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