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

LECTUREPAGE
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

FIGUREPAGE
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 Ito face page[112]
Plate II" "[114]
Plate III" "[116]