• EVOLUTION w CHAPTERS ON EVOLUTION BY ANDREW WILSON, PH.D. F.R.S.E. F.L.S. &c. AUTHOR OF ' LEISURE-TIME STUDIES' 'LEAVES FROM A NATURALIST'S NOTE-BOOK ' ETC SEEN BY PRESERVATION SERVICES JAN 7 DATE.. WITH 259 ILLUSTRATIONS NEW YORK G. P. PUTNAM'S SONS 1883 TO SIR JOHN LUBBOCK, BART. M.P. IX. D. F.R.S. &c. VICE-CHANCELLOR OF THE UNIVERSITY OF LONDON, PRESIDENT OF THE LINN-BAN SOCIETY, ETC. IN ADMIRATION OF HIS WORK AND LABOURS IN THE CAUSE OF SCIENTIFIC EDUCATION, THIS VOLUME IS RESPECTFULLY DEDICATED. PREFACE. As the objects intended to be subserved by this work are explained in the introductory chapter, there is little need for a formal preface. It may, however, be well to state that the chief aim of the work is to present, in a popular and readily understood form, the chief evidences of the evolution of living beings. In this view, whilst I have been content to assume the reality of that process, I have also endeavoured to marshal the more prominent facts of zoology and botany, which serve to prove that evolution, broadly considered, is not merely a name for an unknown tendency in nature, but is an actual factor in the work of moulding the life with which the universe teems. A considerable experience as a biological teacher has long since con- vinced me that the hesitancy with which evolution is accepted, and the doubt with which even cultured persons are occasionally apt to view this conception of nature, arise chiefly from lack of knowledge concerning the overwhelming evidences of its existence which natural history presents. Doubtless a training in botany and zoology is required before the case for evolution can be fully mastered, but there need be no difficulty in the way of any intelligent person forming a just estimate of evolution upon even an elementary acquaintance with the facts of biology. I have accordingly sought to bring such facts prominently before the notice of my readers, and I would fain hope that even the complex topic of " development," itself a strong pillar of the theory of evolution, is susceptible of easy appreciation when the facts and inferences to be drawn therefrom are x PREFACE. plainly stated. It would be invidious to mention any special sources to which I have been indebted for aid in the production of the present work : the field is so vast, that one must needs gather details from the stores of many workers : but I cannot refrain from expressing my indebtedness to the works of the late distinguished author of the theory of " Natural Selection," and to those of Professor Huxley and of Sir John Lubbock. My best thanks are due to the latter for his kind permission to use several illustrations from his interesting work on the relations between insects and flower- fertilisa- tion. The illustrations as a whole will, I trust, be found to materially assist the comprehension of the most important points discussed in the various chapters. A. W. EDINBURGH : October 1882. CONTENTS. PAGE I. THE PROBLEM STATED i II. THE STUDY OF BIOLOGY . 14 III. THE CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS . 36 IV. CONCERNING PROTOPLASM .61 V. THE EVIDENCE FROM RUDIMENTARY ORGANS .... 80 VI. THE EVIDENCE FROM THE TAILS, LIMBS, AND LUNGS OF ANIMALS 97 VII. THE EVIDENCE FURNISHED BY THE SCIENCE OF LIKENESSES . 121 VIII. THE EVIDENCE FROM MISSING LINKS . . . . . 143 IX. THE EVIDENCE FROM DEVELOPMENT. — i. THE EARLIER STAGES IN THE LlFE-HlSTORY OF ANIMALS 167 X. THE EVIDENCE FROM DEVELOPMENT. — 2. THE LIFE-HISTORIES OF STAR-FISHES AND CRUSTACEANS 191 XI. THE EVIDENCE FROM DEVELOPMENT..— 3. THE DEVELOPMENT OF MOLLUSCS, AMPHIBIANS, &c 220 XII. THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS . . 252 XIII. THE EVIDENCE FROM THE CONSTITUTION OF COLONIAL OR COMPOUND ANIMALS . 273 XIV. THE FERTILISATION OF FLOWERS 308 XV. THE EVIDENCE FROM DEGENERATION 342 XVI. GEOLOGY AND EVOLUTION 366 INDEX 377 LIST OF ILLUSTRATIONS. 1. Cross-section of Vertebrate and Inver- tebrate 39 2. Joints of Lobster's Body . 3. Diagram of Lobster's Structure 4. ,, an Annulose Animal 5. Section of Vertebrate .... 6. Diagram of Mollusc .... 7. „ Echinoderm, Ccelenter- ate, and Protozoon .... 8. Hydrae 9. Zoophytes 45 10. Jelly-fish 45 n. Amoebae 12. Foraminifera .... 13. Lancelet 14. Sea-squirt 15. Bean in Section 57 16. Leaf of Dead-nettle . 17. Tulip in Section 57 18. Leaf of Tulip .... 19. Yeast Plants .... 20. Amoeba 64 21. Cell of a plant (Tradescantia), drawn at intervals, and showing changes in the contained protoplasm . 22. Various Cells 23. Structure of Wallflower . 24. Flower of Frog's-mouth 25. Sentinel Crab 26. Apteryx 27. Penguin 88 28. Dodo 29. Solitaire .... 30. Bones of Man's Arm . 31. „ Bird's Wing 32. „ Horse's Fore-limb 33. Skeleton of Hind-limb of Horse 34. Fore-feet and Hind-feet 35. Spider Monkey . . 36. Side View of Human Spine GE FIG. 37- 39 38. 41 39- 4i 40. 42 41. ' 43 42. 43 43- 44- 44 45- 44 46. 45 45 47- 46 46 48. 53 49- 54 5°. 57 Si- 57 52- 57 53- 58 54- 58 55- 64 56. 57- 58. 68 59- 70 60. 82 61. 83 62. 86 63- 87 64. 88 65- 89 66. 89 67 90 90 68. 9i 69. 92 70. 94 7i. 98 72. 99 73- PAGE . IOO . IO2 . 102 , IO2 . 103 . 103 Pig, Calf, Rabbit, Man . Perch Horizontal Tail of Whale . Fish showing an Equal-lobed Tail Thresher or Fox-Shark Skeleton of Salmon's Tail . Development of the Tail in Flounder 104 Lancelet 105 Lamprey and its Breathing Apparatus 106 Pterichthys : a Fossil Fish (Old Red Sandstone) 107 Fore-limbs of various Vertebrate Ani- mals 108 Skeleton of Frog . . . .no Feet of Marsupials . . . . in The Ceratodus or Barramunda . . 113 Fin of Ceratodus . . . .114 Air-bladder of Carp . . . .115 Air-bladders of Fishes . . .116 Lepidosiren or Mud-fish . . .117 Development of Frog . . . .118 Wallflower 131 Stamens changing to Petals . . 132 Gooseberry Leaves becoming Scales . 132 Double-flowering Cherry . . .133 Tortoise .... . . 134 Jaws of Vertebrata . . . -135 A Leaf and its Parts . . . . 136 Leaf of Pea 137 Tendrils of a Vine .... 137 Leaves of Smilax . . . .138 Yellow Vetch 138 Sloe and Rose, with Thorns and Prickles 139 Strawberry 140 Rose Fruit 141 Section of Fig 141 Skull of Dinoceras .... 152 Palaotherium (restored) . . .153 Restoration of Anoplotherium . . 153 XIV LIST OF ILLUSTRATIONS. FIG. PAGB 74. Flying Dragon *54 75. Skeleton of Bird's Wing . . . iSS 76. „ Bird . . . -156 77. Leg and Ankle of Bird . . . »57 7 \ Fossil Footprints from Triassic Rocks 157 79- ' 80. Fossil Remains of Archxopteryx . 158 8t. Hesperornis Jaw .... 160 82. Ichthyornis Jaw 160 83. Odontopteryx (restored) . . .161 84. Restoration of Compsognathus . . 162 85. Hindlimbs of Bird, Extinct Reptile, and Crocodile 163 86. Skeleton of Pterodactyl . . .164 87. Development of a Sponge (Olynthus) 173 88. Sea-squirt i?4 89. Development of Sea-squirt . . 175 90. Appendicularia 176 91. Amphioxus, or Lancelet . . . 176 92. Development of Lancelet . . . '77 93. „ Vertebrate . .179 94. Section through a developing Verte- brate i?9 95. Development of Chick . . .180 96. Embryo-chick 180 97. Embryo-vertebrates .... 181 98. Development of a Protozoon . .184 go,. „ Bear-animalcule . 185 100. Segmentation of Vertebrate Egg . 185 101. „ Frog's Egg . . 186 102. Gastrulas of various Animals . . 186 103. Development of a Fish . . . 187 104. Embryos of Quadrupeds . . . 188 105. Tadpoles of Frog . . . .189 1 06. Sea-urchins 195 107. Starfishes 195 108. Sea-cucumbers 196 109. Crinoid 197 no. Larvae of Starfish .... 197 in. Development of Sea-urchin . . 198 112. Rosy Feather-star and Young . . 198 113. Development of Crinoid . . . 199 114. „ Sea-cucumber . . 200 115. Crab 201 116. Water-fleas 202 117. Barnacles 203 118. Sacculina 204 1 19. Young of Barnacle .... 205 120. Development of Barnacles, &c. . . 205 121. Nauplius of Sacculina . . . 206 122. „ Cyclops .... 207 123. Fish-louse and its Nauplius . . 208 124. King Crab . .... 209 FIG. PAGE 125. Trilobites 210 Iz6- 1 Larvae of King Crab and Trilobite . 210 127. ) 128. Brine Shrimp and Young . . .211 129. Development of Lobster . . .211 130. „ Crab . . .212 131. Mysis 213 132. Penaeus 2I4 133. Nauplius of Penaeus .... 215 134. Zoea of Penaeus 215 135. Mysis-stage of Penaeus . . . 216 136. Mussel 22° 137. Snail 221 138. Slugs 221 139. Chitons 222 140. Pteropoda 222 141. Cuttlefishes 223 142. Development of Cockle and Ship- worm 223 143. Teredo, or Ship-worm . . . 224 144. Dentalium and its Structure . . 224 145. Development of Dentalium . . 225 146. „ Chiton (Loven) . 226 147. Pond Snail, Gastrula-stage . . 226 148. Development of Trochus . . . 227 149. Doris 228 150. JEolis 228 151. Bulla 228 152. Young of ./Eolis and Adult Pteropod 228 153. Larval, or Young Pteropod . . 229 154. Brachiopoda and Development . . 229 155. Lob-worm (A renicola) . . .231 156. Nereis : a Marine Worm . . . 231 157. Development of Worms . . . 231 158. Serpula 232 159. Development of Frog . . . 235 160. Axolotl, showing the External Gills . 236 161. Newts 238 162. Hippocampus, or Sea-horse . . 239 163. Amblystoma 242 164. Cecidomyia 246 165. Sitaris and its Development . . 247 166. Humming-bird 249 167. Swifts 250 168. Sun-bird 250 169. Young Kangaroo .... 254 170. The Rosy Feather-star's Develop- ment 256 171. Chloeon ...... 257 172. Metamorphosis or Swallow-tailed Butterfly 258 173. Cockroaches 259 174. Dragon-fly and its Metamorphosis . 261 LIST OF ILLUSTRATIONS. FIG. PAGE FIG. PAGE 175. Grasshopper 264 217. Stamen of Amaryllis .... 3H 176. Cricket 264 218. Pistil of Chinese Primrose . 3'4 177. Plant Lice 265 219. Strawberry ..... 3'4 178. Red Ants 265 220. Pollen-grains emitting Pollen-Tubes 3»« 179. Aquatic Insect Larva? . . 266 221. Pollen-masses of Orchid . 3,6 1 80. Polynema ...... 267 222. Pollen-grain of Evening Primrose 181. Campodea • 269 (magnified) 9rf 182. Lindia 270 223. Pollen-grain of Melon emitting Con- 183. Gregarina and its Development 275 tents 316 184. Different Forms of Amoebae 276 224. Pollen-tubes of Datura penetrating 185. Foraminifera , 277 the style (magnified) 3i7 1 86. Volvox and various Animalcules 278 225. Section of Bean 3iS 187. Sponge and its Development 279 226. Fertilisation of Primrose . 322 188. Hydrae 280 227. Arum, or Cuckoo Pint 327 189. Zoophytes 281 228. Carnation, showing the ripe Pistil . 327 190. Flustra, or Sea-mat .... 284 229. Myosotis in its early and later 191. Tapeworm ...... 285 329 192. NaTs, or Freshwater Worm 286 230. Dead-nettle in Section 330 193. Joints of Lobster .... 286 231. Flower and Stamens of Salvia . 331 194. Development of Julus . . . 287 232. Fertilisation of Salvia 33i 195. Starfishes 288 233. Section of Fuchsia .... 332 289 T , -> 197. Comparison of Development in a 235. Flower of Sage J> J" 3.53 Flowering Plant, a Zoophyte, and 236. „ Pea dissected 333 a Colony of Plant- Lice . 290 237. Section of Pea 333 198. White Corpuscles of the Blood . 295 238. Orchid Flower 336 199. Daisy . . . 300 239. Pollen Masses of Orchid . . . 336 200. Section of Daisy .... 301 240. Section of Orchid Flower . 336 201. Dandelion 302 241. Development of Frog 343 202. Centaurea cyanus, or Corn Blue- 242. Embryo-vertebrates .... 344 bottle ..... OO-5 243 Brachiopods •J A - 203. Head of Thistle, showing numerous j^j 244. King Crab . . . J4O 345 Florets 3<>4 245. Beryx ....... 34« 204. Simple Umbel of Cherry and Com- 246. Ichthyosaurus and Plesiosaurus 34<5 pound Umbel of Fool's Parsley . 304 247. Pearly Nautilus 347 205. Wallflower ...... 3O9 248. Globigerina, etc. . • . . . 348 206. Foxglove ...... 309 249. Common Tapeworm .... 350 OOQ ?:: I 208. Primroses . ..... j^y 310 251. Young Sacculina .... J3 l 351 ?= I 210. Female or Pistillate Flowers of Willow 311 J3 l :? r -i 211. Male or Staminute Flowers of Willow 311 254. Demodex (magnified) OJJ 355 212. „ „ „ Oak . 312 255. Linguatulina 355 213. Pistillate Flowers of Oak . 313 256. Sea-squirt 356 214. Parts of a Flower (Campanula) . 313 257. Development of Sea-squirt 357 215. Flower of Saxifrage in Section . 313 258. Hydrae 360 216. Stamens of Iris 314 259. Rotifera 361 CHAPTERS ON EVOLUTION, i. THE PROBLEM STATED. THE year 1858 may be said to mark a distinct era in the science of biology, or that dealing with the structure, functions, development, and general history of animals and plants. On July i, 1858, two papers were read before the Linnsean Society of London, which were destined to evoke and to direct an amount of criticism and research unparalleled in the annals of scientific history. It was then that Mr. Darwin and Mr. Alfred Russel Wallace laid before the scientific world the results of independent observations and reflections con- cerning the origin of the varied species of animals and plants which form the diverse population of the globe. Considering that the views expressed in the papers referred to had been formed and elaborated in entire independence of thought, and, indeed, in well-nigh opposite regions of the earth's surface, the harmonious nature of the conclusions arrived at by the authors was both interesting and surprising. Mr. Darwin's paper dealt with the Origin of Species ; that of Mr. Wallace bore the title "On the Tendency of Varieties to depart indefinitely from the Original Type." The former, as naturalist on board H.M.S. " Beagle," had been " struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent." Mr. Darwin further tells us that " these facts seemed to throw some light on the origin of species — that mystery of mysteries, as it has been called by one of our greatest philosophers." Mr. Wallace, on the other hand, exploring the Malay Archipelago, and interesting himself in the problems which the varied flora and fauna of the East suggested to the mind, formed opinions concerning the origin of species which, as we have seen, practically coincided with those of Darwin. In each case the inspiration, so to speak, came direct from nature, and from the unbiassed observation of the world of life itself — an origin this, as suggestive as it was appropriate for specula- tions including in their sweep and extent the entire organic universe. 2 CHAPTERS ON EVOLUTION. The leading ideas of 1858 may be briefly and plainly stated. Mr. Wallace's conclusion may be summed up in his own expression, " that there is a tendency in nature to the continued progression of certain classes of -varieties further and further from the original type — a progression to which there appears no reason to assign any definite limits — and that the same principle which produces this result in a state of nature will also explain why domestic varieties have a tendency, when they become wild, to revert to the original type. This progression," continues Mr. Wallace, "by minute steps in various directions, but always checked and balanced by the neces- sary conditions, subject to which alone existence can be preserved, may, it is believed, be followed out so as to agree with all the phenomena presented by organised beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit." Mr. Darwin's views were no less lucidly expressed. He agreed essentially with Mr. Wallace in attributing the origin of new species to the modification of already existent animals and plants. The " Origin of Species " itself — a work first published in November 1859, and at present in its " thirteenth thousand " — represents the expansion and elaboration of Mr. Darwin's views of 1858, the publication of which raised at once a multitude of scientific critics, and invoked, it may be added, the rancour, bigotry, and often insensate, because ignorant, opposition of many persons outside the ranks of biological science. To understand the meaning of the opposition which the views of Darwin and Wallace at first provoked, it is needful simply to take a brief retrospective view of the history of man's ideas regarding the origin of living nature, including, of course, the history of his own genesis. The opinions of 1858 were at first simply branded with the heterodox stamp, as preceding opinions had been similarly treated from the time of Lamarck in 1801, and, indeed, as every other statement which was not thoroughly " nail'd wi' ScripturY' had been treated with the " apostolic blows and knocks " of those who seemed to claim a monopoly of all truth concerning the past, present, and future of the universe. The reason for the stormy reception of views concerning the species of animals and plants, promulgated as a matter of strict science, and formulated without any reference to other or more venerable opinions, can be readily enough understood, when it is added that the chief opposition to the " Origin of Species " came from the theological camp. Mr. Spencer remarks that " early ideas are not usually true ideas." He might have added with equal truth that early ideas, when woven into the texture of religious systems, are not given to lose their vitality with increasing age. At all events, the opposition to the views of Darwin, and to the evolution theory at large, were chiefly combated, THE PROBLEM STATED. 3 not from any inherent error they were believed to contain, but simply because they ran in direct opposition to the older and more primitive conceptions of the origin of species which, formulated in creeds, and elaborated from pulpits, had come to be received as an article of unquestioning faith by cultured and uncultured alike. Two theories, and only two, concerning the origin of animals and plants, present themselves for examination and acceptation by the human intellect. Of these two theories, one dates from a pre-scientific period, when this earth was believed to be the centre of the universe, when this world was believed to possess a round and flattened surface, and when the sky was believed to be a solid roof environing the earth above, and constituting at the same time the floor of an upper and celestial sphere. Such rude ideas of cosmogony and astronomy were fully paralleled by as primitive a biological system. The various species of animals and plants were believed, according to the Mosaic cosmogony, to have originated each as a complete and "special creation." As man was conceived to have been formed of the dust of the earth, and as all the intricacies and complexities, struc- tural, physical, and chemical, of the human organism were believed to have been set in action at once and perfectly, through the opera- tion of a mysterious, supernatural fiat; so the varied species of animals and plants, from the monad to the elephant, from the plant- specks in the pool to the giant pine or lordly oak, were similarly held to have originated each as a " special creation." In this way a creative interference, capable of originating living beings ex nihilo^ and therefore capable of literally creating matter — itself an incon- ceivable act — was credited on the first theory, as it may still be credited in creed and dogma, with the production of the entire universe of living things. The genesis and development of such a theory has naturally been laid stress upon by most writers who have criticised, from an a priori point of view,4the worthiness and acceptation of itself and its opponent hypothesis. The fact that the " special creation " theory was framed in an age when primitive ideas and mythologies, now completely consigned to the limbo reserved for exploded myths, constituted the philosophy of mankind, naturally militates against the truth and probability of the hypothesis in question. Being a primitive imagining, it would, according to Mr. Spencer's view, be most likely a wrong and untrue one. " If the interpretations of nature given by aboriginal men were erroneous in other directions," says that author, " they were most likely erroneous in this direction. It would be strange if, whilst these aboriginal men failed to reach the truth in so many cases where it is comparatively conspicuous, they yet reached the truth in a case where it is comparatively hidden." As we have to-day rejected the astronomy of the B 2 4 CHAPTERS ON EVOLUTION. ancients, and as we no longer utilise their geology as serviceable or true, we can afford to dispense with their biological views, and we therefore turn hopefully to the second and scientific concep- tion of the origin of living beings. This conception is the theory of " Derivation," " Descent," or '« Evolution." According to the evolutionist, the universe of life, instead of being composed of a series of fixed and unchangeable units — unvarying as when they were first " created " on the former theory of life's origin — is the theatre of incessant variation and change. Each " species " or " kind " of animals and plants, instead of existing as a stable unvarying group, as the older naturalists defined it, is seen to vary to a greater or less degree, according to internal and con- stitutional, or to external conditions, or under the influence of both combined. The progeny do not rigidly resemble the parents, but continually exhibit differences in colour, size, and other peculiarities. Thus " variations " in species are produced ; and these variations may appear of singularly wide character when conditions favouring change have operated in their production. In this way the existing " species " are modified, and the new " varieties " thus produced, in time give origin to new species. These latter are, therefore, viewed as having been " evolved " by natural descent, that is by the ordinary laws of generation and reproduction, from the older species. The animal and plant worlds regarded in this light are liable to perpetual modification, and the experience of every-day life — seen familiarly in the culture of plants, and in the breeding of horses, cattle, sheep, dogs, and pigeons — amply testifies to the mobility and plasticity of the animal and plant constitutions. That is to say, man, in the process of breeding animals, and by selecting the parents of his domestic races, can " evolve " animals which, in time, differ from the original stock far more widely than ordinary and so-called " species " differ from one another. But the plasticity of " species " is far from being the only prop and support of the theory of evolution. When the naturalist attempts to classify animals or plants, he discovers that instead of exhibiting each a specific and individualised structure, as might be presumed were the " special creation " theory true, the various groups of animals are linked together in such a fashion as to suggest the existence of some natural bond of relationship between them. With the plant world the case is analogous. The tribes of plants are harmoniously connected together in such a manner as to indicate a relationship which, as in the case of the animals, is only satisfactorily explained on the idea of connected descent. What explanation, for example, satisfying to the rational mind, can be given of such a striking feature as that illustrated in the literally marvellous correspondence which exists between the fore and hind THE PROBLEM STATED. 5 limbs respectively of all vertebrate animals ? How, on any other hypothesis save that of evolution, and of the common origin of the .animals in question, can we explain why the arm of man, the wing of the bird, the horse's fore-limb, the dog's fore-leg, and the whale's paddle, are constructed on a common plan ? Or, again, why should the bodies and appendages of lobsters, insects, spiders, and centipedes, be similarly identical in fundamental structure, unless on the theory of their common origin? Again, from the region of Development, the evolutionist derives a whole host of cogent reasons for the faith he entertains in the sound- ness of his conclusions. All animals begin life under a similar guise — or, to come to actual details, as protoplasmic specks. In their earliest stages, the germs of a man and of an animalcule are indistinguishable. Furthermore, as human development proceeds along its lines, it assumes its own and special phases only after passing through stages which correspond more or less completely with permanent forms of lower animals. At first each quadruped is thus fish-like, and after successive developments leading it upwards through reptile and bird phases, it attains the quadruped type. But, even as a quadruped, the human organism itself declares its nobility of blood, only as a final feature in its early history. Of all other animals, the same recital holds good. Each animal comes to assume its own place as an adult through stages of development which repeat, as in a moving panorama, the phases of the lower life through which its ancestry has passed. The development of the individual animal is thus the brief and condensed recapitulation, often more or less obscured, of the development of the race or species. If facts like these be not admitted to prove the reality of evolution, then development as a whole must present itself as a series of the most meaningless paradoxes which it has been the fate of man to discover in the universe around. Such are a few of the considerations — to be fully illustrated in succeeding chapters — which suggest that evolution is a great truth and a sober fact of living nature. Other topics of equal importance — such as the occurrence of rudimentary and useless organs in animals and plants, the existence of "links" between distinct groups, the results of degeneration, and other subjects — will also be found fully detailed in the following pages, which partake, indeed, of the cha- racter of a continuous series of proofs of the truth of the evolution theory. It requires, however, to be pointed out in the present instance, that whilst the general truth of evolution is now admitted by all competent biologists, there exists considerable diversity of opinion regarding the exact factors to which the processes of modi- fication are due. Thus the title of Mr. Darwin's classic work is •" The Origin of Species by Means of Natural Selection ; or, the 6 CHAPTERS ON EVOLUTION. Preservation of Favoured Races in the Struggle for Life ; " and such a designation indicates with sufficient clearness that it was to "natural selection " that Mr. Darwin attributed the chief power in evolving new species through the modification of the old. Mr. Wallace accepts " natural selection " as a true factor, but he does not regard it as operating to the same extent in evolution as did Mr. Darwin. Other biologists, again, are inclined to adopt the idea that the evo- lution of living beings follows particular lines, along which the process is guided or directed partly by internal causes inherent in the constitution of the living being, and partly by external causes and by the surroundings of life. Concerning the relative importance of the various factors which biologists regard as of importance in determining the process of evolution, Huxley remarks that the exact place and power of " natural selection " " remains to be seen. Few can doubt that, if not the whole cause, it is a very important factor in that operation, and that it must play a great part in the sorting out of varieties into those which are transitory and those which are permanent. But," continues this high authority, " the causes and conditions of variation have yet to be thoroughly explored, and the importance of natural selection will not be impaired, even if further inquiries should prove that variability is definite, and is determined in certain directions rather than in others by conditions- inherent in that which varies. It is quite conceivable that every species tends to produce varieties of a limited number and kind,, and that one effect of natural selection is to favour the development of some of these, while it opposes the development of others along their predetermined lines of modification." It forms no part of the purpose of this volume to discuss the merits of these varied views respecting the exact nature of the factors to which evolution owes its force and power. Perhaps any exhaustive account of this aspect of the subject is at present im- possible with the materials at command. That which is infinitely more important in the first instance is the appreciation, firstly, of what evolution at large is and implies ; and, secondly, of the proofs and arguments on which the existence and operation of this process- may legitimately be based. A brief statement of the Darwinian theory of evolution may, however, be given, inasmuch as this aspect of the theory is that most frequently discussed and criticised both in scientific and in popular circles. It should be clearly borne in mind that the broad idea of evolution forms a foundation for every theory of the special fashion in which that process may be conceived to operate. " Darwinism " in this light is therefore to be regarded merely as one, but also as probably the strongest phase of those speculative endeavours to show the " how " of living nature, just as- evolution itself has supplied the answer to most of the biological, " whys." THE PROBLEM STATED J The term " natural selection," applied by Mr. Darwin to his theory of evolution, is in itself a highly expressive designation. It indicates an analogy with that process of " selection " whereby man chooses the animals he intends to breed from. As by human agency, the special features of any given race may be brought to the front in the progeny, or as other characteristics may similarly be obliterated by gradual changes in the appearance, size, colour, and structure of the animal and plant units, so it is contended an analo- gous principle — that of "natural selection" — is traceable in the world around us. This process naturally tends to effect in nature the same or allied variations in species which man produces for a given end. In this view, natural selection is simply the natural result of a series of interactions between animal and plant life and its surroundings ; and the gist of the process may be summed up in the statement that in the process of selection the weeded- out units die off, whilst the " selected " and stronger units, coming to the front, perpetuate their race, and thus tend, through their superiority and strength, to evolve new races and species. It is an easy matter to summarise, in a series of propositions, the chief data upon which Mr. Darwin's theory rests. These proposi- tions are as follows : — Firstly. Every species of animals and plants tends to vary to a greater or less degree from the specific type. No two indi- viduals are alike in every respect; each inherits from its parents a general likeness or resemblance to the species, whilst it tends at the same time to diverge from the parental form. Secondly. These variations are capable of being transmitted to offspring ; in other words, by natural laws of inheritance, the variations of the parents appear in the progeny along with the natural characters of the species. This much is proved in the " artificial selection " by man, for breeding, of those animals whose characters it is desired should be transmitted to offspring. Thirdly. More animals and plants are produced than can pos- sibly survive. Each species tends to increase in geometrical progression, and all the individuals produced could not find food, or even surface-area whereon to dwell. Fourthly. The world itself (i.e. the surroundings of animals and plants) is continually undergoing alteration and change, represented by climatal variation, the rising and sinking of land, &c. Fifthly. There ensues a " struggle for existence " on the part of 8. CHAPTERS ON' EVOLUTION. living beings. Over-population means a struggle for food and for other conditions of life, such a consideration being really the doctrines of " Parson Malthus " applied to the animal and plant worlds at large. Hence it follows that as some forms will be better adapted (by variation) than others to their surroundings, the former will come to the front in the struggle. Nature, so to speak, will "select" those individuals which best adapt themselves to their surroundings, and will leave the rest to perish. This is the " survival of the fittest" The change of surroundings, already postulated, will further induce and perpetuate variations in those individuals which survive. Sixthly. A premium is thus set by nature upon variatio'n, inas- much as the varying and surviving individuals will transmit their peculiarities to their offspring. Seventhly. Thus " varieties " of a species are first produced ; the "varieties" becoming permanent, form "races;" and the " races," in time, differ so markedly from the original species whence they were derived, as to constitute new "species." Eighthly. Past time has been, to all intents and purposes, infinite. Hence it is probable that the existent species of animals and plants have been evolved (through " natural selection," acting through long periods of time) from a few primitive and simple forms of life, or possibly from one such form alone. Such is a summarised statement of Mr. Darwin's views. His theory of " Sexual Selection " may be viewed as supplementary to that of natural selection, and as serving likewise to account for certain phenomena of which the former takes little heed. The pro- cess of sexual selection is that whereby the males of many species secure the females after contests. The result of these contests is that the stronger and victorious males will transmit to their offspring any peculiarities of form or constitution which they themselves pos- sess, and in virtue of which they became victors over others. In this way variation is again seen to be favoured. Then, secondly, the "selection "of a mate is often determined, not by the males, but by the females. In such a case it is assumed that those males which exhibit (as seen typically amongst birds) special features in the way of colour, plumage, size, or ornamentation, will be preferred and chosen. Variations are thus once more produced ; since the special characters of the male will be reproduced in the offspring, whilst the perpetuated accumulation of such characters will in due time modify the species and evolve new races therefrom. By aid of the theory of 7 HE PROBLEM STATED. g Asexual selection" Mr. Darwin accounts for many of the special features and possessions of animal races. Thus, the song of birds, •the brilliant plumage and colours of many species, and the curious and peculiar ornamentation of many forms, altogether inexplicable on any ordinary theory of utility, are seen to be useful or necessary .adjuncts, on the theory of " sexual selection," to the modification of species and to the evolution of new races. The foregoing statement of the Darwinian theory will enable the reader to follow with greater advantage the arguments and illustra- tions adduced in the succeeding chapters in support of the evolution theory at large. It only remains in the present instance to indicate the order and succession in which the evidences of evolution are herein presented. An account of the methods in which the study of modern biology or natural history is carried out, forms the subject of the second chapter. Such an account will serve to place the reader in posses- sion of the chief data, from a knowledge of which the naturalist is enabled to constructs reasonable and harmonious series of details respecting the living denizens of the globe. ' The special inquiries of the biologist are duly noted, and the divisions of biology which supply answers to the pertinent queries of the scientific investigator are also detailed. Incidentally, the bearings of ordinary biological details on evolution are also discussed, and a suitable introduction is thus afforded for succeeding studies. In the next and third chapter, the reader is made acquainted with the constitution of the animal and plant worlds. The know- ledge of the general relationship of animals and of plants to each other, viewed in groups and as individuals, forms a necessary founda- tion for all biological studies, whether viewed in reference to the theory of evolution, or merely as a part of ordinary information re- specting the universe of life as a whole. In this chapter, the bearings of the constitution of the animal and plant kingdoms on the theory of descent are duly detailed ; and a sketch of the primary classifications of animals and plants is also included in the general history of the worlds of life. The fourth chapter introduces the subject of " protoplasm." On the due appreciation of the relations of this substance as the "physical basis of life" to the constitution of the living body, rests the clear understanding of many fundamental points in connection with animal development. Similarly, the inferences which the evolu- tionist is led to draw from the universality of protoplasm as the common material of living beings, are only appreciable when the nature of this curious and all-pervading substance is set forth in detail. No step is possible in biological advance until the facts relating to protoplasm and its relations to life are mastered ; and in lo CHAPTERS ON EVOLUTION. the discussion of such a topic certain fundamental truths and propo- sitions of biology therefore fall to be discussed. Thus fortified and prepared by these introductory details, the evidences of evolution as the great process which summarises in itself the forces and tendencies of living beings fall to be noted. The first of these evidences is constituted by " rudimentary organs," and the tale they tell of animal and plant modification. Here the curious nature of these apparently useless parts is seen to be fully borne out by the idea that they refer " to a former state of things," and that they represent the natural, but deteriorated and vanishing remains of structures once useful in the ancestors of the animals that now possess them. The sixth chapter strikes a somewhat related key-note to that touched in the preceding section. The evidence deducible from the modifications which animal structures have undergone is largely in favour of evolution. The structures specially selected for treatment in this chapter are the tails, limbs, and lungs. It is attempted to be shown that these organs illustrate in the clearest manner how adap- tation to new ways of life is induced by alterations in the habits and surroundings of animal forms. Incidentally, information is likewise afforded respecting certain interesting aspects of the structure of higher animals. The science of likenesses (or homology) forms the special topic of the succeeding section. Herein the general deductions of " homology " are discussed and illustrated from both animal and vegetable worlds. The broad likenesses between animals which were discussed in the third chapter, are here specialised, and the natural correspondence existing between parts and organs, often of the most diverse appearance, is duly dwelt upon. In its general tenor, this chapter will be found to follow out the line of argument specially selected in chapter sixth. The subject of "missing links" is treated in the eighth chapter. No topic in all the wide range of evolution demands more detailed treatment than that of the "links" between apparently distinct groups of animals the existence of which the theory itself postulates, and the necessity for which is a matter of popular notoriety. The higher animals have been specially selected for treatment in this chapter, not merely because the case for evolution is more likely to- be duly appreciated when these forms are selected for discussion, but because the evidence is overwhelmingly clear in favour of evolution when the higher groups are examined, and also because links in lower life are duly treated in succeeding chapters under the head of " Development." The succeeding three chapters deal with the evidence afforded by development in favour of evolution. All evolutionists may THE PROBLEM STATED. II be said to regard the deductions of embryology amongst the chief supports of their hypothesis. Hence, as the subject is not merely important in itself, but also somewhat technical in details, it has been judged advisable to discuss the problems of development at some length. In chapter ninth, the earlier stages in the development of animals at large form the chief topics treated. In the tenth section, two special groups — the Echinoderms or star-fishes, &c., and the Crustaceans (or crabs, lobsters, and their allies) — are selected for discussion ; whilst in the succeeding section attention is directed to the special features observable in the development of the Molluscs, and of higher animals still. The twelfth chapter, devoted to the "metamorphosis " of insects, is intended specially to show how the development of these animals presents us with a series of highly interesting illustrations of certain modifications affecting the young of animals as well as the adults. The origin of the wings of insects, and other details incidental to the structure and physiology of these animals, are also discussed in this chapter. The thirteenth chapter revises, somewhat at length, certain problems in the constitution of animals which appear worthy of study ; whilst incidentally the nature of the plant-constitution is also treated. Both topics are related to evolution in a broad sense ; since the factors which determine the intimate constitution of the animal or plant must also perforce possess a large share of influence in modifying the worlds of life at large. The fourteenth chapter, dealing with.the "fertilisation of flowers," is intended to illustrate certain of the methods whereby, in the physiology and life of plants, the evolution of new races is favoured and assisted. No more typical examples of ways and means adapted to aid and inaugurate the primary conditions on which evolution depends and to ensure variation, could well be cited than this department of botanical science. The deductions from flower- fertilisation tend very powerfully, moreover, to support the doctrine of descent in other phases than those which are connected merely with plant-reproduction at large. The fifteenth chapter, devoted to the subject of "degeneration," exemplifies the axiom that the ways of evolution include backsliding and retrogression as well as advance. Many animals and plants have developed all their characteristic features through their adoption of, and adaptation to, a lower way of life than that pursued by their ancestors ; whilst whole groups of animals present features to the naturalist which could not be accounted for by any ordinary phase of evolution, but which the idea of degeneration, as a factor in working out the ways of life, has fully explained. The concluding chapter deals with the relations of geological 12 CHAPTERS 'mON E VOL UTION. science to evolution, and sums up certain geological matters and aspects of evolution which have been cursorily alluded to in the preceding sections. The general development of life on the earth, as well as the more special phases with which the geologist has to deal, are shown to support evolution fully and completely. The history of life in the past correlates itself so completely and fully with that of life as it exists to-day, that the geological side of the argument in favour of evolution has come prominently to the front in every system which has had for its aim the exposition of the theory of descent. It should, lastly, be borne in mind that the evidence for or against the theory of evolution must be judged chiefly by biological standards, and from the biological standpoint, if an accurate esti- mate of its probabilities, excellencies, and powers to explain satisfac- torily the phenomena of life and structure is to be formed. The theory of descent has been frequently criticised, with scant success, however, from other points of view than the biological But as a theory which, above all else, purports to present us with a rational account of the origin and modifications of living beings, it is evident that its weakness and its strength alike must be sought for within the domain which the naturalist claims as his own. Hence the succeeding pages may be viewed as an attempt to summarise in a popular form the chief details of the evidence, on the fair and rational interpretation of which the evolutionist is well content to rest the claims of his doctrine for intellectual assent and acceptance. In such a study, moreover, may be most readily found the materials for a compre- hension of those aspects of the subject which lie somewhat apart from the main pathways of biological study. The interest of the whole topic need hardly be alluded to in closing these introductory remarks. No subject which can engage the attention of the thinker in these latter days presents so many and varied avenues, leading to allied fields of inquiry, as the doctrine of descent As applied to man alone, the evolution theory teems with interest, and suggests endless problems for the consideration of the metaphysician, the ethical philosopher, and the sociologist, not to speak of the multifarious features of anatomy, physiology, and geology, which the purely human phase of the theory presents to view. The concluding words of Mr. Darwin in the " Origin of Species " elo- quently describe the varied interests which the subject evokes, and also summarise his own conclusions concerning the agencies which have wrought out the existing order of living nature. " It is interesting," says Mr. Darwin, " to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different THE PROBLEM STATED. 13 from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction ; Inheritance, which is almost implied by reproduction ; Variability from the indirect and direct action of the conditions of life, and from use and disuse ; a Ratio of Increase so high as to lead to a Struggle for Life, and, as a consequence, to Natural Selection, entailing Divergence of Character, and the Extinction of less improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the pro- duction of the higher animals, directly follows. There is grandeur," concludes Mr. Darwin, " in this view of life, with its several powers having been originally breathed by the Creator into a few forms or into one ; and that whilst this planet has gone cycling on accord- ing to the fixed law of gravity, from so simple a beginning endless forms, most beautiful and most wonderful, have been, and are being evolved." I4 CHAPTERS ON EVOLUTION. II. THE STUDY OF BIOLOGY. IT may reasonably be supposed that every intelligent person is perfectly conversant with the term "Natural History," and with the common meaning usually attached thereto. As employed in ordinary life, or even in scientific circles, where exactness of language is a necessity for the clear expression of thought, the term has come to signify the study of the animal world. Hence, popularly, a " natural historian " is believed to be a person who is much at home in zoological gardens, in aquaria, and in all places where animal life is presented to view, for purposes of study, serious or otherwise. To correct popular and long-standing ideas, is a task for which no sensible person can have any great liking. Albeit that the task is often necessary, and in matters more serious than the nomenclature of science has to be undertaken as a matter of conscience, the work of reforming old-established notions of things is frequently the labour, not of one lifetime, but of many generations. Still, effort is, and must be, cumulative in its effects ; and if in the present instance I can succeed in showing the rational use of the name " Natural History," I may perchance not merely preface this chapter by a necessary and appropriate explanation, but likewise aid in diffusing better, because truer, ideas of the aim and scope of natural science. The term " Natural History " finds different meanings according to the latitude in which it is used, and according to the prevailing ideas which the name has been accustomed to convey to the minds of those using the name. In the north, for instance, in academic circles, the name is used to signify "zoology," or the study of animals alone. A student who, in a northern university, attends a class of " Natural History," is understood to concern himself solely with the animal population of the globe. Elsewhere the name has been used to indicate the study of plants and animals together ; the student of " Natural History " in this latter sense, extending his researches into the field of " Botany," in addition to that of " Zoology." But a third meaning of the name comes to hand in which it is used, in strict accordance with its etymological signi- ficance, to signify, not the study of any one or two departments of nature, but to denote the whole range of natural science studies. Employed in this latter sense, the name " Natural History " is found to include not merely the knowledge of animals and plants, but the THE STUDY OF BIOLOGY. 15 study of minerals and of the inorganic or non-living world at large ; whilst it may also be shown to include the study of the planets, because, as a history of nature, it is bound to take account of everything whereof nature consists. To be a "Natural Historian " in this latter sense would imply a man's knowledge of the whole universe. But as human life, in one view at least, is conveniently short, and as wisdom and knowledge are apt to linger long, the most ardent devotees of science may reasonably shrink from laying claim to a full or even moderate knowledge of "Natural History" as thus defined. The " Admirable Crichton " in these days is an unknown creature ; and although now and then a master-mind sweeps across the horizon of knowledge — although an occasional century may see a Darwin or a Helmholtz with a profound knowledge of nature- science in well-nigh all its branches — still, the bounds of this wide science of " Natural History," as we have defined it, threaten to prove beyond the powers and grasp of any one mind amongst us. It will thus be seen that the correct use of the name " Natural History " is that in which it is employed to mean a knowledge of universal nature. This being so, what are the branches which this great science may be said to include? I have already indicated that geology and mineralogy, in addition to astronomy and natural philosophy (or physics), find a natural place within its limits. Chemistry is as truly a branch of natural history as geology, and when we have placed these sciences in the category of the " Natural Historian," there yet remains an important branch which in one sense may be said to unite the others, and which concerns itself with the living things of this world. The child in his elementary lessons is accustomed to speak of the three kingdoms of nature. This division into animals, plants, and minerals is a perfectly correct method of parcelling out nature's belongings. Although possessing obvious relations with the animals and plants, the sciences of chemistry, geology, and mineralogy deal chiefly with the mineral, or lifeless, section of nature, as does natural philosophy, and its offspring astronomy. It becomes clear, then, that the interests of living things require to be considered under a special department of natural science. In former days, as we have seen, the "Natural Historian" was the scientific guardian of the animal and plant interests. Abolish- ing this phrase, what term, it may be inquired, do we now employ to indicate the study of living beings? The answer to this ques- tion may fitly conclude these introductory remarks. As Huxley has shown in his lecture "On the Study of Biology," whilst the name " Natural History " was used in the broad sense to include all departments of natural knowledge up to the middle of the seventeenth century, the growing specialisation of scientific studies 16 CHAPTERS ON EVOLUTION. tended thereafter to separate the sciences into the sciences of mathe- matics and experiment (such as chemistry, astronomy, and physics), whilst the sciences of observation (geology, mineralogy, zoology, and botany) remained to represent the wider " Natural History " of olden days. Buffon and Linnaeus wrote their " Natural Histories " under this latter idea, namely, that they professed the study of rocks, fossils, plants, and animals. Further limitation of scientific aims and names was, however, soon necessitated by the increase of knowledge. It was clearly perceived that, as living things, the animals and plants remained more closely connected than did the geological and other branches of natural history. Hence, in due course, a new name crept into use to indicate the sciences which specially select life and living beings as subjects of stud)'. In 1801 Lamarck, the French naturalist, first used the name " Biologic " to indicate the collection of sciences dealing with the manifold relations of animals and plants. There seems to be a faculty in the human mind for acquiring a liking for a name or method which exhibits a special appropriateness in its description of the objects it is destined to describe. And we find that, despite the firm hold which the name " Natural History " had obtained as descriptive of the study of life, it is being gradually superseded by the name " Biology " — in every sense a most appro- priate term. Although chiefly in the northern parts of these islands we still cling with a striking proclivity, favoured by a reverence for antiquity, to the name "Natural History," the term "Biology" has already gained a secure hold as a scientific expression. To-day, when we study " Natural History," we should be understood to take the widest possible view of natural things ; and we may include in our studies subjects as diverse as the origin of chalk-flints, the anatomy of the brain, the liquefaction of gases, and the fertilisation of flowers. But when we assert that we study "Biology," we thus limit, with some degree of exactness, the objects of research. Then, we take for granted that our studies limit us to the fields of life — to the history of animals and plants — a history which, be it remarked, however, stretches its interests far afield, and relates itself in many and diverse ways to other and even widely separated branches of knowledge. Thus much may be said by way of introduction to the nature of biological study. In the field before us lie the manifold concerns of the world of life ; and it is straining no analogy to assert, with Mr. Herbert Spencer, that "preparation in biology" may after all be the best preliminary for the successful study of the human race, and for the understanding and regulation of its interests, whether regarded as . pertaining to the individual, the family, the race, or the nation at large. It is no startling thought that the laws of human life and society can be demonstrated to be founded upon wider laws which prevail in animal life at large, and that the analogies and resemblances betwixt THE STUDY OF BIOLOGY. 17 the ways of humanity and the acts of lower life are too close to admit a doubt of their intimate relationship. Spencer is stating no mythical idea but a solid fact, when he remarks that " the Science of Life yields to the Science of Society certain great generalisations, without which there can be no Science of Society at all." Nor is the statement to be viewed as aught else" than reasonable, that " all social actions being determined by the actions of individuals, and all actions of individuals being vital actions that conform to the laws of life at large, a rational interpretation of social actions implies knowledge of the laws of life." Such a subject, however — the connexus between biology and human interests — would require a volume to itself; and at present I merely mention the fact of such relationship to impress the idea that the future of biology will undoubtedly include in its scope much of human affairs that now appears wholly at a distance from the interests of animals and plants at large. Nor have I the inten- tion, at present, of discussing the relations of biology to religion, or of trenching even cursorily upon those modifications in religious opinion and in theological reasoning which, of all the sciences, biology has been most plainly instrumental in inaugurating and fostering. At present, therefore, we may simply endeavour to dis- cover how biology is to be studied, to what that study leads, and the nature and direction of the paths wherein the modern biologist pursues his research. If, according to Spencer, "preparation in biology" is the great necessity for a true knowledge of the laws which govern human society, so, for us, preparation in the methods of the science of life is a needful preliminary for an understanding of the influence which modern biology has exerted upon men's ideas concerning the order and origin of living nature. The study of the standpoints of biology may be fitly com- menced by a reference to the manner in which the investigations of the biologist into the history of animals and plants are carried on. It is the province of science to be exact j it is the first and highest duty of its professors to secure correctness in their methods of dis- covering facts. In science we are not at liberty to begin anywhere, as, in truth, our researches, if pursued completely, will terminate in a definite fashion. Organised method is, in short, the great essen- tial for scientific success in the pursuit and discovery of truth ; and it is in his adoption of such methods that the scientific investigator differs most notably from the student in many other departments of thought. We may note in passing that another and equally important characteristic of scientific investigation exists in the fact that, having no prejudices to defend or prepossessions to consult, the man of science stands in no dread of the results to which he may be led, and is placed at no disadvantage when he replaces c 1 8 CHAPTERS ON EVOLUTION. beliefs, however time-honoured they may be, by the newer phases of thought to which his studies have led. Four very definite questions may be said to contain in their replies, the materials for constructing the full history of any living being. The queries to which I allude are such as the child might well ask respecting any object presented for the first time to his view; and it is worthy of note that the methods of inquiry through which the cumulative experience of ordinary life is gained find in the ques- tionings of science a striking parallel. First, and most naturally, we inquire concerning the living being, " What is it ? " Next in order comes the question, " How does it live ? " Thirdly, the query, " Where is it found ? " appears as a most natural inquiry ; and the question, " How has it come to be what it is ? " may fitly close the list of scientific interrogations. It may be said that, could we perfectly and fully answer these four queries as applied to any living thing, the history of such a form might be regarded as being in every sense complete. Its present history, its past existence, its way of life, its bodily mechanism, its evolution and descent — these, and other points in which the life and being of an animal or plant is summed up, are included in the replies to our four queries. Answer these questions fully, I repeat, respecting an animal or plant, and you leave no item in its history unexplained. When they shall have been fully answered respecting the known organic world, then will dawn a millennium in biological and other sciences, of which, however, not the remotest shadow of a dream has yet crossed the scientific expectation. Full as our knowledge is on many points of structure and life history, biologists too frankly recognise the gaps in their information to hope for or expect the completion of their science even in the most distant years that from the present horizon we care to scan. Still, the labour of investigation proceeds apace — slowly, it may be, yet hopefully ; and every scientific advance which the present sees or the future may know, may assuredly be regarded as filling up, wholly or in part, one or more of the replies to the four questions wherein, as we have seen, the gist of biology is comprised. The principle of the division of labour which has wrought such wonderful changes and improvements in human affairs, political, social, and commercial, has extended its advantages to the domain of life-science, in that each query possesses its allotted branch as the agent for supplying its answer. Part of the excellence of bio- logical reasoning, and of scientific method at large, consists in the fact that the labour of investigation is divided amongst three well-marked branches of inquiry ; whilst the answers to the fourth and last ques- tion on our list are in reality supplied by the concentrated knowledge of the three preceding replies. Thus, to the question "What is it ?" THE STUDY OF ^BIOLOGY, ig the science we name "Morphology" gives us an answer. This depart- ment of biology concerns itself with structure alone. Under this head we gain a complete knowledge of the mechanism of the living being. A watchmaker, taking a watch or clock to pieces to ascertain the struc- ture of the timepiece, investigates its " morphology." An engineer, describing to a bystander the principles of the mechanism he has constructed, is similarly detailing its morphological composition. The structure and build of the living body — animal or plant, high or low organism, be it remembered — is investigated under this first head of inquiry. It is morphology which places before us the few facts of structure perceptible in the animalcule ; and it is this science, in its highest development, which investigates the complexities of the human organisation itself. But "morphology" can readily be shown to possess a subdivision into three important branches, each dealing with a special phase of living structure. There exists, firstly, the subdivision Anatomy, which deals with the structure of the fully developed (or adult) animal or plant. Next in order comes Development — a study all-important, as we shall hereafter see, in the eyes of modern biologists. Through development we obtain a knowledge of the manner in which the adult body, which " anatomy " investigates, came to assume its perfect and completed form. Development, in short, initiates us into Nature's manufactories, -and shows us her methods of evolving living organisms. Just as even a rapid run through a watch-manufactory, and a glance at this table and that, or a look at the various stages in the progress of the watch towards perfection, would afford an idea of the fashioning and forming of the watch, so development gives us an insight into the process and method employed and followed in the formation of the animal or plant The pin or pen we think so little of, came to be what it is through a highly complex process of manufac- ture. To thoroughly know what the pin or the pen is, we should naturally require a knowledge of how it was made. Just so in nature ; development teaches us how the animal and the plant is made — nay, more, it tells us also, by the way, a wondrous tale respecting the causes of the manufacture, and the circumstances which have led Nature to frame her living possessions according to one fashion or another, and to relate, it may be, apparently diverse articles of her handiwork in the closest bond of intimacy and union. Last of all, a third department of morphology, or the science of structure, exists in the shape of Taxonomy or Classification. It is the plainest of truisms, that we can only classify and arrange any set of objects truly and satisfactorily when we really know the objects, and when we possess a perfect acquaintance with their structure. Hence "classification" falls into a most natural place when, after the acquirement of knowledge concerning the structure and nature of C2 20 CHAPTERS ON EVOLUTION, living beings, we are able as a consequence to place together those which are truly alike, and to separate those which are unlike. By way of illustrating the application of morphology, and on the principle that example is better than precept, let us select as an example of scientific inquiry the history of a fish. Under the head of morphology, the biologist is bound to take account of every detail of structure which that animal exhibits. Through the aid of " anatomy " he will make its acquaintance as a fully formed being ; he will ascertain the full details of its structure ; note the form, number, position, and relation of its organs; and in general obtain a thorough knowledge of its composition and bodily mechanism. But anatomy does not inform him of the prior history of the fish ; hence he turns to development as a means of showing him the manner in which the fish-body grew and was fashioned. Beginning as a small speck of protoplasm, indistinguishable from the matter 1 which forms the whole body of the lower animalcule, he would trace for us the evolution of the complex body from materials of extreme simplicity. Hour by hour, and day by day, he would chronicle the changes in the division of the egg, the first appearance of the embryo, the beginnings of the heart-pulse, the formation of brain and nerve, and the outlining of body at large. And, finally, he would show how the completed being, evolved by strange artifice from literal nothingness, grows to its adult form and takes its place amongst the finished products of nature. Such are the details of development. Finally, asking himself concerning the place and rank of the fish in the scale of creation, the biologist would turn to " classification " to aid him in his search. Ascertaining the structure and develop- ment of other fishes, he would know accurately enough the proper sphere to which science calls, and in which science places, the form before him. He would find cause to utterly reject classifications and systems of arrangement not founded upon a true knowledge of structure. The whale, for instance, is classified as a fish by primi- tive man — and, I may add, also by persons amongst ourselves, whose culture professes to be by no means of a low grade. It is fish-like in form and appearance ; it inhabits the sea ; its conditions of life are evidently those of the fish. Why, then, asks popular opinion, is the whale not a fish, seeing that in any case the latter is " very like a whale " ? To this question the biologist can but reply, that if nature has modelled whale and fish on the same lines, he can have no quarrel with nature on that account. His, however, is the duty to assure himself that the fish and whale are really alike. Through anatomy he learns that, outwardly alike as the two animals are, things in this instance are really not what they seem. The fish, his study of morphology informs him, has cold blood, and a heart con- THE STUDY OF BIOLOGY. 21 sisting of but two cavities or chambers : the whale, he finds, has warm blood, and a heart constructed on the same type as that of the biologist himself, and consisting of four chambers. The fish is covered with scales : the whale's body-covering consists typically of hairs ; and whilst the fish out of water dies, as a rule, because its gills are then removed from the medium from which they derive the oxygen for breathing, the whale breathes by lungs, and, as every one knows, requires to ascend periodically to the surface of the water to inhale the air directly from the atmosphere, like ourselves. The whole internal economy of the fish, albeit that it exhibits the same general type as that of the whale, is of much less complex kind. And, not to penetrate more deeply into the distinctions which separate the whale race from the fish tribe, we may lay stress on one last fact of primary importance in distinguishing the two animals — namely, that whilst the fish was developed from an egg which was hatched externally to the parent body, the whale was born alive and was nourished in its early life by the milk-secretion of its parent Now, all of these characteristics infallibly demonstrate to the merest tyro in zoology that, so far from a whale being in any sense a fish, it is a true " quadruped" or mammal like ourselves. It finds refuge in the same class which includes the kangaroos and their neighbours as its lowest members or democracy, and apes and man as its aristocrats. The whale, in short, is a 'mammal with but two well-developed limbs, and occasionally rudiments of two other members ; the two front and developed limbs being converted into swimming-paddles or " nippers." It is a quadruped modified for an aquatic life, and resembles the fish only in the fact that its body is built up on one and the same general type, and in its outward modification as a tenant of the " vasty deep." Thus clearly do we observe that the true position of an animal or plant in the living series can only be determined by a reference to the facts of structure. Classification, in •other words, is the natural termination to the work begun by the .anatomist and the student of development. Turning to the second question asked by biological science regarding every living being — " How does it live?" — we find the science of Physiology credited with furnishing the reply to this latter •query. Physiology is the " science of functions," a term translatable into meaning that branch of inquiry which shows us how the living mechanism works, and how life is supported in virtue of defined actions which it is the duty of that mechanism to perform. The watchmaker or other artificer who, setting the mechanism he has •constructed in motion, professed to instruct us in the manner of its working, would be showing us the " physiology" of the machine — just as previously, when describing its structure, he taught us its •" morphology." We may go further still, and add, that, without a 22 CHAPTERS ON EVOLUTION. preliminary knowledge of structure, the intelligent appreciation of function, or working, is impossible of attainment. The exact manner in which a watch performs its duties can only be comprehended after an examination of its anatomy or the disposition of its parts. Hence, in living beings, " how life is carried on " is a question only to be answered from the knowledge and by the aid of the considerations- which the examination of their structure affords and supplies. Summing up the history of the living being in action which physiology writes for us, we may say that three great functions are performed by every animal and by every plant. The living being has first to nourish itself; to provide for the continual wear and tear to which, in the mere act of living and being, its frame is subjected. The first function of Nutrition thus provides for the support of the individual animal or plant. But death is continually thinning the ranks of animal and plant species. As local death, or the decay of the particles of the individual body, is a constant concomitant of individual life, no less true is it that general death is an invariable accompaniment of the life of the race or species. As nutrition — the act of taking and assimilating food — repairs individual loss, so the function of Reproduction repairs the loss and fills the gaps which death has made in the ranks of the race. New beings, through the exercise of this latter function, are brought into the world to take the place on the stage of life of the actors whose parts in the biological drama have already been played out. Lastly, in the exercise of its living powers, the animal or plant is found to possess certain means for acquiring relations of more or less definite kind with its surroundings. An amoeba — in its way a mere speck of protoplasm — is seen under the microscope to contract its jelly-like body when a food-particle touches its substance ; and, as the result of the contact, the protoplasmic speck engulfs the atom in question and duly assimilates it. But for this property of sensitiveness, the life of the animalcule would be equivalent to the existence of the mineral ; its power of nourishing its frame and of receiving food really depends on its sensitiveness to the outward impressions produced by the chance contact with its body of the external particles on which it feeds. Withdraw from the protoplasm this sensitiveness, and your animalcule would starve. Sensation and a power of acting, like human units of official nature, upon " infor- mation received " through sensation, is a universal attribute of life. Even the fixed plant may, as in the Venus's fly-trap (Dioncea)^ develop a more sensitive and elaborate apparatus for the capture of prey than many animals of tolerably high grade ; and in all plants there exists living protoplasm which, as its first characteristic, exhibits sensitiveness and a power of contraction. A snail, irritated by touching the tip of its tentacles, withdraws into the obscurity of THE STUDY OF BIOLOGY. 23 private life for a while, and indicates that it possesses not merely a nervous apparatus analogous to our own, but that such apparatus is used in an exactly similar fashion. A broad likeness exists between a snail's retirement into its shell when touched, and the human act of withdrawing the head from a threatened blow. And so we find that from the animalcule to man, from the lowest plant to the highest member of the vegetable kingdom, there exist means whereby the living being, through the property of sensitiveness or " irritability " (as we may term the general function of nervous tissue or its represen- tative), is brought into relation with its surroundings. This act of relating itself to the outer world in which it lives, constitutes the third function of life wherever found. The nerve-acts whereby man is enabled to think, feel, and move; the actions whereby a daisy closes its florets when the chill of evening falls upon the world ; the act of a Venus's fly-trap or a sundew in capturing the insects on which, like vegetable spiders, these plants feed ; and the humbler manifestations of sensation seen in the sluggish movement of an animalcule or in the cells of a seaweed — are bound together in one harmonious function, which we name that of Relation, Innervation, or Irritability. To nourish itself, to reproduce its kind, and to maintain relations with the world in which it lives — such is the whole physiological duty of man and animalcule alike ; and in the survey of these three functions is comprehended the answer to our second question, " How does the animal or plant live ? " The third inquiry of the biologist, as we have seen, relates to the place and position of the living being on the surface of the world — whether it be found on the earth itself or in the waters under the earth, whence by deep-sea research the knowledge of its habitat has been drawn. Every animal and every plant, besides a name and designation, possesses a " local habitation " on the earth's surface. The study of structure and the knowledge afforded by physiology take no account of the dwelling-places of animals and plants. " Where is it found?" is thus a question which must also be asked of the biologist ; and for the answer we depend upon a third branch of biology, to which the name of Distribution has been given. The purport of the inquiry, " Where is it found ? " requires no explanation. The most natural of queries concerning a living being is that which the child might ask concerning the native habitation of an animal or pknt Outward nature appeals too forcibly to us to render the question, " Where does it come from ? " an unnatural one when applied to the animal or plant ; the difference between our own land and habitation and those of other men being included in some such interrogation as that involved in the questions which the science of Distribution professes to answer. No more interesting queries can well be imagined within the whole range of natural-history study 24 CHAPTERS ON EVOLUTION. than those included within the sphere of this third division of biology. Why, for instance, are kangaroos and animals of like grade only found in Australia and adjacent islands ? Why are the opossums — near relations of the kangaroos — absent from the Australian home of their nearest kith and kin ? and why do they occur in America, when natural expectation would have placed them in Australia ? Why are antelopes well-nigh confined to Africa, which has no true deer, whilst the deers are otherwise world-wide in their distribution ? Why are humming-birds only found in the New World, over the length and breadth of which they are widely distributed ? Why are the monkeys of America absolutely different from those of the Old World ? and why are those found in Madagascar, in turn, so varied from their neighbours of Asia and Africa? Why are sloths and armadillos only found in South America ? Such are a very few of the queries which Distribution asks, and to which this science endea- vours to supply an answer. We thus perceive, clearly enough, that the situation and position of an animal or plant on the surface of the earth is no mere matter of chance, but is as much the result of law, and has been as clearly brought about by the circumstances which regulate existence as a whole, as its structure is the result of laws of development acting in definite fashion and ordered sequence. Distribution, it is true, is a biological science as yet in its infancy. It presents us, we may note, with two aspects, under one of which we settle the place and position of an animal in space, that is, in the world as it now exists — such is Geographical Distribution. Through the other aspect of this science, we determine, by the aid of the history of fossils, whether it had an existence in the past history of our earth, and if so, under what conditions it lived. This latter phase of the subject is named Geological Distribution, or distribution in time. The importance of distribution as a branch of biology grows and increases daily, as we perceive that the answers to many puzzles and problems of life are bound up in the replies we are able to furnish to the question, "Where is the animal (or plant) found ?" At this stage of biological investigation many naturalists might be tempted to call a halt Having ascertained, as fully as may be, the structure, physiology, and distribution of an animal or plant, the investigation of the living form might be regarded as complete. Contrariwise, however, the tendency of the biology of past years has been to lay increasing stress on a fourth inquiry concerning every living thing — namely, " How has it come to be what it is?" Such a question is tantamount to the inquiry, " How and why was the living being created so ? " — an interrogation which, even a few years back, would have sounded as an attempt to probe the mystery of divine intent, and which, as such, would have been relegated to the domain THE STUDY OF BIOLOGY. 25 of the unscientific, if not to that of the impious as well. But con- siderations of theoretical impiety have no effect in face of the need for knowledge. If the speculation how any planet was framed, and if the formation of a nebular hypothesis, or the promulgation of a theory of elliptical orbits, was a warrantable procedure — nay, even a necessity — of astronomical knowledge, one may well be excused for failing to discover the unwarrantableness of speculation concerning the origin of animals and plants. Especially, too, if the way of crea- tion, as biological science believes, has not been through successive acts of supernatural interference with the matter of life and the manner of living, but through the modification — slow, gradual, natural, and pro- longed— of pre-existing species, the justification for the query, "How has this animal or that plant assumed its form and place in the world?" lies on the face of nature itself. If, as is apparent to all biologists at least, the way of creation is traceable in the forms and develop- ments of living beings, we are bound to investigate that history, as a part of the duty laid upon scientific truth- seeking and upon biological investigation. The impiety so much talked of in past years, but of which one happily hears but little now, if it exists at all, is illustrated solely in the absolute scepticism of those who refuse to admit and believe in the right of man to read and construe, as reason dictates, the records written in the fair face of creation itself. Persons who deem it impious in the scientist to assert that he can trace the evo- lution of this animal or that plant, present the best possible frame of tnind for the development of the very scepticism the existence of which they are the first to deplore. The wilful folding of the hands in deprecation of scientific investigation, and the shutting of the eyes in a so-called " orthodox " and slumbering ignorance of the facts of nature, is the procedure of all others best calculated to sap the foundations of religion itself. It is such ideas which Dr. Martineau, with his accustomed ability, has ably denounced when he says, " What, indeed, have we found by moving out along all radii into the Infinite ? — that the whole is woven together in one sublime tissue of intellectual relations, geometric and physical — the realised original, of which all our science is but the partial copy. That science is the crowning product and supreme expression of human reason. . . Unless, there- fore, it takes more mental faculty to construe a universe than to cause it, to read the Book of Nature than to write it, we must more than ever look upon its sublime face as the living appeal of thought to thought." These are words worth reflecting upon ; and they certainly admit from the side of liberal theology the full, free, and unrestrained right of science to investigate fully and hopefully whatever facts or aspects of Nature lie. to her hand. They present, if need exists for such apology, the fullest justification of the scientific investigator's work, 26 CHAPTERS ON EVOLUTION. when he endeavours to trace through the mazes and byways of evolu- tion the manner in which the living world and all that is therein com- prised has been formed, moulded, and perfected as we now find it If, therefore, as we shall hereafter see, there are means and ways, clues and traces, to be found in nature for the study of the method through which living beings have come to assume their existing order, it were but folly to deny our right to utilise1 such means to the full, and to extend that knowledge, the increase of which Bacon wisely declared tended to the relief of man's estate. " ^Etiology" or the " science of causes," thus supplies us with the reply to the last of the four queries which concern the nature of animals and plants. In itself, this branch of inquiry connects the other three departments. It utilises the knowledge which structure, physiology, and distribution collect and systematise. It supplies the natural termination to all inquiries respecting the history of living beings. Since we believe that the causes which have wrought out the existing order of nature have left traces of their operation in the living universe ; which traces, like the silver thread running through the many-coloured pattern, serve to link together the interests and to show the harmonies which underlie the varied warp and woof of life. To fix these methods of biological study the more firmly upon our minds, we may select, as the subject of a brief exposition, the natural history of a kangaroo — an animal form sufficiently distinct and spe- cialised to render the details of its biological study a matter of easy com- prehension. No animal form is more familiar as a being foreign to our own country than the kangaroo ; and its history, like that of every other living being, familiar or otherwise, must be investigated along the lines we have just laid down. The question " What is it?" is answered by morphology ; and a large number of very interesting replies would be found amongst the answers to the questions of the science of structure. We should thus be informed, as a primary fact of kangaroo history, that it is a Vertebrate, or " backboned" animal ; that it agrees in the general type of its body with all fishes, reptiles, birds, and quadrupeds ; and we should, moreover, speedily discover by even a cursory ana- tomical examination that it belongs to the quadruped class, and presents essentially the same general characteristics which all mammals or quadrupeds, from the whale upwards to the lion, dog, rat, sheep, ape, and man, agree in possessing. But the more personal history of our kangaroo would show wide differences in structure from the organisa- tion of ordinary quadrupeds. We should be struck by the low type of its brain, as compared with the brain of ordinary quadrupeds. We should note two curious bones unknown in common animals, and which arise from the front of the kangaroo's haunch-bones. These are the so-called " marsupial bones," on which the " pouch " these animals possess is supported. In connection with this fact of kangaroo- THE STUDY OF BIOLOGY. 27 structure, we should also discover that the young kangaroo is born in an immature condition, that it is thereafter transferred to the pouch of its mother, and that it exists therein for many days after birth, being duly nourished by the secretion of the milk-glands which open into the pouch. We might also note that the kangaroos, as every visitor to the Zoological Gardens knows, possess hind limbs which are de- veloped out of all proportion to the fore-legs. In its resting posture, it sits upon a kind of tripod, or three-legged stool, formed by the tail and two hind limbs; and when the skeleton of the hind limb is examined, we find, further, that the great apparent length of the foot is in reality due to the elongation of the animal's instep bones. The foot, we may lastly note, possesses four toes, whereof one (the fourth toe) is very large and conspicuous. The fifth toe is smaller than the fourth ; and the remaining two (placed to the inner side of the other toes) are very small, and united together by a fold of skin. There is no first or great toe in the kangaroo ; and the two large toes form- ing the bulk of the animal's foot are the fourth and fifth toes : the two small and rudimentary toes corresponding to the second and third toes in ourselves. Thus much a brief study of " anatomy " would teach us about the kangaroo. Of its development, nothing need be said beyond noting the fact that it is formed and fashioned after the manner, firstly, of all Vertebrates in general, and, secondly, of all other quadrupeds in particular. Kangaroo-development stops short, so to speak, at a lower level than the development of such an animal as a dog, and at a considerably lower level than that of an ape or a man. But, if any proof of the exact nature of the kangaroo were wanting, such facts as those elucidated by its development would at once and indisputably settle its relationship to ourselves, as a low member of our own great class. Next as to its "classification." What, it maybe asked, is the kan- garoo's place in nature ? As the claims of structure settled the place and position of whale and fish in the animal series, so the morphology of the kangaroo allocates to it a situation in the quadruped class. The structure of many other animals is found to present a striking likeness to that of the kangaroo. The opossums, the wombats, the native "bears" and "hyenas" of Australian colonists, the kangaroo rats, the phalangers, the bandicoots, and allied forms — all, with the exception of the opossums, confined to the Australian province — exhibit evident affinities to kangaroo structure. Relying upon structure — and development would be found to strengthen the evidence of morphology — we should place these animals along with the kangaroo in a special order of quadrupeds to which we give the name of Marsupials, or " pouched " animals. These animals would agree with the kangaroo not merely in lowness of brain 28 CHAPTERS ON EVOLUTION. structure, in the possession of the curious " marsupial bones," in the general arrangement and even special form of internal organs, and in the peculiar shape of the lower jaw, but also in the matter of the foot structure. Very striking is it to observe the prevalence of the one type in the feet of this varied assortment of quadrupeds. " How curious it is," says Mr. Darwin, " that the hind feet of the kangaroo, which are so well fitted for bounding over the open plains — those of the climbing, leaf-eating koala, equally well fitted for grasping the branches of trees — those of the ground-dwelling, insect- or root-eating bandicoots — and those of some other Australian mar- supials— should all be constructed on the same extraordinary type, namely, with the bones of the second and third digits extremely slender and enveloped within the same skin, so that they appear like a single toe furnished with two claws ! Notwithstanding this simi- larity of pattern, it is obvious that the hind feet of these several animals are used for as widely different purposes as it is possible to conceive. The case is rendered all the more striking by the Ameri- can opossums, which follow nearly the same habits of life, having feet constructed on the ordinary plan." The science of structure thus settles the questions which natur- ally arise respecting the relationships of the kangaroo, by uniting it, in classification, with those forms which truly resemble it in structure. So also with its physiology. The second question, " How does it live? " would be answered in an exact fashion by the investigation of the life-processes of the animal, and by the knowledge which physiology would bring to bear upon the manner in which kangaroo-existence is divided, like that of all other animals, between supporting its frame, increasing its race, and maintaining relations with the world around. The question, " Where is it found ? " involves in its reply, in the case of the kangaroo, a large number of highly interesting and instruc- tive considerations. Kangaroos are found in Australia and adjacent islands alone. Why are they limited to this region of the earth's surface ? and why, to put this question more generally, has Australia no native quadrupeds other than these marsupials and their near rela- tions ? — for it need hardly be added that the horse, cow, sheep, and allied animals are all of recent introduction by the hand of enter- prising, colonising man. Looking at a zoological map of the world — a chart prepared solely with reference to the distribution of animal life — we should observe that the animals peculiar to Australia stop short on one side of a line called " Wallace's Line," which passes in one part of its course between the little islands of Bali and Lombok in the Eastern Archipelago. The Straits of Lombok are about fifteen miles in width, yet that narrow sea divides the land of marsupials — Australia and adjacent islands — from other lands and islands in which no marsupials are found. THE STUDY OF BIOLOGY. 29 Why, then, should the kangaroos and their marsupial kith and kin stop short at " Wallace's Line " ? The answer to this query involves considerations which extend over the whole domain of life-science. The briefest possible explanation of the kangaroos' distribution must therefore suffice for our present purpose. Let us go back in imagina- tion to that far-back time in the history of our earth when the Triassic rocks were being formed. That period existed ages before the Chalk in point of time. It was the period, moreover, when the first quadrupeds appeared on the earth's surface. These primitive animals were wholly of marsupial kind, and entirely of the type of which our kangaroos and other Australian mammals are the existing representatives. Not a single higher mammaHhus graced the Triassic forests ; no elephants roamed in Triassic jungles ; the plains of these early times were unenlivened by the agile deer, or by the grace of the antelope herds ; no carnivora roamed about to slay and devour the weaker races ; and the humblest quadrupeds were lords of animal creation, and represented in themselves the fulness of the mammalian life which the later ages were destined to see. Over the whole land surfaces then in existence these low marsupial quadrupeds of the Trias in due course spread. In Britain, on the Continent, in the New World, the fossil remains of these early Triassic quadrupeds are found ; the best known of them being represented most nearly by the little "banded ant-eater" (Myrmecobitis) living in Australia to-day. In the Triassic period, also, Australia obtained its marsupials. For that island-continent was then part of the Asiatic or Palsearctic mainland, and the con- necting land was not then broken up into the islands of the Eastern Archipelago of to-day. The next phase in the drama of Australian quadruped-life shows us that, at the close of the Triassic and of the succeeding Oolitic periods, that land became disjointed from the mainland. Geological change made Australia the island-continent we see it to-day. And what of its quadrupeds ? These early marsupials, left to themselves, shut off from all possible invasion by and competition with higher and later quadrupeds, flourished and grew apace in the Australian land. Elsewhere, and in the rest of the world, the early marsupials were distanced in the "struggle for existence" which ensued on the evolution of higher types of life. Elsewhere than in Australia, they were killed off ; and at the close of the Oolite age (or that immediately succeeding the Trias) hardly a remnant of the great marsupial life of these two periods was left to bear witness to the first beginnings of mammals on the earth. In Australia how different was, and still is, the quadruped-life ! In the "recent" bone-caves of Australia we meet with the remains of giant marsupials, compared with which the largest kangaroo of to-day appears a pigmy form. These are the 30 CHAPTERS ON EVOLUTION. lineal descendants of the first mammalian population which Australia obtained from the Triassic period. Thus left unopposed, until the advent of the colonists, the marsupials have lived and flourished in Australia, which still retains the main features of its Triassic and Oolitic life. For in its seas swims the Port Jackson shark, elsewhere known only by fossil representatives from the Oolitic rocks. In its rivers lives the curious fish Ceratodus, whose teeth occur fossil in Triassic and Oolitic formations. The cycads and araucarias, repre- senting a typical and universal plant vegetation of the Oolitic times, still flourish in Australian soil, though elsewhere scanty or non- existent ; and even the shell-fish on the shores of Australia belong to types which flourished in our own Oolitic seas, but which have since practically died out over the world, save the Australian shores. Thus, Australian life of to-day is merely the survival of the general life which prevailed over the world in the Trias and Oolitic periods. The history of the kangaroo points out clearly enough that only on the theory of evolution having given rise to new species from the ancient and original Triassic stock, can we account for the persistence in a corner of our existing world, of the otherwise lost and extinct population of the first quadrupeds. Lastly, the opossums — which, as a family of marsupials, we should have expected to find in Australia — are discovered, as already remarked, in America. " How came they, then, to inhabit the New World?" is a question worth answering, along with that which inquires into the distribution of the kangaroo. The opossums, firstly, represent a family which never entered Australia, They were plentifully existent in Europe and elsewhere in the Oolitic period; and even nearer our own day — namely, in the Eocene and Miocene formations — the opossums lived in the Old World. These facts are accurately told us by the history of their fossil remains. Thence their range extended to the New World; and, when a subsequent irruption of higher quadrupeds killed off the opossum-race elsewhere, these animals continued to flourish and grow in the New World, presumably because the struggle for exist- ence was and is less severe in the latter region. As the kangaroos are survivals of a quadruped-life, world-wide in Triassic and Oolitic times, so the opossums are survivals in their turn of later marsupials than the Australian animals. Finding in the New World, to which they migrated, a suitable home, the opossums, distanced in the competition in the Old World and now extinct therein, have flourished apace across the sea, and have extended their bounds even into the northern part of the American continent The deep water of the narrow "Wallace's Line" between Bali and Lombok, therefore, indicates a channel of great antiquity, which severed Australia from the nearest land, and which, presenting an , THE STUDY OF BIOLOGY. 31 impassable gulf to migrating forms, has kept the original quadruped- life of that island-continent free, separate, and unmingled with the higher types of life evolved since Triassic and Oolitic times. Thus do we answer the question, "Where is the kangaroo found ? " The remaining question, " How has it come to be what it is ? " or, in other words, " How has it assumed its present place in the organic series ? " has been answered in greater part by the preceding observations. If the first quadruped population of Australia was, as we know it to have been, of marsupial nature, our existing kangaroo must be the descendant of pre-existing species. Laws of descent, affected by variation, have unquestionably produced and evolved the existing kangaroo from ancestors more or less resembling itself. This much is clear, at least — that although the exact lines of descent and variation of the marsupial families of to-day are as yet un- determined, the great principle of descent through variation from pre-existing species, remains, not a theory merely, but an inferred and unmistakable fact from the circumstances of the case. As the various opossums now inhabiting America are the descendants of the one or more primitive species which first colonised the New World, so the varied marsupial life of Australia is the legitimate outcome, through variation, of the primitive quadrupeds which first peopled that strange land in the old Triassic days. As Professor . Flower has remarked, ' even the likeness between the feet of marsupials is too close to admit of any doubt of their derived relationship, " of inheritance from a common ancestor." And the causes which have produced the striking likeness of this one feature in marsupial history are simply those which have also evolved, from a common origin, the varied features and new offshoots which mark the marsupial life of to-day. The somewhat extended survey thus taken of the means and methods of biological study obviates any necessity for extending more fully our researches into the remaining characteristics of modern biology. What remains to be said on this latter head may, however, be shortly summed up in the light of previous remarks. Natural history science, as prosecuted of old, was a mere collection of descriptions of species. It was a science in which the search after new species, merely for the sake of adding to the number of known forms, was the paramount aim of the zoologist and botanist. Classifications grew apace ; but the relations of one species to another, of group to group, or the general plan upon which the animal world was constructed and organised, were either undreamt of as subjects of study, or were cursorily dismissed from scientific view. We have but to open a volume of natural history lore of the past decade of zoology to realise the truth of this statement. We may readily perceive that attention to outside characters and to the 32 CHAPTERS ON EVOLUTION. construction of artificial systems of classification represented the chief labours of the biologists of past years. But impelled by the researches of Cuvier, who laid the foundations of morphology, and who clearly mapped out the animal world into four great types — three of which to this day remain much as his genius left them — biology awoke to a new lease of life. Placed in possession of some definite aim in the investigation of animal structure, zoologists began the systematic examination of the great divisions of the animal world which Cuvier had mapped out. Next in order came the era marked by the speculations of Lamarck, in turn succeeded that characterised by the imperishable deductions and suggestions of Darwin. Then was supplied the guiding clue, for want of which zoology and botany had been left to progress in slow and desultory fashion. The impetus given by Darwinism and evolution to biology may be fully appreciated when we reflect that in evolution we perceive the suggestion of a rational purpose in the researches we undertake into the structure, physiology, and distribution of living beings. When we discover that life everywhere exhibits progress, that the development of animals and plants has been a work of progress in the past, that modification proceeds apace even now, and that it is possible to discover the clear plan and method of creation in the forms and development of living things, we may readily appreciate the incentives to research in all directions which the idea of evolution, as the method of nature, has given to the biology of to-day. Understanding something of the theory of the living universe, the biologist can set himself to work hopefully to unravel many of the so-called mysteries of life. Asking himself regarding every living thing the question, " How has it come to occupy this or that place in nature ? " he firstly studies its development as a clue to its descent and origin. The modern biologist looks to development, above all else, to teach him the true nature and relationships of animals and plants. If a sea-squirt's development runs in parallel lines to that of the lowest fish, then he naturally concludes that like results in this case follow from similarity of origin, and fishes and sea- squirts become organically connected through community of descent. If a Sacculina (existing as a mere parasitic bag of eggs on a hermit crab) passes through essentially the same stages in its development as a shrimp, a water-flea, a barnacle, a crab, and all other crus- taceans, he feels bound to believe that these varied forms have sprung from one and the same root-stock. If he finds that a frog in its early life is essentially a fish in structure and physiology, he assumes that he is being taught the descent of the frog-race from aquatic and fish-like ancestors ; otherwise, why, he may reason- ably ask, should nature trouble herself to develop a fish-stage in the formation and growth of the land-inhabiting frog ? If he finds THE STUDY OF BIOLOGY. 33 that man's development proceeds along the same lines as those of all other vertebrate animals ; if he knows that man, like the fish, has gill-clefts in his neck in early life, which clefts are of no use whatever to their possessor ; if he finds that other structures, found permanently in lower animals, have a temporary existence in human development — is he not morally bound to believe that, human development being a moving panorama of lower forms of life, man himself has had his beginning in some pre-existing and lower form ? If he finds that it is impossible in early life to distinguish the human embryo from that of other quadrupeds, is he not logically bound to regard such likeness as a proof of man's lowly origin ? Such are the queries which the biologist of to-day is forced to face. And when the facts of develop- ment are fairly stated, the answer is not for a moment doubtful, if only from the overwhelming conviction that Nature has written her method and way of creation in our evolution, and that it is, or should be, our highest pride and glory to read aright that " strange eventful history." No less powerfully are the deductions and studies of the modern biologist aided by such considerations as those which deal with variation in species as a great fact of life. Formerly, when the fixity of species was deemed a grand fact of biology, the idea that variation might exist was unwillingly entertained, if allowed to have any weight at all. Now, with exact' knowledge that variation exists to a greater or less extent in every living species — that change is the law, and fixity in species the exception — we can clearly discern Nature's purport in inaugurating such change, as the preliminary to the formation of new races and species. We know that variation pro- ceeds apace in the existing world of life. We ourselves evolve at large, new " races " of cattle and sheep, of pigeons and dogs and horses ; and even if it be fully and freely admitted that the causes of variation are still obscure, there will be found no competent biologist to deny either the reality of the changes in species now proceeding in the world, or the results such changes have wrought in the past. Subsidiary methods and aids in studying the biology of to-day exist in such subjects as rudimentary organs, homologies, missing links, and the like. If we discover that a whalebone whale, which has no teeth in the adult state, develops, before birth, teeth which never cut the gum and are gradually absorbed, we must either assume that Nature is woefully improvident in developing useless structures, or that these useless teeth have a meaning. If we find that, whilst a horse walks upon the single toe of each foot, it possesses other two rudimentary and useless toes in its "splint bones," the same idea of meaning or no-meaning comes vividly before our minds. Rudimentary organs teach us, like development, valuable lessons concerning the past history of the race which D 34 CHAPTERS ON EVOLUTION. possesses them. The useless teeth of whales represent organs once well developed in the ancestors of our existing toothless cetaceans ; and when we find in our horse rudiments of two toes, we expect that that single-toed animal is descended from a three-toed race. Is such an idea probable ? may be asked. If we visit Yale College, in America, and observe the array of fossil horses there displayed, we shall be able to trace the evolution of the horse in time, from not only three-toed, but four-toed and five-toed ancestors. There, placed in a graduated series, is the proof that evolution is a stable fact. No " missing links " require to be supplied in the series of Yale College : and those who can maintain, in the face of such an array of testimony, that evolution is an impossibility and development a myth, may be regarded as possessing a hardness of heart against honest conviction, compared with which the Egyptian obstinacy against which Moses declaimed and Aaron battled, is mildness indeed. Homology, or the " science of likenesses," again, teaches us that when organs are built upon the same type, like the feet of marsupials or the limbs of all vertebrates, from the arm of man to the wing of the bird and the breast-fin of the fish, they must have had a common origin. The true nature of organs and parts in animals and plants is only discoverable after a careful study and comparison of their structure and affinities as declared by homology. Such are a few of the aids to biological study which the modern naturalist has at his command. Under the light and countenance of evolution, every new fact fits sooner or later into an appropriate niche in the biological fabric. No one fact remains isolated and distinct, as in days of old, but all our knowledge of the past and present of living beings tends to supply us with a rational under- standing of their origin and progress towards their existing structure and position in nature. Evolution thus takes its stand on the rational interpretation of the facts of nature. Its reasonable aspect presents its strongest claim to support : its rational explanation of former mysteries com- mends it to the unbiassed truth-seeker as the key to the former mysteries and inexplicable problems of the past. Founding its data upon observed facts, the evolution theory holds that the living species of this world are in a state of constant change and variation. It maintains that animals and plants are produced in greater numbers than can obtain the necessaries of life. It postulates, what observa- tion confirms — the operation of a " struggle for existence," in which the weakest forms (which are those that do not vary) go to the wall, whilst the strong (those that do vary) survive. It holds that Nature thus appears to set a' premium on variation, that she encourages change in species, and that firstly new varieties, then new races, and Jastly new species, are thus produced by the modification of the old. THE STUDY OF BIOLOGY. 35 The theory thus presented, calls to its aid all the facts of biological science. It shows by development, that the way of nature is that of progressing from the general to the special. It notes that extinct forms of life can frequently be shown to be intermediate between living forms, and that "missing links" are capable of being supplied as knowledge grows and as research advances. It correlates out- ward or physical changes in land and sea with the change in species, and shows how varying conditions of life modify the living form. It enlists, as we have seen, the facts of geographical distribution in its favour, and proves, by an appeal to geology as well, that the modifi- cation of life through the changes of land and sea accounts for the otherwise puzzling phenomena viewed in the distribution of living beings over the world's surface. Laying hold of every detail of natural science, this theory of nature has thus wrought a mighty revolution in biology ; whilst geology and other sciences have moulded their conceptions on the consistent theory of the universe which evolution lays down. It is the pride and boast of evolution that the avenues to which knowledge leads through this theory of the universe are illimitable — that knowledge may truly " grow from more to more " under its benign influence. And, best of all, whilst science is thus made the handmaid of truth, we also find that the spirit of reverence in face of the facts of nature is also inculcated by the study of development. There is no room for the idea of arbitrary interference with the laws of nature when evolution has fairly asserted its right to be heard. As in the inorganic world around us law reigns supreme, as planets revolve in their cycles with unchanging regularity, so in the world of life there is demon- strated to us the existence of law and ordered sequence which prevails in lowest as in highest spheres of being, which directs the destinies and development of man equally with the movements of the animalcule, and which as fully explains the evolution of a leaf, as it does the formation of a world. 36 CHAPTERS ON EVOLUTION. III. THE CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. THE intelligent foreigner, visiting a country which to him is practi- cally a terra incognita, and desirous of acquainting himself as fully as possible with the constitution of the land wherein he intends to sojourn, would contrive, before departing from his native coasts, to gain some adequate idea of the new country itself, its government and laws, its social, political, and religious condition, its geographical and geological features, and its general history in so far as these details were necessary for the comprehension of what he expected to see and hear during his foreign tour. If to the details of its present condition he was able to add information concerning its past — if he could trace its history along the lines of centuries, and discover how this event or that occurrence had tended to mould the country and its constitution into its existing form, his appreciation of the strange land, as presented to his view to-day, would tend to become of still more complete nature. And if, lastly, from his study of the past and present of the foreign territory, he ventured to indulge in any reflections on its possible or probable future, and on its chances of further development or possible decline, such reflections would possess every claim to rank as rational thoughts, deducible from his knowledge of the land as it was and is. The parallelism between the process of acquiring an adequate knowledge of a foreign state, and that of gaining some idea of the constitution of the worlds of living beings, can readily be shown to be of the closest possible description. The most superficial acquaintance with the study of zoology and botany, if carried out in any fashion worthy the name of a scientific and intellectual exercise, must proceed along lines which follow out in all essential details the pathways whereby we gain an intelligent idea of a foreign land. No study of animals or of plants can be satisfactorily carried out without, at least, a brief preliminary discussion of the constitution of the worlds of life, and without some acquaintance with their mutual relationships and their fundamental characters. In the light of recent researches concerning the " why and where- fore " of the animal and plant kingdoms, such preliminary knowledge becomes not merely of high importance, but of absolutely essential CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 37 nature. In the days before " evolution " was anything but a name, and ere " Natural Selection " had become a striking reality to the biological mind, such knowledge formed the basis of every study of zoology and botany worthy the name of a scientific investigation. To-day, when the burning questions of biology centre around the evolution of the living universe, and include in their sway and limits the details of the development and structure alike of man and monad, it need hardly be urged that some acquaintance with the general constitution of the animal and plant worlds is absolutely necessary for the intelligent comprehension of all that is interesting in the study of life. If the " making of England," to quote the ex- pressive phraseology of a historical authority, be regarded as at once the summation and foundation of all knowledge of the genesis of the English race, so the fundamental nature of animals and plants and a knowledge of their existing relations may be legitimately viewed as the only sound preparation for a knowledge of the great questions that deal with the becoming and making of living things. The most cursory survey of the worlds of animal and plant life leaves, as the prevailing impression on the observer's mind, the idea of extraordinary variety and diversity of form, colour, and habitat. From the grand Sequoia (or Wellingtonia) of California to the hum- ble moss that covers a rock, the grey lichens of the walls, or the minute Alga that colour the pools, there is an endless variety in the ranks of the plant kingdom.- No less distinctly is the diversity seen in the hordes of animal life. From the giant quadrupeds that find a home in the tropical jungles, through the teeming life of the waters, to the insect life that everywhere surrounds us, and to the animal- cular swarms that find a world in the water-drop, there is to be viewed endless and well-nigh undeterminable variation in every feature of existence. Indeed, so wide is the range of the naturalist's sphere of observation, that one might be readily tempted to believe that, save for the one common belonging and possession of life, there seems no bond of union which may link together the hosts that people the earth. The variety in question tends somewhat to puzzle the uninitiated observer when he attempts to form some adequate ideas regarding the relations of animals to each other, or concerning the bonds that connect the apparently diverse forms of plant life. It is this variety also, which in some degree tends to discourage the popular study of natural history — the apparent hopelessness of overtaking in a human lifetime even a small portion of the inex- haustible fields of research, having its own share in the work of discouragement, and in demonstrating the theoretical vanity of human knowledge. But the student of living nature is destined to find a speedy and satisfactory solution of many of these preliminary difficulties at the very outset of his studies. The first tendency of 38 CHAPTERS ON EVOLUTION. scientific investigation is to correlate the objects of its research ; or in other words, to effect a classification and arrangement of subjects destined for investigation. When the child groups the objects by which he is surrounded into animals, vegetables, and minerals, he is unconsciously laying the foundations of a scientific system ; and, in reality, the naturalist simply enlarges the conceptions of the child when he shows that differences, as fundamental in their nature as those the child learns to note, can be determined between the varied tribes of animals and the equally diverse groups of plants. Prior to the time of Cuvier, naturalists concerned themselves chiefly with the description of the different species of animals and plants, and with the determination of the characters whereby one species was distinguished from another. The writings of Linnaeus, for example, are largely composed of such descriptions, and if we add to such details, others dealing with the habits and distribution of animals and plants, we shall have completed the enumeration of the chief aims of naturalists in bygone days. The popular zoology and botany of to-day, which do not concern themselves with matters beyond form and the recognition of species or the description of habits, reflect, in a very characteristic and exact fashion, the natural history studies of the past It should be remembered, however, that the classic naturalists, amongst whom Aristotle stands out con- spicuously, dived somewhat more deeply into the history of the animal kingdom than their modern successors. But it may be fairly assumed that the ordinary naturalist, prior to Cuvier's time, con- cerned himself not so much with the structure or morphology of living beings as with the description of their external forms, peculiarities, habits, and habitations. With Cuvier, a new and higher era of natural history study dawned. Linnaeus had mapped out the animal world into (i) Mammalia, (2) Aves, or birds, (3) Amphibia (reptiles, frogs, &c.), (4) Pisces, or fishes, (5) Insecta (insects, spiders, &c.), and (6) Vermes, or worms — this latter group being, like that of the Linnaean " Insects," a most heterogeneous division, and including all known and lower forms of animal life, from the " worms " themselves downwards as far literally as the senses could reach. It has well been remarked that such a classification as the foregoing possesses a representative in the vocabulary of well-nigh every language. In this view it might be maintained that a popular conception of a unity of animals underlying their obvious diversity was early formed in the human mind. This is undoubtedly true, since the division of the animal world into beasts, birds, fishes, insects, and worms is a step in the construction of animal "types," to one or other of which any animal may be referred. But the system in question exhibits, after all, but little advance on the classification of FIG. i. — CROSS-SECTION OF VERTEBRATE (A) AND INVERTEBRATE (B). CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 39 childhood ; and it serves, moreover, to indicate very cursorily indeed the scientific and further delineation of the animal constitution. Lamarck, whose name is associated with views concerning the transformation and evolution of species, contributed a very decided addition to the knowledge of the constitution of the animal world, when, about the close of last century, he showed that the beasts, birds, reptiles, and fishes, instead of being regarded as distinct and unconnected divisions of animals, might be grouped together to form a large and characteristic division of the kingdom. He pointed out that each and all of these animals, as he knew them, possessed, firstly, a spine or backbone. Within this spine (Fig. i, A px ), whereof the skull formed merely a front expansion, the nervous system («2) was con- tained as within a tube ; whilst below that system, and contained within the body (/2) itself (as bounded, say, by the ribs), were the heart (//), digestive system (a), and other organs. Lamarck, com- menting upon this arrangement of parts — which a glance at the carcase of a sheep, as vertically bisected in the butcher's shop, will illustrate — demonstrated that no other animals, save mammals, birds, reptiles, and fishes, possessed such a disposition of their organs. The worm or the insect, for instance, possesses a body (Fig. i, B) we may legitimately compare with the lower tube (A) of the fish or beast, since neither the worm nor insect has a spine containing a nervous system. Hence Lamarck, taking his chief character from the spine or backbone, composed of separate bones or vertebra, named the beasts, birds, reptiles, and fishes the Vertebrata, whilst all other animals became accordingly known as Invertebrata. That Lamarck's discovery and his subsequent arrangement of the animal world into these two leading divisions marked a distinct era in zoology no one can doubt. Best of all, his deduction laid the foundation of the method which a little later — that is, about 1795 — Cuvier so successfully enunciated and followed out to a practical result. Other hands, in addition, laboured at the scientific edifice, which was practically completed when Cuvier laid before the world his elementary scheme of the history of animals, and showed that at least three common types or plans could be instituted amongst the inver- tebrate animals. Placed in tabular order, then, the main outlines of the animal world, according to Cuvier, might be thus rendered : — I. VERTEBRATA ( " backboned " animals) (fishes, frogs, reptiles, birds, and mammals). 40 CHAPTERS ON EVOLUTION. II. MOLLUSCA (" soft-bodied " animals) (cuttle-fishes and " shell- fish " at large). III. ARTICULATA (" jointed " animals) (insects, crustaceans, worms, &c.). IV. RADIATA (" rayed " animals) (star-fishes, corals, jelly-fishes, zoophytes, and all lower animals). Cuvier's own words, expressive of the nature of these types, may be quoted : " It will be found that there exist four principal forms, four general plans, if it may thus be expressed, on which all animals appear to have been modelled ; and the ulterior divisions of which, under whatever title naturalists may have designated them, are merely slight modifications, founded on the development or addition of certain parts." It may be added that the distinguished embry- ologist Von Baer, attacking the problems of animal form from the standpoint of development, and watching the phases observable in the early history of animals as they advanced from the egg towards their perfect forms, came to the same conclusion as the great French anatomist According to Von Baer, also, there were four types or plans in the animal world, the distinctive nature of the type to which any given animal belonged being indicated at an early stage in its development So that, as early as the beginning of the present century, it became clear to the minds of naturalists that, instead of each animal being built up on a type peculiar to itself, it fell into one or other of four groups ; in a word, it was found to possess a broad and fundamental plan or type of structure, with which a greater or less number of other animals agreed. To render the " type " constitution of the animal world plainer and more readily appreciated, we may select one or two examples by way of illustrating, also, how, with the increase of knowledge since Cuvier's days, the original types have remained stable in some respects, whilst they have undergone modifications in others. No two animals can well appear more varied in form, nature, appearance, and habits — and inferentially in structure likewise — than a lobster and a butterfly. The aerial habits of the one contrast very markedly with the slow aquatic life of the other, whilst the general constitution of the former appears to be separated by antipodean differences from that of the other. Are there any bonds of common nature which can link together beings so diverse ? and can the butterfly and the lobster be shown to possess any relationships in common ? are questions which it is reserved for the scientific but plainly understood deductions of zoology to answer. The superficial examination of the lobster would show that its body consists essentially of a series of some twenty joints, each possessing a pair of appendages modelled, despite their apparent differences, on one and the same CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 41 plan. So that, although the amalgamated joints of the animal's head and chest (Fig. 2, ca) are seemingly different from those of its tail (Fig. 2, 1-6), the zoologist could readily show the uniformity of the series by a comparatively simple dissection, wherein, aided by the knowledge of the animal's development, legs, "nippers," jaws, feelers, and even eye-stalks would be referred to modifications of one and the same type. Furthermore, our dissection of the lobster would show that, whilst its heart (Fig. 3, n) lies on its back, its digestive system (s,f) runs through the middle line of its frame ; and its nervous system (e, £•), in the characteristic form of an essentially double chain of nerve- knots, lies on the floor of its body. So that we might diagrammatise with strict accuracy the essential build of a lobster's body by con- structing a jointed figure (Fig. 4), wherein the heart (a) lay highest, the nervous system (c) lowest, and the digestive system (b] between the two. Now this figure, it may be remarked, would accurately represent every known lobster. It would also stand for the essential structure of every crab — which is merely a tailless lobster — and of every shrimp, barnacle, water-flea, slater, and a host of allied animals as well. Turning now to the butterfly, we should dis- FIG. 2.— JOINTS OF LOBSTER'S cover from even a rough examination of the insect's frame that it possesses an essentially similar disposition of parts to those of the lobster. The butterfly's heart lies on its back, its digestive system occupies the middle position, and its nervous system lies on the floor r. of the body, and more- -f \ ./* over consists of the same knotted and double chain we see in the lobster. Again, the appendages of the butterfly-body are in pairs, and resemble those of the lobster in all essen- tial particulars, although they are less numerous, in the adult state at least. So that, beyond and beneath all differences in appearance, form, and habits — material as these differences appear 42 CHAPTERS ON EVOLUTION. to be — we discover the great truth that both animals are built up on the same fundamental type. In a word, the ideal diagram (Fig. 4) we have constructed of the lobster's body will serve equally well to indicate the broad features of butterfly-structure. And further, as all crustaceans can be shown to possess bodies modelled on the lobster type, so all insects — numbering many thousands of species — FIG. 4. — DIAGRAM OF AN ANNULOSE ANIMAL. i. Diagrammatic longitudinal section of Annulose animal : a, blood or haemal system ; b, digestive system : c, neural or nervous system. 2. Nervous system of Annulose animal, viewed from above, and showing the double ventral nervous chain. 3. Transverse section of Annulose animal : a, blood system; b, digestive system; c, nervous system; dd, gills or breathing organs ; e e sa&ff, "oars" or locomotive organs. may, without exception, be referred to the butterfly type. From which declarations a third may naturally be drawn, namely, that the bodies of all insects and all crustaceans are built up on one and the same fundamental plan. Nor is this all. The diagram (Fig. 3) which, as we have seen, conveys to our mind the essential features in the anatomy of a lobster and a butterfly, and which, through these animals, presents us with a general idea of every insect and every crustacean, can be shown to possess a more extended application still. Every spider, scorpion, and mite agrees with the lobster and insect in its essential structure ; and every centipede and millipede likewise has its heart above, its nervous system below, and its diges- tive system in the middle of its body; whilst if we, lastly, examine the worms themselves, we shall find that our diagram still serves to show the main details of the structure of that extensive class. In this way, therefore, the language of the zoologist becomes clear when he states that all of the foregoing animals constitute " a type of animals." The type in question is, in fact, Cuvier's Articulata, or, as it is rendered in modern zoology, that of the Annulosa. And there remains yet one important addition to the zoological statement, namely, that no other animals, save the Annulosa or Articulates, exhibit the arrangement of parts just noted.. The heart above and the nervous system below (Fig. 4, i) are characters as distinctive of these animals as is the CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 43 particular impress on coins of the country or territory from which they were issued. Yet another illustration may be given of the constitution of animal types by way of impressing their distinctive character on the mind. If we bisect the body of a fish by splitting it through the spine from head to tail, we discover a highly characteristic disposition FIG. 5. — SECTION OF VERTEBRATE. e, the Diagrammatic plan of Vertebrate type, a a a, vertebral column or spine, upper arches of which enclose the brain and spinal cord; bbb, "cerebro- spinal " nervous system or axis, consisting of the brain and spinal cord ; cc, digestive or alimentary system; d, anus; e, heart; fff, "sympathetic" or "ganglionic " nervous system. of its organs and parts. Lying along the back, and enclosed within the skull and spine (Fig. 5, a a) as within a tube, we find the nervous system (b b}, consisting of brain and spinal cord. Lowest down, and lying on the floor of the body, is the heart (e) ; above the heart and in the middle position is the digestive system (c) • and above this system in turn is a second nervous system, dis- tinct from the brain and spinal cord, and known as the sym- pathetic system (//). Thus the positions of organs in the fish, with the exception of the digestive system, are exactly reversed from those of the lobster and butterfly; whilst in that its chief nervous system FlG ^-DIAGRAM OF MOLLUSC. (b, b) is enclosed within the bony tube formed by skull and spine (a, a), the fish presents a most material difference from both animals. Furthermore, we should find that the " fins " of the fish which represent the limbs of higher animals are never more than four in number, and that they are disposed in pairs. A simple diagram, then, might be constructed of the fish (Fig. 5), showing the positions of the various systems as just narrated, whilst a similar idea of vertebrate structure is afforded by the cross-section in Fig. i, A ; and such a diagram would hold true 44 CHAPTERS ON EVOLUTION. for every known fish, just as the diagram of the jointed animal, constructed from the details of the lobster, served to represent the essential anatomy of every known crustacean. If, now, we examine the structure of a frog or other amphibian, or that of any reptile, we shall find that, like the fish, these animals have a nervous system lying along the back, and enclosed within a bony tube. Again, their hearts are lowest, their digestive system a B FIG. 7. — DIAGRAM OF ECHINODERM, CCELENTERATE, AND PROTOZOON. occupies the median position, and their limbs are never more than four, and are invariably developed in pairs. Hence the diagram of the fish represents the essential anatomy of other two distinct classes, namely, frogs and reptiles. But it is easy to show that the fish type is also represented in animals still more widely removed from all apparent relationship with the finny tribes. Between a bird and a fish there seems at first to be no relationship, absolute or comparative ; yet the diagram of the fish will serve to express all the structural features of the bird. The latter exhibits, in short, the same arrange- ment of its nervous, digestive, and blood systems as does the fish ; its nervous system is similarly protected by a bony axis ; and its limbs are likewise in pairs. And ascending, last of all, to the highest CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 4$ confines of the animal world, and entering into the domain of the quadrupeds or mammals, at the head of which latter group stands the human subject, we find the type of fish, frog, reptile, and bird to be accurately adhered to as the fundamental plan of the quadruped FIG. 9. — ZOOPHYTES (b and d represent portions of a; and c, magnified). body itself; so that the diagram of the fish may be left to express accurately and truly the broad outlines of human anatomy. In this way we discover that a type of animals — the Vertebrata — first outlined by Lamarck, as already noted, can be constructed on precisely the same lines, and for the same structural reasons that made the constitution of the Articulate type a reality of living nature. Fishes, frogs, reptiles, birds, and quadrupeds or mammals, includ- ing man, thus possess bodies con- structed on the same fundamental type or plan. To continue the illustration of the formation of natural groups or "types" of animal life through the discovery of broad or fundamental correspondence in the structure of the body, were an easy matter. It may readily be shown that all ordin- ary shell- fish — such as the oysters, cockles, snails, whelks — along with the sea-butterflies (Pteropoda\ and the cuttle-fishes — form another well-marked type, distinguished by the peculiar disposition of the FIG. 10. — JELLY-FISH. CHAPTERS ON EVOLUTION. nervous system in three great masses (Fig. 6, h, i, £), as well as by other definite characters writ large enough in the textbooks of zoology. These animals form the type of the Mollusca — a group from which many animals therein included by Cuvier have been weeded out to form other divisions, or to find a place in other types. Similarly, the fact that the star-fishes, sea-urchins, sea-lilies, sea-cucumbers, and the like, form another and distinct type (Echinodermata), dis- tinguished by the "radiate" shape of the body, and by other characters, might be dwelt upon. A diagram of the FIG. IT.— AMCEB^. star-fish type is shown a, Amoeba radiosa; b. Amoeba diffluens, in various stages of • -r- i j contraction. in Pig. 7, C. It WOUld merely extend our illustrations to show how the hydras (Fig. 8), zoophytes (Fig. 9), jelly-fishes (Fig. 10), corals, and sea-anemones form another " type " ( Calenterata), noted for its curious digestive system (Fig. 7, B e), which communicates freely with the internal cavity of the body (B/). Whilst, last and lowest of all, the Protozoa, represented by the sponges, Amoeba (Fig. n), the Foraminifera or chalk-animal- cules (Fig. 12), and many other and equally simple forms of animal life, constitute the lowest type. These animals are distinguished rather by their want of organs and tissues than by the posses- sion of the belongings of higher animals, and exhibit bodies which consist, for the most part, of simple masses of protoplasm, but which, nevertheless, exhibit all the fundamental characteristics of living organisms. The diagram of a Protozoon might thus be adequately enough rendered by a simple figure representing an in- definite mass of protoplasm, such as is represented in Fig. 7, A. It will thus be seen that the four Cuvierian " types " have become largely extended and modified by modern research. But, notwith- standing these modifications, the principles whereon that great anatomist laid the foundations of the constitution of the animal world, remain solid and enduring as of old. If to-day our list of types is of a more extensive character than was the Cuvierian repertoire, it must be borne in mind that such a result was in- Fic. 12.— FORAMINIFERA. CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 47 evitable from the improvement of the ways and means of scientific research. What remains to be effected in biological research, is the enlargement and extension of the types themselves — an increase of knowledge which has, indeed, been carried out along the very lines which led Cuvier to those remarkable generalisations that have formed the basis of modern natural history studies. Although naturalists are by no means agreed concerning the exact number and relationship of the " types " represented in the animal world, the following table may serve to show the fundamental divisions of the animal kingdom as expressed in modern systems •of zoological classification. VERTEBRATA (Fishes, Reptiles, Birds, Quadrupeds). MOLLUSCA .(Cuttle-fishes, Shell-fish, &c.). MOLLUSCOIDA (Polyzoa, Sea-squirts). ARTICULATA (Insects, Spiders, Crustacea). ECHINODERMATA (Star-fishes, Sea-urchins, &c.). VERMES (Worms). CCELENTERATA t (Corals, Anemones, Hydrae, Zoophytes). Sponges. Infusorians. PROTOZOA (Lowest Animals— Amoebae, £c.). 48 CHAPTERS ON EVOLUTION. Having thus endeavoured to show the chief types of animal life, we may now glance at the conclusions bearing upon the constitution of the animal world to which our researches may legitimately be presumed to lead. It thus seems clear, so far as our studies have led us, that the constitution of the animal world is one in which the development of its included units has followed a series of definite plans or types, leading to the construction of the six or seven primary groups into which the naturalist is accustomed to divide the hosts of animal life. These " types," it must further be noted, are not in any sense theoretical groups, but are founded, as we have seen, on exact and fundamental likenesses in structure. Nor must we lose sight of the exact meaning of the word " fundamental " as thus employed. The use of this term implies that the likeness and similarity in the plan admits of variation in the carrying out of its details. The lobster and the butterfly, for example, are fundamentally alike ; their bodies are constructed on an essentially similar plan ; and the positions of their organs are identical. But whilst these are fundamental likenesses, they do not imply that of necessity the two bodies should be similar throughout. The tissues of a butterfly may be more complex than those of a lobster, or vice versd ; just as the heart or brain of a frog is a more complex organ than that of a fish, and as each organ of a bird or a man shows, in turn, an advance upon that of the frog. The variations, however, are all more or less plain and evident elaborations of one type. As Cuvier put it, " the ulterior divisions " of each type, or, in other words, its arrangement into subordinate groups, drawn upon differences in the included animals, are founded upon " slight modifications " of the type, or by " the development or addition of certain parts " — which parts, it may be added, can, as a rale, be shown to be represented in one form or another in the original constitution of the type. A second consideration of importance in discussing the constitu- tion of the animal world consists in the emphatic declaration of the modern naturalist that it is impossible to arrange animals in a linear series, beginning with the lowest form and ending with man. The nature of the constitution of the animal world, in short, does not admit of any such arrangement ; since it would be manifestly im- possible to determine in very many instances which, of two animals or of two groups, should be ranked the higher. It would be a puzzling, if not an impossible task for any naturalist to determine, for example, whether a cockroach or a cuttle-fish should be ranked highest in the scale. The fact that the body of the one is constructed on an utterly different type from that of the other, constitutes a primary difficulty of no mean order ; and there intervenes a second considera- tion, namely, that of the impossibility of settling any standard whereby the organisation of the one might be legitimately compared with that of CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 49 the other. As Professor Huxley has graphically remarked, "regarded as machines for doing certain kinds of work, animals differ from one another in the extent to which this work is subdivided. Each subordinate group of actions or functions is allotted to a particular portion of the body, which thus becomes the organ of those functions ; and the extent to which this division of physiological labour is carried differs in degree within the limits of each common plan, and is the chief cause of the diversity in the working out of the common plan of a group exhibited by its members. Moreover, there are certain types which never attain the same degree of physiological perfection as others do." These words indicate clearly enough that the high or low character of any animal in a type, depends chiefly upon the complexity of the functions, and necessarily of the organs, whereby life is main- tained. As the household whereof the labour is performed by a maid- of-all-work, is functionally less complex than that whose work is per- formed by a retinue of servants, each discharging a special duty, so in the animal world, the rank of any one of its members or of its groups can only be determined by the complexity of body, and by the corresponding degree of intricacy with which the functions of the body are performed. Furthermore, it is the development of this complexity, or the reverse, from the common plan, which, as Huxley has so well expressed it, is the actual cause of the variations we see in each type. The frog is not higher than the fish because of its type — since both exhibit the same fundamental plan ; but because the frog's functions are more specialised — there is a more minute physiological " division of labour " in the frog, and there exists a more complex staff of organs (developed from the common plan of fish and frog) to discharge the increased work. Of the differences between a frog and a bird, and between both and a man, precisely the same remark may be made. The higher or more complex life involves and demands from the common type, the more complex frame. To quote Professor Huxley's words once more, "a mill with ten pairs of mill-stones need not be a more complicated machine than a mill with one pair ; but if a mill have two pairs of mill- stones, one for coarse and one for fine grinding, so arranged that the substance ground passes from one to the other, then it is a more complicated machine — a machine of higher order — than that with ten pairs of similar grindstones. In other words, it is not mere multiplication of organs which constitutes physiological differentiation ; but the multi- plication of different organs for different functions in the first place, and the degree in which they are co-ordinated, so as to work to a common end, in the second place. Thus a lobster is a higher animal, from a physiological point of view, than a Cyclops (or water- flea), not because it has more distinguishable organs, but because K 50 CHAPTERS ON EVOLUTION, these organs are so modified as to perform a much greater variety of functions, while they are all co-ordinated towards the maintenance of the animal by its well-developed nervous system and sense organs. But," concludes Huxley, "it is impossible to say that, e.g., the Arthropoda (insects, spiders, centipedes, &c.) as a whole are physiologically higher than the Mollusca (shell-fish and cuttle-fishes), inasmuch as the simplest embodiments of the common plan of the Arthropoda are less differentiated physiologically than the great majority of Mollusks." Whilst the difficulties which lie in the way of determining the higher or lower rank of many organisms are thus apparent, it may be remarked that the means already specified — namely, the physiological perfection of the animal — may in turn assist us in assigning to many forms their relative place in any type or group. Thus the possession of air-breathing organs, or lungs, is admit- tedly a mark of a higher organisation than that which possesses gills \ the life of the air-breather being, as a matter of fact, associated with a structural perfection excelling that of the aquatic and gill- bearing animal. So also the degradation of organs and parts which accompanies parasitism, naturally lowers an animal in the series as compared with its non-parasitic neighbours. Thus within the limits of any one type of animals, we may discover many examples of tendencies to higher as well as to lower development, these tend- encies determining the position of the organism within its own type, and either elevating itself or its group collectively, or, on the other hand, degrading it, and assigning it to a low place in the type. The impossibility of any scientific or natural arrangement of animals in a linear series can thus be shown to depend simply upon the con- stitution of the animal kingdom as a whole. If any arrangement of the great types it presents to view is permissible — and naturalists are agreed that some such relationship is embodied and included in the constitution of the kingdom — such an arrangement will find its clearest expression in the metaphor of a tree. As represented, indeed, in the foregoing table (page 47), the various types may be regarded as the great branches of the animal tree, rising here and there from a common stem or root, and developing, each along its own special line and type, into the variety and fulness of form exhibited before our eyes to-day. The impression which is liable to be left on the mind of the observer who, thus far, has traced out the constitution of the animal world into its fundamental types or plans, will undoubtedly take the form of the idea that the mere existence of these types or plans as we behold them represented in living animals would appear to indicate the separate and disconnected nature of the great groups in question. Considerations of this nature inevitably lead to others, CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 51 dealing with the origin of these plans of animal life ; and the conclusion that these types have each had an independent origin might seem, at first sight, to possess actual warranty for acceptation and belief. But a fuller consideration of the constitution of the animal world will tend to dispel any such tacit agreement concerning the actual independence and distinctness, or regarding the separate origin of the animal types already noted. On the contrary, a deeper acquaintance with facts as they stand, will inevitably tend to show us, firstly, that the limits of the types are by no means so rigidly circumscribed as the older naturalists supposed. Whilst, secondly, we may discover that evidence exists to show, not merely that the various types are by no means sharply demarcated from each other, but that in the nature of things they exhibit relationships of the highest importance in the attempt to discover the exact nature and constitution of the animal world. With regard to these latter contentions it is easy to show, for example, that the great " types " of animal life, whilst remaining distinct enough to constitute divisions of utility in classifying animals, nevertheless often merge into one another, and become con- nected by " intermediate forms " — that is, are linked together by animals or by groups which may be termed " transitional " in every respect. If the existence of such links between any of these types be proved, the distinct and utterly separated character of all may logically be denied. The fact that the types are connected in any fashion, must also be held as showing that some form of progression from one to the other must be postulated as an essential part and feature of the animal constitution. In other words, we are led to believe in the continuity of these types, as opposed to the idea of their separate origin. We are led to espouse the idea of an uninterrupted development, as opposed to that of the separate and independent origin of the great plans of animal structure. It is interesting, however, in the first place, to find that there is an unmistakable reflection of such a continuous development to be discovered within the limits of each type ; and to this latter aspect, or that concerning the types themselves, it may now be well to direct our attention. If we select any type, from the lowest to the highest, we may readily discover that its included animals exhibit amongst themselves a connected relationship such as the mere fact of their bodies being built upon one and the same plan would of itself be sufficient to suggest. Amongst the Articulate animals, for instance, this relation- ship is plainly seen ; and it is no less evident amongst the Vertebrates and Molluscs, as will be more plainly shown in succeeding chapters. Why, it may be asked, should the segments or joints of the lobster's body (Fig. 2) and of the insect frame be constructed on one and the $2 CHAPTERS ON EVOLUTION. same plan ? Or, again, why should the appendages of the bodies of these animals, which resemble each other far more closely in the early stages than in the adults, present a striking correspondence of type that is only marked by the modifications they undergo through adaptation to varied ends ? Why, again, should the mouth-parts of a butterfly, adapted, as every one knows, for suction, be formed of essentially similar and corresponding parts to those which are found in the biting mouth of a beetle ? And why should the arm of man, the wing of the bird or bat, the fore-limb of the horse, the paddle of the whale or dolphin, and the fore-limb of the frog, as will be more fully shown in a future chapter, be constructed on one and the same type ? The answer to these pertinent inquiries can only be found in some conception which demands and postulates some intelligible relationship between the varied and yet fundamentally similar parts. Mr. Spencer, speaking of similar facts in the structure of Articulate animals, asks, " What, now, can be the meaning of this community of structure among these hundreds of thousands of species filling the air, burrowing in the earth, swimming in the water, creeping about among the seaweed, and having such enormous differences of size, outline, and substance, as that no community would be suspected between them ? Why under the down-covered body of the moth, and under the hard wing-cases of the beetle, should there be discovered the same number of divisions as in the calcareous frame- work of the lobster? It cannot be by chance" continues Mr. Spencer, " that there exist just twenty segments in all these hundreds of thousands of species. There is no reason to think that it was necessary, in the sense that no other number would have made a possible organism. And to say that it is the result of design — to say that the Creator folio wed this pattern throughout, merely for the purpose of maintaining the pattern — is to assign a motive which, if avowed by a human being, we should call whimsical No rational interpre- tation of this and hosts of like morphological truths, can be given except by the hypothesis of evolution ; and from the hypothesis of evolution they are corollaries. If organic forms have arisen from common stocks by perpetual divergences and redivergences — if they have continued to inherit, more or less clearly, the characters of ancestral races ; then there will naturally result these communities of fundamental structure among extensive assemblages of creatures that have severally become modified in countless ways and degrees, in adaptation to their respective modes of life." Choosing thus the doctrine of evolution, we can clearly enough account for the general likeness exhibited by the various members of an animal type. The animals of each type resemble each other because they are descended from a common stock. " Descent with modification " is the key CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 53 which unlocks whatever mysteries hedge about the fundamental •likeness we see in each type of animal life. In a future chapter we shall endeavour to trace the evidence which has already been gathered in favour of the accumulation of transitional forms, between the main divisions of the vertebrate type, as illustrative of " missing links " at large. In the present instance we may sum up the testimony which tends to support and prove the biological declaration that, between the types themselves, there exist intermediate forms, the presence of which tends to substitute the idea of the gradual and continuous nature of animal development as opposed to that of interrupted or " special creations." For example, it is a comparatively easy matter to demonstrate that the gulf between Vertebrate animals and their Invertebrate neighbours has been largely bridged over, so that to-day no competent naturalist doubts the connection of the highest type of animal life with lower forms. The evidence of such a connection will be more fully detailed hereafter, but it is permissible to refer to its main details in the present instance. The Vertebrate animals have already been shown to be those which alone possess a spine enclosing the nervous system, and which, moreover, of all animals, are those having the heart lowest, and possessing never more than four limbs, these latter appendages being developed in pairs. But when we pass to the lower confines of this group, we discover that the lowest fish (the lancelet [or Amphioxus\, Fig. 13) presents us with a clear-bodied organism, attaining a length of only an inch or two, and desti- tute of nearly all the special belongings of the fishes themselves. In place of a spine and skeleton, it possesses a soft cellular rod (the notochord), such as every other vertebrate develops in early life, but which in all, save a few fishes, is replaced by the backbone. It breathes by an enlargement of the throat ; its nervous system, lying upon the "notochord," is a mere nervous cord destitute of a brain ; its eyes are mere specks of colour ; and it wants a heart, kidneys, spleen, and also the sympathetic nervous system found in all other vertebrates. When, therefore, we attempt to place the lancelet in an animal type, we are met by the difficulty that whilst, in the pos- session of certain important characters, it is undoubtedly a vertebrate, in the want of other characters it appears to lie outside that type. Again, we discover an equally important fact when we learn that FIG. 13. — LANCELET. a, head ; b, the fish viewed from the side ; c, filaments surrounding the mouth. 54 CHAPTERS O.V EVOLUTION. the lancelet presents distinct affinities with the sea-squirts (Fig. 14),. or Tunicates, which belong to the molluscoid type (see table, page 47),. and the commoner species of which may be compared each to a veritable bag, or " leather bottel," firmly attached to rocks, shells, and like objects. These likenesses, to be more fully discussed hereafter (see Chapter IX.), are seen, not merely when the structure of the adult sea-squirt and lancelet are compared, but are still more clearly discernible when the development of the two animals is studied. The lancelet, in short, resembles a fish, or lower verte- brate, whose development has been arrested, so to speak ; and it is equally interesting to discover that there exist certain sea-squirts which, in their special features, approach very nearly to the lancelet, and in which the " notochord " — long supposed to be the special possession of the young of vertebrate animals — remains, as in the lancelet, persistent throughout life. Thus the lancelet remains before us, constituting, in every sense of the term, a link between vertebrates and invertebrates. It agrees wholly neither with the highest type nor with the molluscoids or sea-squirts themselves, but exhibits a series of characters strictly intermediate between the two types. We may readily enough understand, on these grounds, why this little clear-bodied animal, which at first was regarded as a worm, and then as a kind of slug, should, from the peculiarity of its position as the apparent root of the vertebrate type, have been styled, " next to man,, the most important vertebrate." As Professor Huxley has pertinently remarked, "in 1859 there appeared to be a very sharp and clear hiatus between vertebrated and invertebrated animals, not only in their structure, but, what was more important, in their development. I do not think that we even yet know the precise links of connection between the two ; but the investigations of Kowalewsky and others upon the development of Amphioxus and the Tunicata prove, beyond a doubt, that the dif- ferences which were supposed to constitute a barrier between the two are non-existent. There is no longer any difficulty in un- derstanding how the vertebrate type may have arisen from the invertebrate, though the full proof of the manner in which the transition was actually effected may still be lacking." For these weighty reasons, the vertebrate type, in the tabular view of the types of animal life (page 47), is represented as having its root laid within the sea-squirt or " tunicate " line of descent. If at random we selected other types of animal life, we should similarly discover that they exhibit more or less distinct relationships. CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 55 with other divisions or plans of the kingdom. The molluscoids or " sea-squirts " themselves, for example, appear to be related, through a curious worm-like creature, named Baldnoglossus, to the worms on the one hand, and to the star-fish group (Echinodermata) on the other. Or, if we select the last-named group itself, we may discover that the star-fishes and sea-urchins are not more isolated from other types than are the vertebrates. The star-fishes, in fact, present . many points of affinity to certain worm-like forms ; and their development, to be hereafter alluded to, clearly relates them, in the eyes of the naturalists, to lower types of animal life. Again, the lowest animals, or Protozoa, appear to be linked to the Cxlenteratc type (or that of the zoophytes, corals, sea-anemones, &c.) through the sponges, which unite, in a most characteristic fashion, the fea- tures of the lowest forms with organisms of a higher grade. And, lastly, as amongst the worms we find the roots of the star-fish type, so in that class also we discover the beginnings of the great Articulate plan, which possesses the insects, crustaceans, and allied animals as its chief representatives. As has well been remarked, " it may reasonably be doubted whether any form of animal life remains to be discovered which will not be found to accord with one or other of the common plans now known. But, at the same time, this increase of knowledge has abolished the broad lines of demarcation which formerly appeared to separate one common plan from another." Lastly, it will be shown in future chapters that the various animal types start in their development from a common basis, and agree in the earlier and essential stages of their progress towards their adult forms. There is a literally amazing likeness to be dis- cerned between the early stages of the development of many animals which, as adults, and as belonging to different types, present not the slightest resemblances to one another. Each animal, in fact, traced backwards in its history, " approaches the earlier stages of all the rest ; " that is to say, " all start from a common morphological type, and, even in their extremest divergence, retain traces of their primitive unity." Such unity will form the special subject of the succeeding chapter, when the common and universal matter of life, or protoplasm, is discussed in detail. It may thus be demonstrated as a fact, and as a matter removed entirely from the domain of theory and hypothesis, that, whilst the great world of animal life exhibits a constitution, in the study of which its component elements are seen to be resolvable into several distinct " types " or " plans " of structure, the development of these types has followed a pathway and progress comparable to the growth of a tree. The connections between types and the existence of intermediate and transitional groups of animals, apparently belonging to one type when 56 CHAPTERS ON EVOLUTION. studied from one aspect, but exhibiting the closest alliance with another type or plan when different details of structure are regarded, prove in the clearest fashion that continuous development has been the " way of life " in the animal world. Whilst, lastly, the bare fact that, as we trace the histories of all the types backward towards their_ early life, the likeness grows in exactitude until it merges in absolute identity, constitutes in itself a detail which is all-eloquent in favour of the idea that only on one theory can the entire constitution of the animal world be explained. That idea, it is needless to remark, is embodied in the theory of evolution, which postulates descent from a common root or stock with subsequent modification as the only satisfactory explanation of the constitution of the animal world. The constitution of the plant world may be briefly alluded to, by way of close to these observations, because the issues of botanical science tend to support, in the plainest fashion, the deductions and generalisations just detailed concerning the origin of the types of animal life. The variety of plant life is not less profuse than the diversity presented by the tribes of animals ; but, like their neighbour organisms, the plants exhibit certain broad types, to one or other of which it is possible to refer any single plant or group. If a table of the plant-types be constructed, it will assume such a form as that indicated : — Js|] I. THALLOPHYTES (e.g. Alga (or Seaweeds, &c.) and ?r%- « II \ fungi). I £ 8 II. MUSCINE.E (e.g. Liverworts and Mosses). 'III. PTERIDOPHYTES (e.g. Ferns, Horsetails, Club-mosses). "A. GYMNOSPERMS, or those having no") seed-vessels (e.g. Firs, Pines, &c.). t, 3 IV. PHANEROGAMS^ (higher plants) : B. ANGIOSPERMS, or those having distinct seed-vessels : (a] Monocotyledons (e.g. Palm, Lily). ' (b} Dicotyledons (e.g. Oak, Prim- rose, &c.). .MSB 3 «•> % ' Discarding all botanical technicalities, save those absolutely necessary, the types of plant life may be readily enough appre- ciated. If we examine such a plant as an oak, a primrose, a buttercup, a palm, a lily, or, indeed, any ordinary member of the plant series, we may discover that it possesses conspicuous flowers, and that accordingly it may be distinguished from such plants as the ferns, mosses, and fungi, in which no flowers are •CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 57 FIG. 15. — BEAN IN SECTION. c, one of the cotyledons ; p, young stem ; r, the young root. •developed. Such a state of matters suffices, along with other and equally distinctive points of structure, to separate the higher plants (or Phanerogams'] from the lower or flowerless plants ( Cryptogams']. But, selecting the flowering and higher plants themselves, we may readily discover that certain highly distinct types are represented within their limits. Thus, when we watch the development of an oak, a bean (Fig. 15), a primrose, or a buttercup, for example, we discover that the young plant •develops or possesses two primitive leaves, named " seed-leaves " — the cotyledons (c) of the botanist (Fig. 15). Again, such plants have their flower-parts arranged in fours or fives ; and, whilst their stems grow outwards, the leaves present us with the network of veins <(Fig. 1 6) so well seen in "skeleton leaves." These characters suffice to group the highest plants into a type known as that of the Dicotyledons. If now we examine a palm, a lily, a tulip (Fig. 17), or a hyacinth, we shall find that only one "cotyledon" or "seed-leaf" is developed by the young plant. Furthermore, the leaves have parallel veins — a conformation well illus- trated in the tulip leaf (Fig. 18) and the onion leaf, for example. Again, the stem of these plants is an " inward-growing " structure, and the parts of the flowers are developed in threes or in multiples of that number. Hence a second type of plants is constituted by the palms, grasses, lilies, and their allies, and to this type the name of Monocotyledons is applied. A third type of plants is also included in the group of " flowering plants. " This latter type is constituted by the conifers, or " cone-bearing " plants — such as the larches, firs, cedars, cycads, araucarias, cypresses, junipers, &c. — and presents in many respects clear evidence of its title to be regarded as a highly dis- tinctive and specialised group of plants. The chief characters of thes£ plants con- sist in the peculiarity of their flowering arrangements, which are represented in the well-known "cones." Again, the seeds are not contained within a seed- FlG- 17-— TULIP IN SECTION. vessel, as in ordinary plants (e.g. pea), but are borne on the cones. FIG. 16. — LEAF OF DEAD- NBTTLE. CHAPTERS ON EVOLUTION. Hence arises the technical name of Gymnosperms (" naked-seeded ")r applied to the pines, firs, and their allies. The foregoing types constitute collectively the "Flowering" plants of the botanist. Ranking below these plants, however, is a number of types containing plant- organ isms of highly characteristic nature. Thus the Ferns, Horsetails (Equiseta), and Club- mosses or Lycopods, form one section of the " Flowerless " plants. In these forms the true plant arises not directly from a seed, as in the higher orders of plant-life, but from a curious leaf-like structure called a prothallus, which in its turn arises from the spore or germ of the parent plant. Thus from the spore of a fern, falling from the back of its frond, springs the leaf-like " prothallus." On the under surface of this body are developed organs giving rise in turn to the young fern, which is thus developed intermediately and indirectly from its parent. This character, united with others, which need not be specified here, serves to render the fern type clear and indivi- dualised. It may be added, however, that the stem in such plants grows chiefly at its summit, and that its leaves or " fronds," which bear the reproductive organs,, exhibit a forked arrangement of their veins. Equally " flowerless " with the ferns and their neigh- bours are plants which, however, rank below these well- known forms in the botanical scale. Thus the Muscinea, or Mosses and Liverworts, appear as a distinct type of lower plants which are composed solely of ." cells," and which do not possess true " roots " comparable with those of higher plants. And in the lowest of the plant world we meet with the Seaweeds, Fungi, and a host of LEAF'OF TULIP, microscopic plants, equally "flowerless," equally cellular in composition, and which, moreover, do not develop the stem and leaves of the mosses. Many of these lower plants are represented by single " cells ; " the well-known Diatoms, the Yeast Plant (Fig. 19), and many others illustrating such a constitution; whilst a mushroom or other fungus is simply a mass of cells, and nothing more. These details prove that the plant world exhibits a constitution in which "types" ap- pear as prominently as in the animal kingdom. FIG. IQ. — YBAST PLANTS. T-. .1 -^ j-i i_ i_ ^i. . *.\. Furthermore, it can readily be shown that the plant types are not more distinctly separated from one another than are those of the animal world. It is demonstrable, for instance, that the Alga or Seaweeds are connected by intermediate forms with the FIG. 18. CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 59 Lichens and Fungi ; whilst no botanist questions the idea that the ferns, club-mosses, and their neighbours, lead the way from the lower or "flowerless" plants to the Gymnosperms (or firs, pines, &c.) amongst the " flowering " plants. Between the Monocotyledons and Dicotyledons, again, there are obvious links, and hence we discover that the whole plant kingdom may be regarded as being bound together after the actual fashion of its own product seen in the tree, which, whilst possessing its individual parts, likewise exhibits a continuity of development that forms one of the chief characteristics alike of the single organism, and of its relationship with its neigh- bours. In the lowest deeps of plant life we may discover organisms which possess at the best a doubtful title to be regarded as the objects of botanical study. In the animal world, likewise, are included lower organisms which may be regarded in certain aspects as possessing true relationships with plants. Modern biology to-day frankly admits its inability to pronounce whether certain lowest forms of life are animals or plants. Certain " monads," for example, consisting each of a speck of protoplasm provided with the microscopic whip- tails, exhibit a highly confusing identity of structure and function, which renders their exact nature indeter- minable, or at least highly doubtful. Hence we discover that apparently at the lowest confines of the animal and plant realms we enter a " biological ' No man's land,' " whereof the included in- habitants may legitimately claim relationship with both kingdoms. They exhibit in this latter respect, in the eyes of the biologist, the actual survivals of that early epoch in the history of life's develop- ment when the specialised kingdoms of animals and plants were not, and when existence passed placidly along the common lines which were soon to diverge into the two great series of living beings that environ our footsteps to-day. The great lesson which a study of the constitution of the living worlds is calculated to teach the independent observer may be summed up in the contention that the entire subject testifies to the continuous and connected nature of the development of life at large. The beginnings of higher and lower life alike, are represented by humble stages, wherein specks of protoplasm, or at the most simple " cells," discharge all the functions of existence. From such simple beginnings the highest being is developed. The difference between the highest and lowest organism is therefore not so much one of kind as of the degree of perfection to which elaboration and develop- ment has carried the living form. We may be unable definitely to indicate why one organism speeds along this pathway to assume a place in one type, or why another, apparently identical in its early life with the first, should develop into a widely different being. 60 CHAPTERS ON EVOLUTION. But beyond such questions lies the biological surety that to under- stand the way of the becoming of both animal and plant is to deny any independence of creation, and to assert that unless the phenomena of life be without meaning, all nature testifies to continuous develop- ment as the main feature of the living constitution. To collate the evidence which widely different branches of inquiry supply in favour of this view, is the chief aim of the succeeding chapters. But Mr. Spencer's words may be once again quoted by way of showing succinctly and plainly the general conclusion of the present study. " The general truths of morphology," says Spencer, " thus coincide in their implications. Unity of type maintained under extreme dissimilarities of form and mode of life, is explicable as resulting from descent with modification ; but is otherwise inexplicable. The like- ness disguised by unlikenesses, which the comparative anatomist discovers between various organs in the same organism, are worse than meaningless if it be supposed that organisms were severally framed as we now see them ; but they fit in quite harmoniously with the belief that each kind of organism is a product of accumulated modifications upon modifications." 6i IV. CONCERNING PROTOPLASM. THE nature of that curious collocation of actions we commonly denominate "life," and the connection which exists between life and the bodies it invests and whose interests it directs, have ever formed subjects of extreme speculative interest to cultured mankind. In the classic ages such speculation was rife, and modern biology but repeats the procedure of the ancient world, when, with additional sources of knowledge and wealth of research, it proceeds to discuss anew the great question of the origin and nature of life. Each year brings its own quota of detail and argument concerning this im- portant and fundamental matter of modern life-science, and in more than one aspect it may be said to be the pivot around which the research of to-day turns. The subject of the origin of species, itself a burning question of biology, leads directly backwards to the origin of those powers and properties in virtue of which the species retains its hold on the world, and which lie at the root and foundation of the universe of animals and plants. Investigate the development of a liv- ing being, and you are led directly backwards to the germ from which it springs, and to the consideration of the power in virtue of which the shapeless evolves the formed, and the general grows to become the special. Study the differences and distinctions or the likenesses and resemblances that biology brings to view between animals and plants, and you will inevitably touch upon the subject of the nature of the common life which invests both regions of living beings, and which even in its most varied aspects appears to present features of strange , and confusing identity between the animal kingdom on the one hand, and the plant creation on the other. Pass to consider " the records of the rocks " themselves, and in due course the question of the first beginnings of life on our planet — the when, whence, and whither of vitality — will crop up like some unperceived, but felt, presence, which hovers around the biological arcanum. The subject of life and its nature thus awaits us at the beginning of existence, as it faces us at its close. We cannot therefore feel surprise that of all questions of philosophy the nature of life should be deemed the most important, and that those who sit in high places in temples biological should so often dwell upon its varied aspects as a fit and proper theme for philosophic consideration by both gentle and simple, learned and unlearned, in scientific ways. 62 CHAPTERS ON EVOLUTION. The investigation of life from any point of view leads us to seek in the lower confines of the living worlds, the subjects which are most likely to present us with the simplest and most elementary manifesta- tions of living force. The life-history of the higher animal and plant appears before us as the acme of intricate operations, and as a complex collection of manufactories and organisations, the working of which may well puzzle and perplex us even in its plainest details. The mere study of a single function in the higher organism is beset with difficulties of greater or less kind. The circulation of the blood, the elaboration of sap — not to speak of the problems involved in con- sidering animal and plant sensibility and the functions of nerves — are illustrations of points in the history of the high animal or plant which involve problems of well-nigh inexplicable nature in their study. Hence the prevailing tendency in research of the kind before us has been indicated by the selection of the lowest fields of life as the ground best adapted to yield promising results to the scientific inquirer. The lower animal or plant, as we shall presently see, makes its appearance before us as a body apparently of extremely simple structure and nature. Presenting us at the most with the appear- ance of a single "cell " (Fig. 19), the lower organism might be thought to yield to scientific scrutiny some clear knowledge of the nature of the powers which rule its destinies. And such a supposition might likewise be presumed to gather strength in the hopefulness of the idea that, as the higher animal or plant is but an aggregation of units, each representing the single " cell " of lower life, the study of the low organism should reveal to us, as by deputy, the secrets of the higher organisation. But the problem is hardly resolvable into con- ditions such as have just been indicated. The living being in higher life is not a mere collection of units, the disposition of which can be mathematically calculated and mechanically analysed. The condi- tions which might well enough bound the discovery of the mechanical contrivances of mankind, are not those which environ the puzzle of life. The problem which faces us as we gaze at the complex . organism with its multifarious functions, is just as recondite as when, by aid of the microscope, we can look through and through the speck of protoplasm which seems hardly to warrant the term " animalcule " bestowed upon it. Thus the mere environments of the problem of living and being, constitute a difficulty of no ordinary kind, and hedge the nature of the life which is in the animal or plant with a mystery that appears to loom darkly enough, even before the shining lights of these latter days. Although the solution of the problem concerning the nature of life may be said in some respects, therefore, to have gained but little aid from researches into the lower worlds of life that people the stagnant drop — beings which find a home in dimensions that CONCERNING PROTOPLASM. 63 would hardly have contained even the convenient Angels of the ."Schoolmen, whose ability to accommodate themselves within the limits of the minute is matter of common knowledge — still the extension of biological knowledge concerning lower organisms has teen fraught with importance in certain easily discernible ways. If we have not been enabled to shout out " Eureka " to the waiting races of to-day, we have nevertheless gained some useful ideas re- garding the true directions in which our difficulties must be attacked. Through the comprehension of what the lowest animals and plants ;are, we have been led to form certain reasonable ideas concerning what life may be. The knowledge of the conditions required to perpetuate the normal existence of living beings, has led us to recog- nise, in some measure, the true nature and extent of the problem that awaits the fuller knowledge of coming years for its solution. Let us, therefore, in the first place, endeavour briefly to gain some adequate ideas concerning the conditions or environments demanded for the exhibition of life in its lowest grades; since, haply, we may find in such a study a clue which may lead us towards the understanding, in theory at least, of the nature of the forces which make and control the living organism. One of the first decided steps towards the simplification of a theory of life was taken when the living con- tents of vegetable cells were discovered to present a striking similarity to the substance representing the essentially living part of the cells of animals. Mulder thus recognised the vegetable pro- toplasm, as he termed the soft, gelatinous matter of the vegetable cell ; and Remak in turn described the animal " protoplasm." Need- less to remark that this substance, described as locked up within the cells or units composing the tissues of the higher organism — animal or plant — and as constituting the active or vital parts of every living being, was identical with the matter, closely resembling white of egg in appearance, which Dujardin had named "sarcode," and of which the bodies of the lowest animals are entirely composed. Max Schultze had indeed shown that the protoplasm of animals was chemically and microscopically indistinguishable from that of plants ; and that beneath the variations of form, and the diversities of life, there thus remained a curious uniformity of substance in living organisms. The life and growth of the animal was seen .to depend on a substance which was apparently identical with that consti- tuting the living basis of the plant. A curious community of sub- stance was thus proved to underlie wide and apparently irreconcil- able differences of life and habit ; and out of this primary fact grew new and bolder conceptions of the nature of life than had before been ventilated by biologists at large. To appreciate clearly and fully what is implied by the statement that the substance now widely known as " protoplasm " is a sine qu& CHAPTERS ON EVOLUTION. non for the manifestation of life and vital action, let us examine a few of the aspects in which this substance makes its appearance as the medium for the exhibition of living actions. It is by no means unusual to find that familiarity with a name in the abstract implies a total inability to appreciate the concrete aspects of the substance which the name describes. Despite the wide acceptation of the name " protoplasm," it is matter of common observation that the nature of" the substance itself, as well as its qualities and traits, are frequently unknown by those to whom the term is as a " household word" As a preliminary study, then, the discussion of protoplasm itself, and its varied phases, will not be without its value in the determination of its importance as " the physical basis of life." What protoplasm is, chemically and physically, may be very briefly and readily de- scribed. Chemically, it stands as the type of a class of compounds to which Mulder gave the name of "proteine" substances. Of such substances, common albumen seen in white of egg is a familiar example ; and white of egg, indeed, hardly differs, save in minute chemical particulars, from protoplasm itself. The latter substance is resolvable by chemical analysis into the elements carbon, hydrogen, oxygen, and nitrogen, along with mere traces of sulphur and phos- phorus. Physically, protoplasm presents itself as a clear, viscid, and semi-fluid substance, often highly granular from the presence within its substance of fatty or other particles. By immersion in a carmine solution, dead protoplasm may be stained deeply, whilst living protoplasm resists all such contact with colour ; and when we have added that protoplasm can be made to contract under electrical stimulus, and that it coagulates at from 40° to 50° Cent, we shall have completed our examination of its readily observed properties. Let us now turn to consider some of its living aspects and characters. The low-life deeps which it is the province of the micro- scope to explore, present us with a suitable starting-point for our inquiries ; and the stagnant pool, or decomposing infu- sion, maybe made to render from their un- savoury depths the means for scientific sweetness and light. Wandering, in its own erratic fashion, ever in search of fields and pastures new, stumbling over the fragments of weed that lie in its miniature path, and presenting us with a substance which may be paradoxically described as exhibiting every con- FIG. 20.— AMCEDA. COA'CEAKING PROTOPLASM. 65 ceivable form, or as possessing no definite shape at all, we see the animalcule known as the Amoeba (Fig. 20) — a form which has had the honourable distinction of providing a most typical illustration of lower life for all stages of biological teaching and instruction. The name Amoeba signifies change. Of old, the being in question, drawn from the stagnant drop, and placed under the object-glass of our micro- scope, was named the " Proteus animalcule ; " and its more modern cognomen testifies to the same characteristics of alteration and change described by the Protean simile of former days. A mere microscopic speck is the being before us, its size being measurable only in the hun- dredths of an inch. It will require some diligent looking ere its trans- parent body be clearly discerned. For it seems now and then to merge into the water amid which it lives and moves, and appears frequently to fade away into physical nothingness, just as in the sense of its vitality it may be said to hover on the verge of existence itself. When the eye lights upon the Amceba, and becomes accustomed to the dim outlines it exhibits, we are enabled likewise to note the prevailing characteristic of the animalcule in the continual tendency to well-marked physical change and contraction which its body exhibits. At no one period can it be described as exactly resem- bling its look or appearance at any previous stage of existence. Each moment brings new changes of shape (Fig. 20, b) and transmuta- tions of outline. Now it has launched forth its soft body in one direction until it appears in a long-drawn-out line ; now it has drawn this same body forwards, and has protruded its soft substance on each side into so many processes, that it resembles some solitary island with capes, headlands, and promontories jutting out in a sea of its own. We note an animalcule, of, it may be, higher organisation than itself, to approach the Amoeba. There is a momentary contact of the foreign body with the soft protoplasm of the Amceba, and instantly the latter extends its frame outwards so as to encompass the living particle, which is shortly engulfed within the contractile mass. Protoplasm is thus seen to live on protoplasm — a procedure which, by the way, in higher animal life is exactly repeated and imitated in its essential details. By this process of surrounding and enclosing its food-particles within its body, our Amceba obtains its nutriment; and one may well imagine the horror which the appearance of this gelati- nous monster, engulfing, like some formless octopus, all that came in its way, would excite in lower life, were the processes of thought and thinking extant among the animalcular worlds. Thus, also, we see how the Amceba, like so many of its near neighbours, nourishes itself in the absence of a mouth and digestive system ; feels, whilst it wants even the first beginnings of nerves ; and moves, despite the fact that no organs of motion are developed. Watch the food-particle that has just been enclosed within the soft frame, and in due time you F 66 CHAPTERS ON EVOLUTION. may perceive a little space to surround it, as if the particle were being separated from the surrounding protoplasm. Soon, the particle, if digestible at all, will disappear through the solution of its substance ; and you will see it no more, save for the little space that remains awhile to mark the place where the work of digestion was carried on. Thus the process of nutrition is subserved by any part of the interior of the animalcule's frame, just as, through any part of the body, the food, in the absence of a mouth, may be ingested and received. Nor is it less important to note how the simple acts of sensation in the Amoeba are performed similarly by means which appear all inadequate for their performance. That which distinguishes the animalcule most conclusively from the great majority of its plant-neighbours,is this power of receiving sensations and of acting upon them. But for this power, the animalcule would be essentially in the position of an inorganic or lifeless mass. A solid particle floating about in the miniature sea which contains the Amoeba and its neighbours, impinges upon the soft protoplasm of its body. Upon such a stimulus, the protoplasm, as we have seen, contracts, and the food-particle is duly surrounded and engulfed by the living mass of the animalcule. It may truly be affirmed that the first nervous acts are strictly utilitarian in their nature. Their use and purport is that of enabling the animalcule to obtain its food. Sensation is thus unquestionably present in this low form of animal life. Indeed, there are few, if any, naturalists who would not assent to the statement that an Amoeba, lowly organised as it is, is more highly sensitive than a tapeworm possessing an organisation of some complexity — or a Sacculina, which attaches itself to the bodies of crabs, and whose only sign of life consists in the slow pulsations of its bag-like body. But this power of receiving sensations is not the only likeness which the Amoeba, in respect of its innervation, exhibits to higher animal life. Its protoplasm not only receives sensations ; it is also able to act upon information received. The mere contact of the food-particle with the protoplasmic body, is but the prelude to the active contractions of its mass, which are directed towards the seizure of nutriment. And thus we become aware of the fact that not only is this power of " contractility," or of acting upon sensations received, the distinctive property of protoplasm, but that in such a power the actions of higher life are closely imitated. The nervous phenomena which, when occurring in higher existence, are collectively named " reflex action," are essentially of a kind similar to those acts which we see taking place in a body composed of a speck of protoplasm. There is the closest parallelism between our acts of withdrawing our head from a blow, or of closing our eyelids from the same cause, and the action of the animalcule in ingesting its food. Both higher and lower organisms " experience " a sensation, and are capable of acting CONCERNING PROTOPLASM. 67 upon it. The real difference exists in the complexity of the mechanism which responds, and not in the manner in which the stimulus is received or the corresponding act performed. Summing up the facts which a study of the Amoeba has elicited, we learn, firstly, that a minute speck of the sensitive living matter we term " protoplasm" may of itself constitute a living being, capable per- fectly of maintaining its existence and its relations with the external world, and presenting in its life-history many striking analogies with life in its higher and more complicated developments. We next see simplicity of structure united to a complex physiology or way of life ; and we learn that, even in its simplest and most primitive condition, this " protoplasm " of ours may present us, in the endeavour to explain its actions and behaviour, with problems whose solution is practically the despair of many minds amongst us. If it puzzles such minds to see the connection between the molecular stirrage of the human brain-cells and consciousness, the question, " How does a sensation received by the soft protoplasm of an Amoeba become converted into contraction of that body ? " must be regarded as equally unanswerable. Nay, we may go further, and affirm that the difficulty of reply arises primarily because of the identity of the two problems. As we shall presently see, both questions involve like considerations ; both deal with states of protoplasm ; both consider the problem of protoplasmic molecules and their movements as re- lated to actions and motions, which exhibit in higher life the addendum termed " consciousness " — although whether the latter term may not, after all, be simply a name implying another phase of protoplasmic motion is a suggestion worth our consideration. Suffice it to say, however, that, as yet, there is as much mystery involved in the explanation of the movements of an Amoeba as in the molecular play of the brain-cells of a man. And although the admission may furnish considerations which inveigh against the theory of the evolu- tion of the higher mind from the lower sensations, the argument is two-edged after all. If so much that is inexplicable, and apparently complex, exists within the narrow compass of the animalcule's irrita- bility, it may be reasonably said that, of all things, it were most foolish to deny the possibility of these as yet unknown beginnings of nerve- force having been the forerunners of brain and mind. Eliminate these beginnings from view, indeed, and you will find it hard on any save a theory of special and independent creation, to account for the origin of the mental powers which successively mark the higher animal and the man. We have, however, been studying but one phase of protoplasmic existence, and as such, our knowledge can afford us but little aid towards the consideration of the wider part which this substance plays in the phenomena of both animal and vegetable existence. F 2 68 CHAPTERS ON EVOLUTION. Selecting the field of plant-life for our next essay on the powers and nature of protoplasm, we find in this particular region abundant proof that the peculiarities of protoplasm are in nowise affected by its forming part of the plant-regime. Suppose we study under the microscope the nature of the protoplasm which is locked up within the " cells " of such plant-organisms as Chara, Tradescantia (Fig. 21), andVallisneria, or within the cells comprising the stinging hair of the nettle's leaf. We may readily see that active and incessant motion is the attribute of the imprisoned living matter of the plant-cells. Ceaseless currents of particles agitate the plant-protoplasm, which, but for the insidious operation of " osmosis," whereby fluids pass in and out of the cells, would seem to be literally shut out from all participation in outward or external affairs. The cell of the leaf-hair of Tradescantia (Fig. 21), for instance, exhibits an incessant flow of protoplasmic granules hurrying steadily in definite direc- tions, like the ordered traffic in the streets of a great city. Stream of protoplasmic currents unites FIG. 21.^11 of a plant, (7Vo^««/zvo, drawn at intervals, and showing changes in with Stream, and CeaseleSS the contained protoplasm. Qf ^ CQntents of the CQntents is the result. In the nettle-hair the same phenomenon meets the gaze of the microscopist. Here we find the same protoplasmic substance lining the woody matter that forms the external wall of the cell. Constantly does this living lining alter and change its shape with wave-like contractions of its substance, and the granules which exist in the fluid contents of the cell, hurry in various directions with the same activity that we remarked in the cell of Tradescantia. We thus awaken to the fact that in the seemingly inert and uncon- scious field of plant-life there is activity enough, if we may but fortify our seeing powers with the microscope, and peer awhile into the inner recesses, and into the nooks and crannies of the vegetable world. Nor may we neglect to note in passing that, upon some higher development of this same protoplasmic sensitiveness and activity than is usual and common in vegetables, the marked powers of sensation of such plants as the Venus's fly-trap and the Sensitive Plants must depend. Locked up within the hard cell-wall, which, as a rule, it is the business of plant-growth as distinguished from animal increase to develop, there is little wonder that we have come to regard the plant as an organism which feels not, and which is apparently as destitute of all sensation as the world of inorganic things. But the deeper view of plant-existence shows us the fallacy of the common notion regarding the non-sensitiveness CONCERNING PROTOPLASM. 69 of plants. Their protoplasm is as highly contractile under stimulus as is that of the animal. Conceive of a vegetable cell being rup- tured— as, indeed, takes place in certain phases of lower plant-life — and we should find escaping therefrom protoplasm as active as that of our Amoeba, and which, indeed, would comport itself in an exactly similar fashion to that animalcule. Consider, for instance, what takes place in the multiplication of the lower plant-life that forms " the green mantle of the stagnant pool." Here, in due season, the proto- plasm, found in the interior of the cells of which these green Conferva of the stagnant pool are composed, will break up into minute parti- cles, which are duly discharged from custody by the rupture of the cell-wall that formerly imprisoned them. These minute bodies, thus liberated, are named " zoospores." They flit about in the water, and exhibit as free and active an existence as the animalcules which disport themselves side by side with these plant-germs; and they like- wise exhibit an identity of protoplasmic composition with the lower animals that people the stagnant depths. After a period spent in this active existence, the zoospores settle down and grow each into a new plant resembling that from which it sprang. Or, mayhap, meeting with a fellow-spore, a more intricate relationship may be induced ; a third and new body may be produced as the result of this contact ; and from this new body — foreshadowing the "seed" of the higher plant — the adult Conferva will in due time grow. Thus we find that, in addition to the resemblance between the protoplasm of the animal and that of the plant in respect of appearance and composition, there exists a closer likeness still in the common movements which proto- plasm, whether derived from the animal or the vegetable, exhibits. It is not necessary that we should dwell upon other examples of the marked irritability of protoplasm in lower plant-life to demon- strate the community of phenomena which this substance is every- where seen to exhibit in its simple and primitive condition. The life-history of the commonest seaweed that fringes the rocks, would show phenomena of similar kind, and would convince us that power of motion, by common consent the exclusive right and property of the animal, is rather to be viewed as a quality of the protoplasm which forms the living parts of both series of organisms. For, like many of its lower neighbours, the seaweed begins its existence as a minute speck of protoplasm that possesses from nature a roving commission, and swims about freely in its native waters by means of cilia, or fila- ments, resembling those by which the animalcules propel themselves. Ultimately this roving life is abandoned for the stay-at-home exist- ence of the mature seaweed, which in due course arises by cell- growth and protoplasmic multiplication from the once active "spore." Whether studied in the lower animal or in the plant, protoplasm is thus seen to possess essentially the same qualities and properties 70 CHAPTERS ON EVOLUTION. which everywhere and primarily distinguish it as living matter. It remains to be seen whether the examination of higher animal life will destroy the analogies and similarities which are so plainly apparent in the lower confines of the kingdom of living nature. In its complex entirety, the body of a man appears to present us with no features of structural kind which can serve in the least degree to approximate the higher type to lower forms and types of life. Organ and parts in systems and series more or less compli- cated, constitute the framework of the body, whose physiology or functional activity is in turn of a correspondingly intricate character. The simplest tissue of man's frame would, at first sight, appear to present a complexity defying reconciliation with any simpler phase of structure or life. What seems true of the human type may be held to be equally correct when applied to the case of much lower animals, which appear to be far enough removed in their own way from the primitive simplicity of the protoplasmic Amosba and its allies. A snail or a worm, at first sight, appears, in fact, to be as distant from the protoplasmic and primitive stage of organisation as man him- self, in that each is built up of organs exhibiting a complicated structure and highly specialised arrangement of parts. In such a case, what are the likenesses or differences between the higher and lower organisms which the scientific examination of the complex frame reveals? Let anatomy and physiology together furnish the reply. The microscopic anatomy of the tissues of which man's body con- sists, reveals to us a fundamental unity of organisation, which is both striking and important in all its particulars and aspects. Every primer of physiology teaches us the lesson that man's body, like the FIG. aa.— VARIOUS CELLS : a, diagram of a cell ; b, fat cells ; c, d, nerve-cells ; /, cartilage-cells ; /, pigment-cell ; g, a plant-cell ; A, liver-cells ; i, cartilage-cell. frames of all other animals above the rank of the Amceba and its nearest kith and kin, consists of definite layers of minute " cells " (Fig. 2 2), grouped together to form the definite " tissues " of the body. When CONCERNING PROTOPLASM. 71 we speak of the skin, for instance, we are merely indicating a layer of microscopic cells. When we speak of brain tissue we are again dis- coursing of cells (Fig. 22, f, (f) ; and bone itself, in its essential and living parts, is a true cellular tissue. In the human body, it is true, there are muscular fibres, nerve fibres, tendon fibres, and other structures of like nature ; but the physiologist points out that the presence of these latter elements does not invalidate his previous statement concerning the universal cellular composition of the frame. For the body at first consists entirely of cells, and most of the fibres of the body — as, for example, the fibres of muscle by means of which we move, or those of the crystalline lens of the eye — can be shown to be formed directly from cells by the elongation or modifica- tion of the latter ; whilst the growth and renewal of all fibres take place through the production of new cell-elements. It may be assumed as an axiom of physiology that the bodies of all animals, man included, are formed of cells, which become differentiated to form cellular tissues in the one case, or still further modified to form fibres in the other. Such information, all-important as it undoubtedly is, leaves us, however, on the mere confines of our physiological and anatomical study of the higher frame. To understand clearly the relations of the primitive protoplasmic animalcule with the " lord of creation " himself, it is needful to pay a little attention to some further details of microscopical study. Suppose that we examine under the microscope a transverse section of bone. In such research we shall assuredly light upon some facts of interest as assisting our compre- hension of the true typical structure of the most complicated organism in nature. A cross section of bone shows us that the apparently solid tissue is everywhere perforated by the minute " canals " to which Clopton Havers gave his name, and which contain and protect the blood-vessels that nourish the bone. Each Haversian canal of bone is seen to be surrounded by concentric circles of bony matter. When these circles are minutely examined, each is found to consist of elongated spaces, called " lacunse," placed at irregular intervals, and which communicate with each other by minute processes called " canaliculi." Imagine a central lake to be surrounded by circles of smaller lakes, the latter communicating with each other by a complex series of branching rivers, and a fair idea will be gained respect- ing the arrangement of the minute elements of a bone. In a living bone the disposition of parts is not altered from that dis- closed in its microscopic section. The blood-vessels ministering to the nutrition of the bone traverse the Haversian canals already mentioned. Each " lacuna " or lake, however, is occupied by a minute mass of protoplasm, which in all essential respects might be compared to an Amoeba; and the protoplasm of one lacuna sends out minute pro- 72 .CHAPTERS ON EVOLUTION. cesses of its substance along the communicating channels already alluded to, and thus communicates with the living matter of the neighbouring spaces. So that, could we obtain a perfect view of the living protoplasm of a bone, we should find f.hat, when removed from the lacunae, these living parts would appear before us somewhat like a spider's web, and as a connected series of Amceba-like masses of protoplasm, adhering together by the minute processes just described, and roughly reproducing for us the form and outline of the bone. These masses of protoplasm are the " cells " of the bone on which depend the life, nourishment, and general welfare of that structure. We thus learn the curious fact that the most solid and enduring tissue of our body, in its essential nature, represents a collection of Amceba- like masses of protoplasm absolutely undistinguishable, be it also remarked, in nature from the similar matter which moves and gropes in the gutters of our housetops or in the stagnant pools. As the plant-cell imprisons its protoplasm within a thick cell-wall, so our bone-cells in like manner form our skeleton by their special manner of growth and development And it requires no great depth of thought to perceive the similarity of the elements of the human tissue to those which constitute the essentials of lower life at large. Not less striking are the revelations which research into the funda- mental structure of the nervous system displays. Nerve-cells (Fig. 22, t,d)a.nd nerve-fibres together comprise the body's telegraph system; the fibres of nerves being primitively formed like other fibres of the body from cells. The nerve-cell has come to be fully recognised as that part of the nervous mechanism which produces and evolves nerve-force — that subtlest of life's forces, now seen to be represented in the movement of a limb, and now in the impassioned utterances of mind. The nerve-fibre simply carries and distributes the nerve-force gene- rated by the cells, but possesses on its own account no power of evolving the characteristic force that in varied fashions rules the wide universe of human life and of lower existence as well. When the structure of the brain and spinal cord, as the two chief nerve- centres of the body, is examined, both cells and fibres are found to enter into their composition; but the cells alone exist in those parts — such as the grey or external layer of the brain — in which nerve-force is evolved. Nerve-cells vary in size and shape. They may be simple (Fig. 22, d) or complex (c] in form, and range from the round or spherical to the branched and irregular in form. Some of the " multipolar " (c) nerve-cells — as those possessing a plurality of pro- cesses are named — might well enough suggest to the imaginative mind a resemblance to Amoeba in shape, as they of a certainty are related to that animalcule in the protoplasmic nature of their contents and structure. For the essential element in the nerve-cell is protoplasm, pure and simple ; undistinguishable in its chemistry and histology CONCERNING PROTOPLASM. 73 from the substance which we discern in the animalcule or in the bone-cell. Whatever mental powers are exhibited by man, or by animals which possess a brain or nerve-centres of any kind, are the direct products of the nerve- energy stowed up within the cells of the nerve-centres ; and, as we have seen, protoplasm constitutes the essential materies of these cells. That differences of function, wide and apparent, exist between the protoplasm of the bone-cell and that of the nerve-cell need not be even alluded to as a fact of primary significance when considering the physiology of these varied organs. But sufficient for our present purpose is the still broader fact which demonstrates the community of protoplasm as the one living essential of the human frame, whether concerned in the work of forming bone, secreting bile, producing movement, or evolving thought. Thus it remains a stable fact of human existence that on the special qualities and properties of the protoplasm or living contents of cells, depend all the actions and the total activity and individuality of our lives. It is by means of protoplasm that the cells of the liver secrete bile ; it is through the properties of protoplasm producing new cells that a scratch heals, or other breach of bodily continuity is repaired ; and it is by means of a peculiar functional development of this same substance that we are enabled " to lay the flattering unction to our souls" that we are the possessors of mind, intelligence, and will. It might also be shown, as one of the most curious facts of physi- ology, that we harbour in our arteries and veins thousands of proto- plasmic specks which, when viewed under the microscope, behave as do veritable Amoebae. Such are the " white corpuscles " of the blood, which may be seen to undergo mutations of form strictly comparable to the changes of shape that give to the Amceba its characteristic aspect, and which alterations, from this resemblance, have been named " amoeboid " by the physiologist. Enough has already been said of the structural composition of the human body to show that it derives its living activity from the protoplasm which is everywhere scattered throughout its tissues, and which represents the typical living centre of each cell or tissue in which it occurs. But the case for the univer- sality of protoplasm, as the true and only medium by which life is exhibited, increases in importance when the early outlines and fore- casts of development are even briefly chronicled. The nearer we approach the primitive condition of living organisms, the more apparent does the similarity between the earliest stages of all organisms become. An Amoeba gives origin to new animalcules by simply dividing its body in two, when each half swims away as an independent being, to begin life on its own account. Here, there is an absolute and necessary identity of substance between the pro- ducer and the produced. But even in higher grades of life, where the process of development is by no means so simply carried out as 74 CHAPTERS ON EVOLUTION. in Amoeba, there is a wonderful similarity between the individual germs of higher animals, as well as between such germs and the adult and permanent stages of animalcular life. No anatomist could ven- ture, for instance, to express an opinion as to the identity of the germs of the highest class of animals. A protoplasmic germ, pre- senting essentially the same structure and appearance as that of the dog and sheep, gives origin to man himself ; and the stages of develop- ment which evolve the one are strictly comparable in all save the very latest to those that produce the other. Thus man arises from a germ of protoplasm measuring about the one-hundred-and-twentieth part of an inch in diameter, the material substance of which cannot be distinguished by any microscopic or chemical tests from that which is destined to give origin to his canine friend, or from that of which the shapeless frame of the Amoeba is composed. Indeed, the eggs and germs of many animals are strictly Amoeba-like in their nature and motions. The germ of a sponge creeps about within the parent organism in a fashion undistinguishable from the familiar animalcule ; and there are zoophytes and other animals whose eggs exhibit the same exact Amoeba-like appearance which man's own white blood-corpuscles evince. It is thus a plain fact that whatever complexities of body or of mind we find exhibited in the animal world, arise from like matter and similar substance. That man, equally with the monad and the Conferva, owes his origin to a proto- plasmic germ/ in which are contained all the potentialities and pos- sibilities of his after-development, is no piece of scientific romance, but demonstrable truth. Protoplasm begins our life, as it continues that existence for us ; and in this respect the Amoeba may be re- garded as the type of all living things, or, like the famous freebooter of the ballad, as veritable " lord of all" that lives. The universality of protoplasm as the basis of life may be held as fully proved. Apart from the presence of this substance, life is unknown to exist It is seen constituting the essential living parts of animals and plants, from lowest to highest. Whale and animal- cule, triton and minnow, tjie giant pine and the lichen, each and all owe to protoplasm their primary vitality and the powers which mark their varied lives. As Dr. Allman puts it, in an address to the British Association, " we are thus led to the conception of an essential unity in the two great kingdoms of organic nature — a struc- tural unity in the fact that every living being has protoplasm as the essential matter of every living element of its structure, and a physio- logical unity in the universal attribute of irritability which has its seat in this same protoplasm, and is the prime mover in every phenomenon of life. We have seen," continues Dr. Allman, " how little mere form has to do with the essential properties of protoplasm. This may shape itself into cells, and the cells may combine into organs in CONCERNING PROTOPLASM. 75 ever- in creasing complexity, and protoplasm -force may thus be in- tensified, and, by the mechanism of organisation, turned to the best possible account ; but we must still go back to protoplasm as a naked, formless plasma, if we would find, freed from all non-essential complications, the agent to which has been assigned the duty of build- ing up structure, and of transforming the energy of lifeless matter into that of living." How much nearer to the great question of the origin and nature of life do such considerations lead us? is a justifiable query which faces us at the close of these inquiries, as it formed the key-note with which we began our brief study of the mystery of living and being. It cannot be doubted that the research of recent years has at least brought us nearer to our real difficulties than before. It counts for something in a subject like the present that even the boundaries of our knowledge and the environments of our ignorance should be clearly perceived ; and this much, at least, the inquiries concern- ing protoplasm have accomplished. We now know that at last we are face to face with the final stage in the question before us — that the puzzles of protoplasm collectively constitute the one mystery of life. To such a decision, every fact of recent research seems to lead. The knowledge that there is not one life of the animal and another existence of the plant, but that both lives are really similar in their essential manifestations, is one fact which leads us directly to regard proto- plasm and its constitution as the repositories of the secret of life's nature. One consideration which merits special remark in con' nection with the subject of protoplasm and its relations to life exists in the apparent truism that all forms of protoplasm, how- ever alike in appearance and composition science may and does declare them to be, are not identical in their potentialities. They do not, in other words, all possess similar powers of becoming similar organisms. The speck which remains an Amceba, has no power of evolving from its substance a higher form of life. The protoplasmic spore of a seaweed is a seaweed still, despite its similarity to other or higher forms of plant-germs. The germ of the sponge, again, remains possessed of the powers which can convert it into a sponge alone. And the differences between such protoplasmic specks and the germ which is destined to evolve the human frame can only be declared as of immense extent, and as equalling in their nature the wide structural and functional distinctions we draw betwixt the sponge and the man. Of such differences in the inherent nature of protoplasm under different conditions we are as yet in complete ignorance. Their elucidation is really the explanation of heredity or the law of likeness. The mystery why family face and features, along with even habits and gestures, should be rigidly and perfectly transmitted from parent to offspring, really includes the puzzle which 76 CHAPTERS ON EVOLUTION. besets the real differences between one speck of protoplasm and another and apparently similar speck. But our want of knowledge of such points may not leave untouched the primary question concerning the nature of life, to which all the pro- perties and qualities of protoplasm, all the varied forms and faces of living beings, are due. On the contrary, it is possible by analogy to arrive at some broad views concerning the nature of life at large, and to such considerations we may now shortly attend. Physiology points out to us that the properties of protoplasm and all its powers of being and becoming are resident within its own substance, and are dependent upon the energy of which it is the seat Supply appropriate conditions, and the forces of the protoplasm will convert the primitive germ into the form of its progenitor. There is a transformation of force and matter of one kind, into force and matter of another kind therein involved Such facts point to material powers and forces resident in, and peculiar to, protoplasm as the prime movers of the changes and developments that substance undergoes. As clearly, too, does the transmission of parental likeness from generation to generation argue for the existence of some material and physical basis for the carriage, by the protoplasm-germ, of the features of the species. If s'o much be admitted, it seems illogical to deny that whatever properties the protoplasm of germ or adult exhibits depend, strictly speaking, upon the chemical and physical properties of that substance. Thus we approach the idea that this mysterious " life," which no one has yet successfully defined — for the plain reason that the terms of the definition are unknown — simply represents the sum- total of the energies of the physical, chemical, and other properties of protoplasm. Nowhere do we find life dissociated from protoplasm; and this fact alone argues in favour of the view that the " vital force " of the scientist or the " vital spark " of the poet, is in each case merely the convenient and summary expression of that high form of energy, which corresponds to no one force in nature, but to all combined. If this hypothesis be deemed essentially materialistic — as un- questionably it will be from certain points of view — its supporters still possess a distinct coign of vantage in a simple and logical appeal to the facts and phenomena of nature and life as they stand. In addition to the pregnant fact just mentioned, namely, that life requires for its exhibition a material basis seen in protoplasm, the mere considerations that this substance is composed of no unknown elements, but of well-defined and common substances, and that its composition is not ethereal but material, support the view that life is no mysterious aura, but a collocation of the forces and energies and of the material substances which make protoplasm. Life is a property of protoplasm — such is the latest product of scientific thought and research. The forces which make protoplasm are regarded as those CONCERNING PROTOPLASM. 77 which make life ; and although the exact relationship of these forces is as yet unknown, analogy leads us to believe that they are not materially different, if they are different at all, from those which have made the world of inorganic matter what it is. It is analogy, too, which reminds us that certain forces produce, under combination, very different results from those which they exert when acting in separate array. The relationship and correlation of the physical forces not merely teems with examples of such results, but leads us to think of the possibility and probability that life remains a mystery to us simply because the terms under which its component forces are combined are as yet unknown. In any case, we require to postulate a " life- force " of one kind or another. It remains for us to choose between the " vital force " of former decades of biology — a term committing itself to no explanation of vital phenomena whatever — and the idea that in the properties of protoplasm — derived whence and how we, as yet, know not — we find the true nature of life. But analogy rests not here. An extension of thoughts like the foregoing leads us towards the world of inorganic matter with the view of inquiring whether there exist any links or connections between that lifeless universe and the living world which claims protoplasm as its universal substratum. The forces which act upon the lifeless world are those which also affect animals and plants ; but the latter are enabled to resist, alter, and modify the action of these forces in greater or less degree, whilst lifeless matter exists and is acted upon without response. Otherwise, however, the phenomena of the inorganic world, despite their sharp demarcation from the phases of life, may be regarded as presenting us with many facts of origin as inexplicable as those exhibited by living beings. It has well been remarked that the growth of the crystal, taking place in virtue of physical laws, to attain an exact and unvarying form, is as mysterious as the growth of the tree ; and that common salt should crystallise in the form of the cube is as profound a mystery as that an acorn should become an oak, or another protoplasmic germ evolve the human form. If we are to assume that the forces which rule the world of life are inexplicable simply because they are living forces, it might equally well be maintained that the inorganic world and its ways should be the subjects of similar mysticism. Far more rational, because more likely to be true, are the ideas which lead us to note in the living world the highest term to which matter may attain. As the living world is dependent on the non-living for its support, as we are both in the earth and of the earth, so may we conceive that the forces which mould the world, which disperse the waters and rule the clouds, have contributed in their highest mani- festations to combine matter into its most subtle combinations in the form of the animal and in the guise of the plant. Huxley's words 78 CHAPTERS ON EVOLUTION. are worth weighing when he says : " It must not be supposed that the differences between living and non-living matter are such as to bear out the assumption that the forces at work in the one are different from those which are to be met with in the other. Con- sidered apart from the phenomena of consciousness, the phenomena of life are all dependent upon the working of the same physical and chemical forces as those which are active in the rest of the world. It may be convenient to use the terms 'vitality' and 'vital force ' to denote the causes of certain great groups of natural operations, as we employ the names of ' electricity ' and ' electrical force ' to denote others ; but it ceases to be proper to do so if such a name implies the absurd assumption that either ' electricity ' or ' vitality ' is an entity playing the part of an efficient cause of electrical or vital phenomena. A mass of living protoplasm is simply a molecular machine of great complexity, the total results of the working of which, or its vital phenomena, depend, on the one hand, upon its construction, and on the other upon the energy supplied to it ; and to speak of ' vitality ' as anything but the name of a series of operations, is as if one should talk of the 'horologity' of a clock." Although research has not placed the puzzle of life and its solution at our feet, our inquiries have at least served to indicate the direc- tion towards which modern scientific faith is slowly but surely tending. The search after a material cause for phenomena, formerly regarded as thoroughly occult or supernatural in origin, is not a feature limited to life-science alone. Such a characteristic of modern research in- dicates with sufficient clearness the fact that, as biology and physics become more intimately connected, the explanations of the phe- nomena of life will rest more and more firmly upon a purely physical and appreciable basis. That life has had a distinct beginning upon this earth's surface is proved by astronomical and geological de- ductions. That life appeared on this world's surface not in its present fulness, but in an order leading from simple forms to those of an ever-increasing complexity, is an inference which geology proves, and which the study of animal and plant development fully sup- ports. That the first traces of life existed in the form of protoplasmic germs, represented to-day by the lowest of animal and plant forms — or rather by those organisms occupying the debatable territory between the animal and plant worlds — is well-nigh as warrantable a supposition as any of the preceding. And last of all, that these first traces of protoplasm were formed by the intercalation of new combinations of the matter and force already and previously existing in the universe, is no mere unsupported speculation, but one to which chemistry and physics lend a willing countenance. Living beings depend on the outer world for the means of subsistence to-day. Is it more wonderful or less logical to conceive that, at the beginning, CONCERNING PROTOPLASM. 79 the living worlds derived their substance and their energy wholly from the same source ? The affirmative answer seems to be that which science tends to supply, with the qualification that, once intro- duced into the universe, living matter is capable of indefinite self-re- production, without necessitating any appeal for aid, by way of fresh " creation " of protoplasm, to the inorganic world. As Dr. Allman has remarked, it is certain "that every living creature, from the simplest dweller on the confines of organisation up to the mightiest and most complex organism, has its origin in pre-existent living matter — that the protoplasm of to-day is but the continuation of the protoplasm of other ages, handed down to us through periods of indefinable and indeterminable time." The harmony of these inferences with the doctrine of evolution is manifest. The common origin of animal and •vegetable life, and the further unity of nature involved in the idea that the living worlds are in reality the outcome of the lifeless past, constitute thoughts which leave no break in the harmony of creation. " There is grandeur," to quote Mr. Darwin's words, " in this view of life," which, founded upon scientific research, simply commits its supporters to the wholesome philosophic truth, that the ways of all living beings are ordered in conformity with the great system of natural law, whose operation is seen with equal clearness in the formation, of a world or the falling of a tear. 8o CHAPTERS ON EVOLUTION. V. THE EVIDENCE FROM RUDIMENTARY ORGANS. IN the exercise of his scientific attainments, there is one aspect in which the naturalist of to-day bears a certain likeness to the detective officer. The latter is perpetually endeavouring to " strike the trail " of the offender through his dexterity in the discovery of clues to the movements of the pursued, and attains his end most surely and speedily when the traces he has selected are of trustworthy kind. The naturalist, on his part, has frequently to follow the history of an animal or plant, or it may be that of a single organ or part in either, through a literal maze of difficulties and possibilities. His search after the relationship of an animal may be fraught with as great difficulty as that which attends the discovery of a "missing heir " or lost relative in actual life ; and his success in his mission is found to depend, as does that of the detective's work, simply on the excellence and trustworthiness of the clues he possesses, and on the judicious use to which he puts his " information received." It cannot be denied, however, that modern aspects of science and present-day tendencies in research have largely increased the resem- blance between the enforced duties of the criminal investigator and the self-imposed task of the biologist. When, formerly, the order of nature was regarded as being of unaltering kind and of stable consti- tution, naturalists regarded animals and plants simply as they existed. There was of old no looking into the questions of biology in the light of "what might have been;" because the day was not yet when change and evolution were regarded as representing the true order of the world. When, however, the idea that the universe both of living and non-living matter had an ordered past dawned upon the minds of scientists, the necessity for tracing that past was forced upon them as a bounden duty. With no written history to guide them, the scientific searchers were forced to read the " sermons in stones" which nature had delivered ages ago. Without clear and unmistaken records to point the way, they had to seek for clues and traces to nature's meaning in the structure and development of animals and plants; ajid, as frequently happens in commonplace history, the earnest searcher often found a helping hand where he least thought it might appear, and frequently discovered an important clue in a circumstance or object of the most unlikely kind. RUDIMENTARY ORGANS. 8 1 Readers whose tastes are not materially scientific have doubtless heard much of " missing links " of nature, especially in connection with the gaps which exist between the human territory and ape- land. Indeed, the phrase has come to be understood as applying almost entirely and specifically to the absence of connecting forms between man and the apes — forms for which, in one sense, no necessity exists, inasmuch as Mr. Darwin's theory does not demand that the gorilla or any of his compeers should be directly connected with man. The gorilla with his nearest relation lives, so to speak, at the top of his own branch in the great tree of life, whilst man exists at the top of another higher and entirely different bough. The connection between the human and lower types is made theoretically to exist at some lower part of the stem when, from a common ancestor, the human and ape types took divergent roads and ways towards the ranks of nature's aristocracy. But although, in some cases, the need for " missing links " is seen, even theoretically, to be non-existent, or at least of a widely different nature from that supposed by the popular mind, there are yet cases in which that need is very apparent, and wherein, through the persistent tracing of the clues nature has afforded, the past history of more than one race of animals and plants has been made plain and apparent. Of such clues — which are really mere traces, and nothing more — there are no better examples than the curious fragments of structures found in many animals and plants, and named " rudimentary organs." An animal or plant is thus found to possess a mere trace of an organ or part which, so far as the highest exercise of human judgment may decide, is of not the slightest utility to the being. It is invariable in its presence, and as fixed in its uselessness. It bears no relation to the existing life or wants of the animal, but may in some cases — as, for example, in a certain little rudimentary pocket in man's digestive system, serving as an inconvenient receptacle for plum-stones and like foreign bodies — prove a source of absolute disadvantage or even danger. On what theory can the presence of such organs and parts be accounted for ? is a question of extremely natural kind. The replies at the command of intelligent humanity are but two. Either the animal was created with the useless appendage in question — a supposition which includes the idea that Nature, after all, is somewhat of a bungler, and that nothing further or more comprehensible than the fiat " it is so," can be said on the subject. Or, secondly, we may elect to explain the puzzle by the assertion that the "rudimentary organ " of the existing animal represents a part once fully developed in that animal's remote ancestors, but now Dwindled to the shortest span. The rudimentary organ or appendage is regarded by evolution G 82 CHAPTERS ON EVOLUTION. as being represented in the animal or plant of to-day as a legitimate heritage derived from its ancestor. It is, in short, a family feature, to which the living being is the " rightful heir," but which has fallen, through the operation of natural laws and conditions, into disuse, and has accordingly suffered in the career of living nature " down the ringing grooves of change." Necessarily, this second and rational explanation of the rudimentary appendages of animals and plants is founded on the supposition that Nature and Nature's creatures are continually undergoing alterations, and that they have been modified in the past, as they will be in all time to come. The explanation thus afforded of the nature and origin of these disused parts is endorsed by the fuller knowledge of their history ; whilst, from a study apparently of insignificant interest, may be shown how certain of our living neighbours, along with ourselves, have, from lower states, and from the dawning epochs of the world, literally taken their place "in the foremost files of time." As most persons who have attentively looked at any common plant can tell, four parts are included in a perfect flower. These parts FIG. 23. — STRUCTURE OF WALLFLOWER. or sets of organs, as seen in the wallflower, consist (Fig. 23), firstly, of an outer covering coloured green, and named the " calyx " (ca}. Then comes the blossom or flower itself, forming the "corolla" (co). Inside the corolla we find certain stalked organs, each bearing a little head or " anther," filled with a yellow dust, the " pollen." These organs are the " stamens " (st). Lastly, in the centre of the flower we note the " pistil" (/>), or organ devoted to the production of "ovules." The latter, when duly fertilised by being brought into contact with the "pollen" of the stamens, become " seeds," and are capable of growing up, when planted, into new plants. Now, the botanist will inform us that it is a matter of common experience to find some individual plants of a species with well- developed petals or blossoms, and other individuals of the same species with petals in a rudimentary condition ; thus proving that the production of imperfect parts in flowers occurs as an ordinary RUDIMENTARY ORGANS. event under our own eyes, and under the common conditions of plant life. The natural order of plants to which Snapdragon belongs presents a peculiarity, inasmuch as, in most of its members one of the five stamens is abortive or rudimentary. It should be borne in mind that the botanist possesses a highly interesting and exact method of ascertaining how many parts or organs should be represented in plants. He places his reliance in this respect on the working of what may be called the " law of symmetry." The operation of this law, which may be said to be founded on wide experience, tends to produce a correspondence in numbers between the parts in the four sets of organs of which we have just noted a flower to be composed. Thus, when we count five parts in the green calyx of a plant, we expect to find five blossoms (or petals) in its corolla ; five stamens (or some multiple of five), and five parts (or some multiple of that number) in the pistil. Where there appears to be a lack of this numerical correspondence, the botanist concludes that some violation of the law of symmetry has taken place, and that some parts or organs which should normally have been developed have been altered or suppressed. His reasoning, in fact, proceeds on the plain basis of first establishing, through experience, the normal number and condition of parts in the flower of any given order of plants, and of thereafter accounting by suppression or non-development for the absence of parts he expected to have been represented. Now, in the Snapdragon tribe we find, as a general rule, five parts in the calyx, five petals in the corolla, but only four stamens. Such a condition of matters is well seen in the flower of frog's-mouth (Antirrhinum), where we find four stamens, two being long and two short (Fig. 24, A, sl s*), as the comple- ment of the flower. We account for the absence of a fifth stamen by say- ing it is abortive ; and the rudiment of this missing stamen may also be found in the flower. But a natural reflection arises at this point, in the form of the query, Have we any means of ascertaining if our ex- pectation that a fifth stamen should be developed is rational and well founded? May not the plant, in other words, have been " created FIG. a4.-FLowER OF FROG'S-MOUT*. so " ? Fortunately for science, na- A- Flower of FrogVmouth ; B, Flower of . J , , , . Figwort or Scrophulai ta. ture gives us a clue to the discovery of the truth in this as in many other cases. In one genus of these plants (Scrophularia) we find a rudiment of a fifth stamen (Fig. 24, B s); and in Snapdragon itself this fifth stamen becomes 02 84 CHAPTERS ON EVOLUTION. occasionally fully developed ; whilst another plant of the order ( Verbascum} possesses five stamens as its constant provision. Unless, therefore, we are to maintain that nature is capricious beyond our utmost belief, we are rationally bound to believe that the rudimentary fifth stamen of Scrap hularia, and the absent fifth stamen of other plants of its order, present us with an example of modification and suppression respectively. The now rudimentary stamen is the repre- sentative of an organ once perfect and fully developed in these flowers, and which is perpetuated by the natural law of inheritance until conditions, to be hereafter noticed, shall have caused it to entirely disappear. The case for the natural modification, and that against the imperfect creation of such flowers, is proved by an ingenious ex- periment of Kolreuter's, upon plants which have the stamens and pistils' situated in different plants, instead of being contained in the same flower, as is ordinarily the case. Some "staminate" or stamen- possessing flowers had the merest rudiment of the pistil developed, whilst another set had a well-developed pistil. When these two species were "crossed "in their cultivation, the "hybrids" or mule progeny thus produced, evinced a marked increase in the development of the abortive organ. This experiment not only proved that, under certain conditions, the rudimentary pistil could be improved and bettered, but also confirmed the identity of the two pistils, and the high probability that the abortive organ in the one flower was simply the degraded representative of the well-developed part of the other. As a final example of the manner in which we receive clues towards the explanation of the modifications of flowers, the case of the wallflower is somewhat interesting. This plant and its neigh- bours possess the parts of the flower in fours (Fig. 23, A). There are four sepals and four petals, whilst six stamens (Fig. 23, B) are de- veloped ; the pistil possessing only two parts. Here the law of symmetry would lead us to expect either four stamens or eight — the latter number being a multiple of four. The clue to this modifi- cation is found . in the arrangement of the stamens. We find that four of the wallflower's stamens are long (Fig. 23. B, j/'), whilst two (si2) are short. The four stamens form a regular inner series or circle, the two short stamens being placed, in a somewhat solitary fashion, outside the others. This condition of matters points probably to the suppression of two of an originally complete outer row of four stamens, and we receive a clue concerning the probability of this view by finding that in some other flowers of the wallflower group the stamens may be numerous.1 It is hardly within the scope of the present chapter to say anything regarding the causes of the conditions 1 It is proper to mention that other explanations of the existence of two short outer stamens in Cruciferez are known to botanists. That here given appears, however, to be equally acceptable with more elaborate theories of this condition. RUDIMENTARY ORGANS. 85 or of the agencies through which the modifications of plants are wrought out. Suffice it to remark that the " law of use and disuse " of organs explains the majority of such cases, by asserting that organs become degraded when they are no longer found to be useful to the economy of their possessors. The degradation of a part is to be looked upon as subservient to the welfare of the animal or plant as a whole, and thus comes to be related to the great law of adaptation in nature which practically ordains that — Whatever is, is right. The animal world presents us, however, with more obvious and better marked examples of rudimentary organs than are exhibited by the modifications of flowers — conspicuous as many of these latter instances undoubtedly are. Turning our attention first to lower life, we find amongst insects some notable and instructive illustrations of abortive organs, and also of the ways and means through which the rudimentary conditions have been attained. In the beetle order, the natural or common condition of the wings — which in insects typically number four — is that whereby the first pair becomes converted into hardened wing-cases, beneath which the hinder and useful wings are concealed when at rest. Now, in some species of beetles we may meet with certain individuals with normally developed wings ; whilst in other individuals of the species we find the wings to be represented by the merest rudiments, which lie concealed beneath wing- cases, the latter being actually firmly and permanently united together. In such a case the modification has been extreme, but there can be no doubt that the ancestors of the beetles with modified wings possessed fully developed appendages ; otherwise we must regard the order of nature as being one long string of strange and incoherent paradoxes. Mr. Darwin has given us some instructive hints regarding the modification of beetles' wings and feet in his remarks on the effects of the use and disuse of parts in the animal economy. Kirby, the famous authority on entomology, long ago noted the fact that, in the males of many of the dung-beetles, the front feet were habitually broken off. Mr. Darwin confirms the observation of Kirby, and further says that in one species (Onites apelles) the feet "are so habitually lost, that the insect has been described as not having them." In the sacred beetle (Ateuchus} of the Egyptians the tarsi are not developed at all. Mr. Darwin remarks that necessarily we cannot, as yet, lay overmuch stress upon the transmission of accidental mutilations from parent to progeny, although, indeed, there is nothing improbable in the supposi- tion ; and, moreover, Brown-Se'quard noted that, in the young of guinea-pigs which had been operated upon, the mutilations were reproduced. Epilepsy, artificially produced in these latter animals, is inherited by their progeny. " Hence," says Darwin, " it will perhaps 86 CHAPTERS ON EVOLUTION. be safest to look at the entire absence of the anterior tarsi (or feet) in Ateuchus, and their rudimentary condition in some other genera, not as cases of inherited mutilations, but as due to the effects of long-con- tinued disuse ; for as many dung-feeding beetles are generally found with their tarsi lost, this must happen in early life ; therefore the tarsi cannot be of much importance, or be much used by these insects." The beetles of Madeira present us with a remarkable state of matters, which very typically illustrates how rudimentary wings may have been produced in insects. Two hundred beetles, out of over 500 species known to inhabit Madeira, are " so far deficient in wings that they cannot fly." Of twenty-nine genera confined to the island, twenty-three genera include species wholly unable to wing their way through the air. Now, beetles are frequently observed to perish when blown out to sea ; and the beetles of Madeira lie concealed until the storm ceases. The proportion of wingless beetles is said by Mr. Wollas- tonto be "larger in the exposed Desertas than in Madeira itself ;" whilst most notable is the fact that several extensive groups of beetles which are numerous elsewhere, which fly well, and which " absolutely require the use of their wings," are almost entirely absent from Madeira. How may the absence of wings in the Madeiran beetles be accounted for ? Let Mr. Darwin reply : " Several considerations make me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, combined probably with disuse. For during many successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly developed, or from indolent habit, will have had the best chance of surviving from not being blown out to sea ; and, on the other hand, those beetles which most readily took to flight would oftenest have been blown to sea, and thus destroyed." An instinct of laziness, so to speak, alone, or aided by a shortness of wing, developed stay-at-home habits ; and such habits would necessarily tend towards the survival and increase of wingless forms. Other Madeiran insects — such as butterflies, moths, and flower- feeding beetles — have well-developed wings, or possess wings relatively larger than they exhibit elsewhere. This observa- tion, remarks Mr. Darwin, is quite in consistency with the theory of the law of CRAB. natural selection which favours the survival of the fittest "For when a new insect first arrived on the island, the RUDIMENTARY ORGANS. 87 tendency of natural selection to enlarge or to reduce the wings would depend on whether a greater number of individuals were saved by successfully battling with the winds, or by giving up the attempt, and rarely or never flying." Amongst animals of higher rank in the scale than insects, the presence of rudimentary organs is frequently to be demonstrated. What explanation, other than that of degradation and decay owing to disuse, can be offered of the case of the crabs from the Kentucky Cave ? Crabs possess compound eyes borne at the extremities of highly movable stalks, these stalks in the sentinel crab (Fig. 25) being extremely elongated. In some of the Mammoth Cave crabs, the stalk remains, but the eye has completely disappeared. As the eyes in such a case could in no sense disappear from any reason connected with injury to the animal, we are absolutely without any reason for their absence other than that of disuse. Professor Silliman captured a Cave rat which, despite its blindness, had large lustrous eyes. After an exposure for about a month to carefully regulated light, the animal began to exercise a feeble sense of sight. Here the modification or darkness had simply affected the function of the eye ; in due time the effects of disuse would certainly alter and render abortive the entire organ of sight. The possession of flying powers is so notable a characteristic of the class of birds, that any exception to this rule, and the want of aerial habits, may be rightly regarded as presenting us with a highly anomalous state of matters. Yet instances of rudimentary wings in birds are far from uncommon; and several groups are, in fact, more notable on account of the absence of powers of flight than for any other structural features. The ostrich, for instance, represents a bird the wings of which are mere apologies for organs of flight, and which are used, as every one knows, simply as aerial paddles. The curious Apteryx or Kiwi-kiwi (Fig. 26) of New Zealand, a near relative of the ostriches and running-birds in general, represents a still more degraded condition of the organs of flight, for the wing is reduced in size to an extraordinary degree, and exists in a highly abortive condition ; whilst only one complete finger is represented in the hand — other birds, as a rule, possessing three modified fingers. The logger-headed duck of South America has wings so reduced that it can but " flap along the surface of the water," a condition of matters 88 CHAPTERS ON EVOLUTION. FIG. 27.— PENGUIN. closely imitated amongst ourselves by the Aylesbury duck — although, indeed, the young ducks are able to fly. The wing of the penguin (Fig. 27) is a mere scaly appendage utterly useless for flight, but useful as a veritable fin, enabling it to swim under water with great facility ; and of the auk's wing the same remark holds good. In the birds, then, there is ample evidence of dete- rioration of organs in the rudimentary nature of the wings of many species. How these conditions have been brought about is not difficult to explain in most instances. In New" Zealand, where we find a singular absence of quad- rupeds, wingless birds — many being extinct — of which the Apteryx is a good example, take the place of the four-footed population. In view of an immunity from the attack of other animals, the ground-feeding habits of these birds would become more and more strongly settled as their special way of life ; and in the pursuit of such habits, the wings, seldom used for flight, would- degenerate as time passed. The later advent of man, in turn, has exterminated certain races of the wingless birds — such as the Dodo (Fig. 28) and Solitaire (Fig. 29) in Mauritius and Rodriguez — whilst the wingless and giant Dinornis of New Zealand, and its contemporaries, have probably been hunted to the death of their species, by their human co-tenants of these strange lands. The ascent to the quadrupeds brings in review before us still more striking illustrations of the apparently incomplete rendering of the structures of animal life. No better instance of the " rudimen- tary organs " of the naturalist can be found than in the group of the whales, and more especially in the species from which we obtain the commercial whalebone and oil — the Greenland or Right Whale. This whale possesses no teeth in its adult state, but before birth teeth are found in the gum. These teeth, however, are gradually absorbed, and utterly disappear from the jaws, the adult whale possessing, as is well known, a great double fringe of " whalebone-" plates depending from the palate. The same remark holds good of RUDIMENTARY ORGANS. 89 the unborn young of ruminants, or animals which "chew the cud ; " these animals in their adult state possessing no front teeth in the upper jaw, but in their immature condition developing these organs — which, by the way, never cut the gum — only to lose them by a natural process of absorption. Now, here there can be no question of use ; and certainly no adequate explanation of their occurrence exists, save that which regards these foetal teeth as the remnants of structures once well developed in the ancestors of the whalebone whales and ruminants. To this supposition the evidence — avowedly incomplete — obtained from geology gives no contradiction, even if it does not by any means supply the " missing links " in an adequate fashion. We do know that amongst the oldest of the great leviathans of the past was the Zeuglodon, of Tertiary rocks, which had teeth developed much in excess of anything we find represented in the dental arrangements of the whales of to-day — a creature this, of FIG. 28. — DODO. FIG. 29.— SOLITAIRE. which, as regards its teeth at least, modern whales are but shadowy reproductions. Whilst under the shelter of great authority, we may declare this ancestor of the whale to have been intermediate in nature between the seals and whales, or between the whales and their neighbours the manatees or sea-cows and dugongs. In either case, the intermediate character of the animal argues in favour of its having been the likely parent of a race dentally degraded in these latter days. There is little need to specialise further instances of the occurrence of rudimentary organs in the higher animals, save to remark that not the least interesting feature of such cases is contained in the fact that the milk-glands of male animals amongst quadrupeds — organs which CHAPTERS ON EVOLUTION. FIG. 30. BONES OK MAN'S ARM. exist in a rudimentary condition — have been known to become func- tionally active and to secrete milk; this peculiarity having been known to occur even in the human subject. Amongst the higher quadrupeds, however, there yet remains for extended notice one special instance of the occurrence of " rudimentary organs," wherein not merely is the nature of the parts thoroughly determined, but the stages of their degradation can be clearly traced through the remarkable and fortunate discovery of the " missing links." Moreover, the case in point, that of the horse, so clearly illustrates what is meant by progressive development or evolution of a species of animals, that it is highly instructive, even if regarded from the latter point of view. When we look at the skeleton of a horse's fore- limb, we are able, without much or any previous acquaintance with the facts of comparative anatomy, to see that it is modelled upon a type similar to that of the arm of man. Were we further to com- pare the wing of the bird, the paddle of the whale, the fore-limb of the bat, and the fore -leg of a lizard or frog, with the equine limb, we should find the same fundamental type of structure to be represented in all. Thus we find in the arm of man (Fig. 30) — to select the most familiar example from the series just mentioned — a single bone, the humerus (3), forming the upper arm: two bones (radius [4] and ulna [8] ) constituting the fore-arm : eight small bones forming the wrist (carpus) : five bones — one for each finger — forming the palm or metacarpus : and five fingers, each composed of three small bones, named phalanges, with the exception of the thumb, in which, by a mere inspection of that digit, we may satisfy our- selves only two joints exist In the wing of the bird (Fig. 31) we similarly find an upper-arm bone or humerus (a) : two bones (radius [c\ and ulna \d~\ ) in the fore-arm : a FIG. 31. wrist (b) : a thumb ( g) : and two fingers BONES OF BIRD'S WING. //"//{ Now, turning to the fore-limb of a horse (Fig. 32) — the hind limb being essentially similar in its general conformation, and correspond- ing as closely with man's lower limb — we find its conformation to correspond in a remarkable fashion to that of man's arm. First, RUDIMENTARY ORGANS. 91 there is the humerus (h\ or bone of the horse's upper arm, concealed, however, beneath the skin and muscles, and being, therefore, incon- spicuous in the living animal. The horse's fore-arm, like that of man, contains two bones — radius (r) and ulna (u), it is true ; but the ulna has degenerated in a marked degree, and exists as a mere strip of bone which is tolerably distinct at its upper end, but unites with and merges into the other bone, the well-developed radius. The wrist (w) of the horse naturally succeeds its fore- arm, but from the fact of the upper arm being concealed beneath the skin and muscles, the wrist is not usually recognised as such. Thus, when a horse chips its "knee," it, in reality, suffers a contusion of its wrist. Man possesses eight bones in his wrist; the horse has only seven, but the equine wrist is readily recognisable as cor- responding with the similar region in man. The greatest difference between the human limb and that of the horse is found in the regions which succeed the wrist, and which constitute the palm and hand. Man has five palm-bones : the horse has apparently but one long bone, the "cannon bone" (a*1^ in place of the five. Now, to which of man's palm-bones does this " cannon bone " corre- spond ? The anatomist replies, " To that supporting the third or middle finger ; " and attached to this single great palm-bone the horse has three joints or " phalanges " (i, 2, 3) composing his third finger. These joints are well known in ordinary life as the " pastern," " coronary," and " coffin bones : " and the last bears the greatly developed nail we call the " hoof." Thus the horse walks upon a single finger or digit — the third ; and it behoves us to ask what has become of the remaining four — five being the highest number of fingers and toes found in mammals or quadrupeds. We find that, with the exception of other two — the second and fourth fingers — the horse's digits have completely disappeared. The second and fourth fingers have left mere traces, it is true, but it is exactly these rudimentary fingers which serve as the chief clues to the whole history of the equine race. On each side of the single palm-bone (ml) of the horse's great finger we see two thin strips of bone (one of which is FIG. 32. BONES OF HORSE'S FORE-LIMB. CHAPTERS ON EVOLUTION. represented at m*, Fig. 32), which veterinary surgeons familiarly term " splint-bones." (See also Fig. 34, A, 2> 3) — supported by a single cannon bone (0*/1); and there are likewise two splint- bones (one depicted at mi* ), representing the rudiments of the second and fourth toes. The horse's heel (A), like his wrist, appears out of place, and is popularly named his "hock." The shin-bone (/) is the chief bone of the leg, and has united to it the other bone (_/£), succeeding the thigh, named the fibula, and which is seen in man's leg, and in that of quadrupeds at large. To the eyes even of an unscientific observer, who sees the skeleton of a horse placed in a OF HORSB. museum, in contrast with the bony frames of other and nearly related animals, the equine type is admittedly a very peculiar and much modified one. In- place of five toes we find but one ; and in the matter of its teeth, as well as in other features of its frame, the horse may be said to present us with an animal form which appears as a literal example of Salanio's remark, that— Nature hath framed strange fellows in her time. A person of a thoroughly sceptical turn of mind might possibly demand to know the exact reasons for the assumption that the splint- \rntt FIG. 33. SKELETON OF HIND LIMB RUDIMENTARY ORGANS. 93 bones of the horse are in reality the rudiments of the fingers we have represented them to be, and might further demand proof positive of their nature. Fortunately, geology and the science of fossils together come to our aid, with as brilliant a demonstration of the steps and stages of the degradation of the horse's fingers as the most sanguine evolutionist could hope to see. From Mother Earth, whose kindly shelter has sufficed to preserve for us the remains of so many of the forms of the past, we obtain the means for constructing a genealogical tree of the equine race, by methods of certain kind, and through the exhibition of fossils, each bearing an impress of its history, which, to use Cuvier's expression, " is a surer mark than all those of Zadig." Our theoretical journey backwards into the ages begins with the Recent or last-formed deposits — those which lie nearest the outer surface of our earth. The Recent or Quaternary period forms a division of the Tertiary period — that is to say, the latest of the three great epochs into which, for purposes of classifying fossil forms by their relative ages, the geologist divides the rock- formations. The Tertiary rocks, commencing the list, with the last- formed or uppermost strata, begin with the Quaternary or Recent deposits ; next in order succeed the older Pliocene rocks ; then come the Miocene formations, and lastly succeed the Eocene rocks. These last are the oldest of the Tertiary period, and lie in natural order upon the Cretaceous or Chalk rocks, which them- selves belong to an entirely different and anterior (Mesozoic) period in the history of our globe. The youngest or last-deceased of the fossil-horses we meet with, are found in the Quaternary and Pliocene, or the last-formed deposits of the Tertiary system. Between these earlier Pliocene horses and our own Equidae there are no material differences ; and the limbs of these forms may therefore be diagrammatised as depicted in Fig. 34, A, A1, and B, B1 ; the cannon bone in all of these figures being marked a; the splint-bones dd; the "pastern" and " coronary " bone £,