The Evolution Theory, Vol. 2 of 2

Volume Ii

THE

EVOLUTION THEORY

DR. AUGUST WEISMANN

PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU

TRANSLATED WITH THE AUTHOR'S CO-OPERATION

J. ARTHUR THOMSON

REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN

AND

MARGARET R. THOMSON

ILLUSTRATED

IN TWO VOLUMES

VOL. II

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

1904

All rights reserved

CONTENTS

LECTURE PAGE

XX. REGENERATION 1

XXI. REGENERATION (continued) 23

XXII. SHARE OF THE PARENTS IN THE BUILDING UP OF THE OFFSPRING 37

XXIII. EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS 62

XXIV. OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS ARE NOT TRANSMITTED 80

XXV. GERMINAL SELECTION 113

XXVI. GERMINAL SELECTION (continued) 136

XXVII. THE BIOGENETIC LAW 159

XXVIII. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 192

XXIX. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (continued) 210

XXX. IN-BREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION, AND THEIR CONSEQUENCES 238

XXXI. THE INFLUENCES OF ENVIRONMENT 265

XXXII. INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES 280

XXXIII. ORIGIN OF THE SPECIFIC TYPE 299

XXXIV. ORIGIN OF THE SPECIFIC TYPE (continued) 330

XXXV. THE ORIGIN AND THE EXTINCTION OF SPECIES 346

XXXVI. SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION 364

INDEX 397

LIST OF ILLUSTRATIONS

FIGURE PAGE

35 B (repeated). Hydra viridis, the Green Freshwater Polyp. 4

96. A Planarian cut transversely into nine pieces 6

97. A Planarian which has been divided into two by a longitudinal cut 14

98. The leg of a Crab, adapted for self-mutilation or autotomy 17

99. Regeneration of the lens in a Newt's eye 21

100. Regeneration of Planarians 25

101. A Starfish arm 27

76 (repeated). Diagram of the maturation divisions of the ovum 39

82 (repeated). Fertilization in the Lily 59

91 (repeated). Hind-leg of a Grasshopper 83

102. Brush and comb on the leg of a Bee 84

103. Claw on the leg of a 'Beach-fly' 85

104. Digging leg of the Mole-cricket 86

105. Ovary of a fertile Queen-Ant and ovaries of a Worker 91

106. Three Workers of the same species of Indian Ant 97

107 A, B. Larva of a Caddis-fly 105

107 C. Leptocephalus stage of an American Eel 133

108. Nauplius larva of one of the lower Crustaceans 161

109 A, B. Metamorphosis of one of the higher Crustacea, a Shrimp 162

109 C. Second Zoæa stage 163

109 D, E. Mysis-stage and fully-formed Shrimp 164

70 (repeated). Daphnella 166

110. The largest of the Daphnids (Leptodora hyalina), with summer ova beneath the shell 166

111. Nauplius larva from the winter egg of Leptodora hyalina 167

112. Development of the parasitic Crustacean Sacculina carcini 168, 242

113. The two sexes of the parasitic Crustacean Chondracanthus gibbosus 170

114. Zoæa-larva of a Crab 171

115. Caterpillar of the Humming-bird Hawk-moth Macroglossa stellatarum 178

3 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 178

4 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 179

8 (repeated). Caterpillars of the Buckthorn Hawk-moth 179

116. Development of the eye-spots in the caterpillar of the Elephant Hawk-moth Chærocampa elpenor 180

117. Caterpillar of the Bed-straw Hawk-moth Deilephila galii 181

118. Two stages in the life-history of the Spurge Hawk-moth Deilephila euphorbiæ 182

119. Caterpillar of the Poplar Hawk-moth Smerinthus populi 184

120. A, Symmetrical, and B, asymmetrical curve of frequency 207

121. Life-cycle of Coccidium lithobii 214

122. Conjugation of a Coccidium (Adelea ovata) 216

123. Conjugation of Coccidium proprium 218

79 (repeated). The two maturation divisions of the 'drone eggs' 236

124. Alternation of generations in a Gall-wasp 245

125. The two kinds of galls formed by the species 246

126. Ovipositor and ovum of the two generations of the same species of Gall-wasp 247

127. Life-cycle of the Vine-pest (Phylloxera vastatrix) 249

128. Heterostylism 254

38 (repeated). A fragment of a Lichen 261

129. Aberration of Arctia caja, produced by low temperature 276

130. Skeleton of a Greenland Whale, with the contour of the body 313

131. Peridineæ: species of Ceratium 325

Lecture Xx

REGENERATION

Budding and division--Every theory of regeneration in the meantime only provisional, a mere 'portmanteau theory'--Regeneration not a primary character--Volvox--Hydra--Vital affinities--Planarians--Heteromorphoses--Enemies of Hydroid-colonies--Regeneration in Plants--In Amphibians--In Earthworms--Different degrees of regenerative capacity according to the liability of the part to injury--Different results of longitudinal halving in Earthworms and in Planarians--Regeneration in Birds--The disappearance of the power of regeneration is very slow--Morgan's experiments on Hermit-crabs--Autotomy in Crustaceans and Insects--Regeneration of the lens in Triton.

We have endeavoured to explain the handing on of the complement of heritable qualities from one generation to another as due to a continuity of the germ-plasm, and we assumed that the germ-cells never arise except from cells in the 'germ-track'; that is, from cells which are equipped, from the fertilized egg-cell onwards, with a complete sample of slumbering germ-plasm, and are thereby enabled to become germ-cells, and, subsequently, new individuals, in which the aggregate of inherited primary constituents implied in the germ-plasm can again attain to development.

We have now to consider other cases of inheritance in relation to the same problem--the origin of their hereditary equipment.

We know, of course, that new individuals may arise apart from germ-cells, that, in many of the lower animals and in plants, they may arise by budding and fission.

For both these cases the germ-plasm theory will suffice, with a somewhat modified form of the same assumption which we made in regard to the formation of germ-cells. The origin of a new individual by budding seems often, indeed, to proceed from any set of somatic cells in the mother animal; but somatic cells, if they contain solely the determinants controlling themselves, cannot possibly give rise to a complete new individual, since this presupposes the presence of all the determinants of the species. But as these determinants cannot be formed de novo, the budding cells must contain in addition to the usual controlling somatic determinants, idioplasm in a latent, inactive state, which only becomes active under certain internal or external influences, and then gives rise to the formation of a bud. The source of this accessory idioplasm must, however, be looked for only in the egg-cell.

In plants this bud-idioplasm must be complete germ-plasm, because the budding starts only from one kind of cell, the cambium-cells; but in animals in which--as it seems--it always proceeds from at least two different kinds of cells--those of the ectoderm and those of the endoderm--the matter is more complex. In this case these two kinds of cells will contain as bud-idioplasm two different groups of determinants, which mutually complete each other and form perfect germ-plasm, and only the co-operation of these two sets will give rise to the formation of a bud. I will not, however, go further into detail in regard to these relations, for the theory can do nothing more here than formulate what has been observed; it is hardly in a position to help us to a better understanding of the facts.

The case is not much clearer in regard to the processes which lead to the replacing of lost parts. The manifold phenomena of regeneration can also be brought into harmony with the theory, if we attribute to those cells from which the replacing or entire reconstruction of the lost part arises an 'accessory-idioplasm,' which, at least, contains the determinants indispensable to the building up of the part. It is possible that the assumed accessory idioplasm frequently contains a much larger complex of determinants, and that it depends on the liberating stimuli which, and how many of these, will become active.

If we take a survey of regenerative phenomena in the animal kingdom, it strikes us at once that the capacity is very different in different species, extraordinarily great in some and very slight in others. In general it is greater in lower animals than in higher, but, nevertheless, the degree of differentiation cannot be the only factor that determines the capacity for regeneration. That unicellular organisms can completely replace lost parts, that even a piece of an infusorian can reconstruct the whole animal if only the piece contain a part of the nucleus, we have already seen when discussing the significance of the nuclear substance. In this case the nucleus must contain the complete germ-plasm, that is, the collective determinants of the species, and these induce the reconstruction of the lost part, though they do so in a way that is still entirely obscure to us. In the meantime, our interpretation will not carry us further, either here or in regard to any other order of vital phenomena. To go further would be little short of propounding a causal theory of life itself; it would mean having a complete and real 'explanation' of what 'life' is. As yet no one has been able to claim this position. We can see the different stages through which every organism passes, and that they arise one out of the other; we can even penetrate down to the succession of those delicate and marvellously complex processes which effect nuclear and cell-division; but we are still far from being able to deduce, except quite empirically, from the present state of a cell what the succeeding one will be, that is, from being able to understand the succession of events as a necessary nexus which could be predicted. How a biophor comes to develop from itself the phenomena of life is quite unknown to us; we know neither the interaction of the ultimate material particles nor the forces which bring it about; we cannot tell what moves the hordes of different kinds of biophors to range themselves together in a particular order, what molecular displacements and variations arise from this, or what influence the external world has, and so forth. We see only the visible outcome of an endless number of invisible movements--growth, division, multiplication, reconstruction, and differentiation.

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