of individuals in the two species is always
varying a little up and down, and that each influences the other so
that a regulative process results. Throughout periods of considerable
length the average remains the same; that is to say, a _normal number_
is established. This normal strength of population is the mean above
and below which the number of individuals is constantly varying. It is,
of course, seldom that the mutual influences and regulations are so
simple as in the example given; usually several or even many species
interact upon each other, and not beasts of prey and their victims
alone, but the most diverse species of animals and plants, which do
not stand in any obvious relation to one another at all. Moreover, the
physical, and especially the climatic conditions, also cause the normal
number of the species to rise and fall.

The inter-relations between species living together on the same area
are so intricate that I should like to give two other illustrations.
Let us first take Darwin's famous instance of the fertility of clover,
which depends on the number of cats. It is of course only an imaginary
one, but the facts it is based upon are quite correct. The number of
cats living in a village to a certain extent determines the number of
field-mice in the neighbourhood. These again destroy the nests of the
humble-bees, which live in holes in the ground, and thus the number
of humble-bees depends on that of the field-mice and cats. But the
clover must be pollinated by insects if it is to produce fertile seed,
and only the humble-bee has a proboscis long enough to effect the
pollination. Therefore the quantity of clover-seed annually produced
depends on the number of humble-bees, and ultimately upon the number of
cats. And, as a matter of fact, humble-bees were introduced into New
Zealand from England, because without them the clover would produce no
fertile seeds.

On the grassy plains of Paraguay there are no wild cattle and horses,
because of the presence of a fly which has a predilection for laying
its eggs in the navel of the newly-born calves and foals, with the
result that the calves or foals are killed by the emerging maggots. We
may reasonably assume that the numerical strength of this fly-species
depends on the distribution of insect-eating birds, whose numbers
in turn are determined by certain beasts of prey. These again vary
in number in relation to the extent of the forest-land, and this is
determined by the number of ruminants which browse on the young growth
of the woods (Darwin).

That forests can actually be totally destroyed by ruminants is proved
by the case of the island of St. Helena among others. On its discovery
the island was covered with thick wood, but in the course of 200 years
it was transformed into a bare rock by goats and pigs, which devoured
the young growth so completely that trees which were felled or which
died were not replaced.

This point is vividly illustrated by Darwin's observation of a
wide heath on which stood only a few groups of old pine-trees. The
mere fencing in of a portion of the heath sufficed to call forth a
thick growth of young seedling pines within the enclosure, and an
examination of the open part of the heath revealed that the grazing
cattle had eaten up all the young pine-trees which sprang from seed,
and that again and again. In one small space thirty-two little trees
stood concealed in the grass, and several of these showed as many as
twenty-six yearly rings.

How definitely the number of individuals in different species living on
the same area mutually limit and thereby regulate each other, Darwin
sought to illustrate also by the case of the primitive forest, where
the numerous species of plants occur, not mixed together irregularly,
but in a definite proportion. We can find examples of the same kind
wherever the plant-growth of a district has been left to itself. If we
walk along the banks of our little river, the Dreisam, we see a wild
confusion of the most diverse trees, shrubs and herbaceous plants. But,
even though it cannot be demonstrated, we may be certain that these are
represented in definite numerical proportions, dependent on the natural
qualities and requirements of each species, on the number of their
seeds and the facilities for their distribution, on the favourable
or unfavourable season at which they ripen, and on their varying
capacity for taking root in the worst ground, and springing quickly
up, &c. They limit each other mutually, so that the whole flora of the
river-bank will be made up of one per cent. of this species, one per
cent. of that, and, it may be, five per cent. of a third, and the same
combination will repeat itself in the same proportions on the banks of
other rivers of our country in as far as the external conditions are
the same. The same must be true of the fauna of such a plant-thicket;
the animal species also limit one another mutually, and thereby
regulate the number of individuals, which becomes relatively stable
over any area on which the conditions remain the same. That is to say,
a 'normal number' is attained and persists.

Thus we see that the capacity for boundless multiplication inherent in
every species is limited by the co-existence of other species; there
is, metaphorically speaking, a continuous struggle going on between
species, plant and animal alike; each seeks as far as possible to
multiply, and each is hemmed in by the others and as far as possible
prevented from doing so. The 'struggle' is by no means only the
_direct_ limitation of the number of individuals, which consists in
the use of one species by another as food, as in beasts of prey and
their victims, or locusts and plants; it is much more the indirect
limitation--figuratively speaking, the struggle for space, for light,
for moisture among plants, for food among animals. But all this,
important as it is, does not yet exhaust the content of that 'struggle
for existence' to which Darwin and Wallace ascribe the rôle of the
breeder in the process of natural selection. The struggle, that is,
the mutual limiting of species, may indeed restrict a species in
its distribution, and may reduce its normal number possibly to nil.
In other words, it may bring about extinction, but it cannot make
a species other than it is. This can only be done by a struggle
within the limits of the species itself, and this struggle is due
to the fact that of the numerous offspring, on an average those
survive--that is, attain to reproduction--which are the most fit,
whose constitution makes it most possible for them to overcome the
difficulties and dangers of life, and so to reach maturity. We see,
in fact, that a large percentage of each generation in all species
always perishes before attaining maturity. If, then, the decision as to
which is to perish and which is to reach maturity is _not a matter of
chance alone_, but is in part due to the constitution of the growing
individual; if the 'fittest' do _on the average_ survive, and the
'least fit' are on the average eliminated, we have here a process of
selection entirely comparable to that of artificial selection, and
one whose result must be the 'improvement' of the species, whether
that depends on one set of characters or on another. The victorious
qualities, which earlier were peculiar to certain individuals, must
gradually become the common property of the species, if in each
generation the individuals which attained to reproduction all possessed
them, and thus could transmit them to their progeny. But those of the
descendants which did not inherit them would again be at a disadvantage
in the struggle for existence, or rather for reaching maturity, if in
each generation a higher percentage of individuals which possess these
characters reach maturity than of those which do not possess them. This
percentage must increase in each generation, because, in each, natural
selection again chooses out the fittest, and it must finally rise to
100 per cent., that is to say, none but individuals of this fittest
type will be left surviving.

This does not yet exhaust the process, however, for we can infer
from the results of artificial breed-forming that the selected
characters may intensify from generation to generation, and that
they will continue to do so as long as it gives them any advantage
in the struggle for existence, for so long will it lead to the more
frequent survival of its possessors. The increase will only stop when
it has reached the highest degree of usefulness, and in this way new
characters may be formed, just as, in artificial selection, the short
upward-turning feathers of the Jacobin pigeon have been intensified
into the peruke, a feather canopy covering the head.

A few examples of natural selection will make the process clearer. Our
hare is well secured from discovery by his fur of mixed brown, yellow,
white, and black, when he cowers in his form among the dry leaves of
the underwood. It is easy to pass close to him without seeing him.
But if the ground and the bushes are covered with snow, he contrasts
conspicuously with them. Suppose, now, that our climate became colder,
and that the winter brought lasting snow, the hares which had the
largest mixture of white in their fur would have an advantage in their
'struggle for existence' over their darker fellows; they would be less
easily discovered by their enemies--the fox, the badger, the horned
owl, and the wild cat. Of the numerous hares which would annually
become the prey of these enemies, there would be, on an average, more
dark than light individuals. The percentage of light-coloured hares
would, therefore, increase from generation to generation, and the
longer the winter the keener would be the selection between dark and
light hares, until finally none but light ones would remain. At the
same time, the colour of the hares would become increasingly light,
first, because it would happen more and more frequently that two light
hares would pair, and secondly, because, after a time, the struggle for
existence would no longer be between light and dark hares, but between
light hares and still lighter ones. Thus ultimately a race of white
hares would arise, as has actually happened in the Arctic regions and
on the Alps.

Or let us think of a herbaceous plant, in appearance something like a
belladonna, rich in leaves and very juicy, but not poisonous. It would
doubtless be a favourite food with the animals of the forest, and it
would not, therefore, attain to more than a sparse occurrence, since
few of the individuals would be able to form seeds. But now let us
assume that a stuff of very unpleasant taste develops in the stem and
leaves of some of the individuals, as may easily happen through very
slight changes in the chemical metabolism of the plant, what, then,
could result but that such individuals would be less readily eaten than
the others? A process of selection must, therefore, ensue, and the
unpleasant-tasting specimens of the plant would be much more frequently
spared, and consequently would bear seed much oftener than the
palatable ones. Thus the number of unpalatable plants would increase
from year to year. If the stuff in question were not only unpalatable
but poisonous, or gradually became so, a plant would in time be evolved
which would be absolutely safe from being devoured by animals, just as
the deadly nightshade (_Atropa belladonna_) actually is.

Or let us suppose that a stretch of water is inhabited by a species of
carp, which have hitherto had no large enemy, and so have become lazy
and slow, and that there migrates from the sea into this stretch of
water a large species of pike. At first numerous carp will fall victims
to the pike, and the pike will rapidly increase in number. But if all
the carp were not equally lazy and dull-witted, if some of them were
quicker and more intelligent, these would, on an average, become more
rarely the victims of the pike, and numerous individuals with these
better qualities would survive in each generation, till ultimately
there were no others, and the useful characters would gradually become
intensified, and so a more active and wary race of carp would arise.

Let us suppose, however, that the increased activity and wariness would
not alone suffice to preserve the colony from extinction; it might
require also an increased fertility to prevent the normal number from
being permanently lowered; but even this could eventually be brought
about by natural selection, if the nature of the species and the
general conditions of its life permitted. For there are variations of
fertility in every species, and if the chance of seeing some of its
eggs become mature animals were greater for the more fertile female
than for the less fertile, _ceteris paribus_, a process of selection
must take place, which would result in an increase of fertility as far
as that was possible.

Obviously, such processes of natural selection can affect all parts
and characters--size and form of the body, as well as isolated parts,
the external skin and its colour, every internal organ--and not bodily
characters alone, but psychical ones as well, such as intelligence and
instincts. According to this principle, it is only characters which
are biologically indifferent that cannot be altered through natural
selection.

Natural selection can also bring about changes at every age, for the
elimination of individuals begins from the egg, and any kind of egg
which is in some way better able to escape elimination will transmit
its useful characters to its descendants, because the resulting young
animals will thus more frequently reach full development than the
young from other eggs. In the same way, at every succeeding stage of
development, every character favourable to the preservation of the
individual will be maintained and intensified.

We see from all this that natural selection is vastly more powerful
than artificial selection by Man. In the latter, only one character at
a time can be caused to change, while natural selection may influence a
whole group of characters at the same time, as well as all the stages
of development. Through the weeding out of the individuals which are
annually exterminated, it is always on an average the 'fittest' which
survive, that is to say, those which have the greatest number of
bodily parts and rudiments of parts in the fittest possible condition
of development at every stage. The longer this process of selection
continues, the smaller will be the deviations of the individual from
this standard, and the more minute will be the differences of fitness
determining which is to be eliminated and which is to survive to
reproduce its characteristics. In the immeasurable periods of time
which are at the disposal of natural selection, and in the inestimable
numbers of individuals on which it may operate, lie the essential
causes of superiority of natural selection over the artificial
selection of Man.

To sum up briefly: Natural selection depends essentially on the
cumulative augmentation of the most minute useful variations in the
direction of their utility; only the useful is developed and increased,
and great effects are brought about slowly through the summing up of
many very minute steps. Natural selection is a self-regulation of the
species which secures its preservation; its result is the ceaseless
adaptation of the species to its life-conditions. As soon as these
vary, natural selection changes its mode of action, for what was
previously the best is now no longer so; parts that before had to be
large must now perhaps be small, or vice versa; muscle-groups which
were weak must now become strong, and so on. The conditions of life
are, so to speak, the mould into which natural selection is continually
pouring the species anew.

But the philosophical significance of natural selection lies in
the fact, that it shows us how to explain the origin of useful,
well-adapted structures purely by mechanical forces and without
having to fall back on a _directive_ force. We are thus for the first
time in a position to understand, in some degree, the marvellous
adaptation of the organism to an end, without having to call to our
aid any supernaturally intrusive force on the part of the Creator. We
understand now how, in a purely mechanical way, through the forces
always at work in nature, all forms of life must conform to, and adapt
themselves precisely to the conditions of their life, since only the
best possible is preserved, and everything less good is continually
being rejected.

Before I go on to expound in detail the phenomena which we refer to
natural selection, I must briefly state that Darwin did not ascribe to
natural selection by any means all the changes which have taken place
in organisms in the course of time. On the one hand, he ascribed a not
inconsiderable importance to the correlated variations we have already
mentioned; still more, however, he relied on the direct influence of
altered conditions of life, whether these consist in climatic and other
changes in the environment, or in the assumption of new habits, and
the increased or diminished use of individual parts and organs thereby
induced. He recognized the principle so strongly emphasized by Lamarck,
of use and disuse as a cause of heritable increase or decrease of the
exercised or neglected part, though he did so with a certain reserve.
I shall return later to these factors of modification, and shall then
attempt to show that these too are to be referred to processes of
selection, which are, however, of a different order from the phenomena
which the Darwin-Wallace principle of natural selection serves to
interpret. But, in the first instance, it appears to me to be necessary
to show how far the Darwin-Wallace interpretation will suffice, and
in the next lectures we shall occupy ourselves with this question
exclusively.




LECTURE IV

THE COLORATION OF ANIMALS AND ITS RELATION TO THE PROCESSES OF SELECTION

 Biological significance of colours--Protective colours of
 eggs--Animals of the snow-region--Animals of the desert--Transparent
 animals--Green animals--Nocturnal animals--Double
 colour-adaptation--Protective marking of caterpillars--Warning
 markings--Dimorphism of colouring in caterpillars--Shunting back
 of colouring in ontogeny--'Sympathetic' colouring in diurnal
 Lepidoptera--In nocturnal Lepidoptera--Theoretical considerations--The
 influence of illumination in the production of protective colouring,
 _Tropidoderus_--Harmony of protective colouring in minute
 details--_Notodonta_--Objections--Imitation of Strange objects,
 _Xylina_--Leaf-butterflies, _Kallima_--_Hebomoja_--Nocturnal
 Lepidoptera with leaf-markings--Orthoptera resembling
 leaves--Caterpillars of the Geometridæ.


WE have seen what Darwin meant by natural selection, and we understand
that this process really implies a transformation of organisms by
slow degrees, in the direction of adaptive fitness--a transformation
which must ensue as necessarily as when a human selector, prompted
by conscious intention, tries to improve an animal in a particular
direction, by always selecting the 'fittest' animals for breeding.
In nature, too, there is selection, because in every generation
the majority succumb in the struggle for life, while on an average
those which survive, attain to reproductive maturity, and transmit
their characters to their descendants, are those which are best
adapted to the conditions of their life--that is, which possess those
variations of most advantage in overcoming the dangers of life.
Since individuals are always variable in some degree, since their
variations can be inherited by their progeny, and since the continually
repeated elimination of the majority of those descendants is a fact,
the inference from these premisses must be correct; there must be a
'natural selection' in the direction of a gradually increasing fitness
and effectiveness of the forms of life.

We cannot, however, directly observe this process of natural selection;
it goes on too slowly, and our powers of observation are neither
comprehensive nor fine enough. How could we set about investigating
the millions of individuals which constitute the numerical strength
of a species on a given area, to find out whether they possess
some variable character in a definite percentage, and whether this
percentage increases in the course of decades or centuries? And
there is, furthermore, the difficulty of estimating the biological
importance of any variation that may occur. Even in cases where we know
its significance quite well in a general way, we cannot estimate its
relative value in reference to the variation of some other character,
though that other may also be quite intelligible. Later on, we shall
speak of protective colouring, and in so doing we shall discuss the
caterpillars of one of the Sphingidæ, which occur in two protective
colours, some being brown, others green. From the greater frequency of
the brown form we may conclude that brown is here a better adaptation
than green, but how could we infer this from the character itself,
or from our merely approximate knowledge of the mode of life of the
species, its habits, and the dangers which threaten it? A direct
estimation of the relative protective value of the two colours is
altogether out of the question. The survival of the fittest cannot be
proved in nature, simply because we are not in a position to decide,
_a priori_, what the fittest is. For this reason I was forced to try
to make the process of natural selection clear by means of imagined
examples, rather than observed ones.

But though we cannot directly follow the uninterrupted process of
natural selection which is going on under natural conditions, there is
another kind of proof for this hypothesis, besides that which consists
in logically deducing a process from correct premisses; I should like
to call this the practical proof. If a hypothesis can be made to
explain a great number of otherwise unintelligible facts, it thereby
gains a high degree of probability, and this is increased when there
are no facts to be found which are in contradiction to it.

Both of these criteria are fulfilled by the selection-hypothesis, and
indeed the phenomena which may be explained by it, and are intelligible
in no other way, present themselves to us in such enormous numbers,
that there can be no doubt whatever as to the correctness of the
principle; all that can be still disputed is, how far it reaches.

Let us now turn our attention to this practical way of proving the
theory by the facts which it serves to interpret, beginning with a
consideration of the external appearance of organisms, their colour and
form.


_The Colour and Form of Organisms._

Erasmus Darwin had in many cases already rightly recognized the
biological significance of the colouring of an animal species, and
we may be sure that many of the numerous good observers of earlier
times had similar ideas. I can even state definitely that Rösel von
Rosenhof, the famous miniature-painter and naturalist of Nürnberg in
the middle of the eighteenth century, recognized clearly, and gave
beautiful descriptions of what we now call colour-adaptation. It is
true that he gave them only as isolated instances, and was far from
recognizing the phenomenon of colour-adaptation in general, or even
from inquiring into its causes. From the time of Linné, the endeavour
to establish new species overshadowed all the finer observation of
life-habits and inter-relations, and, later on, after Blumenbach,
Kielmeyer, Cuvier, and others, the eager investigation of the internal
structure of animals also tended to divert attention from these
œcological relations. In systematic zoology, colour ranked only as a
diagnostic character of subordinate value, because it is often not very
stable, and indeed is sometimes very variable; it was therefore found
preferable to keep to such relatively stable differences as are to be
found in the form, size, and number of parts.

Charles Darwin was the first to redirect attention to the fact that the
colouring of animals is anything but an unimportant matter; that, on
the contrary, in many cases it is of use to the animal, e.g. in making
it inconspicuous; a green insect is not readily seen on green leaves,
nor a grey-brown one on the bark of a tree.

It is plain that the origin of such a so-called 'sympathetic'
coloration, harmonizing with the usual environment of the animal, can
be easily interpreted in terms of the principle of selection; and it is
equally evident that it cannot be explained by the Lamarckian principle
of transformation. Through the accumulation of slight useful variations
in colour, it is quite possible for a green or a brown insect to arise
from a previous colour, but a grey or a brown insect could not possibly
have become a green one simply by getting into the habit of sitting on
a green leaf; and still less can the will of the animal or any kind
of activity have brought the change about. Even if the animal had any
idea that it would be very useful to it to be coloured green, now that
it had got into the habit of sitting on a leaf, it could not have done
anything towards attaining the desirable green colour. Quite recently
the possibility of a kind of colour-photography on the skin of the
animal has been suggested, but there are many species whose colouring
is in contrast to their environment, so that the skin in these cases
does not act as a photographic plate, and it would, therefore, have
to be explained how it comes to pass that it functions as such in
the sympathetically coloured animals. I do not ask for proof of the
chemical composition of the stuff which is supposed to be sensitive to
light. Whether this be iodide of silver or something quite different,
the question remains the same: how comes it that it has only appeared
in animals to which a sympathetic colouring is advantageous in the
struggle for life? And the answer, from our point of view, must read:
it has arisen through natural selection in those species to which a
sympathetic colouring is useful. Thus even if the supposition that
sympathetic colouring is due to automatic photography on the part of
the skin were correct, we should still have to regard it as an outcome
of natural selection; but it is not correct--at least in general--as
the above objection shows, and as will be further apparent from many of
the phenomena of colour-adaptation which I shall now adduce.

To explain sympathetic coloration, then, we must assume, with Darwin
and Wallace, a process of selection due to the fact that, as changes
took place in the course of time in the colouring of the surroundings,
those individuals on an average most easily escaped the persecution of
their enemies which diverged least in colour from their surroundings,
and so, in the course of generations, an ever greater harmony with this
colouring was established. Variations in colouring crop up everywhere,
and as soon as these reached such a degree as to afford their
possessors a more effective protection than the colouring of their
fellows, then natural selection of necessity stepped in, and would only
cease to act when the harmony with the environment had become complete,
or, at least, so nearly so that any increase of it could not heighten
the deception.

Of course, it is presupposed in the working out this selective process
that the species has enemies which see. This is the case, however, with
most animals living on the earth or in the water, unless they are of
microscopic minuteness. Many animals, too, are subject to persecution
not only in their adult state, but at almost every period of their
life, and so, in general, we should expect that many of them would have
attained at each stage that coloration of body that would render them
least liable to discovery by their enemies.

And this is in reality the case: numerous animals are protected in some
measure by so-called sympathetic colouring, from the egg to the adult
state.

Let us begin with the egg, and of course there is no need to speak of
any eggs except those which are laid. Of these many are simply white
in colour, e.g. the eggs of many birds, snakes, and lizards, and this
seems to contradict our prediction; but these eggs are either hidden in
earth, compost, or sand, as in the case of the reptiles, or they are
laid in dome-shaped nests, or concealed in holes in trees, as in many
birds; thus they require no protective colouring.

In other cases, however, numerous eggs, especially of insects and
birds, possess a colouring which makes it very difficult to distinguish
them from their usual surroundings. Our large green grasshopper
(_Locusta viridissima_) lays its eggs in the earth, and they are
brown, exactly like the earth which surrounds them. They are enough
in themselves to refute the hypothesis that sympathetic colouring has
arisen through self-photography, for these eggs lie in total darkness
in the ground. Insect-eggs which are laid on the bark of trees are
often grey-brown or whitish like it, and the eggs of the humming-bird
hawk-moth (_Macroglossa stellatarum_), which are attached singly to the
leaves of the bedstraw, have the same beautiful light-green colour as
these leaves, and, in point of fact, green is a predominant colour of
the eggs in a very large number of insects.

But the eggs of many birds, too, exhibit 'sympathetic' colouring; thus
the curlew (_Numenius arquata_) has green eggs, which are laid in the
grass; but the red grouse (_Lagopus scoticus_) lays blackish-brown
eggs, exactly of the colour of the surrounding moor-soil; and it has
been observed that they remain uncovered for twelve days, for the
hen lays only one egg daily, and does not begin to brood until the
whole number of twelve is complete. Herein lies the reason of the
colour-adaptation, which the eggs would not have required, if they had
always been covered by the brooding bird.

The eggs of birds are frequently not of one colour only; those of the
Alpine ptarmigan (_Lagopus alpinus_), for instance, are ochre-yellow
with brown and red-brown dots, resembling the nest, which is carelessly
constructed of dry parts of plants. Sometimes this mingling of colours
reaches an astonishing degree of resemblance to surroundings, as in the
golden plover (_Charadrius pluvialis_), whose eggs, like those of the
peewit (_Vanellus cristatus_), are laid among stones and grasses, not
in a true nest, but in a flat depression in the sand, and, protected
by a motley speckling with streaking of white, yellow, grey and brown,
are excellently concealed. Perhaps the eggs of the sandpipers and gulls
are even better protected, for their colouring is a mingling of yellow,
brown, and grey, which imitates the sand in which they are laid so
perfectly, that one may easily tread on them before becoming aware of
them.

But let us now turn from eggs to adult animals. Darwin first pointed
out that the fauna of great regions may exhibit one and the same
ground-colouring, as is the case in the Arctic zone and in the deserts.
The most diverse inhabitants of these regions show quite similar
coloration, namely, that which harmonizes with the dominant colour of
the region itself. It is not only the persecuted animals, which need
protection, that are sympathetically coloured in these cases, the
persecutors themselves are likewise adapted, and this need not surprise
us, when we remember that the very existence of a beast of prey
depends on its being able to gain possession of its victims, and that
therefore it must be of the greatest use to it to contrast as little as
possible with its surroundings, and thus be able to steal on its quarry
unperceived. Those that are best adapted in colour will secure the most
abundant food, and will reproduce most prolifically; and they will thus
have a better prospect of transmitting their usual colouring to their
offspring. The Polar bear would starve if he were brown or grey, like
his relatives; among the ice and snow of the Polar regions his victims,
the seals, would see him coming from afar.

In the Arctic zone the adaptation of the colouring of the animals to
the white of the surroundings is particularly striking. Most of the
mammals there are pure white, or approximately white, at least during
the long winter; and it is easily understood that they must be so if
they are to survive in the midst of the snow and ice,--both beasts of
prey and their victims. For the latter the sympathetic colouring is of
'protective' value; for the former, of 'aggressive' value (Poulton).
Thus we find not only the Polar hare and the snow-bunting white, but
also the Arctic fox, the Polar bear, and the great snowy owl; and
though the brown sable is an exception, that is intelligible enough,
for he lives on trees, and is best concealed when he cowers close to
the dark trunk and branches. For him there would be no advantage in
being white, and therefore he has not become so.

Desert animals are also almost all sympathetically coloured, that
is, they are of a peculiarly sandy yellow, or yellowish-brown, or
clayey-yellow, or a mixture of all these colours; and here again the
beasts of prey and their victims are similarly coloured. The lion must
be almost invisible from a short distance, when he steals along towards
his prey, crouching close to the ground; but the camel too, the various
species of antelope, the giraffe, all the smaller mammals, and also the
horned viper (_Vipera cerastes_), the Egyptian spectacled snake (_Naja
haje_), many lizards, geckos, and the great Varanus, numerous birds,
not a few insects, especially locusts, show the colours of the desert.
It is true that the birds often have very conspicuous colours, such as
white on breast and under parts, but the upper surface is coloured like
the desert, and conceals them from pursuers whenever they cower close
to the ground. It has even been observed that a locust of the genus
_Tryxalis_ is of a light sand-colour in the sandy part of the Libyan
desert, but dark brown in its rocky parts, thus illustrating a double
adaptation in the same species.

Another group, which agrees in colour with the general surroundings,
is that of the 'glass-animals,' as they have been called, though
perhaps 'crystal animals' is a better term. A great number of simple
free-swimming marine forms, and a few fresh-water ones, are quite
colourless, and perfectly transparent, or have at most a bluish or
greenish tinge, and on this account they are quite invisible as long as
they remain in the water. In our lakes there lives a little crustacean
about a centimetre in length, of the order of water-fleas (_Leptodora
hyalina_), a mighty hunter among the smallest animals, which swims
forward jerkily with its long swimming-appendages, and widely spreads
its six pairs of claws, armed with thorny bristles, like a weir basket,
to seize its prey. We may have dozens of these in a glass of water
without being able to see a single one, even when we hold the glass
against the light, for the creatures are crystal-clear and transparent,
and have exactly the same refractive power as the water. It requires a
very sharp scrutiny and a knowledge of the animals to be able to detect
in the water little yellowish stripes, which are the stomachs of the
animals filled with food in process of digestion, for which, as we can
readily understand, invisibility cannot very well be arranged. If the
water be then strained through a fine cloth, a little gelatine-like
mass of the bodies of the _Leptodora_ will remain on the sieve.

A great many of the lower marine animals are equally transparent,
and as clear as water; most of the lower Medusæ, the ctenophores,
various molluscs, the barrel-shaped Salpæ, worms, many crustaceans of
quite different orders, and above all an enormous number of larvæ of
the most diverse animal groups. I can remember seeing the sea at the
shore at Mentone so full of Salpæ, that in every glass of sea-water
drawn at random there were many of them, and sometimes a glass held a
positive animal soup. But one did not see them in the glass of water,
and only those who knew what to look for recognized them by the bluish
intestinal sac that lies posteriorly in the invisible body. But when
the water was poured off through a fine net, there remained on the
filter a large mass of a crystalline gelatinous substance.

It is obvious that this must serve as a protective arrangement, for the
animals are not seen by their pursuers; but it is not an _absolute_
protection, for they have many pursuers who do not wait till they see
their prey, but are almost constantly snapping the mouth open and
shut, leaving it to chance to bring them their prey. _No protective
arrangement, however, affords absolute security_; it protects against
some enemies, perhaps against many, but never against all.

But now let us turn to a group of a different colouring, the green
animals. We are familiar with our big grass-green grasshopper, and
we know how easily it is overlooked when it sits quietly on a high
grass-stem, surrounded by grasses and herbage; the light grass-green of
its whole body protects it most effectively from discovery: for myself,
at least, I must confess that in a flowery meadow I have stood right
in front of one, and have looked close to it for a long time without
detecting it. In the same way countless insects of the most diverse
groups--bugs, dipterous flies, sawflies, butterflies--and especially
the larvæ (caterpillars) of the last, are of the same green as the
plants on which they live, and this again applies to the predaceous
species, as well as the species preyed upon. Thus the rapacious
praying-mantis (_Mantis religiosa_) is as green as the grass in which
it lurks motionless for its victim--a dragonfly, a fly, or a butterfly.

There are also green spiders, green amphibians like the edible frog,
and especially the tree-frog, green reptiles like lizards and the
tree-snakes of tropical forests. It is always animals which live among
green that are green in colour.

We may wonder, for a moment, why there are so few green birds, since
they spend so much of their time among the green leaves. But this
paucity of green birds is only true of temperate climates. In Germany
we have only the green woodpecker, the siskin, and a few other little
birds, and even these are not of a bright green, but are rather
greyish-green. The explanation lies in the long winter, when the
trees are leafless. In the evergreen forests of the tropics there are
numerous green birds belonging to very diverse families.

Yet another group with a common colour-adaptation deserves mention--the
beasts of the night. They are all more or less grey, brown, yellowish,
or a mixture of these colours, and it is obvious that, in the
duskiness of night, they must blend better with their environment on
this account. White mice and white rats cannot exist under natural
conditions, since they are conspicuous in the night, and the same would
be true of white bats, nightjars, and owls; but all of these have a
coloration suited to nocturnal habits.

A very remarkable fact is that in many animals the colour-adaptation
is a double one. Thus the Arctic fox is white only in winter, while in
summer he is greyish-brown; the ermine changes in the same way, and the
great white snowy owl of the Arctic regions has in summer a grey-brown
variegated plumage. Many animals which are subject to persecution
also change colour with the seasons, like the mountain hare (_Lepus
variabilis_), which is brown in summer and pure white in winter, the
Lapland lemming, and the ptarmigan (_Lagopus alpinus_), which do the
same. It has been doubted whether natural selection can explain this
double coloration, but I do not know where the difficulty lies, and
there is certainly no other principle whose aid we can evoke. The
mountain hare must have had some sort of colour before it attained to
seasonal dimorphism. Let us assume that it was brown, that the climate
became colder and the winter longer, then those hares would have most
chance of surviving which became lighter in winter, and so a white race
was formed. Poulton has shown that the whiteness is due to the fact
that the dark hairs of the summer coat grow white as they lengthen at
the beginning of winter, and the abundance of new hairs which complete
the winter coat are from the first white throughout. If the white hairs
were to persist throughout the summer it would be very disadvantageous
to their wearer; so a double selection must take place, in summer the
individuals which remain white, in winter those which remain brown,
being most frequently eliminated, so that only those would be left
which were brown in summer and white in winter. This double selection
would be favoured by the fact that there would be, in any case, a
change of fur at the beginning of summer; the winter hairs fall out
and the fur becomes thinner. The process does not differ essentially
from that which takes place in any species when two or more parts or
characters, which are not directly connected, have to be changed, such
as, for instance, colour and fertility. The struggle for existence will
in this case be favourable, on the one hand, to the advantageously
coloured, and on the other to the most fertile, and though the two
characters may at first only occur separately, they will soon be united
by free crossing, until ultimately only those individuals will occur
which are at once the most favourably coloured and the most fertile.
So in this case there remain only those which are brown in summer and
white in winter.

We must ascribe to the influence of the processes of selection the
exact regulation of the duration of the winter and summer dress, which
has been carefully studied in the case of the variable hare. In the
high Alps it remains white for six or seven months, in the south of
Norway for eight months, in Northern Norway for nine months, and in
Northern Greenland it never loses its white coat at all, as there
the snow, even in summer, melts only in some places and for a short
time. But apart from concealment there is certainly another adaptation
involved here--namely, the growth of the hair as a protection against
the cold. From an old experiment made in 1835 by Captain J. Ross, and
recently brought to light again by Poulton, we learn that a captive
lemming kept in a room in winter did not change colour until it was
exposed to the cold. The constitution of animals which become white in
winter is thus so organized that the setting in of cold weather acts
as a stimulus which incites the skin to the production of white hairs.
This predisposition also we must refer to the influence of natural
selection, since it must have been very useful to the species that the
winter coat should grow just when it was necessary as a protection
against cold. This explains at the same time why the predisposition
to respond to the stimulus of cold by a growth of winter fur finds
expression earlier in those colonies of Arctic animals, such as the
hare, which live in Lapland, than in those which live in the south of
Norway.

But that it is not the _direct_ influence of cold which colours the
hair of a furred animal white we can see from our common hare (_Lepus
timidus_), which, in spite of the winter's cold, does not become white,
but retains its brown coat, and not less so from the mountain hare
(_Lepus variabilis_), which in the south of Sweden also remains brown,
although the winter there may be exceedingly cold. But as the covering
of the ground with snow is not so uninterrupted there as in the higher
North, a white coat would be not a better protection than a brown
one, but a worse. The white colouring of Arctic animals is therefore
not directly due to the influence of the climate, as has often been
maintained, but is due to it indirectly, that is, through the operation
of natural selection. I have tried to make this clear by means of this
example, so that we may not have to repeat it in considering those
which are to follow.

       *       *       *       *       *

But all attempts at any other explanation are even more decidedly
excluded when we turn our attention to more complicated cases of
colour-adaptation, which are not confined to the simple, general
coloration, but are helped by markings and colour-patterns, that is, by
schemes of colour.

Thus numerous caterpillars exhibit definite lines and spots on their
ground-colouring, which, in one way or another, aid in protecting them
from their enemies.

The green grass-eating caterpillar of many of our _Satyridæ_ has two or
more darker or lighter lines running down the sides of its body, which
make it much less conspicuous among the grasses on which it feeds than
if it were a uniform green mass (Fig. 2). Not infrequently the colour
and form present a remarkably close resemblance to the inflorescences
or fruit-ears of the grasses. Caterpillars marked thus are never found
on the leaves of trees, where they would immediately catch the eye. It
is true that longitudinal striping often occurs on caterpillars which
live on other plants besides grass, but as these other plants grow
among the grasses the protective efficacy is just the same. This is
the case with the Pieridæ (Garden Whites).

All the caterpillars of our Sphingidæ, on the other hand, which live
on bushes and trees, have on the sides of the segments light oblique
stripes, seven in number, which are disposed to the longitudinal axis
of the body at the same angle as the lateral veins of a leaf of their
food-plant have to the mid-rib. It cannot of course be said that the
caterpillar thereby gains the appearance of a leaf, indeed, if one sees
it apart from its food-plant it does not look in the least like a leaf,
but among the leaves of a bush or tree this marking secures it in a
high degree from discovery. Thus the caterpillar of the eyed hawk-moth
(_Smerinthus ocellatus_), when it is sitting among the crowded foliage
of a willow, is often very difficult to find, because its large green
body does not appear as a single green spot, but is divided by the
oblique lateral stripes into sections like the half of a willow leaf,
so that even a searching glance is led astray, there being nothing to
focus attention on the animal as distinguished from its surroundings
(Fig. 3). As a boy I often had the interesting experience of
overlooking a caterpillar which was sitting just before me, until after
a time I chanced to hit upon the exact spot in the field of vision.

[Illustration: FIG. 2. Longitudinally striped caterpillar of a Satyrid.
After Rösel.]

[Illustration: FIG. 3. Full-grown caterpillar of the Eyed Hawk-moth,
_Smerinthus ocellatus_. _sb_, the subdorsal stripe.]

In the majority of these caterpillars with oblique stripes, the
likeness to the half of a leaf is heightened by the fact that the light
oblique row is accompanied by a broader coloured band, suggesting the
shade of the leaf's mid-rib. The caterpillar of _Sphinx ligustri_
has a lilac band, and that of _Sphinx atropos_ a blue one. In both
cases it is difficult to believe that such striking colours can
secure the animals from discovery, yet among the blending shadows
of the leaf-complex of their food-plant they greatly increase their
resemblance to a leaf-surface. Of the death's-head caterpillar
(_Sphinx atropos_) this sounds almost incredible, for this form is
chiefly a bright golden yellow, and the narrow white oblique stripes
have sky-blue borders becoming darker towards the under side; but it
must not be forgotten that the potato is not the true food-plant of
the species, for it lives, in its true home in Africa, and also in the
south of Spain, on wild solanaceous plants, which, we are informed
by Noll, have precisely these colours--golden-yellow and blue in the
blossom, the fruit, and in part also in the leaves and stem. There the
caterpillars sit the whole day long on the plants, while with us they
have formed the habit of feeding only in the twilight and at night, and
concealing themselves in the earth by day, a habit that is found in
other caterpillars also, and which we must again ascribe to a process
of natural selection.

[Illustration: FIG. 4. Full-grown caterpillar of the Elephant Hawk-moth
(_Chærocampa elpenor_) in its "terrifying attitude."]

Some caterpillars exhibit other, more complex markings, which do not
protect them by rendering them difficult to detect, but by terrifying
the enemy who has discovered them, and warning him away. Such
terrifying or aggressive colours are to be found, for instance, in the
caterpillars of the Sphingid genus _Chærocampa_ in the form of large
eye-like spots, which occur in pairs close together on the fourth
and fifth segments of the animal. Children and those unfamiliar with
animals take these for true eyes; and as the caterpillar, when it is
threatened by an enemy, draws in the head and anterior segments, so
that the fourth one is greatly distended, the eye-spots seem to stand
on a thick head (Fig. 4), and it cannot be wondered at that the smaller
birds, lizards, and other enemies are so terrified that they refrain
from attacking. Even hens hesitate to seize such a caterpillar in its
defiant attitude, and I once looked on for a long time in a hen-coop
while one hen after another rushed to pick up a caterpillar I had
placed there, but, when close to it, hastily drew back the head already
prepared to strike. Even a gallant cock was a long time in making
up his mind to attack the terrible beast, and drew back repeatedly
before he at length ventured to strike a deadly blow with his bill.
After the first stroke the caterpillar, of course, was lost. Thus even
this disguise is only a _relative_ protection, effective only against
smaller enemies. But that these are really frightened away, I had once
an opportunity of observing, when I put a caterpillar of the common
elephant hawk-moth (_Chærocampa elpenor_) in the feeding-trough of a
hencoop, and a sparrow flew down to feed from the trough. It descended
at first with its back to the caterpillar and fed cheerily. But when by
chance it turned round, and spied the caterpillar, it scurried hastily
away.

Among Lepidoptera, too, eye-spots often occur on the wings, and to
some extent, at least, they have in this case also the significance of
warning marks. Take, for instance, the large blue and black eye-spots
on the posterior wings of the eyed hawk-moth (_Smerinthus ocellatus_).
When the insect is sitting quietly the two spots are not visible, as
they are covered by the anterior wings, but as soon as the creature is
alarmed it spreads all four wings, and now both eyes stand boldly out
on the red posterior wings and alarm the assailant, as they give the
impression of the head of a much larger animal (see Fig. 5). There are
also eye-like spots which have not this significance and effect, as,
for instance, the 'eye-spots' on the train-feathers of the peacock and
the Argus pheasant, or the little eye-like spots on the under surface
of many diurnal butterflies. In the first case, it is a matter of
decoration; in the second, perhaps of the mimicry of dewdrops, which
increases still further the resemblance to a withered leaf; but there
are undoubtedly many cases in which the eye-spots serve as means of
frightening off enemies, and these cases are especially common among
butterflies.

[Illustration: FIG. 5. The Eyed Hawk-moth in its 'terrifying attitude.']

Such warning marks are in no way contradictory to the sympathetic
colouring of the rest of the body, and indeed we usually find them
in combination with it. In some cases the eye-spot, though very
conspicuous, is covered, as in the eyed hawk-moth, when at rest, by
the sympathetically coloured parts--in this instance the anterior
wings. In other cases eye-spots of considerable size lie clearly
exposed, but exhibit the same sympathetic colours as the whole of the
rest of the wing-surface. In this case they do not interfere with
the protective influence of general colouring, because they are only
visible from a very short distance. This is the case in the large
_Caligo_ species of South America, which only fly for a short time in
the early morning and in the evening, remaining concealed throughout
the day in dark shadowy places, where the mingled colouring of brown,
grey, yellow, and black on the under surfaces of the wings prevents
their being recognized from a distance as butterflies at all. But even
the best sympathetic colouring is not an absolute protection, and when
the insect is discovered by an enemy near at hand, the terrifying mark,
a large deep-black spot on the posterior wing, comes into effect, and
scares the assailant away.

[Illustration: FIG. 6. Under surface of the wings of _Caligo_.]

In such cases the sympathetic colouring was probably the first to
arise, and the eye-spot was developed later by a new process of
selection, brought about by the necessity of protecting the species
more effectively than by mere inconspicuousness alone. In many cases
it can be proved that the power of scaring off an enemy did not begin
with the formation of the eye-spot, but with the development of a new
instinct. When the caterpillar of _Chærocampa elpenor_ is attacked
it immediately assumes the defiant attitude described above, but the
same striking attitude is assumed by the caterpillars of the allied
American genus _Darapsa_, as I learn from an old illustration by Abbot
and Smith, although this form possesses no eye-spots (Fig. 7). Thus,
then, metaphorically speaking, the caterpillar at first attempted
to scare off its enemy by a terrifying attitude alone, and it was
only subsequently, in the course of the phyletic evolution, that the
eye-spots were added, in the elephant hawk-moths and other species,
to heighten the terrifying effect. But that the eye-spot did not make
its appearance suddenly is proved by several American species of
_Smerinthus_, in which they are much less perfectly developed than in
the European species. In these Sphingidæ, too, the defiant attitude
was evolved earlier than the eye-spots, as we may see from our poplar
hawk-moth (_Smerinthus populi_), which, when alarmed, spreads out all
four wings in the same peculiar manner which in the eyed hawk-moth
(_Smerinthus ocellatus_) displays the eye-spots; it strikes about with
its wings as if to scare off the enemy, an effect which will certainly
be more surely achieved if, at the same time, a pair of eyes becomes
suddenly visible.

Sympathetically coloured caterpillars are, however, by no means the
only ones; there are some with such striking, glaring colours that,
far from rendering their possessors inconspicuous, they make them
visible from a long way off; but this apparent contradiction of the
theory of the colour-adaptation of animals that require protection
has been explained by the acuteness of Alfred Russel Wallace. We know
that among insects, and also among caterpillars, there are many which
have a repulsive taste. In any case, certain caterpillars are rejected
by many birds and lizards. Such species are, therefore, relatively
safe from being devoured. If they were protectively coloured, or if,
moreover, they resembled caterpillars with an agreeable taste, they
would gain little advantage from their unpalatability; for the birds
would at first take them for eatable, and would only discover their
repulsiveness on attempting to eat them. But a caterpillar which has
received a single stroke from a bird's bill is doomed to death. It must
therefore be of the greatest advantage for unpalatable caterpillars,
and unpalatable animals generally, to be in their colouring as
conspicuously distinguishable as possible from the edible species.
Hence, then, the glaring colours, which we can now refer without any
further difficulty to the process of natural selection, for every
individual of an ill-tasting species that is more conspicuously
coloured than its fellows must have an advantage over them, and must
have a better chance of surviving, because it will be less easily
mistaken for a member of an edible species.

[Illustration: FIG. 7. Caterpillar of a North American _Darapsa_ in its
"terrifying attitude" (after Abbot and Smith).]

I should like to discuss one other phenomenon, which is well calculated
to give us a deeper insight into the transformation processes of
organisms--I refer to the remarkable dimorphism of colour which occurs
in many of the species of caterpillar just described.

The caterpillar of the convolvulus hawk-moth (_Sphinx convolvuli_) is
in its full-grown stage green, like the wild convolvulus on which it
lives, or brown like the ground on which its food-plant grows. It thus
shows a double adaptation, each of which is capable of protecting it
to a certain extent, and we might think to the _same_ extent. But that
is not so, the brown colouring is a more effective protection than the
green, as we may learn from two facts. In the first place, the four
young stages of the caterpillar are green, and it only becomes brown in
the last stage, though sometimes even then it remains green. This shows
that the brown is a relatively modern adaptation, and it could not have
arisen had it not been better than the original green. In the second
place, the green-coloured caterpillars of the convolvulus hawk-moth
are nowadays much less numerous than the brown ones, and this implies
that the latter survive oftener in the struggle for existence. We have
here an interesting case of an easily recognizable process of selection
still going on between the old green and the newer brown variety.

It is hardly necessary to ask why the brown colour should in this case
be a better protection than the green, for it is obvious that such a
large green body as that of the full-grown convolvulus-caterpillar
would be but badly concealed among the little leaves of the convolvulus
plant in spite of its green colour; while the brown caterpillar, on
the brown soil, with its pebbles, hollows, and irregular shadows, is
excellently protected, especially if it passes the day concealed in the
ground, as is actually the case.

Our view is materially strengthened by the fact that the same
phenomenon of double colouring occurs in several allied species of
Sphingidæ, but in a manner which shows us that we have to do with a
similar process of transformation, only at a more advanced stage. The
caterpillar of _Chærocampa elpenor_ (Fig. 4) shows the same state of
things as that of the convolvulus hawk-moth; it is brown or green,
and the green form is the less common. But in the two other European
species of _Chærocampa_ the full-grown caterpillar is always brown,
and indeed it becomes brown in the fourth stage, instead of, like
_Chærocampa elpenor_, only in the fifth and last. Another indigenous
sphingid species, _Deilephila vespertilio_, only remains green during
the first two stages, and assumes in the third stage the grey-brown
colour which it afterwards retains. The dark colour has obviously
prevailed among the full-grown caterpillars for a considerable
length of time, for it is in this, the largest and most conspicuous
stage, that the change of colour must have been most necessary, and
consequently the process of selection must have begun in it, and only
after the more protective brown became general would it have extended
to the next stage below, if it were of use there too, and, later on,
to still earlier stages in the life-history.

One might be inclined to ascribe this shunting back of a new character
from the later to the earlier stages of development to purely internal
forces, which brought it about of necessity, and quite independently
of whether the extension of the character was useful or injurious.
We shall come back to this later, and try to find out how far this
is the case, but in the meantime we may regard at least so much as
established, that this shunting back does not take place everywhere and
without limits, but that natural selection calls a halt as soon as its
effect would be injurious.

[Illustration: FIG. 8. Caterpillar of the Buckthorn Hawk-moth,
_Deilephila hippophaës_. _A_, Stage III. _B_, Stage V. _r_, ring-spots.]

There could be no continuance of insect-metamorphosis if every
character of the final stage had to be shunted back to the one next
below, for then, for instance, the characters of the butterfly must,
in the course of the phyletic evolution, be carried back to the pupa
and larva. But even in the larval stage alone it can be seen that
this carrying back is kept within well-defined limits. Thus, for
instance, in the dimorphic caterpillars of the Sphingidæ the brown of
the full-grown stage never comes so far down as the earliest stages,
for the little caterpillars are all green, like the leaves and stems
on which they sit. On the other hand, there are species in which the
green persists, as apparently the most advantageous colour. Thus in
the buckthorn hawk-moth (_Deilephila hippophaës_) (Fig. 8), which
lives in the warm valleys of the Alps, and especially in Valais, the
caterpillars are grey-green in all stages, and are exactly of the
shade of the lower surface of the buckthorn leaves; they possess no
oblique lines, for these would not make them more like the leaves, as
the full-grown caterpillars are much bigger than an individual leaf
of buckthorn, on which, moreover, the lateral veins are not very
conspicuous. Nevertheless the caterpillar enjoys very fair security, as
it does not feed through the day, but only in twilight and at night;
it passes the daytime concealed in the dry leaves and earth about the
base of the bush. Its resemblance to the leaves is very great, and is
increased by the fact that it bears on the last segment a comparatively
large orange-coloured spot (_r_), exactly the colour of the buckthorn
berry, which ripens just at the time that the caterpillar attains its
full growth.

But butterflies are as much persecuted, and have as much need of
protection, as caterpillars, and among them, too, we find many
instances of protective colouring, which are the more interesting in
that they occur, as a rule, only on such parts of the body as remain
visible when the insect is at rest, which is exactly what we should
expect if the coloration has been wrought out in the course of natural
selection. But it is well known that the resting position of diurnal
Lepidoptera is quite different from that of the nocturnal forms, and is
not even the same among all families, and in accordance with this we
find the sympathetic colouring occurs on quite different areas in the
different families.

The reason why the butterflies only require to be protected by their
colour in the sleeping or resting position is that no colour whatever
could make a flying butterfly invisible to its enemies, because the
background against which its body shows is continually changing during
its flight, and, moreover, the movement alone is enough to betray it,
even if it is of a dull colour.

Thus, in general, only those parts of a butterfly's wing that are
invisible at rest could safely bear bright or conspicuous colour, while
the visible portions had to acquire sympathetic coloration through
natural selection.

As the diurnal butterflies, when at rest, turn their wings upward and
bring them together, it is only the under side which is sympathetically
coloured, and that only as far as it is visible, that is, the whole of
the posterior wing, and as much of the anterior one as is not covered
by it. Many diurnal butterflies, when at rest, fold the anterior wing
so far back that only its tip remains visible, and in such cases only
this tip is protectively coloured, while in other forms, which have not
this habit, almost the whole surface of the wing is sympathetically
coloured.

A very simple protective colouring is exhibited by our 'lemon
butterfly' (_Rhodocera rhamni_), in which the under surface is a
whitish yellow, which protects the insect well when it settles on
the dry leaves on the ground in the light woods which it is fond of
frequenting.

Our gayest diurnal butterflies, the species of _Vanessa_, all have the
under surface of a dusky colour, sometimes passing into a blackish
brown, as in the peacock-butterfly, _Vanessa_ (_v. io_), sometimes
more into greyish brown, or brown-yellow, or reddish brown. They are
never simple colours, but always consist of mixtures of different
colour-tones--indeed, there is often a complex mingling of many
colours, as grey, brown, black, white, green, blue, yellow, and red,
made up of dots, strokes, spots, and rings, into a wonderful and
very constant pattern, which, taken as a whole, has the effect of
being uniform, and harmonizes with the soil, or with the highway, on
which the species loves to settle, with much greater accuracy than a
monochrome grey or brown would do. When the 'painted lady' (_Vanessa
cardui_) settles on the ground it is hardly distinguishable from
it, and this species in particular has a preference for settling
on the ground. Other species of _Vanessa_, such as the peacock and
the Camberwell beauty (_Vanessa antiopa_), are underneath of a dark
blackish grey, or even black; when resting they press themselves into
the darkest corners and crevices, and are thus most effectively secured
from discovery.

Many diurnal Lepidoptera, on the other hand, especially the
wood-butterflies of the family Satyridæ, have the habit of resting
on the trunks of trees, as _Satyrus proserpina_ does on the great
beech-trunks of the forest clearings. These large butterflies, coloured
conspicuously on the upper surface in deep velvety black and white, are
marked on the under surface exactly to match the whitish bark of the
great beech, covered over with white, grey, blackish-brown, and yellow
spots, and the butterfly whose flight one has just been carefully
following disappears as it suddenly alights on such a tree-trunk. As I
have already stated, the protective colour only extends over as much
of the insect as is seen when it is at rest. As the anterior wings are
folded far back between the posterior ones, the protective colouring is
limited to the whole surface of the posterior wing, and the tip of the
anterior one, as far as that is visible in the resting attitude; the
protectively coloured area is somewhat sharply bounded, and it is often
of very different extent in quite nearly allied species, according
to whether the species folds the anterior wing far back or not. Thus
in our common small tortoiseshell-butterfly (_Vanessa urticæ_) the
protective area is considerably wider than in the large tortoiseshell
(_Vanessa polychloros_), much as the two resemble each other in other
details.

This harmony between the wing tips and the posterior wings is nowhere
wanting, where the under side is protectively coloured at all, but in
many cases the protective colouring spreads over almost the whole of
the anterior wings, and these are then not folded far back when at
rest, as will be seen later in the so-called leaf-butterflies.

There is one genus of diurnal butterflies which seems to contradict
the law that all the surface that is visible in the resting position
exhibits the protective coloration--the South American wood-butterflies
of the genus _Ageronia_. They have on the upper surface a very
complicated bark-like pattern of mingled grey on grey, and this
confirms the usual rule, for we know that these butterflies--a striking
exception among all the other diurnal forms--settle with outspread
wings on the trunk of a tree in exactly the same attitude as many of
the nocturnal Lepidoptera of the family of the Loopers or Geometridæ,
in which the upper surface is also deceptively like the bark of the
tree on which they rest.

[Illustration: FIG. 9. _Hebomoja glaucippe_, from India; under surface.
_A_, in flight. _B_, in resting attitude.]

In all the nocturnal Lepidoptera it is the _upper_ side of the wing
which is sympathetically coloured, if protective coloration has been
developed at all. In all the Sphingidæ, many 'Owls' and Bombycidæ, the
anterior wings are grey banded with darker zigzag lines, and mottled
with many shades of black, grey, yellow, red, and even violet. As the
anterior wings cover the body and the posterior wings like a roof,
they make the resting insect very inconspicuous when it has settled
on wooden fences, trunks of trees, or even old timber. When bright
colours--red, yellow, or blue--occur in these moths it is always on
the posterior wings, which are covered when at rest. This can best be
observed in the species of the genus _Catocala_.

Let us now, however, interrupt our survey of the facts for a moment,
and let us inquire whether all the cases of protective colouring in
Lepidoptera we have considered can be referred to natural selection, or
whether it is not conceivable that other causes may have evoked them.

[Illustration: FIG. 10. _Xylina vetusta_, after Rösel. _A_, in flight.
_B_, at rest.]

The first thing to be said is that the Lamarckian principle of the
inherited effects of use and disuse cannot here be taken into account,
as the colours of the surface of the body do not exercise any active
function at all; their effect is due simply to their presence, and
it is for them quite indifferent whether and how often they have
opportunity to protect their bearers from enemies, or whether no
enemies ever chance to appear. It has frequently been suggested, too,
that these colorations are associated with the differences in the
strength of the illumination to which the different parts and surfaces
are exposed. But this again is untenable, as is proved even by the
dimorphism frequently occurring in caterpillars, for the green and
the brown individuals are exposed to precisely the same light; and
still more clearly by the sympathetic colouring, which is so exactly
defined and yet so different on the under surface of the diurnal
butterflies. Yet there are isolated cases in which it seems as if
the direct influence of the light had brought about certain striking
differences in the colouring of the parts of an insect, and I shall
describe perhaps the prettiest of these cases, to which Brunner von
Wattenwyl directed attention. It concerns one of the Orthoptera of
Australia, a Phasmid, _Tropidoderus childreni_, Grey, which has a
general colouring of leaf-green, but with singular deviations from it
on certain areas of the body. In this insect the anterior wings which
form the wing covers or elytra (Fig. 11, _V_) are so short that they
scarcely cover the half of the long abdomen. Their place is taken by
the anterior margin of the posterior wing (_H. horn_), which is hard
and horny like the elytra, and in the resting position protects the
whole abdomen. All these covering parts are grass-green, except at the
places where they overlap; on these areas they have a faded look, and
are yellowish instead of green. Brunner says of this: 'The phenomenon
gives the impression that the more brilliant colour is a character due
to daylight. If several sheets of white paper of unequal dimensions be
placed one above the other, ... and exposed to the sun, after a short
time silhouettes of the smaller sheets will appear on the larger ones,
either in a lighter or in a darker colour. Probably this "fading"
of the covered parts in the Phasmid also belongs to this "category
of photographs."' This seems convincing, but analogous phenomena in
other insects prevent our regarding the pretty comparison with the
photograph as a sufficient explanation. If it were a question of a
diurnal butterfly, such an assumption would have to be rejected on this
ground alone, that the wing colouring is developed in the pupa, and
appears perfect and unalterable as soon as the perfect insect emerges.
But in the pupa the position of the wings is exactly the reverse
of that seen in the resting attitude of a butterfly, that is, the
protectively coloured under side of the wing is not turned towards the
light but away from it. Moreover, in the pupa the anterior wings cover
the posterior ones completely, no matter what the wing position may be
later in the perfect insect. Furthermore, the thick and often darkly
coloured sheath of the pupa prevents the light having any effect, and
not a few species pass their pupal stage in such dark places--for
instance, under stones, as in the case of many 'Blues'--that the light
can hardly reach them. And if the light did exercise an influence, how
could it produce such diverse coloration as the protective colours of
diurnal butterflies, on the one side dark, even to blackness, on the
other side, yellow, reddish, and even white and pure green; and how
should the same rays of light call forth complicated colour patterns
on one and the same surface, for instance, the white, sprinkled with
green, of the Aurora butterfly (_Anthocharis cardaminis_)? Finally, we
have only to remember that numerous nocturnal Lepidoptera pass through
their pupa stage underground, although they exhibit brilliant as well
as protective colours in the most appropriate distribution, to reject
once for all the hypothesis that the influence of light plays any
decisive rôle in determining the distribution of the colours on the
wings of Lepidoptera.

But it is otherwise with _Tropidoderus_. In this case the wings grow
gradually during the slow growth of the animal, which takes place in
full light, and the wings of the young insect probably lie one above
the other, in exactly the same position, and cover the same places as
in the full-grown form; we might, therefore, from the facts of the
case, admit the possibility that the yellow of the covered portions is
due to the exclusion of light.

[Illustration: FIG. 11. _Tropidoderus childreni_, after Brunner von
Wattenwyl, in flying pose. _V_ anterior wing. _H. häut_, membranous
part of posterior wing. _H. horn_, horny portion.]

But as soon as the conditions that obtain among Lepidoptera are
also taken into consideration we recognize the insufficiency of the
interpretation suggested, for among butterflies we have precisely the
same phenomenon--sharp limitation of the protective colouring to the
parts visible in the resting position, a fact which, in the case of
the said butterflies, admits of no other interpretation than that of
natural selection. Let us therefore see if we cannot, in the case of
_Tropidoderus_, arrive at some better understanding of the phenomenon
than that implied in the theory of direct light-influence. Obviously,
the yellow parts of the animal do not require to be green, since they
are not visible in the sitting position, and the locust in flight could
not by any device be made invisible. It therefore only remains to be
explained why the yellow parts are not colourless, and why they are
not also green. We cannot at present answer with any confidence; it is
possible that the colouring matter which causes the green only becomes
green under the influence of direct sunlight, and otherwise remains
yellow; it is possible, too, that, as in Lepidoptera (see Fig. 9), the
full protective colour is only developed by natural selection in the
places which are visible in the sitting position, and that the covered
parts take on any indifferent colour, which might be readily afforded
by the metabolism of the insect. But this much is certain, that the
covered parts would be green, if that were advantageous to the survival
of the species, just as the under surface of the wings of some diurnal
butterflies is green. Had it been required, the green colour would have
resulted in the course of natural selection, just as it has resulted
in the most different parts of the most diverse insects, even in those
whose development takes place entirely removed from the influence of
light. Therein lies the difference between our interpretation and that
of Brunner von Wattenwyl: without natural selection no explanation of
this case is possible.

[Illustration: FIG. 12. _Notodonta camelina_, after Rösel. _A_, in
flight. _B_, at rest.]

Hitherto I have spoken only of the diurnal butterflies in which the
anterior wings show an extension of the protective colouring which
marks the whole surface of the posterior wings, and it was always the
tips of the anterior wings that were thus coloured. But among the
nocturnal Lepidoptera there are corresponding cases, in which a little
tip of the posterior wing forms the continuation of the protective
surface of the anterior wing. Some species of _Notodonta_ and allied
genera show in the posterior corner of the otherwise whitish posterior
wings a little grey spot, and a hair tuft which in colour, and--when it
is big enough--in marking, exactly resembles the protectively coloured
anterior wings (Fig. 12). The 'why' is at once clear, when one looks at
the insect in the resting position, for only this little corner of the
wing projects beyond the covering anterior wing. This has been regarded
as telling against natural selection, for such a little spot could not
possibly, by its colour, turn the scale as to the life or death of the
individual, and so could not be selected. But one might say the same of
the tip of the anterior wing in the diurnal forms, although there the
protective surface is larger, often much larger. But who is to decide
how large an exposed, unprotected spot must be in order to attract
the attention of an enemy on the look out for food? Or who can prove
that the best and most familiar protective colouring really protects
its possessors? What if, after all, it is all a game, a joke, which
the Creator is playing with us poor mortals? Did not a trustworthy
observer recently watch carefully, and see how a pair of sparrows daily
cleared a wooden fence on which moths of the genus _Catocala_ and other
species of nocturnal Lepidoptera, excellently furnished with protective
colours, were wont to settle by day? They did their work thoroughly,
and hardly overlooked a single individual. But who has a right to see
anything more in this than--what surely goes without saying--that the
best protective colouring is not an absolute protection, and never
preserves all from destruction, but always only some, and it may be
very few.

How else could there be such a high ratio of elimination, and such a
constancy in the number of individuals of a species on any unchanging
area? These sparrows had simply made full use of an experience,
probably acquired by chance to begin with, and their vision had become
sharpened for this particular species on the almost similarly coloured
wooden fence, just as that of the expert butterfly collector does. It
certainly does not follow from this that the protective colouring was
useless, nor can we regard the harmony between the protruding tip of
the anterior or posterior wing and the large protectively coloured
surface of the covering wing as of no importance. On the contrary, if
the tips were white or conspicuously coloured like the rest of the
posterior wing, they would assuredly attract the sharp eye of hungry
enemies to the spot, and so betray the victim. Instead of this, the
spot in question is not only dark, but, in the case of _Notodonta_, is
furnished with a tuft of hairs, which, in the insect's resting position
(Fig. 12, _B_), lies on the back, and looks like a dark, somewhat
curved projecting tooth, in front of which there stands another, quite
similar, which arises from the anterior wing, and behind there are
other seven, rather smaller, dark teeth of the same kind, springing
from the outer edge of the anterior wing. Taken altogether, they mimic
the dentated edge of a withered leaf, and thus, in spite of their
diverse origins, form a unified picture, and one with a considerable
protective value. How is it possible to doubt that each of these
hair-tufts has arisen under the influence of natural selection, and
that its absence or imperfect development might result in the discovery
and elimination of the insect concerned?

These cases seem to me particularly beautiful proofs of the productive
efficiency of selection. The wing is protected just as far as it
protrudes from beneath the other--not a millimetre further! How should
it be otherwise, when the colouring of the parts just beside these is
indifferent for the species, so that any variations in these parts in
the direction of protective colouring never survive to be transmitted
and accumulated?

It is precisely this restriction to what is absolutely necessary that
is the surest sign, here and elsewhere, that the character in question
has been brought about by natural selection. And if this is the only
possible, and at the same time quite sufficient explanation of the
remarkably well-defined colour deliminations in all Lepidoptera,
there can be no reason why we should try to drag in any other factor
to explain the case of _Tropidoderus_, the less so as here again
selection alone can account for the green of the exposed surfaces;
and furthermore, the modification, common in other Phasmidæ, of the
most anterior green stripe of the posterior wing into a firm cover
protecting the soft abdomen, also points to natural selection; the
cover-wings proper have here become too short, and so the edge of
the posterior wing has been modified into a hard rib, which protects
the soft body of the insect (Fig. 11, _H. horn_). No differences in
illumination, and no _direct_ effect of any external influence whatever
could have brought that about.

How much more I might adduce in this connexion! The manifold diversity
of colour and form adaptation is so great among insects, to which
protection from their enemies is so necessary, and especially among
butterflies, that I should never come to an end if I were to try to
give even an approximate idea of it. Let us, therefore, turn now from
such cases to a higher--the highest--grade of adaptation, that in which
there is not only a mimicry of special and complex coloration, but in
which the whole animal has become like some external object, and is
thereby secured from discovery.

We must first consider the case of our lappet moth (_Gastropacha
quercifolia_), which in its copper-red colour and in the remarkable
shape and dentated edges of its wings, and finally in the quite
extraordinary clucking-hen-like attitude of the wings when at rest,
greatly resembles some dry oak-leaves lying one above the other.

Not unlike this is a 'shark' moth found in this country, _Xylina
obsoleta_, which, as the name indicates, looks when at rest like a
broken bit of half-rotten wood (Fig. 10, p. 77). It 'feigns death,' as
we commonly say, that is, it draws the legs and antennæ close to the
body, and does not move; indeed, one may lift it up and throw it on the
ground without its betraying by a single twitch that it lives. Only
after it has been left undisturbed for some time does it show signs of
life again, and makes off hastily, to find a better hiding-place. The
colouring of this moth is so curiously mingled--brown, whitish, black,
and yellow--and traced with acute-angled lines and curves, that one
cannot distinguish it at sight from a bit of rotten wood. I experienced
that myself once when, passing a hedge, I thought I saw a _Xylina_
sitting on the ground, and picked it up to examine it. I thought it was
a bit of wood, and, disappointed, I threw it down again on the grass,
but then I felt uncertain, and picked it up once more--to find that it
was a moth after all[1]!

[1] Rösel says in this connexion: 'The marvellous form of this Papilio
preserves it from injuries, for, when he hangs freely on a trunk of a
tree, he would be taken ten times sooner for a piece of bark than for
a living creature. By day, too, he is so little sensitive, that if he
be thrown down from his resting-place he falls to the ground as if
lifeless, and remains lying motionless. One may also throw him into
the air, or turn him about, and he will rarely give a sign of life.
I have impaled many of them on needles, without seeing any sign of
sensitiveness on their part. This is the more remarkable that these
birds (sic), after they have submitted to all the torment and misery
one can inflict on them, without showing any sign of feeling, will,
whenever they are left in peace and have no further disturbances
to fear, quickly creep off to a dark corner and attempt to conceal
themselves from future attacks.'--_Insektenbelustigungen_, Nürnberg,
1746, vol. i. p. 52.

This case of _Xylina_ is hardly less remarkable, and its likeness to
the mimicked object is scarcely less wonderful than that of the often
discussed mimicry of a leaf, with stalk, midrib, and lateral veins, by
many of the forest butterflies of South America and India.

[Illustration: FIG. 13. _Kallima paralecta_, from India, right under
side of the butterfly at rest. _K_, head. _Lt_, maxillary palps. _B_,
limbs. _V_, anterior wing. _H_, posterior wing. _St_, 'tail' of the
latter, corresponding to the stalk of the leaf. _gl_^1 and _gl_^2,
transparent spots. _Aufl_, eye-spots. _Sch_, mould-spots.]

The best known of these is the Indian _Kallima paralecta_, which, when
it settles, is deceptively like a dead leaf, or rather like a dry or
a half-withered one, on which brown alternates with red, and on which
there are one or two translucent spots, without scales, presumably
representing dewdrops. The upper surface of this butterfly is simply
marked, but gorgeously coloured--blue-black with a reddish yellow,
or bluish white band--and quite constant. The under surface, on the
other hand, although it always resembles a dead leaf, shows very
varied ground colours, being sometimes greyish, sometimes yellowish,
or reddish yellow, or even greenish. Often it shows the lateral
veining of the leaf quite as distinctly as in Fig. 13, but often quite
indistinctly, and the black, mouldy spots (_Sch_) of our figure may
be more strongly marked, or they may be absent. It would seem as if
the mimicry of different kinds of leaves was here aimed at--so to
speak--just as in the case of the varied and numerous species of the
South American genus _Anæa_, which usually live in the woods, and are
all more or less leaf-like, but each species is like a different leaf,
or like a leaf in a different condition, dry, moist, or decomposing.
It is simply astounding to see this diversity of leaf mimicry, and
the extraordinary faithfulness with which the impression of the leaf
is reproduced. But it is by no means always the venation which causes
the resemblance, for this is often inconspicuous; the high degree
of deceptiveness is due to the silvery-clear yellow, dark yellow,
red-brown to dark black-brown ground-colouring, which is never quite
uniform, and over which there usually spreads a whitish ripple,
combined with the remarkable imitation of the sheen of many leaves. The
upper side of this butterfly is almost always conspicuously decorated
with violet, dark blue or red, but always without any relation to the
under surface. Not in all, but in many of the species of this genus,
we find the round, translucent mirrors on the wing already mentioned
in the case of _Kallima_, and in some species quite remarkable means
are made use of to make the resemblance to a leaf thoroughly deceptive.
Thus _Anæa polyxo_, when sitting, looks like a leaf out of the edge
of which a caterpillar has eaten a little piece; in reality there is
nothing missing from the wing, but on the front margin of the anterior
wing a semicircular spot of a bright, soft, yellow colour stands out so
sharply from the rest of the chestnut-brown wing surface, that it has
the effect of a hole in the leaf.

[Illustration: FIG. 14. _Cœnophlebia archidona_, from Bolivia, in its
resting attitude. _mr_, midrib of the apparent leaf. _st_, the apparent
stalk.]

A modern opponent of the selection theory (Eimer) has suggested
that the marking of the lateral veins, and other resemblances to a
leaf in _Kallima_, represent nothing more than the pattern which
was present in any case, inherited from ancestors, and which in the
course of time arranged itself in a particular manner according to
internal developmental laws. Not selection--that is, adaptation to
surroundings--but the internal developmental impulse has brought about
the resemblance to the leaf. It is astonishing how a preconceived
idea can blind a man and weaken his judgment! It goes without saying
that the adaptations do not start from a _tabula rasa_, but from
what is already present; of course, natural selection makes use of
the markings inherited from ancestors; it takes what already exists,
and alters or extends it as suits best. Thus it is easy to prove
that the clear mirrors (Fig. 13, _gl_^1 and _gl_^2) on the wings of
_Kallima_ have arisen from a modification of the nuclei of eye-spots,
just as the dark mould-spots which often occur, frequently develop
in association with the inherited eye-spots; not always however, for
many such accumulations of black scales occur in spots on which there
has never been an eye-spot. Thus, too, the 'midribs' of the butterfly
have in part arisen from a gradual displacing, extending, and altering
of the direction of inherited stripes as, for instance, is clearly
recognizable in the posterior wing of Fig. 13, but sometimes they are
new formations. But the veining of a leaf is never found on the wing
of any butterfly of a species which has not the habit of resting among
leaves, or which has not had it at one time, and it never corresponds
to the natural marking of any genus which does not live in forests.
This impression of leaf-venation has obviously arisen from quite
different patterns of markings, and it has been reached now by one
way, now by another. We can see this from the fact that, in different
butterflies, it lies in quite different positions on the wing. In
the _Kallima_ species the stalk of the leaf lies in the tail of the
posterior wing, the tip of the midrib lies near the tip of the wing; in
_Cœnophlebia archidona_ it is exactly reversed, the tip of the anterior
wing (Fig. 14) is prolonged, and forms the stalk, while a broad,
dark, stripe, the midrib (_mr_), runs from there across the middle of
both wings, and seems to give off two or three lateral ribs running
outwards. If it be asked whether this butterfly always sits down so
artistically that the 'upward turning leaf-stalk is in juxtaposition
to a twig,' we may answer that a bird flying fast is not likely to
look to see whether every leaf in the profusion of foliage in the
primitive forests is properly fastened to its stalk or not, any more
than we should do in the case of a painted bush, on which many a leaf
has the appearance of floating in the air, just as in nature, or in its
faithful copy, the photograph.

[Illustration: FIG. 15. _Cærois chorinæus_, from the lower Amazon, in
its resting attitude. _V_, anterior wing. _H_, posterior wing. _mr_,
midrib of the apparent leaf. _sr_, lateral veins. _st_, hint of a
leaf-stalk.]

Quite different from the leaf-marking either of _Cœnophlebia_ or
_Kallima_ is that of one of the Satyrides of the lower Amazon valley,
_Cærois chorinæus_ (Fig. 15). If one spreads this butterfly out in the
usual way it does not look in the least like a leaf, and one only sees
a number of curiously placed disconnected stripes on the under surface
of the wing. But if the wings be folded together to correspond with
the sitting position of the butterfly, there appears the figure of a
leaf, of which, however, only half is present, and whose midrib (_mr_)
runs obliquely forward from the inner angle of the posterior wing.
Here, again, it is not difficult to guess that this straight stripe has
arisen, by displacement and straightening, from a curved line inherited
from some remote ancestor, and it is these precise changes which are
the work of the adaptive processes of natural selection. The same
applies to the lateral ribs (_sr_), which are here four in number.

But even the division of the wing surface by a single dark line, such
as that which crosses the middle of the posterior wing of _Hebomoja_
(Fig. 9), an Indian butterfly, heightens not inconsiderably the
resemblance of the resting butterfly to a leaf, a resemblance which
has already been shown in the form and colour. Indeed, even the sharp
division of the wing surface into a darker inner and a lighter outer
portion, which occurs in many species of _Anæa_, gives a very vivid
impression of a leaf crossed by a midrib.

It is not without a purpose that I have lingered so long over the
leaf-butterflies. I wished to make it clear that we have by no means
to do with a few exceptional cases, but with a great number, in all
of which resemblance to a leaf has been aimed at, although it has
been attained in varying degrees, and by very diverse ways. Whoever
surveys this wealth of fact must certainly receive the impression,
that, wherever it was advantageous to the existence of the species, the
evolution of such a deceptive resemblance has also been possible. In
any case one cannot but be convinced that it is not a case of chance
resemblance, as some naturalists have recently tried to maintain.

But I have not yet quite finished my outline-survey of the facts, for I
must not omit to mention that, in the evergreen tropical forests, there
are also large nocturnal Lepidoptera, which mimic leaves, sometimes
green ones, sometimes brown, dead ones.

[Illustration: FIG. 16. _Phyllodes ornata_, from Assam. Upper surface
with leaf-like marking only on the anterior wing, which is the only
part visible when at rest; ⅔ nat. size.]

Fig. 16 gives a good picture, reduced to two-thirds, of such a species,
_Phyllodes ornata_, from Assam. The posterior wings are conspicuously
coloured in deep black and yellow; in the resting position they are
covered by the anterior wings, and these are red-brown with black
markings which precisely and clearly mimic the ribs of a leaf. The
midrib begins near the tip of the anterior wing, but breaks off
half-way across the wing at two silvery white spots, similar to those
in many of the diurnal forms, which also mimic decaying leaves. Three
pairs of side veins go off backwards and forwards with remarkable
regularity from the midrib, almost at the same angle, and parallel
to one another, and three more are indicated by vague shading. Then
the midrib begins again in the internal half of the wing, though only
represented by a broad shading. The whole suggests two torn, rotten
leaves, one partly covering the other; and the deception will certainly
be perfect when the moth rests on the ground or among decaying leaves.

That all these extremely favourable protective colorations find their
explanation in the slow and gradually cumulative effects of natural
selection cannot be disputed; it is beyond doubt that they cannot be
explained, so far as we know, in any other way.

If, however, it were possible for a species of butterfly living in
the forest and among leaves to become, through natural selection,
in any degree, and in a continually increasing degree, like a leaf,
surely many insects living in the woods, and especially in the
tropical woods, would also have followed such an advantageous path of
variation--at least, so we should be inclined to think. And this is
indeed the case; numerous insects, of different orders, if they are
as large as a leaf, have taken on the colour, form, and usually also
the markings, of a leaf. Thus green and also decaying and dead leaves
are most realistically imitated by many tropical Locustidæ. Besides
_Tropidoderus_, figured on p. 79, a _Pterochroa_ of South Brazil
affords a particularly fine illustration of this, for not only does
the ground-colour, brown or green, harmonize with that of a dead or
fresh leaf, but, at the same time, all sorts of details are marked on
the insect, which help to heighten the deceptive impression. Even the
outline of the wings is leaf-like, and leaf-veins are marked on the
wing-covers with the most beautiful distinctness, and finally there
is, especially in the light-green individuals, a spot at the wing tip
which, by means of a mixture of brown, yellow, reddish, and violet
colour-tones, mimics a decaying spot with astonishing fidelity. Here,
again, the origin of this special adaptation can be clearly recognized,
for the vaguely concentric arrangement of the colours indicates
that, in the ancestors of the species, an eye-spot had occurred on
this area, of the same kind as we still see on the posterior wing,
which is covered in the resting position. Thus we can again look
back on the history of the species and conclude that the dissolution
and degeneration of the eye-spot began at the time when the leaf
resemblance was evolved, and this was probably caused by some change of
habitat, which we can now no longer guess at.

Many species of leaf-like Orthoptera, both in the Old and New World,
have tough, green, parchment-like wing-covers which bear a remarkable
resemblance to the thick Magnolia-like leaves of tropical plants. Along
with these we must also mention the 'walking leaf,' which has been well
known for centuries. In its case, not the wing-covers alone, but the
head and thorax, and even the legs, are of the colour and shape of a
leaf.

The stick-insects, too, must not remain unnoticed; those quaint
inhabitants of warm countries, whose elongated brown body looks like a
knotted twig, and whose long legs, likewise stick-like, are stretched
out irregularly at different angles to the body, and usually remain
motionless when the insect is resting. These creatures are vegetarian,
and generally keep so still, that even the naturalist who is on the
look-out for them may easily overlook them. Even such an experienced
student of insects as Alfred Russel Wallace was deceived, for a native
of the Phillipines once brought him a specimen as a 'walking-stick'
insect, which he rejected, saying that this time it was no animal
but really a twig, until the native showed him that it was an insect
whose likeness to a twig was increased by the fact that it bore on
its back a ragged green growth, which looked exactly like a liverwort
(_Jungermannia_), which occurs on the twigs of the trees in that region.

We must also notice here the thorn-bugs, which are numerous on the
prickly shrubs of tropical deserts and plateaux, especially in Mexico.
These bear on the relatively very small body two or three large spines,
which make them look like a part of the thorny bush on which they sit.
But this masking by mimicry of thorns is not confined to insects, it is
seen in lizards as well, notably in _Moloch horridus_, a lizard that
lives in the Australian bush, and is covered all over with thorn-like
scales.

These examples should be enough to show that mimicry of the usual
surroundings on the part of animals which are in need of protection,
or are wont to lurk on the watch for their prey, are not isolated
exceptions, chance resemblances, or, as they used to be called, 'freaks
of nature,' but that, on the contrary, they are the rule, depending on
natural causes, and always occurring when these causes are operative.
That such protective resemblances seem to be much more frequent in
warmer climates than with us is probably a fallacy due to the fact that
the number of species (especially of insects) is very much greater
there, and that many insect types have their representatives of
considerable size of body, which not only makes them more conspicuous
_to us_, but makes some protective device in relation to their enemies
or victims much more necessary.

But we must here take account of one more example which occurs in our
fauna in many modifications: the caterpillars of Geometridæ. Many
of these soft and easily injured caterpillars resemble closely, in
colour and shade, the bark of the tree or shrub on which they live
(Fig. 17). At the same time they have the habit, when at rest, of
stretching themselves out straight and stiff, so that they stand out
free, at an acute angle from the branch, thus seeming like one of its
lateral twigs. In many species the resemblance is heightened by the
extraordinary pose of the head (_K_) and of the claw-like feet (_F_),
which, partly pressed close to the head, partly standing out from it,
give the anterior end of the caterpillar the appearance of two terminal
buds, while various little pointed, knotlike warts, scattered over the
body, represent the sleeping buds of the little twig. Who has not at
one time or other taken such a caterpillar for a little branch, and not
inexpert observers only, but even trained naturalists? Many a time I
have not been able to make quite sure of what I had before me until I
touched it!

[Illustration: FIG. 17. Caterpillar of _Selenia tetralunaria_, seated
on a birch twig. _K_, head. _F_, feet. _m_, tubercle, resembling a
'sleeping bud'; nat. size.]




LECTURE V

TRUE MIMICRY

Mimicry: its discovery by Bates--Heliconiidæ and
Pieridæ--Danaides--_Papilio merope_ and its five females--The females
lead the way--Species with mimicry in both sexes--Objections--Enemies
of butterflies--The immunity of the models--Poisonousness of the
food-plants of immune species--Several mimics of the same immune
species--Persecuted species of the same genus resemble quite different
models--_Elymnias_--Degree of resemblance--Differences between the
caterpillars of the model and the copy--The same resemblance arrived
at by different ways--Transparent-winged butterflies--The gradually
increasing resemblance points to causes operating mechanically--Rarity
of the mimetic species--Danger to the existence of the species not a
necessary condition of mimetic transformation--_Papilio meriones_ and
_Papilio merope_--Comparison with the dimorphic caterpillars--_Papilio
turnus_--'Mimicry rings' of immune species--_Danais erippus_ and
_Limenitis archippus_--Marked divergence of mimetic species from their
nearest relatives--Mimicry in other insects--Imitators of ants and bees.


LET us now turn to the most remarkable of all protective form- and
colour-adaptations, the so-called Mimicry, including all cases of the
imitation of one animal by another, which we came to know first through
Bates, and to a fuller understanding of which A. R. Wallace and Fritz
Müller have especially contributed.

While the English naturalist, Bates[2], was collecting and observing
on the banks of the Amazons--as he did for twelve years--it sometimes
occurred that, among a swarm of those gaily coloured, quaintly shaped
butterflies, the Heliconiidæ (Pl. II, Fig. 13), he caught one which,
on closer examination, proved to be essentially different from its
numerous companions. It was certainly like them both in colour and
form, but it belonged to quite a different family of butterflies,
that of the Pieridæ or Whites (Pl. II, Fig. 19). These whites with
the colours of the Heliconiidæ always occurred singly in swarms of
the latter form, and Bates found that, in the different districts of
the Amazon, they always resembled in a striking manner the species of
Heliconiidæ there prevalent. Many of them had been previously known
to entomologists, and because they diverged so far from the usual
type of the Pieridæ, especially in the form of the wing, the name
Dysmorphia, the 'mis-shapen,' had been given to them, although the
meaning of this 'mis-shapenness' long remained a mystery. The French
Lepidopterist, Boisduval, went a step further when he pointed out as
something remarkable that nature sometimes makes several species of
quite different families exactly alike, and called attention to three
African butterflies, of which we shall have to speak later in detail.
But even he was too much fettered by the old views of the immutability
of species to arrive at a correct interpretation. Thus it was reserved
for Bates to take the decisive step. Observing that the Heliconiidæ
occurred frequently, and usually in large swarms, he concluded
that they must have few enemies, and as he never saw the numerous
insectivorous birds and insects hunting them, he further concluded
that they must have something disagreeable which secured them from the
attacks of these predaceous forms. On the other hand, he found that the
heliconid-like Whites were always rare, and he took this as a sign that
they were much persecuted, and that they must, therefore, be palatable
tit-bits for the insectivores. If it were possible, then, that a
species of Whites with the usual white colour of the family should give
rise to variations, which would make them in any degree resemble the
Heliconiidæ, which are secure from persecution, and if, in addition,
those that exhibited the profitable variation attached themselves to
swarms of the mimicked form, then these variants would be to a certain
extent secured from attack, and more and more so in proportion as the
resemblance to the protected model increased. The great likeness of
these Whites to the Heliconiidæ, Bates further argued, would depend on
a process of selection, based on the fact that, in each generation,
those individuals would on the average survive for reproduction
which were a little more like the model than the rest, and thus the
resemblance, doubtless slight to begin with, would gradually reach its
present degree of perfection.

[2] _Contributions to an Insect Fauna of the Amazon Valley_, Trans.
Linn. Soc., Vol. XXIII, 1862.

Bates's hypotheses have been subsequently confirmed in the most
striking way. The Heliconiidæ do possess a disagreeable taste and
odour, and are utterly rejected by birds, lizards, and other animals.
It has been directly observed that puff-birds, species of _Trogon_,
and other insectivorous birds, looking down from the tops of trees in
search of food, allowed to pass unheeded the swarms of gaily coloured
Heliconiidæ which were fluttering among the leaves, and experiments
with various insectivorous animals yielded the same result: _the
Heliconiidæ are immune_. We can, therefore, not only understand that
it must be advantageous to resemble them, we can also appreciate many
of their peculiar characters, such as their gay coloration, which must
serve as a sign of their disagreeable taste, and their slow, fluttering
flight, as well as their habit of flocking together, which must make
it easier for the birds to recognize them as uneatable. Everything
which marks out these unpalatable morsels, and makes them more readily
recognizable, must be to their advantage, and therefore must have been
favoured by natural selection (Pl. II, Fig. 13).

In the same way, every increase of resemblance on the part of the
mimics would increase their chances of escaping notice, and any one
who is accustomed to observe butterflies in nature can well understand
that even very slight resemblances may have formed the beginning of
the selection process; perhaps even a mere variation in the manner
of flight, combined with the habit of associating with the swarms of
Heliconiidæ. I myself have many times been momentarily deceived in our
own woods by a White of unusually majestic flight, so that I took it
for an _Apatura_ or a _Limenitis_. If, therefore, individual Whites
occurred here and there in the Amazon valley, which flew somewhat after
the manner of the Heliconiidæ, and associated with them, they might
possibly have attained a certain degree of security through that alone,
and it would be greatly increased if at the same time they varied
somewhat in colour in the direction of their companions.

In any case there can be no doubt whatever that in these cases a real
transformation of the species in colour and marking, and perhaps often,
too, in form of wing, has taken place, and that within comparatively
modern times--let us say during the distribution of a species which
required protection over a large continent, or since the last breaking
up of an immune species into local species. Various facts prove this;
above all, the circumstance that it is often only the females which
exhibit this protective mimicry; and that one and the same species may
mimic a different immune species in different areas, but always the one
occurring abundantly in that area, and so on.

Definite examples will make this clearer, and I will only say in
advance that, since the discovery of Bates, numerous cases of mimicry
in butterflies have been found, not only in South America, but in all
tropical countries which have a rich Lepidopteran fauna. And it is not
only between the Heliconiidæ and the Pieridæ that such relations have
been evolved; many much-persecuted, unprotected species of different
families everywhere mimic species which are rejected on account of
their nauseous taste, and these, too, belonging to different families.
The Heliconiidæ are a purely American group, but in the Old World
and in Australia their place is taken by the three great families of
Danaides, Euplœides, and Acræides, since, as it seems, they all taste
unpleasantly, and are rejected by all, or at least by most, of the
insectivorous birds. Numerous species of the genus _Danais_ (Pl. I,
Fig. 8), _Amauris_ (Pl. I, Fig. 5), _Euplœa_ (Pl. III, Fig. 25, 27),
and _Acræa_ (Pl. II, Fig. 2), and also many species of _Papilio_ and
other genera, enjoy the advantage of unpleasant taste, if not even of
poisonousness; they are, therefore, secure from pursuit, and are, in
consequence, much mimicked by palatable butterflies.

As a further example, I now select a diurnal butterfly from Africa,
_Papilio merope_ Cramer[3], which was shown by Trimen in 1868 to be
mimetic. The species has a wide distribution, for, if we except slight
local differences in the marking of the male, its range extends over
the greater part of Africa, from Abyssinia to the Cape, and from East
Africa to the Gold Coast.

[3] The West African form of _Papilio merope_ has been quite recently
distinguished from the southern form and regarded as a distinct
species, the latter being now called _Papilio cenea_. The differences
in the males are very slight--somewhat shorter wings, shorter
wing-tail, and so on--differences which seem relatively unimportant in
comparison with the differences between the males and the females.

The male is a beautiful large butterfly, yellowish white, with a touch
of black, and with little tails to the posterior wings (Pl. I, Fig.
1), like our own swallowtail. A very nearly related species occurs
in Madagascar, and there the female is similarly coloured, though it
may be distinguished by having a little more black on the wing. On
the mainland of Africa, however, the females of _Papilio merope_ are
so different in colour and form of wing that it would be difficult to
believe them of the same species as the male had not both sexes more
than once been reared from the eggs of one mother. The females (Pl. I,
Fig. 6) in South Africa imitate a species of _Amauris_, _A. echeria_
(Pl. I, Fig. 7), of a dark ground-colour with white, or brownish-white,
mirrors and spots, and they resemble it most deceptively. But what
makes the case more interesting in its theoretical aspect is that
_Danais echeria_ of Cape Colony is markedly different from _Danais
echeria_ of Natal, and the female of _Papilio merope_ has followed
those two local varieties, and has likewise a Cape and a Natal local
form. Even this is not all, for in Cape Colony there are two other
females of _Papilio merope_. One of them has a yellow ground-colour,
and resembles _Danais chrysippus_, which is extremely abundant there
(Pl. I, Fig. 3); the other is entirely different (Pl. I, Fig. 4), for
it closely mimics another Danaid occurring in the same districts of
Africa, and also immune, _Amauris niavius_ (Pl. I, Fig. 5), not only in
the beautiful pure white and deep black of the wing surface, but also
in the distribution of these colours to form a pattern.

We have thus in Africa four different females of _Papilio merope_,
each of which mimics a protected species of Danaid. They are not
always locally separate, so that each is exclusively restricted to a
particular region, for their areas of distribution often overlap, and,
at the Cape for instance, one male form and three different forms of
female have been reared from one set of eggs. In addition, we have the
fact that between the two local forms of _Danais echeria_ transition
forms occur, and that the mimetic females of _Papilio merope_ show
the same transition forms locally, and we must admit that all these
facts harmonize most beautifully with the selection interpretation,
but defy any other. And that the last doubt may be dispelled, nature
has preserved _the primitive female form_ on the continent of
Africa--namely, in Abyssinia, where, along with the mimetic females,
there are others which are tailed like the males (Pl. I, Fig. 1), and
are like them in form and colour, a few minor differences excepted.

Thus we have in _Papilio merope_ a species which, in the course of
its distribution through Africa, has scarcely varied at all in the
male sex, but in the female has almost everywhere lost the outward
appearance of a _Papilio_, and has assumed that of a Danaid, which is
protected by being unpalatable, and not even everywhere the appearance
of the same species, but in each place that of the prevailing one, and
sometimes of several in one region. These females thus show at the
present day a polymorphism which consists of four chief mimetic forms,
to which has to be added the primitive form--that resembling the male.
This has survived in Abyssinia alone, and even there it is not the only
one, but occurs along with some of the mimetic forms.

To the question why only the females are mimetic in this and other
cases, Darwin and Wallace have answered that the females are more in
need of protection. In the first place, the males among butterflies
are considerably in the majority, and, secondly, the females must
live longer in order to be able to lay their eggs. Moreover, the
females, which are loaded with numerous eggs, are heavier in flight,
and during the whole period of egg-laying--that is, for a considerable
time--they are exposed to the attacks of numerous enemies. Whether
one of the abundant males is devoured sooner or later is immaterial
to the persistence of the species, since one male is sufficient to
fertilize several females. The death of a single female, on the other
hand, implies a loss of several hundred descendants to the species.
It is, therefore, intelligible that, in species already somewhat
rare, the female must first of all be protected; that is to say, that
all variations tending in the direction of her protection would give
rise to a process of selection resulting in an augmentation of the
protective characters.

But there are also butterflies in which both sexes mimic a protected
model. Thus many imitators of the unpalatable Acræides (Pl. II, Fig.
21) resemble the model in both sexes, and of the South American Whites
which mimic the Heliconiidæ there are some which have the appearance
of the Heliconiidæ even in the male sex (Pl. II, Fig. 18, 19), while
others look like ordinary Whites (for instance, _Archonias potamea_).
But in many of these species, which are mimetic in the female sex, we
find also in the male some indications of the mimetic colouring, but
in the first instance only on the under surface. Thus the females of
_Perhybris pyrrha_ (Pl. II, Fig. 17) resemble in their black, yellow,
and orange-red colour-pattern the immune American Danaid, _Lycorea
halia_ (Pl. II, Fig. 12), but their mates are, on the upper surface,
like our common Whites, though they already show on the under surface
the orange-red transverse stripes of the _Lycorea_ (Pl. II, Fig. 16).
In other mimetic species of Whites a similar beginning is even more
faintly hinted at, and in others, again, the upper surface of the male
is also provided with protective colours, and only a single white spot
on the posterior, or sometimes even on the anterior wing as well, shows
the original white of the Pieridæ (Fig. 18).

I do not know how any one can put any other construction on these
facts than that the females first assumed the protective colouring,
and that the males followed later, and more slowly. Whether this is
due to inheritance on the female side, and thus ensues as a mechanical
necessity, in virtue of laws of inheritance still unknown to us, or
whether it arose because there was a certain advantage in protection
to the males--though not such a marked one--and that these, therefore,
followed independently along the same path of evolution as the females,
has yet to be investigated. Personally, I incline to the latter view,
because there are protected mimetic species, in which the female
mimics one immune model, and the male another, quite different from
the female's. A case in point is that of an Indian butterfly, _Euripus
haliterses_, and also _Hypolimnas scopas_, in the latter of which
the male resembles the male of _Euplœa pyrgion_, and the female is
like the somewhat different female of the same protected species. The
Indian _Papilio paradoxus_, too, seems to show the independence of the
processes of mimetic adaptation, for the male is like the blue male
of the immune _Euplœa binotata_ (Pl. III, Fig. 25), while the female
resembles the radially-striped female of _Euplœa midamus_ (Pl. III,
Fig. 27), and this double adaptation is repeated in another of the
persecuted butterflies, _Elymnias leucocyma_ (Pl. III, Fig. 26, 28).

Many objections have been made to the interpretation of mimicry by
selection. It has been asserted that butterflies are exposed to injury
from birds only to an inconsiderable extent, not sufficient to account
for such an intense and persistent process of selection, because they
are not very welcome morsels, on account of the large and uneatable
wings and the relatively small body. Doubt has also been raised as to
the immunity of the models, which has not been proved in many of the
species in regard to which it is assumed. Finally, it is maintained
that the advantage which resemblance to an immune model brings is
not proved, but is purely hypothetical; and that it is probable that
the birds do not distinguish the colours and markings of the flying
butterflies at all, but are at the most only deceived by resemblances
in their manner of flight.

The last objection contains a certain amount of truth, inasmuch as
the manner of flight always plays a part in the mimicry of a strange
species. We shall see later how much the instincts of a species
contribute to the deception in all cases of protective colouring.
It is, therefore, not improbable that, in many cases, the imitation
of the flight of an immune species, and a gradually increasing
familiarity with the habitats of the same immune species, preceded the
modification of the colour. Indeed, the slow flight of immune species
(Heliconiidæ) has been unanimously emphasized by observers, as a factor
in facilitating the recognition of the butterflies by the sharp-sighted
birds.

That it was not only in earlier ages of the world's history that
butterflies were much persecuted, as some have supposed, but that they
are so still, seems to me indisputable in view of the observations
of the last quarter of a century. Even in this country, where both
butterflies and insect-eating birds are being more and more crowded out
through cultivation, a considerable number of butterflies in flight
fall victims to the birds. Kennel gives observations on this point in
regard to the white-throat; Caspari for the swallows. The latter let
about a hundred little tortoiseshell butterflies (_Vanessa antiopa_)
fly from his window, 'but not ten of them reached the neighbouring
wood,' all the rest being eaten by swallows, 'which congregated in
numbers in front of his window.' Kathariner observed, in the highlands
of Asia Minor, a flock of bee-eaters (_Merops_) which caught in flight
and swallowed a great many individuals of a very beautiful diurnal
butterfly (_Thais cerisyi_).

Finally, Pastor Slevogt has collected much evidence to show that
our indigenous butterflies have a great deal to suffer in the way
of persecution from birds. And in regard to tropical countries, the
chase of butterflies by insectivorous birds has long been known. Thus
Pöppig says that in the primitive forests one can easily recognize
the place which has been selected by one of the Jacamars (Galbulidæ)
as its favourite resting-place, for the wings of the largest and most
beautiful butterflies, whose bodies alone are eaten, lie on the ground
in a circle for a distance of several paces. We owe direct observations
on the hunting of insects by birds of the primitive forest especially
to Dr. Hahnel, who found many opportunities for observation in the
course of his enthusiastic collecting journeys in Central and South
America. He writes: 'No other family of butterflies suffered so much
from birds as the Pieridæ (Whites), and these freebooters often snapped
away the prettiest and freshest specimens from quite close to me. Every
time I was amazed anew at the unfailing security of their flight, and I
gladly paid for the spectacle by the loss of a few specimens.' Of the
pursuit of one of the large _Caligo_ species, whose leaf-like under
surface, marked with eye-spots, I have already described, (Fig. 6, p.
70), he says: 'With incredible skill this fairly large insect avoided
every blow of the bill of the bird which followed it in close chase,
and saved itself by flying from one shrub to another, till at last it
was lost to sight in the thickest tangle of branches, and the exhausted
bird gave up further attempts at pursuit.'

But, in addition to the birds, the butterflies of the primitive forest
have to dread the persecution of other insects, especially of the large
predaceous dragon-flies, which throw themselves upon them in the midst
of their flight. Hahnel often saw a specimen of the large, beautiful,
blue _Morpho cisseis_, which was fluttering peacefully about the crown
of a tree, suddenly shoot head downwards, 'like an ox with horns
lowered, and then reascended apparently with difficulty, after it had
torn itself free from its sudden assailant, whose jaws left distinct
short scars.'

In addition to birds and predatory insects the butterflies are
persecuted by the whole army of lizards. In order to entice the
butterflies, Hahnel laid bait in the wood, 'sugar-cane, little sweet
bananas, and such like.' Various kinds of butterfly settled on it,
'Satyrides, Ageroniæ, _Adelpha_ and other Nymphalidæ.' He saw that they
'were persistently stalked and attacked by greedy lizards, which, in
spite of their plump figure and uncouth gait, showed themselves able
to spring suddenly out and snatch their prey with great adroitness. It
is, however, very wonderful to see the agility such a persecuted insect
displays in evading the repeated attacks of these marauders.' Thus on
one occasion an _Adelpha_ was driven off a dozen times from the exposed
bait by a lizard, which pounced upon it, but it always settled down
for a short time on a leaf, and soon returned to its repast, whereupon
the enemy 'instantaneously rushed upon it in a fury, until at last he
was obliged to give in,' abandoning the attempt to catch a creature so
adept in retreat.

Many butterflies assemble at midday on sandbanks in the middle
of the river, in order to drink, and there, too, the lizards are
always lurking about. Hahnel gives a pretty and undoubtedly accurate
description of the protective value of the long tail borne by many
of the sail-like Papilios at the end of the posterior wing; they
'quite obviously' afford protection against the lizards, 'which, after
snapping, often find themselves obliged to be content with the tail
alone, while the rest of the animal flies away practically uninjured.'

Not only is the great persecution of the butterflies a fact, the
immunity of the known species, which are models for mimicry, is
also certain. For numerous species, at any rate, this has now been
established. First of all--as has already been said--this is true of
the Heliconiidæ, in regard to which Wallace long ago showed that, if
the thorax be pressed, they exude a yellowish juice of unpleasant
smell. This is probably the blood of the insect, but that does not
hinder the repulsive odour of the living butterfly being perceptible at
a distance of 'several paces,' as Seitz observed in _Heliconius besei_.

Repeated experiments have been made, which have shown that such
butterflies are rejected not only by the insectivorous birds of the
primitive forest, but also by tame turkeys, pheasants and partridges,
usually so greedy. Hahnel has recently repeated these experiments in
Brazil with hens, and he obtained the same result. The hens, 'which
otherwise devoured all butterflies eagerly,' rejected all Ithomidæ,
Heliconiidæ, the white Papilios, as also some of the gaily coloured
Heliconiid-like moths which fly by day, such as _Esthema bicolor_ and
_Pericopis lycorea_. Obviously, the gay or conspicuous colour of these
Lepidoptera acts as a warning signal of their unpalatability, and
protects them from attempts on the part of the birds to investigate
their flavour. Hence we find that the under surface of these insects
is coloured like the upper. Even the numbers of these species which
fly about indicates that they must be little decimated, and, in point
of fact, we never find the wings of Heliconiidæ lying on the ground in
the forests of South America, while those of the Nymphalidæ and other
butterflies are by no means uncommonly seen as the remains of birds'
meals.

There is just as little room for doubt, as in the case of the
Heliconiidæ and their allies, that the Danaidæ, Acræidæ, and the
Euplœidæ in the tropical regions of the Old World enjoy a certain
immunity on account of their repulsive odour and taste. Here, too,
observation and experiment have shown that birds, lizards, and
predaceous insects leave the butterflies of these families unmolested.
I need only mention the observation of Trimen that, under an acacia
much visited by butterflies, on which Mantides--the so-called
praying-insects--caught and devoured large numbers, the wings of an
_Acræa_ or a _Danais_ were never found. These unpalatable butterflies
also possess a motley or at least striking dress, recognizable from
afar, and alike on both surfaces; and they also have a slow flight,
by which they are readily recognized. They, too, usually assemble
in large swarms, and both sexes are alike, or resemble each other
closely in colouring, or at least they are both equally conspicuous.
But even these cases do not complete the list of butterflies which are
protected by their unpalatability; among the otherwise much-persecuted
and therefore palatable Pieridæ (Whites) there is an Asiatic genus,
_Delias_, which in all probability belongs to the immune butterflies,
as their gaily coloured under surface indicates, and among the
nocturnal Lepidoptera of different countries and families there are
isolated generations which are very gaily and conspicuously coloured,
and which are rejected by birds, their unpleasant odour being
perceptible at a distance of several feet (Chalcosiidæ and Eusemiidæ).
The latter no longer fly under cover of night, like their relatives,
but have assumed diurnal habits.

It is to be supposed that the repulsiveness of such 'unpalatable'
butterflies is associated with the food-plant on which the caterpillar
lives. Acrid, nauseous, astringent, and actually poisonous substances
are produced in many plants, and we shall see later that this is to
their own advantage; these substances pass into the insect, and they
do so probably in part unaltered, in part certainly altered, but still
they are protective, perhaps even in an increased degree. This is
borne out by the fact that many caterpillars of immune butterflies
live on more or less poisonous plants: the Acræidæ and Heliconiidæ on
Passiflores, which contain nauseous substances; the Danaidæ on the
poisonous Asclepiadæ, which are rich in milky juice or latex; the
Euplœæ on the poisonous species of _Ficus_, the Neotropinæ on the
Solanaceæ, and so on. But there are many genera, rich in species,
and distributed over the whole earth, the caterpillars of which live
on plants of very various families and characters, and of these the
majority of species are palatable, though a few are repulsive in
taste and odour, and therefore immune. This is the case in the genus
_Papilio_. As far back as the sixties Wallace discovered that there
were immune species of _Papilio_, and that these were mimicked
by other species. Later it was shown that these immune species
live chiefly on poisonous plants (in the wide sense), on various
Aristolochiæ; and Haase has recently grouped these together as
poison-eaters (Aristolochia-butterflies or Pharmacophagæ). They are
distinguished by a conspicuous red on the body. In some of them, as in
_Papilio philoxenus_, a repulsive odour as of decomposing urine has
been detected in the living animal.

We see, then, that the much-persecuted and easily injured butterflies
make use of a poisonous substance (in the widest sense), prepared in
the plant for its own protection, and, wherever their own metabolism
makes it possible, they use it to protect themselves. We need not
wonder, therefore, that so many butterflies are immune, nor that among
the numerous palatable species a small proportion have endeavoured to
become like the protected species, as far as natural selection was able
to bring such a resemblance about.

There is hardly any adaptation phenomenon so widely distributed and
diverse in its manifestations, which has been at the same time so much
observed and followed out into all its details, as Mimicry; and it must
surely be regarded as a justification of the validity of interpreting
it in terms of Natural Selection that all the observed phenomena tally
so beautifully with the deductions from the theory. I at least know of
no facts which contradict the theory, but of many which might have been
predicted from it.

For instance, it might have been predicted from the theory alone that
an immune species would often have several mimics, as, in point of
fact, is frequently the case, and it would be easy to give numerous
examples of this. Thus the two Danaids of South and Central Africa,
_Amauris echeria_ and _Amauris niavius_, are mimicked, not only
by the two female forms of _Papilio merope_, as we have already
described in detail, but the latter is also mimicked by Nymphalid,
which requires protection, _Diadema anthedon_, and the former by two
diurnal butterflies of different families, _Diadema nuina_ and _Papilio
echerioides_.

Similarly, the black-and-red coloured _Heliconius melpomene_ in Brazil
is mimicked both by the female of a White (_Archonias teuthamis_),
and by a _Papilio_, which has received the name of _P. euterpinus_ on
account of this resemblance. Thus, too, the immune _Methona psidii_,
Cr. of Brazil, with its half-transparent wings marked with black bands,
has five mimics, belonging to five different genera, and one of these
is not a true diurnal butterfly at all, but one of the day-flying
species of the genus _Castnia_, whose systematic position is doubtful.

[Illustration: FIG. 18. Upper surfaces of _A_, _Acræa egina_, from the
Gold Coast, immune. _B_, _Papilio ridleyanus_, from Gaboon, not immune.
_C_, _Pseudacræa boisduvalii_, from the Gold Coast, not immune.]

The West African immune Acræid, _Acræa gea_ (Pl. II, Fig. 21), is
deceptively mimicked, both as to the narrow, long shape of the wing
and its blackish-brown and white mottled markings, by a Nymphalid,
_Pseudacræa hirce_, by the female of a Papilio (_P. cynorta_) whose
mate is quite different, and by the female of a Satyrid (_Elymnias
phegea_) (Pl. II, Fig. 20). In the _Papilio_ the resemblance extends to
the peculiar pitch-black shining spot on the under side of the base of
the posterior wing, and all three are like the model on both surfaces,
and therefore in flight as well as in the resting attitude.

On the same West African coast occurs the strange greyish-black _Acræa
egina_, with brick-red spots and bands, and coal-black dots (Fig. 18,
_A_). This immune species is deceptively mimicked in its native country
by two other butterflies--a Nymphalid, _Pseudacræa boisduvalii_ (Fig.
18, _C_), and by a female _Papilio_ (_P. ridleyanus_) (Fig. 18, _B_),
by the latter not so exactly as by the former, but quite sufficiently
to be confused with its model in flight.

It would have been less easy to predict with certainty from the theory
that, conversely, the different species of a genus which stood in need
of protection would be able to mimic quite different immune models, for
who would have ventured to prophesy how far the capacity of a species
for variation might go, and how many different kinds of coloration it
was able to assume? But the facts teach us that there is a wide range
of possibility in this respect.

Most interesting in this respect is, perhaps, the Asiatic-African
genus _Elymnias_, a Satyrid whose numerous (over thirty) species
all seem to be in need of protection, for many of them mimic immune
butterflies, while the rest are inconspicuous and are provided with
protective colouring on the under surface. On Plates II and III some
of the former are depicted beside their models. The single African
species (_Elymnias phegea_) (Pl. II, Fig. 20) mimics, as has been
already mentioned, the prevalent _Acræa gea_ (Pl. II, Fig. 21). Many
of the Asiatic Elymniidæ are mimics of the immune Euplœæ, especially
the dark-brown species with steel-blue shimmer, such as _E. patna_ in
India, _E. beza_ in Borneo, and _E. penanga_ in Borneo. In Amboina
there flies an _E. vitellia_, the female of which mimics accurately the
plain, light-brown, inconspicuous _Euplœa climena_ which occurs there.
The male of _Elymnias leucocyma_ (Pl. III, Fig. 26) resembles the brown
and blue shimmering _Euplœa binotata_ (Pl. III, Fig. 25), while the
female mimics the dusky, radially-striped female of _Euplœa midamus_
(Pl. III, Figs. 27 and 28): the male of _Elymnias cassiphone_ resembles
the blackish-brown and deep-blue iridescent _Euplœa claudia_, while the
female is like the female of _Euplœa midamus_. A number of species of
_Elymnias_ copy Danaids: thus both sexes of _E. lais_ are like _Danais
vulgaris_ (Pl. III, Figs. 29 and 30), and _E. ceryx_ and _E. timandra_
are like another similar Danaid, _D. tytia_. The female only of _E.
undularis_ of Ceylon mimics the brown-yellow _D. genutia_ (Pl. II, Fig.
22) in general appearance, though not minutely, while the male (Pl.
II, Fig. 24) seems to attempt an imitation of the blue Euplœæ. A rare
form, not often represented in collections, _Elymnias künstleri_, bears
a striking resemblance to the Danaid, _Ideopsis daos_ Boisd., with its
white wings spotted with black, while three species mimic the probably
immune Pierid genus _Delias_, especially on the under surface, which
is decorated with yellow and red. Perhaps the one which has diverged
farthest from the original type is _Elymnias agondas_ Boisd. (Pl. II,
Fig. 32) of the Papua region and the island of Waigeu, for it bears
two large blue eye-spots on the posterior wings, and thus, especially
in the case of the almost white female, closely resembles _Tenaris
bioculatus_ (Pl. III, Fig. 31). There are thus seven or eight types
of marking and colouring differing from one another, and belonging to
six different genera and a much greater number of species, which are
mimicked by this one genus _Elymnias_.

It is most interesting to note how these mimetic species give up, more
or less, the original sympathetic colouring of the under surface,
and use in establishing their mimicry the marking elements which
were originally directed towards concealment. According to the
beautiful observations of Erich Haase on this genus _Elymnias_, the
ground-colouring on the under surface must have been 'a grey, darkly
mottled protective one,' as still occurs, for instance, in several
mimetic species, such as _Elymnias lais_ (Pl. III, Fig. 30). This
leaf-colouring disappears more and more the more perfect the mimicry of
the model becomes, until, finally, the model is repeated on the under
surface also. Compare, for instance, Figs. 30 and 32. From this we
may conclude that a dress which makes Lepidoptera appear unpalatable
morsels is a more effective protection than resemblance to a leaf. That
might indeed be deduced even from the theory, for resemblance to a leaf
never protects _absolutely_, and does so, in any case, only during
rest, while apparent unpalatability repels assailants at all times.

Those unversed in butterfly lore usually ask, when these mimetic
relations are expounded to them, how we know that copies which are
so like their models really belong to a different genus, or even
family. There are certainly cases in which model and copy resemble
each other so closely that even a zoologist cannot tell one from the
other without close examination, as, for instance, in the case of
certain transparent-winged Heliconiidæ of Brazil (Ithomiides) and their
mimics belonging to the family of Whites. But even in such cases the
likeness only extends as far as is theoretically requisite, that is,
only to those characters that make the butterfly appear to the eye of
its pursuer like another species, known to it to be unpalatable. The
likeness does not extend to details, which can only be seen with a
magnifying-glass or a microscope, and above all, it does not extend
to the caterpillar, pupa, or egg. Thus, in the case cited, we may be
certain that the caterpillar of _Ithomia_ is quite different from
that of the mimicking White, since the former will be, in structure,
of the type of _Ithomia_ caterpillar, and the other of the usual type
of Whites. As yet, indeed, these two species are not known in their
caterpillar stages, but other cases are known. A species belonging to
the same genus as our indigenous 'kingfishers' (_Limenitis populi_), a
diurnal butterfly of North America, _Limenitis archippus_ (Pl. I, Fig.
9), strongly resembles the brown-yellow, immune _Danais erippus_ (Pl.
I, Fig. 8), while the caterpillars of both species are quite different,
that of _Danais erippus_ possessing the remarkable, soft and flexible
horn-like processes of the Danaid caterpillars (Pl. I, Fig. 10_a_),
while the caterpillar of _Limenitis archippus_ (Pl. I, Fig. 11_a_) is
at once recognizable by its blunt, club-shaped and spinose papillæ
as a _Limenitis_ caterpillar. The adaptation of the butterfly to its
protected model has thus exercised no influence upon the caterpillar.
Nor has it affected the pupa, which in both cases exhibits the very
different and quite characteristic form of the _Danais_ pupa and the
_Limenitis_ pupa respectively (Pl. I, Fig. 10_b_, and 11_b_).

But even in the butterfly itself nothing is altered, except what
increases the resemblance to the model. All else has remained
unchanged, above all, the venation of the wings. Since the painstaking
and valuable work of Herrich-Schäfer the venation has been made the
basis of the whole systematic arrangement of butterflies, and it
enables us, in point of fact, to distinguish with precision, not the
families alone, but often even the genera, for the course of the
veins in the different species of a single genus is the same, and
that is true for the mimetic species as well as for others. Thus the
Danaid-like _Limenitis_ has the usual _Limenitis_ venation, of the kind
seen in our own indigenous species of _Limenitis_, and the already
described _Elymnias_ species of the African and Indian forests and
grassy plains have always the venation characteristic of this genus,
whether they be protected only by sympathetic colouring or imitate
an immune _Euplœa_, a _Danais_, an _Acræa_, or a _Tenaris_. However
much the contour of the wing may vary, the venation is unaffected,
and we can distinguish model from copy by this means alone, so that,
even when there is the closest resemblance, no doubt is possible.
In its theoretical aspect this constancy of venation is obviously
important, for as nothing about the organism is incapable of variation,
the veining of the wings might have varied, as indeed it has varied
from genus to genus in the course of the phylogenetic history; but as
changes in venation could not be detected by the butterflies' enemies,
however sharp-sighted, there has been no reason in these cases for
variation in this respect.

In this connexion Poulton has brought forward interesting facts showing
that the mimics of one model, belonging to different genera, often
secure the same effect in quite different ways. Thus the glass-like
transparency of the wings in the Heliconiidæ of the genus _Methona_
depends on a considerable reduction of the size of the scales, which
ordinarily cover both sides of the wing as thickly as the tiles on a
roof, and produce the colour. In another quite similar species, also
transparent-winged, the Danaid _Ituna ilione_, the transparency is due
to the absence of most of the scales, and in a third mimic, _Castnia
linus_, var. _heliconoides_, the scales are not altered either in size
or number, but have become absolutely unpigmented and transparent.
In a fourth mimic, a Pierid, _Dismorphia crise_, the scales have not
decreased in number, but have become quite minute, while in a fifth
case, the nocturnal _Hyelosia heliconoides_ Swains., the same thing
has happened as in _Castnia_, but the scales are also fewer in number.
Thus in each of the mimics the changes which have taken place in the
scales are quite different, but they bring about the same effect, the
glass-like transparency of the wings, on which the resemblance to the
model depends: what we have before us is, therefore, not a similarity
of variation, but only an appearance of similarity in external features.

In the face of such facts there can be no further question of the often
repeated objection, that the resemblance of model and copy depend on
the similarity of external influences upon species living in the same
latitude, even if that were not already sufficiently refuted by the
frequent restriction of the mimicry to the female. And that mimicry
should be a mere matter of chance is negatived even by the single
fact that model and copy always live in the same area, and that the
local varieties of the model are faithfully followed by the mimic.
An interesting example of this is furnished by _Elymnias undularis_,
already mentioned, for in this case the female (Pl. II, Fig. 23)
mimics the brown-yellow _Danais plexippus_ (Pl. II, Fig. 22), not
wherever _E. undularis_ occurs, but only in Ceylon and British India.
In Burmah, where another Danaid, _D. hegesippus_, is common, it mimics
that; and in Malacca it does not copy a Danaid at all, but resembles
the male of its own species, which in India is very different from
it, since there the female mimics one of the blue iridescent Euplœæ
(Pl. III, Fig. 24). It cannot therefore be a matter of 'chance,' and
we should have to give up all attempt at a scientific interpretation
if we were not prepared to accept that of natural selection. Even the
interference of a purposeful Power can hardly be seriously considered
in this case, even by those who are inclined to such a view, for the
_gradual_ approximation to the model, which is a matter of course in a
process of evolution, could only appear, if referred to the benevolent
intelligence of a Creator, as an unworthy trick, designed to lead
humanity astray in its strivings after knowledge. On the other hand,
this gradual increase of resemblance, which becomes apparent when we
compare several mimetic species--this carrying over, step by step,
from the female to the male--and many other facts point to the working
of natural forces according to law, and, if there is to be found
anywhere in living nature a complicated process of self-regulation,
it certainly lies before us here, clearer and less open to objections
than almost anywhere else. I do not mean to say, however, that we
can verify it statistically in detail, as has been demanded by the
fanatical opponents of natural selection. A direct testing of natural
selection is, as has been already shown, nowhere possible: we can never
exactly estimate how great the advantage is which a species requiring
protection derives from a slight increase in the resemblance to an
immune model; and I for one do not know how we could even definitely
prove that a certain species needed a greater degree of protection than
it had previously enjoyed in order to ensure its persistence in the
struggle. It would be necessary to know the total number of individuals
living on a certain area for many generations. If it appeared that
there was a progressive diminution in the number of individuals, we
should be justified in concluding that the species had not an adequate
power of persistence, and that it therefore required a more effective
protection. But it is impossible for us to collect such exact data
for any species living under natural conditions, although we can
often say approximately that a species is progressively decreasing in
numbers. Even this, however, we can usually do only in cases which
are influenced directly or indirectly by the interference of Man in
nature, and in which the falling off in the species occurs so rapidly
that there is no time for the slow counteractive influence of natural
selection. We shall see later that in this way many species have been
eliminated even within historic times.

I have just spoken of the 'need of protection,' and I have a few
remarks to add on that subject. It is a mistake to believe that every
'rare' species, that is, one represented by few individuals, is
already in process of disappearing. It is not the absolute number of
individuals that determines the survival of a species, but the fact
of the number remaining the same. It is equally mistaken to suppose
that an amelioration of the conditions of existence for any species
by natural selection is possible only when its persistence is already
threatened; that is, when the number of individuals (the 'normal
number') is steadily decreasing. On the contrary, it is of the essence
of natural selection that every favourable variation which crops up is,
_ceteris paribus_, preserved, and becomes the common possession of the
species, quite independently of whether this improvement is absolutely
necessary to its preservation or not. In the latter case it will simply
become a commoner species instead of a rare one; and every species
is, so to speak, striving to become common and widely distributed,
since every advantageous variation that can possibly be produced is
accumulated and made the common property of the species. But this has
its limits, not only in the constitution and the structure of each
species, but also in the external conditions of its life. If a species
of butterfly be restricted, in the caterpillar stage, to a single, rare
species of plant, its normal number will be, and must remain, a small
one. But if there arise within it a variation in the food-instinct
whereby a second and it may be a commoner plant becomes available, then
the normal number of the species will rise, and perhaps the original
number of individuals may be more than doubled. It is, however, by no
means necessary to assume that the species was previously in process of
decadence; on the contrary its normal number may have remained quite
constant.

So, in the case of the mimetic butterflies, we do not need to assume
that they all previously required protection in the sense that they
would have become extinct had they not assumed a likeness to an
immune species. We may indeed conclude, on other grounds, that it
was the rarer species which increased their number of individuals
by the mimetic protection, and in doing so they certainly enhanced
at the same time their chance of survival as a species. In the more
abundant species mimetic resemblance to species whose unpalatability
rendered them immune could not have been evolved, as it would have been
disadvantageous, not only for the model, but for the mimicking species
itself, while in species less rich in individuals, such resemblance
would necessarily have a protective value, no matter whether the
species was in danger of extinction or not. The process of selection
must have started simply because the mimetic individuals survived more
frequently than the others, and the mimetic resemblance must have
gone on increasing as long as the increase brought with it a more
effective protection. It is, therefore, a fallacious objection to say
that a species, whose existence was threatened, would, considering the
slowness of the process of selection, have died out altogether before
it could have acquired effective protection by mimicking an immune
species. The assumption is false--the widespread, hazy idea that the
process of natural selection can only begin when the existence of the
species is threatened. On the contrary, every species utilizes every
possibility of improvement; and every improvement for which variation
supplies the necessary material is possible. The augmentation of the
profitable variations follows as a necessity from the more frequent
survival of the best-adapted individuals, and this 'more frequent
survival' will be not only a relative one, due to the fact that the
better adapted individuals will be less decimated, it will also be
absolute, because more individuals of the species will survive than
before. Of this _Papilio merope_ may serve as an example; in Madagascar
it now flies about only slightly varied from the original form, var.
_meriones_. Here, therefore, the species is maintained, without the aid
of mimetic protection. We do not know if the reason for this lies in
the absence of an immune model, or in the non-appearance of suitable
mimetic variants, or in other conditions; but we know that without
mimicry the species holds its own against its enemies. But if, in
Abyssinia, a female of this butterfly exhibited variations which would
make her resemble, in any degree, the unpalatable _Danias chrysippus_,
these mimetic variants would be less decimated than the original form
of female, and would, therefore, gain stability, and gradually increase
both in mimetic resemblance and in the number of individuals. But is
this any reason why the original form of the female should diminish
in numbers? In itself, certainly not; the red mimetic females could
increase in number without causing any decrease of the yellow ones,
for the red are in no way in conflict with the yellow, and we must not
think of the number of individuals as so fixed for each species that it
cannot increase. On the contrary, it _must_ increase, as soon as the
conditions of existence are permanently improved, and this happens, in
this case, through the mimetic protection of the red female. We can
thus easily understand how mimetic and non-mimetic females can live
side by side in Abyssinia.

In all the rest of Africa, however, there are only mimetic females
of _Papilio merope_, and none of the colour of the male; these last,
therefore, have been crowded out by the mimetic form, not actively, but
through the more frequent survival of the mimetic form, so that those
like the male became gradually rarer, and finally died out--that is,
ceased to occur. The matter is not so simple as it seems, and we shall
best understand it by thinking of the dimorphism of the caterpillars
of our hawk-moths, which we discussed before, in which the green form
in the full-grown caterpillar is less well protected than the brown.
In many species the brown form has crowded out the green, in others
brown and green occur side by side, but the green is less abundant, and
in some species very rare. This must be regarded as the simple result
of the circumstance that a higher percentage of the green than of the
brown caterpillars fall victims to enemies, and thus, in the course of
generations, the green form becomes slowly but steadily rarer. This
will be the case even if the newer and better adaptation raises the
number of individuals (the 'normal number') in the species, for this
increase must always be a limited one, even if it be very great, which
is hardly likely in this case. For the normal number is not determined
by the mortality at one stage, but by that at all the stages of life
taken together. Thus a normal number always persists, notwithstanding
the improved conditions for the species, and, on this assumption, the
form under less favourable conditions cannot permanently hold its own
with that under better conditions, but must gradually disappear. We
can understand, then, that the primitive form of the _Papilio merope_
female may persist even for a long time side by side with the mimetic
form in certain habitats. It is, probably, not a mere chance, that
this should have happened just in Abyssinia, for, in that region, the
mimetic female is still tailed--that is, she has not yet reached the
highest degree of resemblance to her immune model. In the whole of the
rest of Africa the process of the transformation of the female has
already reached its highest point, and on the east and west coasts,
as well as in South Africa, the primitive form of the species is now
represented only by the male.

The gradual dying out of the less favourably conditioned forms of a
species is a law which follows as a logical necessity from the essence
of the process of selection, but its reality may be inferred from the
phenomena themselves. On it depends, as far at least as adaptations are
concerned, the transformation of species.

A beautiful example of the crowding out of a less favoured form of a
species by a more favoured one is afforded by a butterfly of North
America, of which the two female forms have long been known, although
the reason for their dimorphism was not understood. A yellow butterfly,
_Papilio turnus_, not unlike our swallow-tail, has yellow females in
the north and east of the United States, but black ones in the south
and west. There was much guessing as to what the cause of this striking
phenomenon might be, and it was for a time thought that this difference
was directly due to the influence of climate, and, later, the black
form of female was regarded as protectively coloured, because of the
supposed greater persecution by birds in the south, since the female
would be less easily recognized if of a dark colour, and would thus be
better protected. This last explanation could hardly be looked upon as
satisfactory, for a black butterfly in flight would be very easily seen
by sharp-sighted birds; indeed, against a light background, it would be
even more readily seen than a light one.

Since we have acquired a more exact knowledge of the immune species
of _Papilio_ this case has become clear to us. For on those stretches
of country on which the black female of _Papilio turnus_ lives there
occurs another _Papilio_ which is black in both sexes, _Papilio
philenor_, and this is one of those species which are protected by
their unpleasant taste and odour. Here, therefore, we have a case of
mimicry, the female of _Papilio turnus_ imitates the immune _Papilio
philenor_, and thereby secures protection for itself; but as the immune
model only occurs in the southern half of the distribution of _Papilio
turnus_ a somewhat sharp separation of the two forms of female
has been evolved; the black, mimetic form, being the most fit, has
completely crowded the primitive yellow form out of the area inhabited
by _Papilio philenor_, while beyond this area, to the north and west,
the yellow form alone prevails. The extensive and careful studies of
Edwards have shown that the two forms occur together only in a very
narrow transition region.

We thus see that the facts, wherever we scrutinize them carefully,
harmonize with the theory. Of course we can only penetrate to a
certain depth with the theory of selection, and we are still far from
having reached the fundamental causes of the phenomena. Indeed, our
understanding must in the meantime stop short before the causes of
variations and their accumulation, but up to that point the theory
gives us clearness, and discloses the causal connexion of phenomena
in the most beautiful way. Although we do not yet understand how the
southern female _Papilio turnus_ was able to produce the advantageous
black, we do see why a black variation, when it did occur, should
increase and be strengthened, until it crowded out the yellow form from
the area of the immune model, and we are able in a general way to refer
the whole complicated phenomena of mimicry to their proximate causes.

This is true also of other phenomena which have had no part in
establishing the theory, since attention was only directed to them
later, and it is true even of some which, at first sight, seem to
contradict the theory altogether. To this class belongs, for instance,
the phenomenon that immune species not unfrequently mimic each other,
as was first observed among the Heliconiid-like butterflies of South
America. In four different families, the Danaidæ, the Neotropidæ, the
Heliconiidæ, and the Acræidæ, there are species, distributed over the
same area, which resemble each other in their conspicuous colouring
and marking, and also in the peculiar shape of the wings. After what
has been said one might be inclined to regard one of these species as
the unpalatable model and the others as the palatable mimics, but they
are all unpalatable, and are not eaten by birds. The puzzle of this
apparent contradiction was solved by Fritz Müller[4], who pointed out
that the aversion to non-edible butterflies is not innate in birds, but
must be acquired. Each young bird has to learn from experience which
victim is good to eat, and which bad. If every inedible species had
its particular and distinctive colour-dress a considerable number of
individuals of each species would fall victims to the experiments of
young birds in each generation, for a butterfly which has once been
pecked at, or squeezed by the bill of a bird, is doomed to die. But if
two inedible species which resemble each other inhabit the same area
they will be regarded by the birds as one and the same, and if five or
more inedible species resemble each other all five will present the
same appearance to the bird, and it will not require to repeat on the
other four the experience of unpalatability it has gained from one.
Thus the total of five species will be no more severely decimated by
the young birds than each of them would have been if it had occurred
alone; the same number of victims of experiment, which are necessary
every year in the education of the young birds, will, when all five
species look alike, be divided among the whole 'mimicry ring,' as we
may say. The advantage of the resemblance is thus obvious, and we
can understand why a process of selection should develop among such
inedible species which should result in their being readily mistaken
for one another; we can understand why, in the neighbourhood of Fritz
Müller's home, Blumenau, in the province of Santa Catarina in South
Brazil, the Danaidæ, species of _Lycorea_; the Heliconiidæ, _Heliconius
eucrate_ and _Eueides isabella_; and the Neotropinæ, _Mechanitis
lysimnia_ and species of _Melinæa_, should all exhibit the same
colours, brown, black and yellow, in a similar pattern, on similarly
shaped wings. The agreement is by no means perfect in detail, but it
can be noticed in all parts of South America inhabited by species of
these genera, and the same differences which distinguish, for instance,
the two species of _Heliconius_ flying in two different regions,
also distinguish the two species of _Eueides_ and the two species
of _Mechanitis_. In Honduras we find the same mutually protective
company of inedible genera as in Santa Catarina, but represented by
other species, which all differ from the species in Santa Catarina
in the same characters, as, for instance, that they have two instead
of one pale yellow cross-stripe on the anterior wings. The species
are: _Lycorea atergatis_, _Heliconius telchinia_, _Eueides dynastes_,
_Mechanitis doryssus_, and _Melinæa imitata_[5]. In the environs of
Bahia this mimicry ring consists of the following species: _Heliconius
eucrate_, _Lycorea halia_, _Mechanitis lysimnia_, and _Melinæa ethra_,
as figured on Pl. II, Fig. 12, iv, and such a mutual assurance society
has always one or other edible species as mimic. The larger the mimetic
assurance company is, the less harm can mimics do to it. In the case
figured it is two Pieridæ already known to us that have fairly well
assumed the Heliconiid guise, namely, _Dismorphia astynome_ (Pl.
II, Figs. 18 and 19) and _Perhybris pyrrha_ (Pl. II, Figs. 16 and 17).
In the latter of these the male still has, on the upper surface, just
the appearance of one of our common Garden-whites, while the female is
coloured quite like the Heliconiidæ, but without having lost the form
of wing of the Whites. The larger the mimetic company is the greater
will be the protection afforded to its palatable mimics, since they
will be the more rarely seized by way of experiment. It is, of course,
obvious that in this kind of mimicry--that is, in the imitation of an
unpalatable and rejected species for protection--it is presupposed as
a general postulate that the edible mimics are considerably in the
minority, as Darwin showed; for if it were otherwise their enemies
would soon discover that among the apparently unpalatable species there
were some which were pleasant to taste. Here, too, the facts bear out
the theory, although exceptions can easily be imagined, and do seem to
occur.

[4] _Kosmos_, vol. v, 1881, p. 260 onwards.

[5] According to Poulton's report in _Nature_, July 6, 1889, of 'Sykes,
Natural Selection in the Lepidoptera,' _Trans. Manchester Microscop.
Soc._ 1897, p. 54.

PLATE I

 FIG

 1. PAPILIO MEROPE, MALE, AFRICA.

 2. THE SAME SPECIES, ONE FORM OF MIMETIC FEMALE.

 3. DANAIS CHRYSIPPUS, AFRICA, IMMUNE MODEL OF FIG. 2.

 4. PAPILIO MEROPE, SECOND FORM OF MIMETIC FEMALE, S. AFRICA.

 5. AMAURIS NIAVIUS, S. AFRICA, IMMUNE MODEL OF FIG 4.

 6. PAPILIO MEROPE, THIRD FORM OF MIMETIC FEMALE, S. AFRICA.

 7. AMAURIS ECHERIA, S. AFRICA, IMMUNE MODEL OF FIG. 6.

 8. DANAIS ERIPPUS, IMMUNE MODEL OF FIG. 9, CENTRAL N. AMERICA.

 9. LIMENITIS ARCHIPPUS, CENTRAL N. AMERICA, MIMICS THE FOREGOING
 SPECIES.

 10. DANAIS ERIPPUS, (_a_) CATERPILLAR, (_b_) PUPA.

 11. LIMENITIS ARCHIPPUS, (_a_) CATERPILLAR, (_b_) PUPA.

_To face Plate I_

[Illustration: PLATE I. LONDON: EDWARD ARNOLD.]

This comparative rarity is true of the imitators of the Heliconiidæ
and their great mimicry ring of unpalatable species, and is very
general. Thus, for instance, there is a series of palatable mimics of
the beautiful blue _Euplœæ_ of the Indo-Malayan region (Pl. III, Figs.
25 and 27), but each of these mimics is rare compared with the hosts
of the blue unpalatable company, for these immune butterflies also
occur in many species, all similar to _Euplœa midamus_ or _binotata_
(Pl. II, Figs. 1 and 3); and the same applies to the mimics of the
Indo-Malayan Danaidæ. There are a great many _Danais_ species, all of
them resembling _Danais vulgaris_ (Pl. III, Fig. 20), which, when they
occur together, form an inedible ring, and this ring is imitated by a
whole series of edible species, each of which is comparatively rare.
And there are no fewer than six species of _Papilio_ which resemble
these Danaids to the point of being easily mistaken for them, while
another rare _Papilio_ effectively copies the iridescence of the blue
_Euplœæ_--a coloration so unusual in the genus that the species has
received the name of _Papilio paradoxus_.

But even in single species of butterflies immune through unpalatability
there is usually a great abundance of individuals. Thus _Danais
chrysippus_, which is distributed over the whole of Africa, is a very
common butterfly wherever it can live at all; and in North America,
in which country there are only two widely distributed species of
_Danais_, these often occur in enormous numbers. The beautiful large
_Danais erippus_ Cramer (Pl. I, Fig. 8), is distributed over almost all
America, and in many places is not only frequent, but occurs in great
swarms. Usually it peoples the broad, open stretches of the western
prairies of the United States, but when violent winds blow, as they
do there in September especially, the insects are driven together
into the small wooded spots of the prairie, and then they cover the
trees in incredibly large crowds, often so thickly that the leaves are
entirely hidden, and the trees look brown instead of green. Millions of
butterflies go to make up such swarms, which have been observed in many
parts of the United States, even quite in the East, in New Jersey, and
elsewhere.

Considering this extraordinary abundance of the immune species, it
is not surprising that its palatable copy, _Limenitis archippus_
(Pl. I, Fig. 9), should also be widely distributed in North America,
and in many places it is not rare, but even abundant. The enormous
majority of _Danais erippus_ will protect the species which resembles
it so closely, even though it is not rare. Any doubt as to this being
a case of mimicry disappears in face of the fact that, in Florida,
there flies a second very similar but much darker brown North American
_Danais_, and that it is accompanied there by an equally dark variety
of _Limenitis archippus_ (_L. eros_).

To prove the correctness of the hypothesis of an actual process of
selection--which we assume in our interpretation of mimicry--I mean
the assumption that the disguise of the species seeking protection
really deceives the enemy, and thus actually affords protection, I
need only cite the evidence of an acute and experienced entomologist
who was himself deceived by it. Seitz[6], to whom we owe many valuable
biological observations on butterflies, relates that, while he was
collecting in the neighbourhood of the town of Bahia, he was surrounded
by swarms of _Catopsiliæ_, similar to our lemon butterfly, especially
the common _Catopsilia argante_, but he took no notice of these, as he
'had already collected as many of them as he wanted.' It was only when
he saw a pair _in copula_ that he caught them in his net. But to his
extreme surprise he found that he had not caught a _Catopsilia_, but a
butterfly of the family Nymphalidæ, one of those _Anææ_ whose numerous
species are distributed over South America. These _Anææ_ are dark, or
beautifully bright on the upper surface, but on the under side are
leaf-coloured, and one of them bears the name _Anæa opalina_, because
it is quite clear and pale, and of opal-like brilliance. The captive
was nearly related to this species. Seitz was so much surprised
by the discovery that the male, which had quickly detached itself from
the female, escaped him, and he could only make out that, 'as it flew
away, it unfolded dark wings, which certainly bore little resemblance
to those of the lemon butterfly.' In the hope of securing more of
this rare booty he then hunted only for _Catopsilia argante_, without
however securing another coveted specimen--he caught no more _Anœæ_,
which shows that in this case, too, the mimetic species was much rarer.

[6] In citing this observation of Seitz, I do not mean to assert that
there is true mimicry between _Anæa opalina_, or its allied species
in Bahia, and the _Catopsilia_, though I regard this as extremely
probable, because of the marked dimorphism between the male and the
female, in conjunction with the very striking resemblance of the female
to the _Catopsilia_. The example was given only to show how very
deceptive such resemblances may be. To assert with confidence that it
is a case of mimicry we should require to know that _Catopsilia_ is
immune, and on that point we have as yet no information.

PLATE II

 FIG.

 12-15 REPRESENT A 'MIMICRY-RING' COMPOSED OF FOUR IMMUNE SPECIES
 BELONGING TO THREE DIFFERENT FAMILIES AND FOUR DIFFERENT GENERA.

 12. HELICONIUS EUCRATE, BAHIA.

 13. LYCOREA HALIA, BAHIA.

 14. MECHANITIS LYSIMNIA, BAHIA.

 15. MELINÆA ETHRA, BAHIA.

 16, 17. PERHYBRIS PYRRHA, MALE AND FEMALE, S. AMERICAN 'WHITES'
 (PIERIDÆ). THE FEMALE MIMICS AN IMMUNE HELICONIID, WHILE THE MALE
 SHOWS ONLY AN INDICATION OF THE MIMETIC COLOURING ON THE UNDER SURFACE.

 18, 19. DISMORPHIA ASTYNOME, MALE AND FEMALE, ALSO BELONGING TO THE
 FAMILY OF 'WHITES,' AND MIMICKING IMMUNE HELICONIIDS; A WHITE SPOT ON
 THE POSTERIOR WING OF THE MALE IS ALL THAT REMAINS OF THE ORIGINAL
 'WHITE' COLORATION.

 20. ELYMNIAS PHEGEA, W. AFRICA, OF THE FAMILY SATYRIDES, MIMICS THE
 FOREGOING SPECIES.

 21. ACRÆA GEA, AN IMMUNE W. AFRICAN SPECIES.

 22. DANAIS GENUTIA, AN IMMUNE DANAID FROM CEYLON.

 23. PLYMNIAS UNDULARIS, FEMALE, ONE OF THE MIMICS OF FIG. 22. THE
 MALE, WHICH IS QUITE DIFFERENT, IS FIGURED ON PLATE III (FIG. 24).

_To face Plate II_

[Illustration: PLATE II. LONDON: EDWARD ARNOLD.]

We see, then, that the need for protection in butterflies has a great
influence on their external appearance, especially as regards their
colour and marking. First, because the resting insect frequently has
the visible surfaces sympathetically coloured, and also, because there
are numerous species, indeed whole families, which contain nauseous,
perhaps even actually poisonous, juices, and these have been subject
to a double process of selection, directed towards the increase of the
nauseousness, and at the same time towards acquiring as conspicuous
a dress as possible. Thus the whole surface of these butterflies
became gaily coloured, and often--as in many of the tropical nocturnal
Lepidoptera which fly by day, the Agaristidæ, Euschemidæ, and
Glaucopidæ--quite glaringly bright. We thus understand the striking
or at least readily recognizable colours of the Heliconiidæ, the
Euplœæ, the Danaidæ, and the Acræidæ. Finally, the unpalatable species
influence many others which are edible, since the latter strive to
resemble an immune species; and how considerable the variations and
colour transformations thus induced can be is shown by the Whites of
the genus _Perhybris_ (Pl. II, Figs. 16 and 17) and _Archonias_, in
which the male has wholly or partially retained the primitive dress of
the Whites, and in which, side by side with wholly mimetic species,
other species occur in which both sexes exhibit the garb of the Whites
unaltered. Such cases tell decidedly against the often expressed view
that mimetic species must have had from the outset a great resemblance
to the model; they show rather that very great deviations in form,
but more especially in colour, have been brought about solely by the
necessity for mimetic adaptation, and that they have come about only
slowly and step by step, as the different grades of resemblance to the
model in different species of the same genus clearly show.

Lepidoptera are by no means the only insects which exhibit the
phenomenon of mimicry, nor are insects the only animals in which it
occurs; and unpleasant taste and odour are not the only protective
characters; there are many others, as, for instance, among insects, the
hardness of the chitinous cuticle.

One of the most beautiful examples of mimicry was discovered by
Gerstäcker, not in free nature, but in the entomological collection at
Berlin. There he found beside a green, metallic weevil-beetle, one of
the Pachyrhynchidæ from the Philippines, two other insects with the
same metallic sheen and very similar form of body. They had been put in
beside the weevil as duplicates, but more careful observation showed
that they were delicate Gryllidæ, which mimicked the hard beetles
so deceptively that even the practised eye of the entomologist was
misled by them. Later on it was shown that these Gryllids live in the
Philippines beside the weevils, and even on the same leaves with them,
and that the beetles are protected from the attacks of birds and other
enemies by the extraordinary hardness of their cuticle. The case is
especially remarkable because in general the Gryllidæ have no metallic
shimmer, and the form of body must have been considerably altered to
make them resemble the beetle. The usually broad head of the Gryllids
is in this case narrower, the usually flat wing-covers are arched and
pear-shaped, and the legs have become quite beetle-like. The security
enjoyed by the weevil must be very perfect, for it is mimicked by three
other species of beetle in the Philippines.

Animals can also be protected from attack by the possession of
dangerous weapons. To this class belong insects with poisonous
stings, like the bees, wasps, and ants, and in some degree also the
ichneumon-flies. We cannot wonder, therefore, that these dreaded
species find imitators. In this case it is not of so much importance
that the copy should be rarer than the model, for anything that looks
like a dangerous insect will be avoided, since close investigation is
in this case attended with danger. So we find that hornets, wasps,
and bees are frequently imitated by other insects, by beetles, flies,
and butterflies; and these must derive a certain advantage, even
when the resemblance is only a general one. Many Longicorns, which
visit flowers, are striped black and yellow, like a wasp, and so are
many flies, like the species of _Syrphus_, and so on. The Longicorn
_Necydalis major_ bears a strong resemblance to a large ichneumon-fly;
it has the same long-drawn-out body, the same swellings on the femur
and tibia, the curved antennæ, the glossy brown colour, and its
wing-covers are quite short, leaving the wings free, so that the
deception is very complete.

Bees, too, are sometimes so well imitated that they are hardly to be
distinguished from their mimics, not in flight only, but also when
visiting flowers. The best and commonest mimic of our honey-bee is
a perfectly harmless fly of the same size and colour, the drone-fly
(_Eristalis tenax_). The two are often to be seen together on the
same flowering shrub, as, for instance, in autumn, on the Japanese
buckwheat of our gardens (_Polygonum sieboldii_), both busily seeking
for honey. I once noticed a boy catching the flies with a net in order
to imprison them, but a bee stung him severely in the finger. He
immediately abandoned the chase, and gave up the flies, perceiving the
dangers of confusion. So the animal enemies of _Eristalis_ will often
prefer to leave it in peace rather than run the risk of being stung.

PLATE III

 FIG.

 24. ELYMNIAS UNDULARIS, MALE OF THE SPECIES OF WHICH THE MIMETIC
 FEMALE IS DEPICTED IN FIG. 23.

 25. EUPLŒA BINOTATA, IMMUNE INDIAN SPECIES, MIMICKED BY

 26. ELYMNIAS LEUCOCYMA, MALE, OF WHICH

 27. EUPLŒA MIDAMUS.

 28. THE FEMALE MIMICS FAIRLY CLOSELY

 29. DANAIS VULGARIS, IMMUNE INDIAN DANAID.

 30. ELYMNIAS LAIS, MIMETIC OF THE FOREGOING SPECIES, BUT ONLY ON THE
 UPPER SURFACE. THE LOWER SURFACE RETAINS THE ORIGINAL PROTECTIVE
 COLOURING REPRESENTING A DECAYING LEAF.

 31. TENARIS BIOCULATUS, FROM THE PAPUA REGION.

 32. ELYMNIAS AGONDAS, MIMICS THE FOREGOING SPECIES FROM THE SAME
 LOCALITY.

_To face Plate III_

[Illustration: PLATE III. LONDON: EDWARD ARNOLD.]

There is still another relation between two species which can be
induced by mimicry--namely, parasitism, when, for instance, the
so-called cuckoo-bees and parasitic humble-bees deceptively resemble
in colour, arrangement of hair, and form of body, the species into
whose nests they smuggle their eggs, to have them brought up at the
expense of the bee or humble-bee in question. In the same way, among
the numerous parasites of ant nests, there are some which copy the ants
themselves, and so secure themselves from molestation, although they
devour the ants' eggs and pupæ. Thus, among the hosts of South American
driver-ants (_Eciton prædator_) there lives a predaceous beetle of the
family Staphylinæ, which has received the name _Mimeciton_ because it
resembles the ant in form and in the nature of the external surface,
though not in colour, which is to be explained by the fact that this
ant has no compound eyes, and is therefore almost blind, or at any rate
cannot see colours.

I should never come to an end were I to attempt to exhibit the great
wealth of observations now available in regard to mimicry. But this
at least may be added, that isolated cases of mimicry have been found
even among Vertebrates. Thus, according to Wallace, the red-and-black
striped poisonous coral snake of South America (_Elaps_) is most
realistically imitated by a non-poisonous snake (_Erythrolampus_) of
the same region. Among birds, Wallace cites a few cases which may be
regarded as mimicry, but none are known among mammals, which is not
to be wondered at when we consider how very much less numerous in
individuals the species are which live together on one area, and how
much less likely it is that two species should be, to begin with, so
near each other in size, habit, and form that the process of natural
selection could bring about a deceptive degree of resemblance.
Without doubt it is among insects that the conditions for mimicry are
especially favourable, partly because of the enormous number of species
which live together and have interrelations on the same area, even in
our latitudes and much more so in the tropics, and also because of
their usually great fecundity, and their rapid multiplication, both of
which are factors favourable to starting and continuing the processes
of natural selection. Furthermore, we have to take into account the
hosts of enemies which depend wholly or in great part on insects for
food, and destroy them in enormous numbers, eliminating them in inverse
proportion to the perfection of their adaptation. Finally, there is the
extreme susceptibility of many insects to injury. This makes it very
desirable that they should have some disguise sufficient to protect
them from even the first attempt at an attack, since that would in many
cases prove fatal.




LECTURE VI

PROTECTIVE ADAPTATIONS IN PLANTS

 Protection against large animals--Poisons--Ethereal oils--Spines and
 thorns--Sharp and stinging-hairs--Felt-hairs--Position of the thorns:
 buckthorn--Tragacanth shrub--Prigana scrub--Alpine shrubs--Protection
 against small enemies--Chemical substances--Mechanical protective
 arrangements--Raphides--Conclusion.


WE have seen in how many different ways animals are able to adapt
themselves to the conditions of life, both protectively and
aggressively; how they approximate in their colour to that of their
surroundings so that they harmonize with it; how they copy lifeless
objects, or parts of plants, leaves, or twigs, or even mimic, in form
and colour, other animals which are in some way protected. When we
consider that by far the greater number of species find protection in
some degree through their colouring, and often through their form, and
when, at the same time, we remember how different this colouring is in
nearly related species, and even within the same species (dimorphism),
we can scarcely avoid the impression that the forms of life are made of
a plastic material, which, like the sculptor's clay, can be kneaded at
will into almost any desired form.

This impression is corroborated when we turn our attention to plants,
and consider the different ways in which they are able to protect
themselves from the attacks of animals.

That plants stand in need of some protection is obvious enough, since
their leaves and other green parts contain much nourishment, and an
endless army of animals, large and small, depends upon these alone for
sustenance. Indeed, the existence of animals depends altogether on the
occurrence of plants, for carnivorous and saprophytic animals could
only arise after vegetarian forms had been already in existence. But if
the green parts of the plants were left defenceless at the mercy of the
multitude of herbivorous animals, it would not be long before they were
exterminated from the face of the earth, for the animals would devour
unsparingly whatever was within their reach, and, as their increase
does not depend on their ratio of elimination alone, but also on their
fertility, and on their rapidity of multiplication, they would go on
increasing in numbers at the expense of the superabundant nourishment
until the plants on which they depended were themselves consumed.

When we inquire into the means whereby plants evade such a fate we are
astonished at the endless diversity of the devices employed.

Let us consider first of all the menace to plants from the larger
herbivores, from elephants and cattle down to the hare and the
roe-deer; we find that many plants are protected by poisons, which
develop in the sap of their stems, leaves, roots, and fruits. The
juicy and beautifully leaved Belladonna (_Atropa belladonna_) is never
touched by roe-deer, stags, or other herbivores, and the same is true
of the thorn-apple (_Datura stramonium_), the henbane (_Hyoscyamus
niger_), the spotted hemlock (_Conium maculatum_), the danewort of
our woods (_Sambucus ebulus_), and many others; they all contain a
poison. Like the unpalatable butterflies, these unpalatable plants are
also furnished with a warning sign of their undesirability, namely, a
disagreeable odour, perceptible even by man, which scares off animals
from touching them. The development of this through natural selection
presents no very serious difficulty.

But, strangely enough, there are not a few poisonous plants in which
we, at least, are unable to detect any such warning sign. Among these
are the blue aconite (_Aconitum_), the black hellebore (_Helleborus
niger_), the meadow-saffron (_Colchicum autumnale_), species of
Gentian, of spurge (_Euphorbia_), and others. Yet these are avoided by
deer, roe-deer, chamois, hares, and marmots, and our cattle, horses,
and sheep also usually leave them untouched. A case has, however, been
reported from the valley of the Aur, on the lower Rhine, which seems to
contradict this. On the rocky grass-slopes of the valley the poisonous
hellebore (_Helleborus viridis_) grows in great abundance, and the
sheep of that region, which were wont to graze on the slopes, avoided
these plants. But some sheep from another part were imported into the
valley, and these ate the hellebore, with the result that many died.
If these poisonous plants, then, were furnished with a warning sign
such as a disagreeable odour, not perceptible to us, we should have to
assume that the imported sheep had a less acute sense of smell than
the others, which is not impossible in domesticated animals. If there
were no such warning sign, then it must have been not an instinct but a
continuous _tradition_ which prevented the native sheep from touching
the inedible plants.

A more naïve interpretation of nature than that of our day would have
regarded the fragrant ethereal oils developed in the seeds of many
plants, as in those of fennel, cummin, and other Umbelliferous plants,
as a peculiarity designed for the use and profit of man. But these
ethereal substances are obviously a means of protection against the
depredations of seed-eating birds, for a sparrow which was allowed to
eat three or four seeds of cummin died very soon afterwards.

Many plants produce bitter substances in their green parts, and so
secure at least some measure of protection, as is the case with the
majority of mosses, the ferns, and species of _Plantago_ and _Linaria_.
Others, again, deposit silicic acid in their cell-walls, or develop in
addition a very thick epidermis, so that they afford at the best an
unpleasant food, e.g. many grasses, the horse-tails, the rhododendron,
and the bilberry. Others, again (_Alchemilla vulgaris_), have
cup-shaped leaves, which retain rain and dew for a long time, and this
protects them from grazing animals, which are unwilling to touch wet
grass and plants.

Especially widely distributed and diverse is the protection of plants
by sharp thorns and spines. It is extremely interesting to note in how
many different and advantageous ways this armature is disposed.

Obvious at once is the fact that thorns and spines only occur on
those parts which are naturally exposed to attack. Thus we find them
particularly strong in young plants, and on the lower parts of older
ones. The holly, for instance, has crenate, spinose leaves only to the
height to which grazing animals can reach; beyond that the leaves are
smooth-edged and spineless, like those of the camelia. It is almost the
same with some wild pear-trees, which are quite covered with thorns as
long as they are low, but afterwards grow a thornless crown.

Similarly, low bushes, when they are armed with thorns or the like at
all, are covered with them all over, like the rose-bush.

When the leaves of a plant are spinose the spines are disposed on
the parts usually attacked; and thus we understand why the enormous
floating leaves of _Victoria regia_ should have on their under surface
long, pointed spines which, especially at the upturned margin, attain a
length of several inches; it is from water animals--water snails--that
danger threatens them.

Thorns are developed in the most diverse ways. In many of the bushes
on the coast of the Mediterranean true leaves are wanting altogether,
the green branches and twigs being themselves the assimilating parts,
and these are so stiff and rigid, so like some kind of thorn, that they
suffice to scare off any greedy herbivore. Among our own bushes the
Broom (_Spartium scoparium_) may be taken as an example of this class.

In other cases the spines are found on the leaves themselves, but
there is great diversity in their mode of arrangement. In many
tropical plants, such as the Yucca and the Aloe, the point of the
long, reed-shaped leaf is transformed into a spine, and this is the
case in many of our native grasses. Kerner von Marilaun notes that, in
the Southern Alps, two such grasses, _Festuca alpestris_ and _Nardus
stricta_, occur frequently in certain localities, and they prick the
muzzles of the cattle so badly that they return bleeding from the
pasture. This prevents these Alpine runs from being made full use of,
so the grasses are as far as possible extirpated by man, and, curiously
enough, also by the cattle themselves, for they seize the grass at the
base of the tuft with their teeth, pull it out, and let it fall, so
that it withers. Kerner saw thousands of such pieces of turf which had
been pulled up by the cattle lying dried and bleached by the sun on
some of the Alpine grazing grounds in the Tyrolese Stubaithal.

Again, in many plants the whole leaf-edge is transformed into a spiny
wall, which may be enlarged by indentations and lobate projections,
as in the holly, and, in a much higher degree, in the thistles
(_Carduus_), in _Eryngium_, in _Acanthus_, and in many Solanaceæ.
Often, too, there are barbed hooks on the leaf-edge, which work like
a saw; or the leaf-edge, though without spines, may be made sharp by
deposits of silicic acid, as in the sedges, whose sharp edges are
moved to and fro in the mouths of ruminants, and thus injure the
mucous membrane. The hook-bristles of the fig-cactus (_Opuntia_),
which, though small, are abundantly provided with barbs, must also be
mentioned; for they are to be found in great numbers surrounding the
buds of these plants, and most effectively protect them from being
eaten away by animals (Fig. 19).

To this category, too, belong the short, prickly bristles of the
rough-leaved plants, which cover the whole plant as with an overcoat
of sharp needles; of these we may mention the adder's tongue (_Echium
vulgare_), the comfrey (_Symphytum officinale_), and the borage
(_Borago officinalis_).

Very well known are the stinging-hairs of the Urticaceæ, long hairs
(Fig. 20) with an elastic base, but with glass-like, brittle, rounded
heads, which break off at the lightest touch, whereupon the sharp
point of the broken hair penetrates the skin of the creature which has
touched it, and the poisonous contents of the hair are poured into
the wound. Even our large stinging-nettle (_Urtica dioica_) can cause
intense irritation, and evoke the 'nettle-rash,' named after it, on
the human skin; but there are many tropical species of nettle, e.g.
_Urtica stimulata_ in Java, and others, which have an effect similar
to that of snake-poison and produce tetanoid spasms, and so on. In
addition to formic acid these hairs contain an undefined ferment, a
so-called Enzyme. It need scarcely be said that these stinging-hairs
must have much more severe effects on the mucous membrane of the mouth
of grazing animals than on the human skin, and that they are therefore
an excellent protection for the plants. As a matter of fact we never
find our nettle patches eaten away, and even the donkey, which eats
thistles freely, turns away from the stinging-nettle. But even these
stinging-hairs, like all other protective devices, do not afford an
_absolute_ protection. The caterpillars of several of our diurnal
butterflies feed exclusively on the stinging-nettle, and they eat up
the leaves, stinging-hairs and all. This is the case with five species
of the genus _Vanessa_, namely: _Vanessa io_, the 'peacock,' _Vanessa
urticæ_, the small tortoiseshell, _Vanessa prorsa_, _Vanessa C. album_,
the C. butterfly, and _Vanessa atalanta_, the admiral.

[Illustration: FIG. 19. Barbed bristles of _Opuntia rafinesquii_;
enlarged.]

[Illustration: FIG. 20. Vertical section through a piece of a leaf of
the Stinging-nettle _(Urtica dioica_), bearing two stinging-hairs;
magnified 85 times; adapted from Kerner and Haberlandt.]

We are all familiar with our mulleins (_Verbascum_), those beautiful
flower-spikes with the thick, soft felt leaves, which grow on stony or
sandy soil. Harmless as they look, they are much disliked by animals
as food, for the thick hairy felt which covers them breaks up in the
mouth, and sticks in the folds of the mucous membrane, causing burning
sensations and other discomforts. They, too, are therefore spared by
grazing animals, but they have smaller enemies, like the caterpillars
of the genus _Cucullia_, which, however, never completely destroy them,
but only eat large holes in their leaves.

Let us now consider in somewhat greater detail the true thorns, the
most conspicuous protection of many plants. It is very remarkable that
these are always so placed, and so regulated as to their length and
character, as to afford protection to the most important and the most
exposed parts of the plant. Thus many bushes, which would otherwise be
in danger of being completely devoured by cattle, are stiff with thorns
which are nothing else than pointed, hard twigs without, or with very
little foliage. Among these are the sloes, the buckthorn (_Rhamnus_),
the sea-buckthorn (_Hippophäe_), and the barberry (_Berberis_). In the
last-named three thorns arise in a group, and protect the young bud
from danger in three directions (Fig. 21).

[Illustration: FIG. 21. A piece of a twig of Barberry (_Berberis
vulgaris_) in spring; after Kerner.]

The fine-leaved mimosas of the tropics have similar but very long and
sharp thorns, and their leaves are movable and sensitive, so that, when
they are touched, they shut up and draw back behind the rampart of
stiff thorns, which are just of the right length to protect them.

In many thorny bushes only the young shoots of each spring remain
green through the summer, and in autumn they become transformed into
thorns, under whose protection the shoots of the following spring will
develop. Sometimes, too, the leaf-stalks are modified in the course of
the summer into thorns, as in Tragacanth (_Astragalus tragacantha_).
In this case the young leaves are protected by a circle of thorns,
consisting of the leaf-stalks of the preceding year which have not
fallen off (Fig. 22, _A_, _B_, _C_).

I should have to go on for a long time with my exposition, even if I
were to confine attention to the essential facts; we shall, therefore,
only recall the well-known phenomenon of the Cactuses, in which the
leaves are entirely transformed into spines, which may attain a length
of eight centimetres, while the fleshy stem alone represents the
green--that is, the assimilating parts of the plant. The species of
Cactus are almost the only plants which grow on the stony, hard, and
hot plateaux of Mexico, and they are protected from desiccation by the
thickness of their epidermis. But, enticing as is the food promised
by the juicy stem, animals rarely venture to approach them, and it is
only when tortured by thirst that horses and asses occasionally knock
off the spines with their hoofs, and so reach the soft tissues rich in
water. For this attempt, however, as Alexander von Humboldt pointed
out, they often suffer, as the sharp spines are apt to pierce the hoof.
In any case, the cactuses are effectively protected from the danger of
extermination by grazing animals.

[Illustration: FIG. 22. Tragacanth (_Astragalus tragacantha_). _A_,
two spring shoots. _B_, a single leaf, from which the three uppermost
leaflets have fallen off. _C_, leaf midrib, from which all the leaflets
have fallen off. After Kerner.]

It must certainly strike every one that many districts, especially
those which are dry, hot, and stony, are conspicuously rich in thorny
plants, and it has often been supposed that the production of thorns
must be a direct result of these peculiar conditions of life; indeed,
the hard, thorny habit of many of these plants has even been regarded
as a protection against desiccation. This, however, is contradicted
by all those thorny plants which, like the cactuses, possess tissues
extremely rich in sap, and in which desiccation is prevented, not
by the thorns, but by the thick epidermis. The only satisfactory
explanation is that afforded in terms of natural selection. In such
hot, and at the same time dry regions, the plant-growth is often very
scanty, and the food available for the grazing animals is, at least at
times, very scarce; on this account, if the plants are to survive there
at all, they must be armed with the most perfect means of protection
possible against the attacks of hungry and thirsty animals. The
struggle for existence in relation to such enemies is much more severe
than in more luxuriant regions, and the protection by thorns has been
developed to the highest possible pitch of perfection; species which
were unable to develop this protection died out altogether. Hence the
cactuses of Mexico, and the many thorny bushes and shrubs of the hot,
and, in the summer, dried-up stony coast-lands of the Mediterranean in
Spain, Corsica, Africa, and other countries. This so-called 'Prigana
scrub' embraces a number of species, whose nearest relatives in our
climate are not provided with spines, as, for instance, _Genista
hispanica_, _Onobrychis cornuta_, _Sonchus cervicornus_, _Euphorbia
spinosa_, _Stachys spinosa_, and others.

Why do so few thorny plants grow on the rich and well-watered Alpine
pastures? Probably because there is to be found there a rich and
luxuriant plant-growth which can never be wholly exterminated by
the grazing of animals, so that an individual species would not, by
developing thorns, have gained any advantage in the way of increased
capacity for existence.

But these Alpine grazing grounds serve well to illustrate how great
may be the advantage which protective devices give to a species.
Much to the annoyance of the herdsmen, who endeavour to extirpate
them as far as possible, enormous masses of rhododendrons often
cover whole stretches, because their hard silicious leaves cannot be
eaten, and many other plants despised of cattle flourish and increase
on the grazing runs, like the repulsively bitter, large _Gentiana
asclepiadea_, the malodorous _Aposeris fœtida_, and various ferns of
disagreeable taste.

The advantage derived by plants from the possession of any kind of
protective device against grazing animals is perhaps best of all seen
in the 'shrubbery,' which on every Alp is to be found in the immediate
neighbourhood of the herdsman's hut. There, where the cattle daily
assemble, and where the soil is continually being richly manured by
them, we always find a large, luxuriantly growing company of the
poisonous aconite, the bitter goosefoot (_Chenopodium bonus henricus_),
the stinging-nettle, the thistle (_Cirsium spinosissimum_), the
ill-smelling _Atriplex_, and some other inedible species, while the
palatable herbs are gradually exterminated by the cattle which daily
gather round the hut (Kerner).

To sum up. We have seen that there is among plants an extraordinary
diversity of protective adaptations, which secures them from
extermination by the larger herbivores.

Since all useful contrivances, or, as we say, all adaptations, are
capable of interpretation in terms of the process of selection, we
must refer this great array of the most diverse protective devices
to natural selection; and again, as among animals, we receive the
impression that the organism is, to a certain extent, really capable
of producing every variation necessary to its maintenance. Literally
speaking, this would not be correct, but at any rate the number of
adaptations possible to each form of life must be an enormous one, so
great, indeed, that ultimately every species does secure protection
for itself in some manner and in some degree, whether it be by the
production of a poison or a nauseous substance within itself, or by
surrounding itself with thorns or spines. And if it be, in a certain
sense, a matter of 'chance' whether a plant has taken to one method of
defence or to another, according as its innate constitution favoured
the production of one rather than of any other, yet it would not
be easy to prove, even in the case of the purely chemical means of
protection, that these would have occurred in the same distribution
and concentration as a necessary result of the metabolism of the
plant, even if they had not been useful and consequently augmented
by selection. But in the case of the mechanical means of protection
this mode of explanation fails as utterly as that of the direct
effect of the conditions of life. Why the holly should have spinose
leaves beneath and smooth ones above can never be deduced from the
constitution of the species.

While the protective adaptations of plants against the larger
herbivores always point to natural selection, our appreciation of the
adaptability of plants, and at the same time of the potency of natural
selection, will be strengthened still more if we turn our attention for
a little to the arrangements which prevent the extermination of plants
by the lower and small animals.

It might indeed be supposed that extermination by these could hardly
be an imminent danger, but if we think of the cockchafer blight, or of
the destruction of whole woods by the caterpillar of the 'white nun,'
or even of the destruction of several successive plantings of young
salad plants which the snails often cause in our gardens, it cannot be
doubted that all plants would be exterminated by insects and snails
alone unless they were protected against them in some degree.

We owe our detailed knowledge of the means by which plants protect
themselves against the menace of the greedy and prolific snails to
the beautiful investigations of Stahl, Professor of Botany in the
University of Jena.

In this case, too, both chemical and mechanical means are made use of.
The minute quantity of tannic acid which is contained in the leaves of
the clover prevents many snails from eating them, as, for instance,
the garden snail (_Helix hortensis_). If the leaves be soaked so as
to wash out the tannin the snail readily accepts them as food. It is
true that the small, whitish field-slug (_Limax agrestis_) does not
object to the presence of the tannin, and eats the fresh leaves of
the clover; indeed, there is no such thing as absolute protection.
In discussing the herbivorous mammals I have already mentioned that
many trees and shrubs, mosses and ferns are effectively protected by
the large amount of tannin they contain; this protection is effective
also against snails, for all these plants are fairly free from their
attacks; and the same is true of many other tannin-containing plants,
species of saxifrage and sedum, the strawberry, many water-plants, like
the pond-weeds (_Potamogeton_), the horn-nut (_Trapa_), the mare's tail
(_Hippuris_). All these plants are only eaten by snails in case of
necessity, or in the washed-out state.

In other plants protection is gained by means of some acid, especially
oxalic acid, like the wood-sorrel (_Oxalis acetosella_), the sorrel
(_Rumex_), and the species of Begonia. When Stahl smeared slices of
carrot, which is a favourite food of snails, with a weak (one per
cent.) solution of oxalate of potassium, they were refused by the
snails, and this is not surprising when we remember that even the
external skin of the snail is very sensitive, and the mucous membrane
of the mouth is not likely to be less so.

Similarly, many plants develop ethereal oils in the hairs which cover
them, as in the herb-Robert (_Geranium robertianum_). Even the almost
omnivorous field-slug (_Limax agrestis_) does not touch this plant, and
if it be placed upon it, escapes with all dispatch from the ethereal
oil, which burns its naked skin, by covering itself with mucus and
letting itself down to the ground by a thread. The mints (_Mentha_) and
the dittany (_Dictamnus albus_) also produce such oils.

Among chemical means of protection must be named the pure bitter
stuffs, such as are found in the species of gentian, the milkwort
(_Polygala amara_), and in many other plants, and also the curious
'oil-bodies' of the liverworts.

But some plants also defend themselves against the attacks of snails by
mechanical means.

First there are the various kinds of bristle arrangements, which
prevent the snails from creeping up the stalks. We never find the
comfrey (_Symphytum officinale_) of our meadows eaten by snails,
for it is thickly covered over with stiff bristles, which are most
disagreeable to the snail, and the stinging-nettle (_Urtica dioica_) is
similarly protected by bristle hairs, while, as we have already seen,
its stinging-hairs secure immunity from the attacks of larger animals.

And although it is true that the majority of plants do not prevent the
snails from creeping up their stalks, yet they do not serve them in any
great degree as food, since the green parts often offer resistance to
mastication and digestion. Thus the lime encrustations which cover the
stoneworts (_Chara_) prevent snails from eating them. If the lime be
dissolved by means of acids, and the plants then offered to the snails,
they will eat them greedily. The same is true of the silicifying of
the cell-walls, so widely distributed among mosses and grasses, and
when this occurs in a high degree it forms an effective protection
even against the large herbivores. Our slightly siliceous grasses are
secure from snails, and that it is really the presence of the silicic
acid which deters them from an otherwise welcome kind of food is proved
by Stahl's experiment of growing maize in pure water, and so obtaining
plants poor in silica. These were devoured without ceremony by the
snails.

Of the many other protective peculiarities which make it difficult for
snails to eat plants I shall only recall the so-called 'Raphides,'
those microscopic crystal-like needles of oxalate of lime, pointed
at both ends, which lie close together in the tissues of many
plants. Cuckoo pint (_Arum maculatum_), the narcissi, the snowdrops
(_Leucojum_), the squill (_Scilla_), and the asparagus contain them,
and all these plants are spared by snails obviously because during
mastication they are unpleasantly affected by the raphides. Even the
voracious field-slug rejects these.

Of course it cannot be said that these raphides protect against all
other enemies. They are effective against rodents and ruminants,
and also against locusts, but a number of caterpillars seek out by
preference just those plants which contain raphides. Thus certain
caterpillars of the Sphingidæ feed on species of _Galium_ and
_Epilobium_, the leaves of the vine, and the wild balsam (_Impatiens_).
The caterpillar of _Chærocampa elpenor_, which especially prefers
_Vitis_ and _Epilobium_, has transferred its affections to the fuchsias
in our gardens, which came from South America; the butterfly not
infrequently lays its eggs on these plants, and the caterpillars devour
them readily; but the fuchsias may also contain raphides.

We may say, indeed, that almost all wild Phanerogams are protected in
some degree against snails, and this almost suggests the question: What
then is left for the snails to feed on if everything is thus armed
against them? But, in the first place, there remain our cultivated
plants, which, like the garden lettuce (_Lactuca_), are quite without
defence; and secondly, the snails often eat the plants only after
they have been rooted up and lie rotting on the ground, that is,
when the protective ingredient has been dissolved out by the rain;
finally, no means of protection, as I have often said already, is
absolute or effective against all snails. Many of these are, as Stahl
calls them, 'specialists.' Thus, the large slug of our woods eats the
poisonous fungi which are rejected by other snails, and in the same
way there are many other specialists which, however, are not likely to
eliminate unaided the plants to which they have adapted themselves.
There are certainly also omnivorous forms, like the field-slug
(_Limax agrestis_), to which we have referred so often, and _Arion
empiricorum_, the red slug, but just because these eat so many kinds of
plant they are less dangerous to any one species.

These manifold devices for protecting plants against the depredations
of snails afford another proof that innumerable details in the
organization of plants, as of animals, must be referred to natural
selection, since they are capable of interpretation in no other way.
If these protective devices were to be found only in isolated plants,
we might perhaps talk of 'chance'; we might refer them to the inborn
constitution of the plant, which made the production of bristles, or
bitter stuffs, or the deposition of silicic acid a necessity, and which
'happened' to make the plants distasteful to certain snails. But as
it appears that all plants are protected against snails, one in this
way, another in that, this objection cannot be sustained. Furthermore,
some of the beautiful experiments made by Stahl to prove the protective
effect of these devices showed, at the same time, that they were not
in themselves indispensable to the existence of the plant; maize, for
instance, develops a plant perfectly capable of life, even though
silicic acid be withheld, and the acid is, therefore, not an element
essential to its constitution, but a means of protection against
voracious animals. The clearest proof of this is afforded by plants
like the lettuce (_Lactuca_), which formed protective stuffs in the
wild state, but have lost them altogether under cultivation, through
disuse, as we shall see more precisely later on. As the eyes of animals
which live in darkness have degenerated, so the plants which have been
taken under the protection of man have lost their natural means of
defence, because these were no longer necessary to the maintenance of
the species. Even the protective bitter substances (tannin-compounds)
are not essential to the constitution of the genus _Lactuca_; their
formation may be discontinued without the plant being otherwise
affected. And in this case it is not a question of the withdrawal
of something which has to be taken in from outside, it is the
non-development of what is purely a product of the internal metabolism.

The adaptations of plants against snails are instructive in another
way, namely, in their extraordinary diversity. Here again we see how
great is the plasticity of organic forms, and how precisely, though
in many very different ways, they adapt themselves to the conditions
of their life, in this case the weaknesses of their greedy enemies,
and all to attain the same end, the security of their existence as a
species. We see at the same time that innumerable minute details in the
structure and character of a species, which may appear unimportant, may
yet have their definite uses--hairs, bristles, and raphides, as well as
bitter substances, ethereal oils, acids, and tannin-compounds. But we
must, of course, have minute and exhaustive investigations, like those
of Stahl, in regard to the biological relations of these peculiarities
before their utility can become clear to us.




LECTURE VII

CARNIVOROUS PLANTS

 Introduction--The Bladderworts or Utriculariæ--Pitcher-plants,
 Nepenthes--The Toothwort, Lathræa--The Butterwort, Pinguicula--The
 Sundew, Drosera--The Flytrap--Aldrovandia--Conclusions.


THAT the principle of selection dominates, to a large extent at least,
all the structural characters of plants, and moulds these in direct
relation to the prospects of greater success which may be offered in
the vicissitudes of the life-conditions of a single species or group
of species, is nowhere more apparent than in the case of the so-called
'insectivorous' or 'carnivorous' plants. Here again it was Charles
Darwin who led the way, for while many plants had long been known on
the sticky leaves of which insects were often caught and killed, it
had occurred to no one to regard this as of any special use for the
plant, much less to look on the peculiar dispositions of such leaves
as especially determined for this purpose. Darwin was the first to
show that there is no small number of plants--we now know about
500--which secure only a portion of their nutritive material by the
usual method of assimilation, and gain another and smaller portion by
dissolving and utilizing animal protoplasm, especially nitrogenous
muscle substance. The correctness of this interpretation was at first
disputed, but Darwin showed that pieces of muscle, or any nitrogenous
organic substance, were really dissolved by the relevant parts of the
plant, and were afterwards absorbed. It can therefore no longer be
doubted that the remarkable contrivances by which animals are laid
hold of by plants--are in a certain sense caught and killed--have
arisen with reference to this particular end; or, to speak less
metaphorically, that existing structural and functional peculiarities
in a plant which caused animals to be held fast were of advantage to
the nutrition of the plant, and were therefore augmented and perfected
by natural selection. That this was possible is obvious from the number
of insectivorous plants which now live upon the earth, and that these
processes of selection ran their courses quite independently of one
another, and even that they started from different parts of the plant,
is shown by the diversity of the contrivances which occur in plants of
several different families. A few of these I wish to discuss in some
detail.

[Illustration: FIG. 23. _Utricularia grafiana_, after Kerner. _A_, a
plant in its natural position, floating in the water. _FA_, traps. _B_,
a trap enlarged four times. _sz_, suctorial cells. _kl_, valve, which
closes the entrance to the trap. _C_, suctorial cells on the internal
wall of the trap, enlarged 250 times.]

The marshes of European countries, and also those of warmer lands,
often contain bladderworts, or Utriculariæ (Fig. 23)--floating
water-plants, without roots, and with horizontally spread,
long-drawn-out, tendril-like shoots, in part thickly covered with
whorls of delicate, needle-shaped leaves, in part bearing sparse
leaves of quite peculiar structure. These are stalked, hollow
bladders (Fig. 23 _A_, _FA_), with quite a narrow entrance at the
apex, which is closed, as far as larger animals are concerned, by
projecting bristle-like hairs (_B_). Small animals, such as water-fleas
(_Daphnia_), species of _Cyclops_, and Ostracods, can swim in between
the bristles, and they then come in contact with a valve which opens
easily inwards (_B_, _kl_) and allows them to penetrate into the
interior of the trap. Once inside they are captives, for the valve
does not open outwards; therefore they soon die and decompose, and are
then taken up by special absorptive cells (_B_, _C_, _sz_) and utilized
as nourishment for the plants. In this way the Utriculariæ catch
numerous little crustaceans and insect larvæ, which slip into their
traps, presumably for concealment.

[Illustration: FIG. 24. Pitcher of _Nepenthes villosa_, after Kerner.
_St_, stalk of the leaf. _Spr_, its apex. _Fk_, the pitcher. _R_, the
margin beset with incurved spines.]

Another example is found in the marsh plants of the genus _Nepenthes_,
some species of which live as climbers on the outskirts of tropical
forests, climbing up the trees and letting their long, thin tendrils
hang downwards, often over ponds and stagnant pools, where swarms of
small flying insects abound. These plants have developed exceedingly
remarkable contrivances for catching insects and using them as food
(Fig. 24). The long stalks (_St_) of their leaves (_Spr_) are first
bent downwards, then they suddenly turn sharply upwards, and the
upturned portion is modified into a pitcher-like structure, in the
bottom of which a fluid gathers, acid in taste, containing pepsin,
and therefore a digestive fluid. Nitrogenous substances, such as
flesh, dissolve in this fluid, and insects which fall into the pitcher
from the rim are killed and dissolved. There are many species of
_Nepenthes_, but not all of them possess the trap-structure in equal
perfection, so that we are able, to some extent, to follow the course
of its evolution, from a broad leaf-stalk, somewhat bent over at the
edges, to the marvellous closed pitcher shown by _Nepenthes villosa_
(Fig. 24) of Borneo. In this species the pitchers attain a length of
fifty centimetres, and are beautifully coloured, resembling in that
respect, as well as in their form, the tobacco-pipe-like flowers of
the tropical Aristolochiæ. When we come to discuss the origin of
flowers, we shall see that the bright, conspicuous colour possesses
a very considerable value in attracting insects; and in the case of
the pitcher-plant, too, the gorgeous colour probably allures insects
to settle on the rim of the pitcher, and they are tempted to dally
the longer since it secretes honey. But the thick, swollen rim of the
pitcher is as smooth as if it were made of polished wax, and resembles
the petals of those magnificent large orchids, the Stanhopeæ; the
inner surface of the pitcher below the margin is also smooth, so that
insects which creep about seeking honey are apt to slip and fall to the
bottom. Even if many of them are not at once killed by the digestive
fluid, but are able to climb up the smooth wall again, they cannot
escape, for beneath the swollen rim, which projects inwards, there is a
circle of strong bristles or teeth, with the points directed downwards,
which, like thorns, prevent the captive's escape. Thus the pitchers
of _Nepenthes_ secure and digest a large number of insects, and we
can easily understand that the plant acquires a considerable amount
of valuable nourishment in this way, for ready-made protoplasm is a
convenient food to which the plant has to do but little in order to
convert it into its own particular kind of living matter.

The toothwort (_Lathræa squamaria_) must also be briefly noticed
here, because it does not catch insects through the medium either of
air or of water, but through the earth. As is well known, this plant
is parasitic on the roots of various foliage-trees. It is of a pale
yellowish colour, and has no green assimilating parts. For such a plant
it must be of particular value to be able to catch animals and to
use them as food. To this end the short, pale leaves, which surround
the creeping, underground stem in the form of closely appressed
scales, have been modified into snares for minute animals. The leaves
have their upper parts recurved downwards, and the edges have grown
together, so that only a small opening is left at the base, and this
leads into a system of tunnels. Aphides, rotifers, bear-animalcules,
but especially springtails (Podurids), creep into these hollow leaves,
are held fast by a sticky secretion, and are dissolved and absorbed.

Another example, also indigenous, is that graceful marsh plant, the
butterwort (_Pinguicula vulgaris_), whose broad, tongue-shaped leaves,
arranged in the form of a rosette, have been modified into an insect
trap by the turning up of their edges, while the middle is deepened
into a longitudinal groove (Fig. 25). The whole upper surface of the
leaf is covered with an enormous number of little mushroom-shaped
glands (_B_, _C_, _Dr_), which secrete a viscid slime. Insects which
settle on the leaf stick fast, and as the glands continue to pour out
more and more slime, while at the same time the edges of the leaf,
stimulated by the struggling of the insect, curl over still farther,
the victims are drowned in the slime, and ultimately absorbed; for
this secretion is so powerful that even fragments of cartilage are
dissolved by it in forty-eight hours. Midges and mayflies in particular
fall victims to this plant, which is common in marshy places both in
mountain and plain.

[Illustration: FIG. 25. Butterwort (_Pinguicula vulgaris_). _A_, the
entire plant, showing the incurved margins of the leaves and some
insects caught by the secretion. _B_, cross-section through a leaf,
enlarged 50 times. _r_, the margin. _Dr_, _Dr_^l, two kinds of glands.
_C_, a portion of the leaf-surface, magnified 180 times.]

We must also mention the sundew (_Drosera rotundifolia_), which takes
its name from the seeming dewdrops that sparkle in the sun on the
leaves, or rather on the rounded extremities of long and rather thick
cilia-like hairs which cover the whole upper surface of the leaf. In
reality the apparent dewdrops consist of a sticky, clear, viscid slime,
which is secreted by the glandular ends of the pin-shaped hairs or
'tentacles.' Insects which settle on the leaf are caught by the slime,
and in this case also an acid, pepsin-containing fluid is secreted,
which gradually effects the digestion of the soluble parts of the
insect. It is especially noteworthy that it is not only those tentacles
which are in contact with the insect that take part in its digestion
and absorption, for all the others gradually alter their position from
the moment when any nitrogenous body, be it a fragment of flesh or an
insect, touches any of them. All begin to curve slowly towards the
stimulating object (Fig. 27), so that, after one to three hours, all
the tentacles have their heads towards it, and collectively pour out
their digestive juice upon it.

[Illustration: FIG. 26. The Sundew (_Drosera rotundifolia_), after
Kerner.]

[Illustration: FIG. 27. A leaf of the Sundew, with half of the
tentacles curved in upon a captured insect; enlarged 4 times.]

The sundew grows in marshes, as, for instance, those of the Black
Forest, and also on the moss-covered ridges there, and it is easy to
observe that a leaf often shows not merely a single gnat, midge, or
little dragon-fly, but several, sometimes as many as a dozen. In this
case, again, the value of the arrangement from the point of view of
nourishment can be no inconsiderable one.

In the case of the sundew we are obviously face to face with an
exceedingly complex adaptation, for not only is there a secretion of
the peculiar digestive juices, which occur only in carnivorous plants,
but the secreting tentacles are actively motile. That the tentacles
more remote from the captive may be excited to curve towards it, it is
necessary that the stimulus exerted by it on the heads of the tentacles
connected with it be conveyed to the base, and thence to the tips of
the other tentacles, for they curve throughout their whole length.
The utility of the contrivance is obvious, but that an arrangement so
divergent from the ordinary dispositions of plants could be brought
about points to the length of time that the processes of natural
selection must have gone on, preserving every new little variation, and
adding it to the rest.

[Illustration: FIG. 28. Leaf of Venus Fly-trap (_Dionæa muscipula_),
after Kerner. _A_, leaf-blade (_Spr_) open. _St_, leaf-stalk. _Stch_,
sensitive hairs. _B_, vertical section through the closed leaf-blade.]

[Illustration: FIG. 29. _Aldrovandia vesiculosa_, a branch with the
traps _FA_.]

Two plants remain to be noticed in conclusion, both possessing movable,
closing traps for catching animals. The so-called Venus fly-trap
(_Dionæa muscipula_) is a marsh plant of North America, the leaves of
which, like those of _Pinguicula_ and _Drosera_, are arranged in a
rosette on the ground. The individual leaf has a spatula-like stalk
and a blade in two halves (Fig. 28, _A_), each edged with long and
strong spinous processes, directed obliquely inwards. The halves of
the blade, when the necessary stimulus is applied to the surface, can
close together in a very short time, from 10 to 30 seconds. The two
rows of marginal spines then cross, as the interlocking fingers of
the hands do, and thus form a cage out of which the imprisoned insect
cannot escape. The appropriate stimulus to set the mechanism in motion
is a light touch, while a more violent shock, or strong pressure,
or a current of air, does not cause the trap to close. But if a fly
comes to creep about on the leaf, and in doing so touches one of six
short jointed hairs rising erect from a minute cushion of cells, then
the leaf closes, quickly indeed, but at the same time so gently and
imperceptibly that the fly is unaware of danger and does not try to
escape. Then numerous purple mucous glands begin to surround the victim
with pepsin-containing, acid, digestive juice which gradually dissolves
it.

One of the water-plants of Southern Europe, _Aldrovandia vesiculosa_,
which is also to be found in swamps on the northern ridge of the
Alps, possesses, in addition to the capturing and digesting apparatus
proper, an active motile apparatus, which is set in motion through
sensitive hairs. When I found the plant for the first time in a swamp
at Lindau, on the Lake of Constance, I took it at first sight for an
_Utricularia_, for the two plants resemble each other in external
appearance (cf. Figs. 22 and 29), but the modification of the leaves
into traps is quite different. On both halves of the leaf-blade there
are numerous bristles (Fig. 30, _A_), and the lightest touch on these
by a little water animal acts as a releasing stimulus to the motile
elements of the leaf (_Stch_). As in the Venus fly-trap, the two halves
of the leaf close together somewhat quickly, but quite quietly, and the
animal is caught. Fig. 30 shows a section of one of these traps in its
closed state. The captive animals cannot escape, because the margins of
the leaf shut quite tightly on one another, and are beset with little
teeth. Numerous little glands (_Dr_) secrete a digestive juice, and
after some days, or even weeks, the insoluble remains of the minute
animals may be found inside the trap.

[Illustration: FIG. 30. _Aldrovandia_: its trap apparatus. _A_, open.
_St_, stalk of the leaf. _Spr_, blade of the leaf. _Stch_, sensitive
bristles. _Dr_, glands. _B_, closed, a cross-section.]

Many more cases of animal-catching plants might be adduced, but
it is far from my intention to try to describe all the existing
contrivances; those already mentioned may suffice to give an idea of
the diversity and of the detailed effectiveness of these adaptations.
They amplify--so it seems to me--our conception of the scope of natural
selection, by showing us that adaptations may arise which are quite
foreign to the original mode of life of the organism in question,
and stand, indeed, in apparent contradiction to its fundamental
physiological processes. It is hardly necessary to enter into a
special argument to show that they can only have been brought about
in the course of natural selection, since every other interpretation
of their occurrence fails. Neither climatic nor any other external
direct influence could have effected these modifications of the parts
of plants, which are all so different, yet all so well suited to
their purpose; they are different even in plants growing quite close
together, like the sundew and the butterwort. The Lamarckian principle
of use and disuse hardly enters into the question at all, since plants
do not possess a will, and we can hardly speak of 'chance' where we
have to do with such complex and diversely combined transformations.
A process of selection actually operative in each of these cases can
easily be thought out, and I shall leave it to my readers themselves
to do this, and shall only indicate that we have to do with increasing
elaboration in two different directions: first, improvements in the
ability to utilize animal substances which happened to stick to the
leaves, and second, an increase in the probability of animals sticking
to the leaves, and so becoming available. Thus there arose, on the one
hand, dissolving and digestive juices, and arrangements for absorption;
and, on the other hand, viscid slime, and traps of various kinds to
secure the animals, as well as honey and bright colours to attract them.

But it is not merely transformations in the form of the stems and
leaves which have come about; there are also important physiological
changes. The sensitiveness to stimulus of various parts of the leaf is
greatly increased, to a certain extent in the butterwort, the edges of
whose leaves turn inwards in response to stimulus, still more in the
sundew, in which the stimulus is conveyed from the tentacles touched to
all the others, but most wonderfully of all in the Venus fly-trap and
_Aldrovandia_, whose sensitive hairs so transmit the stimulus that the
whole leaf is affected by it, and is set in motion, in a manner quite
comparable to the effects of a nerve-stimulus in animals.

Thus the case of carnivorous or insectivorous plants shows us that,
in the course of natural selection, quite new organs can be produced
in a plant by a thoroughgoing transformation of old ones, as, for
instance, the pitchers of _Nepenthes_, and that, furthermore, even
the physiological capacities of the plant may be changed in the most
far-reaching manner, increasing and varying until they come to resemble
the functions of the animal body.




LECTURE VIII

THE INSTINCTS OF ANIMALS

 The robber-wasp--Statement of the problem--Material basis of
 instincts--Instincts are not 'inherited habits'--Instinct of
 self-preservation--Fugitive instinct: death-feigning--Masking
 of crabs--Nutritive instinct--Monophagous caterpillars--Diverse
 modes of acquiring food: May-flies, sea-cucumbers, fishes that
 snare--'Aberration' of instinct--Change of instinct during
 metamorphosis: Eristalis, Sitaris--Imperfection of adaptation points
 to origin through natural selection--Instinct and will--Instincts and
 protective coloration--Leisurely flight of Heliconiidæ--Rapid flight
 of Papilionidæ--Instincts which act only once in a lifetime--Pupation
 of butterflies--Pupation of the Longicorns--Pupation of the
 silk-moth--The emperor moth--The cocoons of Atlas--Oviposition of
 butterflies.


WE have hitherto considered animals with especial regard to the
variation and re-adaptation of morphological characters, e.g.
modifications of form and colour; and we have now to ask whether their
behaviour also is to be referred as to its origin, in whole or in
part, to the principle of selection. All around us we can see that
animals know how to use their parts or organs in a purposeful manner:
the duckling swims at once upon the water; the chicken which has just
been hatched from the egg pecks at the seeds lying on the ground; the
butterfly but newly emerged from the pupa, as soon as its wings have
dried and hardened, knows how to use them in flight; and the predatory
wasp requires no instruction to recognize her victim, a particular
caterpillar, a grasshopper, or some other definite insect; she knows
how to attack it, to paralyse it by stings, and then hesitates not
a moment as to what she has to do next; she drags it to her nest,
deposits it in one of the cells already prepared for her future brood,
lays a single egg upon it, and roofs the cell carefully over. It is
only because all these complex acts are so precisely performed, as
precisely as if the wasp knew why she performed them, that the species
is able to maintain its existence, for only thus can the rearing of the
next generation be secured. Out of the egg there slips a little larva,
which at once makes for the paralysed victim, feeds upon it, and grows
thereby, then, within the shelter of the closed cell, passes through
the pupa stage and is transformed into a perfect wasp. Many species
of these predatory wasps do not lay the egg directly beside or upon
their prey, but lest its movements should endanger their offspring,
they hang the egg above it by a silken thread. It is thus in security,
and the young larva, too, when it appears, can withdraw to its safely
swinging resting-place as soon as danger threatens from the convulsive
struggles of the unfortunate victim at whose body it is gnawing.

Every animal has a great many such 'instincts,' which lead it, indeed
force it, to act appropriately towards an end, without having any
consciousness of that end. For how should the butterfly know what
flying is, or that it possessed the power of flight at all, or who
could have shown the predatory wasp, when she wakened from the pupa
sleep to quite a new kind of life, all that she had to do in order to
procure food for herself and to secure shelter and nourishment for
the brood which was still enclosed within her ovary? Since species
have developed from other species, these regulators of the body, the
instincts, cannot have been the same in earlier times; they must have
evolved out of the instincts of ancestors, and the questions we have to
ask are: By what factors? In what way? Has the principle of selection
been operative here too, or can we refer instincts to the inherited
effects of use and disuse?

Before I enter upon this question it is necessary to consider for a
little the physiological basis of instinct. We can distinguish three
kinds of actions: purely reflex, purely instinctive, and purely
conscious actions. In the case of the first, we see most clearly that
they depend on an existing mechanism, for they follow of necessity on
a particular stimulus, and cannot always be suppressed. Bright light
striking our eye makes the pupil narrower by a contraction of the iris,
and in the same way our eyelids close if a finger be thrust suddenly
towards them. We know, too, the principle of these reflex mechanisms;
they depend on nerve connexions. Sensory nerves are so connected in the
nerve-centres with motor nerves, that a stimulus affecting the former
at the periphery of the body, as at the eye, is carried to certain
nerve-cells of the brain, and from these it excites to activity certain
motor centres, so that definite movements are set up. It is rarely
only one muscle that is thus excited to activity, there are usually
several, and here we have the transition to instinctive action, which
consists in a longer or shorter series of actions, that is, of motor
combinations. These, too, are originally, at least, set a-going by a
sense impression, an external stimulus which affects a sensory nerve
exactly in the same way as in the reflex mechanism, and this stimulus
is carried to a particular group of sensory nerve-cells in the central
nervous organ, and from these transmitted by very fine inter-connexions
to motor centres. There are extraordinarily complex instinctive
actions, and in these the completion of one action is obviously the
stimulus to the second, the completion of the second to the third, and
so on, until the entire chain of inter-dependent movements which make
up the whole performance has been completed.

Instincts have thus a material basis in the cells and fibres of
the nervous system, and through variations in the connexions and
irritability of these nervous parts they too can be modified, like any
of the other characters of the body, such as form and colour.

Conscious actions depend directly on the will, and they have a close
connexion with instinctive actions in as far as these also can be
controlled by the will, that is, can be set a-going or inhibited, and
also, on the other hand, in as far as purely voluntary actions may
become instinctive through frequent repetition. The first case is
illustrated, for instance, when the suckling of a child at the mother's
breast is continued into the second year of life, as not infrequently
happens in the southern countries of Europe. Such a child knows exactly
why it wants the breast, and its action is a conscious one, while the
newborn child seeks about with the mouth instinctively, and when it has
found what it sought performs the somewhat complex sucking movements
automatically. The second case is illustrated, when, for instance, we
have made a habit of winding up a watch on going to bed, and do it when
we happen to change our clothes through the day, although it is then
purposeless and would have been omitted if the action had required
a conscious effort of will. One can often observe on oneself in how
short a time a conscious action may become instinctive. I once sent my
keyless watch to a watchmaker for repairs, and received from him for
the time an ordinary watch, which had to be wound with a key, which
key I kept for safety in my purse. At the end of eight days I got
back my own watch, and on undressing the first night I found myself
'instinctively' taking my purse from my pocket in order to get the key,
which, as I very well knew, I no longer needed. And that a long series
of complex movements, originally performed only consciously, may be
gone through instinctively, is shown by the fact that pieces of music
which have been learnt by heart can often be played without mistake
from beginning to end while the player is thinking of quite other
things. The complex instinctive actions of animals are performed in
quite a similar manner.

There is thus no sharp boundary line between reflex and instinctive
actions, nor between instinctive and conscious actions, but one passes
over into the other, and the thought suggests itself, that in the
phyletic development also transitions from one kind of action to the
other must have taken place. As long as one believes the Lamarckian
principle to be really operative one can suppose that actions, which
were originally dependent on the will, when they were often repeated,
became instinctive, or, in other words, that instincts, many of them at
least, are inherited habits.

I shall endeavour to show later on that this assumption, plausible as
it seems at first sight, cannot be correct; but in the meantime I must
confine myself to saying that there are a great number of instincts
which must be referred to the process of selection, and that the rest
can be similarly interpreted in their essentials at least.

The instinct of self-preservation is universally distributed, and it
is exhibited in many animals by flight from their enemies. The hare
flees from the fox and from men; the bird flies away at the approach of
the cat; the butterfly flies from even the shadow of the net spread to
catch it. These might be regarded as purely conscious actions, and in
the case of the hare and the bird experience and will have undoubtedly
some part in them, but even in these the basis of the action is an
organic impulse; this, and not reflection, causes the animal to flee
at sight of an enemy. In the butterfly, indeed, this must be purely
instinctive, since it is done with the same precision immediately on
leaving the pupa state, before the animal has had any experience. But
even in the case of the hare and the bird, taking to flight would in
most cases come too late if reflection were necessary first; if it is
to be effective it must take place as instantaneously as the shutting
of the lids when danger threatens the eye.

The hermit-crab (Fig. 34, p. 163), which conceals its soft abdomen
in an empty mollusc shell, and drags that about with it on the floor
of the sea, withdraws with lightning-like rapidity into its house
as soon as any suspicious movement catches its eye, and it is very
difficult to grip one of its legs with the forceps in time to draw it
out of its shell. The same is the case with the so-called Serpulids,
worms of the genus _Serpula_, and its allies; it is not easy to seize
them, because, however quick one is with the forceps, their instinct
of fugitive self-preservation acts more quickly still, and they shoot
back into their protecting tubes before one has had time to grasp them.
But this impulse to flee from enemies, though it seems almost a matter
of course, is by no means common to all animals, for in quite a large
number the instinct of self-preservation finds expression in an exactly
contrary manner, namely, in the so-called 'death-feigning,' that is,
remaining absolutely motionless in a definite position precisely
prescribed to the animal by its instinct. In speaking of protective
colouring, I drew attention to the 'wood-moth' (_Xylina_), which
resembles a broken fragment of half-decayed wood so deceptively, and
I pointed out that the colour-resemblance to wood would be in itself
of but little use to the insect if it were not combined with the
instinct to remain motionless in danger, to 'feign death.' The antennæ
and legs are drawn close to the body, so that they rather heighten
the disguise, and, instead of running away, the insect does not move
a muscle until the danger is past. This instinct must have evolved
hand in hand with the resemblance to a piece of wood, and, just as
we sought to interpret the latter from the fact that the moths which
most resembled the wood had always the best chance of surviving, so we
maintain that those moths would profit most by their resemblance which
drew in their legs and antennæ closely and lay most perfectly still.
Thus the brain-mechanism, which effected the keeping still whenever the
senses announced danger, would be more and more firmly established and
perfected in the course of selection.

Even nearly related animals may have quite different instincts
which secure them against danger. Thus in the group of pocket crabs
(Notopoda) there are some species which run away when danger threatens,
but others which anticipate the risk of discovery by masking themselves
to a certain extent. With the last pair of legs they hold over
themselves a large piece of sponge, which then grows till it often
leaves only the limbs and frontal region uncovered. Of course there
can be no question of consciousness in what the crab does, as is
proved by the fact that these crabs will, in case of necessity, take a
transparent piece of glass instead of the sponge; but the impulse to
cover themselves with something is strong in them, and finds expression
not only when they see a really protective substance, but even when
they see one which is transparent and therefore wholly useless for the
purpose. Crabs from which the sponge has been taken away wander about
until they find another; the impulse is thus set up not only by the
sight of the sponge or of a stone, but also by the feeling that their
back is uncovered. The large spider-crab of the Mediterranean (_Maja
squinado_) effects its disguise in a somewhat different manner. It has
peculiar hooked bristles on the back, and on these it hooks little
bunches of seaweed, often many of them, so that it is entirely covered
and looks like a bunch of wrack rather than like an animal. Here again
a bodily variation has gone hand in hand with the development of the
instinct to cover itself: the bristles of the back have become hooked.
Many instincts are accompanied by structural modifications, and in the
crabs which cover themselves with sponge or stone this is the case, for
the last pair of thoracic legs is turned towards the back, instead
of being set at the side of the body, as is usual among crabs. They
are thus enabled to hold the sponge much better and more permanently,
and as this is advantageous we may well ascribe the change to natural
selection.

Let us now turn our attention to another category of instincts, the
most common and most indispensable of all, those which lead to the
seeking and devouring of food.

The chicken just emerged from the egg picks up the seeds thrown to it
with no experience of what eating is, or what can be made to serve it
as food; its instinct for food expresses itself in picking up, and it
is awakened or stimulated to action by sight of the seeds. As Lloyd
Morgan in his excellent book on _Habit and Instinct_ well says, 'It
does not pick at the seeds because instinct says to it that this is
something to be picked up and tested, but because it cannot do anything
else.'

In the same way the instinct to seek for food wakes in the kitten at
the sight of a mouse. I once set before a kitten which had never seen
a mouse a living one in a trap. The kitten became greatly excited, and
when I opened the trap and let the mouse run away she overtook and
caught it in a few bounds. The instinct in this case does not express
itself as in the chicken by the rapid lowering of the head and seizing
the food, but in quite a different combination of movements, in running
after and grasping the fleeing victim. But that is not all that is
included in the instinctive action in the case of the cat, for there
is also the whole wild and gruesome play, the familiar letting go and
catching again, the passionate growling of satisfaction which, in its
wildness, reminds us much more of a blood-thirsty tiger than of a tame
domestic animal.

As the egg-laying instinct of the female butterfly is excited only by
the sight and odour of a particular plant, so also is the food instinct
of the caterpillar. If we put a silkworm caterpillar (_Bombyx mori_)
just out of the egg upon a mulberry leaf it will soon begin to gnaw at
it; but put it on a beech leaf or on that of any other indigenous tree,
shrub, or herb and it will not touch it, but simply die of hunger.
And yet it could quite well eat many of these leaves, and thrive on
them too, but the smell and perhaps also the sight of them is not the
appropriate stimulus to liberate the instinct of eating. There are
many species of caterpillar which are 'monophagous,' that is to say,
restricted to a single species of plant in a country. One may ask how
such a restriction of the liberating stimulus to a single species
could have been brought about by natural selection, since it could not
possibly be advantageous to be so much restricted in food. The answer
to this will be found in the following facts. On the Belladonna plant
there lives a little beetle whose feeding instinct is aroused by this
plant alone. But as _Atropa belladonna_ is avoided entirely by other
animals on account of its poisonousness, this beetle is, so to speak,
sole proprietor of the Belladonna; no other species disputes its food,
and in this there must assuredly be a great advantage, as soon as the
other instincts, above all that of egg-laying, are so regulated as to
secure that the larva shall have access to its food-plant; and this
is the case. The monophagy of many caterpillars is to be understood
in the same way; it is an adaptation to a plant otherwise little
sought after, and it is combined with a more or less complete loss
of sensitiveness to the stimulus of other plants. The establishment
of such a specialized food-instinct depends on its utility, and has
resulted from the preference given, through natural selection, to those
individuals in which the food-instinct responded to the stimulus of
the smallest possible number of plants, and at the same time to those
which showed themselves best adapted to a plant especially favourable
to their kind, whose food-instinct was not only most strongly excited
by this one plant, but also whose stomach and general metabolism made
the best use of it. So we understand why so many caterpillars live
on poisonous plants, not only some of our indigenous Sphingidæ, like
_Deilephila euphorbiæ_, but whole groups of tropical Papilionidæ,
Danaides, Acræides, and Heliconiidæ. With this again is connected the
poisonousness or nauseousness of these butterflies.

How diversely the instinct to procure food may be developed in one and
the same group of animals is shown by the fact that not infrequently
plant-eating, saphrophytic, and flesh-eating animals occur in a single
group of organisms, as, for instance, in the order of water-fleas or
Daphnidae, or in the class of Infusorians. Many species find their
food by making an eddy in the water, which brings a stream towards the
mouth, and with it all sorts of vegetable or dead particles; others
live by preying upon other animals like themselves.

But even when the food-instinct in all the species of a group is
directed towards living prey, the procuring of it may be achieved by
means of quite different instincts. Such finer graduations of the
food-instinct are found not infrequently within quite small groups of
animals, as in the Ephemeridæ or Day-flies. All their larvæ live by
preying on other animals, but those of one family, represented by the
genus _Chloëon_, seek to secure their victims by agility and speed,
while the larvæ of the second family, with the typical genus _Baëtis_,
have the instinct to press their smooth broad bodies, with large-eyed
head, close to the brook pebbles on which they sit. They are exactly
like these in colour, and thus they lurk almost invisible, until a
victim comes within their reach, when they throw themselves upon it
with a bound. The third group, with the typical genus _Ephemera_,
follows its instinct to dig deep tubes in the mud at the bottom of
the water, and to lurk in these for their prey. We have thus within
this small group of Day-flies three distinct modifications of the
food-instinct, which differ essentially from one another, are made
up of quite different combinations of actions, and, consequently,
must have their foundation in essentially different directive
brain-mechanisms. All these cases have only one feature in common; the
animals all throw themselves upon their prey as soon as they are near
enough.

[Illustration: FIG. 31. Sea-cucumber (_Cucumaria_), with expanded
tentacles (_a_), and protruded tube-feet (_b_); after Ludwig.]

But even this common feature is not everywhere part of the
food-instinct. The sea-cucumber (_Cucumaria_) (Fig. 31), according to
the observations made on it by Eisig in the Aquarium of the Zoological
Station at Naples, gets its food in the following manner. The animal
sits half or entirely erect on a projecting piece of rock and unfolds
its ten tree-like tentacles which surround the mouth. These are
branched, and have quite the effect of little tufts of seaweed. They
are probably taken for such by many minute animals; for larvæ of all
kinds, Infusorians, Rotifers and worms settle down on them. But the
sea-cucumber bends inwards first one tentacle and then another, so
slowly as barely to be noticeable, brings the point to its mouth, lets
it glide gradually deeper into the gullet, until the whole tentacle is
within, and after a time draws it out again equally slowly and unfolds
it anew. Obviously it wipes the tentacle inside the gullet, and retains
everything living that was upon it. This performance is repeated
continually, day and night, and it is usually the only externally
visible sign of life which the animal displays.

This remarkable instinct is associated with a structural peculiarity,
for without the arborescent tentacles the fishing would not be nearly
so successful. Other sea-cucumbers or Holothurians have different
tentacles, and use them in quite a different manner, filling the mouth
with mud by means of them.

Very frequently, indeed, there are visible structural changes
associated with the modified food-instinct. Most predatory fishes chase
their prey, like the pike, the perch, and the shark, but there are also
lurkers, and these show in addition to the lurking instinct certain
definite bodily adaptations, without which this instinct could not have
such full play.

Thus in a marine fish known as the 'star-gazer' (_Uranoscopus_) the
eyes are situated not on the sides but on the top of the head, and the
mouth is also directed upwards. Its instinct leads it to bury itself in
the sand so that only the eyes are uncovered. It lies in wait in this
way until a suitable victim comes within reach, and then snaps at it
with a sudden movement. But it also possesses a decoying organ, a soft
worm-shaped flap, which it protrudes from the mouth as soon as little
fishes draw near. They make for this bait, and are thus caught.

Such ingenious fishing, which is quite suggestive of the human method
of catching trout with artificial bait, occurs in many predatory
fishes; but in every case the fish acts instinctively, without
reflection, on becoming aware of approaching prey. The suitability of
the action to the end does not depend upon consciousness of the end,
or upon reflection, but is a purely mechanical action, performed in
response to the stimulus of a sense-impression.

This is best shown by the fact that the instincts may lead their
possessors astray, which always happens when an animal is transferred
to an unnatural situation, to which its instincts are not adapted, so
to speak. The mole-cricket, which is in the habit of escaping pursuit
by burrowing in the earth, makes violent motions with the forelegs,
even if it be placed upon a plate of glass into which it could not
possibly burrow; an ant-lion (_Myrmeleo_), whose instinct impels
it to bore into loose sand by pushing backwards with the abdomen,
goes backwards on a plate of glass as soon as danger threatens, and
endeavours, with the utmost exertions, to bore into it. It knows no
other mode of flight, and its intellect is much too weak to suggest any
novel mode. Even the mode of escape most universal among animals, that
of simply running away, does not occur to it; it acts as it must in
accordance with its inborn instinct; it cannot do otherwise.

The change of instincts in the different stages of development of
one and the same animal have always seemed to me very remarkable; for
instance, the change of the food-instinct in the caterpillar and the
butterfly, where the food-instinct is liberated in the caterpillar
by the leaf of a particular plant, but in the butterfly by the sight
and fragrance of a flower, the nectar of which it sucks. In this case
everything is different in the two stages of development, the whole
apparatus for seeking and taking food, as well as the nerve-mechanism
which determines these modes of action. And how far apart often are the
stimuli which liberate the instinct! The larva of the flower-visiting,
honey-sucking _Eristalis tenax_ is the ugly, white, so-called
rat-tailed larva, well described by Réaumur, which lives swimming
in liquid manure, and feeds on that! What complete and far-reaching
changes, not only in the visible structure, but also in the finer
nervous mechanisms, which we cannot yet verify, must have taken place
in the vicissitudes of time and circumstance during the life-history of
this insect!

[Illustration: FIG. 32. Metamorphosis of _Sitaris humeralis_, an
oil-beetle, after Fabre. _a_, first larval form, much enlarged. _b_,
second larval form. _c_, resting stage of this larva (so-called
'pseudo-pupa'). _d_, third larval form. _e_, pupa.]

Not the food-instinct alone, but the instinct of self-preservation,
of mode of motion, in short, every kind of instinct, may vary in the
course of an individual life. Let us follow the somewhat complex
life-history of a beetle of the family of the Blister-beetles or
Cantharides, as we learnt it first from Fabre. The female of the
red-shouldered bee-beetle (_Sitaris humeralis_) lays its eggs
on the ground in the neighbourhood of the underground nest of a
honey-gathering burrowing-bee (_Anthophora_). The larvæ, when they
emerge, are agile, six-legged, and furnished with a horny head and
biting mouth-parts, as well as with a tail-fork for springing (Fig. 32,
_a_). The little animals have at first no food-instinct, or at least
none manifests itself, but they run about, and as soon as they see a
bee of the genus _Anthophora_ they spring upon it and hide themselves
in its thick, hairy coat. If they have been fortunate the bee is a
female, who founds a new colony and builds cells, in each of which she
deposits some honey and lays an egg upon it. As soon as this has been
done the _Sitaris_ larva leaves its hiding-place, bites the egg of the
bee open, and gradually eats up the contents. Then it moults, and takes
the form of a grub with minute feet and imperfect masticating organs;
the tail-fork, too, is lost, for all these parts are now useless, since
it can obtain liquid nourishment without further change of place, from
the honey in the cell, in exactly the quantity necessary to its growth.
Then it spends the winter in a hardened, pupa-like skin, and it is
not till the next year (the third), after another short larval stage
(_d_) and subsequent true pupahood (_e_), that the fully-formed beetle
emerges. This again possesses biting mouth-parts, and eats leaves, and
has legs to run with and wings to fly with.

In this beetle, then, the food-instinct changes three times in the
course of its life; first the egg of the bee is the liberating
stimulus, then the honey, and finally leaves. The instinct of moving
about varies likewise, expressing itself first in running and jumping
and in catching on, then in lying still within the cell, and, lastly,
in flying and running about on bushes and trees.

We can well understand that, in the course of innumerable generations
and species of insects, the various stages of development would,
by means of selection, become more and more different from each
other, both structurally and in their instincts, as they adapted
themselves better to different conditions of life; and thus ultimately
instincts frequently and markedly divergent have been developed in
the successive stages of life. No other interpretation is possible;
through natural selection alone can we understand even the principle
of such adaptations. An animal can thus very well be compared to a
machine which is so arranged that it works correctly under all ordinary
circumstances, that is to say, it performs all the actions necessary to
the preservation of the individual and of its kind. The parts of the
machine are fitted together in the best possible way, and work on each
other so ingeniously that, under normal circumstances, a result suited
to the end is achieved. We have seen how precisely the liberating
stimulus for an action may be defined, and this secures a far-reaching
specialization of instincts. But as every machine can work only with
the material for which it was constructed, so the instinct can only
call forth an action effectively adjusted to its end when the animal
is under natural conditions. Its specialization has its limits, and
in this lies the reason of its limited purposiveness. For instance,
if the larva of _Sitaris_ were not impelled by the sight of every
bee to spring on it and cling to it, but only by the _females_, then
many of them would be saved from the fate that awaits them if they
attach themselves to male bees, which make no nest, or even to other
flying insects, in which case also there is no possibility of further
development. But both these things happen, although the latter has
not yet, to my knowledge, been recorded of _Sitaris_, but only of its
relative, the larva of _Melöe_.

'Instinct goes astray,' it is often said; but in truth it does not
go astray, but is only not so highly specialized in relation to the
liberating stimulus of the action as seems to us necessary for perfect
purposiveness. But in this very imperfection there lies, as it seems to
me, another proof that we have to do with the results of a process of
selection, for it is of the very nature of these never to be perfect,
but only relatively perfect, that is to say, just as perfect as is
necessary to the maintenance of the species. At the moment at which
this grade of perfection is reached every possibility of a further
increase in the effectiveness of adjustment to the end ceases, because
it would then no longer directly further the end. Why, for instance,
should the liberating stimulus in this case be more highly specialized,
since enough of the _Sitaris_ larvæ already succeed in attaching
themselves to female bees? It is not for nothing that the beetles of
this family are so prolific; what is lacking in the perfection of the
instinct is made up for by the multitude of young larvæ. A single
female of the oil-beetle (_Melöe_) lays several hundred eggs.

In speaking of the animal as a machine, it must be added that it
is a machine which can be altered in varying degrees, which can be
regulated to work at high or low pressure, slowly or quickly, finely or
roughly. This regulating is the work of the intelligence, the limited
'thinking-power,' which must be ascribed to the higher animals in a
very considerable degree, but which in the lower animals becomes less
and less apparent, until finally it is unrecognizable. That instinctive
actions can be modified or inhibited by intelligence and will is proved
by any trained beast of prey which masters its hunger and the impulse
to snap at the piece of flesh held before it, because it knows that
if it does not control itself painful blows will be the consequence.
In a later lecture I shall return to the connexion between will and
instinct; all that concerns us here is to regard instincts as the
outcome of the processes of selection, and as an indirect proof of the
reality of these.

From what I have already said at least so much must be clear, that
nothing, in principle, stands in the way of referring instincts to
selection, since their very essence is their adaptation to an end, and
such purposive changes are precisely those that are preserved in the
struggle for existence. It might, however, be supposed that in all this
the principle of use and disuse also had a share, and that without it
no changes in instincts could have come about.

There are, however, numerous instincts in considering which this can be
entirely excluded.

At an earlier stage we discussed in detail the protective colourings
which secure insects, and especially butterflies, from extermination
by their numerous enemies, and it was mentioned that this was always
accompanied by corresponding instincts, without which the protective
colouring and the deceptive form would have profited nothing, or at any
rate not nearly so much. If the caterpillar of the _Catocala sponsa_,
which resembles the bark of an oak so deceptively, did not possess at
the same time the instinct to creep away from the leaves and hide in
the clefts of the bark on the trunk of the oak-tree, its disguise would
be of very little use to it; and if the predatory and grass-coloured
praying mantis was not impelled by instinct to lie in wait among the
grass for its prey, instead of pursuing it, it would rarely succeed in
seizing any of its victims, because of its somewhat leisurely mode of
movement. This adaptation of the instincts to the protective colouring
is carried into the most minute and apparently trifling details. Thus
different observers have established the fact that the nauseous,
sometimes even poisonous, butterflies, which are distinguished by their
glaring or sharply contrasted colour-pattern, are all slow fliers.
This is the case with the Danaides and Euplœides of the Old World and
the Heliconiides of the New; many of their mimetic imitators also fly
slowly.

If we inquire how this instinct of fluttering, careless flight has
come to be, we may leave habit as _primum movens_ out of the question
altogether, for there are no external conditions which could have
induced the butterfly to take to slower flight than its ancestors
exhibited. That it is now advantageous for it--since it acts as a
signal of its nauseousness--to be as clearly seen and recognized as
possible can exercise no direct influence on its manner of flight,
since it knows nothing about it. Even if we assume that individual
variations cropped up which had an instinct for slower flight, there
would still, without selection, be no reason why this variation in
particular should multiply, still less why the originally slight
slowing of the flight should become more marked in the course of
generations. On the contrary, the butterflies fly a great deal, just
as all other diurnal butterflies do; they exert their power of flight
as long as the sun shines, and if the exercise of one generation
influences the next, they ought to become gradually more capable of
rapid flight. In this case exactly the opposite takes place to what is
ascribed to the Lamarckian principle; more constant use must here have
brought about a diminution of the activity of the relevant parts. It is
quite otherwise when we look at it from the point of view of selection.
The variants which cropped up by chance with slower flight survived
because they were most easily recognized and avoided; they are the most
frequent survivors; they leave descendants which inherit the slower
flight-instinct, and this goes on increasing in them as long as the
increase carries any advantage with it. As soon as this ceases to be
the case the variation comes to a standstill, for it is adapted to the
average of the conditions at a given time.

We may picture to ourselves the thousand kinds of regulations of animal
movements through instinct as having come about in a similar way; in
the majority of cases we _must_ picture it thus. For it is only in the
case of those with high intelligence that we can ask whether the animal
did not by deliberation help in establishing the purposive variation
in its movements. Among insects in any case this could only be taken
into account to a very limited extent, although I do not dispute that
the more intelligent among them may learn, and may make experiments,
and can modify their actions accordingly. But in fleeing from an enemy
experience has nothing to do with it, for the first time it is caught
it pays the penalty with its life. Without care, and with no idea of
the dangers surrounding them on all sides, the butterflies float about,
guided only by their instinct, which, however, is so exactly adapted
to the conditions of their life that a sufficient number of them to
preserve the species always happily escapes all the many dangers. I
may remind you of Hahnel's case of the butterfly, already mentioned,
which escaped the agile lizard by flying rapidly up from the sweet
bait, but settled again upon it without fear immediately afterwards, to
fly from the lizard as before, and did so several times in succession.
We usually judge such actions far too much from the human standpoint;
the butterfly does not wish to escape the death which threatens it; it
knows nothing about death; it is not with it as it was with Dr. Hahnel
himself, who when he was once in danger from a jaguar in a thicket
was so affected by the thought of the death he had happily escaped
that he never cared to pass the place again, but made a long circuit
to his home. The butterfly does not act according to reflection and
imagination; it flies up with lightning-like rapidity when the lizard
rushes at it, because this rapid movement, which it _sees_, acts as
the stimulus which liberates the flight-instinct, and this works so
promptly that in most cases the insect is rescued from danger. Its
disposition, however, is not otherwise affected by its narrow escape,
and it obeys anew the food-instinct which impels it to settle again
on the bait, until the flight-instinct is again set a-going by the
visual impression of the re-advance of the lizard. It is the plaything
of its instincts, a machine which works exactly as it must. That it
is only sense-impressions and not conceptions which here liberate the
actions can be well seen in the case of shy species of butterfly like
our purple emperor (_Apatura iris_), which flies up like lightning from
the moist wood-paths on which it loves to settle as soon as any rapidly
moving visual image, even if it be only a shadow, strikes its eyes. For
this reason the collector tries to approach it so as not to throw his
shadow before him, for then the insect lets the advancing enemy get
quite close, and only flies up when the net is quickly thrust towards
it. In all probability the eye of this insect is particularly well
adapted for perceiving movements, and certainly the flight-instinct
reacts very promptly to such visual impressions, and we can understand
that it must have been so regulated if, as we assume, the regulation
came about through processes of selection, for the enemies of the
butterflies, such as birds, dragon-flies, and lizards shoot quickly
out on their prey, and therefore those butterflies must always have
survived whose instinct impelled them to take to flight most quickly.

In this, then, as in a thousand other cases, the instinct of flight,
or indeed any other mode of movement, cannot be interpreted as an
'inherited habit,' because there is no evidence of the possession of
that degree of intelligence which could have induced the variation in
the previous habit, that is, in manner of movement. The same is true
of animals of low intelligence in regard to all the other instincts,
which otherwise might seem to be explicable in terms of the Lamarckian
principle.

In addition, there is a whole large group of instincts in regard to
which the idea of the Lamarckian principle cannot be entertained, as
I showed years ago, and it consists of all those instincts which are
only exercised once in the course of a lifetime. These cannot possibly
depend on practice in an individual lifetime, and transmission of
the results of this exercise to the following generation; they can
therefore only be interpreted in terms of selection, unless we are to
give up all attempts at a scientific interpretation, and simply accept
them as 'marvels.'

To this class belong all the diverse instincts by which insects
protect themselves against attack during the pupa stage. Even the way
in which the caterpillars of many diurnal butterflies hang themselves
up in pupation is not by any means a very simple instinctive action.
The caterpillar first spins, in a suitable place, a small round disk
of silk threads, to which it then attaches the posterior end of its
body, so securely that it cannot be easily torn away. More complicated
still is the securing of the pupa when it does not hang freely, but
is to remain pressed against a wall or a tree, as is the case in the
Papilionidæ and the Pieridæ. In this case the caterpillar must, in
addition to the usual cradle, spin a thread of silk, in an ingenious
way, diagonally across the thorax, so that it may cross about the
middle of the wing rudiments, and not be too loose, lest the pupa fall
out, yet not too tight, lest the thread cut too deeply into the wing
rudiments and hinder their development. When one remembers that it is
the caterpillar that does all this, before it has taken the form of the
pupa, and that it must all be adapted to the pupa's form, we are amazed
at the extraordinary exactness with which instinct prescribes all the
individual movements which make the whole of the complex performance
effective. And yet, as each caterpillar only accomplishes this
performance once in its life, it could at no time in the development
of the species have become a habit in the case of any individual
caterpillar, and it cannot therefore be an 'inherited habit.'

But however diverse are the methods of securing the safety of the pupæ
in the different families of butterflies, they must all be referred
back to a single root, if the butterfly pedigree can be traced back to
a single ancestral group. The caterpillar of the Sphingidæ does not
creep up walls and trees when it is ready to enter on the pupa stage,
as so many of the caterpillars of the diurnal butterflies do, but
instead its instinct compels it to run about on the ground until it has
found a spot which seems to it suited for boring into the earth, or,
to speak less metaphorically, until it comes to a place which, from
its nature, acts as a liberating stimulus to the instinct to burrow.
Then it penetrates more or less deeply, according to the species, and
makes a small chamber, which it lines with silken threads to prevent
it collapsing; this done, it moults, and enters on the pupa stage.
The exactness with which the individual movements are prescribed
by instinct is seen in the way in which the size of the chamber is
regulated so as to be exactly as large as is necessary to give the
pupa room enough without leaving any superfluous free space. This is
not so simple as it seems, and is not directly conditioned by the
size of the animal, for the caterpillar is longer and altogether of
greater volume than the pupa. The same thing is seen in the stag-beetle
(_Lucanus cervus_), the largest of our indigenous beetles, which gets
its name from the powerful antler-like jaws which distinguish the male.
It also undergoes its pupal metamorphosis in the earth, and makes a
large hard ball of clay, hollow inside, and as smooth as if polished,
and its cavity is exactly the size of the future pupa, or to speak more
precisely, of the fully-formed beetle. For, as Rösel von Rosenhof in
his day 'observed with amazement,' the balls in which the males lie
have a much longer cavity than those built by the females, and for
this reason, that when the fully-formed beetle emerges from the pupa
it must, if it is a male, have room to stretch out its horns, which
have till then lain upon the breast. 'For the beetles do not leave
their dwelling-place until all their parts are sufficiently strong and
properly hardened, and till the season has arrived in which they are
wont to fly about.' The male larva thus makes a much longer pupa-house
than the female larva, in anticipation, so to speak, of the enormous
size of the jaws which will grow out later!

Here the instinct has two modes of expression, according as the bodily
parts are male or female. Here we have to do with an action which is
performed once in a lifetime, and thus the possibility of any other
explanation of the origin of this instinct than through natural
selection is excluded.

Not less significant is the case of the silk-cocoons. The cocoons spun
by the silkworm are egg-shaped, and consist of a single thread many
thousand yards in length, which is wound round the spinning caterpillar
so that not a space is left uncovered. The web is firm, tough, and very
difficult to tear; therefore we must grant that the pupa resting within
will enjoy a very considerable degree of security against injury. But
the moth must be able to get out, and that this may be possible the
caterpillar is impelled by instinct to make its spinning movements such
that the cocoon is eventually looser at the anterior end, so that the
insect, when it is ready to emerge, can tear it asunder with its feet
and make a way out for itself. For this very reason, because the silk
must be torn and spoilt by the emerging insect, silk-breeders kill the
pupating insect before it begins to make its way out.

But there are species whose cocoons are provided from the very start
with an outlet, for the caterpillar spins the silk round itself in
such a way that a round opening is left. But this opening would be not
only a convenient door for the butterfly to emerge by, but an equally
convenient entrance for all its enemies. It is, therefore, closed up.
In the case of the 'emperor moth' (_Saturnia carpini_) this is effected
by means of a circle of stiff bristles of silk on the inside (Fig. 33),
the points of which bend outwards like those of a weir-basket (_r_);
from the inside the emerging moth can easily push aside the bristles,
while the threatening enemy from without is scared off by the stiff
points of the bristles.

[Illustration: FIG. 33. Cocoon of the Emperor Moth (_Saturnia
carpini_), after Rösel. _A_, enclosed pupa. _B_, emerging moth. _r_,
hedge of bristles. _fl_, wings.]

Such a cocoon is comparable to a work of art in which every part
harmonizes with the rest, and all together are adapted as well as
possible to their purpose. And yet it is all accomplished without the
caterpillar having the remotest conception of what it is aiming at when
it winds the endless silken thread about itself in the artistic and
precisely prescribed coils. Nor has it any time for trying experiments
or for learning; it must make all the complex bendings and turnings
of the head which spins the thread, and of the anterior part of the
body which guides the thread, quite exactly and correctly the first
time if a good cocoon is to be produced. Here every possibility of
interpreting this instinct as 'an inherited habit' is excluded, for
each caterpillar becomes a pupa only once; and it is just as impossible
to suppose that it can be directed by intelligence, since it can
neither know that it is about to become a pupa, nor that, in the pupa
stage, it will be in danger from enemies which will attempt to force
their way into the cocoon, nor that the hedge of bristles will protect
it from such enemies. Our only clue to an interpretation is in the slow
process by which minute useful variations in the primitive instinct
of spinning are accumulated through selection; and it is wonderful to
see how exactly these spinning powers are adapted to the particular
life-conditions of individual species.

Thus there are several of the Saturnides whose enormous caterpillars
live on large-leaved trees, and these make use of the large leaves to
form a shelter for the pupa stage, spinning them together so that the
cocoon is for the most part surrounded by leaf. But as the leaf might
easily fall off with the weight of the pupa, they make the leaf-stalk
fast to the twig from which it grows by binding the two firmly together
with a broad, strong, closely-apposed silken band. Seitz relates of
the largest of all these spinners, the Chinese _Attacus atlas_, that
this silk sheath 'is continued to the nearest strong branch, so that
it is impossible with the hand to detach the leaves that conceal an
Atlas-pupa from the tree.' To be sure, this pupa weighs about eleven
grammes!

Since instincts vary, as well as the visible parts of an animal, a
fulcrum is afforded by means of which selection can bring about all
these very special adaptations to given conditions, since it always
preserves for breeding the best suited variations of an already
existing instinct. Any other interpretation is once more excluded.

The same may be said of insects and their egg-laying. This, too,
is in many cases only performed once in a lifetime, and the insect
dies before it has seen the fruit of its labour. Yet egg-laying is
performed in the most effective manner, and with the most perfect
security of result. It seems as if the insect knew, so to speak,
exactly where, in what numbers, and how it should lay its eggs. Many
Mayflies (Ephemeridæ) let their eggs fall all at once into the water
in which the larvæ live; many Lepidoptera, such as _Macroglossa
stellatarum_, lay their eggs singly, and on definite plants--the
humming-bird hawk-moth, just referred to, on _Galium mollugo_; others,
like _Melitæa cinxia_, lay their eggs in heaps on the leaves of the
way-bread (_Plantago media_), or, like _Aglia tau_, on the bark of a
large beech-tree. Nothing in these different modes of egg-laying is
due to chance or caprice; all is determined and regulated by instinct,
and all, as far as we can see, is as well adapted to its purpose as
possible. When, for instance, _Macroglossa stellatarum_ lays her eggs
singly, or in twos or threes, on the green leaves of the food-plant, it
thereby obviates the danger of scarcity of food for the comparatively
large caterpillars, since not many of them could subsist together on
a single plant of Galium, while _Aglia tau_ can place several hundred
eggs on the same beech-tree trunk without having to fear that its
caterpillars will not find abundant nourishment. The precision with
which the egg-laying instinct works is even greater in other species
in which there are more special requirements, e.g. when the eggs have
to be laid on the under side of the leaves, as in _Vanessa prorsa_, or
where they have to be cemented together in a little pillar, so that
they bear a deceptive resemblance to the green flower-buds of the
food-plant (the stinging-nettle).

It is certainly astonishing how exactly the stimulus in these cases is
specialized to the liberation of the instinct. In general the smell
of the food-plant of the caterpillar is enough for most butterflies,
and this attracts the female ready to deposit its eggs, but complete
liberation of the instinct is only effected by the visual impression
of the under side of the leaf. We cannot but be astonished that there
is room for such finely graded nerve-mechanisms in the little brain
of a butterfly, and yet it would be easy enough to adduce still more
complex instincts connected with oviposition in insects. The large
water-beetle, _Hydrophilus piceus_, lays its eggs on a floating raft
made by itself; the gall-wasps must first pierce with their ovipositor
into a particular part of a particular plant to be able to lay the
eggs in the proper place, and this in no haphazard way but with great
carefulness and in a perfectly definite manner. But there is no
necessity to refer here to many or to the most complicated cases of
egg-laying; I only wish to show that, even in the simple cases, such
as that of the butterflies just referred to, there is a precisely
regulated combination of actions which is executed mechanically, and
which cannot be interpreted as inherited habit, because it never was a
habit in any individual of any generation.

It is thus placed beyond the possibility of doubt that very many
instincts, at least, must depend on selection, and it would be useless
to go further in this direction by extending our survey to other
groups of instincts. I shall, however, return later on to the study of
instincts, and, after we have become acquainted with the main features
of the laws of inheritance, it will then be seen that, even among
higher animals, instincts can never be interpreted in terms of the
Lamarckian principle.




LECTURE IX

ORGANIC PARTNERSHIPS OR SYMBIOSIS

 Hermit-crabs and sea-anemones--Hermit-crabs and hydroid polyps--Fishes
 and sea-anemones--Green fresh-water polyps--Green Amœba--Sea-anemones
 and yellow Algæ--Cecropia trees and ants--Lichens--Root fungi--Origin
 of Symbiosis--Nostoc and Azolla apparently contradict the origin
 through natural selection.


WE have already seen, by means of many examples, to what a great degree
animals and plants are able to adapt themselves to new conditions
of life; how animals imitate their surroundings in colour and form,
how instincts have varied in all directions, how plants have made
use of the chance of frequent contact with little animals to obtain
nourishment from them, and have developed contrivances adapted for
bringing as many of these as possible into their power and causing them
to yield them the largest possible amount of food. A great many of
these could only be interpreted in terms of natural selection, and in
others it seemed at least very probable that selection was one of the
factors in bringing them about.

Particularly clear proof of the reality of natural selection is
afforded by those cases where one form of life associates itself with a
very different one so intimately that they are dependent on one another
and cannot live without one another--at least in extreme cases--and
that new organs, and, indeed, new dual organisms, are sometimes
produced by this interdependence of life. This phenomenon--so-called
'Symbiosis'--was discovered by two sharp-sighted botanists, Anton de
Bary and Schwendener. But Symbiosis occurs not only between plants;
it occurs also between plants and animals and between two species
of animal, and we understand by it a life of partnership depending
on mutual benefits, so that each of the two species affords some
advantage to the other, and makes existence easier for it. In this
respect Symbiosis differs from Parasitism, in which one species is
simply preyed upon by another without receiving any benefit from it in
return, and also from the more innocent Commensalism of Van Beneden,
the table-companionship in which one species depends for its existence
on the richly-spread table of another. Symbiosis is particularly
interesting, because, in addition to extreme cases with marked
adaptations, many occur which are of great simplicity, and which seem
to have brought about almost no change in the two associated species.

We shall take our first examples from the Animal Kingdom.

The partnership between certain sea-anemones (Actiniæ) and hermit-crabs
(Paguridæ) had been noticed long before any particular attention
was devoted to it. Many species of hermit-crab frequently carry a
large sea-anemone about with them on the mollusc shell which they
use as a protecting-house; indeed, two or three of these beautiful
many-tentacled polyps are often attached to them, and this is not at
all a matter of chance, but depends upon instinct on the part of both
animals; they have the feeling of belonging to each other. If the
sea-anemone be taken away from the hermit-crab and put in a distant
part of the aquarium, the crab seeks about till it finds it, then
seizes it with its claws and sets it on its house again. The instinct
to cover itself with Actiniæ is so strong within it that it loads
itself with as many of these friends as it can procure, sometimes with
more than there is room for on the shell. The sea-anemone on its part
calmly submits to the crab's manipulations--a fact very surprising to
any one who is aware of the anemone's ordinarily extreme sensitiveness
to contact, and knows how it immediately draws itself together on any
attempt to detach it from the ground, and will often let itself be
torn in pieces rather than give way. The mutual instincts of the two
creatures are thus adapted to each other; but it does not at first
sight seem as if any structural changes had taken place in favour of
the partnership. This is true, indeed, as regards the hermit-crab, but
not as regards the sea-anemone, although the nature of the adaptation
on the sea-anemone's part only becomes apparent when the two animals
are closely observed in their life together.

We owe our understanding of this adaptive change in the sea-anemone,
and, indeed, our knowledge of this whole case of Symbiosis, to the
beautiful observations of Eisig. Starting from the hypothesis that the
mutual relations could only be the outcome of natural selection, Eisig
pointed out that this partnership must offer some advantage not to one
partner only, but to both; otherwise it could not have arisen through
selection. The advantage to the sea-anemone is obvious enough; since of
itself it can only move very slowly, and is usually firmly fixed in one
place, it is easy to see that it would be useful to it to be carried
about on the floor of the sea by the hermit-crab, and to get its share
of the hermit-crab's food. But the service yielded to the hermit-crab
by the sea-anemone in return is not nearly so apparent. Eisig made
an observation in the Zoological Station at Naples which solved this
riddle. He saw an octopus attack the hermit-crab and attempt to draw
it out of its shell with the point of one of its eight arms. But before
this had succeeded there sprang from the body of the sea-anemone a
large number of thin worm-like threads which spread over the arm of
the robber, who immediately let go his hold of the crustacean and
troubled himself no further with it. These threads, called acontia,
are thickly beset with stinging-cells, which must at least cause a
violent smarting on the soft skin of the octopus. Thus we see that the
Actinia instinctively defends its partner from attacks, and does it so
effectively that we need not wonder how the instinct to provide itself
with Actiniæ could have arisen in the hermit-crab. But the acontia seem
to have been greatly strengthened in the course of the sea-anemone's
association with hermit-crabs, for they do not occur in all forms, and
they are most highly developed in those which live in Symbiosis with
crustaceans.

[Illustration: FIG. 34. Hermit-crab (_E_), within a Gasteropod shell,
on which a colony of _Podocoryne carnea_ has established itself.
From the common root-work (which is not clearly shown) there arise
numerous nutritive polyps with tentacles (_np_), among which are
smaller 'blastostyle' polyps with a circle of medusoid buds (_mk_),
spine-like personæ (_stp_), and on the margin of the mollusc shell a
row of defensive individuals (_wp_). _F_, antennæ. _Au_, eyes of the
hermit-crab; slightly enlarged.]

In this case the structural change, the transformation of the
mesenteric filaments that occur in all Actiniæ into projectile
acontia, is comparatively slight, but in another partnership between
hermit-crabs and polyps the latter have undergone a much more marked
adaptation. At Naples _Eupagurus prideauxii_ is one of the commonest
hermit-crabs. It lives at a depth of about a hundred feet, and is
often brought to the Zoological Station by the fishermen in large
quantities. Its borrowed mollusc shell often bears a little polyp,
_Podocoryne carnea_ (Fig. 34), which forms colonies of often several
hundred individuals, arising from a common root-work of stolons which
covers the shell. The polyp colony is composed of different kinds
of individuals or personæ, illustrating the principle of division
of labour: it includes (1) nutritive persons (_np_) which possess a
proboscis, mouth, and tentacles on their club-shaped bodies; (2) much
smaller blastostyles (_bl_), that is to say, polyps with degenerate
mouth and tentacles, which are wholly given over to the production
of buds (_mk_), which then develop into sexual animals, little
free-swimming medusoids; and (3) protective personæ in the form of hard
spines (_stp_), beneath the shelter of which the soft polyps withdraw
when the mollusc shell is rocked about on the sea-floor by the rolling
of the waves. In addition to these three different kinds of individuals
or personæ there are also (4) defensive polyps (_wp_) of long,
thread-like shape, thickly set with stinging-cells, but possessing
neither mouth nor tentacles. It might at first be thought that these
are for the defence of the colony, but this is not so; the fact is
that they rather serve for the direct defence of the hermit-crab.
This is indicated by the position they occupy in the colony; they are
not regularly distributed over the surface, but are ranged round the
edge, and, indeed, only on the edge which surrounds the opening of
the mollusc shell. Here these defensive polyps stand in close array,
sometimes spirally contracted, sometimes hanging loosely down over
the hermit-crab like a fringe. Their function, like the acontia of
Actiniæ, is to defend the crab when an enemy tries to follow it within
the shelter of its domicile. This can easily be demonstrated by drawing
out the hermit-crab from the Gasteropod shell, and, when the colony has
settled down again, seizing the shell with the forceps and drawing it
slowly through the water. The water-stream which then flows upon the
shell mimics the attack of an enemy, and immediately all the defensive
polyps, as at a given signal, strike from above downwards, and repeat
this three or four times; they are scaring off the supposed enemy.

In this species of polyp a special form of individual has developed
with a quite definite position in the colony, and furnished with a
special instinct or reflex mechanism which is directly useful only to
the crab, and has therefore, in a sense, arisen for its advantage. This
can quite well be explained through natural selection, for indirectly
these polyps are also of use to the colony, inasmuch as they protect
their valuable partner, and thus render it possible for the hydroid
colony to make the partnership of use to the hermit-crab as well as to
itself.

This mutual arrangement thus satisfies the requirement which, from
the selectionist point of view, must be made in regard to all that is
new--that it must be useful to its possessor.

If it be asked what service the hermit-crab renders to the polyp colony
in return, the answer is that, as in the symbiosis with sea-anemones,
the hermit-crab carries the polyps to their food, which is also its
own. Hermit-crabs eat all sorts of animal food, living or dead, which
they find on the sea-floor, and the remains of their meal fall to
the share of the polyps. Once, without special intention, I laid a
hermit-crab with its polyp colony in a flat vessel of sea-water beside
a bright green living sponge. After some time the majority of the
polyps had become bright green; they had filled themselves with the
green cells of the sponge.

I do not know how else we should picture to ourselves the origin
of symbiotic instincts in such lowly animals except through the
transmission and augmentation of variations in the instincts of the
two partners--variations which made their possessors more capable of
survival. Mollusc shells, ever since there were any, must have served
as a foundation and point of attachment for polyp colonies; as a
matter of fact, we find to-day on mollusc shells many kinds of polyp
colonies which show no special adaptation to a life of partnership
with hermit-crabs. From such indifferent associations a symbiotic
one must gradually have been evolved in some instances, through the
preservation and augmentation of every useful variation, both of
instincts and reflex actions, as well as of form and structure. I
shall not attempt to trace the course of this evolution in detail, but
it is obvious that the development of defensive polyps, and of their
instinct to defend the crustacean, can be interpreted neither as due
to any direct influence nor as due to the effect of use, but only to
the utility of this arrangement, the beginnings of which--polyps with
stinging-cells--were already present. Their augmentation and perfecting
must be referred entirely to natural selection. It is the same with
adaptations which do not refer directly to the crustacean partner,
but rather to the disposition of the polyps on the shell. The spinous
personæ which protect the softer polyps from being crushed by being
rolled about on the pebbles by the waves cannot possibly be regarded
as the direct result of this crushing. But it is obvious that some
such colonies must have had among their members some with a stronger
external skeleton, and therefore less easily crushed than the rest, and
this would lead to their more frequent survival.

No adaptation seems to have taken place in the hermit-crab in this
case, but that is probably only apparently the case; the probability
is that it would not tolerate the presence of the polyp-colony on the
shell unless its instinct compelled it thereto, just as its instinct
impels it to cover itself with sea-anemones, and fearlessly to grasp
the dangerous animal, which, however, only shows its partner its softer
side. Truly, such transformations of instinct are wonderful enough, but
that they should have come about through intelligence is here quite
inconceivable; there remains nothing but natural selection.

A case in which no apparent corporeal adaptations have occurred, but
which depends altogether on slight modifications of the instincts, is
afforded by the well-known relations between ants and aphides. These
two groups of insects live in a kind of symbiosis, although they are
by no means inseparably connected with each other. Wherever strong
colonies of aphides cover the young shoots of a plant, such as a
stinging-nettle, a rose, or an elder, we almost always find ants which
walk cautiously about among the plant-lice, often in great numbers,
stopping now and again to stroke them with their antennæ, and then
licking up the sweet juice from the intestine which they now give
forth. Darwin showed by experiment that the aphides retain this juice
if no ants are on the spot, and only give it off when ants are put
beside them. Herein lies the proof that we have again to do with a
case of modification of instincts. This juice is, of course, not the
secretion of special glands, as it was still believed to be in Darwin's
time, and it does not come from the so-called 'honey-tubes' situated on
the back of the abdomen of the aphides; it is simply their excrement,
which is liquid like their food, and the voiding of it has become
instinctively connected with the presence of the friendly ants.

That the aphides are not in any way afraid of the ants implies, in
itself, a modification of their instinct, for these poisonous insects,
prone to biting, are otherwise much dreaded in the insect world.
Moreover, the aphides, harmless as they seem, are not quite without
means of defence, although these are never used against the ants.
Other animals which approach them they bespatter with the sticky,
oily secretion prepared in the so-called 'honey-tubes' already noted,
squirting it especially into the eyes of an assailant, so that the
attack is abandoned.

Of course the aphides have no idea wherein the utility of their
friendship for the ants consists, but it is not difficult for us to
discover it, since the ants, by their mere presence in the aphid
colony, frighten and keep off their enemies. We see, then, that the
conditions for a process of natural selection are here afforded:
the instinct to be friendly to the ants is thoroughly useful, and
the instinct of the ants to seek out the aphides, and, instead of
devouring them, to 'milk' them, is also advantageous; it must be an
old acquisition, an instinct early developed, for in several species it
has gone so far that the aphides are carried into the ants' nest, and
are there (as one might say) kept and tended as domesticated animals.

A pretty case of symbiosis between two animals is reported by
Sluiter, and I mention it because it concerns a vertebrate animal,
and intelligence has something to do with it. In the neighbourhood
of Batavia there are frequently to be found on the coral reefs
large yellow sea-anemones, with very numerous and comparatively
long tentacles, and a little brightly-coloured fish, of the genus
_Trachichthys_, makes use of these forests, beset with stinging-cells,
to find security from its enemies. These appear to be numerous, for
in an aquarium, at any rate, the little fish very soon falls a victim
to one or other of them, unless he is supplied with the protective
sea-anemones. When this is the case it swims blithely about among the
tentacles, and the sea-anemone does not sting it; for there has been a
modification of instinct on its part as well as on that of the fish.
The advantage it gains from the fish is, that the latter brings large
morsels of food--in the aquarium, pieces of meat--into the anemone's
mouth. In doing so it tears away fibres for itself, and even if the
Actinia has swallowed pieces too quickly, the fish pulls them half out
of the gullet again, and only relinquishes them to be consumed by its
partner when it has satisfied its own appetite. In this case, again,
the modification of the instinct is the only adaptation which has been
brought about by the symbiosis, and its origin seems difficult to
understand. How can the fish have first formed the habit of putting
its prey into the mouth of the anemone instead of eating it directly?
Although in many cases it is difficult to guess at the beginnings of
a process of selection, because they are scarcely discoverable in
the subsequently accumulated variations, yet in this instance we may
perhaps picture them to ourselves in this way: The fish was in the
habit of letting fall pieces of food which could not be swallowed
whole, and of diving down upon them repeatedly, to tear off a fragment
each time. As the sea-floor in flat places is often covered with
sea-anemones, these pieces would often sink down upon one, which would
welcome it as a dainty, and set about swallowing it, slowly in its own
fashion. The fish must then have found by experience that it could tear
off little bits much more easily from a piece that was held firmly by
the anemone than from one that was lying loose upon the ground, and
this may have caused it to do intentionally what was at first done by
chance. But the sea-anemone, suffering no harm from the fish--indeed,
its association of ideas, if I may use the expression, must rather
have been little fishes and unexpected food--had no cause to shoot
its microscopic arrows at it, and did not do so even when the fish
concealed itself among the tentacles. This latter habit on the part of
the fish would be developed into an instinct through natural selection,
since the individuals that most frequently exhibited it would be the
best protected, and therefore, on an average, the most likely to
survive. Whether the benevolent attitude of the anemone towards the
fish is to be regarded as the expression of an instinct is open to
dispute, for it is quite conceivable that each individual sea-anemone
is disposed to gentleness by the behaviour of the fish, and so the
development of a special hereditary instinct was unnecessary, because
without it each anemone reacted in the manner most likely to secure its
own advantage[7].

[7] Since the above was written Plate has observed several similar
cases in the Red Sea. A little fish lives along with the anemone,
_Crambactis aurantiaca_, a foot in size, and not only conceals itself
among its tentacles, but remains among them when the anemone draws them
in. These fishes, therefore, must be immune against the stinging-cells
of the sea-anemone; and in the same way another species of fish
appears to be immune from the strong poison secreted by sea-urchins
of the genus _Diadema_ from the points of their spines, among which
the fishes live. This relation certainly seems more like a one-sided
adaptation on the part of the fishes than a true symbiosis, but in the
cases observed by Sluiter the return service of the fishes seems to
be regularly rendered. Here, as everywhere else in nature, there are
transition stages, and a one-sided protective relation may gradually,
under favourable circumstances, be transformed into a symbiosis.

The same may be true of the fish as far as laying its booty in the
mouth of the anemone is concerned; there may be no inherited instinct
in this; it may be an intelligent action, which is learnt anew in the
lifetime of each individual.

It might of course be objected to this interpretation that the
beginning of the process, namely, the assumption that chance fragments
from the food of the fish falling just on the anemone is very
improbable; but I once observed that flat rocks washed over by the
sea on the Mediterranean coast (not far from Ajaccio) were so thickly
covered with green anemones that at first I took the green growth for
some strange sea-grass new to me until I had pulled up a little tuft
of the supposed plants and identified them as the soft tentacles of
_Anthea cereus_. Anemones must be equally abundant in the tropical seas
of Java, and a sinking fragment must often alight on the mouth of one
of them.

Much attention and keen discussion have in the last few decades been
focussed on cases of symbiosis between unicellular Algæ and simple
animals. A good example is our green fresh-water polyp, _Hydra viridis_
(Fig. 35, _A_). Its beautiful colour is due to chlorophyll, and it was
long a matter of surprise that animals should produce chlorophyll,
which is a characteristic and fundamental important substance of
assimilating plants, until Geza Entz and M. Braun demonstrated that the
green did not belong to the animal at all, but to unicellular green
Algæ, so-called Zoochlorellæ, which are embedded in the endoderm cells
of the polyps in great numbers (Fig. 35, _zchl_). As these algoid
cells assimilate, and thus liberate oxygen, their presence is of
advantage to the polyp. That--as was at first believed--they also yield
nourishment to the polyp I consider very probable, notwithstanding
the apparently opposed results of the experiments of so acute an
observer as von Graff, for I have seen a large number of these animals
thrive for months, and multiply rapidly by budding in pure water which
contained no food of any kind. In favour of this view, too, are some
observations, to be cited presently, on unicellular animals, in regard
to whose nourishment by the zoochlorellæ living within them there can
be no doubt at all.

[Illustration: FIG. 35. _Hydra viridis_, the Green Fresh-water Polyp.
_A_, the entire animal, greatly enlarged. _M_, the mouth. _t_,
tentacles. _sp_, testis. _ov_, ovary, both in the ectoderm. _Ei_, a
ripe ovum, already green, in process of being extruded. After Leuckart
and Nitsche.

_B_, section of the body-wall, about the position of the ovary in _A_.
_Eiz_, the ovum lying in the ectoderm (_ect_), in which zoochlorellæ
(_zchl_), belonging to the endoderm (_ent_), have already migrated
through the supporting middle lamella (_st_). _eik_, nucleus of ovum.
After Hamann.]

The little algæ on their part find a peaceful and relatively secure
abode within the polyp, and they apparently do not occur outside of
it, at least they do not now migrate from outside into the animal,
but are carried over as a heritable possession of the polyps from
one generation to another, and in a very interesting manner, namely,
by means of the eggs, and by these alone. As Hamann has shown, the
zoochlorellæ migrate at the time when an egg is formed in the outer
layer of the body of the polyp (Fig. 35) from the inner layer outwards,
piercing through the supporting layer between them (st) and penetrating
into the egg (_B_, _Eiz_). They make their way only into the egg, not
into the sperm-cells, which in any case are too small to include them.
Thus they are absent from no young polyp of this species, and it is
easy to understand why earlier experimental attempts to rear colourless
polyps from eggs could never succeed even in the purest water.

[Illustration: FIG. 36. _A_, _Amœba viridis_. _k_, the nucleus.
_cv_, contractile vacuole. _zchl_, the zoochlorellæ. _B_, a single
zoochlorella under high power. After A. Gruber.]

Quite similar green algæ live in symbiosis with unicellular animals,
as, for instance, with an amœba (Fig. 36) and with an Infusorian of the
genus _Bursaria_. In the Zoological Institute in Freiburg there is a
living colony of a green amœba and a green _Bursaria_, both of which
came from America, sent to us some years ago by Professor Wilder, of
Chicago, inside a letter with dried _Sphagnum_, or bog-moss. The plants
came from stagnant water in the Connecticut valley in Massachusetts.
That in this case the zoochlorellæ are of use to the animals within
which they live, not only by giving off oxygen, but also by yielding
food-stuff, has been proved by A. Gruber, who bred the two green
species for seven years in pure water which contained no trace of any
kind of organic food for them. Nevertheless, they multiplied rapidly,
and still form a green scum on the walls of the glass in which they are
kept. They only die away when they are kept in the dark, where the algæ
are unable to assimilate; then one green cell after another wanes and
disappears, and, in consequence, their hosts also die from the double
cause of lack of oxygen and lack of food.

Even in this case the symbiotically united organisms have not remained
unaltered. The algæ at least differ from others of their kind in their
power of resistance to living animal protoplasm. They are not digested
by it, and we may infer from this that they possess some sort of
protective adaptation against the dissolving power of animal digestive
juices; they must, therefore, have undergone some variation, and
adapted themselves to the new situation. Probably their cell-membrane
has become impenetrable to the stuffs which would naturally digest
them, an adaptation which could not be referred to direct effect or to
use, but only to the accumulation of useful variations which cropped
up--in other words, to natural selection. That any adaptive variation
has taken place on the part of the host, whether polyp, amœba, or
Infusorian, cannot be made out. None of these have altered their
original mode of life; they do not depend on the nourishment afforded
by the algæ, but feed on other animals, if these come in their way,
and they live in water rich in oxygen like other species allied to
them, and therefore are not altogether dependent on the algæ in this
connexion; but they can no more help having their partners than the pig
can help having Trichinæ in its muscles.

Similar plant-cells, not green however, but yellow, called
zooxanthellæ, live in great numbers in the endoderm of various
sea-anemones and in the soft plasmic substance of many Radiolarians.
In both these cases we must look for the benefit they confer on their
host in the oxygen they give off, for, like the green zoochlorellæ,
they break up carbonic acid gas in the light, and give off oxygen;
they no longer occur, as far as is known, in a free state, but are
always associated with the host, and they must therefore have altered
in constitution, and have adapted themselves to the conditions of the
symbiosis.

Higher plants, too, sometimes have symbiotic relations with animals;
the most remarkable and best-known example is the relation between ants
and certain trees, in which the ants protect trees which afford them in
return both a dwelling-place and food. We owe our knowledge of these
cases to Thomas Belt and Fritz Müller, and more recently it has been
materially increased by Schimper's researches.

In the forests of South America there grow 'Imbauba,' or
candelabra-trees, species of the genus _Cecropia_, which well deserve
their name, for their bare branches stretch out like candelabra,
and bear little bunches of leaves only at their tips. These leaves
are menaced by the leaf-cutting ants of the genus _Œcodoma_, which
attack numerous species of plants in these regions, often in tens of
thousands, biting off the leaves, cutting them in pieces on the ground,
and carrying them on their backs piece by piece to their nests. There
they use them to make a kind of compost heap, on which fungi, to which
the ants are very partial, readily grow. The candelabra-tree protects
itself from these dangerous robbers, inasmuch as it has established an
association with another ant (_Azteca instabilis_), which finds a safe
dwelling-place in its hollow, chambered stem (Fig. 37, _A_), and feeds
on a brown sap which oozes from the inside. On the stem there are even
little pits regularly arranged in definite places (_E_), through which
the female of _Azteca_ can easily bore her way into the interior. There
she lays her eggs, and soon the whole interior of the trunk teems with
ants, which come trooping out whenever the tree is shaken.

[Illustration: FIG. 37. _A_, a piece of a twig of an Imbauba-tree
(_Cecropia adenopus_), with the leaves cut off. At the leaf-bases
are the hair-cushions (_P_). _E_, the opening for the associated ant
(_Azteca instabilis_). _B_, a piece of the hair-cushion with the
egg-shaped nutritive corpuscles (_nk_). After Schimper.]

This alone would not suffice to protect the tree against the
leaf-cutting ants, for how should the Aztec ants living inside notice
the presence of the lightly climbing leaf-cutters? But that is
provided for, for the Aztecs also frequent the outside of the trunk,
and just where attack would be most disastrous, namely, at the stalks
of the young leaves. At these places there is a peculiar velvet-like
cushion of hair (_P_), from which grow little stalked white papillæ
(Fig. 37, _B_), which are rich in nourishment, and are not only eaten
by the ants, but are harvested by them, being carried off into the
ants' dwellings, presumably to feed their larvæ. In this case, then,
a particular organ, offering special attraction to ants, has been
developed by the plant at the places more especially threatened; while,
as regards the ants, it is probable that only the instincts of feeding
and habitat require to be modified, since courage and thirst for battle
are present in all ants, almost any species being ready at any time to
throw itself on any other which intrudes into its domain.

It should be noted that not all the candelabra-trees live in symbiosis
with ants, and so secure a means of defence against the leaf-cutters.
Schimper found in the primitive forests of South America several
species of _Cecropia_ which never had ants in the chambers of their
hollow stem. But these species did not exhibit the nutritive cushions
at the base of the leaf-stalk; these contrivances for attracting and
retaining the presence of partner ants were altogether absent. Indeed,
only one species, _Cecropia peltata_, has produced these peculiar
structures, and, as they are of no _direct_ use to the tree, we must
say that it has produced them only for the ants. Here, again, natural
selection must have gradually brought about the development of these
nutritive cushions, though as yet we do not know what the beginnings of
the process may have been. In no case can the origin of these cushions
be referred to any direct influence of the environmental conditions.

We may now pass to the association of two species of plants, of
which the lichens furnish the best-known and probably most complete
illustration. Till about twenty years ago the lichens, which in so many
diverse forms clothe the bark of trees, the stones, and the rocks,
were regarded as simple plants like the flowering plants, the ferns,
or the mosses; and many lichenologists occupied themselves with the
exact systematic distinction of about a thousand species, each of which
could be as well and exactly classified, according to form, colour,
habitat, and minute structure, as any other kind of plant. Then De Bary
and Schwendener discovered that the lichens were made up of two kinds
of plants, fungi and algæ, so intimately associated with and adapted
to one another, that on coming together they always assume the same
specific form.

[Illustration: FIG. 38. A fragment of a Lichen (_Ephebe kerneri_),
magnified 450 times. _a_, the green alga-cells. _P_, the fungoid
filaments. After Kerner.]

The framework, and therefore the largest part, and the one which
determines the form of a lichen, is due to the fungus (Fig. 38).
Colourless threads of fungus ramify in a definite manner according
to the species of fungus, and in the network of spaces left by this
ramification green alga-cells (_a_) lie singly, or in rows, or groups.
The fungus is propagated by multitudes of minute spores, which it
produces periodically, and these are disseminated in the air by the
bursting of the sporangia and are carried away by the wind in the form
of fine dust; the alga multiplies simply by continual division into
two, but it also, like the whole lichen, can survive desiccation,
and, after falling to pieces, is likewise carried through the air as
microscopic dust.

The partnership of the two plants rests on a basis of mutual benefit;
the fungus, like all fungi, is without chlorophyll, and cannot
therefore decompose carbonic acid gas or elaborate its own organic
food-stuffs; it receives these from the alga. The alga has in the
network of the fungus a safe shelter and basis of attachment, for the
fungus is able to bore into the bark of trees and even into stones;
besides which it absorbs water and salts, and supplies these to the
partner alga. We here see the mutual advantage derived from the
partnership, which is really an extremely intimate one. Fungus spores,
sown by themselves, spring up and develop some branchings of fungoid
hyphæ, a so-called mycelium, but without the requisite partner alga
these remain weak and soon die away. The alga, on the other hand,
can, in some cases, though not in all, survive without the fungus if
the necessary conditions of its life be supplied to it, but it grows
differently and more luxuriantly in association with the fungus.

The same species of alga may be found associated with different species
of fungi, and then each partnership forms a distinct species of lichen
of definite and characteristic appearance; Stahl even succeeded in
making new species of lichen artificially by bringing the spores of a
lichen-forming fungus into contact with alga-cells, with which they had
never been associated in free nature.

The most remarkable feature of this remarkable association seems to me
to be the formation of common reproductive bodies--an adaptation in
face of which all doubt as to the theory of selection must disappear.
Periodically there are developed in the substance of the lichen small
corpuscles, the so-called soredia, each of which consists of one or
more alga-cells surrounded and kept together by threads of the fungus.
When they are developed in large numbers they form a floury dust over
the maternal lichen, which 'breaks up' and leaves them, like the spores
of the fungus, to be carried away by the wind. If these alight on
favourable soil nothing more is needed than the external conditions
of development, light, warmth, and water, to enable the lichen to
spring up anew. The great advantage to the preservation of 'species' is
obvious, for, when multiplication by the ordinary method occurs among
lichens, the spores of the fungus, even if they have fallen on good
ground, can only develop into a new lichen if chance bring to them the
proper partner alga.

Obviously there must be, in the formation of the soredia, great
advantage for the species, or rather 'for the two species,' for the
fungi as well as the algæ benefit by the arrangement, which ensures the
continuance of the partnership. It was not without reason, however,
that the dual organism was so long regarded as a simple species in the
natural history sense, _for that is what it really is_, although it has
arisen in a manner quite different from the usual origin of species. As
we know species which consist only of single cells, and others which
consist of many cells, differentiated in different ways, and forming
a cell-community or 'person,' and, finally, others which consist of a
community of diversely differentiated personæ, making up a 'stock'; so
in the lichens we see that even different species may combine to form
a new physiological whole, a vital unit, an individual of the highest
order. When, at the outset of these lectures, I said that the theory
of evolution was now no longer a mere hypothesis, and that its general
truth could no longer be doubted by any one acquainted with the facts
available, I had in my mind, among other facts, especially that of
symbiosis, and above all the case of the lichens.

There are many other interesting cases of symbiosis between two
different kinds of plants, and one side of the partnership is
represented by fungi in a relatively large number of instances. The
reason is not far to seek: fungi must always be dependent on other
plants for their food; they must be parasitic, because they cannot
themselves produce the organic substances they require. They must
therefore associate themselves in some way with other organisms, living
or dead, and as a general rule they simply prey upon their associate,
sucking up its juices and killing it. But in not a few cases they can
render services in return, and, as we have seen in the case of the
lichens, symbiosis may then occur. Fungi in general have the power of
discovering and absorbing the least trace of water in the soil, and
with it they absorb the salts necessary to the plant, and in this,
apparently, consists the service which they are able to render even to
large plants fixed deep in the earth, such as shrubs and trees. The
roots of many of our forest trees, e.g. beech, oak, fir, silver poplar,
and bushes like broom, heaths, and rhododendrons, are thickly wrapped
round with a network of fungoid threads, and the mutual relations just
indicated exist between these and the plants in question (Fig. 39,
_A_ and _B_). The plants give to the fungi some contribution from the
superfluity of their food-stuffs, and receive in return water and
salts, which are of value especially in times of drought. Perhaps there
is some connexion between this and the fact that limes wither and lose
their leaves so quickly during great summer-heat; these and many other
of our trees possess no root-fungi or mycorhizæ.

It is easy to understand, therefore, that genuine 'symbiosis' may have
arisen from parasitism. But that this is not the only path that leads
to symbiosis is shown by the cases of animal symbiosis we have already
discussed.

[Illustration: FIG. 39. _A_, fragment of a Silver Poplar root, with an
envelope of symbiotic fungoid filaments (mycelium); after Kerner. _B_,
apex of a Beech root, with the closely enveloping mantle of mycelium;
enlarged 480 times.]

The partnership between polyps and hermit-crabs may have arisen from a
one-sided commensalism, since polyps establishing themselves on mollusc
shells which were often made use of by hermit-crabs would be better fed
than those which settled down on stones. There are still species which
make use of both modes of settlement. Then followed the adaptation
of the crustacean to the polyp, for, first, those hermit-crabs would
thrive best which tolerated the presence of the polyp; then those which
sought its presence, that is to say, which gave a preference to shells
covered with polyps; and, finally, those which would take no others,
and even themselves fixed the sea-anemone upon it, if it chanced to be
removed. Intelligence need not be taken into account in the matter at
all, not even in the hermit-crab's case. We have only to recall the
complex instincts, exercised only once in a lifetime, which compel the
silkworm and the emperor moth to elaborate their effective cocoons.
The elaboration of the spinning-instinct can only be due to natural
selection, for the insect can have had no idea of the utility of its
performance, and the same is true in the case of the sea-anemones
or the hydroid polyps and the hermit-crab. The sea-anemone is quite
unconscious that it is defending its partner, the hermit-crab, when it
lashes out its stinging acontia on any disturbance, and the hermit-crab
is equally unaware that the sea-anemone is contributing to its safety;
both animals act quite unconsciously, purely instinctively, and the
origin of these instincts, on which the symbiosis is based, must be
due, not to intelligent activities which have become habitual, but only
to the survival of the fittest.

According to the principle of natural selection nothing can arise
but that which is of use directly or indirectly to its possessor.
Nevertheless, there are cases in which it appears as if something had
arisen, which was of no use to the species in which the variation
appeared, but only to the species protected by it. This is the case
in the remarkable symbiosis between algæ of the family Nostocaceæ and
the floating, moss-like water-fern _Azolla_. This plant, in external
appearance almost like duckweed, has on the under surface of its leaves
a minute opening, leading into a relatively roomy hair-lined cavity,
and in this cavity there is always, enclosed in jelly, a bluish green
unicellular alga, _Anabæna_. The cavity is present in every leaf, and
the alga is present in every cavity, making its way in from a deposit
of alga-cells which is found on the incurved tip of every young shoot.
As soon as a young leaf of _Azolla_ unfolds from the bud it receives
its _Anabæna_ cells from this deposit, and no one has yet found
either twigs or leaves which were free from the algæ. But no one has
succeeded in discovering any benefit derived by the _Azolla_ from this
partnership.

This looks like a contradiction of the theory of selection, but there
remains the possibility that there is some benefit rendered to the
_Azolla_ by the alga, though we cannot see it as yet. There is also
the possibility that the cavity is an organ which was of use to the
plant at an earlier time, perhaps as an insect-trap, but has now lost
its significance, and is utilized by the alga as a dwelling-place.
This, however, is contradicted by the remarkable distribution of the
four known species of _Azolla_. Two of these are widely distributed in
America; the third lives in Australia, Asia, and Africa; the fourth
in the region of the Nile: all four have cavities in their leaves,
and in all these forms the cavity is inhabited by the same species of
_Anabæna_. This indicates that the leaf-cavity and the partnership
with the alga must have originated in remote antiquity; the symbiosis
must date from a time before the four modern species of _Azolla_ had
split off from a single parent-species. But no rudimentary organ,
that is to say, no organ not of use to the plant itself, would have
been preserved through such a vast period of time, as we shall see
later, for useless organs disappear in the course of ages. As the
cavity has not yet disappeared, we may assume with some probability
that it is useful to the plant, whether by means of the _Anabæna_, or
in some other unknown way. To draw an argument against the reality of
the processes of selection from our lack of knowledge of what this
advantage may be would be as unreasonable as if, notwithstanding our
experience that stones sink in the water, we were to assume of a
particular stone which we did not see sink, because it was hidden from
our sight by bushes, that perhaps it had not sunk, but was capable of
floating.




LECTURE X

THE ORIGIN OF FLOWERS

 Introduction--Precursors of Darwin--Pollination
 by wind--Arrangements in flowers for securing
 cross-fertilization--Salvia, Pedicularis--Flowers visited by
 flies--Aristolochia--Pinguicula--Daphne--Orchids--Flowers are
 built up of adaptations--Mouth-parts of insects--Proboscis of
 butterflies--Mouth-parts of the cockroach--Of the bee--Pollen
 baskets of bees--Origin of flowers--Attraction of insects by
 colour--Limitation of the area visited--Nägeli's objection to
 the theory of selection--Other interpretations excluded--_Viola
 calcarata_--Only those changes which are useful to their possessors
 have persisted--Deceptive flowers--Cypripedium--Pollinia of
 Orchis--The case of the Yucca-moth--The relative imperfection of
 the adaptations tells in favour of their origin through natural
 selection--Honey thieves.


WHEN one species is so intimately bound up with another that neither
can live for any length of time except in partnership, that is
certainly an example of far-reaching mutual adaptation, but there
are innumerable cases of mutual adaptation, in which, although there
is no common life in the same place, yet the first form of life is
adjusted in relation to the peculiarities of the second, and the second
to those of the first. One of the most beautiful, and, in regard to
natural selection, the most instructive of these cases is illustrated
by the relations between insects and the higher plants, relations which
have grown out of the fact that many insects have formed the habit of
visiting the flowers of the plants for the sake of the pollen. In this
connexion the theory of selection has made the most unexpected and
highly interesting disclosures, for it has informed us how the flowers
have arisen.

In earlier times the beauty, the splendour of colour, and the fragrance
of flowers were regarded as phenomena created for the delight of
mankind, or as an outcome of the infinite creative power of Mother
Nature, who loves to run riot in form and colour. Without allowing our
pleasure in all this manifold beauty to be spoilt, we must nowadays
form quite a different conception of the way in which the flowers
have been called into being. Although here, as everywhere else in
Nature, we cannot go back to ultimate causes, yet we can show, on
very satisfactory evidence, that the flowers illustrate the reaction
of the plants to the visits of insects, and that they have been in
large measure evoked by these visits. There might, indeed, have
been blossoms, but there would have been no flowers--that is to say,
blossoms with large, coloured, outer parts, with fragrance, and with
nectar inside, unless the blossoms had been sought out by insects
during the long ages. Flowers are adaptations of the higher flowering
plants to the visits of insects. There can be no doubt about that now,
for--thanks to the numerous and very detailed studies of a small number
of prominent workers--we need not only suppose it, we can prove it
with all the certainty that can be desired. The mutual adaptations of
insects and flowers afford one of the clearest examples of the mode
of operation and the power of natural selection, and the case cannot
therefore be omitted from lectures on the theory of descent.

That bees and many other insects visit flowers for the sake of the
nectar and pollen has been known to men from very early times. But this
fact by itself would only explain why adaptations to flower-visiting
have taken place in these insects to enable them, for instance, to
reach the nectar out of deep corolla-tubes, or to load themselves with
a great quantity of pollen, and to carry it to their hives, as happens
in the case of the bees. But what causes the plants to produce nectar,
and offer it to the insects, since it is of no use to themselves? And
further, what induces them to make the pillage easier to the insects,
by making their blossoms visible from afar through their brilliant
colours, or by sending forth a stream of fragrance that, even during
the night, guides their visitors towards them?

As far back as the end of the eighteenth century a thoughtful and
clear-sighted Berlin naturalist, Christian Konrad Sprengel, took
a great step towards answering this question. In the year 1793 he
published a paper entitled 'The Newly Discovered Secret of Nature in
the Structure and Fertilization of Flowers[8],' in which he quite
correctly recognized and interpreted a great many of the remarkable
adaptations of flowers to the visits of insects. Unfortunately, the
value of these discoveries was not appreciated in Sprengel's own time,
and his work had to wait more than half a century for recognition.

[8] _Das neu-entdeckte Geheimniss der Natur im Bau u. der Befruchtung
der Blumen_, Berlin, 1793.

Sprengel was completely dominated by the idea of an all-wise Creator,
who 'has not created even a single hair without intention,' and, guided
by this idea, he endeavoured to penetrate into the significance of
many little details in the structure of flowers. Thus he recognized
that the hairs which cover the lower surface of the petals of the
wood-cranesbill (_Geranium sylvaticum_) protect the nectar of the
flower from being diluted with rain, and he drew the conclusion,
correct enough, though far removed from our modern ideas as regards the
directly efficient cause, that the nectar was there for the insects.

He was also impressed by the fact that the sky-blue corolla of the
forget-me-not (_Myosotis palustris_) has a beautiful yellow ring round
the entrance to the corolla-tube, and he interpreted this as a means by
which insects were shown the way to the nectar which is concealed in
the depths of the tube.

[Illustration: FIG. 40. _Potentilla verna_, after Hermann Müller. _A_,
seen from above. _Kbl_, sepals. _Bl_, petals. _Nt_, nectaries near the
base of the stamens. _B_, section through the flower. _Gr_, stigma.
_St_, stamen. _Nt_, nectary.]

We now know that such 'honey-guides' are present in most of the flowers
visited by insects, in the form of spots, lines, or other marking,
usually of conspicuous colour, that is, of a colour contrasting with
the ground colour of the flower. Thus, in species of Iris, regular
paths of short hairs lead the way to the place where the nectar lies.
In the spring potentilla (_Potentilla verna_) (Fig. 40) the yellow
petals (_A_, _Bl_) become bright orange-red towards their bases,
and this shows the way to the nectaries, which lie at the bases
of the stamens (_st_), and are protected by hairs, the so-called
'nectar-covers' (_Saftdecke_) of Sprengel, from being washed by rain.

The recognition of the honey-guides led Sprengel on to the idea that
the general colouring of the flower effects on a large scale what the
honey-guides do in a more detailed way--it attracts the attention of
passing insects to where nectar is to be found; indeed, he went an
important step further by recognizing that there are flowers which
cannot fertilize themselves, in which the insect, in its search for
honey, covers itself with pollen, which is then rubbed off on the
stigma of the next flower visited, fertilization being thus effected.
He demonstrated this not only for the Iris, but for many other
flowers, and he drew the conclusion that 'Nature does not seem to have
wished that any flower should be fertilized by its own pollen.' How
near Sprengel was to reaching a complete solution of the problem is
now plain to us, for he even discovered that many flowers, such as
_Hemerocallis fulva_, remained infertile if they were dusted with their
own pollen.

Even the numerous experiments of that admirable German botanist, C. F.
Gärtner, although they advanced matters further, did not suffice to
make the relations between insects and flowers thoroughly clear; for
this the basis of the theory of Descent and Selection was necessary.
Here, again, it was reserved for Charles Darwin to lead the way where
both contemporaries and predecessors had been blindly groping. He
recognized that, _in general_, self-fertilization is disadvantageous
to plants; that they produce fewer seeds, and that these produce
feebler plants, than when they are cross-fertilized; that, therefore,
those flowers which are arranged to secure cross-fertilization have an
advantage over those which are self-fertilized. In many species, as
Sprengel had already pointed out, self-fertilization leads to actual
infertility; only a few plants are as fertile with their own pollen as
with that of another plant; and Darwin believed that, in all flowering
plants, crossing with others of the same kind, at least from time to
time, is necessary if they are not to degenerate.

Thus the advantage which the flowers derive from the visits of
insects lies in the fact that insects are instrumental in the
cross-fertilization of the flowers, and we can now understand how the
plant was able to vary in a manner favourable to the insect-visits, and
to exhibit adaptations which serve exclusively to make these visits
easier; we understand how it was possible that there should develop
among flowers an endless number of contrivances which served solely
to attract insects, and even how, for the same end, the insignificant
blossoms of the oldest Phanerogams must have been transformed into real
flowers.

We must not imagine, however, that the obviously important crossing of
plant-individuals, usually called 'cross-pollination,' can be effected
only by means of insects. There were numerous plants in earlier times,
and there is still a whole series in which cross-fertilization is
effected through the air by the wind; these are the anemophilous or
wind-pollinated Angiosperms.

To these belong most of the catkin-bearers, such as hazel and birch,
and also the grasses and sedges, the hemp and the hop, and so forth.
In these plants there is no real flower, but only an inconspicuous
blossom, without brightly-coloured outer envelopes, without fragrance
or nectar; all of them have smooth pollen grains, which easily
separate into fine dust and are carried away by the wind until they
fall, by chance, far from their place of origin, on the stigma of a
female blossom.

[Illustration: FIG. 41. Flower of Meadow Sage (_Salvia pratensis_),
after H. Müller. _st´_, immature anthers concealed in the 'helmet' of
the flower. _st´´_, mature anther lowered. _gr´_, immature stigma.
_gr´´_, mature stigma. _U_, the lower lip of the corolla, the
landing-stage for the bee.]

By far the greater number of the phanerogams, however, especially
all our indigenous 'flowers,' are, as a rule, fertilized by means of
insects, and it is amazing to see in what diverse ways, often highly
specialized, they have adapted themselves to the visits of insects.
Thus there are flowers in which the nectar lies open to view, and
these can be feasted on by all manner of insects; there are others in
which the nectar is rather more concealed, but still easily found, and
reached by insects with short mouth-parts, e.g. large flowers blooming
by day and bearing much pollen, like the Magnolias. These have been
called beetle-flowers, because they are visited especially by the
honey-loving Longicorns.

Other flowers blooming by day are especially adapted to fertilization
by means of bees; they are always beautifully coloured, often blue;
they are fragrant, and contain nectar deep down in the flower, where
it can only be reached by the comparatively long proboscis of the
bee. Different arrangements in the different flowers secure that
the bee cannot enjoy the nectar without at the same time effecting
the cross-pollination. Thus the stamens of the meadow sage (_Salvia
pratensis_) are at first hidden within the helmet-shaped upper lip of
the flower (Fig. 41, _st´_), but bear lower down on their stalk a short
handle-like process, which turns the pollen-bearing anther downwards
(_st´´_) as soon as it is pressed back by an intruding insect. The
pollen-sacs then strike downwards on the back of the bee, and cover it
with pollen. When the bee visits another more mature flower, the long
style, which was at first hidden within the helmet, has bent downwards
(_gr´´_), and now stands just in front of the entrance to the flower,
so that the bee must rub off a part of the pollen covering its back on
to the stigma, and fertilization is thus effected.

There are other flowers which are specially disposed to suit
the visits of the humble-bees, as, for instance, _Pedicularis
asplenifolia_, the fern-leaved louse-wort, a plant of the high Alps
(Fig. 42). The first thing that strikes us about this plant is the
thickly tufted hair covering on the calyx (_k_), which serves to keep
off little wingless insects from the flower; then there is the strange
left-sided twisting of the individual flowers, whose under lip allows
only a strong insect like the humble-bee to gain access, towards the
left, to the corolla-tube (_kr_), in the depths of which the nectar is
concealed. While the humble-bee is sucking up the nectar it becomes
dusted over with pollen from the anthers, which falls to dust at a
touch, and when it insinuates itself into a second flower its powdered
back comes first into contact with the stigma of the pistil (_gr_)
which projects from the elongated bill-shaped under lip, dusting it
over with the pollen of the first visited flower. Butterflies and
smaller bees cannot rob this flower; it is strictly a humble-bee's
flower.

[Illustration: FIG. 42. Alpine Lousewort (_Pedicularis asplenifolia_).
_A_, flower seen from the left side, enlarged three times; the arrows
show the path by which the humble-bee enters. _B_, the same flower,
seen from the left, after removal of the calyx, the lower lip and the
left half of the upper lip. _C_, ovary (_ov_), nectary (_n_), and base
of style. _D_, tip of style, bearing the stigma. _E_, two anthers
turned towards one another. _o_, upper lip. _u_, lower lip. _gr_,
style. _st_, anthers. _kr_, corolla-tube. _k_, calyx.]

There are not a few of such flowers adapted to a very restricted
circle of visitors, and in all of them we find contrivances which
close the entrance to all except what we may call the welcome insects;
sometimes there are cushions of bristles which prevent little insects
from creeping up from below, or it is the oblique position of the
flower which prevents their getting in from the stem; sometimes it is
the length and narrowness of the corolla-tube, or the deep and hidden
situation of the nectar, which only allows intelligent insects to find
the treasure.

Very remarkable are those flowers which are adapted to the visits of
flies, for they correspond in several respects to the peculiarities
of these insects. In the first place, flies are fond of decaying
substances and the odours given off by these, and so the flowers which
depend for their cross-fertilization on flies have taken on the dull
and ugly colours of decay, and give out a disagreeable smell. But flies
are also shy and restless, turning now hither, now thither, and cannot
be reckoned among the 'constant' insect visitors, that is to say, they
do not persistently visit the same species; it is, therefore, evident
that they might easily carry away the pollen without any useful result
ensuing. Moreover, their intelligence is of a low order, and they do
not seek nectar with the perseverance shown by bees and humble-bees. It
is not surprising, therefore, to find that many of the flowers adapted
for the visits of flies are so constructed that they detain their
visitors until they have done their duty, that is to say, until they
have effected, or at least begun, the process of cross-pollination.

[Illustration: FIG. 43. Flower of Birthwort (_Aristolochia clematitis_)
cut in half. _A_, before pollination by small flies. _b_, the bristles.
_B_, after pollination. _P_, pollen mass. _N_, stigma, _b_, the
bristles. _b´_, their remains. After H. Müller.]

[Illustration: FIG. 44. Alpine Butterwort (_Pinguicula alpina_). _A_,
section through the flower. _K_, calyx. _bh_, bristly prominences.
_sp_, spur. _st_, stamen. _n_, stigma. _B_, stigma and stamen more
magnified. After H. Müller.]

Our birthwort (_Aristolochia clematitis_) and the Cuckoo-pint
(_Arum maculatum_) are pit-fall flowers, whose long corolla-tubes
have an enlargement at the base, in which both pistil and stamens
are contained. In the birthwort (Fig. 43) the narrow entrance-tube
is thickly beset with stiff hairs (_A_, _b_), whose points are
all directed towards the base. Little flies can creep down quite
comfortably into the basal expansion, but once there they are kept
imprisoned until the flower, in consequence of the pollination of
the stigma, begins to wither, the first parts to go being these very
bristles (_B_, _b´_), whose points, like a fish-weir, prevented the
flies from creeping out. Other 'fly-flowers,' as for instance the
Alpine butterwort (_Pinguicula alpina_) (Fig. 44), securely imprison
the plump fly as soon as it has succeeded in forcing itself in far
enough to reach, with its short proboscis, the nectar contained in the
spur (_sp_) of the corolla. The backward-directed bristles hold it fast
for some time, and it is only by hard pressing with the back against
the anthers (_st_) lying above it, and against the stigma (_n_), that
it ultimately succeeds in getting free, but it never does so without
having either loaded itself with pollen, or rubbed off on the stigma
the pollen it brought with it from another similar flower. The Alpine
butterwort is protogynous, that is to say, the pistil ripens first,
the pollen later, so that the possibility of self-fertilization is
altogether excluded.

It would be impossible to give even an approximate idea of the
diversity of the contrivances for securing fertilization in flowers
without spending many hours over them, for they are different in
almost every flower, often widely so, and even in species of the same
genus they are by no means always alike; for not infrequently one
species is adapted to one circle of visitors, and its near relative
to another. Thus the flower of the common Daphne (_Daphne mezereum_)
(Fig. 45, _A_ and _C_) is adapted to the visits of butterflies, bees,
and hover-flies, while its nearest relative (_Daphne striata_) (Fig.
45, _B_ and _D_) has a somewhat narrower and longer corolla-tube, so
that only butterflies can feast upon it. This example shows that there
are exclusively 'butterfly flowers,' but specialization goes further,
for there are flowers adapted to diurnal and others to nocturnal
Lepidoptera. The former have usually bright, often red colours, and
a pleasant aromatic fragrance, and in all of them the nectar lies at
the bottom of a very narrow corolla-tube. To this class belong, for
instance, the species of pink, many orchids, such as _Orchis ustulata_,
and _Nigritella angustifolia_ of the Alps, which smells strongly of
vanilla; also the beautiful campion (_Lychnis diurna_) and the Alpine
primrose (_Primula farinosa_). The flowers adapted to nocturnal
Lepidoptera are characterized by pale, often white colour, and a strong
and pleasant smell, which only begins to stream out after sunset, and
indeed many of these flowers are quite closed by day. This is the case
with the large, white, scentless bindweed (_Convolvulus sepium_), which
is chiefly visited and fertilized by the largest of our hawk-moths
(_Sphinx convolvuli_). The pale soapwort (_Saponaria officinalis_)
exhales a delicate fragrance which attracts the Sphingidæ from afar,
and the sweet smell of the honeysuckle (_Lonicera periclymenum_) is
well known, and has the same effect; an arbour of honeysuckle often
attracts whole companies of our most beautiful Sphingidæ and Noctuidæ
on warm June nights, to the great delight of the moth-collecting youth.

[Illustration: FIG. 45. _Daphne mezereum_ (_A_ and _C_) and Daphne
striata (_B_ and _D_). The former visited by butterflies, bees, and
flies, the latter by butterflies only. _A_ and _B_, vertical sections
of the flowers. _St_, stamens. _Gr_, style. _n_, nectary. _C_ and _D_,
flowers seen from above. After H. Müller.]

I cannot conclude this account of flower-adaptations without
considering the orchids somewhat more in detail, for it is among
them that we find the most far-reaching adaptations to the visits of
insects. Among them, too, great diversity prevails, as is evident
from the fact that Darwin devoted a whole book to the arrangements
for fertilization in orchids, but the main features are very much the
same in the majority. Figure 46 gives a representation of one of our
commonest species (_Orchis mascula_), A shows the flower in side view,
_B_ as it appears from in front. The flower seems as it were to float
on the end of the stalk (_st_), stretching out horizontally the spur
(_sp_) which contains the nectar. Between the large, broad under lip
(_U_), marked with a honey-guide (_sm_), and offering a convenient
alighting surface, and the broad, cushion-like stigma (_n_) lies the
entrance to the spur. Fertilization occurs in the following way:--The
fly or bee, when it is in the act of pushing its proboscis into the
nectar-containing spur, knocks with its head against the so-called
rostellum (_r_), a little beak-like process at the base of the stamens
(_p_). The pollen masses are of very peculiar construction, not falling
to dust, but forming little stalked clubs, with the pollen grains glued
together, and so arranged that they spring off when the rostellum is
touched and attach themselves to the head of the insect, as at _D_ on
the pencil (Fig. 46). When the bee has sucked up the nectar out of the
spur, and then proceeds to penetrate into another flower of the same
species, the pollinia have bent downwards on its forehead (_E_), and
must unfailingly come in contact with the stigma of the second flower,
to which they now remain attached, and effect its fertilization. What
a long chain of purposeful arrangements in a single flower, and no
interpretation of them is available except through natural selection!

[Illustration: FIG. 46. Common Orchis (_Orchis mascula_). _A_, flower
in side view. _st_, stalk. _sp_, spur with the nectary (_n_). _ei_,
entrance to the spur. _U_, lower lip. _B_, flower from in front. _p_,
pollinia. _Sm_, honey-guide. _ei_, entrance to the nectar. _na_,
stigma. _r_, rostellum. _U_, lower lip. _C_, vertical section through
the rostellum (_r_), pollinium (_p_). _ei_, entrance. _D_, the pollinia
removed and standing erect on the tip of a lead-pencil. _E_, the same,
somewhat later, curved downwards.]

And how diversely are these again modified in the different genera
and species of orchids, of which one is adapted to the visits of
butterflies exclusively, as _Orchis ustulata_, another to those of
bees, as _Orchis morio_, and a third to those of flies, as _Ophrys
muscifera_. These flowers are adapted to insect visits in the minutest
details of the form of the petals, which are smooth, as if polished
with wax, where insects are not intended to creep, but velvety or hairy
where the path leads to the nectar, and at the same time to the pollen
and the stigma. And then there is the diversity in the form and colour
of the 'honey-guides' on the 'alighting surface,' that is, the under
lip of the flower, upon which the insect sits and holds fast, while it
pushes its head as far as possible into the spur, so that its proboscis
may reach the nectar lying deep within it! Even though we cannot
pretend to guess at the significance of every curve and colour-spot in
one of the great tropical orchids, such as _Stanhopea tigrina_, yet we
may believe, with Sprengel, that all this has its significance, or has
had it for the ancestors of the plant in question, and in fact that
the flower is made up of nothing but adaptations, either actual or
inherited from its ancestors, although sometimes perhaps no longer of
functional importance.

So far, then, we have illustrated the fact that there are hundreds and
thousands of contrivances in flowers adapted solely to the visits of
insects and to securing cross-fertilization, and these adaptations go
so far that we might almost believe them to be the outcome of the most
exact calculation and the most ingenious reflection. But they all admit
of interpretation through natural selection, for all these details,
which used to be looked upon as merely ornamental, are directly or
indirectly of use to the species; directly, when, for instance, they
concern the dusting of the insect with the pollen; indirectly, when
they are a means of attracting visits.

Moreover, the evidence of the operation of the processes of selection
becomes absolutely convincing when we consider that, as in symbiosis,
there are always two sets of adaptations taking place independently
of one another--those of the flowers to the visits of the insects,
and those of the insects to the habit of visiting the flowers. To
understand this clearly we must turn our attention to the insects, and
try to see in what way they have been changed by adapting themselves to
the diet which the flowers afford.

As is well known, several orders of insects possess mouth-parts which
are suited for sucking up fluids, and these have evolved, through
adaptation to a fluid diet, from the biting mouth-parts of the
primitive insects which we see still surviving in several orders.
Thus the Diptera may have gradually acquired the sucking proboscis
which occurs in many of them by licking up decaying vegetable and
animal matter, and by piercing into and sucking living animals. But
even among the Diptera several families have more recently adapted
themselves quite specially to a flower diet, to honey-sucking, like the
hover-flies, the Syrphidæ, and the Bombyliidæ, whose long thin proboscis
penetrates deep into narrow corolla-tubes, and is able to suck up the
nectar from the very bottom. The transformation was not so important
in this case, since the already existing sucking apparatus only
required to be a little altered.

Again, in the order Hemiptera (Bugs) the suctorial proboscis does not
owe its origin to a diet of flowers, for no member of the group is now
adapted to that mode of obtaining food.

[Illustration: FIG. 47. Head of a Butterfly. _A_, seen from in front.
_au_, eyes. _la_, upper lip. _md_, rudiments of the mandibles. _pm_,
rudimentary maxillary palps. _mx´_, the first maxillæ modified into the
suctorial proboscis. _pl_, palps of labium or second maxillæ, cut off
at the root, remaining in _B_--which is a side view. _at_, antennæ.
Adapted from Savigny.]

The proboscis of the Lepidoptera, on the other hand, depends entirely
on adaptation to honey-sucking, and we may go the length of saying that
the order of Lepidoptera would not exist if there were no flowers.
This large and diverse insect-group is probably descended from the
ancestors of the modern caddis-flies or Phryganidæ, whose weakly
developed jaws were chiefly used for licking up the sugary juices of
plants. But as flowering plants evolved the licking apparatus of the
primitive butterflies developed more and more into a sucking organ, and
was ultimately transformed into the long, spirally coiled suctorial
proboscis as we see it in the modern butterflies (Fig. 47). It has
taken some pains to trace this organ back to the biting mouth-parts of
the primitive insects, for nearly everything about it has degenerated
and become stunted except the maxillæ (_mx´_). Even the palps (_pm_)
of these have become so small and inconspicuous in most of the
Lepidoptera that it is only quite recently that remains of them have
been recognized in a minute protuberance among the hairs. The mandibles
(_md_) have quite degenerated, and even the under lip has disappeared,
and only its palps are well developed (_B_, _pl_). But the first
maxillæ (_mx´_), although very strong and long, are so extraordinarily
altered in shape and structure that they diverge from the maxillæ of
all other insects. They have become hollow, probe-like half-tubes,
which fit together exactly, and thus form a closed sucking-tube of most
complex construction, composed of many very small joints, after the
fashion of a chain-saw, which are all moved by little muscles, and are
subject to the will through nerves, and are also furnished with tactile
and taste papillæ. Except this remarkable sucking proboscis there are
no peculiarities in the body of the butterfly which might be regarded
as adaptations to flower-visiting, with a few isolated exceptions, of
which one will be mentioned later. This is intelligible enough, for
the butterfly has nothing more to seek from the flower beyond food for
itself; it does not carry stores for offspring.

The bees, however, do this, and accordingly we find that in them the
adaptations to flower-visiting are not confined to the mouth-parts.

As far as we can judge now, the flower-visiting bees are descended
from insects which resembled the modern burrowing-wasps. Among these
the females themselves live on nectar and pollen, and build cells in
holes in the ground, and feed their brood. They do not feed them on
honey, however, but on animals--on caterpillars, grasshoppers, and
other insects, which they kill by a sting in the abdomen, or often only
paralyse, so that the victim is brought into the cells of the nest
alive but defenceless, and remains alive until the young larva of the
wasp, which emerges from the egg, sets to work to devour it.

[Illustration: FIG. 48. Mouth-parts of the Cockroach (_Periplaneta
orientalis_), after R. Hertwig. _la_, upper lip or labrum. _md_,
mandibles. _mx_^1, first maxillæ, with _c_, cardo, _st_, stipes, _li_,
internal lobe or lacinia, _le_, external lobe or galea, and _pm_, the
maxillary palp. _mx_^2, the labium or second maxillæ, with similar
detailed parts.]

Before I go on to explain the origin of the sucking proboscis of the
bee from the biting mouth-parts of the primitive insects I must first
briefly consider the latter.

The biting mouth-parts of beetles, Neuroptera, and Orthoptera (Fig.
48), consist of three pairs of jaws, of which the first, the mandibles
(_md_), are simply powerful pincers for seizing and tearing or chewing
the food. They have no part in the development of the suctorial
apparatus either in bees or in butterflies, so they may be left out
of account. The two other pairs of jaws, the first and second maxillæ
(_mx_^1 and _mx_^2), are constructed exactly on the same type, having
a jointed basal portion (_st_) bearing two lobes, an external (_le_)
and an internal (_li_), and a feeler or palp, usually with several
joints, directed outwards from the lobes (_pm_ and _pl_). The second
pair of maxillæ (_mx_^2) differs from the first chiefly in this, that
the components of the pair meet in the median line of the body, and
fuse more or less to form the so-called 'under lip' or labium. In the
example given, the cockroach (_Periplaneta orientalis_), this fusion
is only partial, the lobes having remained separate (_le_ and _li_);
and the same is true of the bee, but in this case the inner lobes have
grown into a long worm-like process which is thrust into the nectar in
the act of sucking.

[Illustration: FIG. 49. Head of the Bee. _Au_, compound eyes. _au_,
ocelli. _at_, antennæ. _la_, upper lip. _md_, mandibles. _mx_^1, first
maxillæ, with _pm_, the rudimentary maxillary palp. _mx_^2, second
maxillæ with the internal lobes (_li_) fused to form the 'tongue.'
_le_, the external lobes of the second maxillæ, known as 'paraglossæ.'
_pl_, labial palp.]

Even the burrowing-wasps exhibit the beginnings of variation in this
direction, for the under lip is somewhat lengthened and modified into
a licking organ. The adaptation has not gone much further than this,
even in one of the true flower-bees, _Prosopis_, which feeds its larvæ
with pollen and honey, and it is only in the true honey-bee that the
adaptation is complete (Fig. 49). Here the so-called 'inner lobe' of
the under lip (_li_) has elongated into the worm-shaped process already
mentioned; it is thickly covered with short bristles, and is called
the 'tongue' of the bee (_li_). The outer lobes of the under lip have
degenerated into little leaf-like organs, the so-called accessory
tongue or paraglossa (_le_), while the palps of the under lip (_pl_)
have elongated to correspond with the tongue, and serve as a sensitive
and probably also as a smelling organ, in contrast to the palps of the
first maxillæ, which have shrunk to minute stumps (_pm_). The whole
of the under lip, which has elongated even in its basal portions,
forms, with the equally long first maxillæ, the proboscis of the bee.
The first maxillæ are sheath-like half-tubes, closely apposed around
the tongue, and form along with it the suctorial tube, through which
the nectar is sucked up. Thus, of the three pairs of jaws in insects,
only the first pair, the mandibles, have remained unaltered, obviously
because the bee requires a biting-organ for eating pollen, for kneading
wax, and for building cells.

But bees do not only feast on nectar and pollen themselves, they carry
these home as food for their larvæ. The form already mentioned,
_Prosopis_, takes up pollen and nectar in its mouth, and afterwards
disgorges the pulp as food for its larvæ, but the rest of the true
bees have special and much more effective collecting-organs, either a
thick covering of hair on the abdomen, or along the whole length of the
posterior legs, or finally, a highly developed collecting apparatus,
such as that possessed by the honey-bee--the basket and brush on the
hind leg. The former is a hollow on the outer surface of the tibia, the
latter a considerable enlargement of the basal tarsal joint, which, at
the same time, is covered on the inner surface with short bristles,
arranged in transverse rows like a brush. The bee kneads the pollen
into the basket, and one can often see bees flying back to the hive
with a thick yellow ball of pollen on the hind leg. In those bees which
collect on the abdomen, like _Osmia_ and _Megachile_, the pollen mass
forms a thick clump on the belly, and in the case of _Andrena_ Sprengel
observed long ago that it sometimes flew with a packet of pollen bigger
than its own body on the hind leg.

All these are contrivances which have gradually originated through the
habit of carrying home pollen for the helpless larvæ shut up in the
cells. They have developed differently in the various groups of bees,
probably because the primary variations with which the process of
selection began were different in the various ancestral forms.

In the ancestors of those which carry pollen on the abdomen there
was probably a thick covering of hair on the ventral surface of the
body, which served as a starting-point for the selection, and, in
consequence, the further course of the adaptation would be concerned
solely with this hair-covered surface, while variations in other less
hairy spots would remain un-utilized.

After all this it will no longer seem a paradoxical statement that the
existence of gaily coloured, diversely formed, and fragrant flowers is
due to the visits of insects, and that, on the other hand, many insects
have undergone essential transformations in their mouth-parts and
otherwise as an adaptation to a flower diet, and that an entire order
of insects with thousands of species--the Lepidoptera--would not be
in existence at all if there had been no flowers. We must now attempt
to show, in a more detailed way, how, by what steps, and under what
conditions, our modern flowers have arisen from the earlier flowering
plants. In this I follow closely the classic exposition which we owe to
Hermann Müller.

The ancestral forms of the modern higher plants, the so-called
'primitive seed plants' or 'Archisperms,' were all anemophilous, as the
Conifers and Cycads are still. Their smooth pollen-grains, produced
in enormous quantities, fell like clouds of dust into the air, were
carried by the wind hither and thither, and some occasionally alighted
on the stigma of a female flower. In these plants the sexes often occur
separately on different trees or individuals, and there must be a
certain advantage in this when the pollination is effected by the wind.

The male flowers of the Archisperms would be visited by insects in
remote ages, just as they are now; but the visitors came to feed upon
the pollen, and did not render any service to the plant in return;
they rather did it harm by reducing its store of pollen. If it was
possible to cause the insect to benefit the plant at the same time
as it was pillaging the pollen, by carrying some of it to female
blossoms and thereby securing cross-fertilization, it would be of
great advantage, for the plant would no longer require to produce such
enormous quantities of pollen, and the fertilization would be much
more certain than when it depended on the wind. It is obvious that the
successful pollination of anemophilous plants implies good weather and
a favourable wind.

[Illustration: FIG. 50. Flowers of the Willow (_Salix cinerea_); after
H. Müller. _A_, the male. _B_, the female catkin. _C_, individual male
flower; _n_, nectary. _D_, individual female flower; _n_, nectary. _E_,
Poplar, an exceptional hermaphrodite flower.]

It is plain that the utilization of the insect-visitors in
fertilization might be secured in either of two ways; the female
blossoms might also offer something attractive to the insects, or
hermaphrodite flowers might be formed. As a matter of fact, both ways
have been followed by Nature. An example of the former is the willow,
the cross-fertilization of which was forced upon the insects by the
development in both female and male blossoms of a nectary (Fig. 50,
_C_ and _D_), a little pit or basin in which nectar was secreted. The
insects flew now to male and now to female willow-catkins, and in doing
so they carried to the stigma of the female blossom the pollen, which
in this case was not dusty but sticky, so that it readily adhered to
their bodies.

The securing of cross-fertilization by the development of hermaphrodite
flowers has, however, occurred much more frequently, and we can
understand that this method secured the advantageous crossing much more
perfectly, for the pollen had necessarily to be carried from blossom
to blossom, while, in cases like that of the willow, countless male
blossoms might be visited for nectar one after the other before the
insect made up its mind to fly to a female blossom of the same species.
The beginnings of the modification of the unisexual flowers in this
direction may be seen in variations which occur even now, for we not
infrequently find, in a male catkin, individual blossoms, which, in
addition to the stamens, possess also a pistil with a stigma. (Fig. 50
_E_ shows such an abnormal hermaphrodite flower from a poplar.)

As soon as hermaphrodite flowers came into existence the struggle
to attract insects began in a more intense degree. Every little
improvement in this direction would form the starting-point of a
process of selection, and would be carried on and increased to the
highest possible pitch of perfection.

It was probably the outer envelopes of the blossoms which first changed
their original green into other colours, usually those which contrasted
strongly with the green, and thus directed the attention of the insects
to the flowers. Variations in the colour of ordinary leaves are
always cropping up from time to time, whether it be that the green is
transformed into yellow or that the chlorophyll disappears more or less
completely and red or blue coloured juices take its place. Many insects
can undoubtedly see colour, and are attracted by the size of coloured
flowers, as Hermann Müller found by counting the visits of insects to
two nearly related species of mallow, one of which, _Malva silvestris_,
has very large bright rose-red flowers visible from afar, while the
other, _Malva rotundifolia_, has very inconspicuous small pale-red
flowers. To the former there were thirty-one different visitors, to the
latter he could only make sure of four. The second species, as is to be
expected, depends chiefly on self-fertilization.

It has recently been disputed from various quarters that insects are
attracted by the colours of the flowers, and these objections are
based chiefly on experiments with artificial flowers. But when, for
instance, Plateau, in the course of such experiments saw bees and
butterflies first fly towards the artificial flowers, and then turn
away and concern themselves no more about them, that only proves that
their sight is sharper than we have given them credit for; for though
they may be deceived at a distance, they are not so when they are near;
it is possible, too, that the sense of smell turns the scale[9]. I
have myself made similar experiments with diurnal butterflies, before
which I placed a single artificial chrysanthemum midst a mass of
natural flowers. It rarely happened indeed that a butterfly settled on
the artificial flower; they usually flew first above it, but did not
alight. Twice, however, I saw them alight on the artificial flower,
and eagerly grope about with the proboscis for a few moments, then fly
quickly away. They had visited the real chrysanthemums or horse-daisies
with evident delight, and eagerly sucked up the honey from the many
individual florets of every flower, and they now endeavoured to do
the same in the artificial flower, and only desisted when the attempt
proved unsuccessful. In this experiment the colours were of course
only white and yellow; with red and blue it is probably more difficult
to give the exact impression of the natural flower-colours; and in
addition there is the absence of the delicate fragrance exhaled by the
flower.

[9] The experiments of Plateau have since been criticized by
Kienitz-Gerloff, who altogether denies their value (1903).

It must be allowed that the colour is certainly not the sole attraction
to the flower; the fragrance helps in most cases, and even this is
not the object of the insect's visits. The real object is the nectar,
to which colour and fragrance only show the way. The development of
fragrance and nectar must, like that of the colour, have been carried
on and increased by processes of selection, which had their basis in
the necessity for securing insect-visits, and as soon as these main
qualities of the flower were established greater refinements would
begin, and flower-forms would be evolved, which would diverge farther
and farther, especially in shape, from the originally simple and
regular form of the blossom.

The reason for this must have lain chiefly in the fact that, after
insect-visits in general were secured by a flower, it would be
advantageous to exclude all insects which would pillage the nectar
without rendering in return the service of cross-fertilization--all
those, therefore, which were unsuited either because of their minute
size or because of the inconstancy of their visits. Before the
butterflies and the bees existed, the regularly formed flat flower with
unconcealed nectar would be visited by a mixed company of caddis-flies,
saw-flies, and ichneumon-flies. But as the nectar changed its place to
the deeper recesses of the flower it was withdrawn from all but the
more intelligent insects, and thus the circle of visitors was already
narrowed to some extent. But when in a particular species the petals
fused into a short tube, all visitors were excluded whose mouth-parts
were too short to reach the nectar; while among those which could
reach it the process of proboscis-formation began; the under lip, or
the first maxillæ, or both parts together, lengthened step for step
with the corolla-tube of the flower, and thus from the caddis-flies
came the butterflies, and from the ichneumon-flies the burrowing-wasps
(_Sphegidæ_) and the bees.

At first sight one might perhaps imagine that it would have been more
advantageous to the flowers to attract a great many visitors, but
this is obviously not the case. On the contrary, specialized flowers,
accessible only to a few visitors, have a much greater certainty of
being pollinated by them, because insects which only fly to a few
species are more certain to visit these, and above all to visit many
flowers of the same species one after another. Hermann Müller observed
that, in four minutes, one of the humming-bird hawk-moths (_Macroglossa
stellatarum_) visited 108 different flowers of the same species, the
beautiful Alpine violet (_Viola calcarata_), one after the other, and
it may have effected an equal number of pollinations in that short time.

It was, therefore, a real advantage to the flowers to narrow their
circle of visitors more and more by varying so that only the useful
visitors could gain access to their nectar, and that the rest should
be excluded. Thus there arose 'bee-flowers,' 'butterfly-flowers,'
'hawk-moth flowers,' and, indeed, in many cases, a species of flower
has become so highly specialized that its fertilization can only
be brought about by a single species of insect. This explains the
remarkable adaptations of the orchids and the enormous length of the
proboscis in certain butterflies. Even our own hawk-moths _Macroglossa
stellatarum_ and _Sphinx convolvuli_ show an astonishing length of
proboscis, which measures 8 cm. in the latter species. In _Macrosilia
cluentius_, in Brazil, the proboscis is 20 cm. in length; and in
Madagascar there grows an orchid with nectaries 30 cm. in length,
filled with nectar to a depth of 2 cm., but the fertilizing hawk-moth
is not yet known.

Thus we may say that the flowers, by varying in one direction or
another, have selected a definite circle of visitors, and, conversely,
that particular insect-groups have selected particular flowers for
themselves, for those transformations of the flowers were always most
advantageous which secured to them the exclusive visits of their best
crossing agents, and these transformations were, on the one hand,
such as kept off unwelcome visitors, and, on the other hand, such as
attracted the most suitable ones.

From the botanical point of view the assumption that flowers and
flower-visiting insects have been adapted to each other by means
of processes of selection has been regarded as untenable, because
every variation in the flower presupposes a corresponding one in the
insect. I should not have mentioned this objection had it not come
from such a famous naturalist as Nägeli, and if it were not both
interesting and useful in our present discussion. Nägeli maintained
that selection could not, for instance, have effected a lengthening
of the corolla-tube of a flower, because the proboscis of the insects
must have lengthened _simultaneously_ with it. If the corolla-tube
had lengthened alone, without the proboscis of the butterfly being
at the same time elongated, the flower would no longer be fertilized
at all, and if the lengthening of the proboscis preceded that of the
corolla-tube it would have no value for the butterfly, and could not
therefore have been the object of a process of selection.

This objection overlooks the facts that a species of plant and of
butterfly consists not of one individual but of thousands or millions,
and that these are not absolutely uniform, but in fact heterogeneous.
It is precisely in this that the struggle for existence consists--that
the individuals of every species differ from one another, and that
some are better, others less well constituted. The elimination of the
latter and the preferring of the former constitutes the process of
selection, which always secures the fitter by continually rejecting the
less fit. In the case we are considering, then, there would be, among
the individuals of the plant-species concerned, flowers with a longer
and flowers with a shorter corolla-tube, and among the butterflies
some with a longer and some with a shorter proboscis. If among the
flowers the longer ones were more certain to be cross-fertilized than
the shorter ones, because hurtful visitors were better excluded, the
longer ones would produce more and better seeds, and would transmit
their character to more descendants; and if, among the butterflies,
those with the longer proboscis had an advantage, because the nectar
in the longer tubes would, so to speak, be reserved for them, and they
would thus be better nourished than those with the shorter proboscis,
the number of individuals with long proboscis must have increased from
generation to generation. Thus the length of the corolla-tube and the
length of the proboscis would go on increasing as long as there was
any advantage in it for the flower, and both parties must of necessity
have varied _pari passu_, since every lengthening of the corolla
was accompanied by a preferring of the longest proboscis variation.
The augmentation of the characters depended on, and could only have
depended on, a guiding of the variations in the direction of utility.
But this is exactly what we call, after Darwin and Wallace, Natural
Selection.

We have, however, in the history of flowers, a means of demonstrating
the reality of the processes of selection in two other ways. In the
first place, it is obvious that no other interpretation can be given
of such simultaneous mutual adaptations of two different kinds of
organisms. If we were to postulate, as Nägeli, for instance, did, an
intrinsic Power of Development in organisms, which produces and guides
their variations, we should, as I have already said, be compelled
also to take for granted a kind of pre-established harmony, such as
Leibnitz assumed to account for the correlation of body and mind:
plant and insect must always have been correspondingly altered so that
they bore the same relation to each other as two clocks which were
so exactly fashioned that they always kept time, though they did not
influence each other. But the case would be more complicated than that
of the clocks, because the changes which must have taken place on both
sides were quite different, and yet at the same time such that they
corresponded as exactly as Will and Action. The whole history of the
earth and of the forms of life must, therefore, have been foreseen
down to the smallest details, and embodied in the postulated Power of
Development.

But such an assumption could hardly lay claim to the rank of a
scientific hypothesis. Although every grain of sand blown about by
the wind on this earth could certainly only have fallen where it
actually did fall, yet it is in the power of any of us to throw a
handful of sand wherever it pleases us, and although even this act of
throwing must have had its sufficient reason in us, yet no one could
maintain that its direction and the places where the grains fell were
predestined in the history of the earth. In other words: That which we
call chance plays a part also in the evolution of organisms, and the
assumption of a Power of Development, predestinating even in detail,
is contradicted by the fact that species are transformed in accordance
with the chance conditions of their life.

This can be clearly demonstrated in the case of flowers. That the wild
pansy (_Viola tricolor_), which lives in the plains and on mountains of
moderate elevation, is fertilized by bees, and the nearly allied _Viola
calcarata_ of the High Alps by Lepidoptera, is readily intelligible,
since bees are very abundant in the lower region, and make the
fertilization of the species a certainty, while this is not so in the
High Alps. There the Lepidoptera are greatly in the majority, as every
one knows who has traversed the flower-decked meads of the High Alps in
July, and has seen the hundreds and thousands of butterflies and moths
which fly from flower to flower. Thus the viola of the High Alps has
become a 'butterfly-flower' by the development of its nectaries into a
long spur, accessible only to the proboscis of a moth or butterfly. The
chance which led certain individuals of the ancestral species to climb
the Alps must also have supplied the incentive to the production of the
changes adapted to the visits of the prevalent insect. The hypothesis
of a predestinating Power of Development suffers utter shipwreck in
face of facts like these.

We have, furthermore, an excellent touchstone for the reality of the
processes of selection in the _quality_ of the variations in flowers
and insects. Natural selection can only bring about those changes
which are of use to the possessors themselves; we should therefore
expect to find among flowers only such arrangements as are, directly or
indirectly, of use to them, and, conversely, among insects only such as
are useful to the insect.

And this is what we actually do find. All the arrangements of the
flowers--their colour, their form, their honey-guides, their hairy
honey-paths (Iris), their fragrance, and their honey itself--are all
indirectly useful to the plant itself, because they all co-operate in
compelling the honey-seeking insect to effect the fertilization of
the flower. This is most clearly seen in the case of the so-called
'Deceptive' flowers, which attract insects by their size and beauty,
their fragrance, and their resemblance to other flowers, and force
their visitors to be the means of their cross-fertilization, although
they contain no nectar at all. This is the case, according to Hermann
Müller, with the most beautiful of our indigenous orchids, the lady's
slipper (_Cypripedium calceolaris_). This flower is visited by bees
of the genus _Andrena_, which creep into the large wooden-shoe-shaped
under lip in the search for honey, only to find themselves prisoners,
for they cannot get out, at least by the way they came in, because of
the steep and smoothly polished walls of the flower. There is only one
way for the bee; it must force itself under the stigma, which it can
only do with great exertion, and not without being smeared with pollen,
which it carries to the next flower into which it creeps. It can only
leave this one in the same way, and thus the pollen is transferred to
the stigma by a mechanical necessity.

Such remarkable cases remind us in some ways of those cases of mimicry
in which the deceptions have to be used with caution or they lose their
effect. One might be disposed to imagine that such an intelligent
insect as a bee would not be deceived by the lady's slipper more than
once, and would not creep into a second flower after discovering that
there was no nectar in the first. But this conclusion is not correct,
for the bees are well accustomed in many flowers to find that the
nectar has already been taken by other bees; they could therefore not
conclude from one unsuccessful visit that the _Cypripedium_ did not
produce nectar at all, but would try again in a second, a third, and a
fourth flower. If these orchids had abundantly covered flower-spikes
like many species of _Orchis_, and if the species were common, the bees
would probably soon learn not to visit them, but the reverse is the
case. There is usually only one or, at most, two open flowers on the
lady's slipper, and the plant is rare, and probably occurs nowhere in
large numbers.

If we could find a flower in which the nectar lay open and accessible
to all insects, and which did not require any service from them
in return, the case could not be interpreted in terms of natural
selection; but we do not know of any such case.

[Illustration: FIG. 51. The Yucca-moth (_Pronuba yuccasella_). _M_,
laying eggs in the ovary of the Yucca flower. _n_, the stigma. After
Riley.]

Conversely, too, there are no adaptations in the insects which are
useful only to the flowers, and which are not of some use, directly or
indirectly, to the insect itself. Bees and butterflies certainly carry
the pollen from one flower to the stigma of another, but they are not
impelled to do this by a special instinct; they are forced to do it
by the structure of the flower, which has its stamens so placed and
arranged that they must shake their pollen over the visitor, or it may
be that the anthers are modified into stalked, viscid pollinia which
spring off at a touch, and fix themselves, so to speak, on the insect's
head. And even this is not all in the case of the orchis, for the
insect would never of its own accord transfer these pollinia on to the
stigma of the next flower; this is effected by the physical peculiarity
which causes the pollinia, after a short time, to bend forwards on the
insect's head.

All this fits in as well as possible with the hypothesis: how could an
instinct to carry pollen from one flower to the stigma of another have
been developed in an insect through natural selection, since the insect
itself has nothing to gain from this proceeding? Accordingly, we never
find in the insect any pincers or any kind of grasping organ adapted
for seizing and transmitting the pollen.

There is, however, one very remarkable case in which this appears to
be so, indeed really is so, and nevertheless it is not contradictory
to, but is corroborative of, the theory of selection. The excellent
American entomologist, Riley, established by means of careful
observations that the large white flowers of the Yucca are fertilized
by a little moth which behaves in a manner otherwise unheard of among
insects. Only the females visit the flowers, and they at once busy
themselves collecting a large ball of pollen. To this end they have
on the maxillary palps (Fig. 52, _C_, _mxp_) a long process (_si_),
curved in the form of a sickle, and covered with hairs, which probably
no other Lepidopteron possesses, with the help of which the moth
very quickly sweeps together a ball of pollen, it may be three times
the size of her own head. With this ball the insect flies to the
next flower, and there she lays her egg, by means of an ovipositor
otherwise unknown among Lepidoptera (Fig. 52, _A_, _op_), in the pods
of the flower. Finally, she pushes the ball of pollen deep into the
funnel-shaped stigmatic opening on the pistil (Fig. 51, _n_), and so
effects the cross-fertilization. The ovules develop, and when the
caterpillars emerge from the egg four to five days later they feed on
these until they are ready to enter on the pupa stage. Each little
caterpillar requires about eighteen or twenty seeds for its nourishment
(Fig. 52, _B_, _r_).

[Illustration: FIG. 52. The fertilization of the Yucca. _A_, ovipositor
of the Yucca-moth. _op_, its sheath. _sp_, its apex. _op_^1, the
protruded oviduct. _B_, two ovaries of the Yucca, showing the holes by
which the young moths escape, and (_r_) a caterpillar in the interior.
_C_, head of the female moth, with the sickle-shaped process (_si_) on
the maxillary palps for sweeping off the pollen and rolling it into a
ball. _mx_^1, the proboscis. _au_, eye. _p_^1 base of first leg. _D_,
longitudinal section through an ovary of the Yucca, soon after the
laying of two eggs (_ei_). _stk_, the canal made by the ovipositor.]

Here, then, we find an adaptation of certain parts of the moth's body
in relation to the fertilization of the flower, but in this case it is
as much in the interest of the moth as of the plant. By carrying the
pollen to the stigma the moths secure the development of the ovules,
which serve their offspring as food, so that we have here to do with
a peculiar form of care for offspring, which is not more remarkable
than many other kinds of brood-care in insects, such as ants, bees,
Sphex-wasps, ichneumon-flies, and gall-flies.

It might be objected that this case of the Yucca is not so much one
of effecting fertilization as of parasitism; but the eggs, which are
laid in the seed-pods, are very few, and the caterpillars which emerge
from them only devour a very small proportion of the seeds, of which
there may be about 200 (Fig. 52, _B_). Thus the plants also derive
an advantage from the moth's procedure, for quite enough seeds are
left. The form and position of the stamens and of the stigma seem to
be as exactly adapted to the visits of the moth as the moth is to the
transference of the pollen, for the Yucca can only be fertilized by
this one moth, and sets no seed if the moth be absent. For this reason
the species of Yucca cultivated in Europe remain sterile.

Thus the apparent contradiction is explained, and the facts everywhere
support the hypothesis that the adaptations between flowers and insects
depend upon processes of selection.

This origin is incontrovertibly proved, it seems to me, in another way,
namely, by the merely _relative_ perfection of the adaptations, or
rather, by their relative imperfection.

I have already pointed out that all adaptations which depend upon
natural selection can only be relatively perfect, as follows from the
nature of their efficient causes, for natural selection only operates
as long as a further increase of the character concerned would be of
advantage to the existence of the species. It cannot be operative
beyond this point, because the existence of the species cannot be more
perfectly secured in this direction, or, to speak more precisely,
because further variations in the direction hitherto followed would no
longer be improvements, even though they might appear so to us.

Thus the corolla of many flowers is suited to the thick, hairy head
and thorax of the bee, for to these only does the pollen adhere in
sufficient quantity to fertilize the next flower; yet the same flowers
are frequently visited by butterflies, and in many of them there
has been no adaptation to prevent these useless visits. Obviously
this is because preventive arrangements could only begin, according
to our theory, when they were necessary to the preservation of the
species; in this case, therefore, only when the pillaging visits
of the butterflies withdrew so many flowers from the influence of
the effective pollinating visitor, the bee, that too few seeds were
formed, and the survival of the species was threatened by the continual
dwindling of the normal number. As long as the bees visit the flowers
frequently enough to ensure the formation of the necessary number of
seeds a process of selection could not set in; but should the bees
find, for instance, that nearly all the flowers had been robbed of
their nectar, and should therefore visit them less diligently, then
every variation of the flower which made honey less accessible to the
butterflies would become the objective of a process of selection.

Everywhere we find similar imperfections of adaptation which indicate
that they must depend on processes of selection. Thus numerous flowers
are visited by insects other than those which pollinate them, and these
bring them no advantage, but merely rob them of nectar and pollen;
the most beautiful contrivances of many flowers, such as _Glycinia_,
which are directed towards cross-fertilization by bees, are rendered
of no effect because wood-bees and humble-bees bite holes into the
nectaries from the outside, and so reach the nectar by the shortest
way. I do not know whether bees in the native land of the _Glycinia_
do the same thing, but in any case they can do no sensible injury to
the species, since otherwise processes of selection would have set in
which would have prevented the damage in some way or other, whether by
the production of stinging-hairs, or hairs with a burning secretion, or
in some other way. If the actual constitution of the plant made this
impossible, the species would become less abundant and would gradually
die out.

Thus the relative imperfection of the flower-adaptations, which in
general are so worthy of admiration, affords a further indication that
their origin is due to processes of selection.


ADDITIONAL NOTE TO CHAPTER X.

It has been remarked that the chapter on the Origin of Flowers in the
German Edition contains no discussion and refutation of the objections
which have up till recently been urged against the theory of flowers
propounded by Darwin and Hermann Müller. I admit that this chapter
seemed to be so harmonious and so well rounded, and at the same time so
convincing as to the reality of the processes of selection, that the
feeble objections to it, and the attempts of opponents to find another
explanation of the phenomena, might well be disregarded in this book.

However, the most important of these objections and counter-theories
may here be briefly mentioned.

Plateau in Ghent was the first to collect _facts_ which appeared to
contradict the Darwinian theory of flowers; he observed that insects
avoided _artificial_ flowers, even when they were indistinguishable
in colour from natural ones as far as our eyes could perceive, and he
concluded from this that it is not the colour which guides the insects
to the flowers, that they find the blossoms less by their sense of
sight than by their sense of smell. But great caution is required
in drawing conclusions from experiments of this kind. I once placed
artificial marguerites (_Chrysanthemum leucanthemum_) among natural
ones in a roomy frame in the open air, and for a considerable time I
was unable to see any of the numerous butterflies (_Vanessa urticæ_),
which were flying about the real chrysanthemums, settle on one of the
artificial flowers. The insects often flew quite close to them without
paying them the least attention, and I was inclined to conclude that
they either perceived the difference at sight, or that they missed the
odour of the natural flowers in the artificial ones. But in the course
of a few days it happened twice in my presence that a butterfly settled
on one of the artificial blooms and _persistently groped about with
fully outstretched tube to find the entrance to the honey_. It was only
after prolonged futile attempts that it desisted and flew away. That
bees are guided by the eye in their visits to flowers has been shown by
A. Forel, who cut off the whole proboscis, together with the antennæ,
from humble-bees which were swarming eagerly about the flowers. He thus
robbed them of the whole apparatus of smell, and nevertheless they flew
down from a considerable height direct to the same flowers. An English
observer, Mr. G. N. Bulman, has been led to believe, with Plateau, that
it is a matter of entire indifference to the bees whether the flowers
are blue, or red, or simply green in colour, if only they contain
honey, and that therefore the bees could have played no part in the
development of blue flowers, as Hermann Müller assumed they had, and
that they could have no preference for blue or any other colour, as Sir
John Lubbock and others had concluded from their experiments. This is
correct in so far that bees feed as eagerly on the greenish blossoms of
the lime-tree as they do on the deep-blue gentian of the Alpine meadows
or the red blossoms of the Weigelia, the dog-roses of our gardens or
the yellow buttercups (_Ranunculus_) of our meadows; they despise
nothing that yields them honey. But it certainly does not follow
from this that the bees may not, under certain circumstances, have
exercised a selecting influence upon the fixation and intensification
of a new colour-variety of a flower. This is less a question of a
_colour-preference_, in the human sense, on the part of the bees
than of the _greater visibility_ of the colour in question in the
environment peculiar to the flower, and of the amount of rivalry the
bees meet with from other insects in regard to the same flower. In
individual cases this would be difficult to demonstrate, especially
since we can form only an approximate idea of the insect's power of
seeing colour, and cannot judge what the colours of the individual
blossoms count for in the mosaic picture of a flowery meadow. Yet this
is the important point, for, as soon as the bees perceive one colour
more readily than another, the preponderance of this colour-variety
over other variations is assured, since it will be more frequently
visited. In the same way we cannot guess in individual cases why one
species of flower should exhale perfume while a nearly related species
does not. But when we remember that many flowers adapted for the visits
of dipterous insects possess a nauseous carrion-like smell, by means
of which they not only attract flies but scare off other insects, we
can readily imagine cases in which it was of importance to a flower
to be able to be easily found by bees without betraying itself by its
pleasant fragrance to other less desirable visitors.

Thus, therefore, we can understand the odourless but intensely blue
species of gentian, if we may assume that its blue colour is more
visible to bees than to other insects. If I were to elaborate in
detail all the principles which here suggest themselves to me I should
require to write a complete section, and I am unwilling to do this
until I can bring forward a much larger number of new observations
than I am at present in a position to do. All I wish to do here is
to exhort doubters to modesty, and to remind them that these matters
are exceedingly complex, and that we should be glad and grateful that
expert observers like Darwin and Hermann Müller have given us some
insight into the principles interconnecting the facts, instead of
imagining whenever we meet with some little apparently contradictory
fact, which may indeed be quite correct in itself, that the whole
theory of the development of flowers through insects has been
overthrown. Let us rather endeavour to understand such facts, and to
arrange them in their places as stones of the new building.

Often the contradiction is merely the result of the imperfect
theoretical conceptions of its discoverer, as we have already shown in
regard to Nägeli. Bulman, too, fancies he has proved that bees do not
distinguish between the different varieties of a flower, but visit them
indiscriminately with the same eagerness, thus causing intercrossing
of all the varieties, and preventing any one from becoming dominant.
But are the varieties which we plant side by side in our gardens of the
kind that are evolved by bees? That is to say, are their _differences
such as will turn the scale for or against the visits of the bees_?
If one were less, another more easily seen by the bees; or if one
were more fragrant, or had a fragrance more agreeable to bees than
the other, the result of the experiment would probably have been very
different.

One more objection has been made. It is said that the bees, although
exclusively restricted, both themselves and their descendants, to
a diet of flowers, are not so constant _to a particular flower_ as
the theory requires. They do indeed exhibit a 'considerable amount
of constancy,' and often visit a large number of flowers of the same
species in succession, but the theory requires that they should
not only confine themselves to this one species, but to a _single
variety_ of this species. These views show that their authors have not
penetrated far towards an understanding of the nature of selection.
Nature does not operate with individual flowers, but with millions and
myriads of them, and not with the flowers of a single spring, but with
those of hundreds and thousands of years. How often a particular bee
may carry pollen uselessly to a strange flower without thereby lowering
the aggregate of seeds so far that the existence of the species seems
imperilled, or how often she may fertilize the pistil of a useful
variation with the pollen of the parent species, without interrupting
or hindering the process of the evolution of the variety, no mortal
can calculate, and what the theory requires can only be formulated in
this way: The constancy of the bees in their visits to the flowers must
be so great that, on an average, the quantity of seeds will be formed
which suffices for the preservation of the species. And in regard
to the transformation of a species, the attraction which the useful
variety has for the bees must, on an average, be _somewhat stronger_
than that of the parent species. As soon as this is the case the seeds
of the variety will be formed in preponderant numbers, although they
may not all be quite pure from the first, and by degrees, in the course
of generations, the plants of the new variety will preponderate more
and more over those of the parent form, and finally will alone remain.
In the first case we have before our eyes the proof that, in spite of
the imperfect constancy of the bees, a sufficient number of seeds is
produced to secure the existence of the species. Or does Mr. Bulman
conclude from the fact that the bees are _not absolutely constant_ that
flowers are not fertilized by bees at all?

I cannot conclude this note without touching briefly upon what the
opponents of the flower theory have contributed, and what explanation
of the facts they are prepared to offer.

In his important work, _Mechanische-physiologische Theorie der
Abstammungslehre_, published in 1884, Nägeli, as a convinced opponent
of the theory of selection, attempted an explanation. He was quite
aware that his assumption of an inward 'perfecting principle' would not
suffice to explain the mutual adaptations of flowers and insects, and
he refers the transformation of the first inconspicuous blossoms into
flowers to the mechanical stimulus which the visiting insects exerted
upon the parts of the blossom. By the pressure of their footsteps,
the pushing and probing with their proboscis, they have, he says,
transformed gradually, for instance, the little covering leaves at the
base of a pollen vessel into large flower petals, caused the conversion
of short flower-tubes into long ones, and of the pollen, once dry and
dusty, into the firmly adhesive mass formed in the anther lobes of
our modern flowers. The colour of the flowers depends, according to
him, upon the influence of light, which certainly no more explains the
yellow ring on a blue ground in the forget-me-not than it does the many
other nectar-guides which show the insect the way to the honey. Nägeli
works with the Lamarckian principle in the most daring way, and with
the same _naïveté_ as Lamarck himself in his time, that is, without
offering any sort of explanation as to how the minute impression made,
say by the foot or by the proboscis of an insect, upon a flower, is to
be handed on to the flowers of succeeding generations. He treats the
unending chain of generations as if it were a single individual, and
operates with his 'secular' stimulus, and with 'weak stimuli, lasting
through countless generations,' as though they were a proved fact. But
I have not even touched upon the question as to whether these 'stimuli'
could produce the changes he ascribes to them, even if they were
continually affecting the flower. How the scale-like covering leaves
of the pollen vessels could become larger and petal-like through the
treading of an insect's foot is as difficult to see as why a honey-tube
should _become longer_ because of the butterfly's honey-sucking: might
it not just as well become _wider_, _narrower_, or even _shorter_? I
see no convincing reason why it should become _longer_! And even if
it did so, it would necessarily continue to lengthen as time went on,
and this is not the case, for we find corolla-tubes of all possible
lengths, but, _it is to be noted, always in harmony with the length of
the proboscis of the visiting insect_. In a similar way Henslow has
recently attempted to refer the origin of flowers to the mechanical
stimulus exercised upon it by the visiting insects. 'An insect hanging
to the lower petal of a flower elongates the same by its weight, and
the lengthened petal is transmitted by heredity.'...'The irritation
caused by its feet in walking along the flower causes the appearance
of colouring matter, and the colour is likewise transmitted.'...'As it
probes for honey it causes a flow of sweet sap to that part, and this
also becomes hereditary!'

In this case, also, it is simply taken for granted that every little
passing irritation not only produces a perceptible effect, but that
this effect is transmissible. In a later lecture we shall have to
discuss in detail the question of the inheritance of functional
modifications. It is enough to say here that, if this kind of
transmission really took place even in the case of such minute and
transitory changes, there could be no dispute as to the correctness
of the 'Lamarckian principle,' since every fairly strong and lasting
irritation could be demonstrated with certainty to produce an effect.
When a butterfly, floating freely in the air, sucks honey from a tube,
the irritation must be almost analogous to that caused by a comb
lightly drawn by some one through our hair, and this is supposed to
effect the gradual lengthening of the corolla-tube of the flower!

The secretion of honey, too, depends upon the persistent irritation
of the proboscis! Then 'deceptive flowers,' like the Cypripedium we
have mentioned, could not exist at all, for they contain no honey,
although the proboscis of the bee must cause the same irritation in
them as in other orchids which do contain honey. This whole 'theory' of
direct effect is, moreover, only a crude and apparent interpretation,
which explains the conditions only in so far as they can be seen from
a distance; it fails as soon as they are more exactly examined; all
the great differences in the position of the honey, its concealment
from intelligent insects, its protection from rain by means of hairs,
and against unwelcome guests by a sticky secretion, the development
of a corolla-tube which corresponds in length to the length of the
visiting insect's proboscis, the development of spurs on the flower,
in short, all the numerous contrivances which have reference to
cross-fertilization by insects remain quite unintelligible in the light
of this theory--it is a mere _pis aller_ explanation for those who
continue to struggle against accepting the theory of selection.




LECTURE XI

SEXUAL SELECTION

 Decorative colouring of male butterflies and birds--Wallace's
 interpretation--Preponderance of males--Choice of the females--Sense
 by sight in butterflies--Attractive odours--Scent-scales--Fragrance of
 the females--The limits of natural and sexual selection not clearly
 defined--Odours of particular species--Odours of other animals at
 the breeding season--Song of the Cicadas, and of birds--Diversity
 of decoration successively acquired--Humming-birds--Substitution
 of other aids to wooing in place of personal decoration--Smelling
 organs of male insects and crabs--Contrivances for seizing and
 holding the female--Small size of certain males--Weapons of males
 used in struggle for the females--Turban eyes of Ephemerids--Hoods
 that can be inflated on the head of birds--Absence of secondary
 sexual characters in lower animals--Transference of male characters
 to the females--Lycæna--Parrots--Fashion operative in the phyletic
 modifications of colour--Pattern of markings on the upper surface of a
 butterfly's wing simpler than on the under side--Conclusion.


WE found in the process of Natural Selection an explanation of
numerous effective adaptations in plants and animals, as regards form,
colouring, and metabolism, of the most diverse weapons and protective
devices, of the existence of those forms of blossoms which we call
flowers, of instincts, and so on. The origin of the most characteristic
parts of whole orders of insects can only be understood as adaptations
to the environment brought about by means of natural selection.
Impressed by this, we have now to ask whether _all_ the transformations
of organisms may not be referred to adaptation to the continually
changing conditions of life? We shall return to this question later,
but in the meantime we are far from being able to answer it in the
affirmative, for there are undoubtedly a great many characters, at
least in animals, which cannot have owed their origin to natural
selection in the form in which we have studied it so far.

How could the splendid plumage of the humming-birds, of the pheasants,
of the parrots, the wonderful colour-patterns of so many diurnal
butterflies, be referred to the process of natural selection, since all
these characters can have no significance for their possessors in the
struggle for existence? Or of what use in the struggle for existence
could the possession of its gorgeous dress of feathers be to the bird
of Paradise; or of what service is the azure blue iridescence of the
_Morpho_ of Brazil, which makes it conspicuous from a distance when
it plays about the crowns of the palm-trees? We might indeed suppose
that they are warning signs of unpalatableness, like those of the
Heliconiides or of the gaily coloured caterpillars, but, in the first
place, these gay creatures are by no means inedible, and are indeed
much persecuted; and, secondly, the females have quite different and
very much darker and simpler colours. The gleaming splendour of all
these birds of Paradise and humming-birds, as well as that of many
butterflies, is found in the male sex only. The females of the birds
just mentioned are dark in colour and without the sparkling decorative
feathers of the males; they are plain--just like the females of many
butterflies. Alfred Russel Wallace has suggested that the explanation
of this lies in the greater need of the females for protection, since,
as is well known, they usually perform the labours of brooding, and are
thus frequently exposed to the attacks of enemies. It is undoubtedly
true that the dark and inconspicuous colouring of many birds and
butterflies depends on this need for protection, but this does not
explain the brilliant colours of the males of these species. Or can it
be that these require no explanation further than that they are, so to
speak, a chance secondary outcome of the structural relations of the
feathers and wing-scales respectively, which brought with it some other
advantage not known to us? Perhaps something in the same way as the red
colour of the blood in all vertebrates, from fishes upwards, cannot be
useful on the ground that it appears red to us, but because it is the
expression of the chemical constitution of the hæmoglobin, a body which
is indispensable to the metabolism, which here has the secondary and
intrinsically quite unimportant peculiarity of reflecting the red rays
of light.

No one can seriously believe this in regard to butterflies who knows
that their colours are dependent on the scales which thickly cover the
wings, and the significance of which, in part at least, is just to give
this or that colour to the wing. They are degenerate or colourless
among the transparent-winged butterflies, and their colour depends
partly on pigment, partly on fluorescence and interference conditioned
by the fine microscopical structure of a system of intercrossing lines
on faintly coloured scales. The scales of our male 'blue' butterflies
(_Lycæna_) only appear blue because of their structure, while the brown
scales of their mates are due to a brown pigment. If the pigment be
removed from the scales of the female by boiling with caustic potash,
and they be then dried, they do not look blue like those of the male;
the scales of the male, therefore, must possess something which those
of the female do not.

Still less will any one be disposed to regard the marvellous splendour
of the plumage of the male bird of Paradise, with its erectile
collars--glistening like burnished metal--on the neck, breast or
shoulders, with its tufts, with its specially decorative feathers
standing singly out from the rest of the plumage, on head, wings, or
tail, with its mane-like bunch of loose, pendulous feathers on the
belly and on the sides, in short, with its extraordinary, diverse, and
unique equipment of feathers, as a mere unintentional accessory effect
of a feather dress designed for flight and protective warmth. Such
conspicuous, diverse, and unusual specializations of plumage must have
some other significance than that just indicated.

Alfred Russel Wallace regards these distinctive features of the male
as an expression of the greater vigour, and the more active metabolism
of the males, but it is unproved that the vigour of the male birds
is greater than that of the females, and it is not easy to see why
a more active metabolism should be necessary for the production of
strikingly bright colours than for that of a dark or protective colour.
Moreover, there are brilliantly coloured females, both among birds
and butterflies, and in nearly allied species the males may be either
gorgeous or quite plain like the females.

Darwin refers the origin of these secondary sexual characters to
processes of selection quite analogous to those of ordinary natural
selection, only that in this case it is not the maintenance of the
species which is aimed at, but the attainment of reproduction by the
single individual. The males are to some extent obliged to struggle
for the possession of the females, and every little variation which
enables a male to gain possession of a female more readily than his
neighbour has for this reason a greater likelihood of being transmitted
to descendants. Thus, attractive variations which once crop up will be
transmitted to more and more numerous males of the species, and among
these it will always be those possessing the character in question
in the highest degree which will have the best chance of securing a
mate, and so the character will continue to be augmented as long as
variations in this direction appear.

Two kinds of preliminary conditions, however, must be assumed. As the
ordinary natural selection could never have operated but for the fact
that in every generation a great many individuals, indeed the majority
of them, perish before they have had time to reproduce, so the process
of sexual selection could never have come into operation if every
male were able ultimately to secure a mate, no matter what degree of
attractiveness to the latter he possessed. If the numbers of males and
females were equal, so that there was always one female to one male,
there could be no choice exercised either by male or female, for there
would always remain individuals enough of both sexes, so that no male
need remain unmated.

But this is not the case: the proportions of the sexes are very rarely
as 1 : 1; there is usually a preponderating number of males, more
rarely of females. Among birds the males are usually in the majority,
still more so among fishes; and among diurnal butterflies there are
often a hundred males to one female (Bates), although there seem to
be a few tropical Papilionidæ among which the females have rather the
preponderance. Darwin called attention to the fact that one could
infer the greater rarity of the females even from the pricelists of
butterflies issued by the late Dr. Staudinger in connexion with his
business, for the females in most species, except the very common ones,
are priced much higher than the males, often twice as high. In the
whole list of many thousands of species there are only eleven species
of nocturnal Lepidoptera in which the males are dearer than the females.

Among the Mayflies or Ephemerides, too, the males are in the majority;
in many of them there are sixty males to one female: but there are
other kinds of insects, such as the dragon-flies (Libellulidæ), in
which the females are three or four times as numerous. There are also,
it may be remembered, some kinds of insects, such as Aphides, which
have become capable of parthenogenetic reproduction, and in which the
males are becoming extinct, e.g. in the case of _Cerataphis_ in British
orchid-houses.

The first postulate implied in 'sexual selection,' namely, that there
be an unequal number of individuals in the two sexes, is therefore
fulfilled in Nature; we have now to inquire whether the second
condition postulated--the power of choice--may also be regarded as a
reality.

This point has been disputed from many sides, and even by one of the
founders of the whole selection theory, Alfred Russel Wallace. This
naturalist doubts whether a choice is exercised among birds by either
sex in regard to pairing, and maintains that, even if there could be
a choice, this could not have produced such differences in colour and
character of the plumage, since that would presuppose the existence of
similar taste in the females through many generations. In a similar way
it has been doubted whether butterflies can be said to exercise any
real power of sexual choice, whether a more beautiful male is as such
preferred to a less beautiful suitor.

It must be admitted that direct observation of choosing is difficult,
and that as yet there is very little that can be said with certainty
on this point. But there are, after all, some precise observations on
mammals and birds which prove that the female shows active inclination
to, or disinclination for, a particular male. If we hold fast to
this fact, and add to it that the distinctive markings of the males
are wonderfully developed during the period of courtship, and are
displayed before the females, and that they only appear in mammals,
birds, amphibians, and fishes at the time of sexual maturity, it seems
to me that there can be no doubt that they are intended to fascinate
the females, and to induce them to yield themselves to the males. The
opponents of the theory of sexual selection attach too much importance
to isolated cases; they imagine that each female must make a choice
between several males. But the theory of sexual selection does not
demand this, any more than the theory of natural selection requires the
assumption that every individual of a species which is better equipped
for the struggle for existence must necessarily survive and attain
to reproduction, or, conversely, that the less well equipped must
necessarily perish.

All that the theory requires is, that the selective and eliminative
processes do, _on an average_, secure their ends, and in the same way
the theory of sexual selection does not need the assumption that every
female is in a position to exercise a scrupulous choice from among a
troop of males, but only that, on an average, the males more agreeable
to the females are selected, and those less agreeable rejected. If this
is the case, it must result in the male characters most attractive to
the females gaining preponderance, and becoming more and more firmly
established in the species, increasing in intensity, and finally
becoming a stable possession of all the males.

When we go more into details we shall see that the _particular
qualities_ of the distinctive masculine characters are exactly such as
they would be if they owed their existence to processes of selection;
in other words, from this point of view the phenomena of the decorative
sexual characters can be understood up to a certain point. It seems
to me that we are bound to accept the process of sexual selection as
really operative, and instead of throwing doubt upon it, because the
choice of the females can rarely be directly established, we should
rather deduce from the numerous sexual characters of the males, which
have a significance only in relation to courtship, that the females of
the species are sensitive to these distinguishing characters, and are
really capable of exercising a choice.

In my mind at least there remains no doubt that the 'sexual selection'
of Darwin is an important factor in the transformation of species, even
if I only take into consideration those secondary sexual characters
which are related to wooing. We shall see, however, that there are
others in regard to whose origin through processes of selection doubt
is still less legitimate, and from which, on this account, we can argue
back to the courtship characters.

The first beginning of transformation is not, even in ordinary natural
selection, to be understood as due to selection, but is to be regarded
as _a given variation_ (the causes of which we shall discuss later on);
it is only the increase of such incipient variations in a definite
direction that can depend on natural selection, and they _must_ depend
on it in so far as the transformations are purposeful. Now, all
secondary sexual characters can be recognized as useful, save only the
decorative distinctions, although these also undoubtedly represent
intensifications of originally unimportant variations. Are we then
to regard these alone as the mere outcome of the internal impulsive
forces of the organism, while in the case of the analogous sexual
characters for tracking, catching, and holding the female, and so
forth, the augmentation and the directing must be referred to processes
of selection? But if there be any utility at all in the decorative
sexual characters it can only lie in their greater attractiveness to
the females, and it can only be of any account if the females have,
in a certain sense, the power of choice. Independently, therefore, of
direct observations as to the actual occurrence of choosing, we should
be compelled by our chain of reasoning to assume that there was such a
power of choice--and I shall immediately discuss it more precisely.

If we consider the decorative, distinctive characters of the males more
closely, we find that they are of very diverse kinds. The males of many
animals are distinguished from the females chiefly by greater beauty
of form, and especially of colour. This is the case in many birds,
some amphibians, like the water-salamander, many fishes, many insects,
and above all, in diurnal Lepidoptera. Especially among birds the
dimorphism between the sexes is in obvious relation to the excess in
the number of male individuals, or--what practically comes to the same
thing--to polygamy. For when a male attaches to himself four or ten
females the result is the same as if the number of female individuals
were divided by four or by ten. Thus the fowls and pheasants, which
are polygamous, are adorned by magnificent colours in the male sex,
while the monogamous partridges and quails exhibit the same colouring
in both sexes. Of course 'beautiful' is a relative term, and we must
not simply assume that what seems beautiful to us appears so to all
animals; yet when we see that all the male birds which are beautifully
decorative according to our taste--whether humming-birds, pheasants,
birds of Paradise, or rock-cocks (_Rupicola crocea_)--unfold their
'feather-wheels, 'fans,' 'collars,' and so forth, before the eyes of
the females in the breeding season, and display them in all their
brilliance, we must conclude that, in these instances at least, human
taste accords with that of the animals. That birds have sharp vision
and distinguish colours is well known; it is not for nothing that the
service berries and many other berries suitable for birds are red, the
mistletoe berries white, in contrast to the evergreen foliage of this
plant, the juniper berries black so that they stand out amid the snows
of winter; in this direction, then, there is no difficulty in the way
of sexual selection.

Even among much lower animals, like the butterflies, there seems to me
no reason for the assumption that they do not see the gorgeous colours
and often very complicated markings, the bars and eye-spots, on the
wings of their fellows of the same species. Of course if each facet of
the insect eye contributed only a single visual impression, as Johannes
Müller supposed, then even an eye with 12,000 facets would give but a
rough and ill-defined picture of objects more than a few feet away, and
I confess that for a long time I regarded this as an obstacle in the
way of referring the sexual dimorphism of butterflies to processes of
selection. But we now know, through Exner, that this is not the case;
we know that each facet gives a little picture, and not an 'inverted'
but an 'upright' one, and experiment with the excised insect eye has
directly shown that it throws on a photographic plate a tolerably clear
image of even distant objects, such as the frame of a window, a large
letter painted on the window, or even a church tower visible through it.

Furthermore, the structure of the eye allows of incomparably clearer
vision of near objects, for in that case the eyes act like lenses, and
reveal much more minute details than we ourselves are able to make out.
Here again, therefore, there is no obstacle to the Darwinian hypothesis
of a choice on the part of the females, for although it cannot be
demonstrated from the structure of the eye itself that insects see
colour, and that colours have a specially exciting influence on them,
yet we can deduce this with certainty from the phenomena of their life.
The butterflies fly to gaily coloured flowers, and as they find in them
their food, the nectar of the flowers, we may take for granted that the
sight of the colour of their food-providing plants is associated with
an agreeable sensation, and this is an indication that similar colours
in their fellows may awaken similar agreeable sensations.

[Illustration: FIG. 53. Scent-scales of diurnal butterflies. _a_, of
Pieris. _b_, of Argynnis paphia. _c_, of a Satyrid. _d_, of Lycæna. All
highly magnified.]

This conclusion is furthermore confirmed by the fact that, in the male
sex, numerous species of butterfly possess another means of exciting
the females, namely, by pleasant odours. Volatile ethereal oils are
secreted by certain cells of the skin, and exhale into the air through
specially constructed scales. Usually the apparatus for dispersing
fragrance occurs on the wing in the form of the so-called scent-scales
(_Duftschuppen_), peculiar modifications of the ordinary colour-scales
of the wing, but sometimes they take the form of brush-like hair-tufts
on the abdomen, and they are in all cases so arranged that the volatile
perfume from the cells of the skin penetrates into them, and then
evaporates through very thin spots on the surface of the scale, or
through brush-like, expanded fringes on their tips. Many of these have
long been known to entomologists, because their divergence in form
from the ordinary scales attracted attention; and it was also observed
that they never occurred on the females, but only on the males. Their
significance, however, remained obscure until, by a happy chance, Fritz
Müller, in his Brazilian garden, discovered the fact that there are
butterflies which give off fragrance like a flower, and then close
investigation revealed to him the connexion between this delicate
odour and the so-called 'male scales.' One can convince oneself of the
correctness of the observation even in some of our own butterflies by
brushing the finger over the wing of a newly caught male Garden White
(_Pieris napi_). The finger will be found covered with a white dust,
the rubbed-off wing-scales, and it will have a delicate perfume of
lemon or balsam, thus proving that the fragrance adheres to the scales.

[Illustration: FIG. 54. A portion of the upper surface of the wing of
a male 'blue' (_Lycæna menalcas_); after Dr. F. Köhler. _bl_, ordinary
blue scales. _d_, scent-scales. Highly magnified.]

[Illustration: FIG. 55. _Zeuxidia wallacei_, male, showing four tufts
of long, bristle-like, bright yellow scent-scales (_d_) on the upper
surface of the posterior wing.]

In the last case, that is, among the Whites (Pieridæ) (Fig. 53, _a_),
the scent-scales are distributed fairly regularly over the upper
surface of the wing, and the same is true of our blue butterflies, the
Lycænnidæ whose minute lute-shaped scales are shown singly in Fig.
53, _d_, but in their natural position among the ordinary scales in
Fig. 54. In many other diurnal, and also in nocturnal Lepidoptera, the
fragrant scales are united into tufts and localized in definite areas.
They then often form fairly large spots, stripes, or brushes, which
are easily visible to the naked eye. Thus the males of our various
species of grass-butterflies (Satyridæ) have velvet-like black spots
on the anterior wings, while the fritillary, _Argynnis paphia_, has
coal-black stripes on four longitudinal ribs of the anterior wing
which are absent in the females, and which are composed of hundreds of
odoriferous scales. Certain large forest butterflies of South America,
resembling our _Apatura_, bear in the middle of the gorgeous green
shimmering posterior wing a thick expansible brush of long, bright
yellow scent-scales, and a similar arrangement obtains in the beautiful
violet butterfly of the Malay Islands, the _Zeuxidia wallacei_ depicted
in Fig. 55. In many of the Danaides, which we have already considered
in relation to mimicry, the scent apparatus is even more perfect, for
it is sunk in a fairly deep pocket on the posterior wings, and in this
the scent-producing, hair-like scales lie concealed until the butterfly
wishes to allow the fragrance to stream forth. In many South American
and Indian species of _Papilio_ the fragrant hairs are disposed in a
sort of mane on a fold of the edge of the posterior wing, and so on.
The diversity of these arrangements is extreme, and they are widely
distributed among both diurnal and nocturnal Lepidoptera, in the
latter sometimes in the form of a thick, glistening, white felt which
fills a folded-over portion of the edge of the posterior wing. In many
cases the perfume can be retained, and then, by a sudden turning out
of the wing-fold, be allowed to stream forth. But there are a great
many species of butterfly which do not possess odoriferous scales, and
they are often wanting in near relatives of fragrant species; they are
obviously of very late origin, and arose only after the majority of
our modern species were already differentiated. It often seems as if
they bore a compensatory relation to beauty of colour, somewhat in the
same way as many modestly coloured flowers develop a strong perfume,
while, conversely, many magnificently coloured flowers have no scent at
all. Although among butterflies, as among flowers, there are species
which possess both beauty and fragrance, yet our most beautiful diurnal
butterflies, the Vanessas, the Apaturas, and Limenitis, possess no
scent-scales; and many inconspicuous, that is, protectively coloured
nocturnal Lepidoptera, are strongly fragrant, like most night-flowers:
I need only mention the convolvulus hawk-moth (_Sphinx convolvuli_),
whose musk-like odour was known to entomologists long before the
discovery of scent-scales.

It is, however, always only in the males that this odoriferous
apparatus is present. It must not be believed on this account that this
fragrance has the significance of a means of attraction comparable to
the perfume of the flowers which induces butterflies to visit them;
indeed, we cannot assume that the odour carries to a distance, for,
as far as we can make out, it is perceptible only within a very short
radius, and this is indicated also by the manifold arrangements of the
odoriferous organs, which are all calculated to retain the fragrance,
and then--in the immediate neighbourhood of the female--to let it
suddenly stream forth. Obviously, this arrangement can have no other
significance than that of a sexual excitant; its use is to incline the
female to the male, to fascinate her, just as do the beautiful colours,
in regard to which we must draw the same inference. It is in this
direction that the already mentioned relation of compensation between
beautiful colours and pleasant odours is particularly interesting, for
it confirms our interpretation of the decorative colours as a means of
sexual excitement. The most delicately fragrant or the most beautifully
coloured males were those which most excited the females, and thus
most easily attained to reproduction. The expression used by Darwin,
that the females 'choose,' must be taken metaphorically; they do not
exercise a conscious choice, but they follow the male which excites
them most strongly. Thus there arises a process of selection among
these distinctively male characteristics.

If the odoriferous organs we have been discussing had merely been a
means of attraction, serving to announce the proximity of a member of
the species, then they should have occurred, not in the males but in
the females, for these are sought out by the males, not conversely.
The males are able to track their desired mates from great distances,
and many remarkable examples of this are known, some of them indeed
sounding almost fabulous. The females must therefore also exhale a
fragrance, and perhaps continually, but it is much more delicate,
carries extraordinarily far, and is quite imperceptible to our weak
sense of smell. It is possible that it streams out from all the scales
covering the wings and body, for, as I long ago pointed out, all the
scales retain a connexion with the living cells of the skin, however
minute these may be, and it is therefore quite possible that the cells
produce scent imperceptible by us, and let it exhale through the
ordinary scales, since the male scent-scales owe their ethereal oil to
the large gland-like cells of the hypodermis on which they are placed.

Here we see very clearly the difference between ordinary natural
selection and sexual selection. The male odoriferous organs depend on
the latter, for they do not serve for the maintenance of the species,
but are of advantage in the courting competitions among the males for
the possession of the females, while the assumed fragrant cells of the
females must depend on natural selection, since they are of general
importance for the mutual discovery of the sexes, which would otherwise
be in most cases impossible. This hypothetical 'species scent,' as
we may call it, is first of all useful in securing the existence of
the species, and must therefore be referred to natural selection. The
other, the 'male scent,' might be, and actually is, wanting in many
species, although it may be necessary to reproduction in cases where it
has become a male specific character, and could not be absent from any
male without dooming him to sterility.

That the 'species scent' really exists admits of no doubt, although
we may be unable to perceive it. Entomologists have long been in the
habit of catching the males of the rarer Lepidoptera, especially of the
nocturnal forms, by freely exposing a captive female. Some years ago
I kept for some time in my study, with a view to certain experiments,
females of the eyed hawk-moth (_Smerinthus ocellatus_), and placed them
at first, without any special intention, in a gauze-covered vessel near
the open window. The very next morning several males had gathered and
were sitting on the window-sill, or on the wall of the room close to
the vessel, and by continuing the experiment I caught, in the course
of nine nights, no fewer than forty-two males of this species, which
I had never believed to be so numerous in the gardens of the town. The
males of the nocturnal Lepidoptera obviously possess an incredibly
delicate organ of smell, and its bearers, the antennæ, are usually
larger and more complex in structure in the male sex than in the female.

Butterflies are by no means the only creatures that produce a peculiar
odour at the breeding season; many other animals do the same, though
in their case it does not seem so pleasant to our sense of smell.
It is true that the scent of the musk-deer and that of the beaver
(_Castoreum_), when much diluted, are agreeable to man, but others,
like the odours exhaled by stags or by beasts of prey, are very
disagreeable to us, though they have for the species that produce them
the same significance as the others, and are therefore to be referred
to sexual selection.

Darwin referred all the different _mechanisms for the production of
sounds_, up to the song of birds, to sexual selection, but it is
probable that natural selection has also to do with this in many ways.
It is certainly only the males which produce the well-known song of the
Cicadas, crickets, grasshoppers and birds, and I do not see any reason
to doubt that this 'music' affects the females by arousing sexual
excitement. To some extent, then, the rivalry among the males for the
possession of the females--that is to say, sexual selection--must have
produced these mechanisms of song; and how long-continued and gradual
the accumulations must have been which produced the song of the thrush
or of the nightingale from the chirping of the sparrow we may learn
from the innumerable species which, as regards beauty of song, may be
ranged between these two extremes.

My assumption that natural selection has also been operative in the
case of the song of insects and birds is based on the fact that many
of our songsters live widely scattered, and that the characteristic
note must be a means by which the two sexes find each other. That
they should find each other is an indispensable condition for the
maintenance of the species. Thus it is well known that each species has
a characteristic 'note' or love-call, which the male utters during the
breeding season, and which is answered by the female. From this simple
love-call the modern song of many species must have developed by means
of sexual selection.

It is remarkable that here again the various distinguishing characters
of the male seem to be often mutually restrictive or mutually
exclusive. The best singers among our birds are inconspicuously
coloured, grey or brown-grey, and this can hardly be regarded as due
to chance, but as the outcome of a greater sensitiveness on the part
of the females either to the song or to the beauty of their mates. And
since, according to the theory, only those characters of the males
could be increased which decided the choice, it therefore seems to
me that this mutual exclusiveness of the two kinds of distinguishing
characters is another indication of the reality of sexual selection.
It proves--so at least I am inclined to believe--that the excitement
of the female has been essentially affected by _only one_ of the
characters of the male, that in the bird of Paradise it was mainly
the brilliance of the plumage which roused excitement, while in the
nightingale it was mainly the song.

It might be objected to this that there are brilliant butterflies which
also possess scent-scales. This is really the case; thus a magnificent
blue iridescent _Apatura_ from Brazil has on the posterior wings a
large yellow brush of scent-hairs, and even the beautiful blue males of
our Lycænids have scent-scales in addition to their beautiful colour.
But this can hardly be considered as a contradiction, but is rather
an exception, which is the easier to explain since the odoriferous
apparatus is a relatively simple arrangement, which did not require
such a long series of generations for its evolution as the complicated
song-box and brain-mechanism of the singing-birds.

Moreover, it may also be that the scent-scales have arisen later
than the decorative colouring, and they would do so the more easily
since the brilliant blue, when once it was perfectly developed, and
was common to all the males of the species in an equal degree, was
no longer distinctive, and would have no specially exciting effect,
while a novel preferential character in the male would have a much
stronger effect. In the same way, the different parts of the body would
be furnished in succession with decorative and, therefore, exciting
distinctive characters. To understand this effect on the opposite
sex we need only think of analogous phenomena in human kind, and of
the strongly exciting effect that the sight of the secondary sexual
characters of the woman has upon the man.

By the successive additions of new decorative characters after the
older ones became general and reached a climax, the origin of the
extraordinary diversity of the decorative plumage in one and the same
species of bird, can be readily understood, and the same is true of
the complicated decorative coloration of the butterflies in so far as
it depends on sexual selection, and not on other factors. The details
did not arise all at once, but one after the other, and every character
went on increasing till it had reached its limit of increase, but
whenever it was common in its highest development to all the males
it was no longer an object of preference or the cause of specially
violent excitement, so that a new process of selection would begin in
reference to some other part of the body. We thus understand how, among
male birds of Paradise and humming-birds, such a marvellous diversity
of colours and of decorative feathers is found combined in one and the
same species.

Whoever has seen the Gould Collection of humming-birds in London must
have observed with amazement that among the 130 or so species of these
beautiful little birds nearly every group of feathers in the body has
been affected by the decorative colouring. In one species the little
feathers on the region of the throat are emerald green, metallic blue,
or rose; in another the feathers of the neck have been transformed into
an erectile collar of rose-coloured feathers with a metallic sheen; or,
again, it is the little feathers round the ear that stand erect and are
brilliantly coloured. Sometimes we find that the feathers of the tail
are lengthened, it may be only two of them, or the various lengths may
be graduated like steps; sometimes the tail has assumed the form of a
wedge, or is fan-like, or is shaped like the tail of a swallow, and all
this in combination with the most diverse colours and patterns, black
and white, ultramarine blue, and so forth. Or it may be the outermost
tail-feathers which are the longest, the inner ones the shortest, or
the four outer feathers are broad, pointed, directed outwards, and only
half as long as the other two, which are very long and straight. Some
species exhibit a sort of fine swan's down on the legs, others have a
gorgeous metallic red cap on the head--in short, the variety is beyond
description, just as we should expect it to be if now this and now that
chance variation attracted the favourable regard of the selecting sex,
and thus attained to its highest pitch of development.

The decorative colouring of male birds may be replaced, not only by
the power of song, but in other ways also. Not all the male birds
of Paradise possess the familiar feather ornaments. The Italian
traveller Beccari has called attention to a species, the males of
which are simply coloured brown, like the females of other species.
This _Amblyornis inornata_ entices its mate to itself in the pairing
time in a very peculiar manner, for it arranges in the midst of the
primitive forests of New Guinea a little 'love garden' or bower, a spot
several feet in extent, strewn with white sand, on which it places
shining stones and shells, and brightly coloured berries. In this case
a special instinct has developed, which has replaced the personal
charm of the bird in the eyes of the female. For this very reason the
case seems to me to have some theoretical importance, for it serves
indirectly to show that the personal excellences do actually function
as a means of exciting and attracting, if any one should still doubt it.

All the distinguishing characters of the male which we have hitherto
considered have had reference to gaining the favour of the female, but
there are many other secondary sexual characters which are employed in
quite a different manner to secure possession of the female. I have
already mentioned that in many butterflies the males possess a much
larger organ of smell. The antennæ of the males of numerous beetles,
such as the cockchafer and its relatives, are also much larger, and
furnished with much broader accessory branches, than those of the
female, and the same is the case in many of the lower crustaceans, like
the large transparent Daphnid of our lakes, _Leptodora hyalina_. Here
the anterior antenna bears (Fig. 56, _A_ and _B_, _at´_) olfactory
filaments; in the female this appendage is small and stump-like, while
in the male (_A_) it grows to a long, somewhat curved rod, which
is extended obliquely into the water, and in addition to the nine
olfactory filaments of the female (_ri_) bears from sixty to ninety
more (_ri´_).

[Illustration: FIG. 56. _Leptodora hyalina._ _A_, head of the male.
_B_, head of the female. _Au_, eye. _g. opt_, optic ganglion. _gh_,
brain. _at´_, first antenna with olfactory filaments _ri_ and _ri´_.
_sr_, œsophageal nerve-ring. _n_, nerve. _m_, muscles.]

In this and many other such cases it is not the struggle of the
species for existence which has so markedly augmented this distinctive
characteristic of the male; it is undoubtedly the struggle of the males
among themselves, their competition for the possession of the females.
In regard to decorative distinctions, the reality of a rivalry in
wooing and the ultimate victory of the most decorative may perhaps be
still doubted; but it is quite certain that, on an average, the male
which can smell and track best will also gain possession of the females
more easily than one less well equipped. Exactly the same is also true
of those cases in which the male distinguishing character does not
refer merely to finding the female, but to holding her fast, or, as we
may say, to capturing her.

Thus the males of the Copepods possess on their anterior antennæ an
arrangement which enables them to throw a long whiplike structure
like a lasso round the head of the female as she rapidly swims away.
The antennæ of the male Daphnids, too, are in one genus (_Moina_)
developed into a grasping apparatus, instead of into smelling organs as
in _Leptodora_. Fig. 57 shows the male, Fig. 58 the female of _Moina
paradoxa_; the first antennæ of the male are not only much longer and
stronger than those of the female (_at_^1), but they are also armed
with claws at the end, so that the males can catch their mates as with
a fork, and hold them fast. And even that was not enough, for, in
addition, the males of most Daphnids possess a large sickle-shaped but
blunt claw on the first pair of legs (Fig. 57, _fkr_), which enables
them to cling to the smooth shell of the female, and to clamber up on
it to get into the proper position for copulation.

[Illustration: FIG. 57. _Moina paradoxa_, male. _at^1_, first antennæ,
with claws at the tip for capturing the female. _at^2_, second antennæ.
_fkr_, claws on the first pair of legs for clambering. _gh_, brain.
_lbr_, upper lip. _md_, mandible. _md_, mid-gut, with the liver lobes
(_lh_). _h_, heart. _sp_, testis. _aft_, anus. _sb_, caudal setæ.
_skr_, caudal claws. _sch_, shell. _schr_, cavity of the shell. _kie_,
gill-plates. Magnified 100 times.]

If we inquire into the manner of the origin of secondary sexual
characters of this kind, we shall find that both may have been
increased by sexual selection, for a male with a better sickle will
succeed more quickly in getting into the proper position for copulation
than one with a less perfect mechanism. This assumption does not rest
on mere theory, for I was once able, by a happy chance, to observe for
a considerable time, under the microscope, a female to whose shell two
males were clinging, each trying to push the other off. Nevertheless it
seems to me very questionable whether the origin of this sickle-claw
can be referred to sexual selection, for without this clamping-organ
copulation in most Daphnids would not be possible. It was thus not as
an advantage which one male had over another that the clamping-sickle
evolved, but rather as a necessary acquisition of the whole family,
which must have developed in all the species at the same time as the
other peculiarities, and notably those of the shell. The competition of
the males among themselves is thus in this case simply an expression
of the struggle for existence on the part of the species as such, and
it is not a question merely of a character which makes it easier for
the males to gain possession of the females, but of one which had
necessarily to arise lest the species should become extinct. In other
words, in this case natural selection and sexual selection coincide.

[Illustration: FIG. 58. _Moina paradoxa_, female. The letters of Fig.
57 apply _mutatis mutandis_. _brr_, brood-pouch. _ov_, ovary. _sr_,
margin of shell.]

The case of the antennæ of _Moina_, which have been modified into
grasping organs, is quite different; these owe their origin not to
natural selection, but to sexual selection, for antennæ of that kind
are not indispensable to the existence of the species, as we can
see from the closely related genera, _Daphnia_ and _Simocephalus_,
where the males have quite short stump-like antennæ, furnished with
olfactory filaments not much more numerous than the females possess.
Just as these supernumerary olfactory filaments were produced by
sexual selection, and not by the ordinary natural selection, because
those males with the more acute sense of smell had an advantage over
those in which it was blunter, so the males of the genus _Moina_
which could grasp most securely had an advantage over those that
gripped less firmly, and thus arose these two different kinds of male
characteristics. Neither of them is of advantage to the species as
such, but only to the males in their competition for the possession of
the females.

But, where the production of a novel character in the male is
concerned, natural selection cannot proceed in a different manner
from sexual selection; the process of selection is exactly the same:
the better equipped males survive, the less well-equipped die without
begetting offspring; the difference lies only in the fact that in the
one case the improvement is in the species as such, in the other case
only in one sex without the existence of the species being thereby
made more secure. Such cases are instructive, because they make a
denial of the process of sexual selection quite impossible if that of
species-selection is admitted. If processes of selection are operative
at all as factors in transformation, they must act even where the
advantage is not to the species but only 'intra-sexual,' and the one
process must often run into the other, so that it is often quite
impossible to draw an exact line of demarcation between them.

Numerous secondary sexual differences probably depend purely on species
selection, that is to say, they include an improvement of the species
in relation to the struggle for existence. We may find a case in point
in the dwarf-like smallness of the males in many parasitic crustaceans,
in some worms, in many Rotifers, and in the Cirripedes. It can hardly
have been of advantage for the individual male to be smaller than
his fellows, but it was of advantage for the species to produce as
many males as possible in order to ensure a meeting with the females,
and, as the enormous production of males made it advantageous for the
species that as little material as possible should be used in their
individual production, we can readily understand the minuteness of the
males, and in some cases, as in the Rotifers and _Bonellia_, their
poor equipment, lack of nutritive organs, and ephemeral existence. The
marine worm, _Bonellia viridis_, whose female may be a foot in length,
is not the only case in which a microscopically small male lives like
a parasite inside the female. Among the round-worms, too, there is a
species called _Trichosomum crassicauda_, discovered by Leuckart in
the rat, the dwarf males of which live in the reproductive organs of
the female. All these are arrangements for securing the propagation of
the species, which might have been endangered if the males had had to
seek out the females, which, in the case of _Bonellia_, live in holes
in the rocks on the sea-floor, and, in the case of _Trichosomum_, are
concealed in the urinary bladder of the rat. Obviously, this is the
reason which, in addition to the one already mentioned, has conditioned
and produced, or helped to produce, the remarkable minuteness of
certain males.

From another category of sexual differences we see in how many ways
species-selection and sexual selection play into each other's hands.
In many species of animals the males are eager for combat, and they
are equipped with special weapons, or excel the females in general
strength of body. As these males struggle, in the literal sense of the
word, for the possession of the females, Darwin referred to sexual
selection those distinguishing characters which gave the stronger male
the victory over the weaker, and thus raised the victorious characters
to the rank of general characters of the species. And it certainly
cannot be doubted that, for instance, the strength and the antlers of
the stag must have been increased through the combats which recurred
every year at the breeding season, for the stronger always win in these
battles. The case is the same with the strength and the weapons of many
other male animals. The lion is effectively protected by his mane from
the bite of a rival, and the same protective arrangement occurs in
quite a different family of mammals--in an eared seal, which is called
the 'sea-lion' for this very reason. Among the seals the secondary
sexual characters are often very strongly developed, at least in all
the polygamous species, for in these the struggle for the females is
very keen. In the 'sea-lions' and 'sea-elephants' there are often fifty
females to one male, and the latter are 'enormously larger' than the
females, while in monogamous species of seal the two sexes are alike in
size.

Darwin has shown that actual combat for the females takes place among
most mammals, not only among stags, lions, and seals, but even among
the moles and the timid hares. Even among birds such combats occur, and
this is sometimes particularly noteworthy in those species in which the
males possess the most decorative colouring, like the humming-birds. In
some cases among birds there has also been a development of weapons.
Witness the spur of the cock, whose merciless combats with his rivals
Man has, as is well known, made positively atrocious for his own
amusement, by preventing the flight of the vanquished.

In Darwin's great work on sexual selection a considerable number of
cases are cited from among lower vertebrates, such as crocodiles
and fishes, and even from insects, in which the males fight for
the possession of the females, and exhibit distinctive masculine
characters adapted to such combats. But I do not propose to enter upon
a discussion of such cases, since my aim is rather to elucidate the
relation between sexual selection and species-selection than to discuss
all the phenomena of the former in detail. But the combats of males
illustrate with particular clearness the relation of sexual selection
and species-selection, since many of the weapons or protective
arrangements which may have arisen through sexual selection imply at
the same time an improvement to the species in relation to the struggle
for existence. Thus greater strength or sharper and larger teeth in the
males mean a gain to the species, and it is indifferent to the species
whether the weaker males succumb to a strange enemy (species-selection)
or to their stronger rivals (sexual selection), provided only that the
better equipped survive and leave descendants similarly endowed.

I have intentionally begun the consideration of sexual selection with
the cases most difficult to interpret on this theory, with those which
have called forth the greatest divergence of opinion--the decorative
colours and forms, the song of birds and of insects, the alluring
odours--in short, all the courtship-adaptations of the males; these are
the most difficult to deal with, because it is not easy to demonstrate
directly that the females _do_ choose. But if we revise them briefly
in reverse order, I believe that all doubt as to the reality of choice
on the part of the females will disappear. Thus the last-mentioned
sexual characters of greater strength and greater perfection of weapons
and defence in the males have been evolved by sexual selection in
close co-operation with species-selection. We should have to deny
species-selection altogether if we were to dispute this form of sexual
selection, which is closely connected with pure species-selection,
such, for instance, as is revealed in the production of dwarf males,
where there does not seem to be any aid from sexual selection at all.

Then came the cases in which the tracking and grasping organs of the
males were strengthened or were increased in number, and here too
species-selection may have had its share, for instance, in evolving
the sickle-claws of the Daphnids, which were inevitably advanced and
perfected through sexual selection, which must in this case have
operated independently of any choice on the part of the female. In
other cases the result may be referred to pure sexual selection,
as in the grasping antennæ of the male _Moina_, or in the highly
developed olfactory antennæ of the male _Leptodora_. That new organs,
too, can arise in this way is shown by the 'turban eyes'--to which
little attention has hitherto been paid--of some Ephemerids of the
genera _Cloë_ and _Potamanthus_, which were long ago described by
Pictet, the monographer of this family. These are large turban-shaped
compound eyes, occurring beside the ordinary eyes in the males alone,
which in these genera are in a majority of sixty to one. Whole swarms
of these males fly about over the water on the search for females,
and their highly developed organ of vision seems to decide matters
for them just as the organ of smell does for _Leptodora_. Neither of
these sense-organs can have any other advantage than that of making
their possessors aware of the female, for the whole activity of the
short-lived adult Ephemerides is limited to reproduction; they take no
food, and have nothing whatever to do except to reproduce.

Finally, when in an enormous number of cases we find in addition to one
or the other of the already mentioned male distinguishing characters
some which do not directly lead to gaining possession of the female,
but do so only by sexually exciting her, can we doubt that the same
principle has been operative, that here too processes of selection
are fundamental, depending on the fact that in the wooing of the
female the successful male is the one who most strongly excites her?
There is no question of æsthetic pleasure in this, as the opponents
of the theory of sexual selection have often urged, but only of
sexual excitement, which may be aroused by very different means, by
colours and shapes, but also by love-calls, songs, or odours. There
are a few tropical birds (_Chasmorhynchus_) which have as the only
distinguishing character of the male sex a hollow and soft appendage
several inches long borne on the head. Usually it hangs down limply at
the side of the head, but during the breeding season it is inflated
from the mouth-cavity, and then stands erect like a spur. One species
of this genus has as many as three of these horns, one of which is
upright, while the other two stand out laterally from the head. Can it
be supposed that these remarkable horns satisfy the female's 'sense
of beauty'? To human beings they appear rather ugly than beautiful,
both when limp and when inflated, but at any rate they are striking,
and will be regarded by the female bird as something out of the
common, and, since they are only fully displayed during the breeding
season, that is, when the male is sexually excited, they will have an
exciting effect on the female too. These inflated horns are symptoms
of excitement, and they arouse it in the female. In exactly the same
way the decorative feathers, the ruby-red and emerald-green feather
collars of the humming-birds and birds of Paradise, are only erected
and displayed when the males are wooing, and they, too, act as signs
of excitement. This is not to say that the gorgeousness of colour,
the eye-spots on the train of the peacock and the Argus pheasant, and
the hundreds of different kinds of beautiful feathers, do not also
exercise a fascinating influence; on the contrary, we cannot avoid
assuming this, since otherwise we could find no sufficient reason for
their origin. But the primary effect in wooing is not due to the mere
pleasure in the sight, or in the odour, or in the song, but to the
contagious excitement which these express. The females do not behave as
dispassionate judges, but as excitable persons which fall to the lot of
the male who is able to excite them most strongly. It may be, however,
that a sense of æsthetic satisfaction in perceiving such symptoms of
excitement may also have been evolved as an accessory effect, at least
in the higher and more intelligent animals.

In the lower animals, which are lacking not only in intelligence but
also in the higher and more complex differentiation of the sensory
system, the development of such secondary sex characters is rare or
altogether absent. Animals which have no sense of hearing can develop
no song, and animals which do not see cannot acquire gorgeous colours
as a means of exciting one sex through the other. But distinctive sex
coloration may arise even in lowly animals, though there can be no
question of æsthetic pleasure associated therewith; if the animals are
able to see the colours at all, sexual excitement may be associated
with these.

We need not wonder, therefore, that in the somewhat stupid fishes, in
the butterflies, and in the lower crustaceans, like the Daphnids, we
still find brilliant colours, which we can hardly interpret otherwise
than as the results of sexual selection. On the other hand, the absence
of such characters in animals of a still lower order, with still
simpler sense-organs, like the Polyps, Medusæ, Echinoderms, most Worms,
and the Sponges, affords an indirect confirmation of the correctness
of our view as to the reality of a sexual selection in the more highly
organized animals.

We see, then, that numerous peculiarities which distinguish the
males of a species from the females depend on the process of sexual
selection. This may be said of ornamental outgrowths, colours,
remarkable feathers and feather-groups, peculiar odoriferous organs,
vocal organs, artistic instincts, and also weapons, like antlers,
tusks, and spurs, notable size and strength of body, and protective
devices like manes; and again, the various organs for catching and
holding the females, or for finding them out by sight or smell, must
also be referred, at least in part, to sexual selection. The diversity
of the male sexual characters is so great that I cannot give more than
a faint idea of them without entering on a long catalogue; whoever
wishes a complete survey has only to consult Darwin's _Descent of Man_.

But the significance of sexual selection is by no means exhausted with
the production of the male sexual characters, for these characters are
often more or less completely transferred to the females, and thus give
rise to a transformation of the whole species, and not only of the male
section of it. This is obviously a very important consequence of sexual
selection, one which, as we shall see, materially deepens our insight
into the mode of origin of new species.

First let us try to determine the facts. Many male characters are not
represented in the female in any degree, and therefore have never
been transmitted to them at all. Such are the mane of the lion, the
grasping antennæ of _Moina_, the turban eyes of the Ephemerides, the
intensification of the sense of smell in _Leptodora_, the lasso-like
antennæ of the Copepods, the scent-scales of the butterflies, and the
musk glands of the alligators and stags. But in other cases there has
been transmission, though only to a slight extent. Thus many female
humming-birds have a faint indication of the magnificent metallic
colouring of the males; many female blue butterflies have a tinge of
the beautiful blue of their mates; the females of the stag-beetle
(_Lucanus cervus_) possess a diminutive suggestion of the antler-like
jaws of the male, and the female crickets, although they do not chirp,
have a slight indication of the 'musical' mechanism of the male on the
wing-coverts, and some of them even produce feeble notes at certain
times.

It can be proved, however, that such transmissions may, in the
course of many successive generations, become intensified until the
characters are exhibited by the females in the same degree as in the
males. I know no better example of this than that afforded by the
beautiful butterflies of the genus _Lycæna_. In this genus, which is
rich in species and widely distributed over the whole earth, and must
therefore be an old one, the upper surface of the wing is blue in by
far the greater number of species, at least in the male sex. But there
are three or four species which are dark-brown, and quite or nearly
alike in the two sexes; such are the species _Lycæna agestis_, _L.
eumedon_, _L. admetus_, and others. Everything indicates that this is
the primitive colour of the genus. Moreover, there are some species
with brown females, in which the males are not completely blue, but
which have a slight bluish tinge, like _L. alsus_, the smallest of
our indigenous Blues. Then follows a host of beautiful species, like
_L. alexis_, _L. adonis_, _L. damon_, _L. corydon_, and many others,
with brown females, and among these there occasionally occur females
more or less tinged with blue. These lead on to _L. meleager_, which
has two forms of female, a common brown and a rarer blue; and thus
we reach _L. tiresias_, _L. optilete_, and _L. argiolus_, in which
all the females are blue, although less intensely and completely so
than their mates. The climax of this evolutionary series is reached
by some species like _L. beatica_, belonging to tropical or at least
warm countries, in which both sexes are of an equally intense blue. As
we know that, in species with an excess of males, sexual characters
always begin in the males, there can be no doubt as to the direction
of evolution--from brown to blue--in this series. Furthermore, the
entire absence of scent-scales in most of the species with brown males
indicates the great age of these species, for, as far as I have been
able to investigate, all the males of the blue species possess them.

Darwin regarded this transferring of the male characters to the females
as due to inheritance, and it really seems as if it were simply a case
of transmission by inheritance to one sex of what has been acquired
by the other. Yet we have to ask whether we can continue to regard
the facts in this light. In any case this 'transmission' is not an
inevitable physiological process, necessarily resulting from the
intrinsic conditions of inheritance, for we see that it often does
not occur, even in many cases in which we can see no external reasons
why it should not do so, though in other cases the failure may be
presumably correlated with the external conditions of life. Thus, for
instance, the persistent retention of the brown colour in the majority
of our female Lycænidæ has probably its reason in the greater need
of protection on the part of the much rarer females, and this must
be so also in the case of many birds in which the brilliant colours
of the males have not been transferred to the females. Wallace first
pointed out that all birds whose females brood in exposed nests are
inconspicuously coloured in the female sex, even if the males are
brightly coloured, while those whose nests are concealed in holes of
trees or the like, or which build domes over them, not rarely exhibit
brilliant colouring in both sexes. This is the case in woodpeckers and
parrots, while the gallinaceous birds, which brood in the open, have
usually inconspicuously coloured females, for the most part very well
adapted to their surroundings.

If we grasp the fact that a transference of the characters which have
arisen through sexual selection can take place, we have a valuable
aid in the interpretation of many phenomena which would otherwise
remain quite inexplicable. What is the meaning of the gay colours of
the parrots, which occur in such incredibly diverse combinations
in this large and widely distributed family? Or of the marvellously
complex markings and colour-patterns of the butterflies? In some cases
they may be protective, as is the green of many parrots; in others,
warning signs of unpalatability, like the bright colours and contrasted
markings of many Heliconiidæ and Eusemiidæ and other butterflies with
a nauseous taste; but there remain a great many cases to which neither
of these explanations applies, which could only be regarded as pure
freaks of nature if we did not know that male sexual characters can
be transferred to the females, and that thus all the individuals of a
species can be totally altered in their colouring.

Thus the occurrence not only of conspicuous, but of complicated,
coloration is explained.

Darwin has shown that, in the equipment developed by the males in their
competition for the possession of the females, it is by no means only
those characters which may be considered 'beautiful' in themselves that
have to be considered; it is rather the striking characteristics which
mark their possessor and distinguish it from others that are primarily
important. In fact, it is the principle of 'mode' or 'fashion' which is
operative; something new is demanded, and as far as possible something
quite different from that which was previously considered beautiful.
Thus the starting-point for such processes of selection may have been
afforded by white spots on a black ground, or, indeed, by any light
spots on a dark ground, which may have been the primitive colour in
most cases. If in the course of a long series of generations these
spots became the common property of all the males, a possibility of
further change was opened up as soon as a new contrast cropped up as
a chance variation, which would then, under favourable conditions, be
the starting-point of a new process of selection. Darwin has cited
some cases in which, from a comparison of the dress of the young bird
with that of the adult, we may conclude that a transformation of the
colouring of the whole plumage must have taken place in the course of
the phylogenetic history.

In other cases the course of the process of selection has been such
that, though the general colouring has not been changed, variations
have appeared in particular regions of the body--spots or stripes which
accumulated through the ages and co-operated to form an increasingly
diverse and complex colour-scheme, such a 'marking' of the animal as we
may observe to-day, especially in butterflies, but also in birds.

It is a fine corroboration of the origin of bright colours through
sexual selection that, even in those groups of the animal kingdom which
are in general sexually monomorphic, there always occur some species
in which male and female are quite different, and a host of species in
which both sexes are alike in the main, yet with differences in certain
minor points. Among the parrots similarity of colouring prevails as
a general rule, but in New Guinea there lives a parrot the female of
which is a gorgeous blood-red and the male a beautiful light-green;
minor differences occur in many species, for instance, the female
of the horned parrot (_Cyanorhamphus cornutus_ Gm.) lacks the two
long black and red feathers on the head, that of the grass-parakeet
(_Melopsittacus undulatus_) is a slightly paler green and has not
the beautiful blue spots on the cheeks which the male possesses.
Innumerable similar instances might be cited, serving to show that all
these distinguishing characters of the males have been acquired step by
step and piece by piece, and are slowly and independently transferred
to the females--if, indeed, at all.

In yet another way the correctness of the Darwinian theory of sexual
selection may be deduced from the markings and coloration of birds and
butterflies.

It has frequently struck me, during the long period in which I have
been studying brightly coloured birds and butterflies, that those
colour-patterns which are referable to sexual selection are much
simpler than those which must be referred to species-selection,
especially in the case of what we call 'sympathetic coloration.' How
crude is the decorative pattern of most parrots, notwithstanding all
the brilliance of their colour. Large tracts of the body are red,
others green, yellow, blue, and occasionally one finds a red and
blue striped feather collar, a head which is red above and yellow
underneath, but it is seldom that the colours vary enough in a small
space to give rise to a delicate decorative pattern. The gayest of
parrots are the Brush Tongues (_Trichoglossus_), and even among them
subtlety of coloration does not go further than the combination of
three colours on one of the long tail-feathers, or the production of
a double band round the neck, and so forth. If we compare with this
the complex markings of the inconspicuously coloured females of the
pheasants, of the partridges, or that of the upper surface of the
many birds in mingled grey, blackish-brown and white, which resemble
the ground or the dried leaves when they crouch, we find that the
colour-pattern in these cases is infinitely finer and more complex.

This seems to me quite intelligible when we remember, on the one hand,
that species-selection must operate far more intensively than sexual
selection, and that in the production of a protective colouring it is
a question of deceiving the eye of a sharp-sighted enemy, while the
aim of sexual selection is to secure the approval of others of the
same species. As long as the enemy on the search for prey perceives
the difference between the markings of its victim and those of the
surroundings, so long will the gradual and steady improvement of the
protective coloration continue, so long will new shades and new lines
be added. We can thus understand how there would be gradually reached
a complexity of marking to which sexual selection can never attain,
or at least only in regard to a few specially favourable points.
The eye-spots on the train feathers of the Argus pheasant and the
peacock are such points, and these occur among polygamous birds in
which sexual selection must be very intense; they are placed, too,
on a part of the body, the wheel-shaped train, which is peculiarly
suited for communicating the excitement of the male to the female, and
must therefore be especially influenced by the latter. In general,
however, we may say on _a priori_ grounds that the intensity of
species-selection is greater than that of sexual selection, because the
former ceaselessly and pitilessly eliminates the less perfect, while
the claims of the latter are in any case less imperative, and are also
often mollified by a variety of chance circumstances.

But in the case of insects, in particular, we have to add that the
protective colours and the decorative colours have been, so to speak,
painted by different artists--the former by birds, lizards, and other
persecutors endowed with well-developed eyes, the latter by the insects
themselves, whose eyes can hardly possess, for objects not quite near,
that acuteness of vision which the bird's eye has. Thus we find that
the protective coloration of butterflies has often a very complex
marking, while the same butterfly may exhibit only a rather crude
though brilliant pattern on its upper surface, where the coloration is
due to sexual selection. Thus the famous _Kallima_ has on its under
surface the likeness of a dry or decayed leaf composed of a number
of colour-tones--quite a complex painting. But if we look at the
upper surface we see a deep brown with a shimmer of steel blue as the
ground-colour of the wings, and on it a broad yellow band and a white
spot: that is the whole pattern. We find a similar state of things
among many of the forest butterflies of Brazil, and also among our
indigenous butterflies. The pattern of our gayest diurnal butterflies,
the red Admiral and the tortoiseshell butterfly (_Vanessa atalanta_ and
_Vanessa cardui_), is somewhat crude on the upper surface, and very
simple compared with the protective colouring of the under surface,
which is made up of hundreds of points, spots, strokes, and lines of
every shape and colour. On the other hand, the upper surface of the
anterior wings in the hawk-moths and the Noctuidæ exhibits protective
coloration, and is made up of curious zigzag complex lines, strokes,
and spots, so that it resembles the bark of a tree or a bit of an old
wooden fence--a painting, like the modern impressionist work, which,
with an apparently meaningless confusion of colour splashes, conveys a
perfect impression even of the details of a landscape. In the owl-moths
(Noctuidæ) the wing surfaces, which are brightly coloured, are simple,
almost crude, in pattern, as in the moths of the genus _Catocala_, with
their red, blue, or yellow posterior wings, traversed by a large black
band; while in the Geometer-moths, whose wings are spread out flat when
at rest, the protective upper surface of all four wings is covered
with a complex pattern of lines, spots, and streaks in different
shades of grey, yellow, white, and black, so that it bears a deceptive
resemblance to the bark of a tree or the side of a wall. For a long
time I could not understand how such a definite and constant pattern
could arise through natural selection if it was a case of mimicking the
impression of bark or of any other irregularly covered surface, the
colours of which are not mingled in exactly the same way everywhere.
But now I think I understand it; for in the apparently meaningless
colour-splashes of an 'impressionist' landscape the different splashes
must be exactly where they are, otherwise on stepping back from the
picture one would see, not a Haarlem hyacinth-field, or an avenue of
poplars with their golden autumn leaves, but a mere unintelligible
daub. It is the _type_ of the colour-pattern that must be attained,
and in nature this is attained very slowly, step by step, spot after
spot, and therefore, obviously, no correct stroke once attained will
be given up again, since, in combination with the rest, it secures
the proper type of colour-pattern. Only thus, it seems to me, can we
understand how apparently meaningless lines, like the figures 1840 on
the under surface of _Vanessa atalanta_, could have become a constant
characteristic of the species.

To sum up briefly, we may say that sexual selection is a much more
powerful factor in transformation than we should at first be inclined
to believe. It cannot, of course, have been operative in the case of
plants, nor can it be taken into consideration in regard to the lower
animals, for these, like the plants, do not pair, or, at any rate,
do so without any possibility of choice. Animals which live on the
sea-floor, or which are attached there, must simply liberate their
reproductive cells into the water, and cannot secure that they unite
with those of this or that individual. This is the case among sponges,
corals, and Hydroid polyps. In some other classes the sense organs are
too poorly developed, and the eyes in particular too imperfect to be
excited in different degrees by any peculiarities in the appearance
or behaviour of the males. This is what Darwin meant when he ascribed
to these animals 'too imperfect senses and much too low intelligence'
'to estimate the beauty or other attractive points of the opposite
sex, or to feel anything like rivalry.' Accordingly, in the Protozoa,
Echinoderms, Medusæ, and Ctenophores, secondary sexual characters are
entirely absent, as pairing also is.

In those worms that pair we first meet with secondary sexual
characters, and from this level upwards they are never quite absent
from any large group, and gradually play an increasingly important rôle.

But the significance of sexual selection lies, as we have seen,
not only in the fact that one sex of a species, usually the male,
is modified, but in the possibility of the transference of this
modification to the females, and further, in the fact that the process
of variation may start afresh at any time, and thus one variation may
be developed upon or alongside of another. In this way we can explain
certain complex and often fantastic forms and colourings which we could
not otherwise understand; thus the extraordinary number of nearly
related species in some animal groups, such as butterflies and birds,
in which the differences mainly concern the colour-patterns.

Darwin has shown convincingly that a surprising number of characters
in animals, from worms upwards, have their roots in sexual selection,
and has pointed out the probability that this process has played an
important part in the evolution of the human race also, though, in this
case, all is not yet so clearly and certainly known as among animals.

To conclude this section, I should like once again to call attention
to the deficiency which is necessarily involved in the assumption
of any selection, sexual selection included, namely, that the first
beginning of the character which has been intensified by selection
remains obscure. Darwin attached importance to the occurrence of
ordinary individual variation, but it is open to question whether the
insignificant variations thus produced could give an adequate advantage
in the competition for the possession of the females; and, further,
whether we have not grounds for the assumption that larger variations
also occur. This question may also be asked in regard to ordinary
natural selection, although in that case we can imagine the beginnings
to be smaller, since here the advantage of a variation lies only in
the fact that it is useful, not in its being appreciated by others.
As a matter of fact, this very difficulty as to the first beginnings
of variations has been frequently urged against both hypotheses of
selection, and rightly so, inasmuch as this must be above all else
the point of attack for further investigations. But it is a mistake
to deny the whole processes of selection simply because this point is
not yet clear. Later on we shall attempt to gain some insight into
the causes of variation, and then we shall return to this question
of the beginnings of the selective processes. In the meantime let it
suffice to say that Darwin was very well aware that, in addition to the
ordinary individual variations, there were also larger deviations which
occurred discontinuously in single forms. He believed, however, that
such occurrences were very rare, and, on the whole, he was not inclined
to ascribe to them any particular importance in the transformation of
species. He rather referred the organic transformations which have
taken place in the course of the earth's history, in the main, to the
intensification of the ordinary individual variations, and I believe
that he was right in so doing, since adaptations from their very nature
cannot have been brought about by sudden chance leaps in organization,
but can only have become exactly suited to chance conditions of life
through a gradual accumulation of minute variations in the direction
of utility. Whether, however, purely sexual distinctions may not have
had their primary roots in discontinuous variations must be inquired
into later. Theoretically, there is nothing against this assumption,
when such characters are not adaptations like the lasso antennæ of
the Copepods, or the turban eyes of the Ephemerids; mere distinctive
markings, decorative coloration, peculiar outgrowths, and the like,
may, if they arose discontinuously, very well have formed the basis for
further sexual selection, as long as they were not prejudicial to the
existence of the species.




LECTURE XII

INTRA-SELECTION OR SELECTION AMONG TISSUES

 Does the Lamarckian principle really play a part in the
 transformations of species?--Darwin's position in regard to this
 question--Doubts expressed by Galton and others--Neo-Lamarckians
 and Neo-Darwinians--Results of exercise and practice: functional
 adaptation--Wilhelm Roux, _Kampf der Theile_.


WE have devoted a whole series of lectures to studying the
Darwin-Wallace principle of Natural Selection and the range of its
operation. It seemed to us to make innumerable adaptations intelligible
up to a certain point. We now understand how the purposefulness, which
we meet with everywhere among organisms, can have arisen without
the direct interference of a Power working intentionally towards an
end--simply as the outcome and result of the survival of the fittest.
The two forms of the processes of selection, 'natural selection' in
the narrower sense, and 'sexual selection,' dominate, so to speak, all
parts and all functions of the organism, and are striving to adapt
these as well as possible to the conditions of their life. And although
the range of operation of Natural Selection is incomparably greater,
because it actually affects every part, yet we must attribute to sexual
selection also, at least among animals, a range of influence by no
means unimportant, since through it, as far as we can see at present,
not only do the secondary sexual characters in all their diversity
arise, but by the transference of these to the other sex that too is
modified, and thus the whole species may be influenced, and may indeed
be started afresh on an unlimited series of further transformations.

But although the processes of selection play such an important part in
the transformations of the forms of life, we have to inquire whether
they are the _sole_ factors in these transformations, whether the
accumulation of chance variations in the direction of utility has been
the sole factor in bringing about the evolution of the animate world,
or whether other factors have not also co-operated with it.

We are all aware that Lamarck regarded the direct influence of use
and disuse as the most essential factor in transformation, and that
Darwin, though hesitatingly and cautiously, recognized and accepted
this factor, which he believed to be indispensable. Indeed, it seems
at first sight to be so. There is a whole range of facts which seem to
be intelligible only in terms of the Lamarckian theory; in particular,
the existence of numberless vestigial or rudimentary organs which
have degenerated through disuse, the remains of eyes in animals which
live in darkness, of wings in running birds, of hind legs in swimming
mammals (whales), and of ear muscles in Man, who no longer points his
ears, and so forth through a long list.

According to Wiedersheim, there are in Man alone about two hundred of
these vestigial or rudimentary organs, and there is no higher animal
which does not possess some. In all, therefore, a piece of the past
history of the species is embodied in the actually existing organism,
and bears witness to the fact that much of what the ancestors possessed
is now superfluous, and is either transformed, or is gradually set
aside, or is still in process of being set aside. It seems obvious that
this gradual dwindling and degeneration of an organ no longer needed
cannot be explained through natural selection in the Darwin-Wallace
sense, for the process goes on so exceedingly slowly that the minute
differences in the size of an organ, which may occur among individuals
of the species at any given time during the retrogressive process,
cannot possibly have a selection value. Whether the degenerate and
now functionless hind leg of the whale is a little larger or a little
smaller can have no importance in the struggle for existence; the
smaller organ cannot be considered either as a lesser hindrance in
swimming or as a greater economy of material, and the case is the same
in regard to most other instances of degeneration through disuse. We
therefore require another interpretation, and at first sight this seems
to be supplied by the Lamarckian principle.

But the reverse process, the strengthening, the enlarging, and the more
perfect development of a part, very often goes on proportionately to
its more frequent use, and here again the Lamarckian principle seems to
afford a simple explanation. For we know that exercise strengthens a
part, as disuse weakens it, and if we could assume that these results
of use and disuse were transmitted from the individual who brought them
about or 'acquired' them in the course of his life to his offspring,
then there would be nothing to object to in the Lamarckian principle.
But it is precisely here that the difficulty lies. Can we assume such
a transmission of 'acquired' characters? Does it exist? Can it be
demonstrated?

That Lamarck did not even put this question to himself, but assumed
such transmission as a matter of course, is readily intelligible
when we consider the time at which he lived. He was himself one of
the first to grasp the idea of the transmutation-hypothesis, and he
was only too glad to have any sort of principle of interpretation
ready to work with. But Charles Darwin, too, attributed a not
inconsiderable influence to this principle, although the transmission
of 'acquired' characters which it took for granted was not accepted
without reflective hesitation. He even directed his own particular
theory of heredity, as we shall see, especially to the explanation of
this supposed form of inheritance, and we can very well understand
this, after what I have said as to the impossibility of explaining
the disappearance of organs which have become superfluous by the
Darwin-Wallace theory of Natural Selection. Darwin needed the
Lamarckian principle for the explanation of these phenomena, and it
was this that decided him to assume the transmission of 'acquired'
characters, although the proofs of it can hardly have satisfied him.
For when we are confronted with facts which we see no possibility
of understanding save on a single hypothesis, even though it be an
undemonstrable one, we are naturally led to accept the hypothesis, at
least until a better one can be found. It is in this way, obviously,
that we are to understand Darwin's attitude to the Lamarckian
principle; he did not reject it, because it seemed to him to offer the
only possible explanation of the disappearance of characters which have
become useless; he adhered to it, although the transmission of acquired
characters which it assumed must have seemed, and, in point of fact,
did seem to him doubtful, or at least not definitely proved. Doubts,
some faint, some stronger, as to this assumed form of inheritance were
hardly expressed till somewhat late in the day--almost twenty years
after the appearance of the _Origin of Species_--first by Francis
Galton (1875), then by His, who definitely declared himself at least
against any inheritance of mutilations, and by Du Bois-Reymond, who,
in his address _Ueber die Uebung_ in 1881, said: 'If we are to be
honest, we must admit that the inheritance of acquired characters is a
hypothesis inferred solely from the facts which have to be explained,
and that it is in itself quite obscure.'

This is how it must appear to every one who examines it simply in
respect of its theoretical possibility, its conceivability. This is how
it appeared to me when I attempted, in 1883, to arrive at clearness on
the subject, and I then expressed my conviction that such a form of
inheritance was not only unproved, but that it was even theoretically
unthinkable, and that we ought to try to explain the fact of the
disappearance of disused parts in some other way, and I attempted to
give an explanation, as will be seen later.

Thus war was declared against the Lamarckian principle of the direct
effect of use and disuse, and there arose a strife which has continued
down to the present day, the strife between the Neo-Lamarckians and the
Neo-Darwinians, as the two disputing parties have been called.

In order to form an independent opinion in regard to this famous
dispute, it is, first of all, necessary to examine what actually takes
place when an organ is exercised or is left inactive, and further,
whether we can assume that the results of this exercise or inaction can
be transmitted to descendants.

That exercise in general has a strengthening, and neglect of it a
weakening influence on the relevant organ has long been known and is
familiar to all; gymnastics make the muscles stronger, the thickness of
the exercised muscle and the number of its fibres increases; the right
arm, which is much more used than the left, is capable of performing
twenty per cent. more work. Similarly, the activity of glands is
increased by exercise, and the glands themselves are increased in
size, as are the milk-glands of the cow through frequent milking; and
that even the nerve-elements can be favourably influenced by exercise
is proved by actors and professors of mnemonics, who have by practice
increased their powers of memory to an almost incredible degree. I have
heard of a singer who had learned by heart 160 operas; and which of us
has not experienced how quickly the capacity for learning by rote can
be again increased by practice, even after it has been neglected or
left unexercised for a long time?

I have always been particularly struck with the practising of a piece
of music, with its long succession of periods of different phrase,
with its changes in melody, rhythm, and harmony, which nevertheless
becomes so firmly stamped on the memory that it can be played, not
only consciously, but quite unconsciously, when the player is thinking
intensely of other things. It is in this case not the memory alone, but
the whole complicated mechanism of successive muscle-impulses, with
all the details of fast and slow, loud and soft, that is engraved on
the brain elements, just like a long series of reflex movements which
set one another a-going. Though in this case we cannot demonstrate
the material changes which have taken place in the nervous elements,
there can be no doubt that changes have taken place, and that these
consist in a strengthening of definite elements and parts of elements.
The strengthening causes certain ganglion-cells to give a stronger
impulse in a particular direction, and this impulse acquires increasing
transmissive power, and so on.

Our first theoretical insight into these relations came through Wilhelm
Roux, who, in 1881, gave expression to what had previously been
an open, if not quite conscious, secret, that 'functional stimulus
strengthens the organ,' that is to say, that an organ increases through
its own specific activity. Up till that time it had been believed that
it was merely the increased flow of blood that caused the increase in
the size of a much-used part. Roux showed that there is a 'quantitative
self-regulation of the organ according to the strength of the stimulus
supplied to it'; that the stimulated organ, that is, the organ which
is performing its normal function, may, in spite of the increased
breaking down or combustion (dissimilation), assimilate all the more
rapidly; that its used-up material is 'over-compensated,' and that
therefore it grows. He called this the 'trophic' or nutritive effect of
the stimulus, and in terms of this he explained the increase and the
heightened functional capacity of the much-used organ. Conversely, he
referred the decrease of a disused organ to 'functional atrophy,' which
sets in when there is a deficient compensation for the substance used
up in the metabolism.

But if we press for deeper analysis, we must ask: 'On what does this
trophic effect of functional stimulus depend?' Roux could not answer
this question when he wrote, nor can we do so now, as Plate has justly
emphasized. We are here face to face with the fundamental phenomenon
of life, metabolism; and, since we do not understand the causes of
this, we are not in a position to say why it varies in this way or in
that according to the 'stimulus.' But the fact itself is certain that
the organs respond up to a certain point to the claims made upon them;
they increase in proportion as they function more frequently or more
vigorously, they are able to respond to increased functional demands,
and this Roux has called 'functional adaptation.' As an animal adapts
itself to the claims of the conditions of its life, for instance, by
taking on a green or a brown protective colour according as it lives
on green or brown parts of plants, so the individual organ adapts
itself to the strength of the stimulus which impels it to function,
and increases or decreases in proportion to it. If _one_ kidney in Man
degenerate, or be surgically removed, the other begins to grow, and
goes on increasing until it has reached nearly twice its former size.
The specific stimulus which is brought to bear upon it by the urea
contained in the blood, and which forces it to grow, is twice as great
in the absence of the other kidney, and therefore the remaining kidney
grows in response to the increased stimulus and its 'trophic effect'
until its increase in size has reduced the functional intensity to the
normal proportion.

Adaptation of an organ in the opposite direction takes place when the
function diminishes or ceases. If a nerve supplying a muscle or a
gland be cut through, the organ concerned begins to degenerate and to
lose its normal structure to a greater or less degree. Sensory nerves
also degenerate in their peripheral part when they are cut through. In
such cases there may be no alteration either in the nutritive mechanism
or in the blood-vessels, &c., but the functional stimulus--in the case
of the muscle, the stimulus from the will--no longer affects the organ,
and its metabolism is so much lowered in consequence that it begins to
degenerate.

When we admit that the fit adaptation of the organism, as far as we
understand it, must depend upon processes of selection, we may refer
this 'functional adaptation' also to primitive processes of selection,
which prevailed at the very beginning of life upon our earth, and
represented, so to speak, the first adaptation that was established,
but we can say nothing with certainty in regard to this matter as long
as we do not understand the essence of assimilation. It is conceivable,
however, that a _primary_ adaptiveness may have arisen, so to speak,
abruptly, through a concurrence of favourable circumstances, as we
shall endeavour to show later on when we discuss the beginnings of life.

Even although we cannot lay bare the primary roots of 'functional
adaptation' we can gain from the fact itself very valuable insight
into phenomena which would otherwise be unintelligible and mysterious:
_the perfectly adapted structure of many tissues and their power of
adaptation to changed conditions_. In this lies, in the main, the
advance in our knowledge which is due to Roux's _Kampf der Theile_.

If a number of embryonic cells of different capacity, say _A_, _B_, and
_C_, be affected by different kinds of functional stimuli, _a_, _b_,
and _c_, those cells will grow most rapidly which are most frequently
affected by the stimulus appropriate to them. The proportion in which
the cells _A_, _B_, and _C_ will ultimately be present in the tissues
will depend upon the frequency with which the stimuli _a_, _b_, and
_c_ act upon the tissue. But the tissue will be still more precisely
determined as to its structure if the three kinds of stimuli affect the
cell-mass, not uniformly all over, but only at certain spots, or along
particular paths, one in this, the other in that. Thus the cells _A_
will predominate over the cells _B_ and _C_ at all the places which
are most frequently affected by the stimulus _a_, the cells _B_ in the
sphere of the stimulus _b_, and the cells _C_ in that of the stimulus
_c_; there they will increase most rapidly and so crowd out the other
kinds of cells, and thus a spatial arrangement will be established
within the tissue, a 'structure' which corresponds and is well adapted
to its end. This is what Roux deduced from his _Struggle of the
Parts_, and I subsequently defined the process as histonal or tissue
selection.

Let us first take an example. The anatomist Hermann Meyer showed in
1869 that the so-called 'spongiosa,' that is, the bony tissue of spongy
structure within the terminal portions of the long bones in Man and
Mammals, has a minute structure conspicuously well adapted to its
office. The thin bone lamellæ of this 'spongiosa' lie precisely in the
direction of the strongest strain or pressure which is exerted upon the
bone at the particular area. Arch-like in form, they are kept apart by
means of buttresses, and no architect could have done better if he had
been entrusted with the task of making a complicated system of arches
with the greatest possible carrying and resisting power combined with
the greatest possible economy of material.

This well-adapted structure is now interpreted through the _Struggle of
the Parts_ as a self-differentiation, for if there be in the rudiments
or primordia of the bone differently endowed elements[10], that is,
cells which respond in diverse ways to different stimuli, these must
arrange themselves locally, owing to the struggle for space and food,
in a manner corresponding to the distribution of the different stimuli
in the bone. The largest amount of bone substance will be formed in the
directions of the strongest strain and the greatest pressure, because
the bone-forming cells are excited by this, their functional stimulus,
to growth and multiplication. Thus the buttress and arch structure
comes about, and between the delicate bone lamellæ spaces will remain
free, and these, being relieved from the burden of strain and pressure
by the aforesaid bony lamellæ, will offer suitable conditions of life
to cells with other functional properties, such as connective tissue
cells or vascular cells.

[10] I do not here enter into the question whether we have not in
this case to do with similar elements, which have the power of
differentiating into one or another kind of cell according to the
nature of the external stimuli by which they are influenced.

The structure of the bone spongiosa is not everywhere the same,
and it is demonstrably related with precision to the conditions of
strain and pressure at each particular region. Thus, just below the
soft cartilaginous covering of the joints there are no long pillars
with short arches, but only rounded meshes, because the pressure is
here almost equally strong from all sides. The long parallel pillars
only occur further down in the bone, and they lie in two directions
which intersect each other obliquely, corresponding to the two main
directions of pressure. But it is only under the functional stimulus
of pressure that the bone-forming cells have an advantage over the
others, and multiply more quickly, thus crowding out those that are not
attuned to the appropriate functional stimulus.

In a similar manner Roux interprets, in the light of the struggle of
the parts, the striking adaptations in the course, the branching, and
the lumen-formation of the blood-vessels, in the direction of the
intersecting connective tissue strands in the tail-fin of the dolphin,
in the direction of the fibres in the tympanum, and in many other
adaptations in the histological structure of complex tissues.

In this there is manifestly an important step of progress, for it is
obvious that the direction of the bone-lamellæ and such like could not
have been determined by individual selection, and the same is true
in regard to many other histological details. It cannot be disputed,
however, that there is a kind of selection-process here also, similar
to that which we think of, with Darwin and Wallace, as occurring
between individual organisms. Just as in the latter, which we shall
henceforward call _personal selection_, variability and inheritance
lead, in the struggle for existence, to the survival of the fittest,
so, in histonal differentiation, the same three factors lead to the
victory of what is best suited to the parts of the body in question.
The tissues and the parts of the tissues have to distribute and
arrange themselves so that each comes to fill the place in which it
is most effectively and frequently affected by its specific stimulus,
that is, the stimulus in regard to which it is superior to other
parts; but these places are also those the occupation of which by
the best re-acting parts makes the whole tissue capable of more
effective function, and therefore makes its structure the fittest.
Variability--in this case that of embryonic cells, with different
primary constituents--must be assumed; inheritance is implied in
the multiplication of the cells by division; and the 'struggle for
existence' here assumes its frequent form of a competition for food
and space; the cells which assimilate more rapidly because of the
more frequent functional stimulus increase more rapidly, draw away
nourishment from the more slowly-multiplying cells around them, and
thus crowd these out to a greater or less extent.

We might even speak of histonal selection among unicellulars, for it
is conceivable that in primitive living substance, such as that of a
moneron, there may be minute differences among the vital particles,
involving also functional distinctions, which, under the influence of
diverse stimuli, may gradually give rise to an increasingly complex
differentiation. For the variations in the primary living substance
most strongly affected by a particular stimulus would tend to
accumulate at the places most frequently reached by that stimulus, and
would crowd out other variations at that spot, just as the body and its
individual parts may be said to have taken their architectural form
in exact response to the demands made upon them by function. In this
case, of course, personal selection and histonal selection co-operate,
for every improvement in the organization of the fundamental living
substance means at the same time a lasting improvement in the whole
individual.

In many-celled organisms, however, we must admit that there is an
essential difference between personal and histonal selection, inasmuch
as the latter can give rise to adaptive structural modifications
corresponding to the needs of the tissue at the moment, but not to
permanent and cumulative changes in the individual elements of the
tissue. If a broken bone heals crookedly, the spongy substance within
the healed portion does not remain as it was before, for the pillars
and arches, which now no longer run in the direction best suited to
their function, break up, and a new system of arches is formed, not
in line with the earlier one, but adapted to the new conditions of
pressure. This is certainly an adaptation through selection, but the
elements, that is the cells which form the bone substance in response
to strain and pressure, or those which in response to the stimulus of
the blood flowing into the spaces form the blood-vessels, or those
which being quite freed from one-sided pressure develop into connective
tissue, must be presupposed. These kinds of cells must be virtually
implied in the germ-rudiment; they are themselves adaptations of the
organism, and can therefore only be referred to _personal selection_.
And this is true of all adaptations of the _elements_ of multicellular
organisms, and thus of the _cells_. Their adaptation according to the
principle of division of labour, their differentiation into muscle,
nerve, and gland cells can only be referred to natural selection in the
Darwin-Wallace sense, and cannot depend upon histonal selection. In
the spongy substance of the bone a better bone-cell does not struggle
with an inferior one and leave behind it by its survival a host of
descendants which are, if possible, better than itself; the struggle
for existence and for descendants, in this case, is between two kinds
of cell which were different from the beginning, and of which one
has the advantage at one spot, another at another. The case may be
compared to that of a flock of nearly allied species of bird, of which
one species thrives best in the plains, another among the hills, and a
third among the mountain forests, all mingled together in a vast new
territory to which they had migrated, and in which all three kinds of
conditions were represented. A struggle would arise among the different
species, in which in every case the particular species would be
victorious which was best adapted to the local conditions. But each
would thrive best in the region in which it was superior to the others,
and very soon the three species would be distributed as they were in
the land from which they came--in the plains, the high lands, and the
mountain forests. This would be the result of a struggle between the
three species, _not between individuals within each species_, and it
could not therefore bring about an improvement of a single species,
but only the local prevalence of one or another. The characters which
made one species adapted for the plain, another for the mountain forest
were _already there_; they can only be referred to personal selection,
which brought about the adaptation of their ancestors in the course
of ages to the conditions of their life. Something similar is true of
the adaptations of the tissues; the differentiation of the individual
kinds of cells is an ancient inheritance, and depends upon personal
selection, but their distribution and arrangement into specially
adapted tissues, so far as there is any plasticity at all, depends
upon histonal selection. Obviously, however, only as far as the tissue
is plastic, that is, with the power of adjusting itself to particular
local conditions. Only adaptations of this kind can be referred to
histonal selection; the ground-plan, even of the most complicated
tissue, such as the large glands of mammals, the kidneys, the liver,
and so on, must have been implicit in the germ, and must therefore be
referred to personal selection. A precise limitation of the respective
spheres of action of personal selection and histonal selection is not
possible as yet, since hardly any investigations on the subject are
available.

Roux undoubtedly over-estimated the influence of his 'struggle
of parts' when he believed that the most delicate adaptations of
the different kinds of cells depended on it. I admit that, for a
considerable time, I made the same mistake, until it became clear to
me, as it did first in regard to the sex-cells, that this is not, and
cannot be the case. How, for instance, could the diverse and minutely
detailed adaptations of the sex-cells--which we are to discuss in a
subsequent lecture--have arisen in this way? As far as the individual
sperm-cell is concerned, it is a matter of indifference whether its
head is a little thinner or thicker, its point a little sharper
or blunter, its tail a little stronger or weaker. This does not
decide whether the cell is to thrive better, or to occur in greater
numbers than some other variety. But it does decide whether it is to
be able to penetrate through the minute micropyle, or through the
firm egg-envelope, into the egg, there to effect fertilization. An
individual with less well formed sperm-cells will be able to fertilize
fewer eggs, and therefore to leave fewer descendants which might
inherit its tendency to produce inferior sperm-cells, and conversely.
Thus it is not the sperm-cells of any one individual which are selected
according to their fitness, it is the individuals themselves which
compete with one another in the production of germ-cells which shall
fertilize best, that is, most certainly. The struggle is thus not
intercellular, but a struggle between persons.

The same is true of all cells differentiated for particular functions;
every new kind of glandular, muscular, or nerve cell, such as have
arisen a thousandfold in the course of phylogeny, can only have
resulted from a struggle between individuals which turned on the
possession of the best cells of a particular kind, _not from a struggle
between the cells themselves_, since these would gain no advantage
from serving the organism, as a whole, better than others of their
kind. In regard to the sex-cells we might admit, in addition to
personal selection, the possibility of an internal struggle between the
sperm-cells or egg-cells of the same individual, inasmuch as each of
these cells is the primordium of a new individual, and as those better
adapted for reproduction might transmit their better quality to these
new individuals. I will not here enter into my reasons for regarding
this idea as erroneous, for in any case this interpretation would not
apply to any other kind of cells. If, for instance, it were a question
of the transformation of an ordinary mucus or salivary gland into a
poison gland, it would not matter in the least to the individual cell
whether it yielded a harmless or a poisonous secretion; but individuals
with many poisonous cells would have an advantage in the struggle for
existence.

I agree so far with Plate when he refers the differentiation of
the tissues entirely to personal selection, but not in his further
conclusion that histonal selection does not exist. The ground-plan
of the architectural structure of the organ depends upon personal
selection, but the realization of the plan in particular cases is
not predetermined down to the minutest details, but is regulated by
histonal selection, and is thus to a certain extent an adaptation to
local conditions of stimulus. The direction, strength, and size of
every single bone lamella is not predetermined from the germ, but only
the occurrence and nature of bone-cells and bone lamellæ in general.
The direction and the strength which these bone lamellæ may assume
depends on the local conditions of strain and pressure which affect the
cell-mass, as is shown very clearly by the spongiosa of an obliquely
healed bone, which we have already described.

But let us now turn to the question which is here most important for
us: _whether functional adaptations can be transmitted_. We must
admit that the insight we have so far gained into the causes of these
adaptations does not make it much easier to answer the question.
Histonal selection is a purely _local_ process of adaptation to the
conditions of stimuli prevailing at the moment, and no one will be
likely to suppose that the distorted position of the spongiosa of
a badly healed fracture could reappear in the straight bone of a
descendant; this would be quite contrary to the principle, for the
crooked lamellæ would in that case no longer be the best adapted. Even
the question _whether the strengthening of a muscle through use can be
transmitted_ cannot be answered in the light of the knowledge we have
hitherto gained. The 'trophic effect of the functional stimulus' is
brought into activity through entirely local influences, through which
only the parts most strongly affected by the stimulus can be caused to
vary. Thus the problem remains unaltered, How can purely local changes,
not based in the germ, but called forth by the chances of life, be
transmitted to descendants?

If all species, even in the highest groups, reproduced by dividing
into two, we might imagine that a direct transmission of the changes
acquired in the course of the individual life through use or disuse
took place, though this would presuppose a much more complicated
mechanism than is apparent at first sight. But it is well known that
multiplication by fission is for the most part restricted to simple
organisms, and that the great majority of modern plants and animals
reproduce by means of germ-cells, which develop within the organism in
regions often very remote from the parts, the results of the exercise
of which are said to be transmitted. Moreover, the germ-cells are of
very simple structure, at least as far as our eyes can discern; for
we see in a germ-cell neither muscles nor bones nor ligaments, glands
nor nerves, but only a cell-body consisting of that semifluid living
matter to which the general name of protoplasm has been given, and of
a nucleus, in regard to which we cannot say that it differs in any
essential or definite way from the nucleus of any other cell. How then
could the changes which take place in a muscle through exercise, or
in the degeneration of a joint in consequence of disuse, communicate
themselves to a germ-cell lying inside the body, and do so in such
a fashion that this germ-cell is able, when it grows into a new
organism, to produce of itself, in the relevant muscle and joint, a
change the same as that which had arisen in the parent through use and
disuse? That is the question which forced itself upon me very early,
and in following it up I have been led to an absolute denial of the
transmission of this kind of 'acquired characters.'

In order to explain how I reached this result, and what it is
based upon, it is indispensable that we should first make ourselves
acquainted with the phenomena of heredity in general, and the
inseparably associated phenomena of reproduction, so that we may form
some sort of theoretic conception of the process of inheritance--a
picture, necessarily provisional and imperfect, of the mechanism
which enables the germ-cell to reproduce the whole organism, and not
merely, like other cells, others like itself. We are thus led to an
investigation of reproduction and heredity, at the conclusion of which
we shall feel justified in returning to the question of the inheritance
of acquired characters, in order to give a verdict as to the retention
or dismissal of the Lamarckian principle.




LECTURE XIII

REPRODUCTION IN UNICELLULAR ORGANISMS

 Reproduction by division--In Amœbæ--In Infusorians--Divisions
 following one another in immediate succession--Formation of germ-cells
 in the Metazoa--Contrast between germ-cells and body-cells--Potential
 immortality of unicellular organisms--Beginning of natural
 death--Budding and division in the Metazoa.


WE wish to consider the reproduction of organisms with special
reference to the problem of heredity, and it is most instructive to
begin with the lowest forms of life--the unicellulars--because their
structure, as far as we can see with the instruments at our command,
is very simple, and, what is even more important, is relatively
homogeneous.

[Illustration: FIG. 59. An Amœba: the process of division. _A_, before
the beginning of the division. _B_, the nucleus divided into two. _C_,
the two daughter-Amœbæ. Magnified about 400 times.]

Suppose that there are bacteria-like organisms of quite homogeneous
structure, and that these multiply by simply dividing into two, each
rod-like creature dividing transversely in the middle of its length,
the two halves would represent independent daughter-organisms, whose
structure would correspond exactly with that of the mother-organism,
could not indeed in any way deviate from it, and consequently would
take over all its characters, that is, would inherit them. The size
of body is the only feature which is not obviously inherited, but
in reality it is potentially heritable, since the structure of the
divided portions involves the capacity and the limits of their possible
growth. Moreover, the size of body is not invariable in any species;
a particular size is only reproduced under similar conditions of
development. Inheritance here consists simply in a continuation of the
mother-organism into its two daughter-cells.

Even in an Amœba (Fig. 59) we might picture the process of inheritance
as equally simple, though in so doing we should probably be making a
fallacious inference, for the structure of these lowest unicellular
animals probably seems to us simpler and more homogeneous than it
really is. Among Infusorians it is quite obvious that inheritance
implies more than the mere halving of the mother-animal into the two
daughter-cells; something more must be involved. For among these
unicellular animals the differentiation of the body is not only
great, but it is unsymmetrical. The posterior and the anterior ends
are different, and the transverse division of the animal, in which
the process of reproduction here consists, does not produce two
halves, but two very unequal portions. In the division of _Stentor_,
the so-called trumpet-animalcule (Fig. 60), the anterior portion
contains the funnel-shaped mouth and gullet with its complicated
nutritive apparatus, the circular peristome with its spirally curved
rows of composite ciliated plates, the so-called membranellæ, and so
forth; the posterior half contains nothing of all this, but possesses
the foot of the mother-Stentor with its attaching organ, which the
anterior half lacks. But each of the two portions possesses the
power of 'regeneration,' that is, it is able to develop anew the
missing parts, mouth or foot, and so on. So that here there is no
longer merely a simple continuance of the maternal organization in
the daughter-animals, there is something new added, something which
requires explanation; we are confronted with the first riddle of
heredity. Simple growth does not explain the phenomenon, for what has
to be added to complete the halved portions has a different structure,
a different form, different accessory apparatus from any that the
halves themselves possess. It in no way affects this state of matters
that in the normal process of division in Infusorians the formation
of the new mouth and peristome-region begins before the halves have
actually separated, for even if a Stentor be cut in two artificially
the cut halves form complete animals. And, indeed, a Stentor may be cut
into three or four pieces, and in certain conditions each piece will
develop into a complete animal. These pieces must therefore possess
something more than the mere power of growth. We shall try later on to
discover whether this marvellous invisible transmission of characters,
this implication of the whole in each of the parts, can be in any
way theoretically expressed and included in our scheme of conceptual
formulation.

[Illustration: FIG. 60. _Stentor rœselii_, trumpet-animalcule.
Process of division. _wsp_, ciliated spiral leading to the mouth
(_m_); _cv_, contractile vacuole. _A_, in preparation for division,
the nucleus (_k_) has coalesced into a long twisted band. _B_, a
second ciliated spiral (_wsp´_) has begun to be formed; the nucleus
(_k_) is contracted. _C_, just before the constricting off of the two
daughter-Stentors. Magnified about 400 times. After Stein.]

Now that we have become familiar with these facts it will no longer
surprise us to learn that the reproduction of unicellular animals does
not always depend on _equal_ division, but that unequal spontaneous
divisions are also possible, so that one or several smaller portions of
the cell-body, containing a portion of the cell-nucleus, can separate
off from the mother-animal. This occurs especially among the suctorial
Infusorians or Acinetæ. In relation to the phenomena of inheritance the
problem raised by the equal division of the Infusorians repeats itself,
and it is in no way affected by the fact that equal division can take
place several times, or many times in succession, so that from one
animal a large number of small pieces of the same size may be produced
in rapid succession. The characteristic marks of the mother-animal are
not infrequently lost sight of, wholly or partially, when this occurs,
and the divided portions seem to consist of a homogeneous cell-body
and nucleus; but they possess the power of regenerating themselves, or
of developing, if the expression be preferred, into animals similar
to the maternal-organism. Such divided portions might very well be
called germs, only it must not be forgotten that the relation of the
mother-animal to these germs is a different one from that of a higher
animal or plant to its germ-cells; the unicellular animal breaks up by
continued division into these 'germs,' while the Metazoon continues its
individual existence unimpaired by the production of its germ-cells.

The beginning of a so-called 'spore-formation' is to be found in many
Infusorians. Thus the holotrichous species, _Holophrya multifiliis_
(Fig. 61), reproduces by first becoming enclosed in a cyst or capsule,
and then dividing many times in rapid succession, so that 2, 4, 8,
16, &c. individuals arise consecutively, and subsequently burst forth
from the cyst (Fig. 61, _B_). In the Gregarines and other Sporozoa the
period of division lasts much longer, and the encysted animal divides
into 128, 256, or even more portions; but in this case also each part
or 'spore' receives a piece of the maternal cell-body and cell-nucleus,
so that there is no difference in principle between this and the simple
division into two exhibited by _Stentor_; as in that case, so here,
it is not the fully differentiated structure of the animal which is
handed on to the divided parts; it is only the power to redevelop this
anew on their own account. Thus here again we are face to face with the
fundamental problem of heredity: How is it possible that the power of
reproducing the complex whole can be inherent in the simple parts?

[Illustration: FIG. 61. _Holophrya multifiliis_, an Infusorian
parasitic on the skin of fishes. _A_, in its usual condition; _ma_,
macronucleus; _mi_, micronucleus; _cv_, contractile vacuole; _m_,
mouth. _B_, after binary fission has been several times repeated within
the cyst (_cy_); _tt_, results of the division. _C_, one of these units
much enlarged.]

In contrast to the unicellular organisms, the great majority of the
multicellulars, the Metazoa and Metaphyta, many-celled animals and
plants, differ not only in the multitude of their cells, but even
more in the manifold differentiation of these cells according to the
principle of division of labour, so that the various functions of
the animal are not performed by all the cells uniformly, but each
function is relegated to a particular set of cells specially organized
with reference to it. Thus there is differentiation between motile,
nutritive, and reproductive cells, and there may also be glandular,
nerve, muscle, and skin cells, and we know how this differentiation
into a great number of different kinds of cells with highly specialized
functions has arisen, especially among the higher animals, in a
multiplicity which cannot easily be overlooked. Thus we find a large
number of the most diverse kinds of cells, all of which serve for
the maintenance of the body, in contrast to the simply reproductive
cells or germ-cells. These alone possess the power of reproducing,
under certain conditions, a new individual of the same species. We can
contrast with these germ-cells, which serve, not for the maintenance of
the individual, but only for that of the species, all the other kinds
of cells under the name of somatic or body-cells. The problem which we
have to solve now lies before us in the question, How comes it that
the germ-cell is able to bring forth from itself all the other cells
in definite sequence and arrangement, and is thus able to build up the
body of a new individual?

[Illustration: FIG. 62. _Pandorina morum_; after Pringsheim. I, A young
colony, consisting of 16 cells. II, Another colony, whose cells have
reproduced daughter-colonies; all the cells uniformly alike. III, A
young Volvox-colony; _sz_, somatic cells; _kz_, germ-cells.]

The similarity of this problem to that formulated in regard to
unicellular organisms is at once obvious, but it becomes still more
emphatic when we remember that the gulf between unicellular organisms
and the higher animals and plants is bridged over by certain transition
forms which are of the greatest interest, especially in relation to the
problems of inheritance.

Among the lower Algæ there is a family, the Volvocineæ, in which the
differentiation of the many-celled body on the principle of division of
labour has just set in; in some genera it has been actually effected,
though in the simplest way imaginable, and in others it has not yet
begun. Thus in the genus _Pandorina_ the individual consists of sixteen
green cells, united into a ball (Fig. 62, I), each one exactly like the
other, and all functioning alike. They are all united into a spherical
body, a whole, by a gelatinous matrix which they all secrete, and thus
they form a cell-colony, a cell-stock, a many-celled individual; but
each of these cells has not only all the typical parts--cell-body,
nucleus, and contractile vacuole--but each possesses a pair of flagella
or motor organs, an eye-spot, and a chlorophyll body which enables
them to assimilate nourishment from the water and the air. Each one
of these cells thus performs all the somatic functions, that is, all
that are necessary to the maintenance of the individual life. But each
also possesses the power of reproducing the whole colony from itself,
that is, it also performs the function of reproduction necessary to
the maintenance of the species. When such a colony, whose sixteen
cells are continually growing, has led for some time a free-swimming
life in the water, the cells retract their flagella, and each begins
to multiply by dividing into 2, 4, 8, finally into 16 cells of the
same kind, which remain together, forming a spherical mass enclosed in
a gelatinous secretion (Fig. 62, II). Thus there are now, instead of
sixteen cells in the mother-colony, sixteen daughter-colonies, each
with sixteen cells which soon acquire flagella and eye-spots, and are
then ready to burst forth from the dissolving jelly of the maternal
stock as independent individuals. This _Pandorina_ shows no trace of
a differentiation of its component cells to particular and different
functions, but a nearly allied genus of the same family, the genus
_Volvox_ (Fig. 62, III), consists of two kinds of cells--on the one
hand of small cells (_sz_) which occur in large numbers and compose the
wall of the hollow gelatinous mass, forming, so to speak, the skeleton
of the _Volvox_; and, on the other hand, of a much smaller number of
cells which are very much larger (_kz_). The former, the 'body' or
'somatic' cells, are green, and have a red 'eye-spot' and two flagella;
they are connected with each other by processes from their cell-bodies,
and are able, by means of the co-ordinated action of their flagella,
to propel the whole colony with a slow rotatory movement through the
water. Many of my readers are doubtless familiar with these light green
spheres, which are quite recognizable with the naked eye, and people
our marsh pools and ponds in Spring in such abundance that it is only
necessary to draw a glass of water to procure a large number of them.

The little flagellated cells just described serve not only for the
locomotion of the colony, but also for nutrition, for the secretion
of the jelly, and for the excretion of waste products; in short, they
perform all the functions necessary to the maintenance of life, but
not that of reproduction. They can, indeed, multiply by dividing when
the colony is young, like the cells of _Pandorina_, but they cannot
reproduce the whole colony but only cells like themselves, that is,
other somatic cells. In _Volvox_ the maintenance of the species, the
production of a daughter-colony, is the function of the second and
larger kind of cells, the reproductive cells, which are contained in
the interior (filled with a watery fluid) of the gelatinous sphere.
They possess no flagella (_kz_), and so take no share in the swimming
movements of the somatic cells. For the present we need not allude to
the fact that there are several kinds of these cells, and need only
state that the simplest among them, the so-called 'Parthenogonidia,'
after they have reached a considerable size, begin a process of
division which results in the formation of a daughter-colony. Usually
there are several of these large reproductive cells in a _Volvox_
colony, and as soon as these have developed into a similar number of
daughter-colonies they burst out through a rupture in the now flaccid
jelly of the maternal sphere and begin to lead an independent life. The
mother-sphere, which now consists only of somatic cells, is unable to
produce new reproductive cells; it gradually loses its spherical form,
sinks to the ground, and dies.

In _Volvox_ we have, for the first time, a cell-colony in which a
distinction has been established between body or somatic cells and
reproductive or germ-cells. In contrast to _Pandorina_, a large number,
indeed the majority of the cells of the colony, have lost the power
of reproducing the whole by division, and only the few reproductive
cells possess this, while they, in turn, have lost other functions,
notably that of locomotion. Their power of reproducing the whole, that
is to say, their hereditary capacity, gives them a greater theoretical
interest than the cells of _Pandorina_, for the latter require only to
produce others like themselves, because there is only one kind of cell
in the colony, while in _Volvox_ the reproductive cell can not only
produce others like itself, by division, but can produce the body-cells
as well. The problem is quite analogous to the one which we have had
to face in regard to the unicellular animals of complex structure, the
Infusorians. The question, How can the part of the trumpet-animalcule
which is mouthless develop from itself a new mouth and ciliated
apparatus? here transforms itself into the question, How can a cell
by division give rise not only to others like itself, but also to the
body-cells, which are of quite different structure? This is, in its
simplest form, the fundamental problem of all reproduction through
germ-cells, to which we must now pass on. But first a short digression.

We have already noted that unicellular organisms multiply by division,
and originally, as well as in the great majority of cases to-day, by
division into two. It follows, therefore, that there is no _natural_
death among them, for, if there were, the species would die out as
the individuals grew old; but this does not happen. The two daughter
organisms which arise from the binary fission of an Infusorian are
in no way different in regard to their power of life; each of them
possesses an equal power of doubling itself again by division, and
so it goes on, as far as we can see, for an unlimited time. Thus the
unicellular organisms are not subject to natural death; their body is
indeed used up in the course of ordinary life so that the formation of
new cilia and so on is necessary, but it is not worn away in the same
sense in which our body is and that of all Metazoa and Metaphytes,
where, through functioning, the organs are gradually worn away until
they become incapable of function. Our body grows old, and can at last
no longer continue to live; but among unicellular organisms there is
no growing old, and no death in the normal course of the development
of the individual. The unicellulars are, as we may say, immortal; that
is, while individuals may be annihilated, by external agencies, boiling
heat, poisons, being crushed, or eaten, and so on, at every period
some individuals escape such a fate, and perpetuate themselves through
succeeding ages. For, strictly speaking, the daughter-individual
is only a continuation of the mother-individual; it contains not
only half of the substance, but also the organization, and life is
continued directly from mother to daughter. The daughter is simply
half of the mother, which is subsequently regenerated; and the other
half of the mother lives on in the other daughter, so that nothing
dies in this multiplication. It may be said that the daughter has to
develop the other half of its body anew, and that therefore it is a
new individuality, and not merely a continuation of the old, and that
therefore the unicellular animals are not immortal. The 'immortality'
of the Protozoa may be scoffed at; the idea may seem absurd that the
'immortal' Protozoa are still the same individuals which lived upon
the earth millions of years ago, but all such objections mean no more
than doctrinaire quibbling with the concepts of 'individual' and
'immortality,' which do not exist in nature at all, but are mere human
abstractions, and therefore only of relative value. My thesis as to the
potential immortality of the Unicellulars aims at nothing more than
impressing on Science the fact that the occurrence of physiological,
that is, natural, death is causally associated with the transition
from single-celled to many-celled organisms; and this is a truth which
will not be overthrown by any sophisms. It is the Volvocineæ which
show us, so to speak, the exact point at which natural death set in,
at which it was introduced into the world of life. In _Pandorina_ the
state of things is still the same as in single-celled organisms, for
each cell is still all in all, each can bring forth the whole, none
dies from physiological causes involved in the course of development,
and they are therefore 'immortal' in the sense stated. But in _Volvox_
the 'individual' dies when it has given off its reproductive cells,
because here the contrast between germ-cells and body has developed.
Only the body is mortal in the sense of being subject to natural death;
the germ-cells possess the potential immortality of the single-celled
animals, and it is necessary that they should possess it if the species
is to continue to exist.

From this alone it does not seem quite clear why the body or soma
should be subject to death, and when I first endeavoured to arrive at
clearness in regard to these matters I tried to find out why a natural
death of the body was necessitated by the course of evolution. I did
not at once discover the true explanation, but without delaying to
discuss my mistakes I shall proceed to expound what I believe to be the
true reason. It lies simply in the fact, which we shall inquire into
later on in more detail, that every function and every organ disappears
as soon as it becomes superfluous for the maintenance of the particular
form of life in question. The power of being able to live on without
limit is useless for the somatic cells, and thus also for the body,
since these cannot produce new reproductive cells after those that had
been present are liberated; and with this the individual ceases to be
of any value for the preservation of the species. What advantage would
it be to the species if the _Volvox_ balls were to continue living for
an unlimited time after the reproductive cells were developed and had
been liberated? Obviously their further fate can have no influence
whatever in determining or preserving the characters of the species,
and it is quite indifferent to the continuance of the species whether
and how long they go on living. Therefore the soma has lost the
capacity which conditions endless continuance of life and continued
renewal of body-cells.

In regard to these views it has been asked jeeringly, how
'immortality,' if it were really a property of the Unicellulars and of
undifferentiated cell-colonies, could be lost, as if the world, which
we believe to be everlasting, should give up its everlastingness.
But the jeer recoils on the superficial outlook which is unable to
distinguish between the immortality dreamed of by the poets, religious
and secular, and the real power that certain forms of life have to
resist being permanently exhausted by their own metabolism. That
we should call this 'immortality' does not seem to me to require
any apology, for the right has always been conceded to science to
transfer popular words and ideas in a restricted and somewhat altered
sense to scientific conceptions when it seems necessary. That the
word 'immortality' in this case expresses the state of matters more
precisely and better than any other cannot be doubted, any more than
we can doubt that there exists in regard to natural death a real
difference, which we must take account of, between the Unicellulars
and the higher organisms. What enables the species in the case of the
higher organisms, like ourselves for instance, to last through ages is
not the immortality of the individual, of the person, but only that
of the germ-cells; these alone, among the cells of the whole body,
have retained the primæval power. A small piece of the individual is
still immortal, but only a minute part, which cannot be considered as
equivalent to the whole, either morphologically or from the point of
view of the conception of individuality. Can anyone consider himself
identical with his children? If any one should imagine this, it would
still not be the case, for he himself would in the course of time
suffer natural death, and his children would continue to live on until
they too had brought forth children, and in their turn also came to
die. It is quite different with an Infusorian, which never lies down to
die, but simply splits itself afresh into two halves which continue to
live.

It is hardly credible that such a simple and clear truth should have
remained so long undiscovered, and it is even more incredible that
since it was enunciated it should have been until quite recently
laughed at as false, as a piece of pseudo-science, and as valueless.
But it is the fate of all knowledge which rests on an intelligent and
comprehensive working up of facts to be attacked, until it gradually
bears down antagonism by the weight of its truth, and compels at least
a silent recognition.

The fact that natural death made its appearance with the appearance of
a 'body,' a soma, as distinguished from the germ-cells, will sooner or
later compel recognition. When I pointed out above that the explanation
of natural death lay in the fact that it would be superfluous for the
soma to continue to live on unlimitedly, after it had discharged its
germ-cells, and so fulfilled its duty to the species, I only intended
to say that this was the general reason for the introduction of natural
death. I have no doubt that the actual beginning of this phenomenon
could have, and probably did come about in other ways. Many kinds of
cells in higher animals perish as a result of their function; it is,
so to speak, their business to perish, to break up; this is the case
with many glandular and epithelial cells. It may very well be that, in
many of the highly differentiated tissue-cells, such as nerve, muscle,
and glandular cells, the high differentiation in itself excludes the
possibility of unlimited length of life and multiplication. Through
this alone, therefore, the exhaustion of the body and an ultimate
death may be explicable from internal causes. But the deeper cause
remains what I have already indicated, for it is obvious that if the
continued life, that is, the immortality of the soma, were necessary
to the preservation of the species it would have survived through
natural selection; that is to say, had it been so, then histological
differentiations incompatible with immortality would not have made
their appearance; they would always have been eliminated on their way
to development, since only that which is adapted to its end survives.
Only if the immortality of the soma were indifferent for the species
could the soma have become so highly organized that it became subject
to death.

Thus the old song of the transitoriness of life does not apply to
all the forms of life: natural death is a phenomenon which made its
appearance comparatively late in the development of the organic world,
a phenomenon which, up to a certain point, we can quite well understand
from the standpoint of purposefulness.

It would take me too far from the goal towards which we are at present
making if I were now to attempt to show, in connexion with natural
death, that the durability of the soma, or what we usually call
the normal duration of life, is also exactly regulated by natural
selection, so that each species possesses exactly that duration of life
which is most favourable to it, according to its physical constitution,
its physiological capacity, and the conditions of life to which it has
to adapt itself[11]. But, interesting as this subject is, I must not
digress further, but return to our proper subject of study, namely,
reproduction in its relation to inheritance.

[11] See Weismann, _Ueber die Dauer des Lebens_, Jena, 1882. Translated
in _Essays on Heredity_.

We digressed from this study after having seen that all, even the most
complex, multicellular plants and animals, in which the differentiation
of the cells into a number of cell-groups with the most diverse
functions has attained the highest degree of complexity, are able
to produce special cells, the germ-cells, which have the power of
reproducing from themselves another organism of the same species, and
with the same complex structure. It might be thought that such cells
must necessarily be very complex in their own structure, but in most
cases nothing of the kind is to be seen, and the germ-cells often
appear simpler in organization than many of the tissue-cells, such as
the glandular-cells; and where there is an unusual size or complexity
of structure in the germ-cell it usually bears no relation to the
grade of organization of the young creature that is to arise from it,
but is due solely to the special conditions imposed on the particular
germ-cell, if a young organism is to be evolved from it. We shall soon
see what is meant by this.

I must note here that plants and animals do not multiply by means of
germ-cells alone, but that many species--the majority of plants and
the simpler forms of animals--also exhibit multiplication by budding
or division. All animals and plants which do not stop short at the
stage of the individual, the 'person,' but rise to the higher stage
of the 'stock' (or corm), illustrate this. The first person from
which the formation of the stock proceeds gives rise by budding or
division to new persons which remain attached to it, and in turn by
repeated production of buds give rise to a third, fourth, or _n_^{th}
generation of persons, all remaining in connexion with the first, and
together forming the composite individuality of the animal-colony or
plant-stock. Such colonies or stocks are seen in polyps and corals,
Siphonophoræ and Bryozoa, and among plants, according to Alexander
Braun, in all phanerogams which do not consist only of a single shoot.
In these cases we find that definite, or perhaps indefinite groups
of cells in the stock may give rise to a new person, and we have to
inquire how this power may be theoretically interpreted.

New stocks may also have their origin from such buds, or from single
persons of the stock. The fresh-water polyp (_Hydra_) gives rise by
budding to a small stock of at most three or four persons; but the
young animals budded off only remain attached to the parent hydra
until they have attained their full development; then they detach
themselves and settle down independently, and begin to bud off in turn
a similar and transitory stock. Among plants there are many which, like
_Dentaria bulbifera_ and _Marchantia polymorpha_, multiply by so-called
'brood-buds,' that is, buds which fall from the stock and grow into new
plants. The whole horticultural propagation of plants by cuttings also
depends on the process of budding, for what is cut off from the parent
plant and stuck into the earth is a single shoot, that is, a 'person'
which possesses the power of sending down roots into the earth, and by
continual budding giving rise to new shoots or persons which together
make up a new plant-stock.

I must not, however, spend much time over this so-called 'asexual'
reproduction by budding and division, because it does not suggest
any way by which we may penetrate more deeply into the processes of
inheritance, and we may be content if we can bring them into harmony
with other theoretical views which we deduce from other phenomena.
These forms of reproduction were long regarded as the oldest and
the simplest, and it is only since the time of Francis Balfour that
the conviction has gradually gained ground that this cannot be so,
but that they are rather secondary methods of multiplication in the
Metazoa and Metaphyta, which therefore rest on a very complex basis.
We have seen that the germ-cells made their appearance along with
the multicellular body, and the step from _Pandorina_ to _Volvox_ is
as small a step as can be well imagined. It is thus proved that the
oldest mode of multiplication among multicellular organisms was that
through germ-cells, at least along this line of evolution. _Volvox_
does not reproduce by dividing, or by the development of buds from
any part of the spherical colony of cells. What is known as budding
among single-celled organisms is only an unequal cell-division, and
has nothing but its external appearance in common with the budding of
higher plants and animals. The latter, therefore, is something new, of
later and independent origin; _the primitive mode is reproduction by
unicellular germs_.




LECTURE XIV

REPRODUCTION BY GERM-CELLS.

 Historical--Differentiation of germ-cells into male and
 female--Pandorina--Volvox--Sperm-cells and ova in Algæ--Zoosperm form
 of the male germ-cells--Zoosperms of the Barnacles--Adaptation of the
 sperm-cells to the conditions of fertilization--Daphnids--Spermatozoa
 in different animal groups--Their minute structure--Form and structure
 of the egg-cell--Adaptation of the ovum to given conditions--Dimorphic
 ova in the same species--Nutritive cells associated with
 egg-cells--Complex structure of the bird's egg.


IF we now turn to the reproduction of the Metazoa and Metaphyta by
means of germ-cells we find that a great number of lowly plants
produce germ-cells which require nothing more for the development
of a new plant beyond certain favourable external conditions, above
all, moisture and warmth. Such, for instance, are the 'spores' of the
ferns, which are formed on the under surface of the fronds in little
clusters of a brown or yellow colour, easily visible to the naked
eye. These spores are individually very small, so that thousands
go to form one spore-cluster or sporangium, and millions of spores
are given off annually by a single fern. Each spore is a germ-cell
enclosed in a protective capsule, and may, if carried by the wind to
a spot favourable to germination, become a young plant, the so-called
prothallium, from which the fern-plant proper subsequently develops.

This reproduction by spores has been regarded as a form of 'asexual
reproduction' so-called, and has been classed along with budding and
fission under this head. But it has nothing in common with these
forms of multiplication except the negative character that the act
of fertilization, which we shall inquire into later on, does not in
this case occur. This mode of classification has no longer any more
justification than the division of the animal kingdom into backboned
and backboneless animals, in which the negative character of the
absence of vertebræ has led to the slumping of quite heterogeneous
forms in one group. I do not mean to dispute that both these
classifications were fully justified in their own time; indeed
they expressed a step of progress. Nowadays, however, the division
'Invertebrata' or 'backboneless animals' as a scientific conception has
been abandoned, and the same should be done with the category 'asexual
reproduction,' since it groups together quite different things, such
as multiplication by single-celled and many-celled 'germs,' and is
moreover based on a quite erroneous idea of what 'fertilization'
really is. Both terms may very well be retained as a mere matter of
convenience, but it is much to be desired that the two apt designations
proposed by Haeckel--Monogony for asexual, and Amphigony for sexual
reproduction--should come into general use.

Meanwhile it is enough to say that reproduction by 'spores' occurs
normally in Algæ, fungi, mosses, and fern-like plants, and that
there are also animals in which the germ-cells possess the power of
giving rise of themselves to a new individual. But the cases which
I am chiefly thinking of are those of so-called virgin birth or
parthenogenesis, which are not to be compared with multiplication by
spores in regard to their mode of origin; there is a peculiarity in
the origin of this mode of multiplication which I can only make clear
after we have studied the normal forms of what is called 'sexual
reproduction.'

We shall therefore pass on to this mode of reproduction. It is well
known that, in all higher animals, just as in Man, an individual cannot
reproduce by itself; the co-operation of two individuals is necessary,
and these--the male and the female--differ essentially from each other
in many particulars. Their union in the act of procreation induces the
development of a new individual, whether this matures within the mother
in a special receptacle, or whether it is deposited as a 'fertilized
egg,' as in birds, the lower vertebrates, and most 'invertebrates.'

As long as Man has lived he has regarded this process of procreation as
the essential factor in the origin of new individuals, and as he had no
insight into the essence of the process he had necessarily to regard
reproduction as something entirely mysterious, and the co-operation of
the two sexes as a _conditio sine qua non_ of reproduction in general;
thus copulation and reproduction seemed identical.

This was in the main the state of opinion at the time of the discovery
of innumerable minute filaments, the so-called 'spermatozoa' in the
'fertilizing' spermatic fluid of the male. The discovery was made in
1677 by Leeuwenhoek in the case of birds, mammals, and many other
animals. Albrecht von Haller (1708-77) was at first inclined to regard
these spermatozoa as the rudiments of the embryo, but later on, in the
course of his long life, he withdrew this theory, and declared them
to be a kind of parasite in the spermatic fluid without anything to
do with fertilization. The same opinion was expressed in 1835 by K.
E. von Baer, in opposition to the opinion of Prevost and Dumas, who
had rightly interpreted the spermatozoa as the essential elements of
the spermatic fluid. When one follows the matter out in detail, one
finds it almost incredible that such a number of mistakes should have
been made, and so many circuitous paths traversed, before even the
limited knowledge current in the middle of the nineteenth century was
attained--that is to say, enough to give ground for the assertion that
fertilization depends upon the contact of the spermatozoon with the
body of the egg. In 1843 Martin Barry had found the spermatozoa within
the egg-envelope of the rabbit ovum, but it was some time later (1852)
that the investigations of Meissner, Bischoff, and Newport established
the fact that the zoosperm penetrates through the egg-envelope. All
else remained quite obscure, and could not be cleared up as long as it
was believed, on the strength of observations which were in themselves
correct enough, that _several_ zoosperms were always necessary to
fertilize one ovum.

To an understanding of the process even in its most general outlines
there was lacking, apart from technical methods, an appreciation of the
morphological value of the ovum and the spermatozoon. It was necessary
to recognize both ovum and spermatozoon as _cells_ before their union
in fertilization could be regarded as the fusion of two cells, as a
copulation or conjugation of two minute elementary organisms. But this
knowledge only gained ground very gradually, and even in the sixties
opinions on the subject were very much divided. Moreover, there was an
entire absence of knowledge in regard to 'sexual' reproduction among
the lower plants, the Algæ, Fungi, Mosses, and Ferns, as well as of
any detailed acquaintance with the processes of fertilization among
flowering plants. All this had to be elucidated by the labours of many
distinguished observers before it was possible to say so much even as
this, that the process of fertilization depends in general on the union
of two cells.

I need not discuss the whole of this long process of scientific
development, and have only touched upon it because I wished to
emphasize that the conception of the process of fertilization was
for a long time quite erroneous, and has only attained to clearness
in recent times. Pairing as it is seen in the higher animals was for
long regarded as the essential part of the process, and a mysterious
life-awakening influence was assumed in regard to it; and even when it
was understood that not the copulation, but the union of two living
units which was always brought about thereby--the union of the male
and the female germ-cells--was the essence of 'fertilization,' this
was still regarded as a life-awakening process, and the way to a true
understanding of the facts was thus once more blocked.

The simplest form of sexual reproduction in many-celled animals
is found, among others, in the Volvocineæ, those green, spherical,
freshwater cell-colonies which we have already studied in relation to
reproduction by asexual germ-cells. Among them it is the rule that,
after a long series of generations producing only 'asexual' germ-cells,
colonies occur in which each germ-cell is no longer able to develop a
new colony alone, but can do so only after it has united with another
germ-cell.

Now, as we have seen, there are Volvocineæ in which the differentiation
of cells into those of the body (soma) and those concerned with
reproduction has not been established, and all the cells are therefore
alike. In these, as for instance in the genus _Pandorina_ (Fig. 62,
p. 257), when sexual reproduction is to occur the whole colony breaks
up into sixteen cells; these burst forth from the gelatinous matrix
in which they have been hitherto enclosed, swim about in the water
with the help of their two flagella, meet other similar free-swimming
cells and conjugate with these. The two swimming cells come close to
each other, draw in their flagella, sink to the ground in consequence,
and fuse completely both as to the cell-body and the nucleus. They
assume a spherical form, lose the eye-spot, become surrounded with a
tough cell-skin or cyst, and so remain for a longer or shorter time as
so-called 'zygotes' or lasting spores. Then they develop by repeated
cell-division into one of the sixteen-celled _Pandorina_ colonies with
which we are already familiar; this bursts forth from the capsule and
swims freely about in the water again.

Here, therefore, the so-called sexual reproduction depends on the
fusion of two cells similar in appearance, and when this phenomenon
was first known it was regarded as something quite different from the
corresponding reproduction in other multicellular organisms. But we
now know that quite nearly related Volvocineæ belonging to the genus
_Volvox_ and to other genera, which exhibit a differentiation into
body-cells and reproductive cells, may reproduce sexually by means of
two _different_ kinds of germ-cells; and we have also learned through
Goebel and others that even genera like _Pandorina_, which consist of
only one kind of cells, may yet produce male and female reproductive
cells differing essentially in form from one another. In _Eudorina_,
for instance, a gelatinous sphere containing sixteen or thirty-two
individual cells, asexual reproduction occurs in exactly the same way
as in _Pandorina_, that is, each of these cells divides four or five
times in rapid succession, and thus forms a new colony, which then
bursts forth; but when the time for sexual reproduction comes the
colonies behave differently, for some become female and some male.
In the former the cells remain as they were before, but in the male
colonies the sixteen or thirty-two cells undergo a peculiar process
of division, which ends in each becoming a mass (16-32) of so-called
'zoosperms,' that is, minute, narrow, longitudinally elongated cells
with two flagella (Fig. 63 at _D_ shows those of _Volvox_). In
_Eudorina_ they differ from the female germ-cells or ova externally
in form and size, as well as by being much more actively motile, and
they contain green and subsequently yellow colouring matter, and a
red eye-spot. We here find, for the first time among multicellular
organisms, the differentiation of male and female germ-cells; and we
learn from this that the essence of fertilization does not lie in this
differentiation, since it may be absent, but that this distinction
of female and male cells is only of secondary moment. From the fact
that the egg-cells are larger and less active, the 'sperm-cells'
or zoosperms smaller and livelier, we can already anticipate what
will be more definitely established as our knowledge of the facts
increases--that a differentiation according to the principle of
division of labour has taken place even in the germ-cells, and that the
first effect of this is to render the meeting of the cells destined for
conjugation easier and more certain. The much smaller and more slender
zoosperms swim about in the water in clusters until they come in
contact with a female colony; then they separate from each other, bore
their way into the soft jelly of the female colony, and 'fertilize' the
egg-cell, that is to say, each male cell fuses with a female cell and
forms a 'lasting spore,' exactly as in _Pandorina_.

[Illustration: FIG. 63. _Volvox aureus_, after Klein and Schenck.
_A_, besides the small flagellate somatic cells of the colony there
are five large egg-cells (_t_) which are capable of parthenogenetic
development, three recently fertilized egg-cells (_o_) and a number of
male germ-cells (_a_) in process of multiplication. From each of these,
by continued division, a bundle of spermatozoa arises. _B_, a bundle
of thirty-two sperm-cells in process of development, seen from above.
_C_, the same seen from the side. Magnified 687 times. _D_ individual
spermatozoa, magnified 824 times.]

In _Volvox_ the state of matters is similar to that in _Eudorina_;
here again, in addition to the 'asexual' reproduction through the
'Parthenogonidia' which are like egg-cells in appearance (Fig. 63,
_A_, _t_), there are also male and female germ-cells which are usually
produced alternately with the former, but sometimes at the same time,
as in Fig. 63. The egg-cells are large and have no flagella, the
sperm-cells lie together in clusters, and after they are mature (_D_)
they swim freely in the water and then bore into another colony, where
each unites with an egg-cell. The difference between the two kinds of
cells consists therefore in the much greater number, the much smaller
size, and the greater activity of the male cells, and in the smaller
number but much larger size of the female cells--a differentiation
in accordance with the principle of division of labour, depending on
the fact that the two kinds of cells must reach each other, and yet
must contain a certain mass of living protoplasm. While the small size
but large number of male cells, combined with their motility, gives
them an advantage in seeking out and boring into the female cells,
the large size of the latter, on the other hand, makes up for the
loss in mass which the fertilized egg would otherwise suffer from the
diminution in size of the male cell. This difference in size may be
greatly accentuated; thus in one of the brown sea-wracks, for instance,
the spermatozoa are only 5 micro-millimetres in length, while the ova
are spherical and have a diameter of 80-100 micro-millimetres, thus
containing a mass 30-60,000 times greater (Möbius). Fig. 64 shows an
ovum of this species surrounded by spermatozoa.

In the course of the evolution of species this contrast between female
and male germ-cells became more and more marked, not always in the same
direction, however, but in one or another according to the conditions
of fertilization. It would be erroneous to suppose that, with the
higher differentiation of the organism as a whole, the differentiation
of the germ-cells became increasingly complex. On the contrary we find
even among Algæ, as the case of _Fucus_ shows, a marked difference
between the sex-cells, which rather decreases than increases among many
of the higher plants. It is not on the more or less complex structure
of the organism itself that the nature and degree of the dimorphism
of the germ-cells depends, but on the special conditions which are
involved in each case, both in the union of the two kinds of sex-cells
and in the subsequent development of the product of this union, the
'fertilized ovum.'

[Illustration: FIG. 64. _Fucus platycarpus_, brown sea-wrack. _Ei_,
ovum, surrounded by swarming sperm-cells (_sp_). After Schenck.]

Thus it comes about that the male or 'sperm-cells' of the lower plants,
of the lower animals, and, again, of the highest animals are similar
in structure. In all these organisms the male germ-cells exhibit the
minuteness, the form, and the activity of the so-called 'zoosperms'
or 'spermatozoa,' that is to say, they are thread-like, very minute
corpuscles, which move rapidly forwards in water or other fluid with
undulatory movements, and penetrate into the ovum with similar boring
movements when they have been fortunate enough to reach their goal. At
the anterior end they possess a more or less conspicuous thickening,
the so-called 'head' in which the nucleus lies, and this is followed
by the 'tail,' a thread-like structure consisting of cytoplasm which
effects undulatory movements comparable to those of the flagella
of Infusorians and Volvocineæ. The whole spermatozoon is thus a
specialized 'flagellate cell.'

When these 'zoosperms' were recognized as the 'fertilizing elements' in
higher animals, and when 'sperm-threads' had been found, not only in
all mammals and birds, reptiles, amphibians, and fishes, but even in
many 'invertebrates,' the conclusion was suggested that the function
of fertilization might be discharged by this lively motile substance;
for until the eighth decade of the nineteenth century fertilization was
still regarded by many as an 'awakening of life' in the egg. Since life
depends on movement, in truth on infinitely fine molecular movements,
of which the movement of the whole germ-cell from place to place is
only a visible outcome, fertilization was pictured, by a not very
luminous process of reasoning, as the awakening of life in the ovum--in
itself incapable of further life--through the transference to it of
movement through the agency of the zoosperm. Some investigators even
went the length of regarding the ovum as 'dead organic material.'

I mention this at this point, though I do not propose in the meantime
to inquire further into the significance of the conjugation of the
sex-cells. But the view just referred to is so completely refuted even
by the external form of the male germ-cells in many groups of plants
and animals, that I cannot discuss these differences in form without at
the same time indicating the conclusions which they directly suggest.

The great majority of plants and animals exhibit the zoosperm form of
male germ-cells, and this must obviously be interpreted in the light of
the fact that the ova to be fertilized are not generally to be found in
direct proximity to the sperms shed by the male organism, but are at
some distance from them. Among Medusæ and Polyps both male and female
germ-cells are liberated into the water, simultaneously it may be,
but separated from each other by distances of some feet or yards. The
spermatozoa then swim about seeking the ova, which are also floating
freely in the sea, guided by a power of attraction on the part of the
latter--an attraction of whose nature we know nothing, though in the
case of certain fern-ova it has been traced to the secretion of malic
acid (Pfeffer).

The same conditions obtain among Sponges. Here, again, the persons or
stocks are either male or female; the latter produce large delicate
ova, which lie in the interior of the sponge and there await the
fertilizing sperms; the former give off the ripe sperms into the water
in such abundance that thousands and millions of zoosperms burst
forth simultaneously in all directions; these seek about for a female
sponge, penetrate into its canal system, and so ultimately reach the
ova. Of course only a very few of them reach their goal; the greater
number are lost in the water and become the prey of Infusorians,
Radiolarians, or other lowly animals. The fact that enormous numbers
thus miss their true aim shows us why these zoosperms must be produced
in such quantities; it is simply an adaptation to the extraordinarily
high ratio of elimination in these cells, just as the number of young
annually produced by an animal, or of seeds by a plant, is regulated
by natural selection according to the ratio of elimination of the
particular species. The more numerous the descendants which succumb
each time to unfavourable circumstances, to enemies, or to lack of
food, the more prolific must the species be. The same holds true of
the regulation of the number of male germ-cells to be produced by an
individual; there must be so many developed that, in spite of the
unavoidable enormous loss, on an average the number of mature ova
necessary to the maintenance of the species always receive spermatozoa.

Also associated with the prodigal production of zoosperms is their
minuteness, for the more zoosperms that can be developed out of a
given mass of organic substance the smaller they are. Each species
is restricted within definite limits of production by its size and
the volume of its body, and there is thus an advantage in having the
zoosperms of the smallest possible size whenever the chance of the
individual sperm successfully reaching an ovum is very small. In all
such cases nature has abstained from burdening the male germ-cell with
an appreciable contribution of material to the result of conjugation,
that is, to the foundation of the new organism; the passive ovum
contains in itself alone almost all that is necessary to the building
up of the embryo. Fertilization of the ovum by the liberation of the
sperm-cells into the water occurs not only in animals of low degree,
such as Sponges, Medusæ, Star-fishes, Sea-urchins and their relatives,
but also in much higher animals, such as many Fishes and Amphibians,
and in these the male cells have the form of motile threads. But the
spermatozoon-form of male cell does not occur only in animals and
plants which live in the water, or in those which, like mosses and many
vascular plants, are at least occasionally covered by a thin layer of
rain or dew, in which the zoosperms can swim to the ova, it occurs also
in a very large number of animals in which the sperms pass directly
into the body of the female, in those, therefore, in which copulation
takes place.

But even where copulation occurs we find that in most cases, as, for
instance, in Vertebrates, Molluscs, and Insects, the zoosperm-form
is retained. The reason for this is obviously twofold: in the first
place, in many cases the sperms do not directly reach the ovum as a
consequence of copulation, but may have to go a long way within the
body of the female, as in mammals; or even when the way is short and
certain, the ovum may be encased in a firm covering or shell difficult
to penetrate, and the thread-like zoosperm has to face the task of
boring its way through this shell, or slipping in through a minute
opening, the so-called micropyle. In either case it would be difficult
to imagine a form of sperm-cell better adapted to the fulfilment of
this task than that of a thread with a thin, pointed head-portion and
a long motile tail, which enables the zoosperm to twist itself like a
screw through a narrow opening in the egg-envelope, whether the opening
was previously present or not.

We can thus understand why, among insects for instance, the male cells
should always occur in the form of zoosperms, although in this case
they reach a special receptacle in the female reproductive organs,
the 'receptaculum seminis,' and are stored up in this. When a mature
ovum gliding downwards through the oviduct comes to the place where
this receptacle opens into it, the liberation of a few sperm-cells
suffices to fertilize it with certainty, provided that they possess the
thread-like form, which allows them to slip in through the very minute
opening in the egg-envelope. It might be inferred from the certainty
with which the ovum must in this case be found by the spermatozoon
that only a small number of the latter would require to be produced,
and yet even here the number is very large, though not so enormous
as in the sea-urchins and other marine animals, which simply allow
the sperm-cells to escape into the water. The large number in insects
is due to the fact that many of the sperms may miss the micropyle;
and also that in many insects a very large number of eggs have to be
fertilized in succession. In the course of a life lasting three or
four years the queen bee lays many thousand of eggs, most of which are
fertilized, and that from a seminal receptacle which has been filled
only once.

There are, however, other sperm-cells of thread-like form which are
not produced in such enormous multitudes, but in a much more moderate
number, perhaps a few hundreds in the testicle. This is so in the
little Crustaceans, known as Ostracods, all the freshwater species of
which possess zoosperms only moderately numerous and of quite unusual
size.

The comparatively small number is explained by the certainty with which
each of them reaches the ovum, and the large size may be accounted
for in part by the small number which suffices, and which, therefore,
admits of the male cell also carrying a considerable portion of the
material for the building up of the embryo. Probably, however, the
thickness and firmness of the covering of the ovum has something to
do with it, for it has no opening for the entrance of the male cell,
and it is fully hardened by the time fertilization takes place.
Perhaps nowhere can we see more clearly how every little detail of the
structure of the organism is dominated by the principle of adaptation
than in the arrangements for fertilization, and notably in those which
obtain in the Ostracods. I pass by the complicated apparatus for
copulation, since we do not yet understand it fully in all particulars.
According to my own investigations and those of my former students,
Dr. Stuhlmann and Dr. Schwarz, the essential point seems to be that
the colossally large zoosperms, which show no activity within the
body of the male, leave it one at a time, so to speak, in single
file. In copulation they are pressed out singly, one after the other,
through a very fine tube, and then they enter, still singly, through
the reproductive aperture of the female into an equally fine passage
with spiral windings, through which they ultimately reach a roomy
pear-shaped receptacle, the 'receptaculum seminis' of the female. There
they lie in a long band composed of several hundreds, and only now
attain their full maturity by throwing off an outer cuticle--moulting,
so to speak. It is only when they get into a fluid medium that they
show the power of undulatory movement, feeble at first, but gradually
more energetic and more violent. And these movements enable them to
penetrate like gimlets into the calcareous egg-shell. In the normal
course it happens that when a mature ovum is deposited from the opening
of the oviduct, one of the giant zoosperms at the same time, or shortly
afterwards, leaves the 'receptaculum seminis' of the female by way of
the spiral passage, and reaches the exterior just behind the ovum. The
actual process of penetration has not been observed as yet, but the
zoosperm has been seen at a slightly later stage spirally coiled inside
the ovum.

[Illustration: FIG. 65. Copulation in a Daphnid (Lyncæid). Emptying of
the sperm (_sp_) into the brood-chamber of the female (♀). _abd_ ♂, the
abdomen of the male. Magnified 100 times.]

In these Ostracods the sperms are often visible with the naked eye, and
in some species they are twice the length of the animal; they are thus
emphatically giant cells, which can develop a very considerable boring
power.

In respect to the various adaptations of the sperm-cells to the
conditions of fertilization there is hardly any group more interesting
than the water-fleas or Daphnids.

It is amazing how greatly the size of the sperms varies among the
Daphnids, and how it stands in inverse proportion to their number,
and how both are obviously regulated in relation to the difficulties
which stand in the way of each sperm-cell before it can reach the ovum.
In some species the sperm-cells are very large, in others extremely
small. In the genera _Daphnia_, _Lynceus_, and others, copulation
occurs as shown in Fig. 65; the sperm-cells (_sp_) are liberated by
the male into the capacious brood-cavity of the female, which at the
moment is closed to some extent by the abdomen of the male, in reality
closed only partially at the posterior end and at the sides. It seems
inevitable that a large proportion of the male elements should stream
out again and be lost because of the violent movements of both animals.
Accordingly, we find that the sperm-cells are only about the hundredth
part of a millimetre in length and of round or rod-like form, and are
voided in multitudes into the brood-cavity. Fig. 66, _f_, _g_, and
_h_, show such cells in different species, as they occur in the testes
to the number of many thousands. But in all the species in which the
brood-cavity is _closed_, and in which therefore there is not such a
serious loss of sperm-cells, the elements are much larger, and at the
same time less numerous. They are largest and least numerous in species
of genera like _Daphnella_, _Polyphemus_, and _Bythotrephes_, in which
the males have a copulatory organ, so that the possibility of loss
of the male cells is excluded. Thus the round, delicate, and viscid
sperm-cells of _Bythotrephes_ (Fig. 66, _b_) are more than a tenth of a
millimetre in length, but they are developed in proportionately smaller
numbers, so that more than twenty are never found in the testis, and
often only six or eight, while in copulation only from three to five
are ejected. But as there are only two eggs to be fertilized at a time,
and as the male cells are expressed into the brood-cavity directly upon
the eggs, so that they immediately adhere to them, this small number is
amply sufficient.

[Illustration: FIG. 66. Spermatozoa of various Daphnids. _a, Sida._
_b, Bythotrephes._ _c, Daphnella._ _d, Moina paradoxa._ _e, Moina
rectirostris._ _f, Eurycercus lamellatus._ _g, Alonella pygmæa._ _h,
Peracantha truncata._ All magnified 300 times.]

It is remarkable how different the sperm-cells sometimes are in quite
nearly related species of Daphnids, as a glance at Fig. 66 will show;
and, on the other hand, how similar they may be in two species which
belong to different families, like _Bythotrephes longimanus_ (_b_),
and _Daphnella hyalina_ (_c_). The last fact may be explained as an
adaptation to similar conditions of fertilization. Both species have
effective copulatory organs, and their large delicate sperm-cells must
immediately adhere when they come into contact with the shell-less
ovum, and penetrate into it by means of amœboid processes. Conversely,
the difference between sperm-cells of allied species like _Sida
crystallina_ (_a_), _Moina rectirostris_ (_e_) and _M. paradoxa_ (_d_)
is related to different adaptations to nearly the same conditions of
fertilization. In _Sida_ (Fig. 66 _a_) the large flat sperm-cells, with
their fringed ends and their large soft surface, adhere easily to the
ova, and the same end is attained in _Moina rectirostris_ by means of
stiff radiating processes, while in the nearly related species, _Moina
paradoxa_, the male cell (_d_) resembles an Australian boomerang and
presses in like a wedge between the ova and the wall of the brood-sac.

[Illustration: FIG. 67. Spermatozoa of various animals, after
Ballowitz, Kölliker, and vom Rath. 1, man. 2, bat (_Vesperugo_). 3,
pig. 4, rat. 5, bullfinch. 6, newt. 7, skate (_Raja_). 8, beetle. 9,
mole-cricket (_Gryllotalpa_). 10, freshwater snail (_Paludina_). 11,
sea-urchin. Much magnified.]

In Fig. 67 a small selection of animal male cells is figured, all of
which take the form of sperm-threads or spermatozoa, and yet they
differ very much from one another in detail. It would undoubtedly
be of great interest to follow out these minute adaptations of the
sperm-cells to the conditions of fertilization, and to demonstrate
that their size, and especially their form, in the different species
of animals are adjusted to the special constitution of the ovum, its
envelope, and its micropyles, and to the ease or difficulty with which
it can be reached; but much information must be forthcoming before we
can even suggest, for instance, why the sperm-cell of the salamander
is so enormously long, large, and pointed at the head, while that of
Man (Fig. 67, 1) is comparatively short, with broad, flat head and a
recently discovered minute apex; or why those of Man and many fishes
(such as _Cobitis_) should be so much alike, and so on. From many
sides, however, we are led to conclude that even down to the minutest
details nothing is in vain, and that everything depends on adaptation.

In general, even the peculiarities of form already indicate this; thus
the spirally coiled structure of the head, which is especially well
developed in the spermatozoa of birds (Fig. 67, 5), in those of the
skate (7), and of the freshwater snail (_Paludina_) (10), works like a
corkscrew, and makes it possible for the spermatozoon to pierce through
the resistant envelope of the ovum. Similarly, the sharply pointed head
of the insect spermatozoon (Fig. 67, 8 & 9) seems adapted for slipping
through the minute pre-formed micropyle in the hard egg-shell.

Of the detailed and complicated structure of spermatozoa we have only
recently been made aware through the increasing perfection of the
microscope and of technical methods of investigation. Fig. 68 shows
one after a diagrammatic figure by Wilson. We see the apical point
(_sp_) for boring into the ovum, the nucleus (_n_) surrounded by a thin
layer of protoplasm, which together form the head, then the middle
portion (_m_) which contains the 'centrosome' (_c_), and the 'tail' or
'flagellum' which effects the movement of the whole and which itself
possesses a complex structure with an 'axial filament' (_ax_) and an
enveloping layer, the latter often drawn out into a spirally twisted,
undulating membrane of the most extreme delicacy, as is most clearly
seen in the newt (Fig. 67, 6).

[Illustration: FIG. 68. Diagram of a spermatozoon, after Wilson. _sp_,
apical point. _n_, nucleus. _c_, centrospere. _m_, middle piece. _ax_,
axial filament. _e_, terminal filament.]

Not in the Daphnids alone, but in other groups of Crustaceans as
well, sperm-cells of quite peculiar form occur, as, for instance, in
the crayfish and its marine relatives, the crabs and the long-tailed
Decapods. In these cases the spermatozoa bear long and stiff thorn-like
processes, which, as in the sperm-cells of _Moina_, make them adhesive,
and, according to Brandes, render it possible for them to cling among
the bristles on the abdomen of the female until one of the many eggs
leaving the oviduct comes within reach. For among these Crustacea
there is no true copulation, but the masses of sperm-cells are packed
together into sperm-packets or 'spermatophores,' and are affixed by the
male near the opening of the oviduct, where they burst and pour forth
their contents between the appendages of the female.

All these remarkable and widely divergent structures and arrangements
depend not upon chance or on the fantastic expression of a 'formative
power,' as an earlier generation was wont to phrase it; they are
undoubtedly without exception adaptations to the most intimate
conditions of fertilization in each individual case. I lay particular
stress upon a recognition of this, because it permits us to infer with
certainty that even the variations of the single cell, if they are
sufficiently important for the species, may be controlled by natural
selection. It is obvious that the adaptations of the sex-cells must
depend not on histonal selection, but only upon personal selection,
since it is indifferent for the individual sperm-cells whether
fertilization is accomplished successfully or not, while it is by
no means indifferent for the species. The organism dies without
descendants if its sperm-cells do not fertilize, and the carrying on
of the species must be left to those of its fellows which produced
sperm-cells which fertilize with more certainty; thus it is not
the sperm-cells themselves, but the individual organisms which are
selected, and that in relation to the quality of the sex-cells they
produce.

In contrast with the great diversity of form exhibited by the
spermatozoa, the differentiation of the ovum appears very uniform,
at least in regard to form and activity. The main form is spherical,
but it is subject to many variations in the way of elongation or
flattening. In the lower forms of life, as, for instance, among the
sponges, and also in the polyps and Medusæ the egg-cells possess, until
they are mature, the locomotor capacity of unicellular organisms;
they creep about after the manner of amœbæ, and indeed, as I showed
years ago, this movement from place to place in many polyps is exactly
regulated; thus at a definite time they may leave the place where they
originated and may, for instance, creep from the outer layer of cells
(ectoderm) of the animal into the inner layer (endoderm) by boring
through the so-called 'supporting lamella,' then they may creep further
in the endoderm, and finally return to quite definite and often remote
spots in the ectoderm (_Eudendrium_, Fig. 95). In another hydroid polyp
(_Corydendrium parasiticum_) the mature egg-cells leave their former
position within the endoderm and creep entirely outside of the animal
which produced them, establishing themselves in a definite spot on its
external surface, where they await the fertilizing zoosperms. Many ova
can accomplish slight amœboid movements, but in most animals these
do not suffice for movement from place to place, and the ova remain
quietly in the spot where they were developed, or are passively pushed
to another. Cases such as that of the polyp I have cited, in which the
ovum actually comes to meet the male element, are quite exceptional,
for in general the ovum is the passive and the spermatozoon the active
or exploring element in fertilization. The female cell is entrusted
with procuring and storing the material necessary to the building up of
the embryo; and its peculiarities depend chiefly on this.

[Illustration: FIG. 69. Ovum of the Sea-urchin, _Toxopneustes lividus_,
after Wilson. _zk_, cell-body. _k_, nucleus or so-called 'germinal
vesicle,' _n_, nucleolus or so-called 'germinal spot.' Below there is
a spermatozoon of the same animal, with the same magnification (750
times).]

It is true that in plants this stored material is seldom considerable,
and that is because the ovum so frequently remains even after
fertilization within the living tissues of the plant, and is thence
supplied, often very abundantly, with food-stuffs; and, moreover,
because the young plant that springs from the fertilized ovum maybe
very small and simple, and yet capable of immediately procuring its
own nourishment. But there are exceptions to this; thus the ova of the
brown sea-wracks, or Fucaceæ, for instance, are quite twenty times
as large as the ordinary cells of the algæ (Fig. 64), and contain a
quantity of food-stuff within themselves. In this case the ova are
liberated into the water even before fertilization, and the nutrition
of the embryo from the mother-plant is excluded.

In these Algæ we meet, for the first time, with a special organ in
which the ova arise. In animals this is much more generally the case,
and from sponges upwards there are always quite definite parts and
tissues of the body which are alone able to develop eggs, and these
are usually well-defined organs of special structure, the ovaries.
Similarly, in male animals the spermatozoa arise in special places, and
usually in special organs, the spermaries or testes.

Animal ova often consist of more than the simple cell-body, the
protoplasm and its nucleus; they almost always contain in the cell-body
a so-called 'Deutoplasm,' as Van Beneden has fittingly named the
yolk-substance. This consists of fats, carbohydrates, or albuminoids,
which often lie in the cell-body in the form of spherules, flakes, or
grains--a nutritive material that is often surrounded and enclosed
by a small quantity of living matter or formative protoplasm. Apart
from these stores of yolk it would be impossible for a young animal
to develop from the ovum of a snake or a bird, for such highly
differentiated animals could not be formed from an egg of microscopic
dimensions if this remained without some supply of food from outside of
itself during the period of development. There is obviously need for a
considerable amount of building material, so that all the organs and
parts, which are composed of thousands and millions of cells, may be
developed.

Thus the size of the animal-ovum depends essentially on the quantity of
yolk that has to be supplied to the egg, and this depends in the main
on whether the egg is still drawing nourishment from the mother during
the development of the young animal. Therefore, as a general rule,
eggs which are laid, and are surrounded and protected by a shell, are
usually much larger than the eggs of animals which go through their
development within the body of the mother. The best known illustration
of this proposition is offered by mammals and birds, animals of equally
high organization and comparable in bodily size. While the eggs of
birds may be as much as 15 centimetres in length, and may weigh 1½
kilogrammes, those of most mammals remain microscopically minute, and
scarcely exceed a length of 0.3 millimetres. The same principle is
often illustrated within one and the same small group of animals, and
even in the same species. Here, again, the Daphnids or water-fleas may
serve as an example.

Among these there are two kinds of eggs, summer and winter eggs, of
which the former go through their development into a young animal
within a brood-cavity on the back of the female, while the others are
liberated into the water, and are surrounded by a hard shell. The
summer eggs receive more or less nourishment from the mother by the
extravasation of the nutritive constituents of the blood into the
brood-cavity, and they thus require a smaller provision of yolk than
the winter eggs, which are thrown entirely upon their own resources.
Accordingly we find that in all Daphnids the summer eggs are at least
a little smaller and have less yolk than the winter eggs, as in the
genus _Daphnella_ (Fig. 70, _A_ and _B_), while in some species, e.g.
of _Bythotrephes_, this difference increases so much that the summer
eggs are almost without yolk, and therefore very minute (Fig. 71, _B_).
The reason of this lies in the fact that in this case the brood-sac
is filled with a nutritive fluid rich in albuminoid substances, so
that the embryo during its development is continually supplied with
concentrated nourishment. This is not the case with the winter eggs,
because these are liberated into the water, and we therefore find that
they are of enormous size and quite filled with yolk (Fig. 71, _A_).

[Illustration: FIG. 70. _Daphnella._ _A_, summer egg. _B_, winter egg.
_Oe_, 'oil-globules' of the summer egg.]

[Illustration: FIG. 71. _Bythotrephes longimanus._ _A_, the brood-sac
(_Br_) of the female containing two winter-ova (_Wei_), on which
five large sperm-cells (_sp_) are lying. _R_, dorsal surface of the
animal. _Dr_, glandular layer which secretes the shell-substance. _BK_,
copulatory canal. _B_, the brood-sac (_Br_) containing two summer-ova
(_Sei_). Both figures under the same magnification (100).]

In this instance, as in all the simpler eggs, the yolk constituents
are secretions of the cell-body of the ovum; but nature employs many
devices, if I may so speak, to bring up the mass of the egg, and
especially of the yolk, to the highest attainable point. Thus in many
orders of Crustaceans, for instance in the water-fleas just mentioned,
there are special egg-nourishing cells, that is, young ovum-cells
which do not differ from the rest either in origin or in appearance,
only they do not become mature eggs, but at a definite time cease to
make progress, and then slowly break up, so that their substance may
be absorbed as food by the true ova. Thus there is a much greater and
at the same time more rapid growth than could be attained through
nourishment from the blood alone. In the Daphnids the ovaries consist
of groups of four cells each, only one of which becomes an ovum (Fig.
72, _Ei_), while the other three (1, 2, and 4) form nutritive cells
which break up. This is so in all summer eggs; but in the winter eggs
a much larger number of nutritive cells may take part in equipping a
single ovum, and in the genus _Moina_ over forty do so. But here the
difference in size between the two kinds of eggs is very marked, the
winter eggs being twice the diameter of the summer eggs.

[Illustration: FIG. 72. _Sida crystallina_, a Daphnid: a fragment of
the ovary showing one of the groups of four cells, of which 1, 2, and 4
are nutritive cells, and only 3 becomes an ovum. Magnified 300 times.]

In many insects also, e.g. in beetles and bees, similar nutritive cells
occur, but there is in these forms a different arrangement which serves
at the same time for the formation of the shell, and the supplying to
the ovum of the necessary yolk-stuffs--the ovum is surrounded with a
dense layer of epithelial cells, a so-called 'follicle.' In mammals and
birds also these 'follicle cells' certainly play an important part in
the nutrition of the ovum, though it is not yet quite clear how they
act--whether they produce within themselves grains of yolk and other
nutritive substances and convey these to the ovum by means of fine
radiating processes, or whether they themselves ultimately migrate into
the ovum and there break up. In any case it is worthy of note that
all these follicular cells in insects and vertebrates have the same
origin as the egg-cells, that is, they are modified germ-cells. The
case is therefore essentially the same as in the nutritive cells of the
Daphnids; nature sacrifices the greater number of the germ-cells in
order to be able to provide more abundantly for the minority. She thus
succeeds in raising the egg beyond itself, so to speak, and provides
the means for a growth which could obviously not be attained by means
of the ordinary nourishment supplied by the blood.

[Illustration: FIG. 73. Diagrammatic longitudinal section of a hen's
egg before incubation, after Allen Thomson. _Bl_, germinal disk. _GD_,
yellow yolk. _WD_, white yolk. _DM_, vitelline membrane. _EW_, albumen.
_Ch_, chalaza. _S_, shell membrane. _KS_, shell. _LR_, air chamber.]

We now understand why the eggs of many animals should be of such
enormous size and often of such complex structure. The eggs of birds
are especially remarkable in this respect, and it has till recently
been disputed whether they are really morphologically equivalent to
a single cell. But this is undoubtedly the case, and though only the
small thin germinal disk (Fig. 73, _Bl_) with its nucleus is the active
part of this cell--the cell-body proper--yet all the rest--the enormous
sphere of yolk with its regular layers of yellow (_GD_) and white
(_WD_) yolk, the concentric layers of fluid albumen (_EW_) round about
this, the chalazæ (_Ch_), and finally, the delicate shell membrane
(_S_) and the limy shell (_KS_)--belong to this cell, and have arisen
in connexion with it (Fig. 73).




LECTURE XV

THE PROCESS OF FERTILIZATION

 Cell-division and nuclear division--The chromatin as the material
 basis of inheritance--The rôle of the centrosphere in the mechanism
 of division--The Chromosomes--Fertilization of the egg of the
 sea-urchin according to Hertwig--Of the egg of Ascaris according
 to Van Beneden--The directive divisions, or the extrusion of the
 polar bodies--Halving of the number of chromosomes--The same in
 the sperm-cell--Reducing division in parthenogenetic eggs--In
 the bee--Exceptional and artificial parthenogenesis--Rôle of the
 centrosphere in fertilization and in parthenogenesis.


NOW that we have made ourselves acquainted with the two kinds of
germ-cells on the union of which 'sexual reproduction' depends, we may
proceed to a more detailed discussion of the process of fertilization
itself. But it is indispensable that we should take account of the
processes of nuclear and cell-division, as these have been gradually
recognized and understood in the course of the last decade. It may
appear strange that the processes of division should throw light on the
apparently opposite processes of cell-union, but it is the case, and
no understanding of the latter is possible without a knowledge of the
former.

From the time of the discovery of the cell until well on in the sixties
the process of cell-division was looked on as a perfectly simple
process, as a mere constriction in the middle of the cell. It was
observed that a cell in the act of dividing (Fig. 59, _A_) stretched
itself out, that its nucleus also became longer, became thinner in the
middle, assumed a dumb-bell form, and was then gradually constricted,
giving rise to two nuclei (_B_), whereupon the body of the cell also
constricted and the two daughter-cells were formed (_C_). In certain
worn-out or highly differentiated cells a cell-division of this kind
really seems to occur--the so-called 'direct' division--but in young
cells, and indeed in all vigorous cells, the process, which looks
simple, is, in reality, exceedingly complex. Not only is the structure
of the nucleus incomparably more complex than was recognized a quarter
of a century ago, but nature has placed within the cell a special and
marvellously intricate apparatus, by means of which the component parts
of the nucleus are divided between the two daughter-nuclei.

For a long time all that was distinguished in the cell-nucleus was
the nuclear membrane and a fluid content in which one or more nuclear
bodies or nucleoli float. But this does not by any means exhaust
what can now be recognized in the structure of the nucleus, and the
most important constituents are not even among these, for recent
researches, especially those of Häcker, have shown that the nucleolus
or the nucleoli, to which there was formerly an inclination to attach
a very high importance, must be regarded as only transient formations
and not living elements--in fact, as mere collections of organic
substance--'bye-products of the metabolism,' which at a definite time,
that is just before the division of the nucleus, disappear from the
nuclear space and are used up. We now know that in the resting cell,
that is, in the cell which is not in the act of dividing (Fig. 74,
_A_), a very fine network of pale threads, often very difficult to
make visible, fills the whole nuclear cavity, like a spider's web or
the finest soap bubbles, and that in this so-called nuclear framework
there are embedded granules of rounded or angular form (_A_, _chr_)
which consist of a substance which stains deeply with such pigments as
carmine, hæmatoxylin, all aniline dyes, &c., and which has therefore
received the name of chromatin. Often, indeed generally, these granules
are exceedingly small, but sometimes they are bigger, and in that case
they are less numerous and more easily made visible; in all cases,
however, they are in a certain sense the most important part of the
nucleus, for we must assume that it is their influence which determines
the nature of the cell, which, so to speak, impresses it with the
specific stamp, and makes the young cell a muscle-cell or a nerve-cell,
which even gives the germ-cell the power of producing, by continued
multiplication through division, a whole multicellular organism of a
particular structure and definite differentiation, in short, a new
individual of the particular species to which the parents belong. We
call the substance of which these chromatin granules consist by the
name first introduced into science by Nägeli, though only to designate
a postulated substance which had not at that time been observed,
but which he imagined to be contained within the cell-body--by the
name _Idioplasm_, that is to say, a living substance determining the
individual nature (εἶδος = form). I am anticipating here, and I reserve
a more detailed explanation until I can gradually bring together all
the facts which justify the conception I have just indicated of the
'chromatin grains' as an 'idioplasm,' or, as we may also call it, a
'hereditary substance.'

That this chromatin must be something quite special we see from the
processes of cell and nuclear division, which I shall now briefly
describe.

[Illustration: FIG. 74. Diagram of nuclear division, adapted from E.
B. Wilson. _A_, resting cell with cell-substance (_zk_), centrosphere
(_csph_) which contains two centrosomes, nucleolus (_kk_); and
chromosomes (_chr_), the last distributed in the nuclear reticulum.
_B_, the chromatin united in a coiled thread; the centrosphere divided
into two and giving off rays which unite the halves. _C_, the nuclear
spindle (_ksp_) formed, the rays more strongly developed, the nuclear
membrane (_km_) in process of dissolution, the chromatin thread
divided into eight similar pieces (_chrs_), the rays are attaching
themselves to the chromosomes. _D_, perfected nuclear spindle with
the two centrospheres at the poles (_csph_) and the eight chromosomes
(_chrs_) in the equator of the spindle, all now longitudinally split.
_E_, daughter-chromosomes diverging from one another, but still united
by filaments, the centrosomes (_cs_) are already doubled for the next
division. _F_, daughter-chromosomes, quite separated from one another,
are already beginning to give off processes; the cell-substance is
beginning to be constricted. _G_, end of the process of division:
two daughter-cells (_tz_) with similar nuclear reticulum (_tk_) and
centrospheres (_csph_), as in _A_.]

When a cell is on the eve of dividing we observe first that the
chromatin grains, which have till then been scattered throughout the
network of the nucleus, approach each other and arrange themselves
into a long thin thread which, irregularly intertwined, forms a loose
skein, the so-called coil-stage (Fig. 74, _B_). The thread then begins
to thicken, and somewhat later it can be seen to have broken up into
a number of pieces of equal length, as if it had been cut into equal
pieces with scissors (_C_).

These pieces or chromosomes become shorter by slowly contracting,
and thus each takes the form of an angular loop, a straight rod, or
a roundish, oval, or spherical body (Fig. 74, _C_, _chrs_). While
this is happening, we can see at the side of the nucleus, and closely
apposed to it, a pale longitudinally striped figure with a swelling,
similar to a handle, at both ends--the so-called nuclear spindle or
central spindle (_ksp_). This is the apparatus for the division of the
nucleus, and it was previously represented by a small body susceptible
to certain stains--the centrosome, which was surrounded by a halo-like
zone, the centrosphere or 'sphere.' This body was long overlooked,
but now the majority of investigators assume that, though it is often
inconspicuous and very difficult to make visible, it is nevertheless
present in every cell which is capable of division, and that it is
therefore a permanent and indispensable constituent of the cell (Fig.
74, _A_ and _B_, _csph_).

When a cell is on the point of dividing, this remarkable cell-organ,
which has hitherto seemed no more than an insignificant, pale, little
sphere, now becomes active. First of all, often before the formation
of the chromatin coil, it doubles by division (_A_ and _B_ _csph_), at
first only as regards the centrosome, and then as regards the sphere
also (_B_); and while division is going on fine protoplasmic filaments
issue from the dividing sphere and radiate like rays from a sun into
the cell-substance. As they only retain their connexion with each other
at the surfaces of the dividing halves of the sphere which are turned
towards each other, we might almost say that fine threads are drawn out
between the two halves as they separate, and these become longer the
further apart the halves diverge. In this manner the much-talked-of
'spindle figure' arises, which was first described in the seventies
through the researches of A. Schneider, Auerbach, and Bütschli, but the
significance and origin of which have claimed the labours of many later
investigators down to our own day.

The processes now to be described do not always take place in exactly
the same manner, but the gist of the business is everywhere the same,
and it consists in this, that the two ends or 'poles' of the spindle
diverge further and further apart, and between them lies the nucleus
whose membrane now disappears (_C_, _km_) while the spindle threads
traverse its interior. Sometimes the membrane is retained, but
nevertheless the spindle threads penetrate into the interior of the
nucleus. But the chromosomes always range themselves quite regularly
in the 'equatorial plane' of the spindle (_D_, _aeq_)--a process the
precise mechanism of which is by no means clearly understood, and
indeed the play of the forces in the whole process of nuclear division
is still very imperfectly revealed to our intelligence.

Thus we have now before us a pale, spindle-shaped figure, which takes
only a faint stain, with the 'suns' (_cs_) at its 'poles,' and in its
equatorial plane the loop- or rod-shaped, or spherical chromosomes
(_chrs_). The whole is designated the 'karyokinetic,' the 'mitotic,' or
the 'nuclear division figure.'

The meaning and importance of this, at first sight, puzzling figure
will at once become clear from what follows. It may be observed at this
stage, if not even long before, that each of the chromatin rods or
loops has split along its whole length like a log of wood, and that the
split halves are beginning slowly and hardly noticeably to move away
from each other, one half towards one, the other towards the other pole
of the spindle (Fig. _D_ and _F_). Directly in front of the centrosome
they make a halt, and now the material for the two daughter-nuclei
is in its proper place (_F_, _chrs_). These develop quickly, each
chromosome group surrounding itself with a nuclear membrane (Fig.
_G_) within which the chromosomes gradually become transformed again
into a nuclear network. Within the chromatin substance proper this is
scattered about in small roundish or angular granules, lying especially
at the intersecting points of the network. It may be stated at once,
though the full significance of the statement can only be appreciated
later, that we may assume with probability that this breaking up of the
chromosomes is only apparent, and that these rods or spheres really
continue to exist in the nuclear network, only in a different form,
greatly spread out, somewhat after the manner of a Rhizopod which
stretches out fine processes in all directions. These processes branch
and anastomose, so that the body, which previously seemed compact,
now appears as a fine network. In point of fact, it can be directly
observed that the chromosomes, after the nucleus is completely divided
into two daughter-nuclei, send out pointed processes (_F_ and _G_)
which gradually increase in length and branch, while the body of the
chromosome itself becomes gradually smaller. It is thus probable
that, when such a daughter-nucleus is on the point of dividing anew,
it may, by a drawing together of the processes or pseudopodia of the
chromosomes, produce the same rods or spheres as those which previously
gave rise to the network. More definite reasons for this interpretation
will be adduced later on. In any case, the chromosomes, even in
their compact rod-like state, consist of two kinds of substance,
the chromatin proper, which stains deeply, and the linin, which is
difficult to stain; and it is the latter which, by breaking up, forms
the pale part of the nuclear network.

Thus we can understand that the number of chromosomes remains the same
in every cell-generation throughout development, as it is the same
in all the individuals of a species. The numbers are known for many
species: in some worms there are only two or four chromosomes, while
in other related worms there are eight; in the grasshopper there are
twelve, and in a marine worm, _Sagitta_, eighteen; in the mouse, the
trout, and the lily there are twenty-four; in some snails thirty-two;
in the sharks thirty-six, and in _Artemia_, a little salt-water
crustacean, 168 chromosomes. In Man the chromosomes are so small that
their normal number is not certain--sixteen have been counted. This
counting can only be done during the process of nuclear division, for
afterwards the chromosomes flow indistinguishably together, or rather
apart, only to reappear, however, in the old form and number whenever
the nucleus again begins to divide.

It remains to be told what becomes of the centrosphere in
cell-division. As soon as the formation of the daughter-nuclei has
been brought about by the divergence of the split halves of the loops,
the spindle figure begins to retrograde, its threads become pale and
gradually disappear, as does the whole radiate halo of the centrosphere
(Fig. _F_ and _G_). The cell-body has by this time also divided in the
equatorial plane of the nuclear spindle, and the centrosome remains
usually as a very inconspicuous pale body lying in the cytoplasm close
to the nucleus, reawakening to renewed activity when cell-division is
about to recommence (_G_, _csph_).

These, briefly, are the remarkable processes of nuclear division. Their
net result is obvious; the chromatin substance is divided between the
daughter-nuclei with the greatest conceivable accuracy.

It is not so easy to understand the mechanism of this partition, and
there are various divergent theories on this point. According to the
older idea of Van Beneden, the spindle fibres work like muscles, and
by contracting draw the halves of the chromosomes which adhere to them
towards the pole, while the rest of the fibres radiating out from
the polar corpuscles act as resisting and supporting elements. This
view, with many modifications however, has still its champions, and M.
Heidenhain in particular has made a notable attempt to establish it
and to work it out in detail. Opposed to it stand the views of those
who, like O. Hertwig, Bütschli, Häcker, and others regard the rays not
as specific elements which were pre-formed in the cell, but as the
expression of the orientation of certain protoplasmic particles--an
orientation evoked by forces which have their seat within the central
corpuscles, and act in the manner of magnetic or electric forces. That
the central corpuscles are centres of attraction seems to me hardly
open to doubt, and I cannot regard the regular arrangement of the
chromosomes in the equatorial plane of the spindle as due to a mere
adhesion to contractile threads. Some still unknown forces--chemotactic
or otherwise--must be at work here. Later on we shall study the
phenomenon of the migration of the sperm-nucleus into the ovum, when it
is accompanied by its central body and its halo of rays. Häcker seems
to me justified in inferring from this phenomenon alone that the sudden
origin of the rays is due to forces resident in the central corpuscle.
But undoubtedly even this 'dynamic' explanation of karyokinesis is
still only at the stage of hypothesis and reasoning from analogy, and
is far removed from a definite knowledge of the forces at work.

For the problems with which we are here chiefly concerned, the problems
of heredity, it is enough to know that the cells of multicellular
organisms possess an extremely complex apparatus for division, whose
chief importance lies in the fact that through it the chromatin units
of the nucleus are divided into precisely equal parts, and so separated
from each other that one half forms one daughter-nucleus, the other
half the other. It is not merely that there is an exact division of the
whole chromatin in the mass, which could have been effected much more
simply, but that there is _a regulated distribution of the different
qualities of the chromatin_, as we shall see later.

It must here be emphasized that the splitting of the chromosomes does
not depend on external forces, but on internal ones involved in their
organization, and in the definite attractions and repulsions of their
component particles which come about in the course of growth. The
chromosomes do not split like a trunk that has been broken open with
an axe, but rather like a tree burst apart by the frost, that is, by
the freezing of the water within itself. I consider it very important
that we should recognize this, even though we do not yet know what
the forces are that have control in this case, because it leads us to
conclude that the structure of the chromosomes is extremely complex,
that they are, so to speak, a world in themselves, that they possess an
infinitely complex and delicate though invisible organization, in which
intrinsic chemico-physical forces produce the regulated succession
of changes which we observe. We shall afterwards see that we are led
to the same conclusion from another direction--that is, from the
phenomena of inheritance. We shall then recognize that the rod- or
loop-shaped chromosomes cannot be simple elements, but are composed
of linear series of 10, 20, or more globular single-chromosomes, each
of which represents a particular kind of chromatin or hereditary
substance. If we consider this carefully, we shall see that it would
hardly be possible to think out a mode of nuclear division which would
so exactly and securely fulfil the purpose of conveying these many
kinds of chromatin to the two daughter-nuclei in like proportions as
does the mechanism of distribution actually brought about by nature.
The longitudinal splitting of the rods halves the chromosomes, and the
spindle apparatus secures the proper distribution of the halves between
the two daughter-nuclei.

So much, at least, is certain, that no such complicated mechanism for
'mitotic' division would have arisen if the very precise division of
a substance _of the highest importance_ had not been concerned, and
in this conclusion lies the first hint of the interpretation of the
chromatin substance as the bearer of the hereditary qualities.

We are now familiar with the cell-nucleus and the apparatus for
its division, and we are thus fully prepared to begin the study of
the phenomena of 'fertilization.' Here also the processes depend
essentially on the behaviour of the cell-nuclei, for even the first
observations made by O. Hertwig on the behaviour of the spermatozoon
after it has penetrated into the ovum led to the suggestion that the
essential fact is the union of two nuclei; and numerous later, more and
more deeply penetrating researches have furnished abundant evidence
that the so-called 'fertilization' _is essentially a nuclear fusion_.

Let us begin with O. Hertwig's observations on the ovum of the
sea-urchin. Eggs of this animal, which have been taken out of the ovary
of the female, may easily be fertilized artificially by pouring over
them spermatic fluid taken from a male, and diluted with sea-water.
Before this is done only one nucleus can be observed in the ovum, but
shortly afterwards two nucleus-like structures of unequal size can
be seen within the ovum, and the smaller is surrounded by a circle
of rays. Hertwig rightly interpreted this smaller nucleus as the
modified remains of the penetrating spermatozoon, which then slowly
approaches the nucleus of the egg, and ultimately fuses with it to form
a 'segmentation nucleus.' From this starts the so-called 'segmentation'
of the ovum, that is, the series of repeated divisions resulting in the
formation of an ordered mass of cells, which by continued division of
cells builds up the embryo.

Simple as this process of nuclear conjugation may seem, it was by no
means so easy to recognize, and several investigators, especially
Auerbach, Schneider, and Bütschli, had seen stages of the process at
an earlier date without arriving at the true interpretation of the
phenomena. This was chiefly due to the fact that, in addition to the
phenomena of fertilization proper, which we have briefly sketched,
other nuclear changes take place in the maturing ovum, and these are
not very easy to distinguish from the former; we refer to the phenomena
of the so-called 'maturation of the ovum.' When the ovum-cell has
attained its full size within the ovary it is not yet capable of being
fertilized, but must first undergo two processes of division, to the
right understanding of which Hertwig's investigations, and afterwards
those of Fol, have contributed much.

For a long time it had been a familiar observation that small
refractive corpuscles were extruded from one pole of the ovum shortly
before the beginning of embryonic development. These were called 'polar
bodies,' because it was believed that they marked the place which would
afterwards be intersected by the first plane of division; it was only
known at that time that they had to be extruded from the egg, but no
one had the remotest idea of their real nature.

We now know that they are cells, and that their origin depends on a
twice repeated division of the egg-cell; but it is a very unequal
division, for these 'directive cells' or 'polar bodies' are always much
smaller than the ovum, and indeed are usually so small that it is easy
to understand why their cellular nature was for so long overlooked.
Yet they have always a cell-body, and in many ova, for instance those
of certain marine Nudibranchs, this is quite considerable; and they
have likewise always a nucleus, which, notwithstanding the smallness of
the cell-body, is in all cases exactly of the same size as the sister
nucleus which remains behind in the ovum after division--a fact which
is in itself enough to indicate that we have here to do essentially
with readjustments and changes in the nucleus of the ovum.

Long before the polar or directive divisions were recognized as
divisions of the egg-cell it was known that the nucleus of the ovum
disappeared as soon as the latter attained to its full size within
the ovary. It was also known that this nucleus--the large so-called
'germinal vesicle' lying in the middle of the ovum--left its central
position and moved to the upper surface of the ovum, there to become
paler and paler, and ultimately to disappear altogether from the
sight of the observer. By many it was believed that it broke up, and
that the 'segmentation nucleus,' which is afterwards obvious, is a
new formation. The truth is that the germinal vesicle, at the time
of its disappearance, is transformed into a division figure which
is invisible without the aid of artificial staining. The nuclear
membrane breaks up; the centrosome of the ovum, which, although hardly
visible, had previously lain beside the germinal vesicle, divides
into two centrosomes and their centrospheres, and these now form the
'mitotic figure' by moving away from each other and sending out their
protoplasmic rays. This nuclear spindle soon ranges itself at right
angles to the surface of the egg, which at the same time arches itself
into a protuberance, and soon two daughter-nuclei are formed, one of
them lying within the protuberance (Fig. 75, _A_, _Rk1_). This soon
separates itself off from the ovum, surrounded by a small quantity
of cell-substance. The other daughter-nucleus remains within the
ovum, but neither of them remains in a state of rest; both are again
transformed into a spindle and divide once more; the minute first
'polar body' dividing into two 'secondary polar bodies' of half the
size (_B_, _Rk1_), while the nuclear spindle within the egg brings
about a second division of the ovum (_B_, _Rk2_) whose unequal products
are the second polar cell and the mature ovum--that is, the ovum ready
for fertilization. The process is now complete; the egg-cell, which
has lost very little plasmic material through the 'polar bodies' and
has not become visibly smaller, has now a nucleus (_B_, _Eik_) which
has become considerably smaller through the two rapidly successive
divisions, and, as we shall see later, has also undergone internal
changes. In this state it is 'ripe,' that is, it is ready to enter into
conjugation with the nucleus of a male cell, and this we have already
recognized as the essential element in the process of fertilization.

These processes of 'maturation of the ovum' are common to all animal
ova which require fertilization, and they follow almost the same
course, only that in many cases the second division of the first
polar body does not take place, so that only two polar bodies in all
are formed. All these processes have nothing directly to do with
fertilization, but it is only through them that the ovum becomes
capable of fertilization. This does not prevent the spermatozoon from
previously making its way into the ovum, for this is usually the case
(Fig. 75, _A_, _sp_); there it waits until the second 'directive
division' of the ovum has been accomplished, utilizing the time to
become transformed in the manner necessary for the conjugation of the
two nuclei. Only in a few species, for example in the sea-urchin, does
the egg complete its polar divisions within the ovary, therefore before
it has come into contact with the sperm at all.

[Illustration: FIG. 75. Process of fertilization in _Ascaris
megalocephala_, the thread-worm of the horse, adapted from Boveri and
Van Beneden. _A_, ovum in process of the first directive division;
_Rk_ 1, first polar body; _sp_, spermatozoon with two chromosomes in
its nucleus, attaching itself to the ovum, and about to penetrate
into it; a protrusion of the egg-protoplasm is meeting it. _B_, the
second directive division has been completed; _Rk2_, the second polar
body; _Eik_, the reduced nucleus of the ovum. The first polar body
(_Rk_ 1) has divided into two daughter-cells, _spk_; the nucleus of
the spermatozoon remains visible with its two centrospheres (_csph_).
_C_, the sperm nucleus (♂_k_) and the ovum nucleus (♀_k_) have grown,
each has two loop-like chromosomes; only the male nucleus has a
centrosphere, which has already divided into two (_csph_). _D_, the two
nuclei lie apposed between the poles of the nuclear spindle. _E_, the
four chromosomes have split longitudinally; the spindle for the first
division of the ovum (the segmentation spindle, _fsp_) has been formed.
_F_, divergence of the daughter-chromosomes towards the two poles;
division of the ovum into the first two cleavage cells or embryonic
cells.]

That we may be able to penetrate still more deeply into the processes
of fertilization, the best illustration to take seems to me to
be, as yet, the ovum of the thread-worm of the horse (_Ascaris_
_megalocephala_), which has become famous through the classical
observations of Ed. van Beneden. Many favourable circumstances
unite in this case to make the essentials of the process clearly
recognizable. Fertilization takes place within the body of the female,
in an enlarged portion of the oviduct, within which a number of the
remarkable sperm-cells are always found in a mature female. They are
remarkable in being not thread-like, but rather spheroidal cells,
bearing, however, a small protuberance something like a pointed horn
(Fig. 75, _A_, _sp_). When such a sperm-cell comes in contact with the
upper surface of an ovum a swelling forms at the place touched, and the
sperm-cell attaches itself firmly to this, and is drawn by it into the
ovum. Without doubt, amœboid movements on the part of the sperm-cell
itself play some part in this, as can be most plainly seen in the
large sperm-cells of many Daphnids which we have already discussed. In
the egg of the thread-worm the whole sperm-cell with its nucleus can
soon be detected within the substance of the ovum, and it then changes
rapidly. Its main body fades more and more completely, until at last
it disappears altogether, while the nucleus becomes vesicle-like and
soon attains a considerable size (Fig. 75, _B_, _spk_). Meanwhile the
residue of the germinal vesicle which remained behind in the ovum after
the second directive division (_B_, _Eik_) has changed into a large
vesicle-like nucleus (_C_, ♀ _k_), which in the ovum of _Ascaris_, as
well as in the spermatozoon, at first contains a nuclear reticulum with
irregular fragments of chromatin. Later on, these form a spiral coil in
the manner we have already described, and finally this breaks up into
two large and relatively thick angular loops or chromosomes (Fig. 75,
_C_ and _D_, _chr_).

At the same time a nuclear division apparatus has formed in the space
between the two nuclei--the so-called male and female 'pronuclei'
(♂ _k_, ♀ _k_)--two centrospheres (_csph_) become visible, at first
lying close together, but afterwards moving apart (_D_) to form the
poles of a nuclear spindle, in the equatorial plane of which the four
chromosomes of the male and female pronuclei are now arranged. The
nuclear membranes disappear, and the two nuclei now unite to form one,
the segmentation nucleus (_D_). A dividing spindle then develops and
brings about the first embryonic cell-division (_E_), and thus the
beginning of the 'segmentation' of the ovum; each of the four chromatin
loops splits longitudinally, and each of the split halves migrates, one
to one, the other to the other daughter-nucleus (_F_). As this same
method of distribution of the chromatin substance is repeated at every
successive cell-division throughout embryogenesis, and indeed through
the whole of development, it follows that the result of fertilization
is, that all the cells of the body of the new animal which develops
from the ovum contain an equal quantity of paternal and of maternal
chromatin. If we are right in regarding the chromatin substance as the
hereditary substance, it becomes immediately apparent that this equal
division is of the most far-reaching importance, for it shows us that
the so-called process of fertilization is the union of equal quantities
of hereditary substance of paternal and maternal origin.

The process of fertilization is now known in all its details in a great
number of animals in the most diverse groups; it is everywhere the same
in its essential features; there is always only one sperm-cell which
normally enters into conjugation with the ovum-nucleus, and in every
case the sperm-cell, however minute it may be to begin with, forms a
nucleus nearly or exactly as large as the nucleus of the ovum, and in
all cases it contains the same number of chromosomes as the nucleus of
the ovum. Of special interest, however, is the fact that this number
is always half the number of the chromosomes exhibited by the somatic
cells of the particular animal in question, and that the reduction of
the number of chromosomes to half the normal is effected in both male
and female germ-cells by the last divisions of these cells, which take
place before they have attained to a state of maturity. In the ovum the
reduction occurs in the directive divisions, to which we must therefore
turn our attention once more, with special reference to the number of
chromosomes.

We saw that, in the full-grown ovarian egg, the germinal vesicle rises
to the surface and there becomes transformed into the first polar
spindle. Now this shows, in its equatorial plane, double the number of
chromosomes normal to the species. This duplication comes about, not
directly before the nuclear division, but much earlier in the young
mother-egg-cell; it is only the change in the time of the splitting
of the chromosomes that is unusual. The first maturation division
takes place nevertheless in accordance with the usual plan of nuclear
division; it is, as I have called it, an 'equation division,' that is,
both daughter-nuclei again receive the same number of chromosomes as
the young mother-egg-cell had to start with, namely, the normal number
of the species. Thus, if the young mother-egg-cell had four chromosomes
(Fig. 76, _A_), this number would double to eight at an early stage
(_B_), but the first maturing division would give each daughter-nucleus
four (_C_ and _D_). In the second maturation division the case is
different, for here no splitting and duplicating of the number of
chromosomes takes place, but the existing number, by being distributed
between the two daughter-nuclei, is reduced to half in each (_E_ and
_F_). For this reason I have called it a 'reducing division.' In our
example, therefore, the ovum, as well as the second polar body, would
contain only two chromosomes (Fig. 76, _F_).

[Illustration: FIG. 76. Diagram of the maturation divisions of the
ovum. _A_, primitive germ-cell. _B_, mother-egg-cell, which has grown
and has doubled the number of its chromosomes. _C_, first maturation
division. _D_, immediately thereafter; _Rk1_, the first directive cell
or polar body. _E_, the second maturation spindle has been formed; the
first polar body has divided into two (2 and 3); the four chromosomes
remaining in the ovum lie in the second directive spindle. _F_,
immediately after the second maturation division; 1, the mature ovum;
2, 3, and 4, the three polar cells, each of these four cells containing
two chromosomes.]

I cannot enter into the details of the process here, for we are
dealing with essentials and not with isolated and, so to speak,
chance details, but I must emphasize the fact that the same process
of reduction of the number of chromosomes takes place in this or an
analogous manner in all animal ova, and can be demonstrated also
in most of the chief groups of plants. Whether it be, as many have
maintained, that the reduction is not always first effected by the
'maturation divisions,' but in some cases takes place earlier in the
primitive egg-cell[12], so much is certain, that the nuclei which come
together for 'fertilization' only contain half the normal number of
chromosomes, and this is true not only of the ovum but also of the
sperm-nucleus.

[12] See the discussion of this point in chapter xxii.

Arguing from general considerations, but especially from the theory
which regards the chromosomes as the bearers of the hereditary
substance, I had come to the conclusion, before there was any full
knowledge of the phenomena of the maturation of the ovum, that a
reduction of the chromosomes by half _must_ take place, and had
postulated a similar 'reducing division' for the sperm-cell, and
further, for plants as well as animals--indeed, for all sexually
reproducing forms of life. The two divisions in the sperm-cell
corresponding to the polar divisions of the ovum with their reduction
of chromosomes were demonstrated by Oscar Hertwig in the case of the
thread-worm of the horse (_Ascaris megalocephala_)--a form which
has proved so very important in relation to the whole theory of
fertilization. It is true that in this case the course of the phenomena
of reduction is less convincing than in some other forms which have
been investigated more recently, as, for instance, the mole-cricket and
the bugs. In these instances, at any rate, a 'reducing division' in
spermatogenesis, quite corresponding to that of the egg-cell, has been
demonstrated, and this demonstration is of particular value owing to
the fact that the development of the sperm-cell, as we shall presently
see, throws an entirely new light on that of the ovum, and especially
on the phyletic significance of the polar bodies.

We began our consideration of the processes of reduction with the
full-grown egg-cell, but now let us go back to the earliest rudiments
of the ovary of the embryo, and we find that it consists of a single
primitive egg-cell, from which, by division, all the other egg-cells
arise. In the same way the first rudiment in the testis or spermary
is formed by a primitive sperm-cell, which does not differ visibly
from the primitive egg-cell. Both now multiply by division for a
considerable time, and in the ovary this is followed by a period of
growth, during which multiplication ceases, and each cell increases
considerably in size and lays in a store of yolk. Each cell thus
ultimately reaches the condition with which we started previously, that
of the full-grown _mother-egg-cell_.

Although the primitive sperm-cells do not exhibit such pronounced
growth as the ova, they have likewise their period of growth, during
which multiplication by division ceases, and the cells increase only in
size (Fig. 77, _A_). When they have attained their maximum of growth
the number of chromosomes is seen to have been doubled by longitudinal
splitting (as in the diagram, Fig. 77, _B_, from four to eight).
From this _mother-sperm-cell_ there now arise by two divisions in
rapid succession (_C_-_F_) four sperm-cells, and the same reduction
of the number of chromosomes to half is effected as in the polar
divisions of the egg-cell. In the first division, four chromosomes
go to each daughter-cell (_D_), in the second, two (_F_). The only
essential difference between the corresponding processes in the
egg-cell and the sperm-cell lies in the fact that the divisions of the
so-called 'spermatocyte' or mother-sperm-cell are equal, so that four
granddaughter-cells of equal size arise, while in the mother-egg-cell
or 'ovocyte' the divisions are very unequal. In the former the result
of the divisions is _four_ cells capable of fertilizing, in the latter
_one_ cell capable of being fertilized and three minute 'polar cells'
which are incapable of conjugating with a sperm-cell and giving rise to
a new individual.

[Illustration: FIG. 77. Diagram of the maturation-divisions of the
sperm-cell, adapted from O. Hertwig. _A_, primitive sperm-cell. _B_,
mother-sperm-cell. _C_, first maturation division. _D_ 1 and 2, the two
daughter-cells. _E_, the second maturation division, by which the four
cells of _F_ arise, each with half the number of chromosomes.]

There can thus be no doubt that the polar cells, as Mark and Bütschli
long ago suggested, are abortive ova, that is, that, at a remote period
in the evolution of animal life, each of these four descendants of a
mother-egg-cell became a germ-cell capable of development. It is not
difficult to infer that the unequal division, which now leads to an
insufficient size in three of these descendants, has gone on _pari
passu_ with the continually increasing size of the mature ovum, and had
its reason in the fact that it was important above all things to store
in the ovum as much protoplasm and yolk as possible. We have already
seen that even the dissolution of a number of the sister-cells of the
ovum is sometimes demanded, so that the ovum may be surrounded by
nutritive follicular cells. In short, the greatest possible quantity of
nourishment is conveyed to the ovum in every conceivable way, and it
is thus stimulated to a growth which no single cell could attain to if
it were dependent on the ordinary nutrition supplied by the blood. And
we can understand that nature--to speak metaphorically--did not wish
to destroy her own work by finally distributing among four ova all the
nourishment she had succeeded in heaping up in all sorts of ways within
the mother-egg-cell.

But it may be asked, Why have all these unnecessary divisions been
maintained up till the present day? Why have they not long ago been
given up, since they can and do only lead to the production of three
abortive ova, which are foredoomed to perish? Are they mere vestiges,
processes which are in themselves meaningless, but have, so to speak,
been maintained by the principle of inertia? This principle is
certainly operative in some sense and to some extent even in living
nature; a process which has been regularly repeated through a long
series of generations does not at once cease to be performed when it is
no longer of use to the organism concerned. The eyes of animals which
have migrated to lightless depths do not disappear all at once and
leave no trace; they degenerate very gradually and only in the course
of many generations; and it would thus be quite possible to defend the
position that these polar or 'maturation divisions' of the ovum are
purely _phyletic reminiscences_ without actual significance.

But I cannot agree with this opinion. If it were actually so we should
expect that the formation of the polar bodies would not still take
place in all cases in almost the same manner, for all rudimentary parts
and processes vary greatly; we should expect that in many animal groups
the polar divisions would not occur, or perhaps that only half the
number would occur. But this is not so; in all multicellular organisms,
from the lowest to the highest, two reducing divisions take place,
and always in almost the same manner, with the exception of a single
category of ova, of which I shall presently have to speak. We shall see
later that even in unicellular organisms analogous processes may be
observed.

But it is also intelligible that this twice repeated division of
the mother-egg-cell is necessary if the reduction in the number of
chromosomes to half is only possible in this way, since _this reduction
is indispensable_. If each of the two conjugating germ-cells contained
the full normal number of chromosomes, the segmentation-nucleus
would contain a double number, and if that went on, the number of
chromosomes would increase in arithmetical proportion from generation
to generation, and would soon become enormous. Even though we were not
otherwise certain that these chromosomes are units of a permanent
nature, which only apparently break up in the nuclear reticulum,
but in reality persist, the fact of reduction would point in this
direction. For if they were not permanent structures and distinct from
one another, and if their number depended solely on the quantity of
chromatin which the nucleus contains, the reduction in number might be
secured if the chromosomes in the growing egg and sperm-cells increased
in size more slowly than the cell-body and the other parts of the cell.
But from the fact that the reduction takes place not in this simple
way, but, in sperm-cells and in ova which require to be fertilized,
only through cell-division and a specific mode of nuclear division, we
may conclude that it cannot happen otherwise, that chromosomes are not
mere aggregates of organic substance, but organs whose number can only
be reduced by the extrusion of some of them from the cell.

It is true that there are ova in which the process of reduction does
not follow the course we have described, but the exceptions only
serve to confirm our view of the reducing significance of the polar
divisions, and of their persistence because of the necessity for
reduction.

As far back as the middle of the nineteenth century it was known
that in various animals the eggs develop without fertilization. This
reproduction by 'parthenogenesis' was first established with certainty
by the German bee-keeper Dzierzon in 1845, and then scientifically
corroborated by Rudolph Leuckart and C. Th. von Siebold. Although
parthenogenesis was at first observed only in a few groups of the
animal kingdom, in bees and some nocturnal Lepidoptera (Psychidæ
and Tineidæ), it has become more and more apparent in the course of
years that this 'virgin reproduction' is by no means a rare form
of reproduction, and that it occurs regularly and normally in many
cases, especially in the very diverse groups of the great series of
Arthropoda. Thus among insects it is found in certain saw-flies,
gall-flies, ichneumon-flies, in the honey bee, and in common wasps, and
it is particularly widespread among plant-lice (Aphides) such as the
vine-aphis (_Phylloxera_), whose prodigious multiplication in a short
time depends partly on the fact that all the generations, with the
exception of one, consist only of females with a parthenogenetic mode
of reproduction.

Among the lower Crustaceans also parthenogenesis plays a large rôle,
and in many species it even occurs as the sole mode of reproduction,
but more often--as is also the case among insects--it occurs
alternately with bi-sexual reproduction. For parthenogenesis must not
be regarded as asexual reproduction, but rather as _unisexual_, that
is, as arising from sexually differentiated individuals (females),
and from germ-cells (true ova), but brought about by the agency of
individuals of only one sex, the female. These parthenogenetic eggs
emancipate themselves, so to speak, from the law that was previously
regarded as without exception, that all ova require fertilization to
enable them to develop. That this law admits of many exceptions is
now universally admitted; thus in the small family of water-fleas
(Daphnids) there are even two kinds of eggs, the summer and winter eggs
we have already mentioned, which are produced by the same female, and
yet the former kind develop without fertilization, while the latter
require to be fertilized before they can develop.

It was obviously important to learn the state of affairs in regard to
reducing divisions in parthenogenetic ova, to find out whether here
also, three, or, in some circumstances, two polar bodies were formed,
and whether the second polar division reduced the number of chromosomes
to half. If the theory previously advanced as to the importance of the
chromatin, and especially of the reducing effect of the second maturing
division be correct, we should expect the second division to be wanting
in parthenogenetic eggs, since otherwise the number of chromosomes
would be reduced to half in each generation, and would thus gradually
disappear or sink to one.

Having directed my attention to this problem, I succeeded in
establishing for a Daphnid, _Polyphemus_, that the second polar
division does not occur, and that only one polar body is formed.
Blochmann found the same in the parthenogenetic eggs of plant-lice or
Aphides, in which, moreover, the eggs requiring fertilization exhibit,
like the winter eggs of Daphnids, two polar divisions. It was thus
established that at least those eggs of Aphides and Daphnids which
are wholly parthenogenetic retain the full number of chromosomes
of their species, as is represented in the diagram, Fig. 78. When
parthenogenesis set in the polar divisions were limited to one, and
that this could happen justifies us in concluding _a posteriori_
that it could have happened also in the case of ova which required
fertilization if that had been necessary or even merely indifferent.
The polar divisions are thus not mere 'vestigial' processes; they have
an immediate significance, and it lies in the reduction of the number
of chromosomes.

But I must make a reservation here; it is not universally true of
parthenogenetic eggs that maturation takes place without the second
polar division. The first exception was observed in the salt-water
crustacean, _Artemia salina_. In this case only one polar body is
actually extruded and the number of chromosomes remains normal, as I
was able to demonstrate with the small number of ova at my disposal;
but according to the investigations of Brauer on more abundant material
it appears that, while the second polar division is suppressed in the
majority of the ova, and the external extrusion of a second polar body
never occurs, the second polar division does nevertheless sometimes
take place. The two daughter-nuclei arising from this division unite
again immediately afterwards to form a single nucleus, and this now
functions as a segmentation nucleus. Of course it again contains the
full number of chromosomes, namely, twice 84=168.

In _Artemia_, therefore, the adaptation of the ova to parthenogenetic
development is not yet fully established, and the complete abandonment
of the second polar division seems to be phyletically striven for,
since, although the division still takes place, its effect is
neutralized immediately afterwards.

[Illustration: FIG. 78. Diagram of the maturation of a parthenogenetic
ovum. The number of chromosomes normal to the species has been assumed
to be four. _Uei_, a primitive germ-cell. _M Eiz_, a mother-egg-cell,
with twice the normal number of chromosomes. _Eiz_, mature ovum after
the separation of the first and only polar body. _Rk_^1.]

Among bees the state of affairs is again exceptional. Here the female,
the so-called queen bee, possesses a capacious sperm-sac, in which
the spermatozoa received in copulation remain living for years,
and the fertilization of an ovum is effected in the usual way from
this sac while the egg from the ovary is passing down the oviduct.
The queen bee has the power of releasing some spermatozoa from the
receptacle, or of not doing so, and thus of fertilizing the egg, or
of not fertilizing it. Since the notable observations of Dzierzon and
the investigations of von Siebold and Leuckart which followed them, it
has been assumed that only those eggs were fertilized which were laid
in the cells destined for rearing females (workers or queens), while
those which were to give rise to 'drones' or males remained normally
unfertilized. Only in the last decade of the past century did the
bee-keepers begin to cast doubt on this so-called 'Dzierzon theory';
various violent and obstinate attacks were made upon it, and these
were supported by new and apparently convincing experiments. Dickel, a
teacher in Darmstadt, has been particularly strenuous in attempting to
overthrow the old theory, by emphasizing the fact that von Siebold's
old investigations on bee eggs afforded no convincing proof. Von
Siebold made his investigations on eggs freshly taken from the hive,
and was never able to find spermatozoa in 'drone eggs' (that is, eggs
laid in drone cells and therefore destined to develop into drones),
while he was often able to demonstrate the presence of from one to
four spermatozoa in 'worker eggs.' But he only examined 'drone eggs'
which were already twelve hours old, and in these, as we now know, he
would not have found spermatozoa in any case, even if they had been
fertilized, because in ova at that stage the development of the embryo
has already fully begun, and nothing remains of the spermatozoa. In
the bee, according to Buttel-Reepen, the fertilizing spermatozoon is
transformed in twenty minutes after it has penetrated into the egg into
a minute 'sperm-nucleus' which is almost invisible even in sections,
and certainly nothing whatever could be seen of it by the old method of
squeezing the fresh ovum.

It had therefore to be admitted that Dzierzon's theory rested on an
insecure foundation, and I accordingly set two of my students at that
time, Dr. Paulcke and Dr. Petrunkewitsch, to examine the eggs of the
bee anew with regard to the point in question, using the greatly
improved methods at their disposal. These investigations have been
carried out in the Freiburg Zoological Institute during the last three
years, and have resulted in establishing the absolute correctness of
Dzierzon's theory: the 'drone eggs' do remain unfertilized, while the
eggs from which females are to develop are fertilized without exception.

In this case, therefore, we have, in the same animal, eggs which
can be fertilized and eggs which, without fertilization, develop
parthenogenetically, and it is therefore of the greatest possible
interest to know the state of matters in them in regard to the
directive divisions and the reduction of the chromosomes.

Dr. Petrunkewitsch's investigations have shown that in both cases,
that is, whether a spermatozoon penetrates into the ovum or does
not, a twice-repeated division of the nuclear material in the ovum
takes place. Moreover, the two daughter-nuclei which result from the
second division do not, as Brauer showed was sometimes the case in
_Artemia_, unite again afterwards; they remain separate, and the
number of chromosomes--there are sixteen of them--is thereby reduced
to half in the segmentation nucleus. But this is not all, for before
embryonic development has begun the normal number can be again seen in
the segmentation nucleus; the chromosomes must therefore have _doubled
their number by division within the nucleus_.

It is probable that something similar takes place in the cases of
exceptional parthenogenesis which have long been known, but this point
has not yet been sufficiently investigated. Nevertheless I cannot pass
them over, as they are instructive from another point of view.

[Illustration: FIG. 79. The two maturation divisions of the 'drone
eggs' (unfertilized eggs) of the Bee, after Petrunkewitsch. _Rsp 1_,
the first directive spindle. _k 1_ and _k 2_, the two daughter-nuclei
of the same. _Rsp 2_, the second directive spindle. _k 3_ and _k 4_,
the two daughter-nuclei. In the next stage _k 2_ and _k 3_ unite to
form the primitive sex-nucleus. Highly magnified.]

In some silk-moths (Bombycidæ) and hawk-moths (Sphingidæ), especially
in the silk-moth proper (_Bombyx mori_), in _Liparis dispar_, and
in quite a number of other Lepidoptera, it sometimes happens that,
out of a large number of unfertilized eggs, a few will develop and
produce caterpillars. This is interesting enough, but it gains
increased importance through the investigations of the Russian
naturalist, Tichomiroff, who succeeded in considerably increasing
the number of unfertilized eggs that developed by gently rubbing
them with a paint-brush, or by dipping them for a little in dilute
sulphuric acid. It is thus possible to make eggs, which would not
ordinarily develop without being fertilized, capable of parthenogenetic
development by means of mechanical or chemical stimulus. This sounds
almost incredible, but it is beyond a doubt, and it is still further
corroborated by the fact that Prof. Jacques Loeb has succeeded in
inciting the eggs of a sea-urchin to parthenogenetic development by
means of a chemical stimulus. When he added to the sea-water in which
the eggs were laid a certain quantity of chloride of magnesium the
ova developed, and not only went through the process of segmentation,
but even reached the stage of the quaint easel-like Pluteus larva.
Quite recently Hans Winkler has made the interesting observation
that from sea-urchin sperms which have been killed by heat it is
possible to extract in aqueous solution a substance capable of exciting
unfertilized sea-urchin eggs to development, although they only go as
far as to the sixteen-cell stage.

From all these results we can at least infer so much, that chemical
changes and influences may determine whether the ripe ovum shall go on
to embryonic development or not, and that these influences, may be very
diverse in nature in different cases. I shall return later to these
important facts.

When we sum up the facts we have cited with reference to the reduction
of the number of chromosomes, it appears that nature is, as it were,
striving to keep the number constant for each species; that in
germ-cells which are destined for amphimixis they are reduced to half
the normal number, but that this halving of the number is suppressed
where fertilization is always absent, or that the reduction to half is
compensated for again in various ways, whether by subsequent fusion
of the two daughter-nuclei, which have arisen from the process of
reduction, or by an independent duplicating of the chromosomes in the
segmentation nucleus.

We might perhaps be inclined to conclude from all this that the
occurrence of development depended on the presence of the normal
number of chromosomes; and I used to regard this as possible. But
facts which have been more recently brought to light have excluded
this view. Above all, we now know that every nuclear division depends
on the presence of a dividing apparatus, a centrosphere, but that this
organ degenerates in the ova of most animals and is completely lost
after the second polar division has been effected. The mature ovum is
therefore in itself incapable of entering on its embryonic development,
no matter how many chromosomes its nucleus contains; it is only capable
of further division when the fertilizing sperm-cell brings with it
its dividing apparatus or centrosphere. In thread-like sperms this
lies in the median portion (Fig. 68_C_), and after the tail-piece has
been dissolved, which happens soon after the sperm enters the egg, the
central corpuscle, at first very small, can be recognized in front of
the sperm-nucleus, where it is soon transformed into an 'aster' and
divides into two. Then both spheres move apart (Fig. 75_D_, p. 296) and
form the nuclear spindle between them by the confluence of their rays.

From this the division of the ovum into the two first embryonic cells
proceeds. The two pronuclei in the ovum, the male and the female, are
thus exactly alike as to number of the chromosomes, and frequently
at least as to size and appearance (Fig. 75_C_). But they differ
in the possession or absence of a dividing apparatus, and in the
great majority of cases it is the male nucleus that brings with it
the central corpuscle which seems to be indispensable to embryonic
development (_B_, _cspt_). Hitherto, at least, only two exceptions to
this are known. In the little segmented worm, _Myzostoma_, which is
parasitic on sea-lilies or Crinoids, Wheeler observed that the ovum
retained its central corpuscle even after the polar divisions, while
the sperm-cell which penetrated into the egg had none. More recently
Conklin made the interesting discovery that in the egg of a marine
Gasteropod (_Crepidula_) both the egg-nucleus and the sperm-nucleus
retain their centrosphere and together form the segmentation spindle,
one lying at one pole and the other at the opposite.

All these observations confirm the view that the sperm and the
egg-cell are alike in this respect also. Each of them can, in certain
circumstances, bring with it the dividing apparatus indispensable to
development, though it is usually the sperm-cell that does so.

I should indeed assume that the sperm-cell and the egg-cell were
essentially alike, even although there were no exception to this
rule, that is, although the centrosome of the ovum perished in
all eggs which were fertilized. For this is obviously a secondary
arrangement, an adaptation to fertilization, that the ovum should be
incapable of development without fertilization, and it is made so by
the disappearance of its centrosome. In all other cells, as far as is
known, the central corpuscle persists after division, so that this
remarkable cell-organ is transmitted from cell to cell just like the
nucleus, and like it, never rises _de novo_. It is only in the egg-cell
that it disappears, though even there often very late, for it may be
present, as an aster, even after the sperm has penetrated into the
ovum and disclosed its own central body, or even brought it the length
of dividing into two (Fig. 80, _A_ and _B_). But the ovum-centrosome
disappears as soon as the second polar division is accomplished.

That this disappearance is really a secondary arrangement, which may be
again departed from, is proved by the case of those eggs which are able
to develop parthenogenetically, for in them the central body does not
disappear, but persists in the ovum after the first polar division, as
Brauer showed in _Artemia_. It then behaves exactly like the sphere of
the sperm-nucleus in the fertilized ovum, that is, it duplicates itself
and forms the segmentation spindle.

[Illustration: FIG. 80. Fertilization of the ovum of a Gasteropod
(_Physa_), after Kostanecki and Wierzejski. _A_, the whole spermatozoon
lies in the ovum. _sp_, its already divided centrosphere. _Rk 1_,
the first polar body. _Rsp 2_, the second directive spindle. _B_,
_spk_, the sperm-nucleus, the second directive spindle still has its
centrosphere, which afterwards disappears. The first polar body (_Rk
1_) has divided into two. Highly magnified.]

Thus the beginning of embryonic development in the ovum depends not on
a definite number of chromosomes, but on the presence of an apparatus
for division. Upon what the awakening of this to activity just at that
time depends cannot as yet be exactly stated; we can only indicate
that all parts of the cell have interrelations with each other, and
that, therefore, the division mechanism is dependent on the condition
of the rest of the cell-parts at the moment, and on the substances
which they contain or produce. From what we know experimentally in
regard to artificial parthenogenesis it is not difficult to imagine
that some sort of chemical substances are necessary to stimulate the
central corpuscle to activity. In any case, the whole nutrition of
the central corpuscle depends on the cell in which it lies, as is
shown by the fact that the sperm-nucleus, whose centrosome before
the entrance of the sperm into the ovum was inactive and scarcely
recognizable, grows rapidly after entrance and forms a large aster
round itself--is, in short, in the highest degree active (Fig. 80). As
the chromosomes certainly play an important part in the life of the
cell, and materially help to determine its various phases, it cannot
be disputed that they also may share in awakening the activity of the
central corpuscle. But this influence is only indirect; it is not the
mere number of chromosomes that decides whether the central corpuscle
is to become active or remain inactive. This cannot be assumed, because
we have in the maturation divisions a proof that division may take
place with a double number of chromosomes as well as with the undoubled
number; while in the divisions of the mother-egg-cells and the
mother-sperm-cells we have proof that a doubled number of chromosomes
does not in itself compel to division.

The exceptional and artificially produced cases of parthenogenesis
which we have discussed above are probably to be interpreted thus:
through slight differences in the constitution of the ovum, or through
certain mechanical or chemical stimuli, the metabolic processes in
the ovum are so altered that the centrosome of the ovum, instead of
breaking up, is stimulated to growth, and thus produces the active
dividing apparatus which is otherwise only brought into it by the
sperm. This is a more exact definition of the interpretation I gave
earlier (1891) of the 'chance' parthenogenesis of the silk-moth, which
was then the only case known, when I said 'the nucleoplasm of some ova
must possess the power of growth in a greater degree than the majority.'

But we are not yet in a position to go further, or to define more
exactly the nature of the processes of metabolism which are involved.




LECTURE XVI

FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS, AND ITS IMMEDIATE
SIGNIFICANCE

 Fertilization in a lichen, Basidiobolus--In Phanerogams--Here too
 there is reduction of the number of chromosomes by a half--'Polar
 cells' in lower and higher plants--Conjugation among unicellular
 organisms--Noctiluca--The maternal and paternal chromosomes
 remain apart--Actinophrys--Infusoria--Sexual differentiation
 of the two conjugates in Vorticella--Importance of the process
 of Amphimixis--Not a 'life-awakening' process--May occur
 independently of multiplication--The Rejuvenescence hypothesis--Pure
 parthenogenesis--The cycle idea--Does Amphimixis prevent natural
 death?--Maupas' experiments with Infusorians--Bütschli's
 view--Potential immortality of unicellular organisms--The
 immortality of unicellular organisms and of the germ-cells depends
 on the fact that there is no time-limit to the multiplication
 of the smallest living particles--Parthenogenesis is not
 self-fertilization--Petrunkewitsch's observations on the ova of
 bees--Is the chromatin really the 'hereditary substance'?--Nägeli's
 conclusion from the difference in size between ovum and
 spermatozoon--Artificial division of Infusorians--Boveri's
 experiments with the fertilization of pieces of ova not containing
 a nucleus--Fertilization gives an impulse to development even to
 non-nucleated pieces of ova--Merogony--The female and male nuclear
 substances are essentially alike--Summary.


I NOW turn to the consideration of the process of fertilization in
plants and unicellular organisms.

With regard to plants, it can now be definitely asserted that in them,
too, fertilization is essentially a conjugation of nuclei; it depends
on the union of the nuclei of the two 'sex-cells.' These sex-cells are
usually very small among lower plants, indeed up to the phanerogams;
this is especially true of the zoosperm-like male germ-cells, but it
usually holds also true of the ovum, which is but seldom burdened
with an abundant supply of yolk. In spite of the many difficulties
which this smallness of size puts in the way of observation, the
untiring exertions of a host of excellent investigators have succeeded
in following the process of fertilization in all the larger groups
of plants--in algæ, fungi, mosses, ferns, and horse-tails among
cryptogams, and in phanerogams.

I shall first give an example from among the lower plants (Fig. 81).
In one of the lichens, _Basidiobolus ranarum_, each of two adjacent
cells in the fungus-thread gives off a bill-like process, and the two
processes become closely apposed (Fig. 81, _a_). The nucleus of each
cell moves into the bill-shaped process, is there transformed into a
nuclear spindle (_B_, _ksp_) and divides, so that one daughter-nucleus
comes to lie in the apex point of the bill, the other at the base.
The cell-body also divides, though very unequally, and the final
outcome of the process is two cells in each, of which one is small
and occupies the apex of the bill, while the other is large and fills
all the rest of the cell-space. The former do not play any further
part of importance, but break up, the latter are the sex-cells, the
cytoplasm of which now coalesces through a gap in the cell-walls, while
their nuclei become closely apposed and ultimately unite (_C_, ♂ and
♀ _k_). From this union arises the fertilized spore, the so-called
'zygote' (_D_). The two small abortive cells so greatly resemble in
their origin the polar cells of the animal ovum that it is difficult to
resist the supposition that they bring about a reduction in the number
of chromosomes. But the number of the chromosomes has not yet been
determined either in them or in the sex-nuclei.

[Illustration: FIG. 81. Formation of polar bodies in a lichen,
_Basidiobolus ranarum_. _A_, the two conjugating cells with the
bill-like processes in which the nuclei lie. _B_, the nuclei dividing.
_ksp_, the nuclear spindle. _C_, after the division into a polar body
(_rk_) and a sex-nucleus (♂ _k_ and ♀ _k_). _D_, after the union of
the nuclei to form a conjugation nucleus (_copk_); the fertilized ovum
is surrounded by envelopes and modified into a lasting spore. After
Fairchild.]

We have come to know the processes of fertilization among phanerogams
chiefly through Strasburger, Guignard, and more recently through the
Japanese botanist Hirase. The agreement with the animal process is
surprisingly great, notwithstanding the notable differences in the
external conditions of fertilization.

As is well known, the male cells in the highest flowering plants are
not zoosperms but roundish cells, each of which, enclosed, together
with a sister-cell--the so-called 'vegetative' cell--in a thick
cellulose capsule constitutes a pollen-grain. The pollen-grains reach
the stigma, under which, buried deep within the 'ovule,' the female
sex-cell rests, enclosed in a long, sac-like structure called the
'embryo-sac' (Fig. 82, _A_). Beside it (_eiz_) there lie several other
cells, usually seven in number, two of which, the so-called 'synergidæ'
(_sy_), have their place at one end of the embryo-sac, just in front of
the ovum (_eiz_). Probably these give off a secretion which exercises
an attractive (chemotactic) influence on the male fertilizing body
('the pollen-tube'), and thus, so to speak, show it the way to the ovum.

[Illustration: FIG. 82. Fertilization in the Lily, _Lilium martagon_,
after Guignard. _A_, the embryo-sac before fertilization; _sy_,
synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei';
_ap_, antipodal cells. _B_, the upper part of the embryo-sac,
into which the pollen-tube (_pschl_) has penetrated with the male
sex-nucleus (♂_k_) and its centrosphere; below that is the ovum
with its (also doubled) centrosphere (_csph_). _C_, remains of the
pollen-tube (_pschl_); the two sex-nuclei are closely apposed. Highly
magnified.]

When a pollen-grain has reached the stigma it sends out a tube, usually
after a few hours, which penetrates into the soft tissue of the
style, and grows deep down into the interior of the ovule, ultimately
penetrating as far as the embryo-sac through a special little opening
in the covering of the ovule, the so-called 'micropyle' (Fig. 82_B_,
_pschl_). Its blunt end is now closely apposed to this, so that the
true sperm-nucleus (_B_, ♂_k_), surrounded by some protoplasm, can
leave the pollen-tube and wander in among the cells of the embryo-sac.
Later on we shall see that two generative nuclei migrate from the
pollen-tube, but in the meantime we shall devote our attention only to
one of them, the fertilizing nucleus, which immediately moves towards
the ovum-nucleus and apposes itself closely to it. Then follows the
fusion or conjugation of the two nuclei, which are alike in size and
appearance, just as in the fertilization of the animal ovum (_C_, ♂ _k_
and ♀ _k_). Whether in this case, too, the sperm-nucleus brings with it
a central corpuscle, or whether, as Guignard believed he observed, the
ovum retains its central corpuscle (_C_, _csph_), or finally, whether
both modes occur, is not yet known with certainty. The fact that, as
a rule, seeds capable of reproduction only form in an ovule when the
stigma has been previously dusted with pollen, leads us to suppose
that, in this case, as among animals, the ovum lacks something that is
necessary to induce embryonic development, only retaining this power
in very exceptional cases, namely, when adapted for parthenogenesis.
And this something may very well be the dividing apparatus of the cell,
the centrosome with the centrosphere. But whether this supposition
prove correct or not, a nuclear spindle always forms simultaneously
with the fusion of the two sex-nuclei into a segmentation nucleus, and
this spindle is the starting-point of the young plant, thus exactly
corresponding to the first segmentation of the animal ovum. It agrees
with it also in the important respect that it again contains the full
number of chromosomes--twenty-four in the lily--while the two nuclei,
male and female, only exhibit half the number each, that is, twelve.

Thus a reduction in the number of chromosomes to half takes place in
plants also, but it is not yet known with certainty whether this is
brought about in the same way as among animals, namely, by reducing
divisions. Without entering more fully into this still unsolved and
very complex problem, I should like to state that I consider this very
probable; indeed, I agree with the view of V. Häcker[13], that the
reducing divisions of plants are only more difficult to recognize as
such, and, furthermore, are often disguised by the fact that they often
occur alongside of, or between divisions which are not reducing. If it
were possible to reduce the number of chromosomes in a cell to half
without the aid of cell-division, if, for instance, only half were to
integrate again from the chromatin-network, this must have been quite
as possible in the case of animal cells, and then, moreover, the single
chromosome would not have had the significance of an individuality,
and no special form of nuclear division would have been introduced to
reduce their number. That it has been introduced seems to me to prove
that it was necessary, and since it was so among animals, it could not
have been dispensed with among plants either.

[13] See V. Häcker, _Praxis und Theorie der Zellen- und
Befruchtungslehre_, Jena, 1899, pp. 144-5.

Moreover, throughout the vegetable kingdom divisions often occur in
connexion with the origin of the sex-cells which can be compared,
in occurrence and result, with the maturation divisions of animal
germ-cells. In the lichen, _Basidiobolus_, we have already seen that an
abortive cell separates itself off from the sex-cell before the latter
becomes capable of reproduction (Fig. 81, _C_). Similar cell-divisions
occur in many if not in all groups of plants. In the marine algæ of
the genus _Fucus_ it has even been proved that the division of the
first primordial cell of the ovary into the so-called 'stalk-cell' and
the primitive egg-cell is a reducing division, and brings down the
number of chromosomes from thirty-two to sixteen. In vascular plants
the reduction is not postponed until the formation of the sex-cells,
but occurs earlier in the formation of the spores, as Calkins has
demonstrated for ferns; in the Conifers and other Gymnosperms
several so-called 'preparatory' divisions precede the formation of
the germ-cells, and we know by comparison with the alternation of
generations in vascular plants that these are related to the gradual
waning of the strictly sexual generation. As the 'polar bodies' or
'directive corpuscles' of the animal ovum are rudimentary egg-cells,
so the cells which, in the pollen-grains, separate themselves from
the sex-cells proper are rudimentary Prothallium-cells, and, like the
animal cells, they perish without playing any further physiological
rôle. I will not assert that it is precisely in these divisions
that the reducing divisions are concealed, for the analogy with the
spore-formation of ferns leads us rather to suppose that it may lie
further back; but in any case there is no lack of opportunity in the
ontogeny of phanerogamic plants for the interpolation of a reducing
division, and as long as it remains unproved that a reduction of the
chromosomes can take place directly, that is, without the help of
nuclear division, we shall continue to expect with confidence that the
reducing divisions of phanerogams will be discovered in the future.
Processes of a similar kind are known among unicellular organisms, and
there, too, they are associated with nuclear divisions.

In passing to the so-called 'sexual reproduction' of unicellular
organisms, I should like first to call attention to the fact that
the expression 'reproduction' is not very suitable in this case, for
the process in question does not always effect an increase in the
number of individuals as reproduction ought to do, but leads, in
fact, in many cases, even to a decrease, when two individuals unite
to form one. Even if the phenomena of sexual 'reproduction' among
higher organisms, which we have already studied, had not made it
clear to us that there are two associated processes, quite different
in nature, the conjugation of unicellular organisms would have led
us to that conclusion. It has long been known that two unicellular
plants or animals occasionally become closely apposed and fuse;
and this process of 'conjugation' was many years ago regarded as
an analogue to 'fertilization,' although it is only through the
laborious investigations of the last two or three decades that
this supposition has been proved to be correct. We now know that
a process quite analogous to that which we have learnt to know as
'fertilization' takes place among unicellulars, only in this case it is
not directly connected with reproduction and multiplication, but occurs
independently of them, and, in its most primitive form, it results, not
in an increase but--for a short time at least--in a diminution of the
number of individuals. This occurrence of the process independently
of reproduction appears to me of inestimable value theoretically, for
it frees us completely from the old deep-rooted preconceptions in the
interpretation of fertilization.

[Illustration: FIG. 83. Conjugation of Noctiluca, after Ischikawa.
_A_, two Noctilucas beginning to coalesce; _pr_, the protoplasm drawn
out into processes which traverse the gelatinous substance of the
cell; _k_, the nucleus. _B_, the cells and their gelatinous substance
have fused; the nuclei, in which the chromosomes are visible, are
closely apposed; _CK_, centrospheres. _C_, the two nuclei are united
in one nuclear spindle; beginning of division. _D_, completion of the
division. Highly magnified.]

First let us briefly sketch the process itself in the main forms of its
occurrence.

The most primitive form of conjugation is undoubtedly the complete
fusion of two unicellular organisms of the same species, as we see it
to-day in unicellular plants, and also among the lowest unicellular
animals, such as the flagellate Infusorians, Gregarines, and Rhizopods.
It is well seen, for instance, in the Noctilucæ, those unicellular
flagellate organisms which cause the familiar marine phosphorescence
extending uniformly over wide surfaces of water (Fig. 83). In these
forms Prof. Ischikawa of Tokio was able to trace the whole process
of conjugation. To begin with, two Noctilucas range themselves side
by side (Fig. 83) and coalesce at the surfaces in contact, both as
to the spherical gelatinous envelope (_A_, _G_) and the protoplasm
(_pr_) itself, which branches in amœboid fashion into the jelly. The
union becomes gradually complete, and the two animals form a single
sphere (_B_) with one cell-body. But the two nuclei (_K_) also place
themselves side by side (_B_), and though they do not actually fuse,
they form together, under the guidance of two centrospheres (_C_), a
single nuclear division-figure, which is obviously analogous to the
segmentation spindle of the fertilized egg. Then follows a division,
by means of which the chromatin substance of the nuclei of both
animals is divided between the two daughter-nuclei, and after this
has been accomplished the united individual again separates into two
independent Noctilucas (_D_). Although I have spoken here--that is, in
referring to the Protozoa--of chromosomes, I must immediately add that
these have not yet been seen with full clearness in Noctiluca itself;
nothing more has been recognized than deeply staining thickenings
of the spindle fibrils, which move from the equator of the nuclear
spindle towards the pole. Since, however, in other Protozoa, as, for
instance, in the beautiful freshwater Rhizopod (_Euglypha alveolata_),
these thickenings of the nuclear spindle fibrils have been clearly
recognized as chromosomes, doubt on this point is hardly justifiable.
Apart from this, the assumption that each of the two daughter-nuclei
receives half the chromosomes of each of the conjugated nuclei rests on
a secure basis, not only because otherwise the whole process would have
no meaning, but because the position of the mitotic figure conditions
this. Even the fact that the two conjugation-nuclei lying side by side
remain apart during nuclear division is not without parallel; Häcker
and Rückert observed it also in the segmentation-nucleus of much higher
animals, the Copepods, and it has no effect in altering the process of
division, but only proves that the chromosomes of maternal and those of
paternal origin in the combination-nucleus remain independent--a fact
the significance of which I shall discuss later on.

The process of conjugation occurs, in the same manner as in
_Noctiluca_, in a freshwater Rhizopod, the well-known Sun-animalcule,
_Actinophrys sol_ (Fig. 84), but in this case complete fusion of the
two nuclei takes place (Fig. 84, _V_) before the formation of the
division-spindle (_VI_, _sp_), which, with the simultaneous division
of the cell-body, gives rise to two new individuals. The process in
this case is especially interesting, because Schaudinn has succeeded in
observing a maturation division (_III_, _Rsp_, directive spindle) as
well as in demonstrating polar bodies (_IV_, _Rk_). Thus the analogy
with the process of fertilization in the Metazoa and the Metaphyta is
almost complete.

But that the conjugation of unicellular organisms, like the
fertilization of multicellular organisms, is essentially a matter of
nuclear conjugation is shown more distinctly still by the ciliated
Infusorians, the most highly organized of the Protozoa.

[Illustration: FIG. 84. Conjugation and polar body formation in
the Sun-animalcule, _Actinophrys sol_, after Schaudinn. _I_, two
free-swimming conjugated individuals, which in _II_ have become
surrounded by a transparent gelatinous cyst. _III_, formation of
the directive spindles (_RSp_). _IV_, the polar bodies are formed
(_RK_); _K_, the two sex-nuclei. _V_, these are fused to form the
conjugation-nucleus (_K_). _VI_, the conjugation-nucleus is transformed
into the division-spindle; the polar bodies (_RK_) have penetrated the
internal cyst-wall, and are in process of degeneration.]

Here there is usually no complete union of the cell-bodies of the
two animals, but only an adhering of the apposed surfaces. In the
relatively large _Paramœcium caudatum_ the process of conjugation is
very exactly known through the beautiful investigations of Maupas
and R. Hertwig. In this case the mouth-surfaces of the two animals
come together and unite over a short area, and then the two animals
swim about together in this conjugated state. During this time very
remarkable changes take place in their nuclei.

It is well known that these Infusorians have a double nucleus, a
large one, the macronucleus (Fig. 85, _ma_), and one which is usually
very small, the micronucleus (_mi_). We may ascribe to the former
of these the guidance and regulation of the everyday processes of
life, that is, briefly, of metabolism, and the preservation of the
integrity of the whole animal. The small nucleus has often been
designated the 'reproductive nucleus,' but as it plays no other
part in reproduction, as far as can be recognized, than that of
dividing into two daughter-nuclei, I cannot regard this designation
as suitable; it obviously originated in the mistaken interpretation,
prevalent till very lately, of conjugation as a 'kind of reproduction,'
and this in its turn depends on the conception, transferred from
multicellular organisms, of fertilization as a 'sexual reproduction.'
We shall immediately see that the micronucleus plays the main part in
conjugation, and from this we may suppose that it otherwise fills no
rôle in the life of the animal, and therefore it may best be designated
the 'supplementary' or reserve nucleus. In every conjugation the
macronucleus, which has hitherto been active, breaks up and becomes
completely absorbed, very much like a ball of food. This of course
takes place slowly; the large nucleus elongates, becomes indented,
falls into several pieces, and these are so gradually absorbed that,
even after the act of conjugation has been accomplished, irregular
fragments of the macronucleus often lie about in the animal (Fig. 85,
9).

But while the macronucleus falls to pieces the previously minute
micronucleus grows enormously and forms a distinct longitudinally
striated spindle (1, _mi_). About the same time these divide in both
animals, and each of the daughter-nuclei immediately divides again,
so that after these two divisions four spindle-shaped descendants
of the micronucleus are to be seen in each animal (Fig. 85, 4). We
have previously noted that the apparatus for nuclear division in
unicellular organisms was similar to that in multicellular organisms,
and yet was different from it. In these ciliated infusorians we see
an essential difference, for the striated spindle, after the division
into daughter-chromosomes has taken place, lengthens out enormously,
and becomes so thin in the middle of its length (2) that the two
daughter-nuclei at the ends of this long stalk suggest the appearance
of a very long and thin dumb-bell, or of a long silk purse. Of asters
(centrospheres) there is nothing to be seen, and the mechanism of
division is still very obscure; it almost seems as if a rapidly growing
substance forced the two groups of chromosomes apart.

Hardly have these four descendants of the micronucleus arisen when
three of them begin to break up and very shortly disappear; only the
fourth is of any further importance, and it divides once more (5),
and so gives rise to the two nuclei which play the chief part in the
process of conjugation--the copulation-nuclei, exactly analogous to
the male and female pronuclei in the fertilized ovum (5, _mi_^4). But
in this case each of the two animals functions doubly, that is, both
as male and female, for each sends one of the two copulation-nuclei
across the bridge formed by the union of the apposed surfaces into
the other animal (6, _mi_ ♂), so that it may form, by union with the
nucleus which has remained there, a double nucleus (7), a structure
which corresponds to the segmentation nucleus of the ovum (_copk_).
From it there then arises by division a new macronucleus and a new
micronucleus, not usually directly, however, that is, not by a single
division, but through several successive nuclear divisions, into the
meaning of which I cannot here enter. Immediately after the union of
the two sex-nuclei the two animals sever their connexion with each
other; each begins again to feed, and is subject to multiplication by
division just as it was before conjugation took place (8 and 9).

[Illustration: FIG. 85. Diagram of the conjugation of an Infusorian,
_Paramœcium_, after R. Hertwig and Maupas. 1, two animals with the
mouth-openings apposed; _ma_, the macronucleus beginning to degenerate;
_mi_^1, the micronucleus has already increased considerably in
size and is beginning to divide. 2. each micronucleus has divided
into two daughter-nuclei (_mi_^2), which are connected only by the
division-strand (_ts_). 3, to the left each of the daughter-micronuclei
(_mi_^2) is beginning to divide; to the right this division is already
completed and the grand-daughter-nuclei of the original micronucleus
hang together by their division-strands (_ts_). 4, in each of the
animals there are now four grand-daughter-micronuclei (_mi_^3). 5,
three of these are in process of dissolution, the fourth is dividing
into two great-grand-daughter-nuclei (_mi_^4), which are the two
sex-nuclei. 6, one (the male) sex-nucleus (_mi_ ♂) migrates into the
other animal, and there unites with the remaining (female) sex-nucleus.
7, the conjugation-nucleus (_copk_) being formed. 8, the animals
have separated; the conjugation-nucleus divides into (9) the new
macronucleus (_n ma_) and the new micronucleus (_n mi_).]

Although the course of this remarkable process exhibits all manner of
differences in detail in different species, it is everywhere the same
in its essential feature, and this essential feature is undoubtedly the
union of an equal quantity of the nuclear substance of two animals to
form a new nucleus. It is thus essentially the same process which we
have already recognized among higher animals as 'fertilization.' The
differences are of minor importance, and they arise partly from the
fact that the sex-cells of multicellular animals are not independent
self-supporting units, and partly from their differentiation into
'male' and 'female' cells. The minuteness of the sperm-cell, for
instance, conditions its penetration of the ovum, which is always much
larger and passive, and also the thorough fusion of its cell-body with
the cell-body of the ovum. That this difference has very little deep
significance is best seen from the fact that, even among Infusorians,
there are forms in which the two conjugating individuals are quite
different, especially in size, and in which the much smaller 'male'
animal fuses completely with the much larger 'female,' and indeed bores
its way into it after the manner of a sperm-cell. This is the case
among the bell-animalcules (Vorticellinæ) (Fig. 86), the conjugating
pairs of which had been observed long before our present insight
into these processes had been attained. Indeed, the facts had been
interpreted as a kind of 'budding process,' the minute and differently
shaped 'male' animal (_mi_), which at the time of conjugation is
attached to the larger 'female' (_ma_), was regarded as its bud. This
supposed bud, however, does not grow out from the animal, but into it!

Thus we see here again that a differentiation of individuals as males
and females may occur among unicellular organisms, just as in the
sex-cells of higher animals and plants, and this proves to us once
more that all these differences of sex, whether in reproductive cells
of multicellular organisms, or in the entire multicellular animal or
plant, or finally, in unicellular organisms, are not of essential,
but only of secondary significance, however important they may be for
securing fertilization or conjugation in each special case. They are
always only adaptations to the special conditions, and only occur where
they are necessary to ensure the union, and always in such a manner
that the union of the two cells is facilitated. In most Infusorians
such a differentiation into male and female animals was not necessary,
because these organisms are very motile, and are thus readily able
to meet and unite; it was therefore sufficient for them to remain
hermaphrodite. The bell-animalcules, however, are sedentary, and for
them it was obviously an advantage that, at the time of conjugation,
smaller, free-swimming, and also more simply organized individuals
should arise, which were able to seek out the larger sedentary forms.
Here, then, as in many other unicellular animals, these little male
individuals only occur when they are necessary, that is, at the time of
conjugation. Similarly, in the green alga, _Volvox_, male and female
cells arise only at the time of conjugation, reproduction being at
other times effected by means of parthenogonidia, that is, by elements
which require no fertilization.

[Illustration: FIG. 86. Conjugation of an Infusorian. _Vorticella
nebulifera_, showing sexual differentiation of the whole organism.
After Greef. _I_, the 'microgonidium' or male individual (_mi_)
attaches itself to the 'macrogonidium' or female individual (_ma_);
_cv_, contractile vacuole; _st_, contractile stalk. _II_, the ciliated
circle on the male individual has disappeared. The male has become
firmly embedded in the female by means of a sucker-like retraction
of its lower end. _III_, the fusion of the two individuals has been
completed; the bristly residue of the male (_ct_) is about to be thrown
off; the stalk (_st_) is contracted into a spiral. Magnified about 300
times.]

As these differences are only adaptations to the necessity that the
animals or cells shall find each other and unite, so also are all the
other differences of a sexual kind, the thousand-fold differences
between the sperm-cell and the egg-cell, and the not less numerous
differences between male and female animals, both in 'primary' and
especially in the diverse 'secondary' sexual characters which we have
previously discussed; all these are only means for bringing about the
process of the union of two germ-cells to form a fertilized 'ovum'
which is capable of development. The essential part of this so-called
'sexual reproduction' does not, however, depend on these differences,
neither on the sexual differences of the germ-cells nor on those of
the whole organism; it lies solely in the actual union of the two
germ-cells. Let us remember the idea we have already emphasized, that
the _essential part_ of the so-called 'sexual reproduction' does not
depend on these differences, and let us hold fast to the idea already
indicated, that the chromosomes of the nucleus are the real bearers of
the hereditary tendencies; then we see that the mingling, or, better,
the union of the hereditary substances of two different individuals,
whether single-celled or many-celled, is the result of the process
which we have hitherto called fertilization or conjugation, but which
we shall henceforward designate by the more general term 'Amphimixis'
which means the mingling of substances contributed from two distinct
sources.

Having made ourselves acquainted with the phenomena of amphimixis in
animals, plants, and unicellular organisms, we have to face the problem
of the significance of this remarkable and complicated process. What is
it that happens, and what meaning can we attach to it?

The first thing to be done is to show that the old and long-prevailing
conception of fertilization as _a life-awakening process_ must be
entirely abandoned. That a new individual can arise even among highly
organized animals, quite independently of fertilization, is proved by
the parthenogenetic eggs of insects and crustaceans; fertilization
is not the spark 'which falls into the powder-cask' and causes the
explosion; it is only an indispensable condition of development. As we
have seen, there are germ-cells which are not sexually differentiated,
such as the spores of the lower plants, which are all capable of
development without amphimixis; and parthenogenetic ova prove that
even differentiated female germ-cells, that is, germ-cells originally
adapted for amphimixis, may in certain circumstances develop without
it; amphimixis is thus not the fundamental cause of development, but
is only, for many germ-cells, one of the conditions which must be
fulfilled before development can set in. It is a condition which, under
certain circumstances, may be dispensed with.

If, then, the multiplication of individuals by single-celled germs
can take place independently of amphimixis, we may conclude that the
establishment of amphimixis has nothing to do with the capacity for
multiplication, that it is not a life-awakening process, but is a
process of a unique kind, which means something quite different. The
whole conception of the awakening of life in the germ is antiquated and
out of harmony with the present state of our knowledge. _Life never
begins anew_, as far as we can see, and apart from the possibility
that, unknown to us, a spontaneous generation (_Urzeugung_) of the
lowest forms of life is still taking place, life is continuous and
consists of an infinite series of living forms between which there
is no real interruption. Life, in fact, is like a continuous stream,
the larger and smaller waves of which are particular species and
individuals. Only a few decennia ago a morphologist, who was rightly
held in high esteem, could champion the idea that the mature ovum of
animals was lifeless material, which had to be quickened in order to
develop, but now such a theory is untenable, since we have become
aware of the phenomena of maturation in the ovum, and know that most
important vital processes, the reducing divisions, take place at the
time of maturation, quite independently of fertilization.

Thus we do not even require to take into account the conjugation of
unicellular organisms to make it clear that amphimixis is not the cause
of the origin of new individuals, but a process, _sui generis_, which
may indeed be associated with the beginning of embryonic development,
but which may also occur independently of it, as we see in the case of
unicellular organisms. If, on the one hand, we see development taking
place in spores and parthenogenetic ova independently of amphimixis,
and on the other hand amphimixis occurring without reproduction in
unicellular organisms, we must regard the two phenomena, amphimixis
and reproduction, as processes of a distinct kind, which may, however,
occur in association with and interdependence upon each other.

It was by chance that human observation brought the latter fact to
light first, and therefore led us for so long to accept the idea that
_fertilization_, that is, amphimixis, and _development_, that is,
reproduction, are one and the same; and thus it happens that even now
there are many naturalists who cannot rid themselves of the idea that
amphimixis, if not a life-awakening, is at least a _life-renewing_
process, a so-called 'process of rejuvenescence.'

More than ten years ago[14] I disputed this view, and since then
the facts which make it untenable have become more and more clear.
Notwithstanding this I see that it is still adhered to, at least in a
modified form, by many esteemed naturalists, and so it does not seem
superfluous to discuss it in more detail.

[14] _Die Bedeutung der sexuellen Fortpflanzung für die
Selektionstheorie_, Jena, 1886.

I have already noted that we see in conjugation an amphimixis without
reproduction, and in spores and parthenogenetic ova reproduction
without amphimixis, and I do not doubt that every unprejudiced critic
will admit this; many among us, however, are not unprejudiced, but are
under the spell of earlier ideas, so that they cannot forget that it
was long believed that fertilization was an indispensable condition
of development; they therefore regard the divisions which recommence
sooner or later after conjugation, and which may be repeated hundreds
of times, _as conditioned by the conjugation which preceded them_,
and compare them to the series of cells which, in the Metazoa, lead
from the fertilized ovum to the fully-formed animal. They regard both
series of cell-generations as a developmental cycle, which leads from
fertilization to fertilization again, from conjugation to conjugation,
and which would be impossible without either fertilization or
conjugation.

This play with the idea of a 'cycle' reminds me vividly of similar
fantastic play from the time of the much-despised 'Naturphilosophie' of
a hundred years ago. As men sought to find the analogues of 'solar' and
'planetary' systems in animal and plant, and believed they had stated
something when they compared the motile animals to planets and the
sedentary plants to the sun (!), so it is now imagined that a deeper
insight has been gained by the recognition of cycles of development.
By all means let us regard the development of a multicellular organism
as cyclic; it returns again to its starting-point, but this no more
explains the forces which produce the cycle, and thus the meaning of
fertilization, than a comparison with the circling planets explains
the causes of locomotion in animals. With quite as much reason the
cycle of development might be made to start from the parthenogenetic
ovum, and then the whole conclusion of the fanciful cycle idea in
regard to the meaning of fertilization falls to the ground, for in
this case the cycle begins without fertilization. Attempts are made to
get over this difficulty by showing that in many cases parthenogenesis
alternates regularly or irregularly with sexual reproduction, as in
the water-fleas (Daphnids), the Aphides, and so on. The mysterious
rejuvenating power of amphimixis is supposed to suffice for several
generations, a purely gratuitous assumption, which is also in open
contradiction to the facts. For there are species which now reproduce
exclusively by parthenogenesis, among plants for instance, a number
of fungi, among animals a few species of Crustaceans. Of the latter
it can be demonstrated that ages ago they reproduced sexually, for
they still possess the sac which serves for receiving spermatozoa, but
this sac remains empty, for there are now no males, at least in any
habitat of the species known to us. To this set belongs an inhabitant
of stagnant water, _Limnadia hermanni_, a species of Crustacean which
was found thirty years ago in hundreds, all of the female sex, near
Strassburg, and also many of the little Ostracods (_Cypris_) which
inhabit especially the muddy bottom of our pools and marshes. I bred
one of these (_Cypris reptans_) in numerous aquaria for sixteen years,
during which there were about eighty generations, and throughout this
time no male ever appeared, nor did the sperm-sac of the female ever
contain spermatozoa. The after-effects of the 'rejuvenating' power of
an amphimixis supposed to have taken place earlier must in this case
have been enduring indeed!

For these reasons it seems to me useless to make comparisons between
the developmental cycle of unicellular organisms and the ontogeny of
multicellular organisms. Both processes have indeed many points of
resemblance--long series of cells, then interruption of the divisions
and the occurrence of amphimixis--so that we may quite well speak of
cyclic development in the physiological sense, in as far as certain
internal conditions periodically recur and compel the organism to
conjugation, but we must not suppose that there is more in this than,
for instance, in the 'cyclic development' of Man, which consists in
the fact that he finds himself periodically impelled to take food. The
feeling of hunger which forces him to do so is the signal which warns
the organism that it is time to supply fresh combustible material to
the metabolism. In the same way, after a long series of generations
of Infusorians the necessity for conjugation arises; the whole colony
suffers an 'epidemic of conjugation,' and the animals unite in pairs;
in the meantime we know not why, and must content ourselves with
formulating what is observable, that _the nuclear substances of two
individuals are thereby mingled in each conjugate_.

Obviously the impulse to conjugation is a signal in the same sense as
the feeling of hunger is, and we know well from the higher animals what
a mighty influence it exerts, an influence hardly less potent than that
of hunger. In Schiller's words, 'Durch Hunger und durch Liebe, erhält
sich dies Weltgetriebe.'

We can see clearly enough why Nature should have given animals the
feeling of hunger, but the reason for the need of conjugation is not so
plain; we can only say in the meantime that it must be of some value in
maintaining the forms of life, for only that which fulfils a purpose
can be permanently established.

I shall return later to the problem of the meaning of 'sexual
reproduction,' and try to probe more deeply into the meaning of its
establishment; in the meantime I must restrict myself to having shown
its significance in the union of the hereditary substances of two
individuals, and at the same time to controverting the theory of the
'rejuvenating power' of amphimixis. I use this expression in its
original sense, which indicates that every life is gradually wearing
itself away and would become extinct were it not fanned to flame again
by amphimixis--by an artifice of Nature, we may say. This conception
rests on the fact that the cells of the multicellular body possess for
the most part only a limited length of life, for they are used up by
the processes of life, and they break up and die, some sooner, some
later. As it is observed that all true somatic cells, among higher
animals at least, are subject to this law of mortality, but that the
germ-cells are not, and that, furthermore, the germ-cells only develop
when they are fertilized, the cause of the potential immortality of
the germ-cells is believed to lie in amphimixis, and a 'rejuvenating'
power in fertilization, or, more generally, in amphimixis, is inferred.
Mystical as this sounds, and little as it agrees with our otherwise
mechanical conceptions of the economy of life, it was until very
recently a widespread view, although perhaps it is now abandoned by
many who formerly held it, and has been imperceptibly modified into
a quite different conception, for which the word 'rejuvenescence' is
retained, but with the altered meaning of a mere 'strengthening of the
metabolism' or 'of the constitution.' By many authors, indeed, the two
meanings of the word are not clearly kept apart. I shall return later
to the modified meaning of the word 'rejuvenescence,' and shall keep
in the meantime to the original meaning of the word, which implies a
renewal of life which would otherwise die out.

This meaning seemed to gain a firm hold, when, about fifteen years ago,
the French investigator Maupas published his remarkable observations
on the conjugation of Infusorians. These seemed to show that colonies
of Infusorians which were artificially prevented from conjugating
gradually died out; not of course at once, but after many, often
several hundred, generations; ultimately a degeneration of all the
animals in such colonies set in, and ended only with their utter
extinction. Maupas himself interpreted this as _a senile degeneration_
which took place because conjugation had been prevented, and he
therefore regarded conjugation as a '_rajeunissement karyogamique_,'
a rejuvenescence, and therefore a means of preventing the ageing and
final dying off of the individuals--of obviating, in short, the natural
death to which in his opinion they would otherwise be subject. This
conception was greeted with general approval, and there are many people
who still regard conjugation as a process by which the capacity for
life is renewed--a view which I must still dispute as emphatically as I
did some years ago.

In the first place, the observations on which this theory is based
admit of another interpretation, quite different from that which has
been assumed to be the only possible one. Maupas prevented conjugation,
not perhaps because he had isolated individuals and their progeny, but
by exposing the whole colony of near relatives to unusual conditions
when conjugation was just about to set in, namely, by supplying
them with particularly abundant food. The need for conjugation then
disappeared, as, conversely, it could be called forth at any time in
a colony by hunger. But these are artificial conditions, and indeed
the breeding of Infusorians for months in a small quantity of water on
the object-glass certainly does not correspond to natural conditions.
We must admire the skill of the investigator who was able to keep his
colonies alive for months and years under such artificial conditions,
but we may venture to doubt whether the fate of extinction which did
ultimately overtake them was really due to the absence of conjugation,
and not to the unnaturalness of the conditions.

In any case a repetition and modification of Maupas' experiments is
very desirable, and would be of lasting value[15].

[15] Since the above was written Calkins has made a series of new
experiments, the results of which differed in several respects from
those yielded by Maupas' experiments. When his infusorian-cultures
began to grow weaker, as happened frequently and at irregular
intervals, he was always able to restore them to more vigorous life by
a change of diet, and especially by substituting grated meat, liver,
and the like for infusions of hay. Certain salts, too, had the same
effect: the animals became perfectly vigorous again. Calkins believes
that chemical agents, and especially salts, must be supplied to the
protoplasm from time to time. He reared 620 generations of Paramœcium
without conjugation. But the 620th was weakly and without energy. The
addition of an extract of sheep's brains made them perfectly fresh
and vigorous again. Further experiments in this direction are to be
desired, but, according to those of Calkins, it is probable that
Infusorians can continue to live for an unlimited time even without
conjugation.

Let us, however, assume for the moment not only that Maupas'
observations were correct, which I do not doubt, but also that they
were rightly interpreted. Would they in that case afford a proof that
amphimixis means a rejuvenescence of the power of life? To my thinking,
not in the remotest degree.

It certainly seems as if this were true at the first glance; the
colony which is prevented from conjugating goes on multiplying for a
considerable time, often indeed for hundreds of generations, but this
may be compared with sufferers from hunger, whose life does not cease
at once if the feeling of hunger is not appeased.

It was certainly made evident by these experiments that Infusorians
which were prevented from conjugating were incapable of unlimited
persistence. But even this in no way proves that amphimixis has a
power of rejuvenating life, but simply that these animals are adapted
for conjugation, and that they degenerate without it, just as the
sperm-cell or the ovum dies if it does not attain to amphimixis.

My opponents take it as axiomatic that the life-movement must come to
a standstill of itself, and that it therefore requires help. Even so
distinguished a specialist on the Protozoa as Bütschli argues that
organisms are not _perpetua mobilia_, and when one remembers the
physicist's theory of the impossibility of a _perpetuum mobile_ this
looks at first sight like a formidable objection. But does the organism
always remain the same as long as it lives, like a pendulum which
friction causes to swing more and more slowly till ultimately it comes
to a standstill? We know surely that the phenomena of life arise from
a continual process of combustion, which is followed by a constant
replacement of the used-up particles by new particles; we know that
life depends on an unceasing metabolism, which brings about changes
in the material basis of the organism every moment, so that it is
constantly becoming new again.

I shall attempt to show later on that the cells cannot be the ultimate
elements of the organism, but that the life-units visible with
the microscope must be made up of smaller invisible units. These,
therefore, undergo 'metabolism,' which conditions their multiplication
and their destruction, and this 'metabolism' is not to be seen only in
the building up and breaking down of 'albuminoid substances,' as the
physiologists say, but in the alternation between the multiplication
and the dissolution of these smallest vital particles. Therefore, it
seems to me that the movement of life, whether in a single-celled or
in a many-celled organism, is not to be compared to one pendulum,
but to an endless number of pendulums which succeed one another
imperceptibly in the course of the metabolism, always producing anew
the same result, which therefore may continue _ad infinitum_. Suppose,
then, that we possessed our present conception of life as a process of
combustion, and of metabolism as the agency which continually provides
new combustible material in the shape of new vital particles, but that
we knew nothing about multicellular organisms and their transitory
existence, but were acquainted only with unicellular organisms and
their unlimited multiplication by division. If we were then to make
the observation that all multicellular organisms are mortal, subject
to natural and inevitable death, it would at first appear to us quite
unintelligible, since we should be aware that in these also the fire
of life is continually being fed by the supply of new combustible
material. Not the potential immortality of unicellular organisms would
then appear to us remarkable and surprising, but the limitation of the
life of multicellular organisms--the occurrence of natural death. Who
knows whether, in that case, many of those investigators trained in
regard to unicellular organisms alone would not say just the opposite
of what Bütschli has said, that there could be no natural death in
many-celled organisms, since single-celled organisms prove to us that
life is an endless chain of transitory minute vital units?

Furthermore, our physiologists are still far from being able to explain
the natural death of many-celled organisms from below--I mean from a
knowledge of its necessary causes; on the contrary, they argue from
the known occurrence of natural death to the causes which underlie
it; and thus they have arrived at the idea, undoubtedly correct, that
the somatic cells of the body are gradually so altered by their own
activity that they are ultimately unable to function any longer and
must die off. Therefore, if we were unacquainted with death, we should
not have been able to infer it from our physiological knowledge, and
still less from our knowledge of the unicellulars.

As our insight has in point of fact grown by starting from the
mortal many-celled organisms, and has only later penetrated down to
the unicellular organisms, so we can understand the genesis of the
conclusion, deduced from the mortality of the many-celled organisms,
that unicellular organisms also are unable to continue without limit
the renewal of material and of vital particles, and that consequently
they would be subject to natural death if nature had not found in
conjugation a 'remedy' for 'the physiological difficulties which ensue
automatically and necessarily from the constitution and from the
continual functioning' even of unicellular organisms.

But we ask in vain for a shadow of proof of this remarkable conception;
it is an axiom deduced from our knowledge of natural death among
multicellular organisms, and bolstered up by a mistaken application of
the idea of 'perpetual motion.' Or may we regard it as a proof of this
if it should be found that all unicellular organisms are adapted for
conjugation?

We shall see later on that amphimixis has certainly quite a different
and, undoubtedly, a very important effect, namely, that it increases
the capacity of the species for adaptation; and a life-renewing
effect in Bütschli's sense could only be ascribed to it in addition
if the assumption of the necessity of a natural death in unicellular
organisms were not directly contrary to the clear facts of the case;
but this is just what it is.

We are acquainted with such contradictory facts, not perhaps among
the unicellulars themselves, where it is difficult to procure direct
proof, but in regard to the germ-cells of many-celled organisms which
correspond to unicellular organisms. We know that under certain
circumstances the ovum is capable of persisting by itself--in cases
of parthenogenesis--how then can we conclude that amphimixis is in
the case of Metazoan germ-cells the cause of their capacity for
development? We can only conclude, it seems to me, that their power of
developing is usually bound up with the occurrence of amphimixis. So we
may conclude in regard to the unicellulars that their unlimited power
of multiplication is bound up with the occurrence of amphimixis, but
not that amphimixis is the cause of this power, or that it implies a
rejuvenescence of life. If unicellular organisms could have been made
immortal through amphimixis, then what I maintain would be proved--that
they possess potential immortality; but if they did not possess it, no
artifice in the world could give it to them; amphimixis could be at
most only the condition with the fulfilment of which the realization of
their immortality was bound up.

One may ask, How then can amphimixis be a condition of their survival?
why should Infusorians which have not conjugated at the proper time be
doomed to extinction? And from the standpoint of our present knowledge
I am as little able to give a precise answer as my opponents. But I
can give one in relation to the amphimixis of multicellular organisms,
for in regard to these we know that each of the germ-cells--male and
female--uniting in fertilization, is of itself incapable of development
and doomed to perish, the sperm-cell because it is too small in mass
to be able to develop the whole organism, and the ovum because, in
order to become capable of being fertilized, it must undergo certain
changes which make it incapable of independent development. We have
seen that after the two maturing divisions in the egg-cell have been
accomplished the ovum no longer contains a mechanism of division,
as the centrosphere breaks up after the second division; embryonic
development can therefore only begin when a new centrosphere has
been introduced into the ovum, and this is normally brought about
by fertilization, that is, by the entrance of the sperm-cell, whose
nucleus is accompanied by a centrosphere.

Thus amphimixis is seen to be really a condition of development. But
we now know that the ovum can emancipate itself from this condition,
by only going through a part of the processes of maturation which are
related to the subsequent amphimixis, and by thus retaining its own
centrosome. Nothing is more instructive in this connexion than the
cases we have already briefly discussed of facultative or occasional
parthenogenesis. We have seen that in some insects, for instance in
the silk-moths, there are sometimes, among thousands of unfertilized
eggs, a few that develop little caterpillars. If we examine a large
number of such unfertilized eggs we not infrequently find among them
several which, although they have not gone through the whole course of
development, have at least gone through the earlier stages, and others
which may have advanced somewhat further and then come to a standstill;
in short, we can see that several of these eggs were capable of
parthenogenetic development, although in varying degrees.

The cause of this parthenogenetic capacity has not as yet been
definitely determined by observation, but we shall hardly go wrong
if we seek it in the fact that the centrosphere of the ovum does
not always perish immediately and completely during maturation, and
may persist, rarely in its integrity, but sometimes in a weakened
state. Future observations will probably reveal some differences in
the size or aster-forming power of the centrospheres of such eggs;
in any case it is of the greatest interest that stimuli of various
kinds--mechanical or chemical--can strengthen the disappearing
centrosphere of the ovum, although as yet we are far from being able to
say how this comes about.

The experiments already mentioned of Tichomiroff, Loeb, and Winkler
give us at least an indication how we must picture to ourselves the
origin of parthenogenesis, namely, through the fact that the breaking
up of the apparatus for division, introduced for the sake of compelling
amphimixis, is prevented. Minute changes in the chemistry of the ovum,
similar to those caused artificially in the ova of the sea-urchin by
the introduction of an infinitesimal quantity of chloride of magnesium
(Loeb), in the ovum of the silk-moth by friction or by sulphuric acid
(Tichomiroff), or in the sea-urchin ovum by an extract of the sperm of
the same animal (H. Winkler), will effect this modification, and normal
parthenogenesis is induced.

For the ovum, therefore, amphimixis is certainly not a life-renewing or
rejuvenating factor; it only appears as such because the process has
in the course of nature been made compulsory by making the two uniting
cells each incapable of developing by itself. As we have seen, this is
true also of the sperm-cell, for although it contains a centrosphere,
and would be capable of division as far as that is concerned, yet in
almost all animals and plants it consists of such a minimal quantity
of living matter that it is unable to build up a new multicellular
organism by itself. Only in one alga (_Ectocarpus siliculosus_) has
it been observed that not only the female germ-cell can develop
parthenogenetically under certain circumstances, but that the male-cell
may also do so. In this case, however, the difference in size between
the two is not great, and it is noteworthy that the male plant, in
correspondence with the smaller size of the zoosperm, tends to be a
somewhat poorly developed organism.

If we are forced to the conclusion in regard to multicellular organisms
that amphimixis does not supply the power of development to the ovum,
but that, on the contrary, the power of development is withdrawn
from the ovum, so that amphimixis can, so to speak, be forced, must
we not assume something similar for unicellular organisms also? May
not amphimixis be made compulsory in their case also, in that the
Infusorians in preparation for conjugation go through changes which
make their unlimited persistence possible only on condition that
they conjugate? In my opinion the division of labour in the nucleus,
which is differentiated into a macronucleus and a micronucleus, and
the transitory nature of the former, may be regarded as an adaptation
in this direction. In any case, it is striking that an organ which
otherwise persists without limit among unicellular organisms, the
nucleus, is here subject to natural death after the manner of the body
of multicellular organisms, that it breaks up and must be reformed
from the micronucleus which in this case is alone endowed with
potential immortality. I am inclined to regard this as an arrangement
for compelling conjugation, since it is only after conjugation that
the micronucleus forms a new macronucleus, although the latter
is indispensable to life, as we see from experiments in dividing
Infusorians artificially.

Suppose we had to create the world of life, and it was said to us that
amphimixis must--wherever possible--be secured periodically to all
unicellular and multicellular organisms, what better could we do than
arrange devices which should exclude individuals which, by chance or
constitution, could not attain to amphimixis from the possibility of
further life? But would amphimixis then be the cause of persistence or
a principle of rejuvenescence?

I do not see that there can be any ground for such an assumption
other than the tenacious and probably usually unconscious adherence
to the inherited and deep-rooted idea of the dynamic significance
of 'fertilization,' no longer, perhaps in its original form, which
regarded the sperm as the vital spark which awakened new life in the
dead ovum, but in the modified form of the 'rejuvenating' power of
amphimixis.

Quite recently an attempt has been made to modify the idea of the
'rejuvenating' effect of amphimixis so that it should mean only
an advantage, not an actual condition of persistence. Hartog, in
particular, admits so much, that the occurrence of purely asexual and
purely parthenogenetic reproduction excludes the possibility of our
regarding the process of amphimixis as a condition of the maintenance
of life. But then we must also cease to regard the 'ageing' and dying
off of Infusorians which have been prevented from conjugating as an
outcome of the primary constitution of the living substance, and should
entirely abandon the misleading expression 'rejuvenescence.'

If we fix our attention on the numberless kinds of cells in higher
organisms and on multicellular organisms as intact unities, we see that
they all die off, that they are subject to a natural death, that is, a
cessation of vital movement from internal causes, yet no one is likely
to refer their transitoriness to the fact that they do not enter into
amphimixis. We find it quite 'intelligible' that the cells of our body
should be used up sooner or later as a result of their own function,
though we are very far from being able to demonstrate the necessity for
this, and so really to 'understand' it.

It is only from the standpoint of utility that we can understand the
occurrence of natural death; we see that the germ-cells _must_ be
potentially immortal like the unicellular organisms, but that the
cells which make up the tissues of the body _may_ be transient, and
indeed _must_ be so in the interests of their differentiation--often
great and in one direction--which determines the services they render
to the body. They required to become so differentiated that they
could not continue to live on without limit, and they did become so
differentiated because only thus could an ever-increasing functional
capacity of the whole organism be rendered possible; but they die not
because 'rejuvenescence through amphimixis is denied them, but because
their physical constitution is what it is.' And we must explain the
death of the whole many-celled individual in a similar way. When we
were trying in a previous study to establish the unlimited continuance,
the potential immortality, of unicellular organisms, we noted that an
eternal continuance of the life of the body of multicellular organisms
could certainly not be a necessity, since the continuance of these
forms of life is secured by their germ-cells. A continuance of the
body cannot even be regarded as useful from any point of view. And
what is not useful for a form of life _does not arise as a lasting
adaptation_, which is of course not to say that an immortality of
multicellular organisms, such as they are now, would even have been
possible. If these organisms were to attain to such a high degree of
functional capacity and of structural complexity as they now exhibit,
they obviously could not also have been adapted at the same time to an
eternal persistence of life.

This is in perfect harmony with our whole conception of the impelling
forces in the development of the organic world; the ever-increasing
functional capacity of the structure arose from the advantage which
this afforded in the struggle for existence, in comparison with which
the apparent advantage of the endless life of the individual was of no
account whatever.

I will not here follow out this idea. I have merely touched on it in
order to make clear that the death of individuals in all multicellular
organisms gives us no ground for thinking of the unlimited life of
the germ-cells as dependent on a special artifice of nature, such as
amphimixis is often supposed to be. Let us always remember that there
is parthenogenesis, and that there are unicellular germs (spores)
which are never fertilized, and that the reproduction of many species
of animals and plants occurs in this way without the intervention of
amphimixis at all.

       *       *       *       *       *

Attempts have recently been made to prove that parthenogenesis is
a kind of self-fertilization, and these have been based on the
observations of Blochmann and Brauer, which showed that in the bee and
in the salt-water Crustacean, _Artemia salina_, the reducing second
maturation division of the ovum-nucleus is not suppressed, but is
regularly accomplished, and that the two daughter-nuclei which result
from this division unite with each other subsequently. I have already
noted that these statements do not hold true, at least with regard to
the bee. In this case the second maturing division takes place without
any subsequent fusion of the two daughter-nuclei. According to the
observations of Dr. Petrunkewitsch, which I have already mentioned, and
for the exactness of which I can vouch, the second maturation-spindle
is unusually long, so that the two daughter-nuclei are pushed very far
apart (Fig. 79, _Rsp 2_), and only the inner of the two nuclei (_K 4_)
becomes a segmentation nucleus, while the outer undergoes a remarkable
fate; it unites with the inner nucleus which results from the division
of the _first maturation cell_ (_K 2_), and from this union the
primitive _genital cells of the animal appear to arise_--an observation
the eventual theoretical importance of which can only be estimated
later.

Meantime all we can gain from it is a certain mistrust of the
interpretation of the processes of maturation in _Artemia_ which have
hitherto been given; at least we are tempted to suppose that the
copulation of two nuclei which Brauer observed in _Artemia_ may not
have led to the formation of the segmentation nucleus there either, but
may have had some other significance.

But, even if we leave this point entirely out of account, there
remain all the cases of regular parthenogenesis in which this mode of
reproduction occurs alone and not in alternation with the sexual mode.
In these only one maturing division is undergone, and only one polar
body is formed, and thus there can lie no possibility of supposing a
self-fertilization of the ovum.

[Illustration: FIG. 79. The two maturation divisions in the
unfertilized (drone-forming) egg of the bee, after Petrunkewitsch.
_Rsp 1_, first polar-body in division. _K 1_ and _K 2_, the two
daughter-nuclei thereof. _Rsp 2_, second directive spindle. _K 3_ and
_K 4_, the two daughter-nuclei thereof. In the subsequent stage _K
2_ and _K 3_ unite to form the primordial sex-cell nucleus. Highly
magnified.]

It is possible that we may yet discover species among unicellular
organisms which multiply without limit in the absence of any
amphimixis. R. Hertwig has recently observed phenomena in Infusorians
which he is inclined to refer to the suppression of an earlier habit of
conjugation, and so to a kind of parthenogenesis. But even if it should
be shown that amphimixis plays a part regularly and without exception
in the life of _all_ unicellular organisms, the facts in regard to
multicellular organisms are not affected; and, finally, the process of
amphimixis is one which we have not the slightest ground for assuming
to be either an awakener or a maintainer of life, and so I return
to the most essential part of the whole problem, the meaning of the
chromatin structures, the combination of which is the undoubted result
of amphimixis. Do they really represent, as we assumed earlier, _the
hereditary substance_, and what do we mean by this term?

As far as I know the literature and the development of biological
theories, the botanist Nägeli was the first to deduce, from the
considerable difference in size between the egg-cell and the
sperm-cell, the conclusion that the material basis on which the
hereditary tendencies depend must be a _minimal_ quantity of
substance. The difference is especially great in animals, even in
those species whose eggs may be called small, for instance, those of
sea-urchins or of mammals; even in these the mass of spermatozoon is
scarcely a thousandth part, often scarcely a hundred-thousandth part
of the mass of the ovum. And yet the inheritance from the father and
from the mother is equally great. Now as we know that vital powers
have always a material basis, a minute quantity, such as is contained,
for instance, in the spermatozoon of Man, must have implicitly in
it all the hereditary tendencies of the father; and the conclusion
is inevitable that in the ovum there can only be an equally minimal
quantity of substance which is the bearer of the hereditary powers, for
if there were a larger quantity of hereditary substance in the ovum its
power of transmission would also be greater[16].

[16] The improbable assumption that the hereditary substance of the
father may be in quality altogether different from that of the mother,
and so may have the same power of transmission, and yet take up much
less room, I leave out of the question altogether.

[Illustration: FIG. 69. Ovum of Sea-urchin (_Toxopneustes lividus_),
after E. B. Wilson, _zk_, cell-substance. _k_, nucleus (so-called
germinal vesicle). _n_, nucleolus (so-called germinal spot). Below
there is a spermatozoon of the same animal (_sp_), magnified in the
same proportion, about 750 times.]

[Illustration: FIG. 68. Diagram of a spermatozoon. After E. B. Wilson.
_sp_, apex. _n_, nucleus. _c_, centrosphere. _m_, middle portion. _ax_,
axial filament. _e_, terminal filament.]

If we inquire as to the part of the spermatozoon which bears this
hereditary substance, we may exclude both the tail-thread and the
middle piece (Fig. 68), the former because it obviously fulfils
quite a specialized physiological function and is histologically
adapted to this function, the latter because, from observation on the
spermatozoon which has made its way into the ovum, we know that it
contains the centrosome, the dividing apparatus of the nucleus. Thus
there only remains the 'head' of the spermatozoon, which includes the
nucleus, as the possible vehicle of the heritable substance. Therefore
we are led to seek for the hereditary substance in the nucleus. But
the hereditary substance cannot be a perishable substance which may
at need be dissolved, in the literal sense of the word, and be formed
anew; therefore we cannot look for it in the nuclear membrane, and just
as little in the 'nuclear sap' which fills the meshes of the nuclear
network, since the material on which heredity depends must necessarily
be solid. Nägeli has clearly shown that we must assume a stable,
that is, a solid molecular architecture. There thus remains only the
nuclear reticulum with its chromatin granules, and when we remember
what we have learnt of the behaviour of this chromatin substance during
division and amphimixis we can entertain no doubt that the sought-for
bearer of the inheritance is contained in the substance of the
chromosomes.

The great care with which the chromosomes are halved by means of the
complicated division apparatus led us earlier to regard them as a
substance of complex and manifold qualities and of great physiological
importance; their constant number in any one species, and the reduction
of that number to half by means of the reducing divisions, justify us
in concluding that they are permanent structures, physiological and
morphological units, which undergo no more than an apparent irregular
dispersion during the resting state of the nucleus. Finally, the fact
that these supposed vehicles of inheritance occur in equal numbers in
each of the conjugating germ-cells, and that this number is _always_,
both in animals and in plants, half of the normal number occurring in
somatic cells, is decisive. The logical necessity that the hereditary
substance of both parents should be transmitted to the offspring in
equal quantity could not be more precisely met than it is by the
fact that half the normal number of chromosomes occurs in each of
the sex-nuclei in the ovum. Personally, I have long been certain, on
these grounds, that the chromosomes of the nucleus are the hereditary
substance, and I expressed my conviction on this point almost
simultaneously with Strasburger and O. Hertwig[17].

[17] More precisely, my conclusions were published several months
later than those of the investigators named (1885). I think, however,
that no one who is familiar with my writings for the years immediately
preceding, which are collected in _Aufsätzen über Vererbung und
verwandte biologische Fragen_ (Jena, 1892), will dispute that the idea
was reached by me independently. I attach importance to this because
all my later work is based upon this idea.

But there is also a physiological proof of the meaning of the nuclear
substance; and this we owe, again, to the simultaneous and independent
researches of two investigators, M. Nussbaum and A. Gruber, the
latter working in the Zoological Institute here (in Freiburg), and
at my request. They made experiments on regeneration in unicellular
organisms, and found that Infusorians which were artificially divided
into two, three, or four pieces were able to build up a whole animal
out of each piece, provided that it contained a portion of the
nucleus (macronucleus). The large blue trumpet-animalcule, _Stentor
cœruleus_, is well suited for such experiments, not only on account of
its size, but because it possesses a very long rosary-like nucleus,
which can be easily cut two or three times. When a piece is cut off
which does not contain a portion of the nucleus, it may indeed live
for some days and swim about and contract, but it is incapable of
reconstructing the lost parts, and thus of forming a whole animal, and
it perishes. It is in the nucleus, therefore, that we have to look
for the substance which stamps the material of the cell-body with a
particular form and organization, namely, the form and organization
of its ancestors. But that is exactly the conception of a hereditary
substance or idioplasm (Nägeli). Some modern biologists deny that there
is any hereditary substance _per se_, and believe that the whole of
the germ-cell, cell-body and nucleus together effects transmission.
But though it must be admitted that the nucleus without the cell-body
cannot express inheritance any more than the cell-body without the
nucleus, this is dependent on the fact that the nucleus cannot live
without the cell-body; if it be removed from the cell and put, say,
into water, it bursts and is dissolved. But the cell-body without the
nucleus lives on, though of course only for a few hours or days, and
its metabolism ceases only when it is brought to a standstill by the
failure to replace by nutrition the used-up material. Thus the argument
used by those who deny the existence of a hereditary substance would be
paralleled if we denied that Man possesses a thinking substance, and
maintained that he thinks with his whole body, and even that the brain
cannot think by itself without the body.

I am convinced that it is just as mistaken to maintain that every
part of an organism must contain the hereditary tendencies in the
same degree, or that in unicellular organisms the cell-body is as
important in inheritance as the nucleus (Conklin). If one feels any
doubt on this point, one has only to call to mind Nägeli's inference,
from the minuteness of the spermatozoon, that the hereditary substance
must be minimal in quantity. But even theoretically there is not the
smallest ground for the assumption that the cell-body as well as the
nucleus contains the hereditary qualities, since we find in general
that functions are distributed among definite substances and parts of
the whole organism, and it is just on this division of labour that
the whole differentiation of the body depends. And why should this
principle not have been employed just here where the most important
of all functions is concerned? Why should all living substance be
hereditary substance? Although Nägeli thought of his 'idioplasm'
otherwise than we now think of hereditary substance, although he
wrongly imagined it in the form of strands running a parallel course
through the cell-substance and forming a connected reticulum throughout
the whole body, he recognized at least so much quite correctly,
that there are two great categories of living substance--hereditary
substance or idioplasm, and 'nutritive substance' or trophoplasm, and
that the former is much smaller in mass than the latter. We now add
to this, that the idioplasm must be sought for in the cell-nucleus,
and indeed in the chromatin granules of the nuclear network and of the
chromosomes.

But incontrovertible proof of the fact that the nuclear substance
_alone_ is the hereditary substance was furnished when it was found
possible to introduce into a non-nucleated piece of a mature ovum of
one species the nucleus of another related species, and when it was
seen that the larva that developed from the ovum so treated belonged
to the _second_ species. Boveri made this experiment with the ovum and
spermatozoon of two species of sea-urchin, and believed that he had
succeeded in getting from non-nucleated pieces of the ovum of the first
species, fertilized with the sperm of the second, larvæ of this second
species; but, unfortunately, later control-experiments made by several
investigators, especially by Seeliger, have shown that this result
cannot be regarded as quite certain and indubitable.

I must emphasize again that I am far from regarding the cell-protoplasm
of the ovum as an indifferent substance. It is certainly not only
important but indispensable for the development of the embryo, and it
has assuredly its own specific character, as in every other kind of
cell. It represents, so to speak, the matrix and nutritive environment
in which alone the hereditary substance can unfold its wonderful
powers; it has developed historically, like every other kind of cell,
but it contains nothing more than the inherited qualities of this one
kind of cell-protoplasm, not those of the other cells of the body.

But although the essence of fertilization lies, as we have seen, in
the union of the hereditary substance of two individuals, and not in
a 'quickening' of the ovum, we may quite well speak of a quickening by
fertilization in another sense, if we mean the impulse to embryonic
development, for this is really supplied by the entrance of the
sperm-nucleus with its centrosphere into the ovum. But even this
impulse can, under certain circumstances, be given in another way,
and certainly the awakening of it is not the _end_ of fertilization,
but only the condition without which the end, the union of two kinds
of nuclear substance, could not be attained. There is no indication
whatever that this 'quickening' of the ovum would be necessary for any
other reason except that _the ovum was previously made incapable of
development_. There would be no 'fertilization' were not the mingling
of hereditary substances of fundamental importance for the organic
world.

Moreover, an ovum, or a fragment of an ovum, may also develop of
itself, having only _one_ of the sex-nuclei, and the union of the
hereditary substance of two cells is therefore not indispensable for
the mere production of a new individual.

What has been observed in regard to fragments of ova is particularly
interesting in this connexion. Ernst Ziegler first succeeded in halving
a newly fertilized sea-urchin ovum, so that one half contained the
female and the other the male pronucleus. The latter alone contained
a centrosphere, and developed a blastula larva. Delage carried these
experiments further, and cut an unfertilized but mature sea-urchin
ovum into pieces, and then 'fertilized' the non-nucleated pieces with
spermatozoa. These pieces developed and yielded young larvæ of the
relevant species; so it is clearly seen that even a piece of mature
ovum-protoplasm may undergo embryonic development, provided that
a nucleus furnished with a dividing apparatus penetrates into it.
Unfortunately it is technically impossible to cut such a non-nucleated
and then fertilized fragment of ovum so that one half shall contain
the male nucleus the other its centrosphere. Even without this
_experimentum crucis_ we may say that the half with the male nucleus
would not multiply by division, and that the other probably would,
though it would not go through the regular course of segmentation
processes, because the hereditary substance absolutely necessary for
these was wanting.

But these and similar experiments prove something more, namely, that
the nuclei of the sperm-cell and egg-cell do not, as was formerly
believed, stand in a primary and essential contrast to each other,
which may be described as male and female, but that both are alike in
their deeper essence, and may replace each other. They only differ from
each other as far as the cells to which they belong differ, in this,
namely, that they are mutually attractive; they find each other and
unite, and then go on to develop, which each was previously unable to
do by itself. Widely as the sperm-cell and egg-cell differ in size,
constitution, and behaviour, in regard to essential character they are
alike; they bear the relation--as I expressed it twenty years ago--of
1:1; that is, _they both contain an equal quantity of essentially
similar hereditary substance_, and the quality of this substance is
only individually variable. We should, therefore, speak not of a 'male'
and 'female,' but of a 'paternal' and a 'maternal' nucleus.

All the more recent experiments on 'merogony,' that is, on the
development of fragments of the ovum, confirm this view. Thus Boveri
had already observed that even small pieces of sea-urchin ova which did
not contain the nucleus of the ovum developed, after the spermatozoon
had entered them, into small but otherwise normal larvæ of the species.
More recently Hans Winkler proved the same thing for the ova of plants,
by dividing the ovum of a marine alga (_Cystosira_) into two pieces,
then fertilizing these with water containing sperms, with the result
that he got from both pieces, the nucleated and the non-nucleated, an
embryo of normal appearance. In the latter it could only have been a
'paternal' nucleus which directed the development.

To sum up. Our investigation into the meaning of amphimixis has led
us to the conclusion that it consists in the union of two equal
complements of hereditary substance, contributed by two different
individuals, into one unified nucleus, and that the sole immediate
result of this is _the combination of the hereditary tendencies of two
individuals in one_. Among multicellular organisms this one individual
of dual origin always implies the beginning of a new life, since
amphimixis is indissolubly associated with reproduction, and even
among unicellular organisms it can hardly be disputed that the two
Infusorians which separate after conjugation are no longer the same
as they were before. After amphimixis they must contain a different
combination of hereditary substance from what they had before, and
this must reproduce the parts of the animal in a somewhat modified
form. This is theoretically beyond doubt, although it can scarcely be
established by observation.

We thus know now what 'fertilization' is. Through the labours of the
last decade the veil has been torn from a mystery of nature which for
thousands of years confronted humanity as unapproachable; a riddle has
been solved for the solution of which a few centuries ago men did not
even dare to hope. Not a few have taken part in these labours; some I
have already named, but it is impossible that I should here mention
all who have shared in the achievement by observation or reflection.
Whoever has helped it on even a single step may say to himself that
he has taken an active part in bringing about what must be called
essential progress in human knowledge.

But in the science of nature every new solution implies the cropping up
of a new riddle, and we are immediately confronted with the problem,
Why should nature, in the course of evolution, have interpolated this
process of the mingling of different hereditary substances almost
everywhere in the organic world? This, however, is a problem which we
cannot attack until we have first made ourselves more fully acquainted
with the phenomena of inheritance, and have attempted to reason back
from these to the nature of the hereditary substance. We must, in
short, think out a theory of heredity.




LECTURE XVII

THE GERM-PLASM THEORY

 Conception of the 'id' deduced from the process of
 fertilization--Hereditary substance, 'idioplasm' and
 'germ-plasm'--'Idants'--Evolution or Epigenesis--Herbert Spencer's
 uniform germinal substance--Determinants--Illustrations: _Lycæna
 agestis_--The leaf-butterflies--Insect metamorphosis, limbs of
 segmented animals--Heterotopia--The ultimate living units or
 biophors--Number of determinants--Stridulating organ of the
 grasshopper.


IN proceeding to expound the theory of heredity which has shaped itself
in my mind in the course of my own scientific development, I should
like to begin by pointing out that the hereditary substance of the
germ-cell of an animal or of a plant contains not only the primary
constituents (_Anlagen_) of a single individual of the species, but
rather those of several, often even of many individuals. That this is
so can be proved in several ways.

I start from what I hold to be the proved proposition, that the
chromatin substance of the nucleus is the hereditary substance. We
have seen that this is present in the germ-cells of every species in
the form of a definite number of chromosomes, and that in germ-cells
destined for fertilization, that is, in sex-cells, this number is first
reduced to half, the reduction being effected, as is now proved in
regard to a whole series of animals, by the two last cell-divisions,
the so-called maturation divisions.

We know that the full number is only reached again through amphimixis,
by which process the half number of chromosomes in the male and female
germ-cells are united in a single cell, the 'fertilized ovum,' and in a
single nucleus, the so-called segmentation nucleus. Thus the hereditary
substance of the child is formed half from the paternal, half from the
maternal hereditary substance, and we have seen that this remains so
during the whole development of the child, since, at every succeeding
cell-division each of the paternal and each of the maternal chromosomes
doubles by dividing, and the resulting halves are distributed between
the two daughter-nuclei.

Now if the complete hereditary substance of a germ-cell before the
reducing divisions contains potentially all the primary constituents
of the body, which it does as a matter of course, then it follows that
after the reduction each germ-cell must either contain only half the
primary constituents of the parents or all the primary constituents
must be contained in the half number of chromosomes. The latter seems
to me the only possible assumption, as I shall immediately proceed to
show, and this is as much as to say that the primary constituents of at
least two complete individuals must be contained in the chromosomes of
the segmentation nucleus.

That this conclusion is correct is obvious from the fact that a whole,
that is, a perfect individual with all its parts, develops from the
ovum, and not a defective one. For suppose that each mature germ-cell
contained only half the primary constituents of the body, it would be
impossible that these halves should always exactly complete themselves
to form a whole embryo when they are brought together in fertilization,
after having been halved by mere chance during the preceding reducing
division; it would be much more likely to happen that they did not
complete themselves, and that their union would therefore result
in an individual with certain parts wanting. If, for instance, in
the sperm-cell only the anterior half of the body was potentially
present, and this united with an ovum which likewise contained only
the primary constituents of the anterior half, the embryo resulting
from their union would lack the posterior half of the body, and so
on. Of course so rough a division of the primary constituents is not
to be thought of, but however fine we can imagine the halving of the
mass of primary constituents to be, there would never be any guarantee
that the two cells uniting in amphimixis would complete the mass of
primary constituents again; indeed, the chance that the two exactly
complementary halves of the mass would meet would rather become less
the finer and more complex one imagines the halving by reducing
divisions to be. A perfect embryo with all its parts would rarely
arise, but now one group of parts, now another would be wanting, while
another group might be developed double, or at least would be doubly
present in the primary constituents.

But in addition to this the facts of inheritance show us that the
resemblance to mother and father may express itself simultaneously
in all the parts, or at least in the same parts of the child, as may
be seen with especial clearness among plant-hybrids, and thus the
conclusion is inevitable that even in the half number of chromosomes
all the primary constituents of the whole body are present.

Let us go a generation further. If the species possess four
chromosomes the child will have in its cells two maternal chromosomes
(_A_) and two paternal chromosomes (_B_); what form will this
proportion take in the germ-cells produced by the child? The maturation
division can effect the reduction to two chromosomes in different ways;
there may, for instance, be two paternal chromosomes (_B_) left in the
one, and two maternal chromosomes (_A_) in the other daughter-cell, or
one paternal (_B_) and one maternal (_A_) in the one, and a similar
combination in the other cell. Let us follow the latter case further.
A sperm-cell which contained the combination _A_ and _B_ might meet
in amphimixis with an egg-cell of different origin also containing a
similar combination of chromosomes, let us say a chromosome _C_ from
the mother, and a chromosome _D_ from the father. We should then have
in the segmentation nucleus of the fertilized ovum four different
chromosomes, each of which contained the hereditary substance of one
grandparent; we should have the four chromosomes, _A_, _B_, _C_, _D_,
as the hereditary substance of the grandchild.

_But since, as we have seen, the halved hereditary substance still
contains the whole mass of primary constituents, each one of these
chromosomes must contain the collective primary constituents of the
whole body of the relevant grandparent_[18]. _The hereditary substance
in the fertilized ovum thus consists of several complexes of primary
constituents (chromosomes) each of which (an 'id') comprises within
itself all the primary constituents of a complete individual._

[18] When I say the 'collective' primary constituents of the whole body
of the grandparent this is not expressing it quite precisely, for, as
we shall see later, each individual must arise from the co-operation
of different chromosomes of different origin, not merely from one of
the chromosomes contained in its germ-plasm. In the example given
above, the body of each grandparent cannot have arisen only from
a single chromosome, which was transmitted to his grandchild, but
from the co-operation of this chromosome with three others, which
have distributed themselves along other genealogical paths. But this
does not affect the above chain of reasoning, for here it is not a
question of whether all the primary constituents of the grandparent
are present in the child--that can never be the case--but whether the
primary constituents transmitted by him represent the whole body of an
individual.

It can be made clear in yet another way that, as a consequence of
sexual reproduction, the germ-plasm of each species must be composed
of several 'ids,' _individually different_. Let us assume that there
was as yet no amphimixis, and that we could look on at its introduction
into the organic world; the hereditary substance of the beings which
had previously lived and multiplied by division would consist of
more or less numerous chromosomes similar to each other, so that,
for instance, each individual would contain sixteen identical 'ids.'
But if amphimixis were now to take place for the first time, in the
same manner as it does to-day--that is, after the reduction of the
number of the ids to half--in the first amphimixis eight paternal
ids would unite with eight maternal ids to form the germ-plasm of the
new individual, as is indicated in Fig. 87 by a circle of spheres,
of which ten are white and ten black as a sign of their difference.
We may think of the figure as representing the 'equatorial plate'
of a nuclear spindle with its ids arranged in a circle. Now, if
two organisms of this generation, with two kinds of ids, unite in
amphimixis after previous reduction of the ids, we have figure _B_,
in which the paternal ids (_pJ_) are seen to the left of the line and
the maternal ids (_mJ_) to the right, while each semicircle is in its
turn made up of two kinds of ids, those of the grandparents (_p_^2_J_
and _m_^2_J_, _p_^2_J_^1 and _m_^2_J_^1). The figures _C_ and _D_ show
the two following generations, in which the number of identical ids is
each time reduced to half, because eight strange ids are again mingled
with them; in _C_ only two ids are still identical, and in _D_ all the
ids are individually different, because they have come from different
ancestors of the same species. Of course this would only be the case
if inbreeding were excluded, because through it the ids of the same
forefathers from two or more sides would meet; but prolonged inbreeding
is a rare exception in free nature.

[Illustration: FIG. 87. Diagram to illustrate the operation of
amphimixis on the composition of the germ-plasm out of diverse
ancestral plasms or 'ids.' _A_-_D_, the ids of the germ-plasm of
four successive generations: _A_, consisting of only two kinds of
ids; _B_, of four; _C_, of eight; _D_, of sixteen kinds. _pJ_ and
_mJ_, paternal and maternal ids. _p_^2_J_, grandpaternal; _p_^3_J_,
great-grandpaternal; _p_^4_J_, great-great-grandpaternal ids. The marks
in the ids themselves indicate their individually distinct characters.]

I shall now call the hereditary substance of a cell its 'idioplasm,'
after Nägeli's example, although he sought it in the cell-substance,
not in the nucleus, and had a different theoretical conception of its
mode of action. It was he, however, who conceived and established
the idea of the idioplasm as the bearer of the primary constituents,
an _Anlagensubstanz_, determining the whole structure of the
organism in contrast to the general nutritive protoplasm. Every cell
contains idioplasm, since every cell-nucleus contains chromatin,
but I call the idioplasm of the germ-cells _germ-plasm_, or the
primary-constituent-substance of the whole organism, and the complexes
of primary constituents necessary to the production of a complete
individual--whose presence we have just shown to be theoretically
necessary--I call _ids_. In many cases these 'ids' might be synonymous
with chromosomes, at least in all the cases in which the chromosomes
are simple, that is, are not composed of several similarly formed
structures. Thus in the salt-water Crustacean, _Artemia salina_,
which possesses 168 minute granular chromosomes, each of these
chromosomes must be regarded as an id, for each can in certain
circumstances be thrown out from the ovum by the reducing division,
or it can be brought into the most various combinations with other
chromosomes by fertilization. Each of them must therefore consist of
perfect germ-plasm in the sense that all the parts of an individual
are virtually contained in it; _each is a biological unity, an id_.
But when we see in many animals larger band-shaped or rod-shaped
'chromosomes,' and when these are composed of a series of granules, as
they are, for instance, in the often mentioned _Ascaris megalocephala_,
each of these granules is to be regarded as an id. In point of fact,
we find, instead of the two or four large rod-shaped chromosomes
of _Ascaris megalocephala_, a larger number of smaller spherical
chromosomes in other species of _Ascaris_.

Compound chromosomes consisting of several ids, such as all rod or
band-like elements of the nuclear substance probably are, I designate
'idants.' That they are composed of several individual ids is not
always clearly apparent because of the smallness of the object, and
even in larger ones this may only be seen in certain stages. Thus
we have in Fig. 88, _A_ and _B_, two 'mother-sperm-cells' of the
salamander; _A_ at an earlier stage, in which the individual ids are
not visible; _B_ at a later stage, in which the band has split, and
the rosary-like structure has become at once apparent. It is not
possible, then, to see at once whether each chromosome corresponds to
one or to several ids. A more exact investigation of the processes
of reducing division has shown that there are chromosomes of simple
spherical form, that is, composed of several ids whose 'plurivalence'
cannot be directly recognized, but can only be inferred from their
further development; there are bivalent chromosomes of double value
and quadrivalent chromosomes of fourfold value, which we have to think
of as made up of two or four ids. It would lead us too far to go into
this more precisely, nor does it fall within the scope and intention
of these lectures to inquire into these intimate and still disputed
details.

The germ-plasm of every species of plant or animal is thus composed
of a larger or smaller number of ids or primary constituents of an
individual, and it is through the co-operation of these that the
individual which develops from the ovum is determined.

[Illustration: FIG. 88. Sperm-mother-cells (spermatocytes) of the
salamander. _A_, cross-section of the cell in the aster-stage; the
chromosomes (_chr_) or idants do not reveal that they are compounded
out of many ids, which are, however, quite distinctly seen in
_B_ (_Jd_), where the chromosomes or idants (_chr_) are already
longitudinally split. _zk_, cell-substance. _csp_, centrosome. _c_,
centrosome in division. After Hermann and Drüner.]

We have further to inquire what conception we can form of the
constitution of an id and of its mode of operation. I have already
spoken of 'primary constituents' (_Anlagen_) of which the germ-plasm
consists, but what right have we to think of the parts of an animal as
already contained in the germ in any form whatever? Is it not equally
possible that the germ consists of parts, none of which bear any
definite relation in advance to the parts of the finished animal? Might
not the germ-cell, along with its nucleus, undergo transformations and
regular changes which would successively give rise to new conditions,
namely, the different stages of development, until finally the complete
animal was attained?

We stand here before an old problem, before the two opposed
interpretations--the theory of 'Evolution' and the theory of
'Epigenesis,' which were first ranged against each other long ago, and
which are a cause of strife even now, although in somewhat different
guise.

The theory of 'Evolution' is especially associated with the name of
Bonnet, who elaborated it in detail in the eighteenth century. It
maintains that the development of the ovum to the perfect animal is not
really a new creation, but only an unfolding of invisible small parts,
which were already present in the ovum. It assumes that the parts of
the perfect organism are already preformed in the ovum, and on this
account it is called the 'Preformation Theory.' Bonnet often speaks
of the preformation of the perfect animal in the germ as a 'miniature
model,' although his conception of 'evolution' was not really so crude
as has been often alleged. He expressly emphasized that this miniature
model was not exactly like the perfect animal, but consisted of
'elementary parts' only, which he thought of as a net whose meshes were
filled up during development and by means of nutrition with an infinite
number of other parts. But after all, his conceptions, and those of his
time generally, were very far removed from the biological thinking of
our own day, as may perhaps be most readily understood when I mention
that he regarded death and decay as an 'involution,' as a folding back,
so to speak, by means of which all the parts gained though nutrition
were removed again, so that the net of the miniature model shrank
together to the invisible minuteness that it had in the ovum. So it
remained, he fancied, till it was reawakened at the resurrection,
using the term in the religious sense! He afterwards dropped this
fancy, because the objection was made to it that human beings who had
lost a leg or an arm in this life would necessarily be maimed at the
resurrection!

In Bonnet's time the facts of development were quite unknown, and
not even the stages of the development of the chick from the egg had
been observed. When this was afterwards done the prevalent theory
of 'evolution' necessarily collapsed, for men saw with their own
eyes that a miniature model of the chick did not gradually grow into
visibility and ultimately into the young chick, but that first of all
parts showed themselves in the egg which bore no resemblance at all
to the chick, that these first rudiments were then altered, and that
through continual new formations and transformations the chick finally
appeared. Upon this K. von Wolff based his theory of 'Epigenesis,' or
development through new formations and transformations. He maintained
that the doctrine of 'Evolutio' was false; that there is no miniature
model invisibly contained within the egg; but that from the simple
egg-substance there arises, through the agency of the formative powers
inherent in it, a long series of stages of development, of which each
succeeding one is more complex than the one before, until ultimately
the perfect animal is reached.

This certainly marked considerable progress, for it meant the
beginning of a science of embryology, that is, the science of the
form-development of the animal or plant from the ovum. The result was
not so important in its theoretical aspect, for though the knowledge
had been gained that the young animal goes through a long series of
different stages, it had not been discovered how nature works this
wonder and causes an animal of complex structure to arise from the
apparently simple substance of the ovum. A solution of the difficulty
was found by attributing to the ovum a formative power, afterwards
called by Blumenbach the _nisus formativus_, which possessed the
capacity of developing a complex animal from the simple 'slime,' or, as
we should say, the simple protoplasm.

If we contrast the strictly theoretical part of the two theories, we
find that Bonnet regarded the ovum as something only apparently simple,
but in reality almost as complex as the animal which developed from it,
and that he thought of the latter, not as being formed anew, but as
being unfolded or evolved. That is to say, he thought that rudiments
present from the outset in the ovum gradually revealed themselves and
became visible. Wolff, on the other hand, regarded the ovum as being
what it seemed, something quite simple, out of which only the _nisus
formativus_ could, by a series of transformations and new formations,
build up a new organism of the relevant species.

Wolff's Epigenesis routed Bonnet's theory so completely from the
field that, until quite recently, epigenesis was regarded as the only
scientifically justifiable theory, and a return to the 'evolutionist'
position would have been looked upon as a retrograde step, as a
reversion to a period of fancy which had been happily passed. I myself
have been repeatedly told, with regard to my own 'evolutionistic'
theory, that the correctness of epigenesis was indisputably
established, that is, was a fact, verifiable at any time by actual
observation!

But what are the facts? Surely only that there is a succession of
numerous developmental stages, which we know very precisely in the
case of a great many animals, and that the miniature model which
Bonnet assumed to be in the egg does not exist. Both these facts
are now no longer called in question. But that does not furnish us
with a theory of development, for theory is not the observation of
phenomena or of a series of phenomena, _it is the interpretation of
them_. Epigenesis, as formulated first by Aristotle and again by
Harvey, Wolff, and Blumenbach, certainly offered an interpretation of
development, not, however, by referring only to what was observable,
but by going far beyond it; on the one hand taking the _appearance_ of
a homogeneous germ-substance for reality, and, on the other, assuming
a special power, which caused a heterogeneous organism to arise from a
homogeneous germ.

We cannot now accept either of these assumptions, for we know that the
germ-substance is not homogeneous, and indeed is not merely a substance
but a living cell of complex structure; and we no longer believe
in a special vital force, and therefore not in a special 'power of
development,' which could only be a modification of the former. We are
thus as little able to accept the old epigenesis as the old evolution,
and we must establish a theory of Development and Heredity on a new
basis.

What this basis must be is in a general way beyond doubt. Since it
is the endeavour of the whole of modern biology to interpret life
more and more through the interactions of the physical and chemical
forces bound up with matter, development, too, comes within this aim,
for development is an expression of life. We seek to understand the
mechanism of life, and, as a part of that, the mechanism of development
and of heredity which is closely associated with it.

If we wished to attack the problem of heredity at its roots we
should first of all have to try to understand the process of life
itself as a series of physico-chemical sequences. Perhaps this will
be achieved up to a certain point in the future, but if we were to
wait for this we should in the meantime have to abandon all attempts
at a theoretical interpretation of the phenomena of development and
heredity, and might indeed have to postpone them to the Greek Kalends.
That would be as though, in the practice and theory of medicine, all
investigation into and speculation regarding disease had to wait until
the normal, healthy processes of life were thoroughly understood. In
that case we should now know nothing of bacteria diseases and the
hundred other acquisitions of pathological science: physiology too
would have remained far behind its present level if it had lacked the
fruitful influence of experience in cases of disease, and the ideas
and theories, true and false, which have been based thereon. In the
same way we require a theory of development and heredity if we are to
penetrate deeper into these phenomena, and must have it in spite of the
fact that we are still very far from having a complete causal knowledge
of the processes of life. For the raw material of observation, which
is to some extent fortuitous, will never bring us any further on;
observation must be guided by an idea, and thus directed towards a
particular goal.

It is, however, quite possible to leave aside for the present all
attempts at an explanation of life, and simply to take the elements of
life for granted, and on this basis to build up a theory of heredity.
We have already taken a step in that direction by establishing that
the whole substance of the fertilized ovum does not take part in
heredity in the same degree, but that only a small part, the chromatin
of the nucleus, is to be looked upon as the bearer of the hereditary
qualities, and by deducing, further, that this chromatin is made
up of a varying number of small but still visible units, the ids,
each of which virtually represents the whole organism, or, as I have
already expressed it, each of which contains within itself, as primary
constituents, all the parts of a perfect animal.

It was these 'primary constituents' which led us to the digression in
regard to Bonnet's theory of 'Evolutio' and Wolff's 'Epigenesis.'

Let us now inquire what must be the constitution of such a chromatin
globule, an id, so that, shut up within the nucleus of a living
reproductive cell, it can direct the development of a new organism
which resembles its parent. Two fundamental assumptions present
themselves, and these can be related to every conception of a
'germ-plasm,' even independently of the assumption of ids. Either we
may think of the id as made up of similar or of different kinds of
parts, none of which has any constant relation to the parts of the
perfect animal, or we think of it as composed of a mass _of different
kinds of parts, each of which bears a relation to a particular part
of the perfect animal_, and so to some extent represents its 'primary
constituents' (_Anlagen_), although there may be no resemblance
between these 'primary constituents' and the finished parts. The
assumption of a germ-plasm composed of similar parts, which has
been made, for instance, by Herbert Spencer, may be called the
modern form of epigenesis, while the other assumption is the modern
form of the 'evolution' theory. As the former theory can no longer
call to its aid a 'formative power' as a _Deus ex machina_, it can
only explain development as induced by the influence of external
conditions--temperature, air, water, gravity, position of parts--upon
the chemical components of the germ-plasm, which are everywhere
uniformly mingled; and it makes no difference whether this uniform
germ-plasm is thought of as composed of many different kinds of parts,
as long as those parts are mingled uniformly to make the germ-plasm
and bear no relation to definite parts of the developing animal.
Oscar Hertwig has recently outlined such a theory. Although I cannot
expound it here I must say at least so much with regard to it, and to
all other theories of development founded on a similar basis, that
they could not be accepted even if they were able to offer a workable
explanation of the development of the individual, and for this reason,
that ontogeny is not an isolated phenomenon which can be interpreted
without reference to the whole evolution of the living world, for it
is most intimately associated with this, being indeed a piece of it,
having, as we shall see, arisen from it, and, furthermore, preparing
for its continued progress. _Ontogeny must be explained in harmony
with phylogeny and on the same principles._ The assumption of a
germ-plasm without primary constituents, or of a completely homogeneous
germ-plasm, as Herbert Spencer maintained, is irreconcilable with this,
for, as will be seen, it contradicts certain facts of inheritance and
variation. Therefore all theories founded on this assumption must be
rejected.

There is another and, I believe, weighty consideration which forbids us
to assume a germ-substance without primary constituents. I shall return
to this later, but in the meantime I wish to build up more completely
my own 'germ-plasm' theory.

I assume that the germ-plasm consists of a large number of different
living parts, each of which stands in a definite relation to particular
cells or kinds of cells in the organism to be developed, that is, they
are 'primary constituents' in the sense that their co-operation in the
production of a particular part of the organism is indispensable, the
part being _determined_ both as to its existence and its nature by the
predestined particles of the germ-plasm. I therefore call these last
_Determinants_ (_Bestimmungsstücke_), and the parts of the complete
organism which they determine _Determinates_, or hereditary parts.

It is easy to show on what basis this assumption rests; the phenomena
of inheritance taken in conjunction with those of variation seem to
me to compel us to it. We know that all the parts of an organism are
variable, and that in one individual the same part may be larger, in
another smaller. Not all variations are transmissible, but many of
them, and some very minute ones, are. Thus, for instance, in many human
families there occurs a small pit, hardly as large as the head of a
pin, in the skin of the ear, whose transmission I have observed from
the grandmother to the son and to several grandchildren. In such a case
there must be a minute something in the germ-plasm, not present in
that of other human beings, which causes the origin, in the course of
development, of this little abnormality in the skin.

There are human families in which individuals occur repeatedly,
and through several generations, who have a white lock of hair, in
a particular spot, on an otherwise dark-haired head. This cannot be
referred to external influences, it must depend on a difference in
the germ, on one, too, which does not affect the whole body, not even
all the hairs of the body, but only those of a particular spot on the
surface of the head. It is a matter of indifference whether the white
colouring of the hair-tuft is produced by an abnormal constitution of
the matrix of the hair, or by other histological elements of the skin,
as of the blood-vessels or nerves. It can only depend ultimately on a
divergently constituted part of the germ-plasm, which can only affect
this one spot on the head, and alter it, if it is itself different from
what is usual. On this account I call _it_ the _determinant_ of the
relevant skin-spot and hair-group. In Man such minute local variations
are usually lost after a number of generations, but in animals there
are innumerable phenomena which prove to us that single minute
deviations can become permanent. Thus there lives in Central Europe a
brown 'blue butterfly,' _Lycæna agestis_, which has a little black spot
in the middle of its wing. The same species also occurs in Scotland,
but there, instead of the black spot, it has a milk-white one, and
so-called 'eye-spots' on the under surface of the wing have also
lost their black centres. The species has thus varied transmissibly,
but only in regard to these particular spots on the wing. A slight
variation must therefore have taken place in the germ-plasm which only
affects these few parts of the body, or, to express it otherwise, the
germ-plasms of the ancestral species and of the variety can only be
distinguished by a difference which determines exclusively the scale
colour of these spots. The two germ-plasms differ, I should say, only
as regards the _determinants_ of these wing-scales.

We know from the artificial selection to which Man has subjected and
still subjects his domesticated animals and useful plants, that any
spots and parts of the body which he chooses can be hereditarily
altered, if the desired variations which present themselves are always
selected for breeding, and that this does not necessarily cause
variation in other parts of the body. When, for instance, in the case
cited by Darwin, the comb of a Spanish cock which had previously hung
downwards was made to stand upright because a prize had been offered
for this character, or when a certain breed of hens was 'furnished with
beards,' the results were permanent variations affecting only the parts
on which the fancier's attention had been fixed. In the same way, when
the tail feathers of the Japanese cock are lengthened to three feet
the rest of the plumage does not alter, still less any other part of
the body. Of course there are numerous 'correlated' variations, and
in very many cases the breeder causes a second or third character, on
which he had not fixed his attention, to vary in addition to the one
he was aiming at. But such concomitant variations are not necessary
or inevitable in all cases; and indeed we need not refer them all to
a true correlation of the parts, but may suppose that they depend not
infrequently on the faultiness of our power of observation, which is
not sufficiently keen to control several parts of the body at one
time, and to notice minimal variations in parts on which we have not
specially fixed our attention.

[Illustration: FIG. 13. _Kallima paralecta_, from India; showing the
right under surface in the resting pose. _K_, head. _Lt_, palps. _B_,
limbs. _V_, fore wing. _H_, hind wing. _St_, 'tail' of the latter,
representing the stalk of the leaf. _gl_^1 and _gl_^2, transparent
spots, _Aufl_, remains of 'eye-spots.' _Sch_, a 'mould'-spot.]

So much, at least, is certain, that in all these cases of the
artificial alteration of individual characters the germ-plasm is in
some way changed, but always in such a way that it differs from that
of the ancestral form through such variations alone, and the effect of
these is that only the altered parts are influenced thereby, and not
the whole organism. This again is but another way of saying that only
the _determinants_ of these parts have altered.

We can see from a thousand cases that exactly the same happens in a
state of nature, that there, too, one part changes after another,
until the highest possible degree of adaptation to the conditions has
been attained. In the mimetic resemblance to leaves exhibited among
butterflies this is most clearly seen, for here we are familiar with
the model--the leaf--and we see how one species approximates to it in
a general way only in the total colour, how others develop a brown
stripe crossing the posterior wing obliquely, so that, to a certain
extent, it resembles the midrib of a leaf, how in a third species this
stripe is continued for some distance forward across the anterior wing,
in a fourth it goes a little further, until, finally, in a fifth, it
is continued on to the tip of the anterior wing. This may be seen,
for instance, in the genus _Anæa_, which is rich in species. But even
then a still further increase of the resemblance is possible, for, as
is well known, there are not infrequently imitations of the lateral
veins of the leaf as well, or dark spots which faithfully reproduce the
mould-spot on a damp, decaying leaf, or colourless transparent spots
which probably simulate dewdrops, and so on. All these are variations
relating to individual and distinct groups of wing-scales, which have
varied transmissibly and independently, that is, each of them has been
produced by a variation in the germ-plasm, which brought about a change
in this particular area of the body and in no other.

Let us for a moment assume the impossible, and suppose that we could
look on at the evolution of such a leaf-butterfly; the beginning of the
leaf-imitation might have its cause in the fact that an ancestral form
of _Kallima_, which had previously lived in the meadows, exhibited on
the part of some of its descendants a migration to the woods, and thus
divided into two groups, with a different manner of life--a meadow form
and a wood form. The latter adapted itself to sitting among leaves, and
the midrib of a leaf developed on its wings. In a germ-plasm without
'primary constituents' this variation could only depend on a uniform
variation of all the parts, for these parts are either alike among
themselves, or at any rate have the same value for every part of the
finished organism. But the germ-plasm of the new breed must somehow
differ from that of the ancestral form, otherwise it could produce no
new variety, but only the ancestral form over again. But how could
an animal differing only in one minute part arise from a germ-plasm
which has varied in all its parts, and how could such little steps of
variation be repeated many times in the course of the phylogeny without
the corresponding variations of the germ-plasm becoming so intense
that not only the wing-markings but everything about the animal would
be altered likewise? And yet these 'leaf-pictures' have not originated
suddenly, but by many small steps, so that the germ-plasm must have
varied _in toto_ a hundred times in succession if there are no primary
constituents.

In the Indian species, _Kallima paralecta_, there are no fewer than
five well-marked varieties, the differences between which depend solely
on the manner in which the leaf-picture on the wing is elaborated,
_for the upper surface of the wing is alike in all_. Even a cursory
observation of a collection of these butterflies shows that the lateral
veins of the leaf-picture are quite different in number, distinctness,
and length in the different individuals. On the right half of the wing
there may be as many as six of them indicated (Fig. 13); and it can
be observed that the three middle ones are the longest, most sharply
defined, and darkest, while those lying near the tip and the base of
the mimic leaf are shorter and often even shadowy. On the left side
the second lateral vein in particular distinctly shows indentations
indicative of the rings, inherited from the ancestral forms, which
surrounded the still visible eye-spots (_Aufl_); the third lateral
vein is quite indefinite and shadowy, but nevertheless it runs exactly
parallel to the first two, and thus heightens the deceptive effect. We
can thus distinguish older and more recent elements in the marking--a
proof of the slow and successive origin of the picture.

This is not reconcilable with the conception of a germ-plasm without
primary constituents, however complex a mixture it may otherwise
be. A substance which had to undergo thousands upon thousands of
variations, arising from each other according to law and in the
strictest succession, in order that it might become a definite
organism, predetermined as to all its thousands of parts down to the
most minute, cannot vary over and over again in its whole constitution
without the consequences showing themselves in numerous, or indeed in
_all_, the parts of the body. Such variations in the germ-plasm would
be comparable to many successive deviations of a ship from her course,
which, although the single ones would only cause a minimal deviation
from the true course, would, when summed up in a voyage of some length,
land the vessel at quite another coast than the one intended. If each
individual adaptation of the species depended on a variation of the
whole germ-plasm the wood _Kallima_ would soon retain no resemblance
to its ancestral form, the meadow species; yet we are acquainted with
species of _Kallima_ which do not show the special resemblance to a
leaf, but, for instance, still exhibit the perfectly developed eye-spot
of the ancestral form, and so forth. It follows, therefore, that the
origin of the leaf-picture has not greatly influenced the general
character of the species; and the fact that the upper surface of the
wings has remained the same in all the varieties is in itself enough to
prove this.

Since, then, the resemblance to a leaf cannot have arisen without
something in the germ-plasm varying, since the germ-plasm of a forest
_Kallima_ and a meadow _Kallima_ must be different in something, and
cannot be any more alike than the germ-plasm of a fantail-pigeon and a
carrier, there _must be 'primary constituents' in the germ-plasm_, that
is, vital units whose variation occasions the variation of definite
parts of the organism, and of these alone.

[Illustration: FIG. 17. Caterpillar of _Selenia tetralunaria_ on a twig
of birch. _K_, head. _F_, feet. _m_, protuberances resembling dormant
buds. Natural size.]

It is on such considerations as these that my assumption, that _the
germ-plasm is composed of determinants_, depends. There must be as many
of these as there are regions in the fully-formed organism capable
of independent and transmissible variation, including all the stages
of development. Every part, for instance, of the butterfly's wing,
which is capable of independent and transmissible variation, must,
so I conclude, be represented in the germ-plasm by an element which
is likewise variable, the determinant; but the same must be true of
every independently and transmissibly variable spot of the caterpillar
from which the butterfly developed. We know how markedly caterpillars
are adapted in form and colour to their environment. Let us assume
that the caterpillar of the butterfly which we chose as an example
of wing-marking had the habit of feeding only by night and during
the daytime of resting on the trunk of a tree, or, more precisely,
in the crevices of the bark. It would then resemble the caterpillar
of the moths of the genus _Catocala_ or the Geometers (Geometridæ),
and possess the colour of the bark of the tree in question; the
determinants of the skin would thus have varied to correspond with this
mode of life on the part of the caterpillar, so that the skin would
appear grey or brown. But there cannot be only _one_ determinant of
the caterpillar skin in the germ-plasm, for the bark-like colour of,
for instance, a Geometer caterpillar is not a uniform grey, but has
darker spots at certain places and lighter whitish spots at others,
such as are to be seen on the bark of the twig on which the caterpillar
is wont to rest, or brown-red spots, like those on the cover-scales of
the buds, or little warts and protuberances which exactly correspond
to similar roughnesses on the twigs, to cracks in the bark, and so on.
All these markings are constant, and are to be found in the same spot
in every caterpillar of the species. A large number of regions of the
caterpillar skin must therefore be independently determined by the
germ-plasm; the germ-plasm must contain parts the variations of which
bring about variations only of an independently variable region of the
caterpillar skin. In other words, in the germ-plasm of the butterfly
ovum there must not only be determinants for many regions of the
butterfly's wing, but also for many regions of the caterpillar's skin.

This line of argument, of course, applies to all the bodily parts and
organs of the butterfly and of the caterpillar, as well as to all the
stages of development of the species as far as these parts are able
to vary in such a way that the variation reappears in the following
generation, that is to say, as far as it is transmissibly variable.

But all parts must be transmissibly variable which have exhibited
independent variation in relation to their ancestors. When, for
instance, the eggs of a butterfly (_Vanessa levana_) bear a deceptive
resemblance to the flower-buds of the stinging-nettle on which
the caterpillar lives, not only in form and colour, but in their
pillar-like arrangement, we may conclude that these eggs have varied
transmissibly from those of their ancestors, which had not acquired
the habit of living on the stinging-nettle, in these three respects
independently, that is, uninfluenced by any other variations the
species may have undergone; and that, consequently, the germ-plasm
must contain determinants for the egg-shell, egg-colouring, and so
on. The manner of laying the eggs in the form of pillars depends on a
modification of the egg-laying instinct, which must in its turn depend
on the variations of certain nerve-centres, and we learn from this that
there must be in the germ-plasm determinants for the individual centres
of the nervous system.

It may, perhaps, be suggested that matters could be explained in a
simpler way--that it is enough to assume the presence in the egg of
determinants for all the parts of the caterpillar, and that those of
the butterfly are only formed within the caterpillar.

This suggestion seems justifiable if we confine ourselves to
superficial considerations. We read in every handbook of entomology
that the wings only arise during the life of the caterpillar, and
in a certain sense this is true, for the primary constituents or
primordia of wings only develop into the fully formed wing during the
larval period. But even if these primordia were only formed during
the caterpillar-stage, what could they develop from? Only out of the
material parts of the caterpillar, that is, from some of its living
cells or cell-groups. The constitution of the wings would therefore
be dependent on that of the cells of the caterpillar from which they
arose, so that if these varied transmissibly through the variation
of their determinants contained in the germ, the determinants of the
butterfly which were just developing would vary with them; every
transmissible variation of the caterpillar would necessarily cause a
similar variation in the butterfly, and this does not happen. If any
one hazarded the assumption that the determinants of the butterfly
develop only in the caterpillar, but quite independently of its
constitution, he would either be making an absurd statement, namely,
that the characters of the butterfly were not transmissible at all,
or he would be unconsciously admitting that the determinants of the
butterfly were already contained in the parts of the caterpillar, and
come direct from the germ-plasm.

That the characters of the butterfly do vary independently of those
of the caterpillar I demonstrated many years ago, when we were still
very far away from the idea of the germ-plasm or of determinants. I
demonstrated then that the constancy of the markings of a species can
be quite different in the two chief stages; that the caterpillar may be
very variable, while the butterfly or the moth may be very constant in
all its markings, or conversely. I called attention to the dimorphic
caterpillars which are green or brown, and yet become the same moth
(for instance, _Deilephila elpenor_ and _Sphinx convolvuli_); I cited
the case of the spurge hawk-moth (_Deilephila euphorbiæ_), whose dark
but at the same time motley caterpillars occur in the Riviera at
Nice as a local variety (_Nicæa_), and there wear quite a different
dress--pale clay-yellow, with a double row of large conspicuous dark
yellow eye-spots--while the moth does not differ from our variety
in a single definite character, except in its larger size. At that
time, too, I instituted experiments with the caterpillars of the
smallest of our indigenous Vanessa species (_Vanessa levana_), of
which the majority are black with black thorns, while a minority are
yellowish-brown with yellow thorns; reared separately, both yielded the
same butterfly, though in this case one would be inclined to suppose
that there was some internal connexion between the colour of the
caterpillar and that of the butterfly, since the butterfly also occurs
in two colours. It was shown, however, that the colour of the butterfly
had nothing to do with that of the caterpillar, for it is known to be
dependent on the season, and is a seasonal dimorphism, 'while the two
forms of caterpillar may occur side by side at all times of the year.'

Subsequently I made a similar experiment with the dimorphic
caterpillars of the 'fire'-butterfly (_Polyommatus phlæas_), and it
yielded the same result. The pure green caterpillars became the same
butterflies as those marked with broad red longitudinal stripes, and in
this case we can definitely describe both colours as protective, for
the green form is adapted to the green under surface of the leaf, the
red-striped to the green red-edged stalk of the lesser sorrel (_Rumex
acetosella_).

There was really no necessity for special proofs that the caterpillar
and butterfly vary transmissibly in complete independence of each
other, for the facts of metamorphosis alone are enough to prove it. How
would it have been possible otherwise that the jaws adapted for biting
should, in the primitive insects, and in the locusts which are nearest
to them, remain as a biting apparatus throughout life, while in the
caterpillar they are modified during its pupal stage into the suctorial
proboscis of the butterfly? The parts of insects, therefore, must be
capable of transmissible variation in the stages of life independently
of each other. Not only have the jaws of the leaf-eating caterpillars
remained unaltered, while in the sexually mature animal they have been
gradually modified into a very long and extremely complex suctorial
apparatus, but when at a much later time this proboscis became
superfluous in a species, because the butterfly or moth, from some
cause or another, lost the habit of taking any nourishment at all, its
degeneration exercised no effect on the jaws of the caterpillar, as we
can observe in many hawk-moths, silk-moths and Geometridæ. How could
such a degeneration become transmissible if the caterpillar's jaws,
from which those of the adult are developed, remain the same? We are
thus forced to assume that there is something in the latter which can
vary from the germ, without the jaws themselves being altered thereby.
This 'something' it is which I call 'determinants,' vital particles,
which--however we may try to picture them--are indeed contained in the
cells of the caterpillar's jaws, but are there inactive and do not
influence the structure of these, while, on the other hand, it is their
constitution which determines the form and structure of the suctorial
proboscis of the butterfly down to the minutest details. It must be
these alone which cause the suctorial proboscis to develop, and in some
cases to degenerate again, without bringing about any change in the
corresponding parts in the caterpillar.

[Illustration: FIG. 89. Anterior region of the larva of a Midge
(_Corethra plumicornis_). _K_, head. _Th_, thorax. _ui_, inferior
imaginal disks. _oi_, superior imaginal disks. _ui_^1, _ui_^2, and
_ui_^3, the primordia of the limbs. _oi_^2 and _oi_^3, the primordia of
the wings and 'balancers.' _g_, brain. _bg_, chain of ventral ganglia
with nerves which enter the imaginal disks. _trb_, tracheal vesicle.
Enlarged about 15 times.]

This example seems to me to be preferable to that of the wings of
insects in this respect, that there is no organ in the caterpillar
with a specific function corresponding to the wing of the butterfly.
Yet the two cases are exactly alike, and it would be a mistake to say
that the first primordium of the wing within the caterpillar is not
a part of the caterpillar at all. At first, certainly, it is only a
group of cells on the skin, occurring at a particular spot on the
dorsal surface of the second and third segments of the caterpillar,
and doubtless arising from a single cell of the embryo, the 'primitive
wing-cell,' which, however, has not as yet been demonstrated. But
it is nevertheless an integral part of the caterpillar, which could
neither be wanting, nor be larger or smaller, and so on; which, in
short, does mean something for the caterpillar, although perhaps not
more than any other of the skin-cells. For the butterfly, however, this
area on the skin means the rudiment of the wing; for from it alone
can there arise by multiplication the aggregate of cells which grows
out into a hollow protuberance, enlarges by degrees into a disk, the
imaginal disk, and eventually develops into the form of wing peculiar
to the species. This imaginal disk is connected very early with nerves
and with tracheæ, as may be beautifully seen especially in dipterous
larvæ (Fig. 89, _oi_), and these become later the nerves and tracheæ
of the wing, while thousands of peculiar scale-like hairs develop
on the upper surface; in short, the rudiment becomes a perfect wing
with its specific venation, and with the marking and colouring which
is often so complicated in Lepidoptera. Almost every little spot and
stripe of the latter is handed down with the most tenacious power
of transmission from generation to generation, and each can at the
same time be transmissibly varied; the same is true of the venation,
which is so important systematically just because it is so strictly
hereditary, yet it too can vary transmissibly, as can also the hooked
bristles, the odoriferous apparatus, and, in short, the whole complex
structure of the wing, with all its specific adaptations to the mode of
flight, to the manner of life, and to the colour of the environment.
How is it possible that all this can develop from a skin-cell? Is it
the influence of position that effects it, and could any other cell
of the caterpillar's skin do the same if it were placed in the same
position? Could any neighbour-cell of the primitive wing-cell replace
it if it were destroyed? It is hardly probable, and I think I can even
prove that this is not so. The experiment of killing such a cell in
the living animal has not yet been made; if it should succeed, we may
venture to say in advance that none of the neighbouring skin-cells
will be able to do its work and take its place in developing a wing;
the wing in question will simply remain undeveloped. In the summer of
1897 I hatched a specimen of _Vanessa antiope_ from the pupa, which,
though otherwise normal and well-developed, lacked the left posterior
wing altogether; no trace of it could be recognized. In this case, from
some cause which could no longer be discovered, the first formative
cell of the wing in the hypodermis, or its descendants, must have been
destroyed, and no substitution of another took place, as the defect
showed.

The young science of developmental mechanics attributes to the position
of a cell in the midst of a group of cells a determining value as
regards its further fate, and as far as the cells of the segmenting
ovum are concerned this seems to be true in certain cases, but the
assumption cannot be generally true except in a very subordinate sense.
The formative cell of the wing does not become what it is because of
its relative position in the organism. If this were so it could not
happen that a wing should develop instead of a leg, as was observed
in a _Zygæna_, nor could there be any of those deformities already
referred to, to which the name 'Heterotopia' is applied, and which
consist in the development of organs of definite normal structure, or
at any rate of apparently normal structure in quite unusual places, e.
g. an antenna on the coxa of a leg, or of a leg instead of an antenna
(in _Sirex_), or instead of a wing. It is therefore not some influence
from without that makes that particular skin-cell of the caterpillar
the rudiment of the wing, but the _reason lies within itself_, in its
own constitution. As the whole mass of determinants for the whole body
and for all the stages of its development must be contained within
the ovum and the sperm-cell, so the primitive cell of the butterfly's
wing must contain all the determinants for the building up of this
complicated part; and if the cell gets into a wrong position in the
course of development because of some disturbance or other, a wing
may develop from it in that position if the conditions are not too
utterly divergent. These heterotopic phenomena afford a further proof
of the existence of determinants, because they are quite unintelligible
without the assumption of 'primary constituents' or _Anlagen_.

The hypothesis of determinants in the germ-plasm is so fundamental to
my theory of development that I should like to adduce another case
in its support and justification. The limbs of the jointed-footed
animals, or Arthropods, originally arose as a pair on each segment
of the body, and they were at first alike or very similar both in
their function and in their form. We find illustration of this in
the millipedes, and still more in the species of the interesting
genus _Peripatus_, which resembles them externally, as well as in the
swimming and creeping bristle-footed marine worms (Chætopods) belonging
to the Annelid phylum. We can quite well picture to ourselves that the
whole series of these appendages was represented in the germ-plasm by
a single determinant or group of determinants, which only required
to be multiplied in development. Without disputing whether this has
really been the case in the primitive Arthropods or not, it is certain
that it can no longer be the case in the germ-plasm of the Arthropods
of to-day. In these each pair of appendages must be represented by
a particular determinant. We must infer this from the fact that
the several pairs of these appendages have varied transmissibly,
independently of each other, for some are jaws, others swimming legs,
or merely bearers of the gills or of the eggs; others are walking legs,
digging legs, or jumping legs. In Crustaceans a forceps-like claw
is often borne by the first of the otherwise similarly constructed
appendages, or also by the second or the third, or there may be no
forceps, and so on; in short, we see that each individual pair has
adapted itself independently to the mode of life of its species. This
could only have been possible if each was represented in the germ-plasm
by an element, whose variations caused _a variation only in that one
pair of legs, and in no other_.

It may perhaps be objected that the differences in the appendages may
quite well have had their origin simply during the development of the
animal, while the primary constituents were the same for all, so that a
single determinant in the germ-plasm would suffice. But this could only
be the case if the differences depended not on internal but on external
causes, that is, if the same primary constituents gave rise to a set
of appendages which became different because they were subject in the
course of their development to different modifying influences. But
this is not the case, at least not to the extent that this supposition
would necessitate. Can it be supposed that, for instance, the jumping
legs of the water-flea (_Gammarus_) are a necessary consequence of the
somewhat divergent form of the segments from which they grow? A direct
proof to the contrary may be found in 'Heterotopia,' for in the place
where a posterior limb, modified for holding the eggs, normally occurs
in the crab an ordinary walking leg may exceptionally develop (Fig. 90,
Bethe), or an appendage resembling an antenna may take the place of an
extirpated eye (Herbst). But if there were really only one determinant
in the germ-plasm for all the appendages these would of necessity be
all alike, apart from the larger or smaller differences which might be
stamped upon them by growing from segments different in size and in
nutrition. Such differences, however, are far from being sufficient to
explain the great deviations seen among the appendages of most kinds
of Crustaceans, and still less to explain their adaptation to quite
different functions.

[Illustration: FIG. 90. The Common Shore-Crab (_Carcinus mænas_), seen
from below, with the abdomen forced back. In place of the swimmeret,
which ought to be borne by the fifth abdominal swimmeret, a walking leg
has grown on the left side, and one which properly should belong to the
right side (6). 1-5, thoracic limbs, _ps_1-4, swimmerets of the right
side. _s_6, _s_7, posterior segments of the abdomen. After Bethe.]

It need not be imagined that my argument can be controverted by
saying that _one_ appendage-determinant in the germ may split
itself in the course of development into a series of different
appendage-determinants. The question would then arise, How is it
able to do so? And the answer can be no other than that the single
first determinant had within it several different kinds of elements,
which subsequently separated to determine in different ways the
various appendages. But that is just another way of saying that this
single determinant actually includes within itself several different
determinants. For a determinant means nothing more than an element
of the germ-substance by whose presence in the germ the specific
development of a particular part of the body is conditioned. If we
could remove the determinants of a particular appendage from the
germ-plasm this appendage would not develop; if we could cause it to
vary the appendage also would turn out differently.

In this general sense the determinants of the germ-plasm are not
hypothetical, but actual; just as surely as if we had seen them with
our eyes, and followed their development. Hypothesis begins when we
attempt to make creatures of flesh and blood out of these mere symbols,
and to say how they are constituted. But even here there are some
things which may be maintained with certainty; for instance, that they
are _not_ miniature models, in Bonnet's sense, of the parts which they
determine; and, further, that they are not lifeless material, mere
substances, but living parts, vital units. If this were not so they
would not remain as they are throughout the course of development,
but would be displaced and destroyed by the metabolism, instead of
dominating it as living matter alone can do--doubtless undergoing
oxidation, but at the same time assimilating material from without,
and thereby growing. There cannot be lifeless determinants; they must
be living units capable of nutrition, growth, and multiplication by
division.

And now we have arrived at the point at which a discussion of
the organization of the living substance in general can best be
interpolated.

The Viennese physiologist, Ernst Brücke, forty years ago promulgated
the theory that living matter could not be a mere mixture of chemical
molecules of any kind whatever; it must be 'organized,' that is, it
must be composed of small, invisible, vital units. If, as we must
certainly assume, the mechanical theory of life is correct, if there
is no vital force in the sense of the 'Natur-Philosophie,' Brücke's
pronouncement is undoubtedly true; for a fortuitous mixture of
molecules could no more produce the phenomena of life than a _single_
molecule could, because, as far as our experience goes, molecules
do not live; they neither assimilate, nor grow, nor multiply. Life
can therefore arise only through a particular combination of diverse
molecules, and all living substance must consist of such definite
groups of molecules. Shortly after Brücke, Herbert Spencer likewise
assumed the reality of such vital 'units,' and the same assumption
has been made in more recent times by Wiesner, De Vries, and myself.
In the meantime we can say nothing more definite about the composition
of these bearers of life, or 'biophors,' as I call them, than that
albumen-molecules, water, salts, and some other substances play the
chief part in their composition. This has been found out by analysis
of dead protoplasm; but in what form these substances are contained in
the biophors, and how they affect each other in order to produce the
phenomena of life by going through a ceaseless cycle of disruptions and
reconstructions, is still entirely hidden from us.

We have, however, nothing to do with that here; we content ourselves
with recognizing in the biophors the characteristics of life, and
picturing to ourselves that all living substance, cell-substance, and
nuclear substance, muscle-, nerve-, and gland-substance, in all their
diverse forms, consist of biophors, though, of course, of the most
varied composition. There must be innumerable kinds of biophors in all
the diverse parts of the millions of forms of life which now live upon
the earth; but all must be constructed on a certain fundamental plan,
which conditions their marvellous capacity for life; all possess the
fundamental characters of life--dissimilation, assimilation, growth,
and multiplication by division. We must also ascribe to them in some
degree the power of movement and sensibility.

As to their size, we can only say that they are far below the limits
of visibility, and that even the minutest granules which we can barely
perceive by means of our most powerful microscopes cannot be small
individual biophors, but must be aggregates of these. On the other
hand, the biophors must be larger than any chemical molecule, because
they themselves consist of a group of molecules, among which are some
of complex composition, and therefore of relatively considerable size.

It may now be asked whether the determinants, whose existence we have
already inferred, are not identical with these 'biophors' or smallest
living particles; but that is not the case, at least not generally. We
called determinants those parts of the germ-substance which determine a
'hereditary character' of the body; that is, whose presence in the germ
determines that a particular part of the body, whether it consists of
a group of cells, a single cell, or a part of a cell, shall develop in
a specific manner, and whose variations cause the variations of these
particular parts alone.

Again, it may be asked how large and how numerous such 'hereditary
parts' may be, whether they correspond to every distinct part of a
cell, or to every cell of the body, or only to the larger cell groups.
Obviously the areas which are individually determined from the germ
must differ in size, according as we have to do with an organism which
is small or large, simple or more complex. Unicellular organisms,
such as Infusorians, probably possess special determinants for a
number of cell-organs and cell-parts, although we cannot directly
observe the independent and transmissible variation of these organs;
lowly multicellular animals, such as the calcareous sponges, will
require a relatively small number of determinants, but in the higher
multicellular organisms, as, for instance, in most Arthropods, the
number must be very high, reaching many thousands if not hundreds of
thousands, for in them almost everything in the body is specialized,
and must have varied through independent variation from the germ. Thus
in many Crustaceans the smelling-hairs occur singly on special joints
of the antennæ, and the number of joints furnished with a smelling-hair
is different in different species; the size, too, of the smelling-hairs
themselves varies greatly, being, for instance, much smaller in our
common Asellus than in the blind form from the depths of our lakes, in
which the absence of sight is compensated for by an increased acuteness
of the sense of smell. Thus the smelling-hairs may vary transmissibly
in themselves, while any joint of the antennæ may also produce one
independently through variation. In this case accordingly we must
assume that there are special determinants for the smelling-hairs, and
for the joints of the antennæ. But we cannot always and everywhere
refer identical or approximately similar organs, when there are many
of them, to a corresponding number of determinants. Certainly the
hairs of mammals or the scales of butterflies' wings do not all vary
individually and independently, but those of a certain region vary
together, and are therefore probably represented in the germ-plasm by
a single determinant. These regions often appear to be very small,
as is best seen by the fine lines, spots, and bands which compose
the marking of a butterfly's wing, and still more in the odoriferous
scales occurring in some butterflies, as, for instance, in the blue
butterflies (_Lycæna_). These little lute-shaped scales do not occur
in all species, and they occur in very unequal numbers even in those
which possess them; there are certain species which exhibit only about
a dozen, and these are all on one little spot of the wing. Since these
odoriferous scales must have arisen as modifications of the ordinary
hair-like scales, as one of my pupils, Dr. Köhler, has demonstrated by
comparative studies, these ordinary hair-like scales must have varied
transmissibly at certain spots, that is, their determinants have varied
while those of the surrounding scales have not.

The case is the same in respect to the sound-producing apparatus of
many insects. Many grasshoppers produce sounds by fiddling with the
thigh of the hind leg on the wing, others by rubbing one anterior wing
upon the other, and, indeed, always with one particular vein in one
upon a particular vein in the other. One of these serves as the bow,
the other as the string, of the violin, and the bow is furnished with
teeth (Fig. 91), ranged beside each other in a long row, which have
the same function as the colophonium of the violin, that is, to grasp
and release the strings alternately, and thus to produce resounding
vibrations. My pupils, Dr. Petrunkewitsch and Dr. Georg von Guaita,
have recently proved that these teeth have arisen as modifications of
the hairs which are scattered everywhere over the wing and leg. But
only in this one place, on the so-called 'stridulating-vein,' have
they been modified to form stridulating teeth (_schr_). Thus this
vein must be capable of transmissible variation by itself alone, that
is, there must be parts contained in the germ-plasm, the variation of
which causes a variation solely of this individual vein and its hairs,
possibly even a variation only on certain hairs on this vein.

[Illustration: FIG. 91. Hind leg of a Locustid (_Stenobothrus
protorma_), after Graber. _fe_, femur. _ti_, tibia. _ta_, tarsal
joints. _schr_, the stridulating ridge.]

On the other hand, there are also large regions, whole cell-masses
of the body, which in all probability vary only _en bloc_, as, for
instance, the milliards of blood-cells in Man, the hundreds of
thousands or millions of cells in the liver and other glandular organs,
the thousands of fibres of a muscle, or of the sinews or fascia, the
cells of a cartilage or a bone, and so on. In all these cases a single
determinant, or at least a few in the germ-plasm, may be enough. But in
numerous cases it is impossible to say how large the region is which
is controlled by a single determinant, and it is, of course, of no
importance to the theory. In unicellular organisms the determinants
will control parts of cells, in multicellular organisms often whole
cells and groups of cells.

Perhaps an inference as to the nature of the determinants may be drawn
from this with some probability, in as far as mere parts of cells may
be supposed to have simpler determinants than whole cells and groups of
cells. The determinants in the chromosomes of unicellular organisms
may therefore often consist of single biophors, so that in this case
the conception of biophors would coincide with that of determinants.
In multicellular organisms, on the other hand, I should be inclined
on the whole to picture the determinant as a group of biophors, which
are bound together by internal forces to form a higher vital unity.
This determinant must live as a whole, that is, assimilate, grow, and
multiply by division, like every vital unit, and its biophors must be
individually variable, so that the separate parts of a cell controlled
by them may also be capable of transmissible variation. That they are
so, every highly differentiated cell of a higher animal teaches us;
even the smelling-hairs of a crab exhibit a stalk, a terminal knob, and
an internal filament, and many muscle-, nerve-, and gland-cells are
much more complex in structure.




LECTURE XVIII

THE GERM-PLASM THEORY (_continued_)

 Structure of the germ-plasm--Vital affinities--Division--O.
 Hertwig's chief objections to this theory--Male and female eggs
 in the Phylloxera show differential division--Dispersal of the
 germ-plasm in the course of Ontogeny--Active and passive state
 of the determinants--Predetermination of cells--There are no
 determinants of characters--Liberation of the determinants--Accessory
 idioplasm--Herbst's lithium larvæ--Plant galls--Cells with several
 facultative determinants--Connective tissue in vertebrates--Mesoderm
 cells of Echinoderms--Sexual dimorphism--Female and male
 ids--Polymorphism (_Papilio merope_)--Ants.


I HAVE endeavoured to prove that the germ-substance proper must be
looked for in the chromatin of the nucleus of the germ-cell, and more
precisely still in those ids or chromosomes which we conceive of as
containing the primary constituents (_Anlagen_) of a complete organism.
Such ids in larger or smaller numbers make up the whole germ-plasm of
a germ-cell, and each id in its turn consists of primary constituents
or determinants, i.e. of vital units, each of which determines the
origin and development of a particular part of the organism. We have
now to make an attempt to picture to ourselves how these determinants
predetermine those cells or cell-groups to which they correspond. In
doing so we have to fall back upon mere hypotheses, and in stating any
such hypothesis I wish expressly to emphasize that I am only following
up one of the possibilities which our imaginative faculty suggests.
Nevertheless, to endeavour to form such a conception is certainly not
without use, for it is only by elaborating a theory to the utmost
that we are able to use it in application to concrete cases, thus
stimulating the search for corroboratory or contradictory facts, and
leading gradually to a recognition of the gaps or mistakes in the
theory.

The first condition that must be fulfilled in order that a determinant
may be able to control a cell or cell-group is that it should
succeed in getting into it. It must be guided through the numerous
cell-divisions of ontogeny so that it shall ultimately come to lie
in the cells which it is to control. This presupposes that each
determinant has from the very beginning its definite position in
relation to the rest, and that the germ-plasm, therefore, is not a mere
loose aggregate of determinants, but that it possesses a structure, an
architecture, in which the individual determinants have each their
definite place. The position of the determinants in relation to one
another cannot be due to chance, but depends partly on their historical
development from earlier ancestral determinants, partly on internal
forces, such as we have already assumed for keeping the determinants
together. We may best designate these hypothetical forces 'affinities,'
and in order to distinguish them from mere chemical affinities we
may call them 'vital.' There must be forces interacting among the
different determinants which bind them together into a living whole,
the id, which can assimilate, grow, and multiply by division, in the
same manner as we were forced to assume for the smaller units, the
biophors and single determinants. In the ids, however, we can observe
the working of these forces quite directly, since each chromosome
splits into two halves of equal size at every nuclear division, and not
through the agency of external forces, e.g. the attraction which we may
assume to be exerted by the fibrils of the nuclear spindle, but through
purely internal forces, often long before the nuclear spindle has been
formed at all.

But if the determinants must separate from each other in the course
of development so as to penetrate singly into the cells they are
to control, the id must not only have the power of dividing into
daughter-ids of identical composition, it must also possess the power
of dividing under certain influences into dissimilar halves, so that
the two daughter-ids contain different complexes of determinants. The
first mode of division of the id, and with it of the nucleus and of the
cell, I call _erbgleich_, or integral, the second _erbungleich_, or
differential. The first form of multiplication is the usual one, which
we observe everywhere when unicellular organisms divide themselves into
two equal daughter-units, or when the cells of multicellular bodies
produce their like by division into two. The second is not directly
observable, because a dissimilarity of the daughter-cells, as long as
it lies only in the idioplasm, cannot be actually seen; it can only
be inferred from the different rôle which the two daughter-cells play
in the building up of the individual. When, for instance, one of two
sister-cells of the embryo gives rise to the cells of the alimentary
canal and the other to those of the skin and the nervous system,
I infer that the mother-cell divided its nuclear substance in a
differential way between the two daughter-cells, so that one contained
the determinants of the endoderm, the other those of the ectoderm; or
when a red and a black spot lie side by side and under exactly the same
conditions on the wing of a butterfly, I conclude that the ancestral
cells of these two spots have divided differentially, so that one
received the 'red,' the other the 'black' determinants. Our eyes can
perceive no difference between the nuclear substance of the two cells,
but the same is true of the chromosomes of the paternal and maternal
nuclei in the fertilized ovum, although we know in this case that
they contain different tendencies. In any case we are not justified
in concluding from the apparent similarity of the chromosome-halves
in nuclear division that there cannot be differential division. The
theoretical possibility that there is such differential division cannot
be disputed; indeed, I am inclined to say that it is more easily
imagined than the division of the ids into absolutely similar halves.
Both are only conceivable on the assumption that there are forces which
control the mutual position of the determinants in the ids, that is, on
the assumption of 'affinities.' I shall not follow this further, but
that there are forces operative within the ids which are still entirely
unknown to us is proved at every nuclear division by the _spontaneous_
splitting of the chromosomes.

It has been objected to my theory that such a complex whole as the
id could not in any case multiply by division, since there is no
apparatus present which can, in the division into two daughter-units,
re-establish the architecture disturbed by the growth. But this
objection is only valid if we refuse to admit the combining forces, the
'vital affinities' within the ids, and the same is true for the smaller
vital units. An ordinary chemical molecule cannot increase by division;
if it be forcibly divided it falls into different molecules altogether;
it is only the living molecule, that is, the biophor, which possesses
this marvellous property of growth and division into two halves similar
to itself and to the ancestral molecule, and we may argue from this
that in the division of the ids forces of attraction and repulsion must
likewise be operative[19].

[19] In my book _The Germ-Plasm_ I have already assumed the existence
of 'forces of attraction' in the determinants and biophors, as in the
cells. I did not, indeed, enter into details, but I argued on the same
basis as now (_Germ-Plasm_, p. 64, English edition). My critics have
overlooked this.

I see no reason why we should not assume the existence of such
forces, when we make the assumption that the hundreds of atoms which,
according to our modern conceptions, compose the molecule of albumen
and determine its nature, are kept by affinities in this definite and
exceedingly complex arrangement. Or must we suppose that between the
atom-complex of the molecule and the next higher atom-complex of the
biophor, determinant, and id there is an absolute line of demarcation,
so that we must assume quite different forces in the latter from those
we conceive of as operative in the former? The biophor is ultimately
only a group of molecules, the determinants a group of biophors, the
id a group of determinants, and all the three inferred stages of vital
organization only become real units through the forces operating within
them and combining them into a whole. What compels the chromatin
granules of the resting nucleus to approach each other at the time of
cell-division, to unite into a long, band-like thread, and what is it
that subsequently causes this thread to break up again into a definite
number of pieces? Obviously only internal forces of which we know
nothing further than that they are operative.

We shall see later that this assumption of vital affinities must be
made not only in regard to the cells, but also in regard to entire
organisms whose parts are united by an internal bond, and whose
co-ordination is regulated by forces of which we have as yet no secure
knowledge. In the meantime we may designate these forces by the name of
'vital affinities.'

It must be admitted, however, that some objections of a fundamental
nature have been urged against the assumption of a differential nuclear
division of the hereditary substance. O. Hertwig holds that the
assumption of differential division is essentially untenable, because
it is contradictory to 'one of the first principles of reproduction,'
for 'a physiologically fundamental character of every living being is
the power of maintaining its species.'

This certainly seems so, but a closer examination shows that this
'principle,' although correct enough when taken in a very general
sense, does not really cover the facts, and is therefore incapable
of supporting the inferences drawn from it. If the proposition
expressed the whole truth there could have been no evolution from
the primitive organisms to higher ones, every living being must have
simply reproduced exact copies of itself. Whether the transformations
of species have been sudden or gradual, whether they have been brought
about by large steps or by very small ones, they could only have come
about by breaking through this so-called 'principle' of like begetting
like. In fact, we may with more justice maintain the exact converse
of the principle, and say that 'no living being is able to produce an
exact copy of itself,' and this is true not only of sexual, but of
asexual reproduction.

In ontogeny we see exactly the same thing. There are no two
daughter-cells of a mother-cell which are exactly alike, and the
differences between them, if they increase in the same direction, may
lead in later descendants to entire differences of structure. Indeed
the whole process of development depends on such an augmentation
of the differences between two daughter-cells--on differences which
proceed from within and are definitely pre-established. Here, again,
the facts do not justify us in making a dogma of the proposition that
it is a 'fundamental power' of every living being to maintain its
species by producing replicas of itself. If we look at two directly
successive cell-generations, we can hardly, it is true, in most cases,
perceive any difference between them, just as in the generations
of species; but if we compare the end of a long cell-lineage with
the beginning, then the difference is marked, and we recognize that
the difference is due to a gradual summing up of minute, invisible
deviations. In my opinion these steps of difference cannot possibly
depend merely on direct external influences; they proceed rather from
the hereditary substance the cell receives from the ovum, which,
therefore, in order to attain to such many-sided and far-reaching
differentiation, must have undergone a frequently repeated splitting
up of its qualities. That this splitting is not merely a variation
to which the whole of the hereditary substance of the daughter-cells
is uniformly subject, according to the influences dependent on their
position in relation to other cells of the embryo, will be made clear
from the case of the Ctenophora referred to in the next lecture. A
scarcely less striking example is that of those animals in which the
ova contain the primary constituents for only one sex, in which, in
other words, there are 'male' ova and 'female' ova. This is the case,
for instance, among Rotifers, and in plant-lice such as the vine-pest,
_Phylloxera_. Here the eggs from which males develop are smaller than
those which produce females. The primary constituents for both male
and female are not, as in most animals, contained in the same ovum,
to be liberated on one side or the other by influences unknown to
us, but in each ovum there is only one of the two sets of primary
constituents present, and in this case, therefore, the development of
hermaphrodites, which not infrequently occur in other animals, would
be impossible. But all these ova have been produced by one primitive
reproductive cell, and consequently, at one of the divisions implied
in the multiplication of this first cell, a separation of the male
from the female primary constituents must have taken place, that is, a
differential division of hereditary substance, for which no external
and no intercellular influences can possibly account.

If there is, then, a differential division of the ids and with them
of the whole idioplasm, the germ-plasm of the fertilized ovum must
be broken up in the course of ontogeny into ever smaller groups of
determinants. I conceive of this as happening in the following manner.

In many animals the fertilized ovum divides at the first segmentation
into two cells, one of which gives rise predominantly to the outer,
the other to the inner germinal layer, as in molluscs, for instance.
Let us now assume that this is the case altogether, so that one of the
first two blastomeres gives rise to the whole of the ectoderm, the
other to the whole of the endoderm; we should here have a differential
division, for the developmental import (the 'prospective' of Driesch)
of the primitive ectoderm-cell is quite different from that of the
primitive endoderm-cell, the former giving origin to the skin and the
nervous system, with the sense organs, while the second gives rise
to the alimentary canal, with the liver, &c. Through this step in
segmentation, I conclude, the determinants of all the ectoderm-cells
become separated from those of the endoderm-cells; the determinant
architecture of the ids must be so constructed in such species that
it can be segregated at the first egg-cleavage into ectodermal and
endodermal groups of determinants. Such differential divisions will
always occur in embryogenesis when it is necessary to divide a cell
into two daughter-cells having dissimilar developmental import,
and consequently they will continue to occur until the determinant
architecture of the ids is completely analysed or segregated out into
its different kinds of determinants, so that each cell ultimately
contains only one kind of determinant, the one by which its own
particular character is determined. This character of course consists
not merely in its morphological structure and chemical content, but
also in its collective physiological capacity, including its power of
division and duration of life[20].

[20] Emery has lately called attention to another direct proof of the
existence of differential cell- and nucleus-division. According to
observations made by Giardina, in the water-beetle (_Dytiscus_), one
primitive ovum-cell gives rise, through four successive divisions,
to fifteen nutritive cells and one well-defined ovum-cell. But only
half of the nuclear substance takes part in these divisions, the rest
remains inactive in a condensed, cloudy condition. 'The meaning of
the whole process is obviously that the germ-plasm mass as a whole is
handed over to the ovum-cell, while the nutritive cells receive only
the nuclear constituents which belong to them' (_Biol. Centralbl._, May
15, 1903).

But embryogenesis does not proceed by differential divisions alone,
for integral divisions are often interpolated between them, always,
for instance, when in a bilateral animal an embryonic cell has to
produce by division into two a corresponding organ for the right and
left sides of the body; for instance, in the division of the primitive
genital cell into the rudiments of the right and left reproductive
organs, or in the division of the primitive mesoderm-cell into the
right and left initial mesoderm-cell, but also later on in the course
of embryogenesis, when, for instance, the right or the left primitive
reproductive cell multiplies into a large number of primitive
germ-cells, or in the multiplication of the blood-cells, or of the
epithelial cells of a particular region; in short, whenever mother and
daughter-cells have the same developmental import, that is, when they
are to become nothing more than they already are. In all such cases a
similar group of determinants, or a similar single determinant, must in
the nuclear division penetrate into each of the two daughter-cells.

It is in this way, it seems to me, that the determinants gain entrance
into the cells they are to control, by a regulated splitting up of the
ids into ever smaller groups of determinants, by a gradual analysis
or segregation of the germ-plasm into the idioplasms of the different
ontogenetic stages. When I first developed this idea I assumed that the
splitting process would in all cases set in at the same time, namely,
at the first division of the ovum. But since then, in the controversies
excited by the theory, many facts have been brought to light which
prove that the ova of the different animal groups behave differently,
and that the splitting up of the aggregate of primary constituents may
sometimes begin later--but I shall return to this later on.

If we accept the segregation hypothesis, which is similar in purport
to that advanced by Roux as the' mosaic theory,' it must strike us
as remarkable that the chromatin mass of the nucleus does not become
notably smaller in the course of ontogeny, and even ultimately sink to
invisibility. Determinants lie far below the limits of visibility, and
if there were really only a single determinant to control each cell
there would be no chromatin visible in such a case. This objection has
in point of fact been urged against me, although I expressly emphasized
in advance the assumption that the determinants are continually
multiplying throughout the whole ontogeny, so that in proportion as the
number of the _kinds_ of determinants lying within a cell diminishes
the number of resting determinants of each kind increases. When,
finally, only one kind of determinant is present there is a whole army
of determinants of that kind.

It follows from this conception of the gradual segregation of the
components of the id in the course of development that we must
attribute to the determinants two different states, at least in regard
to their effect upon the cell in which they lie: an active state,
in which they control the cell, and a passive state, in which they
exert no influence upon the cell, although they multiply. From the
egg onwards, therefore, a mass of determinants is handed on by the
cell-divisions of embryogenesis, which will only later become active.

My conception of the manner in which the determinants become active is
similar to that suggested by De Vries in regard to his 'Pangens,' very
minute vital particles which play a determining part in his 'pangen
theory,' similar to that filled by the determinants in my germ-plasm
theory. It seems to me that the determinants must ultimately break
up into the smallest vital elements of which they are composed, the
biophors, and that these migrate through the nuclear membrane into the
cell-substance. But there a struggle for food and space must take place
between the protoplasmic elements already present and the newcomers,
and this gives rise to a more or less marked modification of the
cell-structure.

It might be supposed that the structure of these biophors corresponded
in advance to certain constituent parts of the cell, that there were,
for instance, muscle biophors, which make the muscle what it is, or
that the plant-cells acquired their chlorophyll-making organs through
chlorophyll biophors. De Vries gave expression to this view in his
'pangen theory,' and I confess that at the time there seemed to me
much to be said for it, but I am now doubtful whether its general
applicability can be admitted. In the first place, it does not seem
to me theoretically necessary to assume that the particles which
migrate into the cell-bodies should themselves be chlorophyll or muscle
particles; they may quite well be only the architects of these, that
is to say, particles which by their co-operation with the elements
already present in the cell-body give rise to chlorophyll or muscle
substance. As we are as yet unacquainted with the forces which dominate
these smallest vital particles, as well as the processes which lead
to the histological differentiation of the cells, it is useless in
the meantime to make any further hypotheses in regard to them. But in
any case the biophors which transform the general character of the
embryonic cells into the specific character of a particular tissue-cell
must themselves possess a specific structure different from that of
other biophors, for they must keep up the continuity of the structures
handed on from ancestors, chlorophyll and muscle-substance and the
like, since we cannot assume that these structures, so peculiar and so
complex in their chemical and physical constitution, are formed afresh,
so to speak, by spontaneous generation in each new being, as De Vries
has very rightly emphasized. A specific biophor, for instance, of
muscle substance will produce this substance as soon as it has found
its way into the appropriate cell-body, even though it may not be a
contractile element itself.

To this must be added that the structure of the body and the
distinctive features of an organism do not depend merely on the
histological differentiation of the cells, but quite as much on
their number and arrangement, and on the size and on the frequency
of repetition of certain parts. These distinctive characters are just
as constant and as strictly transmissible, and may be as heritably
variable as those which depend on specific cell-differentiation, and
they must therefore likewise be determinable by definite elements of
the germ-plasm. Obviously enough, however, these elements are not
of the same nature as the known specific histological elementary
particles; they can be neither nerve-, muscle-, nor gland-biophors.
They must rather be vital units of such a kind that they communicate
to the cells and lineage of cells, into whose bodies they migrate from
within the nucleus, a definite vital power, that is, an organization
which regulates the size, form, number of divisions, and so on, of
these cells--in short their whole prospective significance. Always,
however, they act in co-operation with the cell-body into which they
have penetrated.

Throughout we must hold ourselves aloof from the idea that 'characters'
are transmissible. It is customary, indeed, to speak as if this
were so, and it is also necessary, because we can only recognize
the 'characters' of a body, and not the essential 'nature' on which
these characters depend; but the determinants are not seed-grains of
individual characters, but co-determinants of the nature of the parts
which they influence. There are not special determinants of the size
of a cell, others of its specific histological differentiation, and
still others of its duration of life, power of multiplication, and so
on; there are only determinants of the whole physiological nature of a
cell, on which all these and many other 'characters' depend. For this
reason alone I should object to the assumption that the determinants of
the germ are ready-made histological substances. That is as unlikely
as that their groups in the germ-plasm are 'miniature models' of the
finished parts of the body.

I conceive of the process of cell-differentiation as follows: at every
cell-stage in the ontogeny determinants attain to maturity, and break
up so that their biophors can migrate into the cell-bodies, so that the
quality of each cell is thus kept continually under control, and may
be more or less modified, or may remain the same. By the 'maturity'
of a determinant I mean its condition when by continual division it
has increased in number to such a point that its disintegration into
biophors and their migration into the cell-substance can take place.

One more point I must touch upon here, the question of the 'liberation'
or 'stimulation' of the determinants. The activity of an organ never
depends on itself alone; the contraction of a muscle is induced by
a nerve stimulus or by an electric current; the activity of the
nerve-cells of the brain requires the continual stimulus of the
blood-stream, and cannot continue to exist without it; the specific
sensory-nerves and sense-cells of the eye, ear, olfactory organ, and
so on, are all prompted to activity by adequate stimuli. The same is
true in regard to the determinants, they must be 'liberated' if they
are to distribute themselves and migrate into the cell-body; and we
have to ask how that happens, whether it is possibly due only to their
own internal condition, which again would, of course, depend on the
nutritive conditions of the cell in which they lie, or whether it is
perhaps due to some specific stimulus which is necessary in addition to
the fact of 'maturity,' just as a muscle is always ready to contract,
yet only does so when it is affected by a specific stimulus.

From the very first, therefore, I have considered whether it would
not be better to elaborate the determinant theory in such a way that
it would not be necessary to assume a disintegration of the id in
the course of ontogeny, but simply to conceive of every expression
of activity on the part of a determinant as dependent on a specific
stimulus, which in many cases can only be supplied by a definite cell,
that is, by internal influences, and in other cases may be due to
external influences.

Darwin assumed the first of these alternatives in his theory of
Pangenesis, which we have still to outline. In it he attributes to
his 'gemmules' the power of giving rise to particular cells, which,
however, they can only accomplish when they reach the cells which are
the genetic antecedents of those which the gemmules are to control.
Translated into the language of our theory this view would read
as follows: the whole complex of determinants is contained within
every cell, as it is contained in the germ-cell, but at every stage
of ontogeny, that is, in each of the developing cells, only the
determinant which is to control the immediately successive cells is
'liberated,' and that through the stimulus which the specific nature
of the cell supplies to the determinant. In that case there would
necessarily be in every species of animal as many specific stimuli for
determinants as there are determinants. This appeared to me improbable,
and I rejected the hypothesis because of the enormous number of
specific stimuli which it demands, but also on other grounds which will
be touched upon in the course of these lectures.

Although the assumption of an autonomic dissolution of the determinant
complexes of the id in the course of ontogeny seems to me imperative,
I do not by any means reject the interposition of liberating stimuli,
indeed I regard their co-operation as indispensable. Later on we shall
discuss cases in which it is definitely demonstrable that there may be
two alternative sets of homologous determinants present in a cell, but
that on any occasion only one of these becomes active, a fact which we
can only explain on the assumption that only one of these is affected
by the specific liberating stimulus. The phenomena of regeneration, of
polymorphism, of germ-cell formation, &c., compel us to the assumption
that numerous cells, even after the completion of the building up of
the body, contain two or more kinds of determinants, as in a sense
inactive 'accessory idioplasm,' each of which could control the cell
alone, though in reality it only does control it when it is affected by
the appropriate liberating stimulus. I stated this view some years ago
when I attempted to define more precisely the rôle played by 'external
influences as developmental stimuli[21]'. It is not, then, that I
underrate the importance of external influences on the organism, but
I believe that a still larger part of the determination of what shall
happen at a particular point depends on the primary constituents, and
that these are not alike at all parts of the body.

[21] _Äussere Einflüsse als Entwicklungsreize_ [External Influences as
Stimuli to Development], Jena, 1894.

All living processes, therefore, both those of growing and of
differentiation, depend always upon the interaction of external and
internal factors, of the environment and the living substance, and
the resultants of the interaction, namely, the structure of the body
and its parts must necessarily turn out differently, not only when
the germ-substance is different, but when the essential conditions
of development are changed. But that the constitution of the germ is
by far the most potent factor, and that the nature of the results of
development depends on it in a much greater degree than on the external
conditions, has long been known. The conditions, such as warmth, may
vary within certain limits, and yet the frog's egg becomes a frog;
though it does not follow that the result of development may not be
modified through certain changes in the conditions. The interesting
experiments made by Herbst with the eggs of sea-urchins have shown
that, in artificially altered sea-water in which sodium-salts are to a
slight extent replaced by lithium-salts, these eggs develop into larvæ
which only remotely suggest the normal structure, and diverge widely
from it both in external shape and in the form of the skeleton.

Such larvæ are not able to survive, but soon perish; they are,
however, of great interest from the point of view of our theory, for
they show that determinants do not bring forth the same structure
under all circumstances, but that, as I have already said, they are
vital units of specific composition, which play a part in the course
of development, and give rise under normal external influences to
normal parts, while under unusual influences, if these are not such
as to prohibit development altogether, they may give rise to an
abnormally formed part. It must not be forgotten that most composite
parts--indeed, strictly speaking, all the parts--of an animal are not
controlled by a single determinant, but by the many which successively
determine the character of the cells and define the path of development
of the part in question. There are no determinants of 'characters,'
but only of parts; the germ-plasm no more contains the determinants
of a 'crooked nose' than it does those of a butterfly's tailed wing,
but it contains a number of determinants which so control the whole
cell-group in all its successive stages, leading on to the development
of the nose, that ultimately the crooked nose must result, just as the
butterfly's wing with all its veins, membranes, tracheæ, glandular
cells, scales, pigment deposits, and pointed tail arises through the
successive interposition of numerous determinants in the course of
cell-multiplication.

But in both processes we must presuppose _normal conditions of
development_. In regard to the butterfly we know that abnormal
conditions, such as cold during the pupal period, can cause
considerable variation in the colour and marking of the wing, and in
regard to the nose it can scarcely be doubted that, for instance,
persistent pressure on the nasal region would result in a considerable
deviation from the hereditary form.

The case of the lithium-larvæ is similar. Here the chemical conditions
of the first segmentation-cells are modified by the presence of the
lithium-salts, and the determinants which make their way out of the
nucleus in the first and in subsequent cell-generations find an
unusual soil for their activity, which diverges further and further
from the normal with each successive cell-generation. Thus the whole
animal is abnormally formed. The process may perhaps be compared to a
plant which is negatively geotropic and positively heliotropic, that
is, the stem of which tends to grow straight upwards, while all its
green parts grow towards the light. If a plant of this kind have light
shed on it from one side only, the stem with its leaves will grow
obliquely towards that side. If the plant be then turned round so that
it receives light from the other side, the stem in its further growth
will curve in a direction opposite to that which it took before, and so
by continually changing the position of the plant in relation to the
light one could--theoretically at least--produce a plant with a zigzag
stem. But this would not furnish any evidence against the presence of
determinants; there are no 'upright determinants' any more than there
are 'zigzag determinants' or 'crooked nose determinants,' but there are
determinants controlling the nature of the cells which give rise, under
normal conditions of development, to the straight stem, under abnormal
conditions to the zigzag stem, or to a flat nose instead of a crooked
one, and so on.

This consideration should make it clear that plant-galls are not in
the remotest degree a stone of stumbling for the determinant theory,
as some have supposed. Of course there can be no 'gall-determinants,'
for galls are not transmissible adaptations of the plants on which they
occur; they arise solely through the larvæ of the gall-insect which
has laid its eggs within the tissues of the plant. But the specific
nature of the different kinds of plant-cells, predetermined by their
determinants, is such that, through the abnormal influences exercised
upon them by the larvæ, they are compelled to a special reaction which
results in the formation of galls. It is marvellous enough that these
abnormal stimuli should be so precisely graded and adjusted that such a
specifically definite structure should result, and in this case there
is obviously a very different state of matters from that obtaining in
most other processes of development, in which the chief determining
factor is rather implied in the nature of the idioplasm, that is, of
the determinants, than in the nature of the external influences. Here,
however, the specific structure of the gall depends mainly on the
quality, variety, and successive effects of the external influences or
stimuli. In discussing the influences of surroundings I shall return
once more to the galls.

My determinants have generally been regarded as if they were like
grains of seed, from which either nothing may arise, under unfavourable
conditions, or just the particular kind of plant from which the seed
itself originated.

This simile is, however, to be taken _cum grano salis_. The whole ovum
is certainly comparable to a grain of seed, but single determinants
or groups of determinants will always be able to adapt themselves
to different influences, and to remain active even under slightly
abnormal conditions, though in that case the resulting structures may
be somewhat divergent. This relative plasticity is indispensable even
in relation to the ceaseless mutual adaptations of the growing parts
of the organism. Not only do the cells which live beside each other
at the same time influence each other mutually, but the influence
extends to the whole cell-lineage. No cell or group of cells develops
independently of all the others in the body, but each has its ancestral
series of cells on whose determinants it is so far dependent, since
these have taken part in determining its own nature, in, so to speak,
supplying the soil in which ultimately its own determinants will
be sown from the nucleus, and whose influence modifies these last
according to its quality. We might therefore say that every part is
determined by all the determinants of its cell-ancestors.

If there be urged against the doctrine of determinants the undoubted
fact of the dependence of individual development on external
conditions, or the capacity that organisms have of functional
adaptation, or especially the power that some parts of the organism
have of taking a different form in response to different stimuli, I can
only say that I see no reason why certain cells and masses of cells
should not be adapted from the first for responding differently to
different stimuli.

Therefore I see no contradiction of the determinant theory when, for
instance, among the higher vertebrates, the cells of the connective
tissue exhibit a great diversity of form, becoming a loose 'filling'
connective tissue in one place, a tense fascia, ligament, or tendon
tissue in another, according as they are subjected to slight pressure
on all sides or to stronger pressure on one side. I see no difficulty
in the fact that this connective tissue forms in one case bone-tissue
with the most accurate adaptation of its microscopic structure to the
conditions of stress and pressure which affect the relevant spot,
or in another case cartilaginous tissue, when the cells are exposed
to varying pressure (as on the surface of joints), or even that it
gives rise to blood-vessels when the pressure of the circulating
blood and the tension of the surrounding tissues supply the necessary
stimulus. It is easy to see how important, indeed how necessary, the
many-sidedness of these cells is for the organism, even leaving out
of account such violent interference as the breaking of a bone, the
irregular healing of broken ends of bones, new joint formation, and
so on, and thinking only of the normal phenomena of growth. While the
bone grows it is continually breaking up in the inside and forming anew
on the surface, and this occurs through the power of the connective
tissue-cells to form different tissues under different influences or
stimuli.

We must therefore assume that there are side by side in the connective
cells of higher vertebrates determinants of bone, of cartilage, of
connective tissue in the narrower sense, and of blood-vessels, and
that one or other of these is liberated to activity according to the
stimulus affecting it. Phenomena occur also in the development of lower
animals which lead us to the same assumption.

Among these is the remarkable behaviour of the primary mesoderm-cells
in the young embryo (gastrula) of the Echinoderms (Fig. 92). At the
point where the primitive gut or archenteron invaginates into the
interior of the hitherto single-layered blastula (Fig. 92, _A_), some
cells are separated off (_M_), and move independently, constantly
multiplying the while, into the clear gelatinous fluid (_G_) which
fills the cavity of the larva, and there they fix themselves, some
on the outer ectodermic layer, others to the various regions and
outgrowths of the archenteron (_Ms_). According as these cells have
established themselves at one or another point, they become connective
tissue, muscle, or skeleton cells of the dermis, or contribute to
the muscular layer of the food-canal and water-vascular system, or,
finally, become skeleton-forming cells of the calcareous ring which
surrounds the gullet of the sea-cucumber. In all this there is nothing
to indicate a determination of the cells in one direction; on the
contrary it seems as if the fate of the individual cells depended on
the chance conditions which may lead them to one place or to another.

[Illustration: FIG. 92. Echinoderm-larvæ. _A_, blastula-stage; the
primary mesoderm-cells (_M_) are being formed at the subsequent
invagination-area of the endoderm (_Ent_). _Ekt_, the ectoderm. _B_,
gastrula-stage; the archenteron (_UD_) has been invaginated (_Ent_),
and between it and the ectoderm (_Ekt_) the mesoderm-cells (_Ms_)
migrate into the gelatinous fluid which fills this cavity. There they
attach themselves partly to the ectoderm, and partly to the endoderm.
After Selenka.]

There are thus three possibilities of development, three kinds of
reaction, implied in these cells, which are all outwardly alike, and
we can only understand their rôle in the building up of this very
symmetrical animal if we assume that of these three only one is in
each case liberated, by the specific stimulus exerted by the immediate
surroundings of the cell, so that it may become, according to the
chance position it takes up after its migration, either a skin-cell, a
muscle-cell, or a skeleton-forming cell.

This case may be compared in some respects with the permanent
colour-adaptation of those caterpillars, in regard to which Poulton
demonstrated that they become almost black if they are reared on
blackish-brown bark, light brown on light bark, and green if they are
kept among leaves, and in all cases permanently so. In this case also
the implicated pigment-cells of the skin may develop in three ways,
according to whether this or that quality of the light releases this or
that determinant.

But in many cases we do not know the quality of the liberating
stimulus, and must content ourselves with imagining it. This is so in
the case of dimorphism of the sexes. It is clear that in the males
of a species the germ-cells develop quite otherwise than they do in
the females, that different determining elements attain to activity
in each sex, and since the primary constituents of both sexes must be
contained in most animals in the ovum and in the spermatozoon, we must
assume that in both there are at once 'ovogenic' and 'spermogenic'
determinants, of which, however, only _one_ kind becomes active in a
given individual. There are, however, both among plants and animals
hermaphrodite individuals, in which both kinds of sexual products are
developed simultaneously or successively.

It is not only the primary sexual characters, however, that compel
us to the assumption of double determinants in the germ-plasm, the
secondary sexual characters do so too. We know very well in relation
to ourselves that 'the beautiful soprano voice of the mother may be
transmitted through the son to the grand-daughter, and that the black
beard of the father may pass through the daughter to the grandson.'
Thus both kinds of sexual characters _must be present in every sexually
differentiated being_, some visible, others latent. In animals the
determinants are sometimes handed on from germ-plasm to germ-plasm
through several generations in a latent state, and only make their
appearance again in a subsequent generation. This is the case in the
water-fleas (Daphnids) and the plant-lice (Aphides), in which several
exclusively female generations succeed one another, and only in the
last of them do males occur again side by side with the females.

The germ-plasm of the ovum which is ripe for development must thus
contain not only the determinants of the specific ova and sperms of the
species, but also those of all the male and female sexual characters,
which we discussed at length in the section on sexual selection. I
then showed that these secondary sexual characters differ greatly
in range and in strength, that among lower animals they are almost
entirely absent, and that among higher forms, such Crustaceans,
Insects, and Birds, they attain to very different grades of development
even among the same species. Thus the birds of Paradise are in most
species brilliantly coloured and adorned with decorative feathers
only in the male sex, while the females are simply blackish-grey, but
there is a single species in which the males are almost as soberly
coloured as the females. Conversely, too, we find that in parrots both
sexes are usually coloured alike, but a few species exhibit a totally
different colouring in the two sexes. In the same way the secondary sex
differences may affect only a few parts of the animal or many, while
in a few species the sexes are so divergent in structure that almost
everything about them may be called different. Examples of this are
the dwarf males of most Rotifers, and the males, more minute still in
proportion to the females, of the marine worm _Bonellia viridis_ (p.
227).

We have now to inquire what theoretical explanation of these facts we
can arrive at in accordance with the germ-plasm theory. That double
determinants, male and female, for the differently formed parts of the
two sexes must be assumed to exist in the germ-plasm has been already
said, and we have to suppose that the same stimulus--usually unknown
to us--which incites the determinants of the primary sexual characters
to activity also liberates those of the secondary characters. But we
may safely go a step further and conclude that there are male and
female _ids_, that is, that the male and female determinants belong
to different ids. I infer this from the fact that in some groups,
such as the Rotifers and certain plant-lice, the ova are sexually
differentiated even at the time of their origin. Males and females
of these animals arise from different kinds of eggs, which are even
externally recognizable. Both develop parthenogenetically, so that
fertilization has nothing to do with it; from the first, therefore,
they must contain ids which consist of determinants of one sex alone.

If this conclusion be correct, then the sexual equipment of the
determinants of the sexual characters must have taken place in the
course of phylogeny in such a way that each id was affected in one
direction only, and we should thus have to assume male and female ids,
even before the separation of the sexes as males and females, and the
same conclusion must be extended to the primary sexual characters. Only
in this way can we understand the fact that differences between the
sexes, at first small, have increased in the course of phylogeny to
such complete divergence of structure as is now exhibited in the forms
we have named, _Bonellia_, the Rotifers, and some parasitic worms.

But there is not only sexual dimorphism, there is also dimorphism
of larvæ, e.g. green and brown caterpillars in certain species of
hawk-moth (_Sphinx_), and there are sometimes not only two but three
or more forms of a species; and in all these cases determinants of the
differential parts must be represented twice, thrice, or several times
in each germ-plasm, in each fertilized ovum, at least in all cases in
which the different forms live together on the same area. In discussing
mimicry we spoke of species of butterfly which were everywhere alike
or nearly so in the male sex, while the females were not only quite
different from the males, but differed greatly in many respects among
themselves. Three different forms of females of _Papilio merope_ occur
in the same region of Cape Colony, each of these resembling a protected
model. All three forms have been obtained from the eggs of one female.
In this case the female ids of the germ-plasm must be represented by
three different sets, one of which, when it is in the majority in the
fertilized ovum, gives rise to the _Danais_-form, the second to the
_Niavius_-form, and the third to the _Echeria_-form of the species.
Phylogenetically considered, it is probable that each of these three
kinds of ids originated by itself, on a more limited area on which the
protected model lived in abundance; but with a wider distribution the
different female ids mingled together, were united through the males
into a single germ-plasm, and now occasionally exhibit all three forms
on the same area. I doubt whether there is any other theory that can
offer an interpretation of these facts, and I regard them, therefore,
as affording further evidence of the real existence of ids.

The polymorphism of social insects must be thought of as similarly
based in the germ-plasm.

In bees there are in addition to the males and females the so-called
workers, and this can only depend on the existence of special kinds
of ids. Those of the workers were originally truly female, but as
many of their determinants underwent variations advantageous for
the maintenance of the species, they were modified into special
'worker-ids.' I postpone for the present any inquiry into the causes by
which these ids come to dominate the ontogeny; obviously it cannot be
by the mere fact of being in a majority over the rest of the ids, as I
indicated in the case of the butterflies with polymorphic females.

In many ants the division of labour goes further still; there are two
kinds of workers in the colony, the ordinary workers and the so-called
'soldiers,' and in this case the worker-id must have developed in two
different directions in the course of phylogeny, and have separated
into two kinds of ids, so that the germ-plasm of these species must
contain four kinds of ids.

I might cite many more cases in regard to which the assumption of two
or more kinds of determinants seems imperative, but I believe that what
has been said is enough to enable any one to think out other cases for
himself.




LECTURE XIX

THE GERM-PLASM THEORY (_continued_)

 Co-operation of the determinants to form an organ: insect
 appendages--Venation of the insect-wing--Deformities in
 Man--Apex of the fly's leg--Proofs of the existence of
 determinants--Claws and adhesive lobes--Difference between a
 theory of development and a theory of heredity--Metamorphosis of
 the food-canal in insects--Delage's theory--Reinke's theory of the
 organism-machine--Fechner's views--Apparent contradiction by the facts
 of developmental mechanics--Formation of the germ-cells--Displacement
 of the germinal areas in the hydro-medusoid polyps, a proof of the
 existence of germ-tracks.


IT would be futile to attempt to guess at the arrangement of the
determinants in the germ-plasm, but so much at least we may say, that
the determinants do not lie beside each other in the same disposition
as their determinates exhibit in the fully-formed organism. This may be
inferred from the complex formative processes of embryogenesis in which
many groups of cells, which in their origin were far apart, combine
together to form an organ. Thus the arrangement of the determinants
in the germ-plasm does not correspond to the subsequent arrangement
of the whole animal, nor are primary constituents of the _complete_
organs contained within the germ-plasm. The organ is undoubtedly
_predetermined_ in the germ-plasm, but it is not _preformed_ as such.

Here, again, the history of development gives us a certain basis of
fact from which to work. Let us consider, for instance, the origin
of the appendages in those insects which in the larval state possess
neither legs nor wings, but exhibit a gradual emergence of these
structures from concealment underneath the integumentary skeleton.
In these cases, as I have already shown in regard to the wings, the
development of the limbs arises from definite groups of cells in the
skin. These must therefore be regarded as the formative, and therefore
as the most important and indispensable, parts of the rudiments, and
may be designated the imaginal disks, as I many years ago proposed[22]
(Fig. 89, _ui_ and _oi_).

[22] _Die Entwicklung der Dipteren_, Leipzig, 1864.

But these disks of cells do not contain the _whole_ leg, but only the
skin-layer of it, the 'hypodermis,' which, however, in this case
undoubtedly determines the form. But the internal parts of the leg,
especially the nerves, tracheæ, and probably also the muscles, are
formed from other cell-groups and grow into the imaginal disk from
outside. Something similar probably takes place in the case of all
organs which are made up of many parts; they are, so to speak, shot
together from several points of origin, from various primordia; and
determinants are brought into co-operation whose relative value in
determining the form and function of the organ may be very diverse.

[Illustration: FIG. 89. Anterior region of the larva of a Midge
(_Corethra plumicornis_). _K_, head. _Th_, thorax. _ui_, inferior
imaginal disks. _oi_, superior imaginal disks. _ui_^1, _ui_^2, and
_ui_^3, the primordia of the limbs. _oi_^2 and _oi_^3, the primordia of
the wings and 'balancers.' _g_, brain. _bg_, chain of ventral ganglia
with nerves which enter the imaginal disks. _trb_, tracheal vesicle.
Enlarged about 15 times.]

For it is undoubtedly a very different matter whether a cell bears
within it the elements which compel it in the course of growth
to develop an organ, for instance a leg, of quite definite size,
sculpture, number of joints, and so on, or whether it only bears the
somewhat vague power of determining that connective tissue or fatty
tissue is to be produced. In the first case it controls the whole
formation of the part, in the second it only fills up gaps or lays
down fat or other substances within itself if these be presented
to it. Between these two extremes of determining power there are
many intermediate stages. Cells which contain the determinants of
blood-vessels, tracheæ, or nerves need not be so definitely determined
that they always give rise to precisely the same blood-vessels, the
same branching of the tracheæ, or the same bifurcation of nerves; they
may probably possess no more than the general tendency to the formation
of such parts, and the special form taken by the nerves, tracheæ, or
blood-vessels may be essentially determined by their environment. Thus
in the morbid tumours of Man, nerves, and especially blood-vessels,
may develop in a quite characteristic manner, which was certainly not
determined in advance, but has been called forth by the stimulus, the
pressure, and other influences of the cellular basis of the tumour.
In short, the cells were only determined to this extent, that they
contained the tendency to give rise to blood-vessels under particular
influences.

It would be a mistake, however, to think of the primary constituents
of all cell-groups as so indefinite. Let us call to mind, for instance,
the venation of the insect wing. It is well known that this is not
only quite different in beetles, bugs, and Diptera from that in the
Hymenoptera, and different again in the butterflies, but that it is
quite characteristic in every individual family of butterflies, and
indeed in every genus. We cannot conceive of the absolute certainty of
development of these very characteristic and constant branchings as
having its roots elsewhere than in the determinants of the germ-plasm,
which, lying within certain series of cells, ultimately cause
particular cell-series of the wing-rudiment to become the wing-veins.
If this were not so, how would it be possible to understand the fact
that every minute deviation in the course of these veins is repeated
in exactly the same way in all the individuals of a genus, while in
all the individuals of an allied genus the venation turns out slightly
different with equal constancy.

But it is quite certain that all determinations are in some degree
susceptible to modifying influences, that they are in very different
degrees capable of variation.

Many deformities of particular parts in Man and the higher animals
may be referred to imperfect or inhibited nutrition of the part
in question during embryonic development; the determinants alone
cannot make the part, they must have a supply of formative material,
and according as this material is afforded more abundantly or more
scantily the part will turn out larger or smaller. In the same way the
pressure conditions of the surrounding parts must in many cases have a
furthering or inhibiting influence, or may even determine the shape.
But it is quite possible, indeed even probable, that other specific
influences are exerted by the cells or cell-aggregates surrounding an
organ which is in process of being formed, just as the stake on which a
twining plant is growing may prompt it to coil. If the stake be absent,
the predetermined twining of the plant cannot attain to more than very
imperfect expression, if indeed it finds any. The spirally coiled
sheath of muscle-cells which occurs so often around blood-vessels
in worms, Echinoderms, and Vertebrates is probably due to similar
processes, that is, on the one hand, to a specific mode of reaction
characteristic of these cells, and predetermined from the germ; on the
other hand, to the external influence of the cell-surroundings without
which the determination of the muscle-cell is not liberated, that is,
is not excited to activity.

[Illustration: FIG. 93. The development of a limb in the pupa of a Fly
(_Sarcophaga carnaria_). _A_, apex of the limb from a pupa four days
old; the jointing is hinted at; _hy_, hypodermis; _ps_, pupal sheath;
_ph_, phagocytes; _tr_, tracheal branch. _B_, the same on the fifth
day; the lumen of the limb is quite filled with phagocytes (_ph_); the
last tarsal joint (_t_^5) is beginning to show a bifid apex. _C_, the
same on the seventh day; the claws (_Kr_) and the adhesive lobes (_hl_)
are formed.]

But even if every determinant requires a stimulus to liberate it,
whether this stimulus consists in currents of particular nutritive
fluids, in contact with other cells, or, conversely, on the removal
of some pressure previously exerted on the cell by its surroundings,
the material cause of a structure is to be sought for not in these
conditions of its appearance, but in the primary constituents which
have been handed on to the relevant cell or cell-group from the germ,
in other words, through its determinants. How, for instance, could
the blunt rounded knob of the rough and clumsily jointed sac of cells
which represents the insect's leg at the beginning of the pupal period
(Fig. 93, _A_) be incited to thicken, to constrict at the root (_B_),
and to form a joint-surface, to broaden out at the end, and produce
two sharply cut points (_C_), which become incurved and form claws
(_kr_), while beneath these a broad flat lobe (_hl_) grows forward, and
with its regularly disposed cells gradually forms the characteristic
adhesive organ of the fly--how could all this happen if there were not
contained within these cells special formative forces which determine
them not only in their form and the rest of their constitution, but
above all in their power of multiplication? No special external
stimulus affects the still unfinished knob of the fly's leg unless it
be the removal of pressure; but this operates regularly, and cannot
be the cause of the growth, at definite places, of claws and adhesive
lobes with all their characteristically placed hairs.

We require to assume that each of the cells composing the primary
rudiment of the limb possessed a determining power which made it grow
and multiply under the given conditions of nutrition and pressure in a
prescribed manner and at a prescribed rate; and we must make the same
assumption in regard to all the daughter and grand-daughter-cells,
and so on. The strictest regulation of the power of multiplication of
each of the implicated cells is a necessary condition of the constant
production of the same two claws and adhesive lobes, the same form of
tarsal joint, the same regular covering of hair, and so on. This exact
determination of the cells can only take place through material vital
particles, and it is these which I call determinants.

I have already said so much about the assumed 'determinants' of the
germ-plasm that it might perhaps be supposed that we have now exhausted
the topic; but the assumption of such 'primary constituents' is so
fundamental, not only for my own germ-plasm theory of to-day and
to-morrow, but also--unless I am much mistaken--for all future theories
of development and inheritance. In point of fact, the conception of
determinants has as yet penetrated so little into the consciousness of
biologists, that I cannot remain content with what I have already said,
but must endeavour to test and to corroborate my thesis by additional
illustrations.

As far as I am aware, only a few zoologists have expressly and
unconditionally agreed with the assumption of determinants; on the
other hand, several biologists have rejected it as fanciful and
untenable, while others have set it aside as a useless playing with
ideas. The last, I am inclined to believe, have not taken the trouble
to think out what the idea is. It has even been objected that there
can be no determinants because we can see nothing of them, and that
they must therefore be pure figments of the imagination, invented to
explain facts which could be explained much more easily and simply in
some other way. From the very first I have stated emphatically that
they have not been, and never will be seen, because they lie far below
the limit of visibility, and thus can at best only become visible
when they are collected in large aggregates like chromatin granules.
Nor have I any objections to make if any one chooses to describe all
the details of their activity as mere hypotheses, such, for instance,
as their distribution during development, their 'maturation,' their
migration from the nucleus, and the manner in which they control the
cell. All this is really an imaginative picture which may be correct
to a certain degree, but may also be erroneous; no formal proof of it
can be obtained at present; and I am content if it be simply admitted
to be possible. On the other hand, the existence of determinants seems
to me to be, in the sense indicated, indubitable and demonstrable.

Let us return for a moment to the claws and adhesive lobes which
are developed on the foot of the fly. It may perhaps be thought
that it is possible to do without the assumption of determinants
for these parts, by assuming that although 'external' influences in
the ordinary sense could not possibly have determined that certain
cells of the apex of the leg should form claws and others adhesive
lobes, the result might be due to the differences of intercellular
pressure within the apical knob; these may have been stronger in one
direction, weaker in another, thus prompting the cells to grow here
into claws and there into adhesive lobes. If we had merely to explain
from the constitution of the germ-plasm the ontogeny or development
of these parts in an individual fly there might perhaps be no radical
objection to this view, though it would hardly be possible to explain
the assumed differences in pressure otherwise than as due to a
different intensity of growth in the cells in the various regions of
the limb-apex, which again would have to be referred to differences in
the germ-plasm. But when we reflect that these parts vary hereditarily
and independently of other parts, and owe their present form to their
power of doing so, and that they are differently formed in every
genus and species, we see at once that they must be represented in
the germ-plasm by particular vital particles, which are the roots of
their transmissible variability, that is, which must have previously
undergone a corresponding variation if the relevant parts themselves
are to vary. Without previous variation of the determinants of the germ
no transmissible independent deviation on the part of the claws or
adhesive lobes of the animal is conceivable.

All the opponents of my theory have overlooked this fact; both Oscar
Hertwig and Kassowitz have forgotten that a theory of development
is not a theory of heredity; they only aim at the former, and
they therefore dispute the logical necessity for an assumption of
determinants.

But as this is the very foundation of the theory, let me further submit
the following considerations in its favour.

In insects which undergo metamorphosis, not only the external but the
internal parts of the caterpillar or larva go through a more or less
complete transformation. In the flies (Muscidæ), for instance, the
whole intestinal tract of the larva is reconstructed in the pupa; in
fact it breaks up into a loose, flocculent, dead, but still coherent
mass of tissue. Within this there arises a new intestine, as I have
shown in an early work (1864); and Kowalewsky and Van Rees have since
made us aware of the interesting details of this reconstruction,
showing that the new intestine arises from definite cells of the
old one, which are present in the larval gut at certain fairly wide
distances, and which do not share in the general destruction, but
remain alive, grow, and multiply, and form islands of cells in the dead
mass. These living islands, continually extending, ultimately come
into contact and again form a closed intestinal canal which differs
entirely from that of the larva in its form, in its various areas,
and in its differentiation. In this case those formative cells of the
imago-intestine must have contained the elements which determined
their descendants in number, power of multiplication, arrangement,
and histological differentiation. In other words, each of these
cells must contain the determinants of a particular limited section
of the intestine of the imago. The other cells of the intestinal
epithelium could not do this, even though they were under exactly the
same conditions, were included in the same intimate cell-aggregate,
and had the same nutritional opportunities. They break up when the
formative cells begin to be active, for till then the latter had
remained inactive, and had not multiplied, although they lay regularly
distributed among the other cells. Whence, then, could the entire
difference in the behaviour of these two sets of cells arise, if it
does not depend on the _nature of the cells themselves_, and how could
this difference of nature have developed during the racial history of
insect-metamorphosis if determinants did not reach the cell from the
germ-plasm--determinants which conditioned that some cells should be
hereditarily modified into the cells of the imago-intestine and others
into the larval intestine? Quite similar processes have been recently
demonstrated in regard to the reconstruction of the larval intestine
in other insect-groups. Deegener has done this, for instance, for the
water-beetle (_Hydrophilus piceus_); and it is certain that all these
reconstructions start from particular cells, which lie indifferently
between the active cells during the larval period, and contain the
primary constituents for the formation of a section of the intestine,
but which only become active when their hitherto living neighbours die
and break up.

The whole of the reconstruction of the external form of the fly takes
place in a similar manner. Not only the limb, the head, the stigmata,
but the skin itself is formed anew from imaginal disks. In each of
the abdominal segments three pairs of little cell-islands are formed
during larval life, and these only enter on the stage of formative
activity after pupation, when they multiply rapidly and grow together
to form a segment, whose size, form, and external nature is determined
by them. But it is well known that the abdominal segments of the fly
differ from those of the larva very markedly and in every respect,
so that each cell-island must contain determinants which are quite
different from those in the skin-cells of the corresponding larval
segments. These last break up at the beginning of pupahood, while the
former begin to grow vigorously, and to spread themselves out. The most
remarkable fact about the whole business, and it seems to me also the
most instructive, is that these imaginal disks frequently appear for
the first time during larval life, as I found in the case of a midge,
_Coretha plumicornis_, in regard to the disks of the thorax, and as
Bruno Wahl[23] has recently demonstrated in the case of the abdominal
cell-islands. Since in the young larva the position of the subsequent
imaginal disks is occupied by cells which apparently in no way differ
from the rest of the skin-cells, and are also exposed to precisely the
same external and internal influences, the origination of the imaginal
cells from these can only depend on differential cell-division; the
primordial cell of each imaginal disk must have separated at the
beginning of disk-formation into a larval and an imaginal skin-cell.

[23] Bruno Wahl, _Ueber die Entwickelung der hypodermalen
Imaginalscheiben im Thorax und Abdomen der Larve von 'Eristalis' L.,
Zeitschr. f. wiss. Zool._, Bd. lxx. 1901.

In insects in which the larva and the imago differ widely, the perfect
insect, as regards all its principal parts, is already represented
in the larva, namely, in particular cells which lie among those of
the corresponding larval parts, and do not visibly differ from these,
although they are equipped with quite different determinants, and
consequently enter on their formative activity much later, and give
rise to quite different structures. As the determinants of the whole
animal with all its parts are contained in the ovum, so those of
the parts of its imaginal phase are contained in these cells of the
imaginal disks.

In addition to all this, we have incontrovertible evidence in favour
of the theory of determinants in the independent phyletic variations
of the individual stages of development, on which depends the whole
phenomenon of 'metamorphosis' which we have just been considering. How
could the larval stage have become so different from the imago-stage,
if the one were not alterable by variation arising in the germ
without the other being affected? If this absolute independence of
the transmissible variability of the individual stages were not an
indispensable assumption in the explanation of metamorphosis and other
phenomena of development, I should regard an attempt at a theory of
development without determinants as justifiable. But I am forced to
see in this fact alone an invalidation of all epigenetic theories of
development, that is, of all theories which assume a germ-substance
without primary constituents, which can produce the complicated
body solely by varying step by step under the influence of external
influences, both extra- and intra-somatic. It is possible to conceive
of an ovum in which the living substance is of such a kind that it
must vary in a definite manner under the influence of warmth, air,
pressure, and so on, that it must divide into similar, and subsequently
also into dissimilar parts, which then interact upon each other in
diverse ways and give rise to further variations, which in their turn
result in differentiations and variations, till ultimately we have the
whole complicated organic machine complete and 'finished' in every
detail. Certainly no mortal could make any pronouncement as to the
constitution of such a substance, but even if we assume it, for the
nonce, as possible, how can we account for the transmissible variation
of the individual parts and developmental stages, on which the whole
phylogenetic evolution depends?

As the development of the butterfly exhibits the three main stages of
caterpillar, pupa, and perfect insect, each of which is independently
and hereditarily variable, and therefore implies a something in the
germ, whose variation brings about a change in the one stage only, so
the ontogeny of every higher animal is made up of numerous stages,
which are all capable of independent and transmissible variation. How
else should we human beings, in our embryonic phase, still possess the
gill-arches of our fish-like ancestors, although much modified and
without the gills? Truly, he who would seek to deny that the stages of
individual development are capable of independent and transmissible
variation must know very little about embryology. But if the facts
are as stated, how can they be reconciled with the conception of a
germinal substance developing in epigenetic fashion? Every variation
in this substance would affect not only the whole _succession of
stages, but the whole organism with all its parts_. In this way too,
then, we are driven to the conclusion that there must be something in
the germ whose variation causes variation only in a particular part
of a particular stage. This something we define in our conception of
the 'primary constituents' (_Anlagen_)--the determinants. These are
not to be thought of either as 'miniature models,' or even as the
'seeds' of the parts; they alone cannot produce the part which they
determine, but they effect changes in the cell in which they become
active, causing it to vary in such a manner that the formation of the
relevant part results. While I conceive of development as a continuous
process, I supplement this with the idea that from within, namely, from
the nuclear substance, new, directive, 'determining' influences are
continually being exerted on the developing cells.

I can hardly think of a better proof of the necessity of this
assumption than that furnished by Delage, one of the most acute
biologists of France, who, in his comprehensive book on _Heredity_, has
striven to replace the theory of determinants by something simpler.
Delage rejects all 'primary constituents' (_Anlagen_) in the germ, all
'particules représentatives,' as much too complicated an assumption,
and thinks it possible to work with the conception of a germ-plasm
which is about as simple as the cell-substance of a Rhizopod, that
is to say, a protoplasm of definite chemico-physical constitution
and composition. Leaving out of account the consideration that the
protoplasm of an amœba is scarcely of such extreme simplicity, but is
certainly made up of numerous differentiated and definitely arranged
biophors, how could such an extremely simple ('éminemment simple')
constitution of the ovum as is here assumed give rise to such a
complicated organism, the individual parts of which are capable of
independent and transmissible variation? According to Delage it does so
because the ovum, though not containing 'all the factors requisite for
its ultimate resultant,' does contain 'un certain nombre des facteurs
nécessaires à la détermination de chaque partie et de chaque caractère
de l'organisme futur'! Determinants after all, it may be said, but
that is far from the truth! It is not primary constituents that the
germ contains, according to Delage, it is chemical substances, for
instance muscle substances, probably 'les substances caractéristiques
des principales catégories de cellules, c'est-à-dire, celles qui, dans
ces cellules, sont la condition principale de leur fonctionnement.' All
these must be contained in the ovum. How they are to reach their proper
place in the organism, how the 'characteristic chemical substance' of a
mole is to land just behind the right or left ear of the fully formed
man, is not stated. But apart from this, there is a much deeper error
in this assumption of specific chemical substances in the ovum as an
explanation of the phenomena of local hereditary variation, and I have
already touched upon it: chemical substances are not vital units, which
feed and reproduce, which assimilate and which bear a charm against
the assimilating power of the surrounding protoplasm. They would
necessarily be modified and displaced in the course of ontogeny, and
would therefore--no matter where they had been placed at first--be
incapable of performing all that Delage ascribes to them. Either the
germ contains 'living' primary constituents, or it is, as Delage
maintains, determined chemico-physically; but in the latter case there
is no scope for hereditary local variation. Delage must either renounce
the attempt to explain this, or he must transform his 'substances
chimiques' into real and actually living determinants.

Thus from all sides we are forced to the conclusion that the
germ-substance on the whole owes its marvellous power of development
not only to its chemico-physical constitution, whether that be
eminently simple or marvellously complex, but to the fact that it
consists of many and different kinds of 'primary constituents'
(_Anlagen_), that is, of groups of vital units equipped with the forces
of life, and capable of interposing actively and in a specific manner,
but also capable of remaining latent in a passive state, until they
are affected by a liberating stimulus, and on this account able to
interpose successively in development. The germ-cell cannot be merely
a simple organism, it must be a fabric made up of many different
organisms or units, a microcosm.

Yet another train of thought leads us to the same idea, and this has
its roots in the extraordinary complexity of the machine which we call
the organism.

The botanist Reinke has recently called attention once again to
the fact that machines cannot be directly made up of primary
physico-chemical forces or energies, but that, as Lotze said, forces of
a superior order are indispensable, which so dispose the fundamental
chemico-physical forces that they must act in the way aimed at by
the purpose of the machine. To produce a watch it is not enough to
bring together brass, steel, gold, and stones; to produce a piano it
is not enough to lay wood, iron, leather, ivory, steel, &c., side by
side, but these stuffs must be brought together in a definite form and
combination. In the same way, the mere juxtaposition of carbon and
water does not result in a carbohydrate like sugar or illuminating gas;
the component elements only yield what is desired when they are placed
in a particular and absolutely definite relation to each other, in
which they so act upon and with one another that sugar or illuminating
gas results, and the same is true of the component elements of a watch
or of a piano. In the watch and in the piano this relation is arranged
by human intelligence, by the workmen who form the different materials
and put them together in the proper manner. In this case, then, human
intelligence is, as Reinke says, the 'superior force' which compels the
energies to work together in a particular way.

But organisms also are machines which perform a particular and
purposeful kind of work, and they are only capable of doing so because
the energies which perform the work are forced into definite paths by
superior forces; these superior forces are thus 'the steersmen of the
energies.' There is undoubtedly a kernel of truth in this view, and I
shall return to it. Reinke, however, uses it in a way which I cannot
follow; that is, he infers from it a 'cosmic intelligence' which puts
these superior forces into the organisms, and thus controls these
machines to purposeful work, as the watchmaker puts 'superior forces'
into the watch by means of wheels, cylinders, and levers. In one case
it is human intelligence which controls the 'superior forces,' in
the other 'cosmic' intelligence. I cannot regard this reasoning from
analogy as convincing, because, in the first place, these 'superior
forces' are not 'forces' at all. They are constellations of energy,
co-ordinations of matter and the energies immanent therein under
complex and precisely defined conditions, and it is a matter of
indifference whether chance or human intelligence has brought them
together. If we take Reinke's own example of carbohydrates it is
certain that our coal-gas is due to the intelligence of man, which
brings together the carbon and the water in such a way that coal-gas
must arise. The 'superior forces' must here be looked for in the
arrangements of the coke-stove, and, in the second place, in the
intelligence of man. But when decaying plants in the marsh form another
carbon-compound, marsh-gas, where do the directing 'superior forces'
come in? Surely only in the fortuitous concomitance of the necessary
materials and the necessary conditions. Or may 'cosmic' intelligence
have established this laboratory in the marsh? If not, what can compel
us to refer the formation of dextrin or starch in the cells of the
green leaves of plants to 'superior forces' which are placed in them by
'cosmic' intelligence? I am far from believing that the great and deep
problem here touched upon can be put aside in any off-hand manner, but
I feel sure that it will never be solved by word-play about energies
and 'superior forces.'

Let us return to the kernel of truth in Reinke's thesis; it lies in
this, that, while the working of a machine does really depend on
the forces or energies which are bound up with the stuffs of which
it consists, it also depends on a particular combination of these
stuffs and forces, on a particular 'constellation' of them, as Fechner
expressed it. In the watch these 'constellations' are the springs,
the wheels, &c., and their position in relation to each other; but in
the organism they are the organs, down to the cells and cell-parts;
for the cell too is a machine, indeed a very complex one, as its
functions prove. There are thousands of kinds of 'constellations' of
elementary substances and forces which condition the activity of the
living machine, and only when all these constellations are present in
the proper manner and in the proper relations to each other can the
functions of the organism be properly discharged.

But the living machine differs essentially from other machines in
the fact that it constructs itself; it arises by development from a
cell, by going through numerous 'stages of development.' But none
of these stages is a dead thing, each is itself a living organism
whose chief function is to give rise to the next stage. Thus each
stage of the development may be compared to a machine whose function
consists in producing a similar but more complex machine. Each stage
is thus composed, just like the complete organism, of a number of such
'constellations' of elementary substances and elementary forces, whose
number in the beginning is relatively small, but increases rapidly with
each new stage.

But whence come these 'constellations' or, to keep to our metaphor,
the levers, wheels, and cranks of each successive stage in the making
of the organic machine? The epigenetic theory of a germ-plasm without
primary constituents answers by pointing to internal and external
influences which cause the germ-plasm, originally homogeneous, to
differentiate gradually more and more, bringing it into the most
diverse 'constellations.' But how can such influences introduce new
springs, levers, and wheels of a quite specific kind, as must be the
case if apparently similar germinal substances are to give rise to two
such different animals as a domestic duck and a teal? The cause must
lie in the invisible differences in the protoplasm, opponents will
answer, and we with them. But our studies up to this point have shown
us that the differences cannot be merely elementary differences, cannot
be merely of a physico-chemical nature depending on the composition of
the raw material and the implicated energies; they must depend on the
definite co-ordination of substances and energies, in other words, on
the occurrence of 'constellations' of these. Thus the germ-plasm must
be composed of definite and very diverse combinations of living units,
which are themselves bound up in a higher 'constellation,' so that they
act as a living machine at the first stage of development, and liberate
into activity the already existing constellations of the second
stage. The second stage in the series of living machines which arise
successively from each other liberates the sleeping 'constellations'
for the third, and so on.

These 'constellations' of matter and energy are the biophors, the
determinants, and the 'groups of determinants' which we may think
of as disposed in a manifold overlapping series. That they do not
enter into activity all at once, but successively take their part in
development, seems to me a necessary consequence of their successive
origin in the phylogeny; and the ontogeny, as we shall see later,
arises through a modified condensation of the phylogeny. Now since
every new determinant that arises in the course of phylogeny can only
develop by division and subsequent variation from the determinants
which were previously active at the same place in the organism, it is
quite intelligible that later on, when the phylogeny has been condensed
in the ontogeny, they should not enter upon their active stage at
the same time as their phyletic predecessors, but after them. The
theory of Oscar Hertwig, who starts from a germ-plasm without primary
constituents, that all parts of the germ-plasm become active at the
same time, seems to me quite untenable. How could the wheels, levers,
and springs of the complete vital machine, which arose so very slowly
in the course of phylogeny, arise to-day in the ontogeny in such rapid
succession unless they were already present in the germ-plasm and only
required to be incited to activity, that is, liberated by the stage
preceding them? Even Fechner supported this view when he supposed that
the interaction and mutual influences of the parts in the organism,
that is, of the 'constellations,' gave rise of themselves to the
succeeding stage, that is to say, to the new constellations peculiar
to the succeeding stage. To this Reinke reasonably objected that it
was like expecting the window frames of a house in process of building
to produce the panes of glass. The panes in the organism only develop
in the window frames if their determinants have been present in the
germ-plasm from the beginning, and are liberated by the development of
the frames, just as the activity of the glazier is liberated by the
sight of the completed frames. Neither new panes nor new determinants
could be produced rapidly; the former must be manufactured in the
glass factory, the latter in the developmental workshop of the form
of life in question, which workshop we call its phylogeny. But just
as it is unnecessary to erect a new glass factory for each new house
that is built, so the development of each individual does not require
the establishment each time of those numberless life-factories--the
constellations--whose business it is to produce anew the wheels,
levers, springs, and cylinders of the developmental machinery at each
stage, for they are all provided for in the germ-plasm, and it is only
on this account that they are capable of hereditary variation.

I have already directed attention to some embryological facts which
seem to be contradictory, if not to the germ-plasm theory itself, at
least to the assumption it makes that the germ-plasm is analysed out
during the ontogeny; and something more must be said on this head.
I refer to the numerous facts brought to light through the science
of developmental mechanics founded by Wilhelm Roux, and particularly
to the investigations as to the prospective significance of the
segmentation-cells of the animal ovum.

Among these investigations we find experiments in compressing
certain eggs (sea-urchin's) in the early stages of segmentation.
The blastomeres are prevented by artificial pressure from grouping
themselves in the normal manner; they are compelled to spread out side
by side in the _same plane_. If the pressure is removed, they change
their grouping, and yield a normal embryo. I will not here discuss
whether these results can only be interpreted as showing that each
segmentation-cell has the same prospective significance, and that it
is only its relative position which decides what part of the embryo is
to be formed from it; this could not be done without going into great
detail; I therefore assume it to be true, and confine my survey to the
second group of experiments, those on isolated segmentation-cells.

It has been shown that in the eggs of the most diverse animals,
for instance in the sea-urchin once more, each of the two first
blastomeres, if separated from one another, can develop into a complete
larva. Indeed, in the eggs of sea-urchin and some other animals each
of the first four, or any of the first eight, blastomeres, and indeed
any segmentation-cell during the earlier stages, possesses the power
of developing to a certain point, namely, as far as the so-called
'blastula-larva.' This seems to contradict a theory which assumes that
the primary constituents become separated in the successive stages of
ontogeny. But in the first place the blastomeres of all animals do
not behave in this way, and, moreover, the facts can be quite well
explained without entirely renouncing the assumption of the segregation
of the determinant-complexes. It is only necessary to assume that the
segmentation-cells, which develop in the isolated condition as if they
were intact eggs, still contain the complete germ-plasm, and that the
differential segregation into groups of determinants with dissimilar
hereditary tendencies takes place later. This would certainly load
the theory with further complications, and I shall not enter into the
question here, since the facts which we should have to consider are as
yet by no means undisputed.

But in any case the facts of developmental mechanics referred to, which
we owe to numerous excellent observers of the last decade,--I need only
name W. Roux, O. Hertwig, Chun, Driesch, Barfurth, Morgan, Conklin,
Wilson, Crampton, and Fischel--not only leave the essential part of the
germ-plasm theory untouched, but rather strengthen than endanger its
more subordinate points, such as the assumption of a segregation of the
components of the germ-plasm in the course of ontogenesis.

As to the fundamental ideas expressed in the theory, I have already
shown that these remain unaltered, even if we do not assume a
disintegration or segregation of the germ-plasm, but think of all the
developing cells as equipped with the complete germ-plasm. In that case
the determinants would be liberated to activity solely by specific
stimuli. But in regard to the assumption of disintegration, it must be
noted that the facts cited relative to the sea-urchin's ova do not by
any means hold true of the eggs of all animals.

In various animal types each of the first two segmentation-cells,
when separated from its neighbour, produces only a half-embryo, and
any one of the first four cells a quarter-embryo. This 'fractional
embryo' is, however, in some cases able later to develop into a whole
embryo (to 'postgenerate' itself, as W. Roux says). The isolated
blastomere shows, to begin with, an activity of only a half of the
primary constituents of the animal, as was first established by W. Roux
and maintained conclusively, in spite of many attacks, until it was
established beyond doubt by the detailed corroboratory investigations
of Endres. The secondary completion of the embryo, which, however, is
still disputed, must be regarded as a regeneration, and, to explain it,
a co-operation of the complete but not yet wholly active germ-plasm in
both segmentation-cells must therefore be assumed.

It would carry us too far if I were to deal in detail even with the
most important of the numerous facts that the last decade has brought
to light; I shall restrict myself to the most essential.

That isolated segmentation-cells have the capacity of developing into
embryos which are complete but correspondingly smaller in size has been
demonstrated in animals of various groups, though it does not seem to
go to the same length in all. In the Medusæ we find that not only one
of the first two, but one of the first four, eight, and even sixteen
segmentation-cells may develop a whole larva when isolated (Zoja). In
the sea-urchin at least any one of the first eight blastomeres may do
so. And Driesch's experiments in cutting up the young larvæ at the
blastula-stage (a single-layered ball of cells) leads us to assume that
each of these cells still possesses the complete germ-plasm. Beyond
that stage, however, the primary constituents obviously divide into
those of the ectoderm and those of the endoderm, for the subsequent
two-layered stage in the sea-urchin's development, the gastrula, does
not complete itself if it be artificially divided into fragments which
consist only of cells from the outer, or only of cells from the inner
layer. In corroboration of this experiment made by Barfurth, Samassa
was able to demonstrate in regard to the egg of the frog that, even
after the third division of the ovum, the segmentation-cells are so
different from each other in respect of their primary constituents
that they were not able to replace each other mutually. When this
investigator killed the ectoderm-cells alone by means of an induction
current, or the endoderm-cells alone, the dead half could not be
replaced by the half which remained alive, and the whole ovum perished.

If these facts may be adduced in favour of a separation of the primary
constituents at an earlier or later stage, we find even stronger proofs
among the Ctenophores, Gastropods, Bivalves, and Annelids. In the
last-named group Wilson has shown it to be probable that development
is really a 'mosaic work,' as Roux and I had assumed. The older
observations made by Chun at an earlier date on the Ctenophora, and
the more recent experiments of Fischel on the same animals, prove the
same thing for this group. In this case complete larvæ are easily
distinguished from mere 'partial developments' by the number of the
characteristic 'ciliated meridional rows' or ribs, which extend from
one pole of the larva to another. In the complete larva there are
eight of these, but in larvæ from one of the first two blastomeres
(isolated) there are only four, and in those which have arisen from one
of the first four blastomeres there are only two. If an ovum at the
eight-cell stage can be successfully divided into separate blastomeres,
each of these will form an 'eighth larva,' always with only one
ciliated rib. Even in the succeeding sixteen-cell stage it could still
be demonstrated that the substance responsible for the formation of
the ribs only lies in particular places and always suffices only for
eight ribs. The sixteen-cell stage consists of eight large cells and
eight small ones, the 'macromeres' and the 'micromeres'; if an ovum
at this stage be cut so that one piece contains five macromeres and
five micromeres, a partial larva will develop which possesses only
five ribs, while the larva from the other portion will have only
three. But the localizing of the rib-determinants can be followed
still further, for in larvæ in which individual micromeres have
been displaced from their normal position there is a correlated
displacement of the corresponding ribs, and a dislocation of their
ciliated comb-plates. The determinants of the ribs must therefore
lie in the micromeres, and we must conclude that at the antecedent
division they were only imparted to one daughter-nucleus, while the
other, that of the macromere, did not receive this kind of determinant.
Here then we have an example of dissimilar or differential division.
Those who oppose this theory of qualitative division will hardly be
likely to admit this, but will rather seek to maintain that 'external
influences,' such as relative position, determine which cells are to
give rise to the ciliated ribs and which are not. But the fact that
artificial displacement of the micromeres leads to a disarrangement of
the ciliated comb-plates, of which the ribs are made up, invalidates
this suggestion, and at the same time overthrows the interpretation
that it may be the cells which lie on particular meridians that are
determined by this position to the production of ciliated plates.
Obviously, the converse of this is true; those cells which contain
the rib-determinants come to lie in the regular course of development
in these eight meridians, and the cells lying between them, though of
the same descent (from micromeres), contain no such determinants and
therefore form no ribs. But if those cells which are equipped with
rib-determinants be artificially displaced, then they give rise to
swimming-plates elsewhere than on the aforesaid meridians.

The experiments made by Crampton on a marine Gastropod, _Ilyanassa_,
likewise go to prove that a disintegration or segregation of the
primary constituents does occur in the course of development. In
this case, when the first two or first four segmentation-cells were
artificially separated from each other, they developed exactly as
if they still belonged to the complete ovum, that is, each isolated
segmentation-cell yielded, respectively, a half or a quarter-embryo.
And these 'partial embryos' were not able in this case to give rise
subsequently to the missing parts or to form complete embryos.

There are thus two contrasted groups of animals, in one of which a
segregation of the mass of primary constituents apparently takes place
at the very beginning, while in the other it does not take place in the
first stages of development, but apparently occurs later on. We may
distinguish these two groups, with Heider, as those having 'regulation
ova' and those having 'mosaic ova.' But I do not see that this affords
any reason why we should give up our conception of the successive
segregation of the germ-plasm into its determinants, even although, as
I said before, I may modify it so far as to say that the segregation
does not necessarily take place in all groups and species of animals at
the same time, but occurs earlier in some and later in others.

Now that I have shown how the germ-plasm theory may be brought into
harmony with the phenomena of ontogeny, I wish to go on to show what
the theory can accomplish in clarifying our understanding of the
phenomena of reproduction and heredity. I shall at the same time give a
brief exposition of some of the most important of these phenomena.

First, a few words in regard to the development of the reproductive
cells. We may leave aside in the meantime the question whether they
are sexually differentiated or not; we are only concerned just now
with the main problem: How is it possible for the organism to produce
germ-cells, that is, cells which contain the complete germ-plasm with
all its determinants, when the building up of the body in ontogeny,
according to our theory, involves a disintegration or segregation of
the determinant-architecture into smaller and smaller groups? It is
impossible that specific determinants should arise _de novo_, just
as an animal cannot arise otherwise than from its germ, nor a cell
otherwise than from a cell, nor a nucleus otherwise than from an
already existing nucleus. If vital units ever originate _de novo_
at all, it is only conceivable in the case of the very simplest
biophors, as we shall see later when we come to speak of 'Spontaneous
Generation.' Specific biophors and the determinants composed of them
have behind them a phylogeny, a history, which conditions that they
shall arise only from their like.

Thus we see that germ-cells can only arise where all the determinants
of the relevant species arranged as ids are already present. If we
could assume that the ovum, just beginning to develop, divides at
its first cleavage into two cells, one of which gives rise to the
whole body (soma) and the other only to the germ-cells lying in this
body, the matter would be theoretically simple. We should say, the
germ-plasm of the ovum first doubles itself by growth, as the nuclear
substance does at every nuclear division, and then divides into two
similar halves, one of which, lying in the primordial somatic cell,
becomes at once active and breaks up into smaller and smaller groups
of determinants corresponding to the building up of the body, while
the germ-plasm in the other remains in a more or less 'bound' or 'set'
condition, and is only active to the extent of gradually stamping as
germ-cells the cells which arise from the primordial germ-cell.

As yet, however, only one group of animals is known to behave
demonstrably in this manner, the Diptera among insects; in all others
the cell from which the germ-cells exclusively arise, the 'primordial
germ-cell,' makes its appearance later in development, usually
during embryogenesis and often very early in it, after the first few
divisions of the ovum, but sometimes not till long after the end of
embryogenesis, and not even in the individual which arises from the
ovum, but in descendants which arise from it by budding. This last
case occurs especially in the colonial hydroid polyps, which multiply
by budding. Here the primordial germ-cell is separated from the ovum
by a long series of cell-generations, and the sole possibility of
explaining the presence of germ-plasm in this primordial cell is to
be found in the assumption that in the divisions of the ovum the
whole of the germ-plasm originally contained in it was not broken up
into determinant groups, but that a part, perhaps the greater part,
was handed on in a latent state from cell to cell, till sooner or
later it reached a cell which it stamped as the primordial germ-cell.
Theoretically it makes no difference whether these 'germ-tracks,' that
is, the cell-generations which lead from the ovum to the primordial
germ-cell, are short or very long, whether they consist of three or
six or sixteen cells, or of hundreds and thousands of cells. That
all the cells of the germ-track do not take on the character of
germ-cells must, in accordance with our conception of the 'maturing'
of determinants, be referred to the internal conditions of the cells
and of the germ-plasm, perhaps in part also to an associated quantum
of somatic idioplasm which is only overpowered in the course of the
cell-divisions.

This splitting up of the substance of the ovum into a somatic half,
which directs the development of the individual, and a propagative
half, which reaches the germ-cells and there remains inactive, and
later gives rise to the succeeding generation, constitutes _the theory
of the continuity of the germ-plasm_, which I first stated in a work
which appeared in the year 1885. Its fundamental idea had already been
expressed much earlier by Francis Galton (1872), without however being
fully appreciated at the time or having any influence on the course
of science, and the same is true with the later theoretical views of
Jäger, Rauber, and Nussbaum, all of whom reached the same idea quite
independently of each other, and sought to elaborate it more or less
fully.

The hypothesis does not depend for support merely on a recognition of
its theoretical necessity; on the contrary, there is a whole series of
facts which may be adduced as strongly in its favour.

Thus, even the familiar fact that the excision of the reproductive
organs in all animals produces sterility proves that no other cells
of the body are able to give rise to germ-cells; germ-plasm cannot be
produced _de novo_. An unmistakable corroboration of this, it seems
to me, is to be found in the conditions of germ-cell formation in
the medusoids and hydroid polyps, for here it is apparent that the
birthplace of the germs, that is, the place at which the germ-cells
of the animal are formed, has been shifted backwards in the course
of phylogenetic evolution, that is, has been moved nearer to the
starting-point of development. This shifting has exactly followed
the 'germ-tracks.' as we shall see, although in some cases it would
have been more advantageous if the birthplace of the germ-cells could
have lain outside of these. Obviously, then, it is only the existing
cell-generations of the germ-track which were able to give rise to
germ-cells, or, in other words, they alone contained the indispensable
germ-plasm. With the help of Figs. 94 and 95 I hope to be able to make
this matter clear.

[Illustration: FIG. 94. Diagram to illustrate the phylogenetic shifting
back of the origins of the germ-cells in medusoids and hydroids. A
composite picture. _A_, branch of a polyp colony. _P_, polyp-head with
mouth (_m_) and tentacles. _St_, stalk of the polyp. _M_, medusoid-bud
with the bell (_Gl_). _T_, marginal tentacle. _m_, mouth. _Mst_,
manubrium. _GphK_, a gonophore-bud. _GH_, gastric cavity. _ekt_,
ectoderm. _ent_, endoderm. _st_, supporting lamella. The germ-cells
(_kz_) arise in the medusoid in the ectoderm of the manubrium--first
phyletic stage--where they also attain maturity. In the gonophore-bud
(_GphK_) they arise in the ectoderm (_kz´_), or further down in the
stalk of the polyp at _kz´´_--third phyletic stage, or in the ectoderm
of the branch from which the polyp has arisen, at _kz´´´_--fourth
phyletic stage of the shunting of the originative area of the
germ-cells. In the two last cases the germ-cells migrate until they
reach their primitive place of origination in the medusoid, or in the
corresponding layer of the medusoid gonophore, as may be more clearly
seen in Fig. 95. Drawn from my sketch by Dr. Petrunkewitsch.]

In the hydroid polyps and their medusoids the germ-cells always
arise in the ectoderm; in species which produce sexual medusoids by
budding, the germ-cells arise in the ectoderm of the manubrium of these
medusoids (Fig. 94, _M_, _kz_). But in many species these sexual stages
have degenerated in the course of phylogeny into so-called gonophores,
that is, to medusoids which still exhibit more or less complete bells,
but neither mouth (_m_) nor marginal tentacles (_T_), and which no
longer break away from the colony to swim freely about, to feed
independently, and to produce and ripen germ-cells. The degeneration
of the 'gonophores' often goes even further; in many the medusoid bell
is represented only by a thin layer of cells, and in some even this
token of descent from medusoid ancestry is absent, and they are mere
single-layered closed brood-sacs (Fig. 95, _Gph_).

The adherence of the sexual animal to the hydroid colony has,
however, made a more rapid ripening of the germ-cells possible, and
nature has taken advantage of this possibility in all the cases
known to me, for the germ-cells no longer arise in the manubrium
of the mature degenerate medusoid, that is, of the gonophore, but
_earlier_, before the bud which becomes a gonophore possesses a
manubrium. The birthplace of the germ-cells is thus shifted back from
the manubrium of the medusoid to the young gonophore-bud (Fig. 94,
_M_, _kz_). The same thing occurs in species in which the medusoids
are liberated, but live only for a short time, for instance, in the
genus _Podocoryne_. Although perfect medusoids are formed, these have
their germ-cells fully developed at the time of their liberation from
the hydroid colony. But in species in which the medusoid-buds have
really degenerated and are no longer liberated, the birthplace of the
germ-cells is shifted _even further back_, and in the first place into
the stalk (_St_, _kz´´_) of the polyp from the gonophore-buds. This
is the case in the genus _Hydractinia_. In the further course of the
process the birthplace of the germ-cells has shifted as far back as to
the branch from which the polyp has grown out (Fig. 94, _A_, _kz´´´_);
and finally, in the cases in which the medusoid has degenerated to
a mere brood-sac (Fig. 95, _Gph_), even to the generation of polyps
immediately before, that is, into the polyp-stem from which the branch
arises that bears the polyps producing the gonophore-bud (Fig. 95,
_kz´´´_). Then we find the birthplace of the germ-cells _still_ further
back (Fig. 95, _kz´´´´_), for the egg and sperm-cells arise in the stem
of the principal polyps (the main stem of the colony). The advantage of
this arrangement is easily seen, for the principal polyp is present
earlier than those of the secondary branches, and these again earlier
than the polyp which bears the sexual buds, and this, finally, earlier
than the sexual bud which it bears. Thus this shunting backwards of the
birthplace of the germ-cells means an earlier origin of the primordium
(_Anlage_) of the germ-cells, and consequently an earlier maturing of
these.

[Illustration: FIG. 95. Diagram to illustrate the migration of the
germ-cells in hydro-medusæ from their remotely shunted place of origin
to their primitive place of origin in the gonophore, in which they
attain to maturity. The state of affairs in Eudendrium is taken as the
basis of the diagram. _HP_, one of the principal polyps. _mu_, mouth.
_ma_, gut-cavity. _t_, tentacle. _Sta_, its stem. _A_, a branch of the
polyp colony. _SP_, lateral polyp. _Gph_, a medusoid-bud completely
degenerated into a mere gonophore. _Ei_, ovum. _GH_, gastric cavity.
_st_, supporting lamella. The originative area of the germ-cells
lies in the stem of the principal polyp at _kz´´´´_, whence the
germ-cells first migrate into the endoderm of the branch (_A_) at
_kz´´´_, creeping within which they reach _kz´´_ in the lateral polyp
(blastostyle), finally reaching the gonophore (_kz_) and passing again
into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch.]

But none of all these germ-cells come to maturity in the birthplace to
which they have been shifted, for they migrate independently from it to
the place at which they primitively arose, namely, into the manubrium
of the medusoid, which is still present even when great degeneration
has occurred, or even--in the most extreme cases of degeneration--into
the ectoderm of the brood-sac. This is the case in the genus
_Eudendrium_, of which Fig. 95 gives a diagrammatic representation.

The most interesting feature of this migration of the germ-cells is
that the cells invariably arise in the ectoderm (_kz´´´´_), then pierce
through the supporting lamella (_st_) into the endoderm (_kz´´´_), and
then creep along it to their maturing-place. Once there they break
through again to the outer layer of cells, the ectoderm (_kz_), and
come to maturity (_Ei_). That they make their way through the endoderm
is probably to be explained by the fact that they are there in direct
proximity to the food-stream which flows through the colony (_GH_ =
gastric cavity), and they are thus more richly nourished there than
in the ectoderm. But although this is the case, they never arise in
the endoderm; in no single case is the birthplace of the germ-cells to
be found in the endoderm, but always in the ectoderm, no matter how
far back it may have been shunted. Even when the germ-cells migrate
through the endoderm, their first recognizable appearance is invariably
in the ectoderm, as, for instance, in _Podocoryne_ and _Hydractinia_.
The course of affairs is thus exactly what it would necessarily be if
our supposition were correct, that only definite cell-generations--in
this case the ectoderm-cells--contain the complete germ-plasm. If the
endoderm-cells also contained germ-plasm it would be hard to understand
why the germ-cells never arise from them, since their situation offers
much better conditions for their further development than that of
the ectoderm-cells. It would also be hard to understand why such a
circuitous route was chosen as that exhibited by the migration of
the young germ-cells into the endoderm. Something must be lacking in
the endoderm that is necessary to make a cell into a germ-cell: that
something is the germ-plasm.

If we accept the theory of the continuity of the germ-plasm as in the
main correct, it appears that higher animals and plants are constructed
of two kinds of elements, the somatic cells and the germ-cells; both
owe their being to the germ-plasm of the ovum, but the former do not
contain it complete but only in individual determinants[24], and
therefore can never give rise again to the rank of germ-cells; the
others contain the latent germ-plasm intact, and can therefore produce
not only cells like themselves for a certain time by division, but
have also the power, when they are mature and the necessary conditions
have been fulfilled, of bringing forth a new individual of the same
species. The former have only a limited length of life, they die--they
must necessarily die--when the life of the individual to which they
belong is at an end; the latter are potentially immortal, like the
unicellular organisms, that is, they can in favourable circumstances
give rise to the germ-cells of a new individual, and so on for all
time, as far as we can see. The germ-plasm of a species is thus never
formed _de novo_, but it grows and increases ceaselessly; it is handed
on from one generation to another like a long root creeping through
the earth, from which at regular distances shoots grow up and become
plants, the individuals of the successive generations. If these
conditions be considered from the point of view of reproduction, the
germ-cells appear the most important part of the individual, for they
alone maintain the species, and the body sinks down almost to the
level of a mere cradle for the germ-cells, a place in which they are
formed, and under favourable conditions are nourished, multiply, and
attain to maturity. But the matter can also be looked at in an opposite
light, and then the endless root of the germ-plasm, with its germ-cells
ever forming new individuals, may be regarded as the means by which
alone nature was able to create multicellular organisms, individuals
of higher and higher differentiation and capacity, able to adapt
themselves to all possible conditions, and to make the fullest use of
all the possibilities of life.

[24] Boveri has recently made an observation upon the thread-worm of
the horse, which points to the correctness of the conception of the
germ-plasm. The two first segmentation-cells both receive the four
chromosomes of the species, but, in one of the two, a portion of the
chromatin breaks off and degenerates, or dissolves, at least as far
as can be seen. The other cell retains the whole mass of chromatin,
and from this there arise later the primitive genital-cells. In the
germ-track, therefore--so we must interpret it--the whole of the
germ-plasm is retained, while a part of it is withdrawn from the soma.
I have only partly described the process, and I do not wish to enter in
detail on an interpretation of it, since it seems to me obscure and to
require further observations before an interpretation can be attempted
with any confidence.




MR. EDWARD ARNOLD'S

LIST OF

Scientific and Technical Books.


 =FOOD AND THE PRINCIPLES OF DIETETICS.= By ROBERT HUTCHISON, M.D.
 Edin., F.R.C.P., Assistant Physician to the London Hospital. Seventh
 and Revised Edition. Illustrated. Demy 8vo., 16s. net.

 =LECTURES ON DISEASES OF CHILDREN.= By ROBERT HUTCHISON, M.D. (Edin.),
 F.R.C.P., Assistant Physician to the London Hospital and to the
 Hospital for Sick Children, Great Ormond Street, London. With numerous
 Illustrations. Second Impression. Crown 8vo., 8s. 6d. net.

 =PRACTICAL PHYSIOLOGY.= By M. S. PEMBREY, M.A., M.D., Lecturer on
 Physiology at Guy's Hospital; A. P. BEDDARD, M.A., M.D., Demonstrator
 of Physiology, Guy's Hospital; J. S. EDKINS, M.A., M.B., Lecturer in
 Physiology and Demonstrator of Physiology, St. Bartholomew's Hospital;
 LEONARD HILL, M.B., F.R.S., Lecturer on Physiology, London Hospital
 Medical School; and J. J. R. MACLEOD, M.B. Copiously Illustrated. New
 and Revised Edition. Demy 8vo., 12s. 6d. net.

 =RECENT ADVANCES IN PHYSIOLOGY AND BIO-CHEMISTRY.= By LEONARD HILL,
 M.B., F.R.S.; A. P. BEDDARD, M.A., M.B.; J. J. R. MACLEOD, M.B.;
 BENJAMIN MOORE, M.A., D.Sc.; and M. S. PEMBREY, M.A., M.D. Demy 8vo.,
 18s. net.

 =HUMAN EMBRYOLOGY AND MORPHOLOGY.= By A. KEITH, M.D., F.R.C.S. Eng.,
 Lecturer on Anatomy at the London Hospital Medical College. With 316
 Illustrations. New, Revised, and Enlarged Edition. Demy 8vo., 12s. 6d.
 net.

 =SURGICAL NURSING AND THE PRINCIPLES OF SURGERY FOR NURSES.= By
 RUSSELL HOWARD, M.B., M.S., F.R.C.S., Lecturer on Surgical Nursing at
 the London Hospital. Crown 8vo., with Illustrations, 6s.

 =THE PHYSIOLOGICAL ACTION OF DRUGS.= An Introduction to Practical
 Pharmacology. By M. S. PEMBREY, M.A., M.D., Lecturer on Physiology
 in Guy's Hospital Medical School; and C. D. F. PHILLIPS, M.D., LL.D.
 Fully Illustrated. Demy 8vo., 4s. 6d. net.

 =PHOTOTHERAPY.= By N. R. FINSEN. Translated by J. H. SEQUEIRA, M.D.
 With Illustrations. Demy 8vo., 4s. 6d. net.

 =A MANUAL OF HUMAN PHYSIOLOGY.= By LEONARD HILL, M.B., F.R.S. With 173
 Illustrations. xii + 484 pages. Crown 8vo., cloth, 6s.

 =A PRIMER OF PHYSIOLOGY.= By LEONARD HILL, M.B. 1s.

 =A MANUAL OF PHARMACOLOGY FOR STUDENTS.= By W. E. DIXON, M.A., M.D.,
 B.S., B.Sc. Lond., etc., Assistant to the Downing Professor of
 Medicine in the University of Cambridge. With Numerous Diagrams. Demy
 8vo., 15s. net.

 =THE CHEMICAL SYNTHESIS OF VITAL PRODUCTS AND THE INTER-RELATIONS
 BETWEEN ORGANIC COMPOUNDS.= By Professor RAPHAEL MELDOLA, F.R.S., of
 the City and Guilds of London Technical College, Finsbury. Two vols.
 Vol. I. now ready. Super Royal 8vo., 21s. net.

 =LECTURES ON THEORETICAL AND PHYSICAL CHEMISTRY.= By Dr. J. H. VAN 'T
 HOFF, Professor of Chemistry at the University of Berlin. Translated
 by Dr. R. A. LEHFELDT. In three volumes. Illustrated. Demy 8vo., 28s.
 net; or obtainable separately, as follows:

  Vol. I.--=Chemical Dynamics.= 12s. net.
  Vol. II.--=Chemical Statics.= 8s. 6d. net.
  Vol. III.--=Relations between Properties and Composition.=
                                               7s. 6d. net.

 =THE ELEMENTS OF INORGANIC CHEMISTRY.= For use in Schools and
 Colleges. By W. A. SHENSTONE, Lecturer in Chemistry at Clifton
 College. 554 pages. New Edition, Revised and Enlarged. 4s. 6d.

 =A COURSE OF PRACTICAL CHEMISTRY.= Being a Revised Edition of 'A
 Laboratory Companion for Use with Shenstone's Inorganic Chemistry.' By
 W. A. SHENSTONE. 144 pages. Crown 8vo., 1s. 6d.

 =A TEXT-BOOK OF PHYSICAL CHEMISTRY.= By Dr. R. A. LEHFELDT,
 Professor of Physics at the East London Technical College. With 40
 Illustrations. Crown 8vo., cloth, 7s. 6d.

 =A TEXT-BOOK OF PHYSICS.= With Sections on the Applications of Physics
 to Physiology and Medicine. By Dr. R. A. LEHFELDT. Fully Illustrated.
 Crown 8vo., cloth, 6s.

 =PHYSICAL CHEMISTRY FOR BEGINNERS.= By Dr. Ch. M. VAN DEVENTER.
 Translated by Dr. R. A. LEHFELDT. 2s. 6d.

 =A FIRST YEAR'S COURSE OF EXPERIMENTAL WORK IN CHEMISTRY.= By ERNEST
 H. COOK, D.Sc., F.I.C., Principal of the Clifton Laboratory, Bristol.
 With 26 Illustrations. Crown 8vo., cloth, 1s. 6d.

 =AN EXPERIMENTAL COURSE OF CHEMISTRY FOR AGRICULTURAL STUDENTS.= By T.
 S. DYMOND, F.I.C., Lecturer on Agricultural Chemistry in the County
 Technical Laboratories, Chelmsford. With 50 Illustrations. 192 pages.
 Crown 8vo., cloth, 2s. 6d.

 =THE STANDARD COURSE OF ELEMENTARY CHEMISTRY.= By E. J. COX, F.C.S.
 With 90 Illustrations. 350 pages. Crown 8vo., cloth, 3s. Also
 obtainable in five parts, limp cloth. Parts I.-IV., 7d. each; Part V.,
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 =A PRELIMINARY COURSE OF PRACTICAL PHYSICS.= By C. E. ASHFORD, M.A.,
 Headmaster of the Royal Naval College, Dartmouth. Fcap. 4to., 1s. 6d.

 =ADVANCED EXAMPLES IN PHYSICS.= By A. O. ALLEN, B.A., B.Sc.,
 A.R.C.Sc., Lond., Assistant Lecturer in Physics at Leeds University.
 1s. 6d.

 =PHYSICAL DETERMINATIONS.= A Manual of Laboratory Instructions for
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 A.I.E.E., Principal of the Taunton Technical College. 4s. 6d.

 =ELECTROLYTIC PREPARATIONS.= Exercises for use in the laboratory
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 =THE ELECTRIC FURNACE.= By HENRI MOISSAN, Professor of Chemistry at
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 =MECHANICS.= By W. D. EGGAR, Science Master at Eton College. Crown
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 =MAGNETISM AND ELECTRICITY.= An Elementary Treatise for Junior
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 =VALVES AND VALVE-GEAR MECHANISMS.= By W. E. DALBY, M.Inst.C.E.,
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 =ELECTRIC AND MAGNETIC CIRCUITS.= By ELLIS H. CRAPPER, M.I.E.E., Head
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 Sheffield, viii + 380 pages. Demy 8vo., 10s. 6d. net.

 =AN INTRODUCTION TO THE THEORY OF OPTICS.= By Professor ARTHUR
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 Manchester. With numerous Diagrams. Demy 8vo., 15s. net.

 =ASTRONOMICAL DISCOVERY.= By H. H. TURNER, Savilian Professor of
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 by G. C. TURNER, Goldsmith Institute. Crown 8vo., cloth. 4s. 6d.

 =THE PRINCIPLES OF MECHANISM.= By H. A. GARRATT, A.M.I.C.E., Head of
 the Engineering Department of the Northern Polytechnic Institute,
 Holloway. Crown 8vo., cloth, 3s. 6d.

 =ELEMENTARY PLANE AND SOLID MENSURATION.= By R. W. K. EDWARDS, M.A.,
 Lecturer on Mathematics at King's College, London. For use in Schools,
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 =FIRST STEPS IN QUANTITATIVE ANALYSIS.= By J. C. GREGORY, B.Sc.,
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 =GEOMETRICAL CONICS.= By G. W. CAUNT, M.A., Lecturer in Mathematics,
 Armstrong College, Newcastle-on-Tyne, and C. M. JESSOP, M.A.,
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 =FIVE-FIGURE TABLES OF MATHEMATICAL FUNCTIONS.= By J. B. DALE, M.A.
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 =LOGARITHMIC AND TRIGONOMETRIC TABLES.= (To Five Places of Decimals.)
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 =PRELIMINARY PRACTICAL MATHEMATICS.= By S. G. STARLING, A.R.C.Sc.,
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 Municipal Technical Institute; and F. C. CLARKE, A.R.C.Sc., B.Sc. 1s.
 6d.

 =A NOTE-BOOK OF EXPERIMENTAL MATHEMATICS.= By C. GODFREY, M.A.,
 Headmaster of Royal Naval College, Osborne, and G. M. BELL, B.A.,
 Senior Mathematical Master, Winchester College. Fcap. 4to., paper
 boards, 2s.

 =HOUSE, GARDEN, AND FIELD.= A Collection of Short Nature Studies. By
 L. C. MIALL, F.R.S., Professor of Biology, University of Leeds. With
 numerous Diagrams. Crown 8vo., 6s.

 =THE EVOLUTION THEORY.= By AUGUST WEISMANN, Professor of Zoology in
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 Coloured Plates. Two Vols. Royal 8vo., 32s. net.

 =ANIMAL BEHAVIOUR.= By C. LLOYD MORGAN, LL.D., F.R.S., Principal of
 University College Bristol, author of 'Animal Life and Intelligence,'
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 =HABIT AND INSTINCT.= By C. LLOYD MORGAN, LL.D., F.R.S. With
 Photogravure Frontispiece, viii + 352 pages. Demy 8vo., cloth, 16s.

 =A TEXT-BOOK OF ZOOLOGY.= By G. P. MUDGE, A.R.C.Sc. Lond., Lecturer
 on Biology at the London Hospital Medical College. With about 200
 original Illustrations. Crown 8vo., cloth, 7s. 6d.

 =ELEMENTARY NATURAL PHILOSOPHY.= By ALFRED EARL, M.A., Assistant
 Master at Tonbridge School. Crown 8vo., cloth, 4s. 6d.

 =A CLASS-BOOK OF BOTANY.= By G. P. MUDGE, A.R.C.Sc. Lond., F.Z.S., and
 A. J. MASLEN, F.L.S., Lecturer on Botany at the Woolwich Polytechnic.
 With over 200 Illustrations. Crown 8vo., 7s. 6d.

 =THE BECQUEREL RAYS AND THE PROPERTIES OF RADIUM.= By the Hon. R. J.
 STRUTT, Fellow of Trinity College, Cambridge. With Diagrams. Demy 8vo.
 8s. 6d. net.

 =A MANUAL OF ALCOHOLIC FERMENTATION AND THE ALLIED INDUSTRIES.= By
 CHARLES G. MATTHEWS, F.I.C., F.C.S., etc. Fully Illustrated. Crown
 8vo., cloth, 7s. 6d. net.

 =WOOD.= A Manual of the Natural History and Industrial Applications
 of the Timbers of Commerce. By G. S. BOULGER, F.L.S., F.G.S. Fully
 Illustrated. Crown 8vo., 7s. 6d. net.

 =PSYCHOLOGY FOR TEACHERS.= By C. LLOYD MORGAN, LL.D., F.R.S. xii + 251
 pages. Crown 8vo., 3s. 6d.




_LONDON: EDWARD ARNOLD, 41 & 43 MADDOX STREET, W._




                         Transcriber's Notes:

    On page 104, the link for 'Elymnias lais' has been corrected
    from PlII to PlIII.

    Illustrations have been moved out of mid-paragraph.

    Variations in spelling and hyphenation are retained.

    Punctuation has been retained as published.

    Bold type is shown as =strong=.

    Italics are shown thus: _sloping_.

    Small capitals have been capitalised.