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.