•
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, , d.) But these " splints "
bear no finger-bones, and the condition of the horse's " hand," or
fore-foot, is therefore seen to be of most noteworthy and curious
conformation. It may, indeed, sometimes happen that two small
pieces of gristle or cartilage may be found at the base of the
splint-bones, and comparative anatomists incline to regard these
gristly pieces as the representatives of the
first and fifth fingers. The ordinary condi-
tion of the horse's hand may be summed
up by saying that the animal walks on one
well-developed finger — the third — and pos-
sesses the rudiments, in the form of the
" splint-bones," of other two fingers, the
second and fourth. These latter, it need
hardly be added, are completely concealed
beneath the skin and other tissues of the
limb. In the hind limb of the horse (Fig. 33)
a similar modification is observed. The
thigh-bone (fe) and knee-cap (/) are readily
observed. There is but one toe — the third
(*> 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 £, ). If the
stamen be in reality a leaf, it is also certain that it resembles a leaf
much less closely than the sepal or the petal. The stamen is a
EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 133
stalked organ, as we have seen, and bears in its head or "anther," the
yellow pollen. This head seems to represent the folded blade of the
staminal leaf, but have we any proof that our conjecture is probable
or correct ? Let the facts of botany reply. Here is a Petunia, for
instance, in which the stamens are replaced by stalked leaves ; and
here a leaf (Fig. 58) degrading to become a mere scale. There
a white Water Lily (Fig. 57, B) and a Double-rose
(Fig. 57, A), in both of which cases we may observe
the transition stage whereby the stamen (4, 6) be-
comes a petal ; whilst the petal in the rose may
become in its turn a sepal (Fig. 57, A, i). So, too,
in the common tulip, the three parts of the pistil and
the six stamens may all be transformed into petals.
Nor does the central organ of all, the seed-produc-
ing pistil, escape these metamorphic changes. The
double-flowering Cherry (Fig. 59) shows its carpel in
the shape of a green leaf (b}. The willow flowers
show us gradations from the leaf-like carpel to the
altered stamen, and thence to the ordinary leaf;
and we may, lastly, find in some plants, as in the FlG- 59-— DOUBLE-
" . •" c T\ ± i. /~«i j.u j FLOWERING CHERRY.
monstrous specimens of Dutch Clover, that every
part of the flower becomes a leaf. Goethe's own words regarding
the pistil succinctly express the true state of matters regarding its
abnormal history : " If we keep in view the observations which have
now been made, we shall not fail to recognise the leaf in all seed-
vessels, notwithstanding their manifold forms, their variable structure,
and different combinations." Thus Goethe's generalisation finds its
best proof in the facts of vegetable monstrosities. And the science
of likenesses, tracing nature in her bypaths of development, discovers
that, whatever may be said of the first beginnings of plant life on the
globe, the later development which has given us the flowering plants
has apparently been directed wholly, or in greater part, towards the
elaboration of the leaf. To the evolution of the leaf, as the science
of likeness proves, we owe the wondrous beauty of the flowers,
which, like the stars of the poet, brighten earth's otherwise dull
firmament.
It is both interesting and important to note that some botanists
hold the view that the original organs of the flower consisted simply
of stamens and pistil, these organs being alone necessary for the
production of seeds. The petals are to be viewed as " flags," serving
to attract insects for fertilisation, as will hereafter be explained. The
petals are thus regarded as modified stamens, and yellow is consi-
dered to be the original colour of flowers. Unquestionably, the
most simple and least modified flowers are yellow, and we know that
stamens are extremely apt to develop into petals, or, at least, into
134 CHAPTERS ON EVOLUTION.
leaf-like structures. But such considerations do not affect the
general axiom of Goethe that the leaf is the type of the whole plant ;
for before stamens and pistils were developed at all, there must have
existed leaf-like organs adapted for nourishment, and, as we see in
lower plants, exercising reproductive functions as well. Stamens
and petals, in other words, are secondary developments ; the leaf
remains, as before, the type of the whole plant-kingdom.
The flower, however, is not the only part of the plant which has
received abundant elucidation at the hands of the science of like-
nesses. The ingenuity of Nature and the prolific nature of the
expedients by which she has developed structures to serve her varied
ends, formed of old two of the stereotyped sources of wonder by
the recital of which philosophers were wont to regale their auditors.
This fertility of device in using simple means to effect important ends
receives a new reading from the study of homology. We now perceive
that the modifications effected by nature represent the utilisation
of like parts in divers ways. Just as essentially similar limbs may
be employed in the animal world for very different purposes, so the
variations of similar parts in plants may illustrate what is meant by
" homoplastic " organs — that is, the adaptation to new and varied ways
of life, of the common belongings of the plant world. Our com-
prehension of this truth may be firstly assisted by an example
culled from the animal world. The idea that Nature, " in framing
her strange fellows," and in developing the unusual and unwonted,
should effect her purpose by the creation of new structures and fresh
parts, is an idea for which there apparently exists the warrant of
common sense. But let us see if the way of Nature in such a case
is not rather by the elaboration and modification of already existing
parts. Take as an illustrative case the Tortoise (Fig. 60) and its
structure. No single animal form stands apparently more aloof from
its neighbours of the reptile class than the sluggish chelonian. En-
closed in a bony box, its structure seems to be unique, and its relations
to the serpent, lizard, or crocodile extremely unapparent. But what
has comparative anatomy to
say respecting the building of
the chelonian house? Look at
the roof formed by the greatly
expanded ribs and solid spine.
Regard its sides formed by the
cartilages or ends of the ribs;
and its floor formed by certain
skin-bones comparable roughly
„, in their nature to the large
FIG. 60.— TORTOISE. , e ,, j-i > j
scales of the crocodile s under
surface, and in any case presenting us with no structures unusual or
EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 135
foreign to the reptile class. The boxlike body of the animal is, in
short, formed by so much of its skeleton, and by so many of its scales,
altered and modified to suit the animal's way of life. It presents
us, thus, with no new thing in the way of structure, but with an
elaboration of the common elements of the reptile body.
More interesting, perhaps, because more complex in their re-
lations, are the changes which occur in the lower jaw and ear, as we
ascend from the fishes as the lowest vertebrates to Man and
quadrupeds as the highest. We could not find a better example of
the manner in which Nature moulds the same elements into widely
different forms than the latter subject. Homology teaches us clearly
enough that in the elaboration of the skull, as in the modification of
the tortoise-skeleton as a whole, new parts and new organs are evolved
simply and for the most part by the alteration and higher develop-
ment of the original type. When we examine the lower jaw and
its connections with the skull in any vertebrate animal below the
rank of the quadruped, we find that the jaw is attached to the skull
by the intervention of a special bone called the " quadrate bone.*
The manner in which lower jaw and skull are connected in Man
and quadrupeds is very different from the latter arrangement. In
Man, as every one knows, the lower jaw works upon the skull directly
and of itself, and the " quadrate bone," which one sees so distinctly in
the reptile, bird, frog, or fish, is apparently wanting in higher verte-
brate life. Is the skull of the quadruped, then, modelled, as regards
its lower jaw and articulations thereof, on a different type from that
seen in the lower vertebrate? Comparative anatomy supplies the
answer in a highly interesting fashion.
Attend for a moment to the disposition of the parts of the
internal ear, which in quadrupeds we find to exist within the
skull and just above the lower jaw. We find three small bones
(Fig. 61, A, m, i, c,) to connect the "drum" of the ear with the
internal hearing apparatus. Of these three bones, one shaped
somewhat like a hammer is named the malleus (m), and to this
A B C
FIG. 61.— JAWS OF VERTEBRATA. A, Quadruped ; B, Lizard ; C, Fish.
bone our attention must be specially directed. For when we trace
this bone downwards through the reptiles and birds towards the
fishes, we discover that it alters its relations to the ear and assumes
new ones with the lower jaw. In reptiles and birds, for example, we
136
CHAPTERS
EVOLUTION.
find the malleus to be of large size, and to be divided so that one part
(B, m) becomes transformed into the " quadrate bone," and another
(B, ml) into the upper part of the lower jaw (/) itself. In the fish a
third bone (c, tn") may actually appear in connection with the lower
jaw (/), and as the result of the division of the part representing the
" malleus " of Man and quadrupeds. So that, divesting the subject
of all technicality, we may say that, as we first enter the vertebrate
sub-kingdom, we find the " malleus " to be represented in the fishes
by no less than three bones (c, m, m', m") which are connected
with the upper part of the lower jaw and lie outside the ear altogether.
Next, in the reptile and bird we find a modification of this arrange-
ment to hold good. Here the malleus is divided into two portions
(m, m*) only; these parts, however, being still concerned in the
articulation of the lower jaw (/). But in Man and his neighbour-
quadrupeds (A), these outside bones become pushed upwards in the
course of development, and are finally enclosed within the skull. They
thus appear as the " malleus " of the ear (A, ni), having no connection
*with the jaw, and being concerned in the higher function of conveying
impressions of sound to the internal ear. The upper part of the lower
jaw of the lower vertebrate is in fact taken into the interior of the
skull and ear, when we reach the quadruped class. The two com-
panion bones (A, c, /) of the malleus in the ear, likewise represent separate
parts of the skull, which in higher life become modified for the
hearing function. And a glance at the accompanying diagram will
serve to show how the other bones — "incus" (/) and
"stapes" (c) — of the quadruped ear 'are represented
wholly cr in part in lower life, and how they attain
their higher place and function simply as the result
of modification, and of the evolution of a new structure
from the materials of an already existing type. Such
modification is simply part of the wider process we
see everywhere illustrated in animal life at large,
whereby complication and diversity of structure and
form are the results of no new creations, but of the
development, the splitting up, and differentiation of
already existing parts.
So is it also, with plants in some of their most
unusual aspects. The strange features in animals
and plants are in reality but the altered "cominon-
62. A LEAF place of nature." By way of illustration, the subject
AND ITS PARTS. r f ., ., ,,., ,. J , <, „ r , . J ...
of the threadlike " tendrils of plants presents itself
in a prominent manner. It would be hard to discover any organs of
plants which are better known than these. Poetic allegory itself has
ever found in the simile of the " tendrils," the best guise under which
the affections of mankind might be shadowed forth ; and that weak-
EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 137
FIG. 63.— LEAF OF PEA.
stemmed plants climb by the aid of these organs, is not a matter
requiring even a primer of botany for its verification. Now, plants
of very varied nature possess these organs ; and the question arises,
are these tendrils new and special organs in such plants as possess
them, or are they but modifications, like the home
of the Tortoise, of familiar structures ? Let the
science of likenesses reply, by directing our
attention to the general form of the leaf. Every
ordinary leaf (Fig. 62) consists, as we know, of
a stalk or petiole (/) and a blade or lamina (/),
and when we look at the apple leaf (Fig. 62), or
at a rose leaf, we may see at the point where
the leaf-stalk leaves the stem, two little wing-
like appendages, called stipules (s s), and which
are probably to be regarded as normal parts
and appendages of the leaf. These stipules are
large in the pansy tribe, and are also prominent
in the beans and peas, whilst in one of the
vetches (Fig. 66) — Lathyrus aphaca, the Yellow
Vetch — the stipules, as we shall see, may ac-
tually represent the leaves. In many other
plants, on the contrary, no stipules occur.
Now let us examine the leaf of the Common Pea (Fig. 63). It
is a compound leaf, and we notice that the tendrils seem to grow
out at the sides and at the end of the leaf-stalk. The tendrils (tf)
here, are at once seen to exist in the place
of some of the leaflets (/), and are formed
by the end of the leaf-stalk also. We find
a very simple modification to be thus re-
presented ; certain parts of a leaf, in other
words, become altered to enable the plant
to climb. Tendrils here are "homolo-
gous" with leaflets and leaf-stalk. In the
lentil, it is the leaf-stalk itself which is
long drawn out to form the climbing
thread. The vine (Fig. 64) or passion-
flower may be selected as our next ex-
ample. Here the tendrils appear to be
formed in a very different fashion from
that seen in the pea. Apparently the
tendril (t t) in the vine and passion-flower
is a modified branch ; such an opinion being arrived at from a study
of the relations of the tendril to the stem and normal branches of
the plant. The Virginia Creeper likewise climbs by means of its
altered tendril-like branches. Once again we meet with a similar
FIG. 64.— TENDRILS OF A VINE.
CHAPTERS ON EVOLUTION.
end — that of forming a climbing support — served by a different
means, when we turn to the Smilax (Fig. 65), which in Southern
Europe replaces the Bryony of our English hedgerows. The leaves
of Smilax are heart-shaped, and when we look at the points at which
the leaves spring from the stem, we detect two tendrils (//), which
pass to the surrounding plants there to entwine
themselves in complex fashion. Now, what are the
tendrils of Smilax ? Our knowledge of the leaf and
our observation of the position of our tendrils enable
us to answer the question. What organs arise from
the base of the leaf-stalk? The reply, illustrated by
a reference to Fig. 62, is " stipules " (s s) ; and stipules
are paired organs. Therefore, we conclude that the
tendrils of Smilax are simply altered stipules. The
LEAVES OF SMILAX. Yellow Vetch (Fig. 66), which adorns our cornfields,
reverses the conditions of Smilax. The stipules (s s)
remain in the Vetch to represent the leaves, whilst the leaf-stalk
itself and its leaflets become altered as in the Pea, to form tendrils
(/ 1) and to enable Lathyrus to indulge its climbing propensities.
Thus does a study of tendrils illustrate in apt fashion the bearings
of homology. But for this science of likenesses we should not be
enabled to unravel some of the complexities which beset the study
of how a plant climbs ; and we again
note how modification and adapta-
tion, as distinguished from new crea-
tions, form the way of the world of life.
No less interesting in certain of its
aspects is the study of the " thorns "
and " prickles " which " set the rose-
bud," or give to the hawthorn its
characteristic name and feature. The
popular botany of every-day life is
content to consider prickles and
thorns to represent one and the same
kind of structure. But the science of likenesses is careful to ask us
to make a very decided distinction between their nature as between
the tendrils themselves. Examine the Sloe (Fig. 67, A), for instance,
or the Hawthorn, and you will readily determine the nature of the
"thorns" which these plants bear. You will note that from the
thorns (a a) leaves spring, and in this observation lies the key to the
understanding of their relationship with other parts of the plant
Leaves are only borne on the stem itself or on the appendages of
the stem we familiarly call branches. Therefore the presence of
leaves on the thorns, plainly tells us that these appendages of Sloe
and Hawthorn are in reality stunted branches. Nor are we left in
the slightest doubt as to the nature of these objects ; for many of the
FIG. 66. — YELLOW VETCH.
EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 139
FIG. 67.— SLOE AND ROSE, WITH THORNS AND PRICKLES.
plants which in a wild state possess thorns alone produce full-grown
branches under cultivation. "Spinosse arbores cultura ssepius de-
ponunt spinas in hortis," said Linnaeus, and the Sloe itself illustrates
the remark. But the prickles of the Rose (Fig. 67, B), which might
readily be deemed thorns in miniature, now demand attention. The
prickle has no intimate j ,
connection with the y J
stem. On the contrary,
it is merely a hardened
appendage of the skin
of the stem or leaf as
the case may be. A
prickle causes no
trouble in its detach-
ment from the stem,
and the botanist would
inform us that these
appendages in their true nature correspond to hardened hairs.
Lastly, we may meet with double prickles, or spines, which spring
from the axils of leaves and from the base of the leaf-stalk. In the
Acacias and the American Prickly Ash (Echinopanax) we may see
spines the origin of which is not hard to trace, and which spring from
the bases of the leaves. Just as the tendrils of the Smilax were formed
from " stipules," so we perceive in the Acacias how these latter
organs may be altered to form the " spines," or " prickles," of these
plants.
Passing from leaves and flowers to fruits, we enter a new but
equally interesting field of speculation with the last. Let us firstly
inquire what is the nature of the structure to which the botanist
gives the name of " fruit." It is perfectly evident from the common
knowledge of Nature's processes which ordinary observation affords
that the fruit is merely part of the flower. The buds of springtime
and the blossoms of summer must precede the fruit of the autumn ;
and the promise of " a golden reaping " is heralded by the early
growth of the vernal season. Without the flower, then, the fruit
would be non-existent, and considering that within the vast majority
of fruits we find the seeds, we can readily construct a definition of
the botanical fruit by defining it as " the ripe pistil." Such is the
invariable nature of the fruit in the mind of the botanist. Popu-
larly, however, "fruits" are only to be so called when they are
edible. The mental and scientific concept of the man of science
vanishes before the practical matter-of-fact definition of a fruit as
that which is good to eat ; and perhaps each definition meets in
its own way the exigencies and circumstances which called it forth.
But the study of fruits from the botanical side, presents us with a
highly interesting illustration of the value of " homology," as showing
I4o CHAPTERS ON EVOLUTION.
us how the modification of simple and well-known parts of the
flower may become transformed so as to be well-
nigh unrecognisable in the fruit No better illus-
tration of the latter fact can be found than in the
Strawberries (Fig. 68), which secured the full
admiration of Dr. Boteler, who declared that
" Doubtless God could have made a better berry,
but doubtless God never did" — a remark the
correctness of which will probably be viewed
proportionately by the individual minds and
tastes which may consider the saying. Glancing
at the Strawberry flower, we see no promise therein of the toothsome
fruit which the summer brings ; and we may well be puzzled to dis-
cover the true nature of our berry, even after a close examination of
its substance. The apple cut across is seen to contain seeds — therefore
we may reasonably enough imagine that, whatever growth has pro-
duced the fleshy fruit from the apple blossom, we find the seed-
producing pistil of the flower to be represented in its interior.
But no seeds are to be found in the interior of Dr. Boteler's berry.
Where, then, is the true fruit — the ripened pistil — of the Strawberry,
and what is the nature of the succulent mass we eat ? The science
of likenesses answers the question by a reference to the growth
of the Strawberry itself. In the flower, the pistil is seen to be
composed of a great many little parts, called " carpels " — equally well
seen in the pistil of a buttercup. As the flower fades and the
pistil ripens, the end of the flower-stalk (called in botany the
receptacle) begins to swell out and to exceed the rest of the flower in
its growth. Soon it becomes red and succulent, and the little green
carpels of the pistil, each containing a single seed, come in due
time to be separated from each other, and to be embedded in the
juicy mass on which, when it was the simple end of the flower-stalk,
it was set. Thus to offer a friend the " botanical fruit " of the
Strawberry would be a proceeding tantamount to invite him to a
Barmecide's feast : since, to fulfil the promise, we should simply
require to pick out from the surface of the berry the little green
carpels (f) which represent the ripe pistil of the flower — the popular
" fruit," as we have seen, being merely the enlarged end of the flower-
stalk. In such a case, one might well be excused for preferring the
common construction of the term " fruit " to the scientific, and for
neglecting the intellectual aspect of the berry in favour of the exer-
cise of practical aesthetics as applied to the end of the flower-stalk.
The Strawberry does not stand alone in its illustration of the
curious facts concerning the transformation of flowers which the study
of homologies elicits. What, for example, is to be said of the Rose-
fruit (Fig. 69) itself, save that the familiar red "hip" of our hedgerows
is formed by the enlarged and hollowed flower-stalk (c), along with
EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 141
the calyx ( s) or outer and green part of the flower ; or, according to
some botanists, by the calyx alone, whose green leaves become
thickened, red, and glistening as the summer passes into the autumn,
and come to enclose the true fruit (fr) in the form
of the little " carpels " similar in nature to those
on the outside of the Strawberry. So that the
difference, in one botanical theory at least, be-
tween the " hip " of the Rose and the Strawberry,
simply consists in the fact that the Rose flower-
stalk is hollow and has the fruits inside, whilst the
end of the Strawberry flower-stalk is solid, and
has its fruits outside. The Apple and Pear like-
wise exhibit much the same arrangement as the
Rose and Strawberry in respect of their fruits.
If we suppose the hip of the Rose to have its
walls extremely thickened and fleshy, we should FIG. 69.— ROSE FRL-IT.
convert it into a form of fruit resembling the Apple
or Pear. No less interesting is the nature of the Fig, which, to be
properly understood, should be examined as it grows in the hothouse.
Slice your fig longwise (Fig. 70 a), and you will see in its interior, not
seeds, but "flowers"; some with stamens (b} alone, others (c] with
pistils alone. The Fig appears before us as another example of the
hollowing of the flower-stalk, with this important difference, that not
merely the fruits but the flowers are contained in its interior.
It only remains for us to sum up the results and general conclu-
sions to which our brief study of the science of likenesses may be
said legitimately to lead us. Turning
firstly to the features we have just
been discussing, we have noted, for
instance, that the leaf was the type
of the whole plant, and that as the
leaf became modified to form the
" flower," so that flower and its parts,
still representing leaves, became further
altered to form the " fruit " under all
its varied aspects and forms. From a
simple structure — the leaf — we thus
discover, by the aid of the science of likenesses, complex and
elaborate organs and parts to be developed. What lessons do such
examples teach us concerning the order of Nature at large ? Do
these lessons argue in favour of evolution or against that theory of
Nature ? The answer is not for a single moment doubtful. If, as
our inquiry shows, it is the way of Nature to produce many and
varied structures by the modification of one simple organ or part,
surely there is no greater wonder involved in the idea, that by the
same process of development she has woven from simple forms, the
FIG. 70.— SECTION OF FIG.
142 CHAPTERS ON EVOLUTION.
whole complex warp and woof of the living world. When we see
Nature in her abnormal methods of development revealing to us,
under the guise of her sports and freaks amidst the flowers, the true
composition of the pistil and stamens, or altering the same structure
to form the varied fruits ; when we discover that the complex skull
has apparently been built up through slow and gradual modifications
from skulls of simpler type, which vanish away, in the lowest confines
of the vertebrate animals, and disappear in the barely defined skul-
less "cord" of the lowest fish, we may not esteem it an impossibility
that all organic forms have been evolved under like conditions of
development
Nor must we omit to think of another important point involved
in the study of homologies. If Nature is, as we have shown,
liable to modify and alter continually the work of her hands,
can such a practice be held to favour the origin of new species by
the way which evolution points out ? When the flower returns to
the leaf-type, or when it exhibits variations from its usual form and
structure, is Nature going back or reverting to former conditions ?
or is she initiating paths which lead to new species? The answer to
both of these queries may be given in the affirmative. When the flower
grows into its leaves, that is a " reversion," a stepping backward to
the primitive and simple type. When, on the other hand, the plant
shows a tendency towards complexity, instead of simplicity — to alter
in favour of increased development — then is seen the tendency to
progression and elaboration of the type. Both tendencies hold sway
in Nature, and the one is as inexplicable as the other, save on the
theory of Evolution. From the monstrosity of the flower a new
"variety" springs, and in time the variety becomes a "race," and
the race in turn a new " species." Thus, whilst the course of Nature
before our eyes runs not smoothly but in an apparent irregularity,
the deeper faith in a law-governed universe, not as yet fully compre-
hended or known, convinces us that with the higher knowledge of
to-morrow, the irregularities of to-day will resolve themselves into
parts of an ordered system. It is not without good reason for
believing in the reality of the convictions which nature-studies inspire
respecting the government of this world's order, that we find Professor
Parker maintaining that " the study of animal morphology leads to
continually grander and more reverential views of creation and of a
Creator. Each fresh advance shows us further fields for conquest,
and at the same time deepens the conviction, that, while results and
secondary operations may be discoverable by human intelligence,
4 no man can find out the work that God maketh from the beginning
to the end.' We live as in a twilight of knowledge, charged with
revelations of order and beauty ; we steadfastly look for a perfect
light which shall reveal perfect order and beauty."
143
VIII.
THE EVIDENCE FROM MISSING LINKS.
WHEN the Darwinian theory of the origin of living species and
other theories of evolution were yet in their infancy, the subject-
matters of the present paper had attained notoriety, if not fame.
The early critics of the hypotheses of evolution were not slow
to fix upon " missing links " and their nature, their assumed
absence, and the impossibility of supplying them, as weapons of
satisfactory kind and lasting strength, against such ideas of the order
in which the living universe had been formed. Especially has the
phrase found favour in the eyes of critics of an unscientific cast of
mind — those " old ladies of both sexes," to use Huxley's words, who
consider the "Origin of Species" "a decidedly dangerous book,"
and who regard most contributions to the literature of evolution as
works of darkness in the most literal sense of the term. Persons
who would have been puzzled had they been asked to mention a
single example of a case where " missing links " were required,
nevertheless were found ready with much unction to declare that Mr.
Darwin could never be expected to fill the gaps in question ; and the
argument as against evolution, in the early days of which we are
speaking, was frequently supposed to be clenched with the trium-
phant query, " Where are the missing links?" A feature of Darwinism
and evolution, not to speak of natural history at large, so apparently
familiar as the subject before us, deserves some detailed examination.
It is not too much to say, that even with the lapse of years, and with
the better understanding by cultured persons at large of evolution,
its weaknesses and its strength, the nature of " missing links " is often
imperfectly understood. Apart from the necessity for some clear
understanding of what is demanded by the opponents of evolution,
and of what evolutionists and naturalists are able to present in reply
to these demands, the present topic may be said to have grown in
importance with the most recent discoveries in geological science.
Its true nature, and its attitude to the existing phases of evolution,
are therefore matters for careful inquiry; since their investigation
may powerfully aid the solution of the great problem which evolution
endeavours in one phase to solve — the how and why of living Nature
and her ways.
The widespread recognition, even in the popular mind, of the
importance of the discovery of " missing links " between existing
144 CHAPTERS ON EVOLUTION.
species of animals, in so far as the welfare of evolution-theories is con-
cerned, is not difficult to trace or account for. Taking for granted
the very reasonable and obvious admission that any theory of evolution
must rest upon the idea of the production of new species by the
modification of the old, it follows that in our examination of living
nature we should expect to find evidence of the connection between
the varied forms of life in existence. From the monad up to man,
the evolutionist postulates an unbroken series — not, indeed, as many
suppose, in one straight undeviating line, but rather after the idea of
a great tree with countless branches, offshoots, and diverging twigs,
which, however, unite in their lower limits in a common stem. Now,
is it possible, when we look around at the varied forms of animal and
plant life, to trace this unbroken sequence, this continuity of structure,
and this connected relationship ? The common observation of nature,
not to speak of even an elementary acquaintance with popular
zoology, forbids the idea, and at once negatives the supposition.
The forms of life, animals and plants, fall into groups and divisions
of varying extent and different rank in the scale of creation. In
each large group we include a number of lesser divisions, the
members of which are united by certain common characters. But
even in the smallest of our classes or orders, the gaps betwixt the
included forms are many and wide; and Nature, as we observe her pro-
cesses, does not appear to supply the " missing links," in the existing
order of affairs at least. In that great sub-kingdom of the animal world
which zoologists have parcelled out as the Vertebrata, — or the territory
wherein man and quadrupeds reign as the aristocrats, birds and
reptiles as the middle classes with their varied estates and ranks, and
frogs, toads, and fishes as the lower orders and substrata of verte-
brate society, — the gaps existing between the various classes are very
patent and clear to the merest tyro in natural history. Not even the
proverbial old lady with a marked partiality to a belief in the mar-
vellous in natural history, or towards a literal interpretation of the
compound zoological character of certain wondrous beasts mentioned
. in ancient fables, could be brought to entertain seriously the idea
of the existence of an animal half-reptile, half-bird. Still less
easy is it for the popular mind to conceive the existence of a
creature midway as to structure between the bird and the quadruped.
Whilst certain small jokers — a race happily becoming, as regards
scientific matters, well-nigh extinct — might be perfectly safe in chal-
lenging zoologists at large to produce the " missing links " between
man and his nearest animal relations ; or to show on Lord Monboddo's
hypothesis, the various stages in the decline of man's caudal appendage,
upon the disappearance of which that witty savant is presumed to
have founded a large part of man's physical and moral supremacy.
Amongst lower forms of life the gaps are equally apparent, and the
THE EVIDENCE FROM MISSING LINKS. 145
continued distinctness of each species would seem to argue power-
fully at once in favour of the "special creation" of the varied kinds
of animals and plants, and against the evolution of new species from
the old, and against the hereditary connection of species one with
another. The argument derived from the visible gaps between even
nearly related kinds of animals, was therefore too apparent to be over-
looked by popular critics of evolution, and it was also too important
to be made light of by evolutionists themselves. " Distinct now,
distinct always," was the opinion which was duly expressed regarding
the nature of species, in the early days of the historical controversy
concerning their origin. We may not be surprised, therefore, to find
Mr. Darwin, in speaking of this subject, saying that one objection to
his theory, " namely, the distinctness of specific forms, and their not
being blended together by innumerable transitional links, is a very
obvious difficulty;" and again, "Why is not all nature in confusion,
instead of the species being, as we see them, well defined?" Alike
grave, then, to evolutionists and their opponents is the question of
" missing links." Let us endeavour to examine this question in the
light of recent research, with the view of determining to which side
the balance of evidence, duly weighed^ will lead us.
Amongst the procedures commonly witnessed in our courts of
law there is one which I believe is styled, in legal parlance,. " taking
an objection to the relevancy of the record or indictment." The
essential feature of that procedure consists in one of the interested
parties showing that certain parts of the statement of facts made by
the opposing side involve items which may be absolutely untrue or
incorrect, and which therefore require to be expunged from the list
of matters involving litigation. In this way the details of a lawsuit
become simplified, and the chariot- wheels of justice are enabled to roll
easily onwards in that glorious ease and uncertainty of movement
which is one of the most ancient if also unsatisfactory characteristics
of legal science and practice. The contention before us at present in
one respect admits of its issues being amended through an objection
to their relevancy. The chief points for discussion are those con-
cerning the need for "missing links" according to the theory of
evolution, and the ability or inability of the evolutionist to supply
them. Let us suppose, however, that counsel for the evolutionist
moves the relevancy of these points. The following will be his line
of argument : — " It is demanded that we produce the ' missing links,'
or transitional forms between existing species. Unquestionably the
demand is a just one ; and in furnishing its reply, it is clear we must
point out such links either in the existing world, or in the fossils
found in rock- formations, as representing the life-systems of the
past. We shall be able presently to demonstrate that whatever
evidence geology has to show is all in our favour, and that where a
L
146 CHAPTERS ON EVOLUTION.
want of evidence exists, such deficiency is no fault of ours, but
depends on the ' imperfection of the geological record.' But there
exists an equally important consideration for our opponents, in the
fact that the very circumstances under which new species are produced
may frequently obviate the necessity for the existence of missing links
and transitional forms. This latter contention can be supported by
the plainest evidence, and on this preliminary point — namely, the
reason for the justifiable and natural absence of transitional forms —
we may firstly lead evidence."
Is it necessary, then, that we should, by the laws of, and in the
very nature of the origin of species by, evolution, or by Mr. Darwin's
principle of " natural selection," always expect to find transitional
forms connecting existing species ? Mr. Darwin's reply to this question
is a negative. The new varieties or species which appear will tend,
by the very conditions of evolution, to present improvements on the
species which preceded them; and, on the principle that " the weakest
goto the wall," the ancestors of existing species will in many cases have
become exterminated by their successors being better adapted than
themselves to survive in the " struggle for existence." The parent-
species will fail in the competition involved in the struggle with its
offspring. Viewing each species as usually the product of an improved
constitution, we may naturally expect the parent- form and the tran-
sitional links to have become exterminated, as Mr. Darwin remarks,
" by the very process of the formation and perfection of the new
form." But extinct animals are liable to be preserved as "fossils"
in the rocks composing the crust of the earth, and yet "missing
links " are not discoverable in any adequate proportions. This latter
fact has already been mentioned, and the reason assigned in the
fragmentary condition of nature's great geological museum. Neglecting
the geological evidence for the nonce, it might still be contended that
living species as noted by us to-day should be more closely Con-
nected than they are, were their creation by evolution and descent a
probable theory.
Now, the pith of the evolutionist's reply consists in showing that
such connecting species or forms are by no means to be expected as
a matter of course, and that their absence is, in fact, actually favour-
able to his views and opinions. Consider a well-known and proved
case of the origin of very different varieties from a common stock,
that of the Pigeons. The various breeds or races of pigeons, of which
the four best known are the pouters, fantails, carriers, and tumblers,
may be certainly regarded as having descended from the Rock Pigeon
( Columba livta). Between the various breeds of pigeons, the differences
are so marked as to be of "specific" character. Their variations are
so plain and distinct, that had these birds been met with in a wild
state and been examined by ornithologists, they would have been
THE EVIDENCE FROM MISSING LINKS. 147
assuredly classified as distinct " species," and not as mere " varieties "
of one species — so apparent are the differences in size, in colour,
in feather-arrangement, and even in the skeleton. Such an instance
stands, therefore, as a most typical case of the origin of new races or
of new species by the modification of the old ; and its consideration
will show us the futility of the demand that the original stock should
resemble the descendants to which it has given origin. There exists
no necessity that the rock pigeon should be intermediate between
any two of the four breeds just mentioned, or that any two of these
races — say the fantails and pouters — should in turn evince combina-
tions of the characters of each other.
Mr. Darwin remarks of the pigeons and their history, that " if
we could collect all the pigeons which have ever lived, from before
the time of the Romans to the present day, we should be able to
group them in several lines, diverging from the parent rock pigeon.
Each line would consist of almost insensible steps, occasionally
broken by some slightly greater variation or sport, and each would
culminate in one of our present highly modified forms. Of the many
former connecting links, some would be found to have become abso-
lutely extinct, without having left any issue ; whilst others, though
extinct, would be seen to be the progenitors of the existing races. I
have heard it remarked as a strange circumstance," he continues,
" that we occasionally hear of the local or complete extinction of
domestic races, whilst we hear nothing of their origin. How, it has
been asked, can these losses be compensated, and more than com-
pensated?— for we know that with almost all domesticated animals the
races have largely increased in number since the time of the Romans.
But on the view here given we can understand this apparent contra-
diction. The extinction of a race within historical times is an event
likely to be noticed ; but its gradual and scarcely sensible modifica-
tion, through unconscious selection, and its subsequent divergence,
either in the same, or more commonly in distant, countries into two
or more strains, and their gradual conversion into sub-breeds, and
these into well-marked breeds, are events which would rarely be
noticed. The death of a tree that has attained gigantic dimensions
is recorded ; the slow growth of smaller trees and their increase in
number excite no attention."
The true view of the matter really consists in our recognising
that the likeness and relation of new species or races to their parent
stock depend on the circumstances of human observation, and on
the exact lines along which the variation has proceeded. Occa-
sionally each likeness is apparent ; at other times, by the very manner
of development of the new species, it is non-existent. Nor must we
forget one all-important consideration, which, according to Pro-
fessor Huxley, Mr. Darwin himself somewhat overlooked. It is a
L 2
i$8 CHAPTERS ON EVOLUTION.
frequent fact, hereafter to be noted, that, despite the Linnaean aphorism
Natura non facit saltum, Nature may and sometimes does take not
merely a jump, but a running leap from one species to another.
What would be thought of the history of the Ancon or Otter sheep,
which about the close of last century was born of an ordinary ewe
as the progeny of an equally commonplace male parent ; both, along
with fourteen other ewes, having been the property of a certain Seth
Wright, a Massachusetts farmer? This Ancon sheep differed most
materially from its parents and from the ovine race at large, in possess-
ing a large body and proportionally short legs. For sundry reason s, con-
nected with the over-lively habits of his long-legged sheep in leaping
over their fences, Wright from this one Ancon sheep, in due time, bred
a whole flock of pure Otter sheep ; the breed being allowed to die out
on the introduction of the Merino sheep. Presuming that, in ignorance
of its true and sudden origin, the history of the Ancon breed had been
made the subject of biological speculation, how would the demand
for " missing links," and the evolutionist's inability to reply to the
demand, have been construed ? Simply as against the transmutation
of the sheep species or race, and as against the origin of the Ancon
by the variation and modification of the ordinary sheep. And yet
the Ancon race had certainly its beginning in the sudden modification
of an existing race such as utterly precluded the possibility of any
" connecting links " having been developed or required.
Such considerations, we may submit, will tend to weaken the rele-
vancy of the demand for "missing links " and transitional forms. But it
may be worth our while to hear a little further testimony on the same
point. Taking Mr. Darwin's own examples, we find him citing the
instance of a journey from north to south, over a great continent,
in the course of which we meet with closely related or representative
species, which represent each other in their respective regions or
habitats. Such species are found to meet and interlock, and there-
after, as our journey proceeds, one species is found to become less
frequent, until it is completely replaced by the other. Even in the
common or middle region where these two species intermingle, the
members of the one group are as absolutely distinct from the other,
as if specimens had been selected for comparison from the head-
quarters of each species. Yet, says Mr. Darwin, " by my theory, these
altered species are descended from a common parent ; " each in the
process of descent having exterminated the parent species and also
the transitional forms. Once again — leaving the extinct and fossil
species out of consideration for the present — the question crops up,
why do the species not intermingle in the middle region, with inter-
mediate conditions of life ? Here geology steps in to reconcile the
discrepancy. Because your continent is continuous from north to
south to-day, it is not lawful to infer that this continuity of land-
THE EVIDENCE FROM MISSING LINKS. 149
surface always existed. Changes of land, and the separation of even
our great continents into detached portions of territory, are not theo-
ries, but facts of geology. And, admitting the existence of separate
islands or disconnected portions of land-surface, the distinction of
species by such separation, and the absence of intermediate forms,
would be fully accounted for. Nor must it be lost sight of that the
neutral territory or " No Man's Land " common to two species, is
usually small and ill-defined as compared with the wider territory or
area of the distribution of each group. And again, the range and
extension of a species, and its power of commingling with other
species, will be materially affected by the range of distribution of
other and already well-defined groups. The species will be preyed
upon by these latter groups, and the tendency to mix and unite with
its nearest allies is thus lessened and limited ; whilst the fact has been
already noted that the narrow and limited character of the common
area is by no means favourable to a blending of the characters of
the nearly related groups. Conversely, in a larger area, with less
risk of destructive competition from other species, we find the repre-
sentative group attaining the maximum of its development, and, even
in point of greater numbers alone, attaining a marked and charac-
teristic personality, as do the representative species alluded to in the
north and south of a large land-surface. Each species thus " fighting
for its own hand," and either aided, or on the other hand weakened,
by surrounding conditions, improves or decays, without mixing with
neighbouring groups.
Summing up these preliminary observations on the theory that
" missing links " are by no means so necessary on a fair showing of
Nature's ways and polity as might be supposed, we may submit,
firstly, that the favourable variation of a species is a slow process,
depending not merely on changes in the constitution of the included
animals or plants, but on many other external causes, such as changes
of climate, and the like. Secondly, in connection with this first
discouragement to the mixing of specific characters, we must re-
member that detachment of land-surfaces will account for the
absence of intermediate forms ; and in cases where such forms have
existed, they would be developed, as we have seen, in fewer numbers
than the species they would tend to connect ; lesser numbers imply-
ing few chances of either actual or geological preservation.
But we may not forget that up to the present stage we have been
merely contending for the relevancy of the indictment. Supposing
our objections to the invariable necessity for " missing links " have
been maintained, there yet remain very many instances wherein, as
the evolutionist would freely admit, such connections require to be
supplied, theoretically or actually, for the support of his case. The
connected chain of life which the evolutionist postulates, implies the
ISO CHAPTERS ON EVOLUTION.
presence of numerous links; the chief question relating to the exact
stages or points at which these links are demanded — and this question
again depending on another, "What is or was the exact sequence and
order of development?" Suppose Mr. Browning to be as correct in
his poetic rendering of the " Descent of Man " as he is — judged by
ordinary theories of evolution — absolutely incorrect, when he says
in " Prince Hohenstiel Schwangau " —
That mass man sprang from was a jelly lump
Once on a time ; he kept an after course
Through fish and insect, reptile, bird, and beast,
Till he attained to be an ape at last,
Or last but one, —
then, according to the poet's rendering of man's evolution, his
descent would imply connecting links between the amoeboid or
protoplasm stage of his existence and the " after course," and also
between the successive stages of which that " after course " is alleged
to consist. Fortunately for scientific criticism, poetry possesses an
invaluable commodity known as " licence ; " and it may suffice in
the present instance to remark that the sequence and succession
of life indicated by the most psychological of modern poets, are
certainly not those held by Mr. Darwin, or by any other competent
biologist. Man's descent from the gorilla — the chief element in the
evolutionist's creed as propounded by popular notions and by a dog-
matic but unlearned theology — is, after all, but " the baseless fabric " of
a vision, from which a better acquaintance with the facts of nature, and
with theories explanatory of these facts, will most effectually awaken the
unconvinced. The knowledge of what evolution really teaches and
reasonably demands constitutes, therefore, the first condition for ascer-
taining what "missing links " are required. To bridge over the gulf be-
tween the gorilla or any other anthropoid ape and the human type, may
be the mental bane and lifelong worry of unscientific minds contorting
the demands of evolution — such a task is certainly no business or labour
of Mr. Darwin and his followers, or of any other school of evolution.
And Mr. Darwin, writing in his " Descent of Man," and after a
review of man's theoretical origin, is careful to add, " But we must not
fall into the error of supposing that the early progenitor of the whole
Simian (or ape-like) stock, including man, was identical with, or even
closely resembled, any existing ape or monkey." We must, in truth,
look backwards along the " files of time " to the point whence, from a
common origin, the human and ape branches diverged each towards its
own peculiar line of growth and development on the great tree of life.
Thus much by way of caution in alleging how or what " missing
links " are to be supplied. The contention that, even on the showing
of the evolutionist, the connecting links between distinct groups of
THE EVIDENCE FROM MISSING LINKS. 151
living beings are not supplied even to the extent he himself requires,
is answered in the expression of Mr. Darwin already quoted, namely,
" the imperfection of the geological record." No fact of geology is
more patent than that, to use Sir Charles Lyell's words, " it is not
part of the plan of Nature to write everywhere, and at all times, her
autobiographical memoirs. On the contrary," continues this late
distinguished scientist, " her annals are local and exceptional from
the first, and portions of them are afterwards ground into mud, sand,
and pebbles, to furnish materials for new strata," The very process
of rock-formation consists in the rearrangement of the particles of
previously formed materials, and the manufacture of new strata
implies the destruction of the old with the included " fossils " of the
latter. The geological series is thus certainly a detached and discon-
tinuous collection of formations, interrupted by gaps of considerable
and often undeterminable extent. Of the contemporaneous life-his-
tory of the globe, during the periods of time represented by such gaps,
we have no record whatever. But even when the materials for form-
ing a detailed history of any past period of our globe are found iii
tolerable plenty, the record is never complete. "We can never hope,"
says Lyell in a most emphatic passage on breaks in the sequence of
rock formations, " to compile a consecutive history by gathering to-
gether monuments which were originally detached and scattered over
the globe. For, as the species of organic beings contemporaneously
inhabiting remote regions are distinct, the fossils of the first of several
periods which may be preserved in any one country, as in America,
for example, will have no connection with those of a second period
found in India, and will, therefore, no more enable us to trace the
signs of a gradual change in the living creation, than a fragment of
Chinese history will fill up a blank in the political annals of Europe."
Add to these considerations the brief chronicle of a long and
important chapter of geological history, namely, that soft -bodied
animals and plants are rarely preserved as fossils ; that land animals
are but sparsely represented in any formations as compared with
marine forms ; and that even " metamorphism," or the alteration of
rocks subsequent to their formation, is known to alter and obliterate
their fossil contents, — and we find reasons of the most stable and
satisfactory kind for the imperfect nature of even the fullest records
of rocks and of their fossils that man has been able to obtain.
But in what direction does the positive evidence we have been able
to obtain lead ? Clearly to the side of evolution, and towards the
supply of " missing links " in a fashion which even the most sanguine
expectations of scientific ardour could scarcely have hoped to see
realised. Bearing in mind that vast tracts of rock-formations are as yet
absolutely unexplored, the present subject is seen to be one to which
each year brings its quota of new and strange revelations. And at
152 CHAPTERS ON EVOLUTION'.
the most, any one record of what has been done towards supplying
" missing links " must be held to be merely provisional, and to serve
but as a prelude to the discoveries of a succeeding period. Espe-
cially within the last few years, however, has the evidence of the
existence of animals which may fairly be deemed " missing links "
accumulated in a very marked degree, and in some cases in a very
astonishing fashion. The reader has but to become informed of
recent discoveries amidst the Tertiary rocks of North America, to
learn the surprising revelations concerning intermediate forms between
existing groups of mammals or quadrupeds, which, chiefly through the
researches of Professor Marsh, have been added to the conquests of
science. What, for example, is to be said of the zoological position
of the huge Dinoceras (Fig. 71) and its allies, creatures as large as
existing elephants, and which, from the examination of their skeletal
remains, can at the best be regarded as intermediate bet.wixt the
elephants themselves, and the odd-toed Ungulates (or hoofed quad-
rupeds), such as the rhino-
ceroses, &c. ? Dinoceras thus
possessed two large canine
teeth (c c\ six small molars (m)
on each side, and four horn-
cores (h l h 2), besides a pair of
similar structures in front of
the upper jaw. Or, again,
which rank, save that of an
intermediate position, and as
a veritable group of "missing
links," can be assigned to the
. 71.— SKULL OF DINOCERAS. extinct quadrupeds, included
by Marsh under the collective
name Tillodontia, the remains of which occur in the Eocene Tertiaries
of the United States ? For how else should we -classify animals with
great front teeth like the Rodents or "gnawers," grinders like the
Ungulates or hoofed quadrupeds, and a skull and skeleton generally
like that of the carnivorous Bears ? Or, once more, what can be
said of the affinities or relationship of the extinct Toxodonts, also
from American deposits, in which the characters of Rodents are
united to those of Ungulates and Edentates — the latter being a
group of animals represented by the existing sloths, armadillos, and
ant-eaters ? Nor is the list of extinct quadrupeds which fall into
no existing group, but .present a union of the characters of several
distinct divisions, exhausted with the foregoing brief chronicle.
Again drawing upon the well-nigh inexhaustible treasure-house of
geological specimens in the recent deposits of the New World, we find
the extinct Marauchenia connecting the odd-toed hoofed mammals
THE EVIDENCE FROM MISSING LINKS.
153
with the even-toed division. Passing to the whales and their kin,
we find the extinct Zeuglodon with its well-developed teeth — a feature
unusual in living whales — appearing to connect the whale tribe with
the seals and their
allies. Similarly,
the curious Anoplo-
therium (Fig. 73) of
the Eocene Tertiary
deposits appears to
connect the swine
race with the true
cud-chewers or Ru-
minants, just as the
Pal&otherium (Fig.
FIG. 72. — PAL^EOTHERIUM (RESTORED).
72) itself — one of
the first animals
whose remains were disinterred from Montmartre — connects the pigs
and tapirs with the apparently far-removed rhinoceros. The case for
the existence of " missing links," wherewith the at present distinct
orders and sub-orders of quadrupeds may be connected, would thus
seem to be very strong. There would appear to be more than
sufficient cause to account for 'the hopeful spirit of the evolutionist,
whose scientific prophecy, that philosophic research into the nature
of fossil organisms — begun by Cuvier, in the now classical quarries of
Montmartre — is destined to powerfully aid his cause, seems likely to
be realised. When it lies in the power of the naturalist to point, as
well he may, with pride, to the perfect series of forms and missing
links which connect the one-
toed horse of to-day with the
curious three, four, and five-
toed steeds of the past, one
may overlook the jubilant tone
of the evolutionist in the more
silent and deeper satisfaction
with which mankind at large is
given to welcome the demon-
stration of a great truth. It is FIG. 73.— RESTORATION OF ANOPLOTHERIUM.
of such a demonstration that
Huxley writes : " On the evidence of palaeontology, the evolution
of many existing forms of animal life from their predecessors is no
longer an hypothesis, but an historical fact ; it is only," he adds,
" the nature of the physiological factors to which that evolution is
due which is still open to discussion."
But not merely in the highest class of the animal world have
" intermediate forms " been discovered. The case for evolution
CHAPTERS ON EVOLUTION.
grows in interest when we learn that in lower ranks of Vertebrate life,
groups of animals, separated apparently by the widest of intervals,
are now being linked together by the discovery of intermediate fossil
forms. The best-known example of the latter facts is found in the
relationship which may be now regarded as being clearly proved to
exist between reptiles and birds. Were we to search the whole
animal kingdom through for examples of creatures of thoroughly
different appearance, habits, and general conformation, no two
groups would fall more familiarly to hand than birds and reptiles.
There would, indeed, appear to be no similarity or likeness between
the secretary bird, which daily devours its quota of snakes, and
the prey upon which it lives ; or, reversing the comparison, betwixt
the unfortunate bird and the serpent whose stony gaze has allured
it literally to a living death. Activity of organisation on the one
hand would be opposed by a torpidity of action on the other ;
beauty of form and colour, by appearances frequently grotesque,
and often, in popular estimation at least, repulsive. The
contrast is one which, in the popular view, would be complete
and perfect in every respect. Birds are warm-blooded, and have
a four-chambered heart : reptiles possess a slow circulation, a low
blood-temperature, and a three-chambered heart, which, however, in
the crocodiles becomes four-
chambered. The former class
is covered with feathers, the
latter with scales, bony plates,
or both. The fore-limbs, modi-
fied for flight in the bird, are
never thus used in reptiles —
the so-called " flying lizards "
(Fig. 74) possessing no true
powers of flight, but being
enabled by a parachute-like
arrangement of their front ribs
to take flying leaps from tree to
tree. Birds, as we well know,
want teeth; and although in
tortoises and turtles, as typical
enough reptiles, a dental ap-
paratus is also wanting, the
reptilian character tends de-
cidedly towards a large and perfect display of teeth.
A closer inspection and comparison of the skeletons of the two
groups, such as may be made in a very general review of their bony
possessions, would reveal several interesting points of likeness and
also of divergence. Thus both classes have a lower jaw which may
FIG. 74.— FLYING DRAGON.
THE EVIDENCE FROM MISSING LINKS.
'55
9.
be called "compound ;" since, unlike the simple two-halved lower jaw
of quadrupeds, that of birds and reptiles is composed of numerous
pieces united to form the single bone. Then, also, this lower
jaw is joined to the skull, not of itself and directly, as in man and
quadrupeds, but by a special bone named the quadrate, which,
curiously enough, by a wonderful process, of alteration and meta-
morphosis, becomes represented in man and quadrupeds by one
(the malleus] of the small bones of the ear (see page 135). Such,
among others, are a few points of agreement between reptiles and birds.
But plain grounds of distinction are apparent within the same region
of " dry bones." A bird has never more than three fingers (thumb [g\,
and two next digits \d, f^f~\) in its "hand " or wing (Fig. 75); and the
supporting bones of these fingers, correspond-
ing to our "palm," are united together. The
reptile's fingers are never so few as three, and
their pa 1m -bones, moreover, are not ossified
together. The "merrythought" of the bird
(Fig. 76, /_/), indissolubly associated with
mystic forebodings of hymeneal nature,
consists of the two united "collar-bones;"
such a disposition of the collar-bones being
unknown in the more prosaic reptilians ; and
the great "keel" (/) seen on the bird's
breast- bone (g) is wanting on that of living
reptiles. Next in order, we find that the
sacrum, or bone wedged in between the
haunch-bones, consists, in birds, of a goodly
number of vertebrae or joints of the spine,
whereas, in the reptile, one or two vertebras
form the sacrum. In all birds, save the
ostrich tribe, the two haunch-bones (Fig. 76,
/, r) are not united below or in front in the middle line. In
reptiles such a union does take place, this union, indeed, being
also seen in man and quadrupeds. In birds, the tail terminates in
a "ploughshare-bone" (Fig. 76, d), giving support to the oil gland,
the secretion of which is used in preening the feathers. In reptiles
no such bone exists, and the joints of the tail simply taper towards
the extremity of the appendage. The axis of the thigh-bone (/) in
the bird, like that of quadrupeds, lies parallel with the median plane
or axis of the body ; but in reptiles, the axis of the thigh makes an
open angle of varying dimensions with the median plane.
The ankle of the bird (Fig. 77) is peculiarly formed, inasmuch as
theupperhalf of the ankle, or " tarsus" (a), becomes united to the lower
end of the shin-bone or leg (/) ; whilst the lower half of the ankle
unites with the bones corresponding to those of man's instep, the union
FIG.
SKELETON OF
IRD'S WING.
156
CHAPTERS ON EVOLUTION.
producing the so-called " tarso-metatarsal " bone (Fig. 76, w). It is
this bone which becomes so greatly elongated in the waders, such as
the storks and ibises. As seen in the young fowl (Fig. 776), the shin or
leg-bone (/) bears at its lower extremity the " astragalus " (a) of the
ankle, shortly to be firmly
united to the leg by bony
union (A). The latter con-
dition is seen in the left-
hand figure (A), where the
astragalus (a) has become
united to the tibia, or
chief leg-bone (/); the
other bone of the leg, or
fibula (/), being rudi-
mentary. Such a com-
plete union of ankle-bones
with the leg is not seen in
any living reptiles (see Fig.
85, c). Whilst the latter
have four toes as their
least complement, birds
have never more than
four, the fifth toe being
invariably wanting. And
whilst in birds, the bones
of the instep unite with
the lower half of the ankle
to form a single bone
(Figs. 85, A, tm, and 76,
#'), in reptiles the instep
FIG. 76.-sKEtETON OF BIRD. bones (or metatarsals)
(Fig. 85 c, i, 2, 3, 4) are not united together, and are distinct from
those of the ankle.
Thus much for dry details. The reader who has taken the trouble
to follow this category of the personal characters of birds as compared
with those of reptiles, will probably find that the somewhat extended
examination will assist his comprehension of certain abnormalities
in the structure of several extinct forms of bird and reptilian life ;
since many of the characteristic features of each class just detailed
will be found to have been curiously modified and often united in
the "missing links" which bind these two groups of animals together.
It may be firstly asserted that the ostriches, cassowaries, and their
relatives, differ from all other birds in possessing a flat shield-like
breast-bone instead of the normal "keeled" structure (Fig. 76, f,g)
proper to the class. Their " merrythought " is likewise incomplete,
THE EVIDENCE FROM MISSING LINKS.
157
and their haunch-bones are united below or in front, instead of
remaining open as in other birds. But he would be worse than an
over-bold zoologist who would venture to maintain
that such points of difference meant more than the
merest tendency reptilewards ; and the ostriches and
their neighbours can hardly be denominated links
which appreciably narrow the gulf betwixt reptiles
and their bird kith and kin. But presuming that the
zoologist, dealing with the birds of to-day, refuses /
assent to the idea that he can supply us with missing
links between reptiles and birds, can the contents of
the geologist's aviary be shown to be better adapted
to supply the gap ? Research here may proceed in
two directions. Either we may try to discover if any
extinct birds are nearer reptiles than their living
allies ; or, we may endeavour to ascertain if any fossil FIG. 77.— LEG AND
reptiles exhibit a closer relationship with birds than A ^'^ °fuiwr^wn
the reptiles of to-day. We may very profitably dis- biVd ; B, in the young
cuss, in brief detail, both aspects of the case.
Fossil birds make their first appearance in the Upper Oolite
FIG. 78. FIG. 79.
FOSSIL FOOTPRINTS FROM TRIASSIC ROCKS.
rocks — formations lying in their natural order just below the chalk.
158
CHAPTERS ON EVOLUTION.
Prior to the Oolitic epoch, however, and in the Triassic rocks of
America, certain large footprints (Figs. 78 and 79), supposed by some
authorities to be those of birds, are found. But these footprints may,
at the same time, be those of reptiles, and it is safer at present to
hold their exact nature as undetermined, and to assert that the first
unmistakable bird-fossil belongs to the Oolitic period.
The Lithographic Slates of Solenhofen, in Bavaria, are rocks
resulting from the consolidation of the finely powdered mud which
once coated an ancient Oolitic sea- or lake-bed. On this fine-grained
deposit, belonging to the Upper Oolite series, the merest traces and
most delicate impressions of living organisms have been preserved
— the impress of even a filmy jelly-fish having thus been brought to
light. In 1 86 1 the impression of a single feather was found, and later
FIG. 80. — FOSSIL REMAINS OF ARCH^OPTERYX.
on in the same year, a Dr. Haberlein of Pappenheim brought to light
the fragments of a skeleton which was soon discovered to be of a
thoroughly unique kind. This scientific treasure was duly purchased
for the British Museum, and was named the Arc/uzopteryx macrura
(Fig. 80). The skull of Archaeopteryx was wanting in this first
specimen, but the leg, foot, pelvis, tail, shoulder, and some of the
feathers are well preserved, and by these relics the materials for a
strange history was in part supplied. Of the bird-nature of this creature
no doubt exists. In the matter of its feathers and feet it is wholly
bird-like. But it is also discovered to differ very materially from all
known birds. Thus, firstly, Archaeopteryx possessed a long tail
(Fig. 80), exactly resembling that of a lizard, consisting of some twenty
THE EVIDENCE FROM MISSING LINKS. 159
joints, each of which supported a pair of quill feathers. Then,
secondly, no plough share- bone (Fig. 76 d] was developed. The
fingers, united by bony union in existing birds, were free and reptile-
like in Archaeopteryx, and, whatever their number may have been, it
is certain that these fingers were provided with reptile-like claws,
such as are seen in no living bird.
Such were the details of Archaeopteryx structure at hand till
within the last three years or so. In 1879 Professor Carl Vogt made
a communication concerning a fresh specimen of this ancient bird,
found in the same deposits which afforded the previous specimen.
The new specimen was singularly complete ; and its wings were
unfolded, as if death and fossilisation had overtaken it in the act of
flight. Its examination revealed certain startling features, which only
serve to confirm in an unmistakable manner the thoroughly " inter-
mediate " nature of this animal. Its upper jaw bore two small conical
teeth ; the breast-bone is " reduced to zero ; " and whilst its arm-
bones " present no features peculiar to reptiles or to birds," its hand
can be compared neither to that of a bird nor to that of a ptero-
dactyl, but to that of a three-toed lizard. " If the feathers had not
been preserved," says Vogt, "no one could have ever suspected
that, from the examination of the skeleton alone of Archceopteryx >
this animal was furnished with wings when alive." Head, neck, chest
and ribs, tail, shoulder-girdle, and arm or wing, are all built on a rep-
tilian type ; the haunch is more reptilian than bird-like ; but the hind
limbs are those of a bird. The reptile characters unquestionably
predominate in the skeleton, just as the bird-characters come to the
front in the feathers.
Professor Vogt strenuously asserts that a study of Archceopteryx
shows that it is neither bird nor reptile, but that it is a decided
"link" betwixt the two classes. It is a bird by its feathers and
hind limbs; it is a reptile by the rest of its structure; and it is,
moreover, a bird only in so far as we regard its type as having
emerged from a reptilian stock. The birds to be presently described
from the American Chalk are later developments. As such, they
are nearer the birds of to-day; but they retain the reptilian
teeth, whilst the rest of their organisation has been evolved along
the lines of bird- structure. Professor Vogt further insists on
the fact that the adaptation to flight is not necessarily combined
with an erect position, since the extinct pterodactyls and the living
bats illustrate cases in which that position was and is not maintained.
The bird-like hind feet of the Archceopteryx must be viewed as
having been independent of flight, and as related to the possibility of
sustaining the body on the hinder feet alone. In other words, we
are not specially entitled to concern ourselves with the question of
flight in this ancient animal ; and the consideration is worth attention
i6o
CHAPTERS ON EVOLUTION.
in dealing with the affinities of the Archaopteryx. Finally, as
Vogt points out, there is a complete affinity betwixt the scale of the
reptile and the feather of the bird. The feather is, in fact, the
further modification of the scale ; and we may, therefore, " imagine
the ancestors of the Archaeopteryx as lizard-like terrestrial reptiles,
having feet with fine, hooked, free digits, showing no modification in
t-v their skeleton, but having the skin furnished at different
points with elongated warts, downy plumes, and rudi-
mentary feathers, not yet fitted for flight, but susceptible
of further development in the course of generations."
But that this odd relic of the Oolite leads us
decidedly in the direction of the reptiles by its tail
and its hand there can exist no reasonable doubt.
Scepticism may exist on this latter point, but the doubt
is neither of a learned nor of a scientific kind. We
may not say that Archaeopteryx actually leads us from
any one bird to any one group of
'reptiles. It rather stands inter-
mediately and alone; but even
in its solitary position it certainly
makes the gulf betwixt the two
classes seem less formidable.
Next in order from the aviary
of the geologist may be produced
evidence of the existence of reptile-
like birds in a most interesting series
of fossils obtained from the Chalk
of Western America by Professor
Marsh. About 1871 a headless bird
skeleton was discovered in the
Upper Chalk of Western Kansas.
This bird evidently resembled
our living divers, and was duly christened Hes-
perornis regalis. Like our living ostriches, emeus,
and their allies, this extinct bird possessed no
keel on its breast-bone. It had the merest
rudiments of wings; and certain reptile-like
resemblances seen in its haunch-bones made
geologists naturally anxious for the realisation
of their hopes in the discovery of a complete
skeleton. In 1872 fresh discoveries rewarded
the patient and indefatigable search of Professor
Marsh. Not only were the missing parts of the Hesperornis duly
obtained, but the remains of another and still more remarkable
species (Ichthyornis dispar) of extinct birds, were duly brought to
FIG. 82.
ICHTHYORNIS JAW.
THE EVIDENCE FROM MISSING LINKS, 161
light. By the new discovery both Hesperornis and Ichthyornis were
found to possess teeth; the former (Fig. 81) having its curved teeth
(B) set in a common groove in the jaw-bones.; whilst Ichthyornis
(Fig. 82) makes a further advance towards perfection in dental
apparatus, in that its twenty or so teeth of each jaw were lodged
in distinct sockets. The importance of these facts as bearing on
true reptile-like characters in birds may be readily imagined. No
living bird possesses even the semblance of teeth, if we except the
horny ridges of the Merganser's bill. Prior to Marsh's discoveries,
no fossil bird was known to have been provided with true teeth —
although indeed, in certain bird- remains, described by Owen, from
the London clay (Eocene) of Sheppey, under, the name of Odontopteryx
(Fig. 83), the jaws were provided with bony projections. These pro-
jections, however, are not true teeth — which, as many readers may
know, do not resemble bones, either in development or structure, being
developed from the "gum" or lining membrane of the mouth, and not
from cartilage, as true bones usually are. Doubtless these bony projec-
tions aided Odontopteryx to catch its finny prey, as the horny ridges
FIG. 83. — ODONTOPTERYX (RESTORED)..
of the Mergansers enable them to retain the fishes they so dexterously
capture. One curious bird (Phytotoma), a South American Leaf-
cutter, certainly possesses a double row of bony projections on its
palate. But even this novel and unusual addition to the list of bird-
possessions bears but a faint resemblance to the bony teeth of Odont-
opteryx, as these latter in turn are an entirely different and relatively
modern feature of the bird- type, when, compared with the true teeth
of their " American cousins " of the Western Chalk.
The Ichthyornis, moreover, diminishes the distance betwixt birds
and reptiles in yet another fashion — the joints of its spine (Fig. 82, B)
were concave at either end (<:), a conformation familiar to us in the joints
of the fish-backbone, utterly unknown in living birds, but common
enough in reptiles. This character alone, in the eyes of the naturalist,
becomes invested with an importance hardly to be over-estimated as
regards its reptilian relationships ; and in Hesperornis, also, certain
features in addition to those already noted show unmistakable marks
of affinity to the reptile type. The teeth of this latter bird, set, as
M
162
CHAPTERS ON EVOLUTION.
already remarked, in a common groove, strongly remind one of the
manner in which the teeth of certain lizards are fixed in their jaws.
Some of the teeth of this curious bird also exhibit the manner in which
one series of teeth was replaced by another — for, as most readers
know, reptiles and fishes possess an unlimited supply and a continual
succession of teeth. The old teeth are ousted from their sockets
by new teeth which are developed at their bases ; and in the jaws
of Hesperornis such a manner of tooth-formation, exactly imitating
a common reptilian mode of renewal, is to be plainly seen. The tail
of this great diver of the Chalk seas was, lastly, like that of the
Archseopteryx of the Oolite epoch, and exhibited a very different
structure from the caudal appendage of existing and of other fossil
birds. At its middle and under parts the joints of the tail present
long projections of flattened shape, which strongly suggest the idea
of the tail having been a rigid unyielding member in so far as a side
movement was concerned, but, like that of the beaver, being probably
mobile in a vertical direction, and being thus of use in the diving
movements of its possessor. The last joints of the tail were massed
together, but in a fashion different from that in which the " plough-
share-bone " of living birds is formed.
In so far as the birds themselves have rendered an account of
their past history, it is clearly seen that
their affinities to reptiles become very
strongly marked in various directions,
especially in the structure of the spine,
and in the possession of true teeth.
Ichthyornis, in the matter of its hol-
lowed spine-bones (Fig. 82, B, c\ and
in that of its socket-implanted teeth, is
a more modified and more truly reptile-
like bird than Hesperornis. This latter
again, approaches much nearer reptiles
than Odontopteryx (Fig. 83) of the
London Clay, which latter, as becomes
its nearer approach to the existing order
of affairs, presents a less marked rela-
tionship with " the dragons of the
prime."
But what evidence, we may lastly
FJG- 84. ask, do the reptiles afford on their side
RESTORATION OF COMPSOGNATHUS. f ' , , ,1 , • •>
of any tendency towards the bird type ?
Have the reptiles remained as passive to advance and evolution, as
they would appear at first sight to remain to-day ; or does their history
but repeat the changes and variations exhibited by their bird-neigh-
bours ? Let the history of the reptile class in the past answer these
THE EVIDENCE FROM MISSING LINKS.
163
queries. A considerable number of fossil reptiles are ranked to form
a distinct order or division, marked by various near approaches to
the structure of birds. A single example of this curious group will
suffice to show the intermediate nature of its included forms. Once
again the Lithographic Slates of Solenhofen yield a rich reward to
geological investigation, and present us with the fossil skeleton of an
animal which in the flesh attained a length of about two feet. This
is the Compsognathus (Fig. 84) of the geologist — a long-necked reptile,
possessing a small head, the jaws of which, however, were armed with
teeth. Its fore limbs were short, its hind limbs being long and bird-
like. Like that of birds, its thigh-bone (Fig. 85, E,fe) is shorter than
its leg-bone. As in birds (Fig. 85 A), the upper half of the ankle-
bone of Compsognathus (Fig. 85, B, as, ca) unites with the lower part
of the leg ; but the lower half of the ankle (td) was not, as in birds,
united with the instep-bones, or metatarsals (i, 2, 3, 4), which are
three or four in number, long and slender, and which, in Compsogna-
thus, support the second, third, and fourth toes. A mere trace of the
instep-bone of the fifth toe exists, and the first or great toe is of
small size. In all birds the fifth toe is entirely wanting. Looking
at the structure of Compsognathus, and of its fossil allies, such as
Iguanodon, little or no doubt can be entertained that these reptiles
were capable of resting on their hind limbs, in bird-like fashion, and
of walking, or hopping, after the fashion of the feathered bipeds, to
FIG. 85. — (A) HINDLIMBS OF BIRD ; (B) EXTINCT REPTILE ; AND (C) CROCODILE.
which indeed, by a use of the imagination, strictly scientific, we may
regard this reptilian group as having in due time given origin. It
is unquestionably to the struthious birds, that is, to the ostriches and
M 2
164
CHAPTERS ON EVOLUTION.
their allies, that this curious reptile bears the closest resemblance ;
and a comparative glance at the hinder extremities of the crocodile,
bird, and its reptilian neighbour (Fig. 85), will suffice to show the
marked resemblances and gradation which connect, and at the same
time distinguish, this curious series of forms. The Compsognathus
limb stands intermediate betwixt the saurian (Fig. 85, c) and the
bird (A) ; and, strictly judged, is comparable most nearly to that of the
unborn chick. Those " dragons of the prime " known as Iguanodon
and Megalosaurus, from the Chalk and Oolite, are near relations of
Compsognathus. When we think of the size of these reptiles, which
attained a length of from forty to sixty feet, and of the probability
that, like their diminutive neighbour, they may have walked on two
legs, the origin of the giant footprints (Figs. 78 and 79) of the Triassic
Sand-stones would appear to present no special difficulties in the
way of satisfactory solution.
Mention must here be made of the curious Pterodactyls (Fig. 86),
or those extinct reptiles of the Lias, Oolite, and Chalk, in which a wing-
membrane or fold of skin, similar to that seen in bats, stretched from an
outer and enormously elongated finger of each hand to the fore-limb,
FIG. 86. — SKELETON OF PTERODACTYL. (The wing membrane in black.
sides of the body, and hind limbs, and between the hind limbs and
tail. By aid of this wing-membrane these literal " flying dragons "
must have winged their way through the air with ease and speed.
Their breast-bone was keeled like that of the bird (Fig. 76, /, g) ; their
shoulder-girdle was bird-like ; and their bones, as in birds, were hollow,
and were filled with air in place of marrow. The Pterodactyl brain
was essentially bird-like, but the hind limbs and pelvis were rep-
THE EVIDENCE FROM MISSING LINKS. 165
tilian, and unlike those of the bird ; and these flying dragons possessed
prominent jaws, usually furnished with socket- implanted teeth. The
Pterodactyls are thus not markedly bird-like in any sense. They do
not lie in the direct line, nor form one in the serie's of links between
birds and reptiles, but apparently represent a bird-like but inde-
pendent offshoot of the reptilian branch. In any view of their nature,
however, they serve to show plainly and forcibly the modification of
the reptilian type for flight It requires but a limited draft upon
speculative philosophy to support the belief that reptile modification
in another direction, and certainly at an epoch anterior to the appear-
ance of the Pterodactyls, probably produced the modified birds of
which our existing ornithology is the collective product.
Space fails us in the endeavour to describe other examples of
animals which from their anomalous structure seem to connect very
diverse types of living forms. Mention might be made of the
interesting fact that the apparently distinct groups of living and
extinct crocodiles are linked together in a very exact fashion by
their bodily structure ; or, conversely, that it is easy to conceive of
the varied crocodiles known to science, as having originated by
modification of a common type. The mere mention of such fishes
as Lepidosiren and Ceratodus, linking their class to that of the frogs
or amphibians ; or of such a mammal as the Ornithorhynchus (the
" Duck-billed Water-mole " of Australia), with its bird-like skeleton,
and other structures of avian nature, suggests to the naturalist the
idea that such anomalies are after all only to be accounted for by a
theory of nature which postulates the necessity for " links " binding
together groups which, at first sight, appear of widely varied and
distinct nature.
Summing up the results of this investigation in search of "missing
links," what may be regarded as the results of our labours ? and to
which side does the weight of evidence lead? — to evolution and
modification as the parent of all that is in living nature, or to rigidity
and fixity of type and form, as the rule and way of life at large ?
Judged by a very ordinary standard of value, the evidence appears
overwhelmingly strong in favour of the former view. The demand
for " missing links," as necessary features of the evolutionist's
scheme of creation, is not left unanswered where just cause is
shown for the production of these connections between the life of
the past and that of the present. There is neither wildness nor
absurdity in the idea that the bird-stock began in animals resembling
Compsognathus and its neighbours, and that through modified bird-
forms — probably resembling the living ostriches and their allies —
the further and higher development of our existing bird life was
gradually evolved. The exact stages of such development we are
unable to picture. The sketch is as yet in meagre outline; but the
166 CHAPTERS ON EVOLUTION.
outlines foreshadow tolerably well the actual details of the finished
work. And what is true of the relations between reptiles and birds,
or of those between the various races of crocodiles — which, it is
important to note, living and extinct, are bound together in a series
almost as graduated and complete as are the horses and their pro-
genitors— what is true of the connecting links betwixt quadrupeds that
to-day appear distinct and separate, must by every consideration, alike
of logic and common sense, be held to apply with equal force to the
entire world of animal and plant life. There is no law of evolution
for one group, and of special creation for another. There can be
logically postulated no evolution for the lower races, and some
process of " creation " for the higher forms of animal life or for man
himself. Uniformity and sequence exist wholly, or not at all. " If
one series of species," says Huxley, " has come into existence by the
operation of natural causes, it seems folly to deny that all may have
arisen in the same way." The unbiassed mind, contemplating the
varied phases of living nature, will stand in no dread of any conclu-
sions respecting the order of this universe, to which evolution may
lead ; for, after all, evolution, in tracing out the ways of nature, is
but the handmaid of truth, and it is with the truth as it is in nature,
that the earnest mind will most desire to close.
i67
IX.
THE EVIDENCE FROM DEVELOPMENT.
I.— THE EARLIER STAGES IN THE LIFE-HISTORY OF ANIMALS.
AMONG the many features which mark the varied universe of
life, none are more universally recognised, or more typical of
the living world, than those which herald the production of a new
being, and which usher a new form upon the stage of existence.
From the shapeless mass of protoplasm that crawls over the water-
weed as a microscopic speck, upwards to man himself, the varied
processes of development are laid down in orderly sequence and
along lines of special kind. Every living being, animal or plant —
animalcule and whale, the humble lichen and the giant sequoia alike
— passes through a definite series of changes before attaining the
form and likeness of the parent which gave it birth. In virtue of
such changes it assumes that parental form. These changes, occur-
ring in orderly array, mark its pathway from shapelessness and
physiological nonentity to the characteristic form of its race. It is
development which moulds —
The baby figure of the giant mass,
and from the minute beginnings of life evolves the highest of earth's
denizens, or directs the production of the teeming swarms of
animalcules that people the stagnant drop, and pass an existence
none the less interesting or important because often all unknown to
the larger and higher world without. It is this same process of
development which, as one phase of living action, draws the sharpest
and clearest of boundary lines between the world of life and that of
non-living matter. Growth and increase are truly represented in
the inorganic world ; but these processes are different in kind from
the actions which stamp the development of the animal or plant.
The birth of a crystal, although regulated by definite laws, is, after
all, a matter of outside regulation alone, and one in which the crystal
itself is but a passive agent. New particles are added to the outside
surfaces of the old and already formed particles ; and crystal and
stalactite thus grow mechanically and by accretion, but without active
participation in the work destined to mould and form their substance.
Very different are the forces and laws which regulate the pro-
duction of the living form. Here the changes of form and the
1 68 CHAPTERS ON EVOLUTION.
building of the frame are marked out in plain and definite path-
ways by laws essentially independent of external conditions. True,
the development of the living form may be retarded by cold or
favoured by warmth, but these conditions leave unaffected the
course and direction in which it is destined to pass towards the form
and belongings of the parent which gave it birth. Stamped in-
effaceably on the pages of its life-history, the way of the animal or
plant towards maturity is written for it, not by it. Internal forces
and hidden but all-powerful laws of life direct its progress, and
ultimately evolve the perfect being from the shapeless germ, in
which its past as derived from its parents, and its future as depend-
ing in some degree at least upon itself, meet in strange and incom-
prehensible union. The development of a living being may be
further shown to be merely a part of the wondrous cycle in which
life appears to direct its possessors. From the egg or germ, develop-
ment leads us to the perfect being. Next in order we consider its
adult or perfected history ; and in due time we may discover the
adult existence to merge into that of the immature state in the pro-
duction of germs, in the development of which its own life-history
will be duly repeated. The period of adult life in this view merely
intervenes 'betwixt one development and another, and serves to
connect those ever-recurring stages in the life-history of the race
which it is the province of development to chronicle and record.
As 'a necessary item in the perfect understanding of animal and
plant history, it may readily be understood how important a place
development occupies in modern biology. Nor is the interest of the
study excelled by its importance. The mystery of life itself might
well be thought by the older physiologists to resolve itself into an
understanding of the fashion in which Nature moulded and formed
her varied offspring. The manner of development might be almost
expected to explain the mystery of being ; but the problem of life is
left as insoluble as before, after the course of development in even
the lowest grades of existence has been traced. The history of
development but environs the puzzles connected with life and its
nature. It leads us to the beginnings of life, it is true, but it leaves
these beginnings unaccounted for, and as mysterious as before. It
explains now this tissue or that, this organ or that, is fashioned and
formed ; and as we watch the formless substance giving birth to the
formed, the indefinite evolving the defined, we might well be tempted
to think that the " why " of nature was explained by the " how."
Yet the springs of life and vital action remain hidden as of yore, and
the exact origin of life is a mystery as insoluble as when the thoughts
of men were first directed to its elucidation.
Apart, however, from the admission that the study of development
has not brought us nearer to the solution of the question, " What is
THE EVIDENCE FROM DEVELOPMENT. 169
life ? " the investigation of the life-histories of animals and plants is
fraught with high importance in another sense and in other aspects
of the scientific interpretation of nature. The early observers hardly
imagined that, in their researches into the formation of the chick, they
were laying the foundation of a study which in future days would be
destined to aid man's comprehension of his own origin and that of
all other living beings. Aristotle's observations upon the developing
chick, and his bestowal of the name punctum saliens, or " beating
point," upon the first beginnings of the heart in the embryo bird, were
in truth fraught with an importance to succeeding generations which
that philosopher could barely have realised had he possessed any
prophetic foresight whatever. And no less would Harvey himself
have been astonished had he beheld the results to which the pursuit
of his favourite study has led in these latter days. It was that great
philosopher himself who first maintained that the chick was developed,
not from the white of the egg, but from a minute speck or scar on
the surface of the yolk, known as the blastoderm or cicatricula. In
felicitous terms Harvey enunciates his opinion that the " Medici,"
or disciples of Galen and Hippocrates, were in error when they sup-
posed that such important structures as brain, heart, and liver were
first produced, simultaneously, as minute sacs or vesicles ; and he
disagrees with Aristotle, in respect that the latter had maintained the
punctum saliens [or punctum sanguineum\, or heart, as the chief agent
in forming the structures of the new being. Harvey ascribed to the
blood itself the formative power in developing the chick. With
Aristotle, however, Harvey is in perfect agreement in believing that
the chick is formed, not by the sudden formation of new parts outside
the already formed organs, nor by the growth of a miniature and
perfectly formed embryo into the larger chick, but by the gradual
development and elaboration of uniform and like matter into the
new and varied parts and organs of the bird.
Such were Harvey's views regarding the nature of development.
Of the supreme interest exhibited by the discoverer of the circula-
tion in this study, no better proof could be cited than his own words
when he maintains " that it is most apparent that, in the generation of
the chicken out of the egge, all things are set up and formed with a
most singular providence, Divine wisdom, and an admirable and
incomprehensible artifice." Harvey's doctrine of development re-
ceived the name of Epigenests, which the physiologist himself defines,
in his forty-first " Exercitation," as " the additament of parts bud-
ding one out of another." Contrasted with this opinion, is that
of such physiologists as Malpighi and Leibnitz. They held that
the body of the chick could be traced in the egg before the first
rudiment of the heart appeared ; and that from the first formation of
the egg, and prior to incubation, the young bird was to be found
170 CHAPTERS ON EVOLUTION.
perfectly formed therein. Thus, by Malpighi's view, the process "of
development was merely one of the expansion, unfolding, and en-
largement of parts already formed ; and this idea became known as
that of Metamorphosis, in contradistinction to Harvey's theory of
" Epigenesis." So, also, Bonnet maintained the existence of a minia-
ture chick in the egg from the first moment of its formation. Subse-
quent growth and nutrition merely expand the elements and parts of
this germ into those of the adult ; and thus Bonnet declares the
process of development to be merely one of " Evolution." Thus
the doctrine of " Epigenesis," as enunciated by Harvey, becomes
opposed to that of " Evolution," as maintained by Bonnet and
Haller — the development of new parts and structures from a struc-
tureless substance, as distinguished from the mere enlargement
and unfolding of the miniature but already formed elements of the
frame.
But when Bonnet, in 1762, in his work entitled "Considerations
sur les Corps organises," was elaborating his theory of " Evolution "
and less rational views on " Emboitement " — a theory holding that
each germ is the receptacle of the germs of all future beings of its
race — Caspar Friederich Wolff had already lent his aid towards
placing the Harveian views on a secure and stable basis. Wolff
showed that the scar on the hen's egg consisted of particles amidst
which no rudiment of an embryo chick could be traced. He further
demonstrated the changes whereby the chick was built up from these
cells, and showed the process of development to be truly one wherein
new parts were formed in succession, and added to the already formed
organs. Succeeding Wolff came Pander, who filled in the outlines
his predecessor had so well sketched out by detailing the earlier
stages and processes seen in the formation of the young bird. From
Pander came the name blastoderm, given then, as now, to the substance
or formative material resulting from early changes in the " egg-scar,"
and from which material all the parts of the young animal are formed.
This observer also cleared the way for his successors by pointing out
the presence of the three layers into which the blastoderm divides ;
each layer bearing an important share in the formation of the tissues
of the developing being. To Pander came in due time a worthy
successor, who may be said to have laid the solid foundations of
the study of development as prosecuted in modern times. This was
Von Baer, whose labours each physiologist and naturalist of to-day
must hold in grateful remembrance. He it was who, besides per-
fecting the details already to hand, discerned the important fact
that the highest animals are developed from eggs or germs re-
sembling in essential nature those of the lowest. But perhaps the
greatest triumph of discovery and research as represented by
Von Baer's labours resulted in the enunciation of his " law of deve-
THE EVIDENCE FROM DEVELOPMENT. 171
lopment," which may be briefly expressed in the phrase that
" development proceeds from the general to the special."
To rightly understand the purport of this axiom, our preliminary
studies in the constitution of the animal kingdom must be borne in
mind. The animal world, as we have seen, is divided into a number of
great types, the most consistent and most firmly established of which
are the Vertebrates, including the " backboned " animals from fishes
to man ; Molluscs, including shell fish, such as oysters, cockles, snails,
and cuttle-fishes ; and Annulose animals, or Articulates, represented
by animals with jointed bodies, such as worms, insects, centipedes,
crustaceans, &c. Now, if the development of a number of animals
belonging to any one of these three divisions is observed, the egg of
each animal is seen to pursue the even tenor of its way, and to pass at
first through exactly the same stages of development. Up to a given
point, the stages in the development of all Vertebrates, for example,
are essentially similar. Sooner or later, however, development begins
to specialise its course, and hence arise the differences which mark
the adult forms. So also with Molluscs, which in their earlier stages
pass through essentially the same changes, but sooner or later diverge
in their course; each organism passing on its own way to assume
the special features which characterise the adult. Such was the im-
portant generalisation of Von Baer. Succeeding research has but
tended to establish Von Baer's doctrine, whilst it has also enlarged
the conception he was the first to promulgate. It is now known that
there are stages in early development which are common to the eggs
or germs of all animals alike ; and that, from the common track thus
pursued up to a given point by animal life at large, each group of
animals ultimately diverges on its own special way of life. Von
Bae'r's generalisation has thus come to include the whole animal
world, and has in recent times tended powerfully to support the
doctrine which would explain the origin of living beings by presuming
their descent from pre-existing forms, and their relation with each
other as twigs, boughs, and branches of a great connected tree.
The relation of development and its study to the hypothesis of
evolution is thus easy of determination. It is a perfectly reasonable
and most natural conception that in the development of a living being
we should obtain some clue to the history of its origin and to the
birth of its race. If its origin be a subject of research at all, any
information concerning the stages through which an animal or plant
becomes the adult organism, and by which it advances from literal
non-existence to the staid solidity of mature form and perfect life,
should, by analogy the most natural, be regarded as a veritable mine
of knowledge concerning its own beginning — and, by further analogy,
regarding the beginnings of the world of life at large. Upon such a
thought is founded the dependence which modern biology is led to
172 CHAPTERS ON EVOLUTION.
place upon development as a clue to the evolution of living beings.
Succinctly expressed, it is thus held by evolutionists in general, that
the development of the individual is a recapitulation in brief of the
development of its species or race. The history of the production of
the individual is viewed as " the abstract and brief chronicle " of the
changes and developments through which its race has passed in prior
ages of this world's existence. It is true that such a history often
appears meagre and imperfect. Some of its phases become altered
in the lapse of time by the influence of surroundings acting favour-
ably or the reverse upon successive generations. As the lines of
human progress are not always easy to trace, so those of animal
advance and evolution frequently appear blurred and indistinct.
But on the whole the record is tolerably complete. The gaps in
animal histories do not affect the main question at issue — namely,
that, as Darwin says, the embryo or young animal " is a picture, more
or less obscured, of the progenitor, either in its adult or larval state, of
all the members of the same great class." That such a study must teem
with interest, is a remark scarcely requiring mention. Nor may it be
regarded as other than a triumph of scientific research, when develop-
ment may be seen to demonstrate how individual history repeats the
history of the race; and how the living world of to-day once existed in
simpler guise, and in the dim obscurity of the past —
Lay hidden, as the music of the moon
Sleeps in the plain eggs of the nightingale.
Although the study of animal development is in many ways an
intricate branch of research, there exists no difficulty in compre-
hending the broad outlines which mark the building of the body in
the higher as well as the lower forms of animal life. If we watch
the development of some animal — such as a sponge — belonging to the
lower grades of organisation, we may be enabled to distinguish
certain stages which not only mark sponge-development, but also
that of animal life at large. The simplest form of a sponge exists as
a cup-shaped body attached to some fixed object. Such a form of
sponge (Olynthus) is depicted in Fig. 87, 7. The walls of this cup,
consisting of two layers, are perforated with holes or " pores " (/),
which open into the substance of the cup, and thence into the interior,
which communicates with the outer world by the wide mouth or
" osculum " (os). This sponge-cup consists of two layers, of which
the inner is provided with delicate filaments, resembling eyelashes in
miniature, and named " cilia." These cilia by their constant move-
ment cause currents of water to flow into the sponge by the outer
"pores" (p\ whilst by the same movement, the water is driven
outwards by the mouth (os) of the cup. In this way the living
particles of the sponge are supplied with nutriment ; and the com-
parison of a sponge to a kind of submarine Venice, with canals and
THE EVIDENCE FROM DEVELOPMENT.
173
waterways, on the banks of which the inhabitants live, is thus seen to
be fully justifiable. The development of such an organism takes place
through the production of eggs (/;), which are developed in the tissues of
the parent sponge, and which are merely specialised portions or cells of
the inner layer of the parent body. The sponge-egg (Fig. 87, i), it must
be remarked, presents the essential elements seen in the eggs or germs
of all animals. It is a little speck of protoplasm, imbedded in which
FIG.' 87. — DEVELOPMENT OF A SPONGE (Olyntkus).
we find a smaller body known as the germinal vesicle (a), and this latter
in turn contains a still more minute particle, the germinal spot (b).
When such an egg is about to undergo development, the first
changes which occur in its substance are those collectively named
" segmentation." The egg is then seen to undergo a process of divi-
sion (2). It divides internally and successively into two, four, eight,
sixteen, &c., cells or divisions ; these portions ultimately becoming so
numerous, that the egg at the close of its segmentation, from its
resemblance to a mulberry, has been named a morula (3). Soon the
outer cells become elongated and provided with cilia (4), and by
means of these filaments the young organism swims freely about in
the water. In this stage it is known as the planula (4). Next in
order a central cavity and then a mouth (5, m) are formed, this aperture
leading into the cavity (e) of the cup. It is now named the gastrula;
and its body is seen to consist of two typical layers, an outer or
ectoderm (c] and an inner or endoderm (tf). These two germ- layers,
as we shall hereafter note, are common to all animals in the course of
their development — indeed, the formation of the embryo takes place
174
CHAPTERS ON EVOLUTION,
through the subsequent development and elaboration of these two
primary structures. Thereafter, this sponge-embryo will attach itself
to some fixed object ; the outside cilia, no longer required for loco-
motion, will disappear, and it will assume its so-called ascula form
(6). Other and new cilia will become developed in the inner or lining
membrane of the body ; the wall of the cup will next become per-
.forated with pores ; and with the inauguration of the inward and
outward circulation of water, the ordinary features of adult sponge-
existence (7) will thus have been attained.
Such being the course of affairs in one of the simplest of animal
developments, we may briefly summarise the stages included therein.
These stages consist firstly of the segmentation of the egg, which
process produces the mulberry-like mass or morula. Next in
order we find the planula with its two layers and its outer cilia.
Then succeeds the gastrula possessing an internal cavity, into
which a mouth shortly opens ; and wrai the formation of pores and
internal cilia, the form of the adult sponge is duly produced.
Selecting a form of animal life widely removed from the sponges,
let us briefly investigate the stages through which the sea-squirts or
Ascidians attain the somewhat prosaic features
which mark their adult existence. The adult and
ordinary sea-squirt presents itself as a bag-shaped
organism (Fig. 88) rooted to stones at low water
mark, and bearing two apertures (Fig. 88, m, a)
on its upper extremity. The resemblance of these
ascidians to an antique wine-jar (askos) is forcible
enough ; and the characters from which the familiar
name "sea-squirt" has been derived are also readily
discernible. When prying humanity, even in the
legitimate guise of the scientific investigator, pre-
sumes to handle the ascidian constitution too
roughly, these animals are given to eject water
from the orifices of their jar-like bodies— a playful
habit the unpoliteness of which, from its reflex
and unconscious nature, even other than scientific
investigators may well excuse. " Sea-squirts " are
usually regarded by naturalists as near relations of the oysters and
other molluscs ; but their differences from the familiar shellfish , are
so numerous and so important that their separation from molluscs
as an aberrant type of animals and their enclosure, even in the Verte-
brate group, is a perfectly legitimate procedure.
Again the aptness of the Harveian motto, "Omne vivum ex
ovo," is apparent, when we find that sea-squirt history begins with
the production and fertilisation of an egg or germ (Fig. 89, i),
which resembles that of the sponge and of all other animals, man
FIG. 88.
SEA-SQUIRT.
THE EVIDENCE FROM DEVELOPMENT.
'75
included, in possessing a germ-vesicle (a) and germ-spot (b\ Once
again, as in the sponge, we meet with the process of egg- segmentation
(2), resulting in the production of a morula (3). Then the cells of the
morula arrange themselves to form the two layers (Fig. 89, 4, <:, en)
as in the sponge, the outer layer being pushed inwards upon itself so
as to form a central cavity (d\ much as a night-cap is so modelled to
fit the head. Thus our " gastrula-stage " (4) once again appears, and
in the life-history of an animal very far removed from the sponge in
structure and relationship.
From this stage, common alike to sea-squirts and sponges,
ascidian development begins to specialise itself. Another opening
or depression (b] appears above the opening which formerly led into
the gastrula-body. Within
this depression, which at first
communicates by an opening
(o) with the exterior, a part 0*
the outer layer is contained,
and finally becomes shut off
from the other portions of
that layer. This separated
and confined part (b} of the
outer layer becomes the
nervous system of the sea-
squirt. Next in order, we
find the body to extend itself
behind, so as to form a well-
marked "tail" (Fig. 89, 5),
within which a rod-like body,
the urochord (n), is formed.
Overlying this body at its
front portion, the nervous
system (/) just mentioned is
further elaborated ; and mus-
cular elements become de-
veloped in connection with
the tail and its contained rod.
Meanwhile the beginnings of a digestive system (d] and of the breath.
ing- sac (g) are being formed, and at this stage the young sea- squirt
appears to be actively mobile, and to swim freely in its tadpole-like
stage of development. Fixing itself thereafter by specially developed
points of attachment, there begins a process of apparent degeneration
in our as yet undeveloped ascidian. The tail wholly disappears, and
the nervous system degenerates until but a mere fragment remains ;
and with an alteration of the form of the body, and some modifica-
tion and further development of the other systems of organs (such as
FIG. 89.— DEVELOPMENT OF SEA-SQUIRT.
CHAPTERS ON EVOLUTION.
the digestive apparatus and heart), the larval ascidian becomes the
mature sea-squirt.
It is of interest to note that in a few aberrant members of the
sea-squirt group the larval or immature characteristics are retained
throughout life. Such are the Appendicularians
(Fig. 90), which, although ranked as veritable sea-
squirts, retain, as a permanent belonging, the tail (/)
which their neighbours possess only in the days of
their youth. Within this permanent tail the noto-
chord (ri) appears developed as in the fleeting ap-
pendage of other sea- squirts, whilst the other organs
of sea-squirt existence (digestive system [j], heart
[<#], &c.) are fully developed. From the possession
of this notochord these curious animals appear as
unique invertebrates, and stand alone amongst their
fellows as presenting the closest resemblance to the
vertebrate animals. In the Appendicularians we
may perceive the existing representatives of the stock
and ancestry which gave origin alike to the fixed
sea-squirt race and to the great vertebrate group
itself. These " permanent larval forms," as Appen-
dicularia and its neighbours are termed, thus present
us with the least modified members of their class, —
with the primitive and unchanged organism whose
development in other directions has produced the
APPEN'DICULARIA. highest races of living beings. Of these organisms
Darwin himself remarks that, "if we may rely on
embryology, ever the safest guide in classification, it seems that we
have at last gained a clue to the source whence the vertebrates
were derived. We should then be justified in believing that at an
extremely remote period
a grouP °f animals ex-
isted, resembling in many
respects the larvae of our
present Ascidians, which
diverged into two great
branches — the one retro-
grading in development
and producing the pre-
sent class of Ascidians,
the other rising to the
crown and summit of the animal kingdom by giving birth to the
Vertebrata."
Ascending now to the confines of the Vertebrate sub-kingdom
of animals, we may trace the development of the curious little fish
known as the Lancelet or Amphioxus (Fig. 91, b) — a form interesting
FIG. 91.— AMPHIOXUS, OR LANCELET.
THE EVIDENCE FROM DEVELOPMENT.
177
not merely as being at once the lowest fish and Vertebrate, but as
evincing in its development a most marked likeness to that of the
sea-squirt, whose manner of entrance upon the stage of life we have
just studied. The lancelet is a little fish attaining a length of one
or two inches, and found inhabiting sandy coasts in various parts of
the world. Its body is pointed at either extremity, and, save for a
narrow fin bordering the upper and part of the lower surface of the
body, no traces of the appendages commonly seen in fishes are
to be found. This fish occupies the position of a very singular and
anomalous member of the Vertebrate series. Unlike most of its
congeners, it has no skeleton or backbone, a mere soft and gela-
tinous chord, termed the notochord, existing in the place of and
representing the spine. It has no paired fins or limbs ; it wants a
heart; it has no skull or brain; and its organs of sense are represented
by mere pigment-spots for
eyes ; whilst the mouth pos-
sesses a series of filaments (c)
probably subserving the sense
of touch. This little animal
would seem thus to hover,
as it were, on the outermost
confines of Vertebrate exist-
ence. Its adult characters
resemble the rudimentary
traits of other Vertebrates ;
and in respect of its entire
structure, and still more so
of its development, it may
be said to be a connecting
link between Invertebrates in
general and sea-squirts in par-
ticular on the one hand, and
the Vertebrate sub-kingdom
on the other.
Like all other animals
above the very lowest, the
lancelet's history begins with
the production of the germ or egg (Fig. 92, i ), which exhibits in its es-
sential structure the closest similarity to that of the sponge or ascidian.
The first changes to be witnessed in the developing egg of the
lancelet consist in the complete division (Fig. 92, 2, 3) of its substance.
Segmentation of the egg of the lancelet is on an exact parallel with
that of the egg of the sponge or the sea-squirt. We shall presently
note that this segmentation is also imitated, completely or in part,
in higher forms of life. As in the sponge, the " blastoderm " is duly
N
FIG. 92. — DEVELOPMENT OF LANCELET.
1 78 CHAPTERS ON EVOLUTION.
formed ; the infolding of this blastoderm and the formation of the
pocket-like "gastrula" (Fig. 92, 4, and Fig. 89, 4) taking place
exactly as in the development of the sea-squirt. Furnished with
its eyelash-like cilia, this gastrula-lancelet swims freely in the sur-
rounding water. Not a trace of its vertebrate character can be
observed at this stage. It might be the forward progeny of a worm,
or might be ranked as a developing snail ; whilst if it were alleged to
be an embryo star-fish, or a baby sea-squirt, the zoologist would pro-
bably own his inability to say which assertion was correct, or most in
accordance with the appearance of this curious organism.
The succeeding course of events to the gastrula stage brings an
elongation of the body, and from the inside or pocket-like cavity of
the gastrula (4, d) the digestive tube of the future lancelet (5, d) is
seen to be gradually fermed. Then, also, appear the first marks and
traces of its vertebrate relationship, and of its kinship with the aris-
tocracy of the animal kingdom. The flattened aspect of the body now
shows a tendency to develop two ridges or projections, which soon
meet and unite in the middle line to form a tube (Fig. 92, 5, b], enclosing
the nervous axis. This nervous tube remains open for a time in the
lancelet, as depicted at o, Fig. -92. The body of the young fish
now assumes somewhat of the appearance of a flattened cylinder.
It resembles closely the young sea squirt, and, like the latter, possesses
in its back region, a rod-like body, the notochord (5, n). In the
lancelet, however, the notochord extends completely from head to
tail. The identity of the two developing bodies may be best
demonstrated by a comparison of their longitudinal sections in Figs.
89 and 92 (5, 5), where the arrangement of parts and organs is seen to
be essentially similar.
The next change results to the fore-part of the body, where the
throat is seen to become cleft or perforated by the gill-slits — a sea-
squirt feature again being apparent in this latter phase of develop-
ment It is equally curious to note that similar clefts — to be more
specially alluded to hereafter — appear in the development of all other
Vertebrates, including man ; these clefts in fishes bearing gills, but in
reptiles, birds, and mammals becoming obliterated. Ultimately the
free-swimming lancelet assumes habits of more staid character. The
notochord, which in most other and higher Vertebrates is replaced by
the spine, remains in the lancelet as the permanent representative of
the backbone, just as in Appendicularia (Fig. 90), among the sea-
squirts, the " urochord " persists throughout life. And with the
appearance of the systems and organs characteristic of its adult
existence, the preparatory stages of lancelet life may be regarded as
having been completed. Thus it is certain that the development of
the lancelet, whilst clearly that of a vertebrate animal, is also seen
to produce a low type of vertebrate organisation, and to present
THE EVIDENCE FROM DEVELOPMENT.
unmistakable affinities and likenesses to the development of the
sea-squirts and of other invertebrate animals. Noting the absolute
likenesses which exist between the development of the sea-squirt and
lancelet, there seems every justification for the scientific belief that
both animals have arisen from a common stock. The gulf between
Vertebrate and Invertebrate life in this view, no longer exists ; and
the lancelet may be legitimately regarded as the parental form of all
Vertebrates, from the fish to man.
Still higher in the vertebrate scale do our researches in develop-
ment lead us when we approach the study of the chick and its early
life-history. And what is true of the chick's development is, with
greater or less modification of details, true of the production of every
F
FIG. 03. — DEVELOPMENT OF VERTEBRATE.
other vertebrate animal, man included. In the developing egg of the
bird, the yolk undergoes segmentation (Fig. 93, A, B, c, D), as in the
sponge, ascidian, and lancelet; but the process is partial in the
bird, whilst it affects the entire egg-mass in the development of lower
life. The blastoderm is duly formed as the result of segmentation,
and from this substance —
seen in the cicatricula, or scar
of the egg — arises the future
fowl ; the great mass of the
yolk and white serving as
nutrient material for the de-
veloping bird. Soon, the
cells of which the blastoderm
is composed are seen to form
themselves in three layers
an outer layer or epiblast; a middle layer or
N 2
SECTION THROUGH
FIG. 94.
A DEVELOPING VERTEBRATE.
(Fig. 94, E, M, H)
i8o
CHAPTERS ON EVOLUTION.
tntsoblast ; and an inner layer oikypoblast. It may be well to remark
at this stage of our inquiries the part played by each of these three
layers in the formation of the young animal. From the epiblast arise the
outer skin and the nervous system. The superficial layer of the body,
and the great internal nerve-centres governing the frame, its move-
ments and vital processes, thus arise from one and the same layer —
a fact appearing to argue in favour of the origin of the nervous system
FIG. 95.— DEVELOPMENT OF CHICK.
of Vertebrates from a layer which in anterior stages of existence (as in
the animalcules of to-day) originally formed the outer and sensitive
margin of the body. From the mesoblast or middle layer arise the
bones, muscles, blood-vessels, the under skin and other parts; whilst
the hypoblast or under layer gives origin to the
lining membrane of the digestive system, and to
such digestive organs as the liver, pancreas, &c.
About the. sixth or eighth hour of incuba-
tion, these three layers of the blastoderm in the
chick are duly formed. Very rapidly succeed
the changes which result in the production of
the chick itself. A groove (Fig. 95, A, <:, and
93, E, e) soon appears on the surface of the
blastoderm, this furrow being known as the
" primitive groove," and constituting the keel
of the body, so to speak. The edges of the
groove finally grow together (Fig. 95, B, ), and
convert the groove into a canal (Fig. 94, c). A
portion of the epiblast is pinched off from the re-
maining portion, and being included within the
tube thus formed, duly gives origin to the brain and spinal cord. As
two projections of the blastoderm grow upwards to form the spinal
THE EVIDENCE FROM DEVELOPMENT. 181
region, so two folds grow downwards, and thus tend to form the body-
walls of the young animal. Contemporaneously with these changes
we find a structure of high importance to be gradually formed in the
back region of the chick. This structure is the " notochord," a rod-
like body (Fig. 94, cfi), composed of a string of cells, which lies just
beneath the first-formed tube (c), or that containing the nervous
rudiments. The formation of this notochord cannot but forcibly call
to mind the similar string of cells which appears in the course of
development in the sea-squirt's larva ; such a similarity being of too
marked a character to admit of its being regarded in the light of a
mere coincidence. On each side of the notochord the elements
of the spine, in the shape of little cubical vertebrae (Figs. 95, c, e;
93, F, /; and 96, pv\ are duly formed from the middle layer or
mesoblast. The notochord itself — a permanent structure in such
fishes as the lancelet, sharks, dog-fishes, &c. — gradually disappears in
the chick ; its retrogression being apparent after the sixth day, whilst
it is found to have entirely disappeared at the time of hatching ;
whatever of its substance remains being absorbed in the formation
of the spine. The folding of the blastoderm in front and behind
soon specialises the head and tail (Fig. 96, f) of the young animal ;
the head extremity presently showing three swellings (fb, mb, hb),
or dilatations — from which the brain is duly formed — and bending
downwards in a highly characteristic fashion. Brain-development is
accompanied by the formation of organs of sense, such as the nose,
eyes (op v], and ears (au /), which arise as pocket-like ingrowths from
the epiblast or outer layer of the body ; whilst the mouth is similarly
formed by an infolding of the outer layer, and is later on placed in
communication with the digestive system.
During the third day of incubation, certain highly important
structures appear in the neck of the chick. Four clefts or slits
(Fig. 97, A, g) are formed in the walls of the throat, these being named
the visceral or branchial clefts. The
upper edges of the clefts form thick
folds, named the branchial 'folds \ five
folds existing to the four clefts, as
the last cleft has its lower border
thickened in addition to its upper
edge. The significance of these
clefts and folds will be hereafter
alluded to ; it may at present, how-
ever, be noted that all the clefts in
the chick Save the first are Closed FIG. 97.— EMBRYO-VERTEBRATES.
by the seventh day of incubation.
The visceral "folds" contribute in an important fashion to the
formation of the jaws and other structures belonging to the skull,
1 82 CHAPTERS ON EVOLUTION.
the two hindermost folds disappearing in the chick, without leav-
ing any traces of their existence. The limbs (Fig. 97, w, /) begin to
be developed about the fourth day, and first appear as little buds
projecting from a ridge — the " Wolffian ridge " — running round the
young being from neck to tail at about its middle portion ; but it
is only about the fifth day that the distinctive characters of the limbs
can be discerned. By the tenth day, however, the wings and feet, in
all their characteristic structure, may be distinguished; The skull
dates its history from the fifth day ; and only during the succeeding
day may the bird-type of the chick be perceived in the characters of
wings, feet, digestive system, and other stnictures — so remarkably
alike are the developing young of higher Vertebrates in their earlier
stages of development. Meanwhile — as early, indeed, as the third
day of life — the lungs have been formed as little pocket-like growths
from the throat; and even before hatching, the chick begins to use its
breathing organs-. With fully formed parts, and perfectly equipped
for the new existence which lies before it, the chick duly breaks the
shell with its armed beak, and, throwing off the shrivelled remnants of
organs once useful in- its earlier stages, enters upon the characteristic
life of its species.
If the development of a quadruped' were -traced, or the stages
of man's physical progress in early life reported upon, much the same
course of development as that described in the case of the chick would
be chronicled. We should see segmentation of the quadruped-germ
(Fig. 93, A, B, c, D), as in the lancelet ; we should note the formation
of a blastoderm and its three layers, of a primitive groove, of a noto-
chord (Fig. 94, ch\ of three brain- vesicles ( Fig. 96, fb, mb, hb}, of visceral
or branchial clefts (Fig. 97,,^), and of other structures similar to those
of the chick. Only in the latest stages, should we be able to trace
the appearance of the higher features of the quadruped or mammal
as distinguished' from those of the bird. Human development, so far
as has been traced, runs parallel with that of lower forms of life, and
exhibits the- " morula" stage (Fig. 93, D) equally with the sponge
(Fig. 87, 3), sea-squirt (Fig. 89, 3), or lancelet (Fig. 92, 3). Man's de-
velopment is in truth but an epitome — condensed and modified, it may
be, but still a recapitulation — of that of lower forms of life. Thus it
is no mere supposition, but a weighty physiological fact, that through
flitting and successive stages, which exactly repeat and represent per-
manent forms in lower life, man finally attains to be the " paragon of
animals." And thus, also, the community of type and general struc-
ture which man shares with the lowest fish is demonstrated anew by
the marvellous history of the manner in which that type is evolved,
alike in its lower and higher phases.
The marshalling of facts to form generalisations, and the stringing
of these facts upon the thread of a connected history, 13 a duty which
THE EVIDENCE FROM DEVELOPMENT. 183
lies next to hand in treating of development and the lessons it is
calculated to teach. Let us, in the first place, try to discover the
place and import of Harvey's teachings concerning development, as
compared with those of succeeding investigators and theories. There
can be no question that the researches of the nineteenth century
have but confirmed and enlarged the observations of Harvey in the
seventeenth. " Epigenesis " is seen to be the method of nature in
developing the animal form with that " admirable and incomprehen-
sible artifice " which Harvey so justly admired. From the primitive
and undifferentiated protoplasm of the egg, modern embryology
beholds the formation of the chick in a fashion strictly corresponding
in all essential details to that outlined by, the genius of Harvey.
Compared with the views of Malpighi — holding that the egg contained
a miniature chick, and that development was merely an unfolding or
expansion of already formed parts — Harvey's description and theory
of development stand forth in marked contrast in respect of their
thorough correspondence with the fruits of modern research. Bonnet's
theory of the " evolution," through the supply of nutrition, of an
already formed chick contained in the egg,, meets a like fate to the
opinion of Malpighi ; whilst his doctrine of " emboitement " — credit-
ing each germ with being the repository of all future germs — when
taken literally, shares a like fate with his ideas regarding the evolution
of the single animal form. As supplementing the ideas of Harvey
by direct observation, we note the philosophic nature of the views of
Wolff, through whose researches the foundations of modern embryology
may be regarded as having been laid. . The line of research leading
from Wolff and Pander to the present day may be held to represent
merely the direct continuation of the " Exercitations "" of Harvey,
whose " philosophising " has thus led to results of which its sage
founder, with all his perspicuity, could have had no warning or idea.
The details of the studies in development outlined in this chapter
must now be briefly summed up ; whilst a glance at their bearings upon
and teachings regarding evolution may form a fitting conclusion to the
present stage of our studies. Firstly, then, it is noteworthy that the
germ or ovum of all animals — excepting the. very lowest, or Protozoa —
appears as a protoplasmic mass, which exhibits all the characters of
the microscopic body known as a" cell" (Figs, 87, 89, .92).. In the
lowest animals just named, the, difficulty -of distinguishing their germs,
and indeed their entire developmental history, arises in great, part
from their ill-defined nature, and from the marvellous analogies and
likenesses they present to their, lower plant-neighbours. In the
"biological no-man's-land" where the lowest . animals and lowest
plants meet in a confusing identity of form and function, distinctness
of germ-elements may neither be expected nor found. But it is at
the same time noteworthy that even in this lowest group of the
i84
CHAPTERS ON EVOLUTION.
animal series the observed phenomena of development occasionally
1 resent a singular resemblance to the primary process about to be
alluded to, and already named segmentation ; such resemblance
being inexplicable save on the supposition that in these lowest forms
the development of the higher is foreshadowed in dim outline.
Take as an example the development of Magosphcera (Fig. 98, i),
a low form of marine animalcule found living on the Norwegian coast
by Haeckel. It resembles -the familiar animalcule known as the
Amoeba; but during the development of new beings the Mago-
sphaera assumes a spherical form (2), within which a nucleus (a) and
nucleolus (b] give it the appearance of a veritable egg (compare
Fig. 87, i). Next in order succeeds a process remarkably like that
known as " segmenta-
tion" in the eggs of
higher animals. In the
course of this process
the Magosphaera divides
(3» 4, 5). ur»til a stage
resembling the " mul-
berry stage," or morula
(6), is attained. There-
after the outer surface
becomes ciliated, and,
liberated from its invest-
ment, the Magosphaera
swims freely (7) in the
sea. Soon this free-
swimming sphere breaks up into detached fragments of protoplasm, at
first ciliated (8), but finally assuming (9) the adult Magosphaera form
(i). In the well-defined groups of the animal world, ranging from
the zoophytes, corals, and their neighbours (Caelenterate animals),
up to Vertebrates, including man himself, we are presented with a
marvellous likeness and an undoubted correspondence in the form
and nature of the germ, and of the processes in virtue of which that
germ is started on its developmental journey.
Next in order, we note the occurrence, in the developing eggs of
all animals, of that process to which the name segmentation has been
given. We have seen that the germ or egg of the sponge, equally with
that of the sea-squirt, lancelet, chick, and also with that of Mammalia,
or quadrupeds, exhibits this process of division. The egg, at first
single-celled, becomes in this way many-celled ; and the variations
observable in the process itself are but insignificant when contrasted
with the extraordinary uniformity in the broad outlines thus exhibited
by the eggs of all animals in their first stages, and in the changes pre-
paratory to the outlining of the future form. But the similarity be-
FIG. 98. — DEVELOPMENT OF A PROTOZOON.
THE EVIDENCE FROM DEVELOPMENT.
185
tween the development of widely different animals ends not thus. If
the process of segmentation is universal, the morula or " mulberry
stage," in which that process culminates, is seen to be no less uniform
and unvarying in its occurrence. Even among the Protozoa, as
we have already remarked, we may perceive stages (Fig. 98, 6) in
development which imitate the " mulberry mass " of higher forms.
We have already traced the occurrence of this stage in the sponge
and in the other life-histories described in this chapter ; whilst
a wider survey of the animal world would serve to show that in the
early history of every group the " mulberry stage " is to be witnessed,
as the first prominent landmark or halting-place on the journey of
life. The egg of such an animal as a " Tardigrade " or Bear-animal-
cule— minute organisms allied to the mites, and found in the gutters
of house-tops — thus exhibits in its development stages of a nature
essentially similar to those seen in the history of both lower and
higher forms of animal life. The egg itself (Fig. 99, i) exhibits a
structure comparable with that of all other germs. In its develop-
ment the germ not only passes through the stages of segmentation
(2, 3) already familiar to us in the sponges, sea-squirts, and vertebrates,
but also arrives in due course at the mulberry epoch (4), or morula,
whence the special fea-
tures of the Tardigrades
are specialised. How per-
fectly these details in the
animalcule correspond
with the stages in the
development of the verte-
brates or highest animals,
is a fact which may be best appreciated by the comparison of the
segmentation of the egg of a vertebrate animal depicted in Fig. 100,
from its commencing development (i) to the attainment of the mul-
berry stage (5); whilst that of the frog (Fig. 101) exhibits essentially
the same phases as the developing germ of bird or mammal. With
FIG. 99. — DEVELOPMENT OF BEAR-ANIMALCULE.
FIG. 100. — SEGMENTATION OF VERTEBRATE EGG.
Professor Allen Thomson we may therefore hold that " the occur-
rence of segmentation and the regularity of its phenomena are so
constant, that we may regard it as one of the best established series
of facts in organic nature."
1 86
CHAPTERS ON EVOLUTION.
But the further stages in development of different animals run
parallel beyond the " mulberry stage " of progress. The morula, as
we have seen, becomes a " planula " — a stage we saw distinctly in the
sponge (Fig. 87, 4), and which is repeated with but little variation in
the development even of the highest animals. Thus the " planula "
appears to be well-nigh as universal in its occurrence as the " morula."
But we saw that the planula in due course became the bag-shaped
structure named gastrula (Figs. 87, 5 ; 89, 4 ; and 92, 4). The wall of
the planula is pushed in upon itself on one side, a central cavity being
thus formed, bounded by a double wall, and communicating with the
outer world by the
mouth. Such is the
"gastrula"; and in its
composition we are
able to discern the
two primitive layers,
named, as we have
seen, ectoderm («•)
and endoderm (en), or
FIG. 101. — SEGMENTATION OF FROG s EGG. ... . . \ ,',
epiblast and hypoblast.
A third layer, the mesoblast, makes its appearance between these two
primary membranes, and from these three layers, as we have already
seen, all the structures of the future animal are in due course deve-
loped. It seems perfectly certain, then, that if the mulberry stage
constitutes a first landmark in the development of the animal kingdom
at large, no less does the
" gastrula-stage " form
a second resting-place
in the track of life.
Since, as Haeckel and
other embryologists
have shown us (Fig.
102), the gastrula stage
of development (with
its primitive mouth \nt]t
body-cavity or stomach
[/], and double layers
\ec and en~\) occurs
equally in the zoophyte
(5) and worm (i); is as
typical of the star-fish
(2) as of the crustacean
(3); and aids as ma-
terially in the formation of the snail (4) as in the development of
the vertebrate (6). After its " gastrula-stage," each animal form may
6 6
FIG. 102.— GASTRULAS OF VARIOUS ANIMALS.
THE EVIDENCE FROM DEVELOPMENT.
187
be said to assume the special features of the group to which it
belongs. At this point the vertebrate will pass towards its own
sub-kingdom, and develop in due time the special features of the
fish, the frog, the reptile, the bird, or the quadruped. Hence, as
from a common point whence numerous ways and paths diverge,
each organism will elaborate or. develop its " gastrula " germ into a
frame more or less complicated, and into belongings and structures
suiting its rank in the great kingdom of animal life. From such
a standpoint we may discern, more clearly perhaps than at any
other stage of our researches, the justice of the comparison which
symbolises the animal world and its origin by the figure of a tree,
whose divergent branches bear at their extremities the apparently
distinct and specialised groups of animals, but whose stem and trunk
from which these branches spring, no less powerfully represents the
common origin and uniform development of its varied parts.
That the evolutionist's case for the common origin and produc-
tion by descent of the forms of animal life is strengthened and sup-
ported by the facts of development, is a statement which can admit
of no question in the eyes of those who fairly face the facts, and who
logically, and without bias or prepossession^ construe their meaning.
On any other supposition than that of the common origin and subse-
quent specialisation of the varied forms of animal life, the fact that a
sponge, a sea-squirt, and a lancelet pass through essentially similar
stages of development, pre-
sents itself simply as an
inexplicable mystery. Conir
munity of development be-
tokens community of origin ;;
otherwise the facts of nature
must present themselves as
absurdities admitting of no
logical construction whatever. .
Why a vertebrate animal in
its earlier history should re-
semble a sponge or sea-squirt
is a query simply unanswer-
able, save on the hypothesis
that vertebrate ancestry was
at one period transmitted
through forms of which the
sponges and sea-squirts are- the existent and it may be the altered
representatives. The development of an animal thus reasonably
stands before usr in the newer interpretation of evolution, as a veri-
table panorama of its descent Often, according to Darwin's already
quoted remark, the series of pictures may here and there be obscured.
FIG. 103.— DEVELOPMENT OF A FISH.
1 88
CHAPTERS ON EVOLUTION.
The continuity of the shifting views may be interrupted by the extinc-
tion of stages through the influence of external conditions or of
unknown causes. But in most cases, the outlines remain clearly and
fairly drawn, and afford us a glimpse into the order of nature, not
only more astonishing, but also more convincing in its teachings, than
the views obtained of the world of life from any other standpoint.
There yet remain for consideration one or two important points
suggested by the details of animal development — these latter points
bearing as intimately, perhaps, on the argument for evolution as the
grand facts of development themselves. First in order, it behoves us
to note the interesting facts concerning the branchial arches and gill-
clefts of vertebrate animals, already noticed, and the conclusions to
which the observation of these facts eventually leads.
The branchial or gill clefts were remarked as being developed in
the neck or throat of the chick (Fig. 97, g) about the third day of
incubation. The part subsequently played by certain of these struc-
tures in the formation of the jaws was duly noted ; the remaining
clefts and folds disappearing in due course, and leaving no trace of
their existence behind. Shortly stated, the history of these gill-
openings shows us that they are of universal occurrence in the
development of the vertebrate group of animals. They appear in
the fish (Fig. 103 A, g) and in the bird (Fig. 97, g). They are
developed as persistently in man's early history as in the develop-
ment of the frog or reptile. No more convincing proof of the
community of development in this respect could be adduced than
the comparison of the early embryos of different vertebrate animals.
In Figs. 97, 103, and 104 such comparisons have been made. The
gill-arches are there seen to be as clearly the natural heritage of
FIG. 104.— EMBRYOS OF QUADRUPEDS.
(A) Pig; (B) Calf; (c) Rabbit ; (D) Man.
man (Fig. 104, D) as of the rabbit (c), calf (B), and pig (A); whilst
they are as typically represented in the chick (Fig. 97, g) and
fish (Fig. 103, g). In the fish and in some newt-like animals,
the visceral clefts and arches become permanent features of their
adult life, and are associated with the " gills " or breathing organs.
THE EVIDENCE FROM DEVELOPMENT.
189
But reptiles, birds, and quadrupeds are lung-breathers, and possess
gills at no period of their life. Why, then, it may be asked, should
they invariably develop in their early life gill-arches and gill-
clefts which bear no relation to the wants of their adult existence ?
The gill-arches of reptiles, birds, and mammals never develop gills ;
and even the gills and gill-clefts of tadpoles (depicted in Fig. 105, g)
disappear when these animals become adult toads, frogs, and newts.
Why, then, it may be repeated, does this seeming irrationality and
useless expenditure of creative power in nature exist ?
The true and only answer to such a pertinent query is, that the
gills and gill-arches of higher Vertebrates bear reference to a former
condition of matters. They relate to anterior stages of vertebrate
existence, when the ancestors of lung-breathing animals were repre-
sented by gill-bearing and aquatic forms. Gill-arches and gill-slits
FIG. 105. — TADPOLES OF FROG.
thus appear as a true legacy and inheritance from an aquatic ancestry.
In the higher Vertebrata the first gill-opening becomes converted
into structures and parts connected with the ear. The remaining
clefts disappear, whilst the gill-arches themselves contribute to form
the tongue-bone (hyoid bone) and the small bones or vesicles of the
internal ear. Only on the theory of descent with modification can
we rationally explain the presence of now useless structures such as
the gill-arches and gill-clefts of lung-breathing Vertebrates. On this
principle, " we may cease marvelling," says Darwin, "at the embryo of
an air-breathing mammal or bird having branchial slits and arteries
running in loops, like those of a fish which has to breathe the
air dissolved in water by the aid of well-developed branchiae."
190 CHAPTERS ON EVOLUTION.
The method of disappearance of the gills and their arches is as
reasonably detailed, when Darwin states that, " in order to under-
stand the existence of rudimentary organs, we have only to suppose
that a former progenitor possessed the parts in question in a perfect
state, and that under changed habits of life they became greatly
reduced, either from simple disuse, or through the natural selection
of those individuals which were least encumbered with a superfluous
part, aided by the other means previously indicated."
Another fact of interest, derived from our studies in development,
relates to the position of the sea-squirt larva and of the lancelet as
together constituting " links " which -bridge the gulf between the
Invertebrates and their higher backboned neighbours the vertebrate
animals. Only when we think of the apparently great gulf fixed
between the fishes as the lowest Vertebrates and all invertebrate
forms, can we realise the gain to evolution of the knowledge which
shows how the development of the sea-squirt and that of the lowest
Vertebrate run in parallel lines. Such a correspondence in de-
velopment, and the discovery of the possession by sea-squirts of
the " notochord " — long thought to be the exclusive possession of the
Vertebrates — constitute together a veritable tower -of strength for the
evolutionist, whence he may survey a formidable gap supplied, and a
" missing link" satisfactorily brought to light.
It should lastly be noted that the facts brought to light by a study
of the early stages in the development of animals, may be regarded
as being thoroughly in favour of the theory of Evolution. Professor
Allen Thomson, in his presidential address to the British Association
(1877), stated the latter fact forcibly when he said, "I consider it
impossible, therefore, for any one to be a faithful student of embry-
ology, in the present state of science, without at the same time
becoming an evolutionist." These are weighty words, but they are
fully justified by the circumstances of the case to which they apply.
And no less apt are the terms in which the same authority in matters
embryological further alludes to the support received by evolution
from daily life histories of living beings. " If," says Professor
Thomson, "we admit the progressive nature of the changes of
development, their similarity in different groups, and their common
characters in all animals — nay, even, in some respects, in both plants
and animals — we can scarcely refuse to recognise the possibility of
continuous derivation in the history of their origin."
igi
X.
THE E VIDENCE FROM DEVELOPMENT (continued).
II.— THE LIFE-HISTORIES OF STAR-FISHES AND CRUSTACEANS.
ALLUSION has already been made in the preceding chapter to that
most fundamental proposition of modern biology which maintains,
that " community in development reveals community of descent"
It has also been shown at length that, in the eyes of modern natural-
ists, the development of an animal or plant is regarded as affording
a clue to the manner of its evolution or descent from pre-existing
forms. The formation of a living being to-day, in other words,
repeats for us the formation of its race and species in time past. So
that, once again to quote Darwin's words, " we can understand how
it is that, in the eyes of most naturalists, the structure of the embryo
is even more important for classification than that of the adult."
Or, again, " embryology (or development) rises greatly in interest
when we look at the embryo as a picture, more or less obscured, of
the progenitor, either in its adult or larval state, of all the members
of the same great class."
Second to none in interest, in the eyes of modern biologists,
are the phenomena presented to them in the formation of the
animal or the plant frame. In former years the mystery of
development was great indeed. There could be offered in the
past decade of biology no reason — appealing sufficiently to the
rational intellect as explanatory of the events in question — why a
frog in its development should appear first as a gill-breathing fish,
later on as a tailed newt-like creature, and ultimately as a tailless
lung-breathing amphibian. Nor could natural historians in the past
venture to account in more lucid fashion for the curious changes
which a butterfly or beetle undergoes in its progress from the days of
its youth towards the adult form, and from the stage of the crawling
grub, through that of the quiescent chrysalis, to the full-fledged
" imago " with its wings. Kirby and Spence summed up and dis-
missed such matters in a manner — unfortunately for the free play of
intellectual vigour, not quite extinct in these latter days — which said
much, perhaps, for faith, but little or nothing for reason and
science. These famous entomologists held that insects passed
through a metamorphosis because "such is the will of the Creator;"
and they supplement this " confession of faith " with an attempt at a
192 CHAPTERS ON EVOLUTION.
scientific explanation by the further assertion that, insects being
voracious in their feeding habits, especially in earlier life, perform an
important function in the economy of nature in that they remove
from the earth's surface " superabundant and decaying animal and
vegetable matter." A further reason for this providential arrange-
ment was given in the fact that, as " unusual powers of multipli-
cation " were indispensable for recruiting the ranks of the insect
scavengers, and as nutrition and reproduction are incompatible
functions, the removal of decaying matter during the youthful stages
of the insect's life was to be regarded as a convenient subdivision
of its labours, seeing that its adult existence is spent in the work of
reproducing its race. But it might easily be shown that, whilst a
goodly number of larval insects do feed upon carrion, a large pro-
portion of the class does not exhibit any such habit ; and it might
reasonably enough be maintained that the argument of Kirby and
Spence is open to the serious objection that, in its character, it
tends to illustrate the post hoc ergo propter hoc fallacy — Decaying
matter exists, therefore insects were designed to pass through a
metamorphosis, and were gifted with voracity of disposition that
they might remove the said matter from the earth's surface — a
proposition vitiated in its exactitude by the fact just mentioned that
many insects do not eat such matter; and also by the further facts
that many do not undergo a metamorphosis at all ; that many vora-
cious caterpillars, instead of eating decaying matter, destroy our trees
and flowers. It might also be added that many of nature's scavengers
of higher and lower rank than the insects, do not pass through a
series of changes in development, but grow, nourish themselves in
the exercise of their sanitary work, and likewise, at the same time, and
as adult forms, reproduce their species and continue their race in time.
Clearly, then, the explanation of Kirby and Spence affords no satis-
faction to the contemplative mind in the natural anxiety and desire
to discover the causes of things. At its very best, such explanation
leaves " the reason why " untouched ; and conversely, it can well be
understood how any other system of thought, which presents a more
satisfactory method of accounting for the facts in question, should find
ready acceptance as expanding and enlarging the thoughts of men.
In the previous chapter we discussed the meaning of the remark-
able likenesses which can be readily proved, as matters of fact and
observation, to exist between the early stages in the development of
very different animals. A sponge, a sea-squirt, a lancelet, and even
higher animals still, appear in the first beginnings of their existence to
pursue a remarkably similar course. Each form parts company with
its fellows at a given stage on the way of development, and thereafter
passes by the special pathway of its race towards the adult and perfect
stage. Von BaeVs axiom that development proceeds from the general
THE EVIDENCE FROM DEVELOPMENT. 193
to the special, thus declares a great truth of nature. Modern biology
appears provided with a host of witnesses to the truth of that axiom,
and supplies a reason for the likeness by assuming similarity of descent
from lower life as the explanation of those common and general
beginnings from which the special and varied forms of animals and
plants are evolved every hour around us. The axiom that the deve-
lopment of the individual (ontogenesis] is the rapid shifting or pano-
ramic recapitulation of the development of the species (phylogenesis],
is now regarded in biology as the keynote of the whole study of
animal and plant formation. If we find, for instance, that the frog in
its development is firstly a fish, then a tailed amphibian or newt, and,
last of all, a tailless, air-breathing frog, we see in such a panoramic
succession of changes — constituting the development of the individual
— the evolution and development of the frog race. We read such a
history as showing us clearly enough that the frogs have been evolved
from some ancient fish-stock, that this fish ancestor became through suc-
ceeding modifications a tailed, newt-like amphibian, and finally, that the
newt in turn became the higher frog. Most reasonable is the supposi-
tion and belief that, if the living hosts have descended from common
ancestors, the appearance of ancestral features in their development is
a most natural expectation and a highly natural law of life. That trans-
mission from parent to offspring of hereditary features, so familiar to us
in human existence — the reproduction of family features by the suc-
cessive descendants of the family stock — is, in truth, but the repeti-
tion in higher life of the likenesses to its ancient ancestry we see in
the developing frog. On such grounds, we may attempt successfully
to explain the mysteries of development ; and on such a principle, we
may note in passing, it is easy to see how important a guide to the
classification and arrangement of living beings their development
affords. If those animals which are descended from a common
ancestry resemble each other in their development, such resem-
blances may be held to represent the truest of those blood relation-
ships which it is the business and aim of classification to express.
The chronicle of the development of animal life is, however, not
completed when the earliest changes seen in the formation of the
animal frames have been noted. Long after the common and earliest
stages, described in the last chapter, have been completed, there
may be produced before us marvellous resemblances and likenesses be-
tween animals which, when adult, would seem to possess no community
either of origin or of other relationship. It is to these later changes
in the animal form that we now purpose to direct attention. The
history of those changes which more immediately precede the
assumption of adult life, affords as valuable evidence of the evolution
of species, as does the chronicle of the very beginnings of existence.
It is only needful to point out at the commencement of such a study,
o
194 CHAPTERS ON EVOLUTION.
that admittedly the panoramic views of evolution we are about to
discuss frequently present breaks and gaps in their succession. The
expanding canvas of life here and there exhibits a blank surface, due
to the part erasure of the picture which, we believe, formerly existed
thereon.
There exists a second principle in nature and evolution, of equal
importance to heredity or that in virtue of which the likeness of the
parent or ancestors is transmitted to the offspring or descendants.
This second principle is that of " modification " by adaptation to sur-
rounding or varying conditions. The living being is a plastic unit,
capable of being affected and impressed in various, and often undeter-
mined fashions, by the forces of the world in which it lives. Such external
conditions — heat and cold, food, habitat, and a host of other circum-
stances— influence its development in the present, as unquestionably
in the past they have modified the history of its race. In truth, the
germ-idea of evolution is that of progressive change and alteration
induced by the great factors — internal or innate, hereditary and vital
forces, and the external or outside circumstances of life. To the
operation and influence, then, of surroundings, acting variously upon
different natures and organisms, we rightly ascribe the deletion of
stages we would naturally expect to meet with in that recapitulation
of the animal evolution exhibited in its development. As the geo-
logical record, through its imperfections — due to the metamorphism
and destruction of fossil-bearing rocks — causes grievous gaps in
the history of past life on the earth, so the history and development of
the life of to-day shows its blanks and imperfections likewise — these
blanks caused chiefly, we believe, by the varying outward conditions
under which the development of the race was carried out. Thus, if
the main outlines of the development of the frog-race be plainly
delineated, the pictures likewise may exhibit here but the dimmest
possible contour, and may there show a blank . The original fish-ancestor
of the race must be sought for amid the fossils — possibly it may never
come to light at all. The successive stages whereby the tailed newt
became the frog, are barely outlined in the animal world of to-day,
and are here and there wanting altogether. But the finger-posts exist
nevertheless, and they guide our mental way satisfactorily enough, so
long as we trust to their indications. Even though we have to wade
through the high tides of difficulty and dimness of knowledge which
obscure the intervening ground, we may walk with confidence in that
sober path which is founded upon the reason that is attainable.
As Huxley pertinently remarks in a recent manual of zoological
instruction : " In practice, however, the reconstruction of the
pedigree of a group from the developmental history of its existing
members is fraught with difficulties. It is highly probable that the
series of developmental stages of the individual organism never pre-
THE EVIDENCE FROM DEVELOPMENT.
195
FIG. 106.— SEA-URCHINS.
•sents more than an abbreviated and condensed summary of ancestral
conditions ; while this summary is often strangely modified by varia-
tion and adaptation to conditions ; and it must be confessed that, in
most cases, we can do little better than guess what is genuine
recapitulation of ancestral forms, and what is the effect of compara-
tively late adaptation. The only perfectly safe foundation for the
doctrine of Evolution," continues Huxley, " lies in the historical, or
rather archaeological, evidence
that particular organisms have
arisen by the gradual modifica-
tion of their predecessors, which
is furnished by fossil remains.
That evidence is daily increas-
ing in amount and in weight;
and it is to be hoped that the
comparison of the actual pedi-
gree of these organisms with
the phenomena of their deve-
lopment may furnish some
criterion by which the validity
of phylogenetic conclusions (or race-development), deduced from the
facts of embryology alone, may be satisfactorily tested."
A survey of some typical groups of animals in relation to their
development will provide
us with satisfactory means
of judging how far and how
plainly the history of the
individual repeats that of
its race. Turning firstly to
some fields of lower life,
we may select the class
(Echinodermata) repre-
sented by the Starfishes
(Fig. 107), Sea-urchins
(Fig. 1 06), Sea-lilies (or
Crinoids) (Fig. 109), and
Sea-cucumbers (Fig. 108),
as a starting-point for our
inquiries. There is little
need that a list of zoological
characters should be enu-
merated by way of impress-
ing the idea of the varied
appearance of the animals
just mentioned. But it may be remarked that, firstly, they all exhibit
o 2
FIG. 107. — STARFISHES.
196
CHAPTERS ON EVOLUTION,
a fundamental likeness in structure, beneath diversity of form ; and,
secondly, that such general or fundamental agreement is seen in
the management of their internal organs — digestive system, heart,
nervous system, &c., and especially in what zoologists term their
" radial symmetry " — that is, their generally rounded form arising
from their bodily elements, so to speak, being moulded around a
central point (Fig. 107), the mouth. However like these animate
may be in general structure, they, at the same time, present us with
very diverse forms. On the hypothesis of special creation, nothing
could appear more rational than the idea that dissimilarity of form
was due to the separate circumstances of their creation. But such art
idea overlooks at the same time their general likeness in structure ;
and it certainly takes no account
and gives no explanation of the
singular uniformity and resem-
blances presented by these ani-
mals in early life. The general
likeness in question, in fact,
simply reiterates and strengthens
the evidence and conclusions
that the varied tribes of Star-
fishes, Sea-urchins, Crinoids, and
Sea-cucumbers have arisen from
a common ancestry. Let the
history of their development
prove the truth and validity of
this conclusion.
Selecting a Starfish as the
most familiar form of the class,
we find its early development to
exhibit those stages of egg-seg-
mentation common to the de-
veloping ovum of all animals, and which have been already discussed.
But the special features of Starfish-development soon begin to
show themselves in the production of a worm-like organism,
utterly different from the Starfish-form, and which swims freely
in the sea by means of the delicate cilia or vibratile processes
with which the sides of its body are provided. This larva
possesses a digestive system, a system of water-tubes and other
structures ; and it would thus seem as if from the egg of the Star-
fish a wholly different progeny was destined to arise. So unlike
is the young organism to the parent, that when first discovered, it
was described by Sars in 1835 as a hitherto unknown form under
the name of Bipinnaria (Fig. no, A). In due time,, however, a
secondary formation begins to appear within this latter body (Fig.
FIG. 108. — SBA-CUCUMBERS.
THE EVIDENCE FROM DEVELOPMENT.
197
1 10, B, a b\ and the curious spectacle is soon beheld of the form of the
young Starfish growing within and absorbing the materials of which
the Bipinnaria-body is composed. So that when development is
•completed, the Bipinnaria's substance has become appropriated by the
new and secondary formation, which latter duly appears as the true
Starfish, destined, after ordinary growth, to assume the adult form.
The study of a Sea-urchin's early life-history reveals a striking
•similarity to the development of a
Starfish. The embryo Sea-urchin,
"in escaping from the egg" (Fig.
n i, A B), says Agassiz, "resembles
•a Starfish embryo, and it would
greatly puzzle any one to perceive
any difference between them. The
formation of the stomach, of the
oesophagus (or gullet), of the intes-
tine, and of the water-tubes takes
place in exactly the same manner
as in the Starfish, the time only at
which these different organs are
differentiated not being the same."
But at a later stage the young Sea-
urchin develops a different phase
and form from those of the Star-
fish. It appears as a curious body,
shaped somewhat after the fashion
•of a painter's easel and formerly
named Pluteus (Fig. 1 1 1, c), under the idea that it represented an
adult and distinct being. Within this Pluteus, a skeleton of limy
rods is developed, and a digestive system is also formed. Then
succeed the final stages in
development. The body of
the Pluteus is absorbed by
the future Sea-urchin (Fig.
in, D), which, as in the
Starfish, is formed within
and from the substance of
this larva — with this differ-
ence, that a portion of the
Pluteus is generally cast off
as useless material, whereas
in the Starfish the whole
larva was utilised in the
manufacture of the perfect
form. There exists a second group of Starfishes, including the
FIG. 109. — CRINOID.
FIG. no.— LARV.E OF STARFISH.
CHAPTERS ON EVOLUTION.
Brittle-stars and Sand-stars (Fig. 107, 5), and exhibiting certain-
differences in structure from the common Starfishes of our sea-
beaches. In their development these Sand-stars and their neigh-
bours approach very nearly indeed to that of the Sea-urchins. Their
larva is also a " Pluteus," and possesses a limy skeleton ; and it
is singular to find that forms so divergent in character as the
Sand-starfishes and Sea-urchins should thus resemble each other
in development.
The interesting
group of the Crinoids,.
or Sea-lilies (Fig.
109) — well known in
a fossil state under
the name of " Encri-
nites " — presents us-
with beings that may
best be described as-
Starfishes borne on
FIG. in.— DEVELOPMENT OF SEA-URCHIN. Stalks. There CxistSr
however, a well-
known free Crinoid in the shape of the Comatula (Antedori) rosacear
or the Rosy Feather-star of our coasts (Fig. 112) ; this form appear-
ing in its adult condition as a free, unattached "Starfish" (Fig. 112, a\
but indubitably proving its Crinoid nature, in that it spends the early
part of its existence in a stalked condition (b\ resembling the per-
manent state of its neighbour Sea-lilies (Fig. 109). Now, in the
development of the Crinoids, we meet with an oval, free-swimming
larva, within which a digestive system duly appears. This organism-
FIG. ii?.— ROSY FEATHER-STAR AND YOUNG.
in due course attaches itself by a stalk, and the future Crinoid is
THE EVIDENCE FROM DEVELOPMENT.
199
developed within this larva ; a new mouth and digestive apparatus
are produced, and the adult stalked form is assumed. In the Rosy
Feather-star such development, with its characteristic modifications,
is well seen. Here we first see the oval larva, with its four bands of
cilia (Fig. 113, A), and a tuft of these organs at the posterior extremity.
Then traces of the future adult (B) appear within this body. As de-
velopment proceeds, the cup or body of the Crinoid is formed, the
tentacles or arms bud forth, and the young Feather-star, already
stalked (c), appears in the likeness of a true Crinoid. Here develop-
ment might be thought to have well-nigh attained its limit. So
thought the discoverer of this
little stalked form, when it was
announced that in the Cove of
Cork a rara avis in the shape of
a British Stalked Crinoid (duly
named Pentacrinus Europceus)
(Fig. 112, b] had been found.
But years afterwards, the little
Pentacrinus was seen to leave its
stalk, and to appear before the
eyes of zoologists in the guise of
an old familiar friend — the Rosy
Feather-star (Fig. 112, a) of the
coasts. Thus we discover, firstly,
that Crinoids resemble their
neighbours the Sea-urchins and
Starfishes in the essential details
of their development ; and we
discover, secondly, in the case of
the Rosy Feather-star, a further
development of the Crinoid race,
in that this latter organism has
advanced to a free-living stage.
Also noteworthy is the fact, that
when existing in its rooted and
stalked stage, the Rosy Feather-star closely resembles the ordinary
fixed Crinoids, and perhaps bears a still closer likeness to certain
fossil members of the group.
The last class of Echinoderms demanding attention is that of
the Holothurians, or Sea-cucumbers (Fig. 108), found around our own
coasts, but developed typically as the Trepangs and Beches-de-Mer
of tropic seas, and in marketable form as the delicacies of the
Chinese. A Sea-cucumber presents us with an elongated body,
bearing a tuft of feathery tentacles at the mouth-extremity, and
moving by aid of tubular "feet," similar to those of the Starfishes
FIG. 113. — DEVELOPMENT OF CRINOID.
200
CHAPTERS ON EVOLUTION.
and their neighbours. Here development resembles that of the
Starfishes, and begins (Fig. 114, A B) with the production of an oval,
ciliated body, which soon acquires a digestive system. The young
Sea-cucumber, in the guise of what is called its " Auricularia-stage "
(Fig. 1 14, c), presents a cylindrical figure, with four or five bands of
cilia, and bears ear-like processes — hence its name. Before this
larva is fully formed, the future Sea-cucumber commences its exist-
ence as a growth existing near the larval stomach. The tentacles of
FIG. 114. — DEVELOPMENT OF SEA-CUCUMBER.
the young Holothuria soon appear (D), the ear-like projections are
absorbed, the Auricularia assumes a cylindrical form, and, becoming
the " pupa," bears a striking resemblance to a worm. When the
process of absorption proceeds to a further stage, the Auricularia
wholly disappears ; and as the new body which has been developed at
its expense elongates, the young Sea-cucumber form is duly evolved.
Such is the course of development in the sea-urchins and their
allies. The chronicle in question is well adapted to supplement the
important considerations advanced at the commencement of this
chapter. There is strikingly seen in the development of these animals,
firstly, that broad resemblance in their earliest stages which augurs
for a common derivation, and which proves, what their adult structure
teaches, that these organisms are simply so many modifications of a
common plan. In each and all, the first larva gives origin to a
second within itself, this second growth becoming the true and adult
form ; so that the first larva produces the new being, as it were
by deputy. And whilst general similarity in development thus
may be taken to mean community of origin, if it has any meaning
at all, there remains illustrated to us the second principle involved
in the study of development at large. The differences between the
early forms of these various groups are readily enough explicable on
the theory that adaptation and variation (acting through undetermined
laws of life, or through the influence of outward conditions, or through
THE EVIDENCE FROM DEVELOPMENT. 201
both of these phases) have been at work, evolving, from the common
larval type, the differences of form perceptible in their present-day
development as well as in their adult structure. This principle
of adaptation is perhaps best illustrated by those cases of " direct "
development — seen in some species of Holothurians and Starfishes,
-&c. — in which the young appear in the likeness of the parental form
without undergoing a metamorphosis or series of changes. In such
a case, the obliteration of these changes has probably depended upon
causes which at present we are unable to trace ; the directly de-
veloped forms probably representing the later products of evolution.
But it is needless to remark that, on the clear evidence afforded by
the typical development of these animals generally, the theory of
their common origin is in nowise affected by the elimination, here
and there, of the ancestral features of the race. Perhaps the sea-
cucumbers and starfishes represent the most typical and least altered
cycles of development, whilst the sea-lilies and sea-urchins present
us with the results of a more modified series of changes. But, theo-
retically, there is little difficulty in assuming that, could we look
backwards in time with definite glance, we should expect to see the
origin of our sea-stars and their allies in a stock which, if anything,
approaches most nearly to the form of some primitive worm than to
that of any other animal form. Such a primitive form is, perhaps, best
outlined in the larva of the Sea-cucumber itself (Fig. 1 14, B c). Indeed,
the evolution of the Echinoderms from some such worm-stock is one
of the well-founded generalisations of modern zoology. There exist, it
may be added, in the develop-
mental history of the worms
themselves, certain features
which go far to support the
idea of a far-back relation-
ship with the sea-urchins and
their neighbours — these latter
forms being apparently re-
moved very far from the
worm-stock as they present
themselves to our view in the
forms of adult and perfect
•existence. There is a curious FIG. HS.-
marine worm, named Balano-
glossus, the larva of which, known as Tornaria, certainly approaches,
of all other known forms, most nearly to the youthful starfishes,
sea-cucumbers and their neighbours. Indeed, the young Balano-
glossus has been described as intermediate between the young of
Echinoderms and the larval forms of molluscs to be hereafter
{Chapter XI.) described. Balanoglossus itself, is peculiar in possess-
202
CHAPTERS ON EVOLUTION.
ing gill-slits resembling those seen in the early life of all Verte-
brates. There seems little reason to doubt that this curious animal
is a survival of a once widely-represented type, which to-day exhibits
decline and decay, whilst preserving for us the important characters
of a common ancestor of several existing groups of animals.
Ranking above the sea-stars, in respect of generally higher
organisation, we find a very numerous and varied assortment of
animals known as the Crustacea. The etymology of this latter term/
might suffice to convey information respecting the typical repre-
sentatives of the group, inasmuch as the presence of a hard crust
or " shell " characterises the higher forms, as well as many lower
members of this class. Such higher forms are the crabs (Fig. 115),
lobsters, shrimps, prawns (Fig. 132), water-fleas (Fig. 116), and
their neighbours, which possess a "shell" — although, as even a
tyro in zoology knows, the " shell " of the crab is a widely different
structure, in nature as it is in appearance, from the " shell " of the
oyster or whelk. The crab's shell is periodically slipped off its
body to admit of the animal's increase in size; whilst that of the
mollusc — oyster, mussel, whelk, &c. — is a permanent structure,
attached by muscles and other organic means to the animal's body,
growing steadily as bones grow in ourselves, and forming, therefore,
a much more important item of bodily belongings than does the
Crustacean's covering. But apart from the nature of the " shell,"
the Crustaceans, as one may see in the jointed tail of the lobster
or shrimp, are very
differently planned,
so to speak, from
the Molluscs. They
are " Articulate " or
" jointed " animals,
and naturally claim
insects, centipedes,
scorpions, spiders, et
hoc genus omne^ as
their relatives and
friends. Now, this
great Crustacean class
includes a very motley
and varied series of
beings. At its head, as we have seen, are the lobsters, crabs,
shrimps, and prawns ; its middle-classes are represented by the
"water-fleas" (Fig. 116), whose name is legion; and its lower orders
are the barnacles (Fig. 117), the sea-acorns, the Sacculinas (Fig. 118),
and a host of allied creatures which certainly present us with the best
examples of degradation in the animal kingdom, in that they exist for
FIG. 116. — WATER-FLEAS.
THE EVIDENCE FROM DEVELOPMENT.
203
the most part as footless, often as mouthless, and frequently as shapeless
organisms, attaching themselves to fishes and to other Crustaceans,
and living the low existence pertaining to the parasite whether of
higher or lower grade. There seems no wider dissimilarity, for
instance, between any two animals, than between the shrimp or prawn
(Fig. 132) and the bag-like Sacculina (Fig. 1 18), which attaches itself to
the bodies of crabs. There is apparently a wide distinction between the
structure of a crab (Fig. 115) and a water-flea (Fig. 116), and still more
between a barnacle (Fig. 117) and a prawn (Fig. 132). Yet in the
classification of zoology these diverse beings are ranked as members
of the same class ; and development, as the great criterion of classifica-
tions, sanctions the arrangement. Let us endeavour to discover the
grounds which warrant the assertion of such near relationship.
No fitter starting-point can be found than the development
of the Barnacle (Fig. 117), which,
attached to floating wood by its fleshy
peduncle or stalk, enclosed within
its shelly habitation, and sweeping
the waters with its set of feathery
plumes or " cirri," lives a life border-
ing nearly on the state of parasitism
itself. From the egg of the barnacle
— and after the preliminary stages
of development which are common
in greater or less degree to the per-
sonal evolution of all animals —
comes forth a little creature (Fig.
119), so utterly unlike its parent
that one might well feel disposed
to reject the claims of the aphorism
" like begets like," so universally
expressive of the relation betwixt
parent and progeny. The body of
the young barnacle is triangular in
shape ; its anterior angles are pro-
truded into horn-like processes ; and
it possesses a mouth and digestive
system, a single median eye-spot, a forked tail, and three pairs of
feet or limbs.
In this stage it is known as a Nauplius ; and it may be well to
keep the characters of this little organism in mind, since we shall
find them to reappear in the progeny of animals of diverse nature
from our Barnacle. The course of Nauplius-life lies in the direction
of frequent moults, and by-and-by it assumes, after a special change
of skin, the form of the " pupa"-barnacle (Fig. 120, B). It passes,.
FIG. 117. — BARNACLES.
204 CHAPTERS ON EVOLUTION,
in other words, from the days of its infancy to the days of its youth.
As the " pupa," its body is enclosed in a bivalve shell or " carapace."
Two compound eyes replace the single organ of vision of the Nau-
plius-stage ; the first pair of legs (Fig. 120, B a) have become enlarged,
and appear as antennae or feelers provided each with a sucker ; whilst
behind the mouth six pairs of " cirri," or small hair-like limbs (/),
are developed. The mouth appears to become abortive in this stage,
in which the resemblance of the young Barnacle to a Water-flea such
as Daphnia (Fig. 1 16, c) or Cypris (B) is sufficiently striking. Darwin
remarks, that in the Nauplius- stage the young barnacles feed actively
and increase in size; whilst in the second stage, their function is "to
search out by their well-developed organs of sense, and to reach
by their active powers of swimming, a proper place on which to
become attached and to undergo their final metamorphosis."
The concluding phases in barnacle-history are not difficult to trace.
The body of the young barnacle becomes somewhat flattened and
compressed, and, as Darwin remarks, resembles in its shape a mussel-
shell or the water-flea known as Cypris (Fig. 116, B). The carapace
or shell appears paramount in the final stages of development, the
limbs and body being hidden and enclosed by the shell ; and although
jaws exist, these organs are covered by integument, and the organism
is thus deprived of the power
of nourishing itself. Certain
remarkable glands now begin
to be developed in the pupa-
barnacle. These organs open
by the so-called "cement-
ducts," in the suckers of the
well-developed first pair of
appendages — the great feelers
or antennae (a) already men-
FIG. IIS.-SACCULINA?* tioned. The pupa in due
time seeks a location and
resting-place, and adheres (Fig. 1 20, A) to its floating log, or to the side
of the ship, by means of its feelers. Thereupon the cement glands
pour out their secretion, which acts as a veritable " marine glue,"
defying the solvent action of the water, and fastening the barnacle
head downwards to the place of attachment. Then the compound
eyes disappear, leaving the future existence of the barnacle sightless.
The characteristic limy formations or plates seen in the "shell" of the
•adult barnacle (Fig. 117) are developed. The six pairs of swimming
feet become the plumes, "cirri," or "glass hand" of the barnacle,
and by their incessant waving draw food particles into the mouth.
With the production of the characteristic fleshy stalk or " peduncle "
•of the full-grown form — which grows from the front part of the body
THE EVIDENCE FROM DEVELOPMENT.
205
— this curious history comes to an end. Barnacle-growth therefore
exhibits as its stages, firstly, a free-swimming larva or " Nauplius '*
(Fig. 119), with its three pairs of legs
or appendages ; then a pupa with its
bivalve shell, its large feelers, its two
eyes, and its six pairs of swimming feet
(Fig. 120, B); and finally the eyeless,
stalked, and degraded adult stage, in
which, to quote the words of authority,
a barnacle appears as a crustacean,
"fixed by its head, and kicking the
food into its mouth with its legs."
From the crustacean array, we may
next select an animal which, whilst it
resembles the Barnacle in many of its
features, and especially in development,
is yet sufficiently distinct to lead towards
forms presenting greater differences in
the adult stage and yet exhibiting close
identity in the early phases of existence. Such a form is the
Sacculina (Fig. 118), a type of Crustaceans of the very lowest
grade, which live an' attached, rooted, and parasitic existence on
FIG. 119. — YOUNG OF BARNACLE.
FIG. 120. — DEVELOPMENT OF BARNACLES, &c.
fishes or on other crustaceans. If a barnacle exhibits " retrograde
development " or physiological backsliding, in that it appears to be a
lower and more modified form when adult than when in the pupa-
stage, the Sacculina and its neighbours exhibit a still more degraded
condition. The organism just named, exists as a sausage-like bag
attached to the bodies of hermit-crabs. There exist no traces of a
mouth — or, as Fritz Miiller remarks, " they lose all their limbs
206 CHAPTERS ON EVOLUTION.
completely, and appear as sausage-like, sack-shaped, or discoidal
excrescences of their host, rilled with ova (or eggs) ; from the point of
attachment closed tubes, ramified like roots (Fig. 118), sink into the
interior of the host, twisting round its intestine, or becoming diffused
among the sac-like tubes of its liver. The only manifestations of life
which persist in these non plus ultras in the series of retrogressively
metamorphosed Crustacea, are powerful contractions of the roots,
and an alternate expansion and contraction of the body, in conse-
quence of which water flows into the brood-cavity and is again
expelled through a wide orifice."
Now, the history of Sacculina-development clearly proves its
relationship with other crustaceans. As an adult, a Sacculina might
literally be anything in the way of animal organisation. It is a bag
filled with eggs, and attached by roots to a hermit-crab. As such,
its true nature is not recognisable by any of the deductions to be
drawn from the ordinary facts of animal structure. Development,
however, not only shows us its descent, but settles its place in
the animal scale by declaring its affinities, not only with the
Barnacles, but with other crustaceans. From each egg contained
within the bag-like body, there is developed a little free-swimming
creature (Fig. 121). This embryo possesses an oval body, ending
in two short processes; three pairs of swimming
feet are developed ; a single eye may or may not
be present ; but we find in the young Sacculina
a clear and unmistakable reproduction of the
"Nauplius" (Fig. 119) of a Barnacle. No mouth
or digestive system, however, exists in the
youthful Sacculina, which shortly changes into
the " pupa " state (Fig. 120, c). Here it closely
resembles the Cypris water-flea (Fig. 116, B),
whose development we shall also presently note. It
FlG- "I-— NAUPLIUS possesses a shell folded down at the edges so as to
OF SACCULINA. ^ . .... p . .
enclose the body. The front pair of limbs, as in the
Barnacle, become modified to form organs of attachment ; the two
remaining pairs of feet are cast off; and, as in the Barnacle, six pairs of
forked swimming feet appear on the body behind, while the forked
tail is also a characteristic feature of the young Sacculina. Then
succeeds the stage of attachment. The front feet, or feelers, serve as
means of fixation to the body of the crab-host The remaining six
pairs of feet are cast off ; the roots are developed from the feelers, and
the animal thus assumes the adult sac-like and degraded form. Thus a
Sacculina and its parasitic neighbours closely resemble barnacles up
to the pupa-stage. At this point the evolution — manifested in
" degradation " — of the Sacculina intervenes, and the six pairs of feet,
which in the Barnacles are converted into the " cirri " or " plumes,"
THE EVIDENCE FROM DEVELOPMENT. 207
are cast aside as useless. The process of extreme modification for
a life of parasitism effectually moulds the remaining features of the
organism in the characteristic ways of Sacculina life — namely, as the
sausage-like sac, fixed to its crab-host. But there can be no question,
that Barnacle growth and Sacculina development run in strictly
parallel grooves.
Allusion has been made to the likeness exhibited by the " pupae "
of the Barnacle and Sacculina to the perfect and adult form of those
water-fleas, which, like Cypris and Daphnia (Fig. 116, B, c), are
familiar tenants of our fresh waters. The development of the " water-
fleas " — under which general name very diverse beings are included
— is highly instructive, in that it leads us to note how the community
of development existing among Crustacea
extends its roots so as to include every
group or order of that class within its limits.
The Cypris (Fig. 116, B) and its neighbours
are known by their possession of a distinct
bivalve shell — that is to say, a shell con-
sisting of two pieces, united along the back
by a membrane serving as a hinge. Two
-or three pairs of feet exist, but these
-creatures appear to swim chiefly by aid
of the tail. Now, the young Cypris leaves T
.-, ,, XT !• « -,i S • FIG. 122.— NAUPLIUS OF CYCLOPS.
the egg as a " Nauplms " with three pairs
of limbs. It possesses, like the Barnacle-nauplius, a single eye,
and it appears to develop a shell likewise. The adult condition
is attained in due course, with the production of the bivalve
shell ; and the three pairs of limbs of the " Nauplius " are con-
verted respectively into the greater and lesser pair of antennae and
into the mandibles or jaws of the adult. The other feet of the full-
grown Cypris are also developed in its later stages of growth, which
are manifested by frequent moultings of the skin. A young Cypris
therefore resembles a young barnacle in its Nauplius-form, and in the
transformation of its anterior limbs into antennae or feelers, which, in
the water-fleas, serve the purpose indicated by the latter name — or may
«ven be used for swimming, as in the Daphnia, or " branch-horned
water-flea " (Fig. 1 16, c). In the correspondence between the bivalved
Cypris and the pupa Barnacle or pupa Sacculina, we may possibly
discover, likewise, the ultimate point of divergence between these
diverse groups of Crustaceans.
Other water-fleas, such as Daphnia and Cyclops (Fig. 116, c, A),
present variations in their early history from the chronicle of Cypris
development. The Cyprides are perhaps the least modified of the
water-flea race ; this conclusion being supported by the greater com-
plexity of other water-fleas as well as by the course of development
208
CHAPTERS ON EVOLUTION.
of the latter. The anatomical investigation of a Cyclops presents
us with an oval body or carapace (Fig. 116, A), bearing a single eye -r
with two pairs of feelers, big and little ; with a jointed tail, forked at
its tip ; and with five pairs of swimming feet. In Cyclops-develop-
ment a singular resemblance is presented to that of certain low
crustaceans parasitic on fishes : and it will be instructive therefore
to compare these early stages
in both groups. The first
stage in Cyclops-history
(Fig. 122) repeats the now
familiar "Nauplius," with
its oval body, its central
eye, and its three pairs of
legs. Next are developed
the chest and tail regions ;
and six feet appear as the
belongings of the latter.
Then appears another pair
of limbs; and the three
limbs of the Nauplius be-
come the greater and lesser
pairs of feelers, and the
great jaws, as in Cypris.
After a series of moults, the
outlines of the Cyclops-
body begin to be apparent ;
but it is worthy of remark,
that beyond the stage in
which the tail-region with
its six feet is developed,
those lower and parasitic
crustaceans — the fish-lice
just referred to — do not
pass. The further history of Cyclops is simply a record of moults
and the growth of new joints and appendages ; that of the fish-lice
is a history of retrogression. The fish-lice are represented by such
forms as Lernccocera (Fig. 123), or Chondrocanthus, which latter in
its maturity may be found sometimes by the dozen in the gill-chamber
of that ungainly fish the Angler or Fishing ~Frog(Lophius piscatorius).
Lernxocera presents us, as an adult, with a shapeless flattened body,
about half an inch long, possessing the merest rudiments of limbs.
Each fish-louse begins life as a Nauplius (Fig. 123, B), essentially
resembling that of the Cyclops water-flea (Fig. 122). It develops
to resemble still more thoroughly the after-stages of Cyclops, but
retrogresses therefrom and becomes modified for a parasitic life.
FIG. 123.— FISH-LOUSE AND ITS NAUPLIUS.
THE EVIDENCE FROM DEVELOPMENT.
209
Still more marked is this modification in other fish-lice (Adheres and
LernoefR) which resemble Cyclops as closely as does Lernoeocera,
but which, sooner or later, become worm-like or otherwise degraded.
The suppositions, entertained by competent authorities, firstly,
that the fish-lice (Fig. 123) and water-fleas of the Cyclops-type
(Fig. 1 1 6, A) have sprung from the same stock; and secondly, that
the fish-lice are simply Cyclopean beings degraded by the adoption
of parasitic habits, are therefore fully warranted by a consideration of
the plain facts presented to us in their development Or once again,
to state a cardinal proposition of Evolution — the passing development
of individuals repeats and reproduces, with modifications, the fixed
and past development of the race and class.
To trace in full the record of Crustacean development would
considerably exceed the limits which the patience of the reader might
bear, and would unnecessarily protract and repeat facts already
exhibited and illustrated by the life-histories just recorded. It might
be highly profitable, for instance, to trace the development of those
peculiar Crustaceans, the King Crabs or Limuli (Fig. 124), which, as
living forms, stand well-nigh alone in their class, and remain con-
nected with other Crustaceans only as the leaves on the extremities
of one branch of a tree may be said to be
connected with those at the tip of another
and widely divergent bough. These crabs at
one stage of their development, and before
leaving the egg — within which all their notable
features are acquired — present a most remark-
able resemblance to certain of those singular
fossil crabs the Trilobites (Fig. 125), (Prest-
wichia), and likewise at another stage to the
larva of certain other Trilobites (Trinudeus).
This resemblance is well seen on comparing
the larva of the King Crab (Fig. 126, B) with
the larval Trilobite (A); and still more striking
is the resemblance between the King Crab at a
later stage (Fig. 127) and the adult Trilobites
(Fig. 125). Thus, whilst the Trilobite-race
and their neighbours (Eurypteridd) of Silurian
age have died out of existence, the King
Crabs, springing presumably from the same
root-stock, have undergone modification as
descent proceeded along " the files of time," and remain to present a
crab-race of an age and type, compared with which our existing crabs
are but as creatures of yesterday. So also we might, did space
permit, strive to show that those curious creatures, the Brine Shrimps
(Artemia [Fig. 128, #]) of the Lymington salt-pans and the Great Salt
FIG. 124. — KING CRAB.
CHAPTERS ON EVOLUTION.
Lake ; the Fairy Shrimps, which, like Crustacean ghosts, flit through
our fresh waters ; or the curious Apus, with its sixty pairs of feet, begin
life each as a Nauplius (Fig. 128,^), bearing either two, or the statutory
three pairs of limbs. And the account of other Crustaceans in which
FIG. 125. — TRILOBITES.
(as in the woodlice tribe) the Nauplius-stage is passed either within
the egg or is altogether suppressed, might similarly bring again before
our mental view the operation of the laws and principle of modification.
It may, however, suffice, if, in drawing Crustacean history to a
close, we select a few examples of development from the highest and
most specialised group of the class — that of the Crabs, Shrimps,
FIG. 126.
LARV.B OF KING CRAB AND TRILOBITE.
FIG. 127.
Prawns, &c. In such a history, we may discover the important fact
that, notwithstanding modification, and despite the high specialisation
of these latter animals from the primitive types and root-stock of
Crustacea, their community of descent with that of all other members
of the class is proved by those clues and traces, which, all-insignificant
as they may appear to the ordinary observer, literally afford to the
zoologist proofs and confirmations of the strongest character of the
truth of the theory of descent.
The higher Crustaceans (or Decapoda, as they are called), includ-
THE EVIDENCE FROM DEVELOPMENT.
211
FIG. 128. — BRINE SHRIMP AND YOUNG.
ing the Crabs (Fig. 115), Lobsters, Shrimps, Prawns, &c., as their
typical representatives, present us with a sufficiently diverse group
of beings viewed as adults, and likewise afford illustration of equal
diversity in their development.
Such diversities may be well
observed in the comparative
study of the early history of
such a series of forms as is
presented by the lobsters and
crayfish, by certain shrimps,
and by the common crabs.
In its development, the cray-
fish apparently presents but
little that is remarkable, as
compared with Crustaceans
of lower nature. Both cray-
fish and lobster come from
the egg (Fig. 129, a) in the essential guise (b, c) of their species or race ;
and the free-swimming " Nauplius-stage," so universal amongst lower
Crustaceans, is apparently unknown in their life-histories. There is
clear evidence, at the
same time, to show that
a " Nauplius " condition
is represented in their
egg-development, and
that this phase is ob-
scured and modified,
presumably through
those causes and con-
ditions which have
placed the lobster and
crayfish amongst the
aristocracy of the Crus-
tacean class. Speaking
of the development
of the Crayfish and
of its Nauplius-stage,
Huxley says, that animal " is wholly incapable of an independent
existence at this stage, and continues its embryonic life within
the egg-case ; but it is a remarkable circumstance that the cells of the
epiblast (or outer layer of the developing body) secrete a delicate
cuticula, which is subsequently shed. It is as if the animal symbolised
a Nauplius condition by the development of the cuticle, as the fcetal
whalebone whale symbolises a toothed condition by developing teeth
which are subsequently lost and never perform any function."
P2
FIG. 129. — DEVELOPMENT OF LOBSTER.
212
CHAPTERS ON EVOLUTION.
Again, speaking of the Crayfish, Huxley says : " In this Crustacean, in
fact, it would appear that the process of development has undergone
its maximum of abbreviation."
As already remarked, the progressive advance and evolution
of a group must naturally include in their course, changes and
modifications in development as part and parcel of the higher
order and structure to which the advancing members of the group
attain. It is not surprising, therefore, to find that the crayfish or
lobster (Fig. 129, £, c) should evince an absence in their develop-
ment of those phases and repetitions of their ancestry, of which
their lower and more primitive neighbours, the barnacles, &c., pre-
sent such typical examples. Whilst, at the same time, it is equally
notable and interesting to discover that in Nature's process oft-
repeated exceptions prove the rule ; and that here and there, the
exceptions to the ordinary development of higher Crustaceans
certainly prove that their original way of evolution has lain through
the pathways so plainly marked out in the lower ranks of the class.
Such exceptions occur within the family circle of the crayfish and
lobster kind ; and are even represented in the early history of that
most familiar of Crustaceans, the common crab itself. This animal
possesses a life-history, which, whilst it presents striking analogies to
that of lower Crustaceans, likewise offers some interesting points of
difference from the develop-
ment of the latter animals.
Within the egg, as in the
case of the crayfish, the
youthful crab appears to
pass its Nauplius-stage, and
sooner or later it emerges
upon the world of waters in
a form with which our pre-
vious researches have not
made us familiar. The
young crab (Fig. 130, a)
possesses a short body,
which at first sight appears
like a huge head, and a
jointed tail. In front and
above, are spinous projections, the upper of which reminds one
of the end of a nightcap long drawn out. A single and simple
eye is placed between two very large compound organs of sight ;
four antennae or feelers exist, and three pairs of jaws are developed —
the young crab thus presenting us with the complete furnishings of
the head of the adult. There likewise exist traces of appendages
which represent foot-jaws in the full-grown crab, but the jointed tail
FIG. 130. — DEVELOPMENT OF CRAB.
THE EVIDENCE FROM DEVELOPMENT. 213
possesses no addenda or belongings save bristle-like processes attached
to its broad and divided extremity.
In 1778 there was figured by a Dutch naturalist a new form of
Crustacean which was met with in 1822 in large numbers in the Cove
of Cork by Mr. Vaughan Thompson. These beings were referred to a
genus Zoea, which had been constructed for their reception. Later re-
search, however, showed that the Zoeas were merely the young or larval
crabs, just described, and the further development of the Zoeas was
in due course satisfactorily traced. For, after repeated moults, the
Zoea becomes the Megalopa (Fig. 130, b). Its body has now assumed a
shape distantly resembling that of the mature crab, and its five pairs
of walking legs are well developed. It possesses, however, an
appendage, unknown in the adult crab, in the shape of a jointed tail
provided with appendages ; and as the Megalopa, the crab bears a
very decided resemblance to one of its tailed neighbours, such as the
hermit-crab, lobster or shrimp. Ultimately the body widens, after
further moultings ; the tail decreases in size, loses its appendages, and
becomes tucked up under the body, to form the characteristic little
" purse " of the adult crab ; and, finally, with the proportional growth
and development of other regions and parts, the features of adult
crab-life (Fig. 130, c) are duly produced. Thus a crab's body really
consists of a greatly broadened head and chest, and the jointed tail
we see in the lobster or shrimp is represented in mature crab-
existence by the little appendage or " purse," which, on examination,
will be found to bear rudiments of the tail-appendages so typically
developed in the long-tailed neighbours of
the crab. It likewise becomes clear from
the foregoing life-history, that the crabs,
in respect of the modification and dis-
appearance of their tail, are a later and
higher race than the lobsters, shrimps,
and prawns. And geology confirms this
surmise, inasmuch as the lobster-races
were developed ages before the crabs. FIG. 131.— MVSIS.
Fossil kith and kin of the lobsters occur
very early in the stratified rocks, the crabs being late productions ;
so that the idea of the crabs having originated from a tailed Zoea-like
or lobster-like race is fully supported by the best of evidence.
The concluding life-histories which may be glanced at, by way of
summarising the ways of the crustacean evolution, are those of the
My sis or opossum-shrimps (Fig. 131), and a peculiar genus of prawns
known as Penaus (Fig. 132). The first-mentioned animals are common
in the lakes of modern Europe and of North America, and also flourish
in the Arctic Seas. It is a warrantable inference that the Mysis
relicta of the lakes, is simply a variety of the Mysis oculata of the
214 CHAPTERS ON EVOLUTION.
Arctic Seas, which has been shut off from a former marine existence
by the conversion of the Baltic fjords or firths into lakes ; geological
changes thus inducing alteration in animal species, and " a primi-
tively marine animal " thus becoming " completely adapted to fresh-
water life." These opossum-shrimps are so called, because the young
are carried during development in special sacs or pouches of the parent
form. They present in their early history a very interesting connection
between the marked change of form in lower crustaceans, and that
direct development of the higher forms of which the crayfish is so
well-marked an example. Within the egg, Mysis, like the crab, passes
through a Nauplius- stage. Thereafter, however, it grows rapidly; and
a remarkable circumstance has to be chronicled, namely, that the
original skin or integument remains unaltered, and is not moulted,
or otherwise made to participate in the succeeding growth of the
body. In this feature, as Huxley remarks, the young opossum-
shrimp might be justly compared to the pupa or chrysalis of an
insect, since it lies, like the latter, within an enveloping skin from
which, in due course, the young shrimp emerges. Here, then, the
Nauplius-stage is represented as a fleeting period in development ;
and we see in the Mysis, when full grown, a being which has no gills,
which possesses a large tail
or abdomen, and a small body
(or head and chest), and which
has but rudimentary appen-
dages to its tail. Notwith-
standing the fact that the
development of the Mysis is
well-nigh direct, we must not
neglect to note the important
facts, firstly, that one of its
nearest relations (Euphausia)
actually leaves the egg as a
FIG. 132.— PEN^US. true Nauplius ; and secondly,
that the form and figure of the
adult Mysis itself is perfectly reproduced in the development of the
crustaceans of higher type.
Thus in the lobster, which so nearly resembles the crayfish in
its direct development (Fig. 129), and in its imperfectly represented
" Nauplius-stage," the young form (named the Zoea fiom its
analogies to the youthful stage (Fig. 130, a) of the crabs), passes
through a Mysis-stage, but thereafter develops into the mature
lobster, with well-developed tail-appendages and " head." The
idea that in the adult Mysis we may see represented a transitory
phase in the evolution of such higher forms as the lobster and
crayfish, is a justifiable assumption. It is one, moreover, which
appears to be fully proved by the study of the life-history of that
THE EVIDENCE FROM DEVELOPMENT.
215
division of the prawn tribe which includes the species of Penceus
(Fig. 132) as its representatives.
The prawns, as everyone knows, are intimately associated with
the lobsters, shrimps, and crayfish
as higher Crustacea. Yet the first
appearance of Penseus is not, as in
the crayfish, as a well-nigh per-
fectly-formed animal, nor, as in the
lobster (Fig. 129), as a Zoe'a, some-
what like the adult ; nor yet, as in
the crab, as a Zoe'a (Fig. 130 a),
widely different from the mature
form. On the contrary, the
youngest stage of Penaeus is a
veritable "Nauplius" (Fig. 133),
with three pairs of appendages,
and a single median eye, accu-
rately reproducing the features
now familiar to us in the Barnacle FlG- W-NAUPLIUS OF PEK^US.
(Fig. 119), Sacculina (Fig. 121), and lower crustacean life (Fig. 123,6)
at large. Next in order, this Nauplius develops a rounded body-shield
(or carapace); the first and second
pairs of appendages becoming the
two pairs of feelers proper to all crusta-
ceans, whilst the third pair becomes
the chief jaws or "mandibles." Then
are developed four pairs of feet, con-
verted in due time into jaws and
foot-jaws ; and behind these appear
other five pairs of appendages which
become the ten walking feet. The
six joints of the tail have as yet no
appendages, but the tail itself ends in
two tufted processes, and we see the
Zoea-form (Fig. 134) thus limned out;
whilst no less remarkable is the resem-
blance of the young prawn at this stage
to an adult Cyclops (Fig. 116, A) water-
flea. Two stalked eyes, in addition to
the single eye of the Nauplius, appear
in the Zoea-form, which alters and
changes through the decrease of the
feelers, till now used for swimming.
The tail now increases in size and replaces the feelers in function ;
and the feelers, each at first double, become single- jointed organs.
The five feet of the chest-region are each provided with two terminal
FIG. 134.— ZOEA OF
216
CHAPTERS ON EVOLUTION.
joints, and the Zoea becomes thus modelled (Fig. 135) into the exact
form of a Mysis or opossum- shrimp (Fig. 131). Finally, the singleand
median eye disappears, the outermost of the two end joints of each
of the chest-limbs disappears, leaving these walking legs (seen so
plainly in shrimp, prawn, crab, and lobster) of single conformation ;
gills are developed within the chest, sense-organs appear, and the full
development of the prawn (Fig. 132) is then completed. Throughout
these varied stages it is not difficult to trace a panoramic succes-
sion of forms accurately reproducing the existing
degrees and forms of the crustacean class. The
early Nauplius (Fig. 133), the zoea or water-flea
stage (Fig. 134), the mysis-form (135), each pro-
duced in definite and advancing succession, pre-
sent us with a perfect picture of the evolution of
the prawn-race from lower crustacean life, and,
presumably also, of the evolution of all other
crustaceans belonging to the same rank and series
in the class.
In summarising the results to which a study
of the development of the echinoderms and
crustaceans leads, there is to be recognised
the operation of the principles already more
than once insisted upon in the preceding pages,
namely, that community of descent is provable
by likeness in development, just as differences
or obliterations and alterations in development
are explicable on the grounds of adaptation and
change acting concurrently with the evolution and
progress of the race. Only by taking into account
these two principles, can the hard ways of develop-
ment be understood. The present subject is one
which may be regarded as lying thoroughly with-
out the province and power of any explanation
not founded upon evolution and upon the idea
;. that progressive change is part and parcel of
the order of nature. Admitting that the only
feasible explanation of these curious phases of development is to
be found in such an idea of nature's constitution, it seems folly to
deny that the general weight of evidence in favour of descent more
than counterbalances any difficulties which may present themselves
in connection with the exact determination of the lines along which
that descent has travelled. That larval or young forms are them-
selves liable to modification from various circumstances must be
admitted. This variation (to be hereafter studied in the insect-class)
of the young form, which we regard as representing the primitive
stock of the class, must unquestionably complicate the study of
FIG. 135.
MVSIS-STA
THE EVIDENCE FROM DEVELOPMENT. 217
evolution and add to the difficulties of constructing a perfect pedi-
gree of the living world. The Pluteus larva of a sea-urchin and the
Bipinnaria larva of a starfish, thus differ in respect that the former
possesses a limy framework which is wanting in the latter. But such
distinctions do not in the least degree militate against the primary fact
underlying all such developments, namely, that the likenesses, not
merely of young forms, but in adult structure, are explicable only on the
theory of a common origin. Indeed, with the best of reason and logic,
it may be argued that, as a condition of evolution, we postulate the
occurrence of variations in the young stages as well as in the adult
form — just as we should legitimately expect to find in living horses
the rudiments of those toes which the ancestors of the existing equine
race possessed. Thus " direct " development, such as we have seen
to occur in some starfishes and sea-cucumbers, whereby the young
pass directly into the form of the adult, and wherein the changes of
structure and appearance are suppressed, is a result of the adapta-
tion of the larvse to new ways of life. Rejecting this view, we should
have to fall back upon the anomalous position of maintaining that
there existed for one echinoderm a law of special creation, and for
another a law of descent — a supposition which no logical mind will
accept, and which the grander idea of the uniformity of nature at once
dispels. As a final remark in connection with the sea-urchin class
and its transformations, we may add that the changes in form are
themselves progressive in nature. The five existing groups of this
class (sea-urchins, starfishes, sand-stars, sea-lilies, and sea-cucumbers)
are unquestionably modifications of a common plan of structure, and
they originate from a larva which is wonderfully similar throughout,
if we consider the diversities of adult structure which arise therefrom.
Further, if this larva were to be arrested in its development and to
represent a mature form in such an arrested stage, it would present a
striking resemblance to some of the lower worms and their allies ; this
fact alone pointing to the probable beginnings of the sea-urchin class
in a worm-stock. No less clearly do we see in the varying degrees
of organisation exhibited by adult echinoderms, the same proof of
progressive advance and modification of an originally primitive type.
The forces and powers which, before our waking eyes to-day, evolve
a sea-urchin from its egg and easel-like larva, or a starfish from its
Bipinnaria, are, if we will only consider the wonderful nature of the
transformations involved, engaged in as evident and intricate a work
of evolution as those which have developed the varied twigs and
branches of the Echinoderm tree in the aeons of the past.
The foregoing conclusions find, perhaps, plainer illustration in the
history of the Crustacean class, wherein exists a uniformity not so clearly
traceable — although its original existence may not be doubted — in the
early life of the echinoderms. The highest members of the Crustacea
are, as we have noted, the lobsters, crayfishes, shrimps, crabs, and
2i8 CHAPTERS ON EVOLUTION.
their allies. We have seen that in the crayfish a " Nauplius "-stage is
represented ; that in the lobster a Zoea-phase is seen ; that Mysis
likewise exhibits a Nauplius, and then settles down as a peculiar
form ; that in the crab's early history, a still better marked Zoea
appears ; and finally, that the shrimp Penaeus actually passes through
a Nauplius phase, a Zoea or water-flea stage, a Mysis form, and finally
assumes the likeness of the shrimp tribe. The history of Penseus,
therefore, is practically an abridged treatise of the evolution of all higher
Crustacea : its development, to parody Pope's line, is " not one, but
all Crustaceans' epitome." And as perfectly are the facts of lower
crustacean life correlated with those of the higher development of
the class. A water-flea, like Cyclops, as an adult, matures its
development and ceases to progress at a stage corresponding to
that at which Penseus has but attained its. youth. The barnacles
and sacculinas exhibit the influence of conditions of parasitism
acting at a definite stage in the course of ordinary development, and
producing the degraded and attached form of the adults. Mysis
advances so far on the way towards the lobster and crayfish type,
but stops short in its development at a point represented in lobster
history, and beyond which the lobster itself passes as we have seen.
Finally, beyond all such stages, and underlying all the variations
and obscurities even of the higher and most modified life-histories,
we see the Nauplius- form continually appearing as the starting-point
of all crustacean history; or as that point, to use Fritz Miiller's
expression, which represents the " extreme outpost of the class,
retiring furthest into the grey mist of primitive time." The Nauplius
appears before us, then, as the founder of the crustacean race. The
Zoea is a modification and advance upon the Nauplius ; and from
this Zoea (as poved by Penaeus-development) were evolved the
higher crustaceans at large. The lobsters and their allies (again
appealing to Penaeus) were evolved from the Zoea-form through an
intermediate stage represented to-day by the Mysis or opossum-
shrimp ; whilst the short-tailed crabs, in all probability, arose directly
from the zoea, without the intervention of a Mysis-stage, seeing that
in their development they exhibit a distinct Zoea-stage, and do not
pass through a Mysis-stage like the lobsters and their long-tailed
neighbours.
Diagrammatically expressed, we may see in the history of crusta-
ceans that tree-like arrangement of their pedigree which best
illustrates the deductions of evolution. The Nauplius exists at the
root of the class. Developed in direct line, we find Penaeus passing
through the Zoea and Mysis-stages. The lobster branch diverges
after the Mysis-stage has been attained, and the crabs depart from
the main stem before the latter phase. The crayfish, with its
obliterated Nauplius-stage, may be presumed to have followed the
course of development resembling that of the lobster ; its history,
THE EVIDENCE FROM DEVELOPMENT.
219
however, being singular in respect of the obliteration of the inter-
mediate stages. The king-crabs have presumably originated m the
common Nauplius-form, and have passed through the Tnlobite-form,
now extinct, to their present position at the extremity of an isolated
branch of the crustacean tree ; although, indeed, some naturalists
hold that the king-crabs are more nearly allied to the spiders and
scorpions, than to the Crustaceans. The barnacles, fish-lice, and
water-fleas, obviously nearly allied, spring from a distinct Nauplms-
stem, but diverge through different ways and paths of life— the former
to exist mostly as degraded parasites, and the latter to develop into
Penaeus (fig. 132).
Lobster (fig. 129).
Crayfish.
Nauplius (figs. 119, 121, 122, 123, 133).
active free-swimming forms. Thus becomes clear to us the meaning
of those singular changes in animal forms which puzzled the older
naturalists. To question the meaning which evolution attaches to
these changes, is to leave them without explanation or meaning. Our
knowledge of the full evolution of the Crustacea or any other animal
group, as already remarked, may be, and often is, far from perfect.
We are, it is true, still in the " grey mists " of many biological
subjects, and the pedigree of animals, amongst others, is still
enveloped in much obscurity ; but, at the same time, we can detect
breaking through the mist, gleams of knowledge — bright forerunners of
that flood of light w'hich the research of after-years will assuredly bring.
CHAPTERS ON EVOLUTION.
XI.
THE E VIDENCE FROM DE VELOPMENT (concluded).
III.— THE DEVELOPMENT OF MOLLUSCS, AMPHIBIANS, &c.
THE attempt has been made in previous chapters to show that in
the development of living beings there lies an enormous store and
fund of evidence which goes either directly to support evolution
as a rational theory of the universe, or which, at any rate, aids us in
comprehending the causes which have, directly or indirectly, made the
world of life the wondrous thing it is. The result of our inquiries
has been to show that in the first beginnings of an animal's develop-
ment, and in its earliest phases of progress, there is an amazing likeness
to the early stages of every other animal's progress towards maturity.
But even after these early similarities have appeared, there may be
demonstrated in many groups a later likeness, which may often be
traced beneath forms of the most diverse kind. The progress of the
living being is unquestionably, as
Von Bae'r aptly put it, one from
the general to the special. Thus a
sponge, a sea-squirt, and a man,
may and do agree in the essential
phases of their earliest develop-
ment. But the special features
of each group of sponges, sea-
squirts, and quadrupeds are soon
respectively assumed, and, finally,
there appear those more defined
structures which mark the comple-
tion of development, and which
land us within the class, order,
or even species to which each
belongs. Development may thus
be compared to a journey in which
all the travellers, or developing animals, start from a common point,
and in which all pursue at first a common path that shortly, however,
branches out into numerous diverging roads and routes, each leading to
the goal or destination of the race. Community of origin is proved by two
animals following the same beaten track for a longer or shorter distance;
dissimilarity arising when their pathways diverge and the route divides.
FIG. 136. — MUSSEL.
Shell opened, showing ligaments, muscles (f c\
and foot (f).
THE EVIDENCE FROM DEVELOPMENT.
221
Thus much for what is observed in the development of animals,
as already illustrated in these pages. What is to be inferred by
the biologist from the facts of early development? The reply
was clearly enough given in the phrase, "development repeats
descent ; " or, otherwise, " the history of an individual's development
presents us with a panoramic or changing picture, more or less
obscured, of the descent
or development of its
race." In the absence of
such a thought, all de-
velopment is a mystery.
Rejecting the idea that
the phases of individual
development repeat the
evolution of the species,
we may only say that the
facts of natural history are
either each a senseless
paradox, or " form a mere
snare to entrap our judgment." Even in the later developments of
animals, we were able to trace, as we have seen, striking likenesses,
provable only on the theory of evolution. The mere reference to
Crustaceans and Echinoderms, will suffice to indicate the grounds
on which the latter assertion is based ; whilst the history of the
insect-class in its developmental
aspect will shortly be shown
to teach the same practical and
pregnant lesson. It might be
thought that the teachings of
development had by these
examples received copious
enough illustration. But there
remain for notice one or two
life-histories which, whilst they
may trench upon fields already
treated, possess yet an interest
of their own. It is to these
FIG. 137. — SNAIL.
FIG. 138.— SLUGS.
latter examples that we now refer by way of a closing reference to
the early history of animals at large.
Above the rank of the insects, or at least in a different group of
the animal world from that in which they are contained, we may find
plain illustration of that connection between apparently different
Classes of animals which evolution explains in rational and con-
sistent fashion. The group of the Mollusca, known popularly as
that of the " shell-fish," and having as its typical members the oysters,
222
CHAPTERS ON EVOLUTION.
mussels (Fig. 136), cockles, snails, whelks, and cuttlefishes — the latter
existing at the head of the group — presents us with one or two
typical examples of the truths and inferences of development.
There are at least four well-marked classes in the Mollusca, and
the names of these four groups may be placed before the reader
by way of enabling us to retain their distinctness clearly in mind.
Thus, firstly, we find the class
Lamellibranchiata, or " bivalves,"
represented by the oysters, cockles,
mussels (Fig. 136), clams, &c. Then
succeed the Gasteropoda, of which
the snails (Fig. 137), slugs (Fig.
138), limpets, whelks, chitons (Fig.
139), &c., are examples. The
Pteropoda form a small class, often
popularly named "sea-butterflies"
(Fig. 140), and of this group the
Clio and Hyalaa (C) may be selected
FIG.ISQ.-CHITONS. as representatives : whilst last and
A. Upper surface, showing the shell. 1-1 *\r*iii*.j
B. Under surface.showing headland foot (/). highest COme the LephatOpOaa, Or
cuttlefishes (Fig. 141), of which the
familiar octopus, the argonaut, and nautilus are examples.
Such is the constitution of the Molluscan type of animals. When
we study the development of the three first-mentioned classes, we
are struck by the similarity they present in their early history. The
FIG. 140. — PTEROPODA.
A. Diagram of structure ; B. Cleodora ; C. Hyalaea.
cuttlefishes, it may be mentioned, differ from the other groups in
development, and present us with an ancient and early specialised
group of beings whose early history and evolution is really a matter
of geological interest, and lies without the limits of the present chapter.
The early stages of a bivalve, such as a cockle, to select a familiar
member of the first of the classes just noted, exhibit the usual process
THE EVIDENCE FROM DEVELOPMENT.
223
of segmentation of the egg common to all animals. Sooner or later,
however, the young bivalve develops a somewhat rounded body
(Fig. 142, A) at the upper or head.-extremity of which appears an
expanded disc — often
described as consisting
of two distinct lobes or
halves — richly fringed
with the minute vibra-
tile processes called
cilia, and named the
velum (v). In the centre
of this velum, an elon-
gated tuft of cilia is
usually found in addi-
tion, the tuft being
known as the flagellum
(/). Thus provided
with its vibratile " hairs,"
the young bivalve
swims freely through
the sea, and is thus said
to exhibit its "veliger
» TVi f tT f FIG. 141. — CUTTLEFISHES.
Stage. I The upper figure represents an Octopus swimming backwards.
substance forms on the
back of the embryo. This becomes the mantle which lines the shell,
and in fact forms the latter structure ; whilst in due course the internal
organs are developed, and the young shellfish assumes the likeness of
the adult The oyster
and cockle are thus
seen to pass through
a veliger stage (Fig.
142, A), each with its
ciliated lobes and its
free-swimming powers,
through the exercise of
which the oyster-spat
may be conveyed to
great distances from
•its birthplace. As we
shall presently note,
the likeness of this wandering embryo to the young of certain lower
animals is distinctly marked.
The curious ship-worm, or Teredo (Fig. 143), which was termed
by Linnaeus "calamitas navium," and which effects an immense
amount of destruction annually on the wood of our piers and
FIG. 142.— DEVELOPMENT OF COCKLE AND SHIP-WORM.
224
CHAPTERS ON EVOLUTION.
FIG. 143. — TEREDO, OR SHIP-WORM,
Showing the shell detached.
harbours, is in reality a bivalve mollusc. Its body is shortened and
its breathing-tubes are extended to form the worm-like body, whilst
its shells (Fig. 143) are rudimentary and serve as boring-organs.
The teredo first undergoes segmentation within the egg (Fig. 142, B),
and then appears as an active free-swimming " veliger" (Fig. 142, C),
differing from the young
cockle only in that there
is no lash-like "flagellum."
Then its mantle and shell
are formed, and when five
and a half days old, the
shells have well-nigh in-
vested the whole body.
Next the "foot" (/) of
the ship-worm is de-
veloped, and the velum
becomes a crown of cilia
(D v\ Then, as the young animal seeks the wooden pile wherein it
is destined to bore and ensconce itself, the shells come into play as
excavating organs, and, with the growth of the elongated body, ship-
worm development may be said to conclude. Thus we find that the
course of bivalve development is
distinctly enough marked. Only in
one or two cases (such as that of
the fresh-water mussels, Unto) is the
" veliger-stage " suppressed. But this
latter fact will cause no surprise to
the student of development, who is
well aware that the effects of varying
conditions on the developing young
are seen in the production of many
changes in an early life-history, and in
rendering obscure many phases in the
panorama of individual evolution.
Coming next to the Gasteropods, of
which the limpets, whelks, snails, slugs
(Figs. 137, 138), and the univalve shell-
fish at large, are examples, we find a
striking similarity in their early history
to the development just sketched. A
mussel, or oyster, or other bivalve has, as every one knows, no
distinct head. This may be the result of degradation. But in
the snails, whelks, and their neighbours, the head is plainly enough
marked, although in certain low forms of the Gasteropod-class
this head-development may not be at all prominent. Such lower
FIG. 144.— DENTALIUM AND ITS
STRUCTURE.
THE EVIDENCE FROM DEVELOPMENT.
225
members are illustrated by the Dentalium or " toothshell," other-
wise often named the "elephant's tusk-shell" (Fig. 144, B), from
its obvious resemblance to the latter structure. In the early
history of Dentalium, we find obvious resemblances to the de-
velopment of the bivalves. First, segmentation or division of the
egg takes place (Fig. 145, A). Next, the young "toothshell" on
escaping from the egg appears as a rounded body, and possesses tufts
of cilia for swimming, and likewise has a "flagellum " (z) in front (B).
The body then lengthens and develops seven circlets of cilia (Fig. 145,
C),the resemblance between the young "toothshell" in this guise and
an embryo worm (Fig. 157, B) being unmistakable. Then the shell is
formed by the " mantle " (Fig. 145, D, a) as before, and the cilia form
a " velum " (z) at the upper extremity of the body, the young condition
of the bivalve being closely imitated at this stage. The shell, at first
open below, unites by its lower edges to form the toothlike structure
of the perfect animal, and with the further growth of the internal
organs (E), Dentalium becomes the mature animal. There cannot
exist a doubt that, as the lowest gasteropod, and as a poor relation
of the higher whelks and snails, Dentalium's life-history shows, as
might be expected, the closest approach, firstly, to animals of lower
grade than mollusca, and, secondly, at a more advanced stage —
that of the " veliger" (Fig. 145, C, D) — to the bivalves themselves.
Equally interesting is the
chronicle of development which
those little limpet-like animals,
the Chitons (Fig. 139), present
to our view. These latter forms
are found adhering to the rocks
and stones at low water, like
the neighbouring limpets. They
agree with the limpets in being
Gasteropods; but their structure
is, if anything, lower than that
of the familiar molluscs just
mentioned), and their shell is
not univalve, but composed of
no fewer than eight pieces (Fig.
1 39, A), arranged one after the other, on the animal's back. No definite
head, however, is found in the chitons, this lack of front extremity being,
as before, a proof of lowness and democracy in the scale of Gasteropod
society. The general aspect of a chiton is unquestionably more like
that of an "articulated" or jointed animal than of a mollusc, in
which latter we do not expect to see segments of any kind represented.
It is likewise a fact of much interest that these chitons are a remark-
ably ancient group of the Gasteropod-class. They may, it is true, be
Q
FIG. 145.— DEVELOPMENT OF DENTALIUM.
226
CHAPTERS ON EVOLUTION.
regarded, by the strict rules of comparative anatotny, as lower
organisms than the whelks and their relations. But if 'antiquity of
origin be esteemed in the Gasteropods, as it is in higher circles, a cri-
terion of respectability, then the chiton race may claim a superior rank
to many of their neighbours, and may maintain that when the univalve
race was but in the infancy of its development, they possessed a stable
and well-founded family connection. The chitons begin their fossil
history in the lower Silurian rocks, and appear at the present time
with but little variation from their past structure. They are, therefore,
unquestionably an ancient series of beings, which have most probably
sprung from a far back root-stock, whence the Gasteropods themselves,
and other molluscs likewise, may have branched off to become the
superior shelled races and tribes of to-day. What, then, is the course
of chiton development? As we should expect, it is much more
primitive, much nearer
the type of the worms
and of Dentalium de-
velopment, than that of
other univalves. The
researches of Loven
have made us ac-
quainted with the early
history of the chiton
group. From the egg,
the infant chiton (Fig.
146, A) issues forth as an oval speck possessing a circle of cilia
surrounding its body near the front extremity, and likewise bearing
a tuft of cilia on its head, The likeness between the young chiton
and the young cockle (Fig. 142, A) is clearly traceable. An eye-
spot soon appears on each side of the ciliated
circlet, and the body next becomes annulated or
ringed in appearance (B), such an aspect remind-
ing one most forcibly of the young stages of the
worms (Fig. 157). Even when the young chiton
exists in this free swimming state, the segments
of the shell begin to appear (C), and correspond
with the rings into which the larval body is
divided ; whilst subsequently the broad " foot "
is developed, and the animal settles down into
a sedentary and placid existence on the rocks and stones of the
coast. Chiton development thus tells a tale of early origin, and of
alliance with the worm stock. In this respect it forms a worthy
companion to Dentalium itself.
The development of the familiar pond snail (Lymneus), as studied
by Professor Ray Lankester and others, may render us acquainted
FIG. 146.— DEVELOPMENT OF CHITON (LOVEN).
FIG. 147. — POND SNAIL,
GASTRULA-STAGE.
THE EVIDENCE FROM DEVELOPMENT. 227
with normal gasteropod development in its higher and most typical
phases. The eggs of the pond snail are to be found in June de-
posited on the under surface of the leaves of water-plants, enclosed
in capsules containing a white jelly-like matter. The egg undergoes
complete yolk segmentation, and then the " gastrula-stage," with its
two layers (Fig. 147, <:, en) — repeated in all animals from sponge to
man — appears. The mouth of this sac closes as the young form
passes to enter the " veliger stage," in which the body is oval, and
possesses a ciliated ridge. This latter stage has also received the
name of "trochosphere." Ultimately the "foot" is developed, then
the shell appears, and in due time the snail-form is assumed. In the
pond snail, as a high form of mollusc, we unquestionably find a
"veliger- stage," reminding us of the similar phase in other and
lower univalves and in bivalves. It is a noteworthy fact that the
land snails and slugs do not show the "velum," notwithstanding
their apparent nearness to the pond snail. The suppression of the
"veliger- stage" here does not surprise us. On the contrary, we are
fully prepared for such lapses and omissions in development by the
consideration, already enforced, that altered ways of life must in-
evitably produce a changed life-history. Such omissions, in fact,
exactly answer the expectation of the evolutionist ; and their absence
would indeed prove a veritable stumbling-block to his hypothesis.
In the " top- shells " (Trochus\ familiar enough as native species, it
may be mentioned that the "veliger-
stage" (Fig. 148, A), or that of the
" trochosphere," is well represented ;
whilst in this stage the embryo is
also marvellously like the young of
certain worms, and also resembles
that of some of the Rotifera or
" wheel animalcules." Later on, the
" velum " (v) of trochus grows larger
(B) and becomes more prominent ;
and as the shell develops, the larva FlG- 148.— DEVELOPMENT OF TROCHUS.
assumes the likeness of the young
" top-shell." Such a life-history is worth recording, even in a cursory
fashion, if only to emphasise the fact that, even in some undoubted
univalves, the likeness to lower worms is remarkable.
Certain other univalves of somewhat different structure from those
whose development has just been described, may now be noticed.
These latter are the so-called " naked " gasteropods, in which a shell
is either rudimentary or wanting altogether. But the curious fact
remains that, whether a shell is present or not, these animals inva-
riably possess that structure in their embryonic state. This shell,
which is thus never destined to be developed, is an illustration of
Q2
228
CHAPTERS ON EVOLUTION.
" rudimentary organs," which, like the teeth of the unborn whalebone
whale — possessing no teeth whatever in its adult state — have a refer-
ence to a past state of things. These teeth and the rudimentary
shell are heritages derived from ancestors which had well-developed
teeth and shells respectively. Otherwise, and on any other theory of
nature, their mere existence is a hopeless and insoluble puzzle. The
shell-less univalves to which we refer are often familiarly named " sea
slugs," " sea-lemons," and the like. By naturalists they are placed in
such genera as Doris (Fig. 149), sEolis (Fig. 150), Aplysia, &c. Other
examples of these molluscs are included in the genus Bulla, or that
of the " Bubbleshells " (Fig. 151), possessing a delicate shell, and
Aplysia, or that of the " Sea-hares," famed of old as an ingredient
in classical poison-cups. Bulla and the Sea-hares possess each a
thin shell, which, however, is a secondary growth, and does not
represent the true shell or that developed in early life. Now, in
these " naked " gasteropods, there is a well-marked " veliger-stage."
Moreover, if the development of such a form as sEolis (Fig. 150), or
its neighbours of the " Sea-lemon " tribe, be studied, the young form
is observed at one stage of its career to present a singular and highly
characteristic appearance. It possesses a velum, consisting of two
well-marked lobes (Fig. 152, A), richly ciliated, and by means of which
it swims rapidly through the sea, whilst the animal's foot and its
shell are also readily observable.
FJG. 149.— DORIS.
FIG. 151.— BULLA.
FIG. 152.— YOUNG OF JEoLis AND ADULT PTEROPCD.
FIG. 150.— J
Far away in the Northern seas, the Arctic voyager may sometimes
sail for days, or rather for nights, through water which may be
discoloured by the innumerable myriads of small organisms floating
on its surface. Each of these beings is of very small size — certainly
under an inch in length as a maximum measurement ; and each
THE EVIDENCE FROM DEVELOPMENT.
229
paddles or flaps its way through the sea by means of a pair of wing-like
fins attached to the sides of the neck. Such are the " Sea-butterflies,"
or Pteropoda (Figs. 140 and 152, B), already mentioned as a class of the
Molluscan group. Their title to be regarded as " shellfish " rests on
the fact that, besides agreeing with other molluscan characters, they
may possess a delicate glassy shell (Fig. 140, C); but this structure may,
at the same time, be wanting, and a head may also be indistinctly
represented — the latter fact indicating, as we have seen, a
position of inferiority in the molluscan scale. Now, when
a Pteropod (Fig. 152, B) is even cursorily regarded in the
possession of its "wings" or fins, borne on the sides of its
neck, its resemblance to the young (Fig. 152, A) of some
of the "naked" gasteropods, such as JEolis (Fig. 150),
is both close and unmistakable. In their development
the pteropods possess a "velum," like most univalves.
This " velum " is believed by good authorities to remain
developed, and to constitute the "wings" or "fins"
(Fig. 140, A, a) of these animals. By other authorities
their " fins " are believed to represent certain side-lobes
of the molluscan body, and as such are regarded by this
second theory as secondary developments. However,
that the pteropods represent a rudimentary or primitive set of beings
no one may doubt. Let us bear in mind that they run through the
same early phases of development as gasteropods, and that not only
is the "velum" or " veliger-
stage" represented in their
history, but that certain
members of their class
present the cilia-girdled
appearance (Fig. 153)
proper to the early phases
of worm development
(Fig. 157). Let us also
reflect that the pteropod
seems to have been ar-
rested in its development
at, or a little beyond, the
" veliger- stage," and we
may readily understand the
position of those naturalists
who, comparing the young
of the " naked "gasteropod
(Fig. 152, A) with the adult pteropod (B), see the closest affinity
and relationship between them. The pteropod in this view repre-
sents a " permanent larval " or arrested gasteropod. Both have
FIG. 154. — BRACHIOPODA AND DEVELOPMENT.
23o CHAPTERS ON EVOLUTION.
arisen, if the story told by development is worthy of credit, from
a common root-slock, of which the " veliger-stage " is the transient
representative. Both have developed in parallel, or, it may be, in
corresponding and similar grooves. But the gasteropod has been
evolved beyond its " veliger-stage " to assume a higher place in the
animal series ; whilst the pteropod has been arrested in its development
at this stage, and has assumed, with possibly a little fixation of its cha-
racters, a larval condition as the badge and mark of adult structure.
Passing, last of all, to a lower group of molluscs — that of the
Brachiopoda, or " Lampshells " (Fig. 154, A) — we may find through
these latter forms a passage to the still lower and more primitive stock
from which the Molluscan group may be presumed to have originated.
The brachiopods form a scarce group of shellfish in our present seas,
bu,t in past epochs of this world's history they were abundantly
represented. The Silurian rocks, to mention but one group of
formations, literally teem with their fossil representatives, whilst the
paucity of these shells in existing waters is matter of zoological
notoriety. These " Lampshells " are, therefore, an extremely ancient
group of living beings. That they are inferior in many phases of
structure to the common bivalves — such as our oysters and cockles —
is matter of fact. Hence the development of these " Lampshells " may
be presumed, on d priori grounds, to be fraught with meaning and
information as to the descent and origin of the Mollusca at large.
Let us, therefore, endeavour to follow out the researches of Morse on
the development of these singularly interesting forms.
Studying one species — Terebratulina, the common "Lampshell"
of the American coast — the first free-swimming stage is that of an elon-
gated body (Fig. 154, B), which divides itself crosswise into three rings
or segments (C, D), the front one of which becomes provided with long
actively-moving cilia. Eye-spots also appear on the front segment, and
the likeness of the young lampshell to an embryo-worm (Fig. 157, C) is
at this stage plainly apparent. Nor is the likeness lessened, when the
middle segment is found to develop four bundles of setae, or bristles,
such as appear in the worms. Then succeeds the stage of fixation.
The young brachiopod now attaches itself by its lower segment^ (E),
and the middle segment increases greatly in size, so as to form a kind
of hood enclosing the front segment in part Then the front segment
(F, G) decreases in size; the middle portion originates the bivalve shell
(H, v, d), which soon comes to enclose the body (c), the lower or third
segment being represented by the disc or stalk of attachment (/).
The technicality of the subject prevents our following out for the
reader the later stages of lampshell growth, in which striking likenesses
are presented, not merely to brachiopods now extinct, but likewise to
the young stage (Fig. 154, I) of those plant-like animals, named
Polyzoa, and of which the Sea-raats (Flustra) (Fig. 190, a) of our
THE EVIDENCE FROM DEVELOPMENT.
231
coasts are good examples. Hence we conclude that the Brachiopods
present us with a group, which has sprung from a worm-like stock,
along with the sea-mats, thus showing us the possibility of higher
molluscs having had a similar origin.
The early history of the worms themselves — belonging to the
Annulose type — forms a concluding phase in these investigations
into the history of the
Molluscan race. If we
study the development
of one of the true sea-
worms, such as Arenicola (Fig. 155) or Nereis (Fig. 156), we shall find
a striking reproduction of some features with which our molluscan
researches have already made us familiar. The young worm (Fig.
157) makes its first appearance as an active free- swimming, barrel-
shaped body, provided with cilia, disposed in various fashions, in
different groups of the class.
Thus, in some embryos (B, C)
there is a first band of cilia
around the body in front of the
mouth, a second band exists
at the opposite extremity, and
tufts of these cilia may also
be developed at the extreme
front of the body. In other
cases numerous bands of cilia
encircle the body at its middle
portion only (C); whilst a third
set of cases exists where a broad zone of cilia occupies the middle
region, with or without a tuft at the head-extremity. Out of such
larval forms, the young worms are gradually developed, the head and
front segments appearing first
in the order of growth. Cer-
tain of those sea- worms which,
like Serpula (Fig. 158), live in
tubes of lime or other matters
which they fabricate from the
minerals of the sea- water,
possess a development equally
characteristic with that of
their free-living neighbours.
In the larvae of these tube-
dwelling WOrmS (Fig. 1157, FIG. 157.— DEVELOPMENT OF WORMS.
A, D), the head is provided
with cilia, disposed chiefly in two rings, one at either extremity
of the body. Soon tentacles are developed from the head portion,
FIG. 156.— NEREIS. A Marine Worm.
232
CHAPTERS ON EVOLUTION'.
the body becomes segmented, and the tentacle which, under the
name of the operculum (Fig. 158, 0), is destined to form a stopper
to the mouth of the tube, may likewise be discerned. At this stage,
with its segmented body, the young tube-dwelling worm resembles
the permanent condition of its free-living neighbour of the sand
(Fig. 156). Hence, when we discover that the tube-dweller finally
secretes a tube, and lodging its body therein, becomes a stationary
form, we conclude, rationally enough, that both kinds of worms have
arisen from one common stock, and that the tube-dwellers represent
the more modified race of the two groups ; whilst they likewise may
be regarded as "degraded" forms
when compared with their free-
living neighbours (Fig. 156).
We have thus had presented
to view a series of developments
extending from those of the mol-
luscs, through the " lampshells,"
and finally ending with that of the
worms themselves. Is there evi-
dence at hand to show that some-
thing more than a theoretical con-
ception of the connection between
these apparently dissimilar forms
is a warrantable thought? The
answer to such a question depends
on the credence we place on what
development teaches. If the truth
of the axiom that " development
FIG 158 — SERPI-LA repeats descent " be not admitted,
it is worse than useless to invite
comparison between the larva of a chiton and that of a worm. Unless
the mind has been prepared to discover in development the shifting
and progressive past history of a species, there can be no benefit of
an intellectual kind in comparing the likeness of the young brachiopod
with the early stages of the worm. But, conversely, when it is admitted
that all development is meaningless unless some idea of its use,
purport, or cause is afforded, and when in the study of the phases
of an animal's growth we are led to see prospects of tracing its past
evolution, the likenesses and analogies of development become forcibly
plain and valuable. Primarily, it may be said that a very large part
of the reasonableness of evolution depends on its rational interpre-
tations of development. Without development and its lessons,
evolution would be well-nigh unprovable. Conversely, without the
idea of evolution, the development of animal or plant is a meaningless
piece of natural transmogrification and change.
THE EVIDENCE FROM DEVELOPMENT. 233
In so far as the life-histories at which we have just glanced are
concerned, the general conclusions to be drawn from their study lie
on the surface of the subject. Beginning with the worms and their
transformations, we find a type of larva, presenting a rounded body
with variously disposed cilia (Fig. 157, B), which simply becomes
segmented, and with little further change becomes the worm. From
the worms to the " Lampshells " is an easy transition, for in the
development of the latter (Fig. 154) we find the clearest reproduction
of the features of the young worm larva (Fig. 157) in the body divided
into its three segments and exhibiting its cilia and eye-spots ; whilst,
as Huxley remarks, the resemblance to the worm-larva is increased
when we find the young lampshell developing bundles of bristles
(Fig. 1 54, F, G), such as the worm possesses, on the middle joint of its
body. From such resemblances, Huxley is more than justified in
remarking that, whilst the lampshells bear a likeness in development
to the plant-like "Sea-mats" (Fig. 190) and their neighbours, their
development " no less strongly testifies to their close relations with the
worms." Thus the evolution of a race of lower shellfish from a
worm-stock is plainly enough taught by development ; and such a
fact testifies directly enough to the possibilities of other molluscan
developments having had a similar origin.
Coming next in order to the Molluscs themselves, we find two
classes — the bivalves and the gasteropods — in each of which certain
primitive forms of development may be traced. The "Veliger-
stage" (Fig. 142, A) may be regarded as common to both groups ; and
the common origin for both classes may reasonably enough be
argued for and maintained on this ground alone, and apart from any
plain agreement in structure. It is, however, in the lowest members
of each group that we may expect to find the most marked likeness to
the primitive type and root-stock from which these classes have been
derived. Hence, it is to Chiton (Fig. 139) and Dentalium (Fig. 144)
that we turn for aid in solving the problem before us. The young
"Toothshell" (Fig. 145, C) is unmistakably a worm. Its barrel-shaped
body, its circlets of cilia, its end-tuft of these appendages — all are
characters which reproduce before us the embryo worm (Fig. 157, B).
Nor is the early history of chiton materially different from that of
the "Toothshell." The young chiton (Fig. 146, A) leaves the egg,
as we have seen, with a ciliated girdle in the middle of its body,
and a long tuft of cilia on its head ; whilst this embryo seems to
proceed even further on the worm-track, when we find its body to
become segmented or divided as in the worms (B, C); these divisions
becoming the shell-plates of the mature Chiton. Thus Chiton may
be regarded, without exaggeration, as a worm-form existing under a
molluscan guise. And when we arrive at the higher gasteropods, with
their " veliger-stage " and " trochosphere," we see produced before us
234 CHAPTERS ON EVOLUTION.
simply a later modification of the worm-stock. The life history of a
sea-butterfly or pteropod, in fact, takes up the narrative where the
development of chiton ends it. Chiton led us to the worm-larva
stage, and thereafter branches off on its lower molluscan path. But
the pteropod may, as we have seen, begin life as the worm (Fig. 153),
and proceeds not merely to develop its " veliger-stage," but remains
permanently therein ; flapping its way over the surface of the sea by
means of the permanent " velum," or its substitute, in the form of
the fins or wings. Last of all, a gasteropod like ALolis presents us
with a multum in parvo of the whole process of gasteropod and
molluscan evolution. Here, we take up the story at the stage where
the pteropod history concluded. ^Eolis and its neighbours (or
Lymneus), passing through the pteropod stage, each with its "velum,"
develop onwards to become the higher and shelled gasteropod, and
represent the furthest evolution of the race. Thus, from the worms
to the lampshells on one hand, and to the chitons and " Toothshells "
on the other ; from these latter, in turn, to the pteropod and thence
to the bivalves and gasteropods, the track of development seems
plainly marked. The whole story it tells is that of a shifting panorama
of the modification of the animal form ; phase succeeding phase, and
each new succession of forms obscuring, or it may be intensifying, the
development of the preceding classes and groups. But, clearly marked
or obscure, understood fully or only suggested to the mind, the
whole process of development reveals to us the operation of a
great law of evolution and progressive change, manifested through
those wondrous cycles and transformations which nature seems never
weary of exhibiting to the earnest mind and seeking eye.
If, finally, one might be tempted to inquire into the origin of these
ciliated worm-larvae themselves, we may find that speculative natural
history is not unprepared with a reply. We are reminded that, as the
early changes of egg- segmentation are not peculiar to the molluscs,
so neither are the veliger-stages the special belongings of that group
of animals. The " velum," or its representative ciliated girdles,
appears before us equally in the development of the echinoderms
or starfish group, of the worm, of the wheel-animalcule or rotifer,
and of the mollusc. The zoologist would further remind us that
these ciliary bands often remain in adult animals, and are represented
by certain stable possessions, such as tentacles or feelers, gills, the
" arms " of lampshells, and like structures. " It is probable enough,"
says Professor Ray Lankester, " that all the ciliated bands of invertebrate
embryos, even of adult organisms, can be explained as derivatives of one
primitive organ." If this thought be fully worked out, it contains a verit-
able " philosopher's stone " for the zoologist ; inasmuch as it enables us
to account for the forms and structures of animals on a rational basis.
That is to say, the particular form and structure of an animal or class, are
THE EVIDENCE FROM DEVELOPMENT.
235
due to the fashion in which the original ciliated bands of the larva and
the embryo itself have been modified by the external and internal
forces which now, as of old, operate on living things. Professor
Lankester has suggestively worked out this idea of the derivation
of all existing embryos from a type-form, to which he has given
the name of " Architroch " — a form represented by deputy, so to
speak, in certain worms and in the sea-mat class, as adult organisms.
Such a theory explains to us, on a basis of a reasonable nature, how
different forms may arise from a similar root-larva. And it may be
added, that should any objection be urged to such views on the
ground that they
are entirely hypo-
thetical, one may
retort that to all
other explanations
of the past of
nature, whether
theological or sci-
entific, exactly simi-
lar opposition may
be offered. Further,
we must reflect, that
in any case we have
to choose between
filling up from our
observation of na-
ture the gaps in our
knowledge which a
philosophical ne-
cessity entails, or
allowing these gaps
to yawn unsatisfac-
torily and perma-
nently unfilled. The
rational mind is not likely to hesitate in its choice of alternatives.
And if, lastly, it be borne in mind that, so far from being merely
shadowy, theories, such ideas of the origin of animal forms are based
on close observation of nature — often the work of many concentrated
lifetimes — the logical standing of a theory which connects the facts
of nature, and by so connecting explains them, needs no justification,
as it fears no honest and unbiassed criticism.
Turning now to the Vertebrate animals, we may find in the class
of frogs and newts (Amphibia) material for illustrating some of the
most important phases in normal development, and in altered life-
histories as well. The life-history of a frog has already been alluded
CHAPTERS ON EVOLUTION.
to in a previous chapter in connection with the evolution of lungs.
It is needful, however, again briefly to refer to this life-history as a
starting-point for the due understanding of other and allied cases of
development. The frog begins its existence as a tadpole (Fig. 159),
breathing first by external and then by internal gills, and possessing a
two-chambered heart, resembling that of the fish. Sooner or later
the hind limbs begin to appear, then the fore limbs are developed,
and the frog's lungs likewise begin to make their appearance. At
this stage, the animal resembles its neighbours, the Proteus and
Axolotl (Fig. 1 60), which are tailed, and which breathe throughout
life by both gills and lungs. Later on, the gills disappear entirely ; the
tail becomesrudiment-
ary; and the frog, leav-
ing the water, becomes
the terrestrial lung-
breather with which
we are so familiar.
To repeat Huxley's
words in reference to
the case for develop-
ment as a guide to the
history of the race :
"If all living beings
have come into exist-
ence by the gradual
modification, through
a long series of genera-
tions, of a primordial living matter, the phenomena of embryonic
development ought to be explicable as particular cases of the
general law of hereditary transmission. On this view a tadpole is
first a fish, and then a tailed amphibian, provided with both gills
and lungs, before it becomes a frog, because the frog was the last
form in a series of modifications whereby some ancient fish
became an urodele (or tailed) amphibian, and the urodele amphibian
became an anurous (or tailless) amphibian. In fact, the develop-
ment of the embryo is a recapitulation of the ancestral history of the
species." That there are "ancient fishes," still represented by living
species, which may have served as the starting point of the frog-race, is
matter of zoological fact.
" Various features in the anatomy of the Tadpole," says the late
Professor F. M. Balfour, " point to its being a repetition of a primi-
tive vertebrate type. The nearest living representative of this type
appears to be the Lamprey." This author also points out how close
are the resemblances between the mouths of the tadpole and
lamprey ; and a still more remarkable fact consists in the observation
FIG. iCo.— AXOLOTL, SHOWING THE EXTERNAL GILLS.
THE EVIDENCE FROM DEVELOPMENT. 237
that many of the peculiarities of the skull of the tadpole are repro-
duced in the skull of the lamprey. Whilst, as Professor Balfour
remarks, these resemblances must be due to deeper causes than
mere adaptation to similar habits, there are yet "no grounds for
supposing that the lamprey itself is closely related to an ancestral
form of the Amphibia." The more feasible opinion is that which
would assume that both lamprey and tadpole are descended from a
common and still more primitive stock. It would seem, indeed, that
the ganoid fishes (of which the sturgeon, bony pike, and the fossil
forms of the Old Red Sandstone, are examples) together with the
allied Lepidosiren and Ceratodus (see page 113, et se.), possess an
allied origin from the common root of lamprey and tadpole. The
facts already detailed (see Chapter VI.) concerning the development
of lungs in these latter fishes, would of themselves seem to support
this idea of their origin. Just as along one line of descent, the gilled
tadpole-race has developed its lungs as the frogs of to-day, so, start-
ing from the same ancestry, but developing along a different pathway,
the lepidosirens and their neighbours, as if animated by like air-
breathing tendencies, have developed lungs likewise. Similar ten-
dencies towards a certain goal in development, in other words, have
produced like results in the evolution of two branches of the
vertebrate tree.
An interesting fact may be added to these considerations, namely,
that one tadpole-form, that of Dactykthra, presents the closest re-
semblance to certain fossil fishes of the Old Red Sandstone period.
These fishes belong to the ganoid type, already mentioned, and the
Dactylethra tadpole, whilst resembling the ganoid stock, also shows
affinities to the shark and dog-fish type. The latter fishes are nearly
allied to the ganoids, and both appear in the geological record at
once as the oldest of fossil fishes and vertebrates. This curious tad-
pole probably represents a period in the evolution of the frog-race,
after that race had been specialised from the lamprey stage, and had
already advanced somewhat on its amphibian pathway.
It therefore requires no stretch of the imagination, but the exercise
of sober reason, to note, firstly, that as all the amphibian class — frogs,
toads, newts (Fig. 161), and their less familiar neighbours — tailed
and tailless, together begin life as tadpoles ; and, secondly, that as
they end, some like the frogs, tailless and gill-less, others like the
proteus or axolotl (Fig. 160), possessing both gills, lungs, and
tails, — the assumption remains clear that these animals have sprung
from a fish ancestry. It is further matter of fact that their develop-
ment has followed two pathways. In the one case the frogs and
toads have passed towards a pure air-breathing existence, and have
emerged from their development as land animals, pure and simple.
In the other case the lower stock of the class, represented by the
CHAPTERS ON EVOLUTION.
proteus and axolotl, &c., have retained many of their lower cha-
racters— most notably gills and tail — and have accordingly taken a
lower and less modified position than the frogs and. toads. The
familiar tailed newts (Fig. 161), which, though living in water,
and beginning life as
gill-bearing tadpoles,
breathe as adults, by
lungs alone, represent
a middle term in the
series, in that they still
retain the larval tail of
early life.
Whilst the ordinary
course of amphibian
development runs as
has just been described,
there are certain excep-
tions of extreme interest
from the evolutionist's
point of view. Firstly,
there are certain cases
of curious development
amongst the frogs them-
selves, which deserve a
passing notice. There
are peculiarities, for in-
stance, in the carrying
of the eggs, which are
eloquent enough in
their testimony to the singular modification of structure and habits
which may accompany alteration of surroundings. Thus the tadpole
form of Pseudis paradoxa attains a very much larger bulk than the
adult. Such a circumstance points either to some nutritive condition
affecting the larva, or shows that the larva reproduces some ancestral
form which was larger than the living frog. The female Q{ Nototrema
marsupiatum, a tree-frog inhabiting America, carries her eggs in a
large pouch which underlies the skin of the back, and opens behind.
The larva of Nototrema are said to want external gills. If correct,
this circumstance points to high modification in the development
of this frog. A like feature to the method of egg-carrying found
in Nototrema is seen in Optsthodelphys, another American frog ; and
Hylodes, likewise an American tree-frog, lays its eggs in the axils of
leaves — that is, in the angle between leaf-stalk and stem — the water
needful for their development being found in the chance drops resting
in that situation. The male of Alytes obstetricans of Europe, winds
FIG. 161.— NEWTS.
THE EVIDENCE FROM DEVELOPMENT.
239
the long chains of eggs laid by the female round his thighs, so that he
seems to possess " trunk hose and puffed breeches," as Mr. St. Mivart
remarks. Dropping, in due course, into the water, the young burst
forth from the egg-coverings, and swim away, leaving their father-frog
once more unencumbered and free.
Another frog (Rhinoderma Darwinii), a denizen of Chili, ex-
hibits another curious modification of a different kind. Rhinoderma,
like the edible frog of Europe, possesses certain " vocal sacs " or bags
placed within the mouth, whereby the resonance of the mouth and
the loudness of the croak are increased.
It is interesting to find, however, that
Rhinoderma has come to use its vocal
sacs as nests; the newly laid eggs
being thus received into the male
parent's pouches, and the young re-
maining therein till they attain a con-
siderable growth. We certainly know
of male fishes in the sea-horse genus
(Hippocampus) (Fig. 162), which carry
the young in a pouch ; and another
male fish (Arius fissus), like the Rhi-
noderma, carries the eggs in his
mouth and therein hatches them. In
Rhinoderma the vocal sacs are greatly
enlarged, and, in fact, extend on to
the flanks and belly of the animal.
From five to fifteen tadpoles were
found by Espada in each sac, the smallest being at the bottom. The
largest was about half-an-inch long, and had well- developed legs.
Neither the old nor the young tadpoles had any traces of gills, and
from their full development, the conclusion that the young are in
some way nourished in these sacs seems by no means far-fetched.
The Rhinoderma presents us, therefore, with a case in which the
organisation of the male has become curiously and permanently
altered to a decidedly new way of life — so much so, indeed, that the
skeleton has become modified from the pressure of the curious egg-
sacs of the mouth.
More curious still, on account of the very singular modification
which must have produced the feature in question, is the female
Pipa Americana or Surinam Toad, the skin of whose back becomes
soft at the breeding season. The eggs are pressed by the male into
this skin, which grows over them and encloses each in a kind of
cell. Very curious is it to find that in the cells of the maternal back,
not only the tadpole stage but the whole metamorphosis of this toad is
passed. The young toads develop a long tail within the egg, this
FIG. 162.— HIPPOCAMPUS, OR SEA-HORSE.
240 CHAPTERS ON EVOLUTION.
appendage being absorbed before they are hatched, whilst the useless
gills disappear at a very early period in larval existence. Over 120
cells have been counted in the back of this toad, and from these cells
the young emerge as miniature facsimiles of the parent Another
noteworthy case of altered development is that of the Hylodes Martin-
icensis, which passes through the whole of its metamorphosis within
the egg, and emerges, as do the young of the Surinam toad, a perfect
frog, which otherwise would require to pass several weeks in water
to complete its development.
Now, to what conclusions do such facts lead us respecting the
modification and alteration of development? It is perfectly clear that,
as frogs and toads are normal tenants of the water in their early and
tadpole stages, and are provided with gills as aquatic forms, those
species which pass the whole of their development in the back of the
mother, or even within the egg, must represent the most modified
members of the frog class. We are therefore entitled to take their
case as illustrating the best marked of the tendencies to alteration
which the race presents. A frog which, like Alytes, carries the eggs,
but drops them into water when they are ready to leave their primary
abode, represents the first stage of modification. We are led a
little farther on the way towards a suppression of metamorphosis
by the case of the Hylodes, which lays its eggs in the axils of leaves,
where moisture is relatively scarce, but where development is never-
theless undergone in due course. More advanced is the Surinam
Toad, where the young pass their entire metamorphosis within the
egg and in the mother's back ; the Hylodes Martinicensis being but a
further development still, seeing that in this frog the whole develop-
ment is carried on within the egg, and metamorphosis is therefore
practically hidden and unseen.
We may not doubt, therefore, that the amphibian class ex-
hibits thus a tendency towards direct development or that without
metamorphosis. Imagjne the result of the later stages of such a
modification of reproductive habits and customs. Hylodes Martin-
icensis, for instance, is now practically in the position of an
animal which undergoes all its changes within the egg, and which,
in all probability, will, in time, further shorten and condense its
life-history. If such changes and modifications are occurring before
our eyes to-day, is it unreasonable to regard all ordinary and
direct developments — and amongst others, those of fish, reptile,
bird and man — as in reality abbreviated and "brief chronicles"
of once extended chapters in animal histories ? A fish or bird
passing through its development within the egg undergoes a meta-
morphosis it is true, but shortened and condensed as compared with
that of the frog. There is no reason against the supposition, but
every circumstance of life favouring it, that once upon a time fish-
THE EVIDENCE FROM DEVELOPMENT. 241
development and descent could have been as plainly seen from the
outside world, as the frog's descent is traceable before our eyes to-day.
Higher development and progressive tendencies invariably tend to
shorten and condense the early stages of growth. Hence the value of
such cases as those of the frogs and their neighbours, which, through
mode of life, habits, and other and unknown conditions, have re-
tained much of their original "way of life," and have revealed to us,
through a literal byeway of development, the original and primitive
phases of that of all other animal forms.
The conditions which favour or retard such developments are often
obscure, or very frequently unknown. The presence of water or its
absence, for instance, would favour or retard the continuance of the
metamorphosis in the frog-class. We must also bear in mind that
geological changes — the rising and sinking of land and the like, the
conversion of swamps and morasses into dry land and similar physical
changes — are powerful factors in producing modifications of habit in
aquatic animals, and, through change of habit, of effecting variations
in structure and form. It is possible to prove the existence and opera-
tion of such changes from many points of view. Both from the zoo-
logical or biological side, and from that of geology itself, the importance
of such alterations of the earth's surface can be proved. This aspect
of the subject finds appropriate illustration in works devoted to the
facts of geographical distribution and their explanation ; whilst we
may not neglect to observe the strictly utilitarian points involved in
such abbreviated life-histories as those we have been discussing. It
has been noted that as we ascend in the scale of animal and plant life,
development becomes more and more condensed and abbreviated.
On a posteriori grounds we might argue that, from the fact of such
condensation accompanying higher life and progressive development,
some obvious advantage in the struggle for existence was thereby
gained. The nature of such advantage is not difficult to discover.
The more prolonged and exposed larval or early existence is, the
more likely are the young forms to succumb from loss of food,
change of surroundings, or from the attack of enemies and numerous
other conditions. On the contrary, with an abbreviated infancy, the
animal obtains a distinct " coign of vantage." There is less risk of
early death, and a greater prospect of an earlier and stronger
maturity. Thus the " selected races " are those which possess the
shorter and more condensed life-history, and these races, therefore,
come to the front in the universal struggle for existence which besets
and surrounds the living hosts to-day as of yore. As Sir John Lubbock
remarks, when speaking of the shortening of the insect's life-history:
"The compression and even disappearance of those embryonal
stages which are no longer adapted to the mode of life — which do
not benefit the animal — is a phenomenon not without a parallel in
&
242 CHAPTERS ON EVOLUTION.
other parts of the animal or even of the vegetable kingdom. Just
as in language long compound words have a tendency to concision,
and single letters sometimes linger on, indicating the history of a
word, like the ' 1' in ' alms,' or the ' b ' in ' debt/ long after they
have ceased to influence the sound ; so in embryology useless stages,
interesting as illustrations of past history, but without direct advan-
tage under present conditions, are rapidly passed through, and even,
as it would appear, in some cases altogether omitted."
We may here refer to the case of the Mexican Axolotl (Fig. 160),
on account of its peculiar development, and also from its bearings
on that of another member of the frog's class — the black salamander
(Salamandra atrd) of the Alps and its curiously modified life-history.
The Axolotl is a Mexican eft or newt, which retains the gills of early
life along with the lungs of the adult stage. It breeds freely in
captivity, and hence was long regarded as a mature and adult animal.
But in 1867 some axolotls were observed to emerge from the water in
the Jardin des Plantes at Paris, to cast their skins, and to become
transformed into a gill-less newt long known as an American genus,
FIG. 163. — AMBLYSTOMA.
and named Amblystoma (Fig. 163). Such a change was almost
equivalent to that whereby a frog could be metamorphosed into a
toad, and hence it excited no small surprise in the zoological world.
By careful experimentation a lady naturalist, Fraulein von Chauvin,
showed that by gradually inuring the Axolotl first to a life amongst
damp moss, and then to an existence entirely removed from the
water, it could be made to assume the Amblystoma-form, with its
black skin and yellow spots. Weismann states that the transfor-
mation occupies fourteen days, and Dume'ril states the period of
metamorphosis as sixteen days.
Fraulein von Chauvin states, in her account of the Axolotl's meta-
THE EVIDENCE FROM DEVELOPMENT. 243
morphosis, that her experiments were begun on June 12, 1874,
with five larvae, about eight days old. At the end of June the front
legs had appeared in the healthiest specimens, and on July 9 the
hind limbs were developed. Towards the close of November, one of
these larvae appeared to remain constantly at the surface of the
water, and from this sign Fraulein von Chauvin concluded that the
proper and natural period had arrived for the assumption of the
Amblystoma-form. This specimen was accordingly placed, on
December i, in a vessel in which its existence was divided between
a water life and a terrestrial one. The water being gradually
diminished, the gills began to shrivel, and on December 4 the
animal left the water, and remained on the earth and moss in the
vessel. At this period, it moulted its skin for the first time. Then,
also, between December i and 4, the dorsal crest, or fin, shrivelled
in addition to the gills, and the tail became rounded like that of
the Amblystoma. The original greyish-brown colour likewise
changed to black, and the spots of the Amblystoma became apparent.
The gill- clefts were open on December 4, when the animal left
the water ; but in about eight days these slits were entirely closed.
In the other specimens experimented upon by Fraulein von Chauvin,
the period occupied in the metamorphosis varied somewhat, the
greatest differences being due, apparently, to the nutrition of these
forms being less active than in those specimens in which the trans-
formation was speedily accomplished. The general conclusion
arrived at from these experiments is that " Axolotl larvae generally,
but not always, complete their metamorphosis if, in the first place,
they emerge sound from the egg and are properly fed ; and if, in the
next place, they are submitted to the necessary treatment for chang-
ing aquatic into aerial respiration. It is obvious that this treatment
must be applied very gradually, and in such a manner as not to
overtax the vital energy of the amphibian." Weismann concludes
that " most Axolotl larvae change into the Amblystoma-form when,
at the age of six to nine months, they are placed in such shallow water
that they are compelled to respire chiefly^by their lungs."
The Axolotl does not, so far as is known, become converted into
the Amblystoma-form in its native region. In the Mexican lakes it
appears to be perpetually and only known as the gilled Axolotl.
Professor Cope also states that these Mexican animals, when bred in
captivity in America, reproduce their like, and do not show any
tendency towards transformation. Yet in France and elsewhere, as
already remarked, the Axolotl becomes transformed into the Ambly-
stoma in its domesticated or captive condition ; but it is necessary
to remark that the exact species which became metamorphosed in
France was either the Siredon lichenoides of Baird, or some allied
form, and not the Siredon Mexicanus of Mexico. Baird states that
R 2
244 CHAPTERS ON EVOLUTION.
the former species, which occurs 7.000 feet above the sea-level in
Wyoming territory, becomes transformed into the Amblystoma
Mavortium ; whilst the first Amblystomas obtained by Dumeril from
the Parisian Axolotls appeared to agree with another species of
Amblystoma (A. tigrinum). If these details be taken into account,
whilst they do not in the least affect the curious nature of the trans-
formation, they would seem to indicate that more than one species
of Axolotl undergoes metamorphosis ; and this result is exactly that
which the naturalist would be led to expect. The general condition
probably affects the whole Axolotl race, and is not confined to any
one species. Regarding the Amblystomas themselves, it has been
already remarked that these newts are well known in America. Over
twenty species are known to inhabit North America, and there can
be no doubt that these animals live and breed as true Amblystomas.
As Weismann remarks : " There are, therefore, true species of Siredon
which regularly assume the Amblystoma-form under their natural con-
ditions of life (i.e., develop into Amblystomas from eggs laid by
Amblystoma- parents), and which propagate in this form ; while, on
the other hand, there are at least two species which, under their
existing natural conditions of life, always propagate as Siredon " (i.e.,
develop Axolotls from eggs laid by Axolotls). Observations by
Professor Baird on the development of Amblystomas show that, as
might be expected, the young gilled larvae, arising from Amblystoma
eggs, present a close likeness to the Axolotl race.
The case of the axolotl illustrates powerfully the effects of a
change of surroundings in metamorphosing a species. A succession
of dry seasons, operating in the past, has most likely been the active
origin of the amblystoma race from the axolotl stock. Presumably
the axolotl, as the " gill-bearing " form, is the primitive stock ; the
amblystomas being a derived race, but nevertheless representing a
true species of which the axolotl, conversely, may be termed the
" larval form." To this relationship, however, reference will be sub-
sequently made. The shrivelling of the gills seen in the experiments
on the axolotl, it is noteworthy, was probably due to a mechanical
cause, that of dryness of the surroundings. Once established, the
new race of Amblystomas would be propagated amidst the conditions
which best suited them, whilst the axolotls have flourished amid
their own aquatic environments.
This case of modification of species, however, leads to a much
more typical one in which the female of the black salamander of
the Alps, a gill-less newt or eft, retains her eggs within her body,
and hatches them ; the young likewise undergoing development,
and casting their gills therein, just as do the young of the modified
frogs already described. Furthermore, out of some 40 or 60 eggs,
only two young are developed ; the latter devouring the remaining
THE EVIDENCE FROM DEVELOPMENT. 245
eggs as food. Thus, whilst the young of the spotted salamander,
a neighbouring species, number 40 or 50 at a birth, those of the
alpine species number but two. Yet the two species are equally
numerous — a fact showing powerfully how one animal, despite
disparity of numbers, may equal in vitality an apparently more
prolific race. For the two young of the alpine salamander, when
born, are large and active, have passed completely through their
development, and possess strong acrid skin-secretions ; whilst those
of the spotted species are comparatively helpless when born, and
have not got rid of their gills. Hence the latter are subject to a
greater mortality, and the proportion of adults to young is therefore
relatively small. On no rational theory of nature could it be believed
that a young newt was provided with gills, and that, thus furnished, it
was destined to be developed ivithin its parent's body. The two facts
of the presence of gills and the development of the alpine sala-
mander within the parent body are in utter opposition to each other.
Further, we know that when taken from the parent body, long prior
to their natural period of birth, and placed in water, the young of the
black salamander live and breathe by their gills, as was undoubtedly the
original habit of the species. Placed in water, the young beings live
for weeks, and ultimately develop from their water life into land sala-
manders. But in this latter experiment, the full development of the
young occurs weeks after the time when they would have been
moving actively on the Alps, had they been left to their development
within the parent frame. Thus we see, firstly, that the modern
development of this animal is clearly acquired — even the curious habit
of the two larvae eating the other eggs clearly proves as much. And
secondly, we again come face to face with a case of shortened and
condensed development, favouring at once an early maturity and the
increase of the race. Probably a rise of land, carrying these sala-
manders farther and farther from water, was the direct cause of the
altered mode of life of the alpine salamander. We know that this
new adaptation is of relatively ancient origin, for the gills of the
salamander, placed in water, shrink by a natural and vital process of
absorption, and not through mere drying and shrivelling as in the
axolotl. The acquired process of gill-absorption has become, in
other words, an inherited matter — has become part and parcel of
the animal's constitution. As, therefore, their .watery pools were left
below by the rise of land, the salamanders would gradually acquire
the habit of retaining the young within the body for more and more
lengthened periods; and in due time, the present state of matters
was evolved — including limitation of numbers and acceleration of
development, along with the novel condition of utilising the remaining
eggs as a food-supply.
An important and interesting feature in connection with the
246
CHAPTERS ON EVOLUTION.
preceding cases of altered development, consists in the observation
that the Mexican axolotl, apparently a mature form, was able to
reproduce its species. It may perhaps prove a truer conception of
the case if we regard the axolotl as a "permanent larval form,"
which has acquired the power of producing young, and which has
therefore assumed the form, life, and constitution of a species.
Analogy supplies us with a valuable series of parallel instances from
the records of natural history science. The example in question, of
a larva acquiring reproductive powers, is by no means singular or
unique. We have seen that practically a pteropod or " Sea butterfly "
(Fig. 152, B) is essentially the larval form of the gasteropod (A),
which has had its immature character fixed, and which has acquired
the power of producing young. Other cases of this peculiarity are
readily found within the confines of the insect class, and
in other divisions of the animal world. Thus, we know
that the larva or maggot — itself an absolutely immature
form — of a fly (Cecidomyia) (Fig. 164), produces other
larva like itself, and these in turn produce others (a, #),
which, finally becoming males and females, produce
normal young through eggs. There is another insect
(Chironomus) of which, as Grimm has shown, the
chrysalis lays eggs ; and we know of cases in which
(as illustrated by the Aphides or plant-lice, and by the
queen bees) perfect young may be produced by the one
insect alone. So likewise the common Newt (Triton
cristatus] of our ponds may, occasionally, when imma-
ture, produce young ; and another species ( T. Alpestris)
has been seen to reproduce its kind when it was still in
the tadpole stage. Amongst the zoophytes, such features
are still more plainly marked. For a plant-like animal
colony gives origin to jellyfishes, which swim freely in
CEciDolfviA. the sea, and later on produce eggs, from each of which a
zoophyte in turn springs. These facts were formerly
included under the head of " alternation of generations ; " but under
whatever name we denote the phenomena, the lesson they teach is
uniform and clear. Such cases as these of the insect larva and the im-
mature axolotls and new s producing young (contrary to the rule that
only adult animals reproduce their species) prove to us that, if condi-
tions be favourable, a young animal's development and constitution
may be so modified and intensified, that it may, whilst still under its
larval guise, produce young, and thus assume the likeness and functions
of a new and distinct species. Such facts further impress the idea that
the young being, equally with the adult, is liable to modification and
change ; and they therefore teach us that the starting points of new
species and races do not always lie within the domain of mature
THE EVIDENCE FROM DEVELOPMENT.
247
life, but may take origin from stages in development prior to the full
period of growth.
Given an ultimate independence of the young form, together
with the power of producing beings resembling itself, and we may
readily conjecture how a new and very different species or race may,
in comparatively rapid fashion, originate from a well-known stock.
Mr. Darwin gives as an example of this possibility, the case of the
beetle Sitaris (Fig. 165 F), of which the first larvae (A) are active
and minute, and possess six legs, two long feelers, and four eyes.
" These larvae are hatched in the nests of bees ; and when the male
bees emerge from their burrows in the spring, which they do before
the females, the larvae spring on them, and afterwards crawl on to
the females whilst paired with the males." Then ensues the laying of
eggs on the surface of the honey in the cells by the female bees, the
FIG. 165.— SITARIS AND ITS DEVELOPMENT.
Sitaris larvae devouring the eggs. Then the latter undergo a meta-
morphosis. The eyes disappear, and the legs and feelers become
rudimentary (B), whilst they feed on the honey. At this stage they
more closely resemble ordinary insect larvae (C, D, E), and after further
transformation emerge as the perfect beetles (F). " Now," adds Mr.
Darwin, "if an insect, undergoing transformations like those of the
Sitaris, were to become the progenitor of a whole new class of insects,
the course of development of the new class would be widely different
from that of our existing insects ; and the first .'arval stage certainly
would not represent the former condition of any adult and ancient
form." " We can see," adds Darwin, " how, by changes of structure
in the young, in conformity with changed habits of life, together with
inheritance at corresponding ages, animals might come to pass
through stages of development perfectly distinct from the primordial
*R4
248 CHAPTERS ON EVOLUTION.
condition of their adult progenitors." On this reasoning, the Axolotl's
later history cannot be expected to coincide with that of the Ambly-
stoma. It is a larval form, which, apparently arrested in develop-
ment, has nevertheless, contrived to develop the lungs which mark
the full growth of all amphibia, whilst it likewise retains the gills of
early life. The relationship between the Axolotl and Amblystoma
presents, besides, one of the most effective refutations of that
common but ignorant remark that no one has yet adduced any
proof of the direct transmutation of one species into another. In the
case before us, not merely is the transformation one in which one
genus of animals apparently becomes another, but the near relation-
ship of two thoroughly distinct forms is thus proved to lie within the
province of exact zoological observation.
It should be added that Dr. Weismann, of Freiburg, who has
devoted much attention to the metamorphosis of the Axolotl, main-
tains that the case in question is one, not of sudden advance in a
species, but of reversion to a lower stage. He believes that " those
Amblystomas which have been developed in captivity in certain
instances from Siredon Mexicanus (S. pisciformis\ as well as from
the Paris Axolotls, are not progressive but reversion forms. "I
believe," concludes Dr. Weismann, " that the Axolotls which now
inhabit the Mexican lakes were Amblystomas at a former geological
(or better, zoological) epoch, but that owing to changes in their
conditions of life, they have reverted to the earlier perennibranchiate
(permanently-gilled) stage." One of the most interesting facts which
lend support to this view of the backward development of the Axolotl
is the discovery that the Axolotls possess a rudimentary intermaxillary
gland furnishing a glutinous secretion, and which serves to aid the
capture of insect-prey. Now, as this gland exists in a perfect shape
in all land amphibians, but is absent in gill-possessing forms, its
presence in the gilled Axolotls would certainly seem to indicate that
these animals retain the gland as a legacy from the higher or Ambly-
stoma stage from which they are believed by Weismann to have
descended and retrograded. In whatever light we regard the case of
the Axolotl, the bare facts of its curious development remain un-
altered. It seems justifiable enough, notwithstanding Dr. Weismann's
opinion, to regard the Axolotl as a " permanent larval form," and as
representing an arrest of the development of Amblystoma, producing
a literally new race of animals able to reproduce their like, and thus
evolving a new group of animals by an easily understood modifica-
tion of an existent species. The presence of the "intermaxillary
gland " may represent the initial stage or beginning of that gland's
development in the Axolotl race, instead of a degeneration from the
perfect gland of the Amblystoma. Inheritance alone, might account
for the development of this structure. But, indeed, whether we
THE EVIDENCE FROM DEVELOPMENT.
249
adopt Dr. Weismann's view of the Axolotl as a case of reversion from
Amblystoma to its lower stage, or whether we regard the Axolotl
races as representing larval forms permanently settled to form a
distinct race and species, the influence of surroundings in evolving
new forms of life receives an apt illustration. Arrest of development,
an altered way of life, change of surroundings, and allied conditions,
are seen to operate powerfully on animals and plants ; and are found
to effect exactly those changes in the constitution of the living world
which evolution postulates, and which meet the biologist at every turn.
One typical case of similarity in early development, as suggestive
of a near or common origin, from the group of birds, may merit
mention in the present chapter. Mr. A. R. Wallace points out in an
interesting fashion how the humming-birds (Fig. 166) of the New
World, placed of old side by side with
the Old- World sun-birds (Fig. 168),
were, in 1850, separated from the latter
forms and placed by Prince Lucien
Bonaparte near the swifts (Fig. 167) in
his system of classification. That this
arrangement was correct — that is, was
one based upon natural affinities, and
not merely upon superficial resem-
blance—is easily provable. Thus the
breast-bone of a humming-bird and
that of a swift are marvellously like.
It is not notched behind in either bird,
whilst this bone in the sun-bird bears
two depressions. In the colour and
number of the eggs, swifts and hum-
ming-birds agree, and they also present
close resemblances in the arrangement
of their feathers. Both have ten tail
feathers and sixteen true quill feathers ;
and in both the first quill is longest.
But whilst the bill of a swift is short,
broad, and flat, the gape wide, and the tongue flat, the humming-
birds have a long, slender, cylindrical bill, and a tubular tongue,
which can be protruded to a great extent, and which is used for
drinking up the nectar of flowers. We shall allow Mr. Wallace to
tell us of the striking resemblance between these two groups of birds,
revealed by a study of the humming-bird at an early stage of growth.
" When on the Amazon," says Mr. Wallace, " I once had a nest
brought me containing two little unfledged humming-birds, apparently
not long hatched. Their beaks were not at all like those of their
parents, but short, triangular, and broad at the base— just the form
FIG. 166.— HUMMING-BIRD.
250
CHAPTERS ON EVOLUTION.
FIG. 167.— SWIFTS.
of the beak of a swallow or swift slightly lengthened. Thinking
(erroneously) that the young birds were fed by their parents on
honey, I tried to feed them with a syrup made of honey and water ;
but though they kept their
mouths constantly open,
as if ravenously hungry,
they would not swallow
the liquid, but threw it
out again, and sometimes
nearly choked themselves
in the effort. At length
I caught some minute
flies, and on dropping
one of these into the
open mouth it instantly
closed, the fly was gulped
down, and the mouth
opened again for more :
and each took in this way
fifteen or twenty little flies in succession before it was satisfied.
They lived thus three or four days, but required more constant care
than I could give them. These little birds were in the swift stage ;
they were pure insect-eaters,
with a bill and mouth adapted
for insect-eating only."
Such an interesting recital
once again illustrates the
maxim, that the likeness be-
tween living beings, imper-
ceptible in the adult stage,
may yet be plainly enough
apparent in the earlier phases
of development. As with the
crustaceans, where we find a
shrimp and a barnacle, utterly
unlike as adults, beginning life
under an essentially similar
guise, so with the swifts and
humming-birds — their like-
nesses, masked by differences
in habits of life, are neverthe-
less traceable without difficulty
in the young state. Mr. Wallace especially reminds us that certain
of the sun-birds themselves, resemble the humming-birds in respect
of their long bills and tubular tongue, adapted, like those of the
FIG. 168. — SUN-BIRD.
THE EVIDENCE FROM DEVELOPMENT, 251
latter birds, to feed upon flower juices and upon the insects that
infest flowers. He emphasises the need for distinguishing clearly
between characters or likenesses which are " structural " — that is, are
part and parcel of an animal's being — and those that are purely
" adaptive " — that is, arise from a similar mode of life, independently
of the origin of the species. The former are transmitted from
ancestors ; the latter are the products of recent modification. The
former indicate the true nature of the animal, because they are part
of its inheritance ; the latter often suggest false resemblances due to
similarity of habits and not to community of origin. Thus, whilst the
humming-birds and swifts possess stmctural and inherited likenesses,
the former and sun-birds are related only through similar adaptive
characters. The ^kull of a cuttlefish, to select another example, is
comparable in its functions to that of a low vertebrate animal, but on
no theory of nature are these two groups connected together. They
have arisen, like the similarities of sun-birds and humming-birds,
entirely independently, in respect probably of simiiar conditions, and
not of inheritance from a common ancestor. The inherited characters
which mark real resemblances are not, as we have seen, always
apparent ; and the adaptive characters through which the life of the
species is carried on may entirely mask and conceal them. As
Mr. Wallace puts it, we arrive at " the seeming paradox, that the less
of direct use is apparent in any peculiarity of structure, the greater
is its value in indicating true, though perhaps remote, affinities ; while
any peculiarity of an organ which seems essential to its possessor's
well-being is often of very little value in indicating its affinity for (to)
other creatures." Thus we are led to the conclusion, favoured again
by development and its lessons, that the humming-birds " are essen-
tially swifts — profoundly modified, it is true, for an aerial and
flower-haunting existence — but still bearing in many important peculi-
arities of structure the unmistakable evidences of a common origin."
252 CHAPTERS ON EVOLUTION.
XII.
THE EVIDENCE FROM THE LIFE-HISTORIES
OF INSECTS.
WHEN the development of an animal or plant is duly studied,
one or two chief aspects of such a subject fail to be considered
hy the biologist. Either the young organism has been converted
directly into the likeness of its parent, or it has assumed the pa-
rental form indirectly and through a series of transformations more
or less distinctly marked. In other words, the young form has
emerged upon the stage of life in the guise of its parent, or it has
appeared first in a shape and under an appearance not recognisable
as belonging to the race it has sprung from. In the latter case,
changes of greater or less extent convert the young being into the
likeness of its progenitors ; and when such transformations occur in
the life-history of an animal or plant, it is said to undergo " meta-
morphosis." Every one, for instance, knows that the butterflies of
the garden do not emerge from the egg as winged insects, whilst
common information is able to assert that they pass through the larval
or "grub" stage and also through the chrysalis form before becoming
the perfect insects. So, also, the flies begin their life as maggots ;
and the bees and beetles, with other insects, exhibit like stages to the
butterflies in the course of their development. Furthermore, a frog,
as we have seen, practically begins life as a fish, breathing first by
external and then by internal gills. Sooner or later, however, limbs
are developed ; the gills are replaced by lungs ; the tail disappears ;
and the tailless condition of the frog race is finally assumed with its
emergence upon the land. Insects and frogs — not to speak of other
animals, such as crustaceans, whose history has been already discussed
in a previous chapter — are therefore said to undergo "metamorphosis."
Sundry questions not unnaturally rise in the mind which atten-
tively considers such phenomena in the animal world. Firstly, there
is the plain question, " Why do some animals undergo metamor-
phosis and others not ? " Then, secondly, may be asked, " What is
the meaning of metamorphosis?" or more primarily, "Can any
meaning be assigned to this process ? " As we have frequently had
occasion to point out, such questions receive no aid or solution from
that philosophy which maintains, as an article of unquestioning faith,
that the living belongings of this world came forth fashioned in all
THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 253
their excellence — or, it may be added, in all their frequent and
apparent imperfections — at the behest of some sudden creative fiat.
There is no need to assume development at all on this hypothesis of
things, which for the man of science has been slain long ago ; though
traces of its influence are not unknown in regions removed from the
active currents and tides of culture. On the reverse side of matters,
stands the theory broadly denominated " evolution," which, seeing
the promise of reading a past and progressive history in the develop-
ments which pass in panoramic review before our eyes to-day,
asserts that a law of progress has guided and still guides life's courses
and ways. On this theory we can understand why development
takes place — namely, because it is a law of life that the progress and
growth of the race should be represented in, and carried out through,
its individual histories. And we can also conceive why development
should run in the grooves marked out so conspicuously in many life-
histories, such as those of insects and crustaceans. This latter fact is
explicable when it is repeated that we see in an animal's early growth,
the lines and stages along which the development of its race has
passed. By the very idea of evolution we expect variety and change to
be represented in the development of living beings ; for such change
is the one great condition which has made this universe what it is.
Agreeing as to the main reasons for development and its ways, we
should find little difficulty in comprehending how these ways and
paths have been followed. As we have already impressed upon the
reader, the picture is not always clearly limned, and its outlines are
often meagre enough. Still, what we do see and know of its form,
convinces us of the correctness of the broad deductions of evolution ;
which deductions being scorned and denied, leave the whole course
of nature a tissue of inexplicable absurdities.
In the present instance, dealing with the meanings of meta-
morphosis, we intend to direct attention to certain details which, for
lack of space, have been omitted in previous chapters, and which,
dealing with matters of special interest to the student of evolution,
may, logically enough, claim attention in a separate section. Such
subjects as the general nature of "metamorphosis," and how that
process is modified by surroundings and other circumstances, as well
as the narration of some life-histories which illustrate very aptly the
general conclusions of evolution, may therefore fitly engage our con-
sideration in the course of our developmental studies.
Firstly, then, the general question of "metamorphosis " demands
notice. Whilst it is perfectly true that, broadly speaking, only such
animals as insects, crustaceans, and frogs — exhibiting very marked
and apparent change of form in passing from the young to the adult
stage — may be said to undergo " metamorphosis," it would be far
more logical, because more true, to assert that the histories of all
254 CHAPTERS ON EVOLUTION.
living beings, without exception, illustrate the process in question.
This remark has been made in reference to the developments we have
already studied. For example, there is not, after all, such an immense
difference between the development of an insect and that of a fish —
or, for that matter, between that of the frog and of man himself — when
the facts of development are fairly faced and duly understood. No
animal or plant is suddenly transformed into the perfect likeness of
its parent. On the contrary, it has not merely to grow, but it has to
be formed from that which is formless ; to become organised by the
development of that which has no structure at all ; and to advance
along lines of development during which it successively assumes a
transient likeness to the forms of other and lower beings. Thus a
quadruped, whilst undergoing development within its parent's body,
in reality passes through as strange and startling a metamorphosis as
does a frog outside its parent's body, and external to its egg likewise.
A quadruped is really at first like a fish and reptile. So alike are the
young of all vertebrates in their early stages, that recognition of the
nature of any particular form may be an impossibility. " Metamor-
phosis " thus occurs in quadrupeds as in frogs ; in snails and oysters as
in insects. The great and prevailing difference simply exists in the
fact that the insect or frog leaves the egg in an' imperfectly developed
condition and at an early stage of its career, passing the remainder
of its development as an independent being. In the quadruped
or fish, or in the bird and reptile, the young animal does not
quit the parent body or egg at such an early period, but remains
within its primitive shelter to undergo its full development— or at any
rate to emerge upon the world of active life tolerably well prepared
for the struggle of living and being. Even amongst the quadrupeds,
as in well-nigh every other group of animals, and as in the plant
world likewise, there may be great differences in the degree and
stage of perfection at which the young organism is ushered into active
or independent existence. No better instance of this could be found
than in the case of the kangaroos and their allies, in which, as lower
quadrupeds, internal development ceases at a very
early period compared with that at which higher
quadrupeds are born. The newly born young of a
kangaroo, which, when full grown, stands 6 or 7
FIG. 169. fee(. high, measures about one inch in length at
YOUNG JVANGAROO. - . *§?i" s \ i -ii !• i i
birth (Fig. 169), and resembles a little red worm
much more nearly than a kangaroo. At birth it is transferred to
the characteristic " pouch " of the mother, wherein for weeks it
is protected and nourished by the milk secretion. If we consider
the effects of growth on such an organism, we may well feel
assured that a " metamorphosis " of very complete kind must be
required to transform the imperfect and feeble being just described,
THE EVIDENCE FROM THE LIFE-HISTORIES- OF INSECTS. 255
into the giant quadruped which takes its leap of twenty feet with the
utmost ease. So, also, we find in the development of birds well-
nigh infinite variety in the stage of perfection at which the young
animal is thrown upon its own resources. Of old, naturalists were
wont to divide the birds into those which could run about and forage
for themselves immediately on leaving the egg, and those which, as
mere fledglings, required parental care and attention for a longer or
shorter period after bursting the shell. A young chicken is a much
more independent being than, say, an infant thrush ; and numerous
other comparisons might similarly be instituted, with a like result of
showing variations in the development of even the animals of a
single class.
It seems, therefore, correct to say that the term " metamorphosis "
is one of very considerable latitude, and one admitting, in fact, of
no rigid definition at all. At the best its value is merely relative, and
those animals may be regarded as really most " metamorphic," so to
speak, which leave the egg in an immature state, and which, through
circumstances which it is our business to trace in this chapter, have
to pass through a definite or well-marked set of changes in form,
shape, and often of size also, before assuming the likeness of the
parental form. If we reflect that every living being springs from a
mere speck of protoplasm, devoid of all structure, which we
call "germ" or "egg," and which contains the potentialities of
becoming what its parent now is ; or if we further consider that
from this speck of albumen there is developed in a few days, as in
the case of the chicken, a creature rejoicing in the possession of a
complex system of bone, muscle, sinew, brain, nerve, and sense
organs — we may well feel inclined to consider such a transformation
and development as thorough an example of " metamorphosis " as, and
as a far higher development than, that of the insect which attracts
our notice simply because it is more evident to our eyes. Another
striking proof that " metamorphosis " must be, after all, a comparative
term, lies in a knowledge of the fact insisted on and illustrated in a
previous chapter — namely, that the eggs of all animals, from sponge to
man, pass through the same stages up to and including a given point,
at which each group branches off, so to speak, on its own pathway
towards adult and specific perfection. Thus, why one animal under-
goes those changes of form we see in the insect, and why another
does not, are circumstances — to come to details — depending, firstly, on
the size of the egg from which it is developed, and concurrently on
the amount of nourishment the egg contains ; and, secondly, upon
the varying circumstances and surroundings of its life, as well as
on the life and history of its race, as temporarily represented by its
parent. Thus a large-sized egg, with a big yolk, will, cateris
paribus, produce an animal in a higher and more perfect stage of
2S6
CHAPTERS ON EVOLUTION.
development than a small egg, in which no provision exists for the
nutrition of the embryo. So much, indeed, may safely be predicted
of the causes which retard or favour an early escape from the egg.
In the latter case, of course, let us bear in mind that the young will not
resemble the parent animal, and we naturally expect to behold
changes of form or " metamorphosis " in its further development, and
ere it attains to the parent size and likeness.
But we must not neglect to note an equally important cause of
alteration in form, which, acting subsequently to the escape of the
immature animal from the egg, will direct its footsteps in different
channels, and clothe its form with varied guises. The surroundings
of an animal's life necessarily affect that animal, and in time its
race, viewing individual and race as consisting each of an adult
being and beings. This much is the plainest of plain truths.
But it is equally true that surroundings and varying conditions
FIG. 170. — THE ROSY FEATHER-STAR'S DEVELOPMENT.
a, adult starfish ; b, young stalked forms.
of life must also affect the young stages of animal existence.
Even more marked and powerful imist be the effect of outward
conditions on the young organism, whose frame and constitution,
not yet fully formed, are infinitely more plastic and facile than those
of the adult. All we know of the effects of environments on living
beings, teaches us this lesson. We know something of the effects of
heat and cold, of a change of medium, and of numerous other
circumstances which materially alter the development of both animals
and plants. Natural- history records teem with examples of these
facts. A young rosy feather-star (Antedon, Fig. 170) may be hurried
through its larval state, and may be made to gallop post-haste through
its " metamorphosis," if it be supplied with pure sea water. If, on
the other hand, such a larva be kept at a low temperature, and in
water not frequently changed, and consequently on a more meagre
dietary, it will delay in its larval progress. Its development may
THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 257
not merely be greatly protracted and prolonged, but it will attain to a
higher stage of independent development than before. So also with
many insect larvae, and so with zoophytes. The effects of varying
conditions on the young and developing animal are plainly traceable.
It remains for us to discover what light such reflections throw on
some well-marked and familiar cases of metamorphosis around us.
The insect world teems with examples of " metamorphosis " at
once striking and interesting. It also, however, illustrates a previous
remark, that in one and the same class we may find great variations
in development and " metamorphosis." For instance, we may find
no metamorphosis at all in some insects. The lice, the bird-lice,
and the spring-tails (Thy-
sanura) thus come from
the egg resembling in every
respect, save in size, the
perfect in sects. They sim-
ply cast or shed their skin
at each successive stage of
growth, but no change of
form is represented in their
development. So also with
manyinsects of higher.rank.
A kind of day-fly ( Chloeon,
Fig. 171) is described by
Sir John Lubbock as under-
going no fewer than twenty
moultings of its skin during
its "metamorphosis," which
is not, however, of marked
or distinct character, since the organs of the young animal are
simply and gradually changed into those of the adult insect. Even
in insects which undergo a much more typical metamorphosis than
the day-flies, the gradual conversion of the larval parts into the
organs of the adult may be witnessed. A young cricket (Fig. 176)
becomes the adult very gradually, and the days of its infancy are
not markedly separated from those of its youth, nor are these latter
in turn sharply defined from the period of adult life.
Turning, however, to actual details, we find a butterfly (Fig. 172),
fly, and beetle respectively to exhibit the so-called " perfect " form of
metamorphosis. Each begins life — that is, comes from the egg, after the
preliminary stages common to all eggs — as a grub, caterpillar, or larva
(a), which spends the first part of its existence in the guise of a worm,
eating voraciously and increasing, as a rule, many times its original
size in bulk. Next this voracious grub settles down and becomes the
chrysalis, » >, Lubbock, in speaking
L'/ 0 . , ^ I ^ ^ 3 ^ of the modification of
the biting to form the
suctorial mouth, " the
transition from the one
condition to the other,
this would no doubt
take place contempo-
raneously with a change
of skin. At such times
we know that, even
when there is no change
in form, the softness of
the organs temporarily
precludes the insect
from feeding for a time,
as, for instance, in the
case of caterpillars.
If, however, any con-
siderable change were
evolved, this period of fasting must be prolonged, and would lead
to the existence of a third condition, that of the pupa, intermediate
between the other two. Since the acquisition of wings is a more
conspicuous change than any relating to the mouth, we are apt to
FIG. 176.— CRICKET.
a, eggs ; b, arvx (natural size) ; c, magnified ; d, chrysalis ;
e, perfect insect.
THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 265
associate with it the existence of a pupa state ; but the case of the
Orthoptera (cricket, Fig. 176, or grasshoppers, Fig. 175, &c.) is
sufficient proof that the development of wings is perfectly compatible
with permanent activity ; the necessity for prolonged rest is in reality
much more intimately connected with the change in the constitution
of the mouth, although in many cases, no doubt, this is accompanied
by changes in the legs and in the internal organisation." The same
authority expresses the
opinion that, whilst the
biting mouth can be modi-
fied to form the suctorial —
a change witnessed in every
developing moth and but-
terfly— the originally biting
mouth of the beetle could
not have been directly
modified, contrariwise, to
form a sucking apparatus,
"because the intermediate
stages would necessarily be
injurious." More probable
is it that both types have
sprung from some more primary form of mouth, which, partaking of
the character of neither, has been therefore capable of modification
in either direction, "by gradual change, without loss of utility."
That such a form of mouth, united to a body of equally convenient
FIG. 177.— PLANT LICE.
a, wingless insect ; b, wingless insect.
FIG. 178.— RED ANTS.
a, male, and 6, female (both winged); f, neuter.
primitiveness, is to be found still represented in the ranks of living
insects, we shall shortly discover. Meanwhile the question of wings
awaits a brief notice.
The nature of an insect's wing, discussed in reply to the question
"What is it? " throws some light on the question of its origin. The
266
CHAPTERS ON EVOLUTION.
physiology or use of a wing is, of course, to serve as an organ of
flight. But the use or function of an organ may be, and often is, a
secondary and adaptive matter, and may be very far from revealing
the original condition of the structure in question. Authority in
matters 'entomological, assures us that the wings, as appendages of
the insect's body, are in reality parts of the animal's breathing
system. They contain branches of the breathing tubes, and expan-
sions of the blood-vessels likewise. " Hence," says Packard, " the
aeration of the blood is carried on in the wings, and thus they serve
the double purpose of lungs and organs of flight." But we must note
that many insects are absolutely wingless. The lice, spring-tails, and
fleas, and even the plant lice (Fig. 177) and neuter ants (Fig. 178, c),
belonging to winged
groups, are destitute of
these organs. No doubt
the wingless condition in
the latter cases is to be
explained on the theory
of disuse causing the
disappearance of these
organs. But the most
primitive insects are with-
out wings, and we may,
therefore, reasonably con-
clude that wings are not
original belongings but
late developments of the
race. Furthermore, many
insects of relatively high
rank, such as the crickets,
grasshoppers, &c., quit the
egg without wings, and
this although they are ex-
tremely active in every
respect. A wingless state
FIG. 179.— AQUATIC INSECT LARVAE, SHOWING THE is on all grounds, includ-
BREATHING GILLS. .1 ' -j
A, Larva of Ephemera or Day-fly ; B, Larva of Che bioculata. mg the evidence Ot de-
velopment, to be regarded
as the original condition of the insect class. We have seen the
intimate connection which exists between the wings and the
breathing of insects. Of the two functions, breathing is, of course,
much more primary and essential to life than flight. Hence we
may well conclude that as many insects, especially the most
primitive, breathe and live without wings, whilst others develop
wings and utilise them for breathing as well as for flight, the
THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 267
breathing function, and not that of flying, was the first use to which
the earliest insect wings were put.
The first beginnings of wings probably existed as we see the
thin skin-folds of the water-living young (Fig. 179) of some insects
to exist to-day — that is, as primitive organs
adapted for breathing air from water (Fig. 179).
One singular little water larva, that of Chloeon
(Fig. 171), one of the Ephemeridce or "day-flies,"
possesses side expansions for breathing, which are
moved by muscles, as are the wings; and from
what is known of other insect larvae inhabiting
water, it seems highly probable that a pair of these
flat "gills" to each joint of the body (Fig. 179)
may have originally been developed. The next
stage in the evolution of the wing from this side
gill — within which, be it noted, the breathing tubes
branch out — would consist in these " gills " being
employed as agents of aquatic flight — that is, flight
under the water. In time, the hinder gills would
alone be devoted to breathing, whilst those of the
middle of the body, being the most advantageously
placed for locomotion, would become the wings.
Probably the first insect wings were used to propel
their possessors under water. Such a state of
FIG. 180.— POLVNEMA. matters is now seen in Polynema natans (Fig. 180),
a, antenna; which Sir John Lubbock discovered in 1862.
c, rudiments of wines : n->i /-, , j ij
d, rudiments ofie|s. i hereafter, to movements under water would
succeed movements on the surface, and as the
muscular developments progressed, the beginnings of aerial flight
would be simply a matter of time. The late acquirement of wings
in the developing insect of to-day, is thus a fact not without its
due significance. Such an event clearly enough shows us, firstly,
that flight was a power superadded to insect locomotion long after
the evolution of the race from some primitive wingless type ; and
secondly, that wing-power was" evolved through the intermediate
stage of gills still represented in the water-living larvae (Fig. 179)
of our day-flies and their near kith and kin.
As Gegenbaur remarks, the wings correspond in nature with the
gill-processes just described, "for they do not only agree with them
in origin, but also in their connection with the body and structure."
" It is quite clear," the same authority continues, " that we must
suppose that the wings did not arise as such, but were developed
from organs which had another function, such as the tracheal gills ;
I mean to say that such a supposition is necessary, for we cannot
imagine that the wings functioned (or acted) as such in the lower
268 CHAPTERS O.V EVOLUTION.
stages of their development, and that they could have been developed
by having such a function."
That this speculation is a highly probable one is proved by the
curious fact that one insect (Pteronarcys regalis) belonging to the
Orthoptera, inhabiting damp places, retains its gill-bearing organs
throughout life. The mere possibility of the aquatic origin of insects
is therefore placed beyond doubt by such an observation, whilst the
fact that Pteronarcys belongs to the ancient order Orthoptera, shows
its alliance with a primitive type of the insect class.
The consideration of the probable original or type form of the insect
class now demands attention. The tyro in natural history knows that
insects, along with spiders and scorpions, centipedes and crustaceans,
form a great division of the animal world, to which the name of Arthro-
poda (" jointed-legged" animals) is given. The latter group in its turn
forms a division of the great Articulate type, of which group the
possession of a jointed body (seen equally well in the insect's body,
in the centipede's frame, or in the lobster's tail) is a chief characteristic.
Now, the origin of the Arthropoda from some lower and worm-like
stock is not a matter which involves any very great draught upon the
speculative faculty. From some such stock the tribes of spiders,
insects, crustaceans, and centipedes have probably originated. There
exists, indeed, a curious animal known as Peripattts, which is in many
respects entitled to be considered as a primitive Arthropod. From
some such form as Peripatus, it is not improbable that at least the
insects, centipedes, and spiders were evolved. We have discussed in
a previous chapter the nature of the form which has probably through
its evolution and development given origin to the crustacean hosts
and legions. This form is the Nauplius, which, in the development
of highest and lowest crustaceans alike, reappears as the root and
stock of the class, and whose modifications form the puzzles of the
philosophical naturalist of modern times. Now, what is so clear in
the case of the Crustacea is well-nigh as patent in the history of insects.
We certainly do possess in existing groups of insects forms which appear
to fulfil the conditions incidental to the purpose of serving as a gene-
ralised type from which insect evolution may have taken place. Such
groups are those known as the Thysanura, or " tuft-tailed " insects,
and the Collembola of Lubbock, both of which orders may be found
on examination to present us with the natural root stock of higher
insects. A brief inquiry into the characters of these latter insects
may appropriately bring this chapter to a close.
Professor Huxley, in a recent manual of comparative anatomy,
speaks of the cockroach as an " insect without metamorphosis " — a
fact already noticed — and remarks upon the obvious difference which
exists between such a form as a butterfly, with its resting chrysalis, and
the young cockroach, active throughout its whole development. " It is
THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 269
obvious," continues Huxley, "that a metamorphosis in this sense (e.g.
the butterfly or moth) is a secondary complication superinduced upon
the direct and gradual process of development exhibited by such insects
as the cockroach (Fig. 173)." It is also laid down as an axiom of
zoology that insects which, like butterflies, undergo a complete meta-
morphosis (Fig. 1 72) are more differentiated and better specialised —
in a word, are the products of a higher phase of evolution — than
those which undergo no metamorphosis. So also we are duly warned
that insects " which never possess wings are less differentiated or
more embryonic than those which are winged. And, finally, insects
with the parts of the mouth in the condition of ordinary gnathites
(or jaws) are less differentiated than those in which such gnathites
are changed in form and function or become confluent." Now, on
this view of matters, a butterfly is bound to be regarded,' as we
have seen on the grounds of its develop-
ment, as a highly modified insect, far
removed from the primitive type. On
the other hand, "the insects which in
this view of their morphological rela-
tives occupy the lowest position in the
group, are the Collembola and Thy-
sanura." To these we may perhaps add
the true lice and bird-lice (Mallophaga),
because these also undergo no meta-
morphosis and possess no wings.
What, then, are these Collembola
and Thysanura, in whose personnel and
development we may expect to find the
primitive form of the insect type ? The
Thysanura, of which the Lepisma and
Campodea (Fig. 181) are good examples,
are small insects, living in dark situa-
tions, such as amongst damp moss and
under stones. The body is either hairy
or (as in Lepisma) covered with minute
scales, which constitute objects used for
testing the defining powers of micro-
scopes. On the whole, the Thysanura very closely resemble the
young of the cockroach. The tail or abdomen is composed of
some ten segments, and bears paired appendages, from seven to
nine in number. They possess breathing tubes, but, as already
remarked, want wings and exhibit no metamorphosis. The Col-
lembola differ from the preceding group in possessing a tail con-
sisting of six joints only, and a curious tube or sucker, by the
viscid secretion of which they attach themselves to fixed objects.
FIG. 181. — CAMPODEA.
A, larva ; B, perfect insect.
270
CHAPTERS ON EVOLUTION.
Their popular name of " spring-tails," derived from the presence of
appendages formed on a "spring and catch" principle, and by means
of which they are enabled to take leaps of considerable extent,
indicates another peculiarity of the group. Only in one genus of
the Collembola, likewise, are breathing tubes found. The jaws or
" gnathites " in Campodea and the Collembola are not very markedly
developed. As Sir J. Lubbock remarks, the jaws "are far from
strong, but still have some freedom of motion, and can be used for
biting and chewing soft substances." Of these lower insects, the
genus Campodea (Fig. 181, B) is particularly interesting, inasmuch
as it seems to combine in its person all the primitive characters
which give to its neighbours their extreme interest in the eyes of
naturalists. Campodea, which occurs in loose damp earth, has an
elongated cylindrical body, long and many-jointed antennae, with
paired appendages on the first seven joints of its tail, and
long tail-appendages likewise. Now, if we compare the
young or larva of Campodea (Fig. 181, A) with the adult
(B), we find little or no difference save in size. Its whole
organisation reminds us forcibly of the young stage in such
insects as the cockroaches and other Orthoptera (Figs. 173,
176); whilst there are larva? in other groups of insects
to which Campodea and its neighbours bear a close re-
semblance. Furthermore, the larva (Fig. 171, A) of the
day-fly (Chloeon), which possesses the gill appendages
already referred to, is exceedingly like this Campodea,
whose mouth-parts appear equally capable of further deve-
lopment to form the jaws "of the beetle, or of modification
to become the suctorial apparatus of the butterfly.
Thus, on all the grounds on which it is possible or
necessary to look for resemblances between Campodea
and the young of higher insects, such likenesses are
discoverable. And the conclusion is thus rendered highly
probable that existing insects have been evolved from an
ancient Campodea-like stock — that is, from an animal form
with a jointed body, three pairs of legs, weak mouth-parts,
one pair of feelers, and a tail provided with jointed ap-
*LINDIA'.' pendages. Hence a mental forecast is prepared to see
this Campodea-form developing in one direction, through
an insect like the young Chloeon (Fig. 171, A), or the water- larvae
(Fig. 179) already described, with their side "gills," into winged and
higher races. Or in another direction, and through less modification
perhaps, we may in our " mind's eye " behold Campodea growing
in time into the stock whence the Orthoptera — our existing crickets
(Fig. 176), grasshoppers (Fig. 1 75), and locusts — themselves a primitive
group of insects, have sprung. Backwards, on the other hand,
THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS. 271
in the scale, retrograding from Campodea, we may even conceive of
the stock from which that insect itself has sprung. Campodea
within the egg must pass through the stages common to all animals
at a like stage of development. There is a stage in the Arthropod
type when the young of the insect or crustacean is little else than a
footless, imperfectly developed worm. There is even a worm-like
larva of an insect allied to the gnats, which corresponds to such a
description ; and such low insect-larvae become in turn obviously
related in form to certain low creatures allied to the worm kith
and kin. One of these low forms (Lindia) is depicted in Fig. 182.
This legless organism is related to the well-known " Bear-animalcules,"
and Rotifers, or Wheel-animalcules. Its jaws resemble those of the
larval flies ; it has a ringed body, and in other respects exhibits a
close likeness to the young of many insects. Possibly, therefore,
in some such primitive root, common to a whole host of animals,
we may find the dim, ill-defined starting-point whence, led towards
Peripatus, and by Campodea, the insect tribes have grown into the
brilliance and aerial grace which mark their ranks to-day.
It may not be unprofitable, at the close of our investigation
into insect history, to remind ourselves of the great problem which
their development has lent its aid in part to solve. At the risk of
apparently unnecessary repetition, let us keep in view that every
such history, however its individual terms are to be accounted
for, forms a link of greater or less importance in demonstrating
the great law of evolution, modification, and adaptation as the
true method whereby Nature has wrought out the endless variety of
the children of life. Especially useful and important, moreover,
is the history of the insect as illustrating the changes which the
adaptation and modification of the young form may effect in the his-
tory of a species. So far from the chrysalis or pupa being a stage in
the ancestry of the insects, we have seen that it represents merely a
secondary and acquired phase of their development. As Fritz
Miiller has succinctly formulated it, "the historical record pre-
served in developmental history is gradually EFFACED as the
development strikes into a constantly straighter course from the
egg to the perfect animal, and it is frequently SOPHISTICATED by the
struggle for existence which the free-living larvae have to undergo."
These words sum up the reason why insects in their metamorphoses
exhibit all gradations and shades, from mere moulting of skin to
complete change of form through a chrysalis state. Primarily they
undergo a metamorphosis because they happen to leave the egg at a
relatively early period of development ; but they share " metamor-
phosis " — using the word in the broad sense — with every other living
being. It is this plainly discerned series of changes which has
chiefly given to the study of entomology its fascination in the past.
272 CHAPTERS ON EVOLUTION.
One may, however, well be regarded as enunciating a veritable truth
when it is stated that the new light which evolution throws on the
" why " and " how " a butterfly develops, and a Campodea remains
inert, is likely to invest insects, and indeed all other forms of life,
with an interest far surpassing that which past years could have
imagined or conceived.
273
XIII.
THE EVIDENCE FROM THE CONSTITUTION OF
COLONIAL OR COMPOUND ANIMALS.
AMONGST the many aspects in which the biologist is accustomed
to View the universe of life, few possess a greater interest than that
which deals with the nature of animal and plant personality, and
with the structural philosophy of the living frame. It is not suffi-
cient for the due investigation of living structures that the forms of
animals and plants be compared, and their more obvious differences
and peculiarities noted and recorded in scientific annals. Such details
and such procedure suffice perfectly for the ordinary run and course
of biological work, and form, no doubt, the source of the every-day
knowledge on which natural-history science grows and progresses.
But a higher era of scientific thought intervenes when philosophy,
in its search after relationships and causes, steps forward to correlate
and utilise the knowledge observation has acquired. The higher
questions of cause and origin are not solved by observation alone.
It requires and demands the power of placing facts in appropriate
light and shade ere the mutual relations of these facts can be deter-
mined, and before their place in the systema natura can be definitely
ascertained. Judged by this criterion and standard, there are some
topics of biology which altogether belong to the region of the abstract
and the transcendental. Patient industry may discover, for instance,
that a crayfish within the egg repeats, as a stage in its development,
the likeness of a form represented to-day by the adult state of some
lower crustacean ; but it requires philosophy of a transcendental
kind to see what that fact means, and what such a discovery implies
to the universe of life around. One may perfectly appreciate by
ordinary observation that a horse walks on the single toe of each
foot, and that its two " splint-bones " represent useless rudiments of
other two toes; but it is through an exercise of abstract science
alone that we can form the concept of a single-toed horse having
arisen from a three-toed one ; and from the latter phase of develop-
ment extend a like thought to that of other living beings. The
applications of philosophy to the facts of nature remind one strongly
of the most singular and mysterious work of that nature in the pro-
duction of the living thing itself. In the performance of that function,
we require a certain quantity of the substance called " formative
T
274 CHAPTERS ON EVOLUTION.
material " by the learned in biology, and " protoplasm " by the
simple-minded amongst us. This material contains all that is
required for the formation of the living frame in so far as the material
of that frame is concerned. But in protoplasm alone, we do not
find all that is demanded for the growth of the new being. We
require, likewise, activity of some kind — potential or real, chemical,
physical, or vital, or all three combined ; and we depend upon this
activity for the combination of the elements of our germ and for the
power whereby that germ will in time blossom out into full fruition.
So is it, in truth, with the application of knowledge, and with the
evolution of the wisdom which arranges our knowledge in its due
array. The knowledge we gain is, after all, in itself pure material,
on which the potential power of philosophy must exert its influence
ere the results of seeking and finding wisdom be fully appreciated.
The evolution of a natural fact, or set of facts, to take its place in the
array of knowledge we name a science, is therefore matter of higher
development than that which merely discovers the facts themselves.
Only when philosophy has touched the inert mass of detail, does the
harmonious and arranged system spring into view with its power of
truly adding to man's knowledge of the universe around and overhead.
Only when the search for causation has begun, can our intellectual
gains be fully appreciated in our labour of
Untwisting all the chains that tie
The hidden soul of harmony.
Such a topic as presents itself to view in the individuality of
animals belongs, it may be with all truth affirmed, to the domain of
the philosophy which applies knowledge, rather than to the sphere of
mere fact and observation itself. This declaration might sufficiently
prejudice the subject in the eyes of readers who might be given to view
with suspicion any opinion which apparently lowered fact in the scale
of credence. But the philosophy we eulogise, bases its existence on
the facts we value. It is the mint-stamp of knowledge, which im-
presses fact with its popular and received value ; inasmuch as, with-
out such impress, the fact itself, however valuable, fails to relate
itself to its neighbour truths. Hence, if, in the present chapter, we
may venture somewhat within the domain of transcendentalism, it
may readily be shown that from the sober basis of facts all our
philosophy in reality takes origin. By way of at once illustrating
this latter proposition, as well as of laying the foundation-stone
of our present study, we may enter upon a recital of the facts of
individuality as represented in the living series around us.
A superficial acquaintance with the facts of natural history
serves to demonstrate the truth of the axiom that every animal
originates, directly or indirectly, from that reproductive body we term
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 275
an " ovum " or " egg." As the result of the development of that
egg, the animal body becomes the adult ; and of the plant the same
truth holds good. The seed or germ undergoing development, and
passing through stages which are, as a rule, of well-defined nature, at
last appears before us as the perfect plant, which, in its turn, will
produce blossom and fruit, and will finally lead us back once more
to the seed and germ. One marked and very obvious difference
between high animals and low animals is found to exist in the dif-
ferent results to which development leads. The lower animal's
growth ceases, and its adult condition is attained, at a stage when
the development of the higher being has barely begun. It takes but
little trouble on nature's part, so to
speak, to convert the matter of life of
a low animal or plant into a form
like itself. On the other hand, the
development of a higher animal means
time and trouble, to use a familiar
expression, and entails the elaboration
and building of a complex body from
that which is invariably in its first
stages uniform and simple in structure.
Such an animal form as a Gregarina
(Fig. 183, d), for instance, presents us
with a good example of that simplicity
of development and that primitiveness
of personality which marks the lower
fields of animal life. A gregarina is a
minute speck of protoplasm found in-
habiting the digestive canal of worms,
insects, and crustaceans, as an internal
parasite. Each gregarina lives what
may be described as the simplest form of existence. Existing in
the digestive system of its host, it literally lies bathed amidst the
nutriment which that host is elaborating for the repair of its own
tissues. Possessing no traces of any of the organs belonging to
higher animal existence, the gregarina lives by the absorption of
the digested fluids of its host ; and save for the slow contractions
which are sometimes seen to pass in waves along the surface of its
body, no movements can be observed whereby its animality might
be popularly confirmed. The course of gregarina-development is by
no means complex. The body itself, in lieu of an egg or germ, will,
sooner or later, become of globular shape (£). The little solid body,
or " nucleus," seen normally (d) in its interior, will vanish by a kind
of physiological necromancy, and the body-substance itself will break
up and divide into spindle-shaped masses (a), for which the thickened
T 2
FIG. 183.— GREGARINA AND ITS
DEVELOPMENT.
276
CHAPTERS ON EVOLUTION.
rim or margin of the body forms a covering. Then this globular
body-margin ruptures ; the little spindles of protoplasm escape there-
from ; and finally each develops, with but little further change, into
a gregarina like that from which it was derived.
Now, such a life-history as this is instructive, especially when
viewed from the stand-point of animal individuality. The single
gregarina is seen to break up into numerous other gregarinae, each
of which repeats at first the single state, and then the process of
division into particles
which characterised its
parent Each gregarina,
then, may, in natural-his-
tory language, be named a
"persona," or "person" —
that is to say, it is a single
or " individual " animal ;
representing in itself, even
as does each of the higher
animals, a defined and
component element of the
animal world. A like re-
mark might be made of
many other lower forms
of animal life. An Amoeba
(Fig. 184), which diners
from a gregarina chiefly in
that it possesses an active
power of locomotion by
pushing out its body sub-
stance into long processes
(Fig. 184), is likewise a
single " individual " animal, which represents, as an oyster or a bird
does, a well-defined unit quantity in the sum total of the living series.
There is, however, one important epoch in the life of both gregarina
and amoeba, when each organism — for both exhibit essentially the same
course of development — shows a tendency to lose ips individuality in
the division of its body to form other individuals. At one stage in its
development, namely, when filled with the miniature " spindles " or
protoplasmic particles (Fig. 183,^) into which it has divided itself, the
gregarinaor amoeba in reality becomes a colony or aggregation of beings.
But such a tendency is at the most transitory, and the temporary colony
speedily resolves itself into a diffused and separated mass of young
organisms, whose individuality, and indeed whose whole existence,
is due to the destruction of those of their parent. In another sense,
the amoeba may occasionally show this tendency to lose its single
FIG. 184.— DIFFERENT FORMS OF AMOZB/S.
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 277
and defined individuality in that of the compound colony. For
occasionally particles or offshoots of the amoeba's protoplasmic body
detach themselves therefrom, and pass away like precocious emigrants
from the parent-frame to assume all the functions of amoebae on their
own account. In this way, and through the exercise of the simplest
reproductive process we know of — namely, that of " fission," or simple
division of an animal's body into two or more new beings — the
amoeba-body converts itself from a single " individual " into a mother-
colony, with offshoots and emigrants seeking a life and existence of
their own. And, last of all, in the gregarina itself, we may find
certain important variations in structure which seem to threaten the
destruction of the individualism of its body, and to merge the
individual in the crowd. For we know not merely of gregarinae
which consist apparently of but one mass of protoplasm, as already
described, but of others which
exhibit a division of body into
two (Fig. 183, d) or even three
compartments. What the signi-
ficance of this tendericy to divi-
sion or segregation may be, is yet
matter of conjecture ; but at first
sight its meaning would seem to
foreshadow the same destruction
of individual constitution which,
in their development, these or-
ganisms unquestionably exhibit.
Even in the lowest animals,
each consisting of a minute mass
of protoplasm, there is thus ob-
served a tendency, at some period
or other of their life-history, to
depart from the single state, and
by division, or, as it is named,
44 segregation," of their substance,
to form a "colonial" or com-
pound organisation. But even
in the lower confines of animal
life, which harbour the amcebse
and gregarinse as typical tenants,
are represented states and phases of organisation which are purely and
typically "colonial." Thus, that low form of life known as Myxo-
dictyum normally exists as a collection of protoplasmic particles, such
as would be exactly imitated if a number of amoebae banded and
fused themselves together. It is equally interesting to note that the
vast majority of the Foraminifera (Fig. 185), or " chalk- animalcules,"
FIG. 185. — FORAMINIFERA.
278
CHAPTERS ON EVOLUTION.
are to be regarded as exhibiting a compound constitution. For, in
these animalcules, which are as a rule of compound nature (Fig. 185,
b], the growth of new divisions of the shell takes place by a process
of budding, and through the production of new protoplasmic units
which remain organically connected with the original mass. Nor
are the lowest plants to be left out of consideration in this recital
of primitive colony-making. The cryptogamic botanist well knows
certain green specks of microscopic "size, each called Chlamydomonas t
which swim freely in fresh waters, by means of two long cilia, or
miniature eye-lashes, projecting from one extremity of the body.
Now, there exist in stagnant waters certain other curious bodies,
long known as "Globe-animalcules," before they were ascertained
to be lower plants. Each of these bodies is scientifically named a
Volvox (Fig. 1 86, ff), and appears to consist of a hollow globe or
sphere, covered with innumerable little specks of bright green, and
FIG. 186.— VOLVOX (J) AND VARIOUS ANIMALCULES.
swimming freely through the water by the waving action of the fine
cilia which fringe its body. More minutely examined, this rolling
globe is found to consist of a collection of little green bodies,
each of which, in all essential details, exactly resembles a single
Chlamydomonas. The filaments fringing the volvox are in reality
pairs of cilia like those of Chlamydomonas, and are attached to
the little green bodies aforesaid. Thus volvox, so far from being
an animal, is, firstly, a rootless lower plant; and, secondly, so far
from being one plant, volvox is in reality a colony of the lowest
members of the vegetable world. There are many other Algce (or
lowest plants) which resemble volvox in their compound nature ; and
thus the beginnings of plant-life appear to present us with a ten-
dency towards colonisation, similar to that which faces us on the
threshold of the other series of living beings.
In the curious group of the sponges (Fig. 187), we may find our next
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 279
convenient halting-place in our researches into animal individuality
and its variations. From forming the bete noire of the naturalist of
former years, who was troubled in his mind as to the animal or
plant nature of the sponges, to occupying a singular and anomalous
position in the animal classifications of to-day, this group of organ-
isms has attained a well-merited celebrity. The living parts of a
sponge — that is to say, the parts which form and make the sponge-
framework, and which alone concern us in our present investigation
— consist of masses of protoplasm, which are in their way strictly
comparable to the minute bodies, or " cells," of which our own
tissues are built up. A sponge, as to its living parts, is a mass of
protoplasmic cells, " some of which," as Huxley puts it, " have all
FIG. 187. — SPONGE AND ITS DEVELOPMENT.
the characters of Amoeba ; while others are no less similar to monads "
— these latter being microscopic masses of protoplasm, furnished,
like chlamydomonas, with two waving cilia. The comparison of a
sponge to a kind of " submarine Venice," with its canals, along the
banks of which the inhabitants (or masses of protoplasm) reside, and
through which flow the water-currents bringing particles of nourish-
ment to these denizens, is therefore seen to be fully justifiable in one
sense. Still more justifiable and appropriate would such a metaphor
be, could we prove that the sponge was in reality what the simile
indicates, namely, a colony of animals — seeing that the comparison
of the sponge to the Adriatic capital, derives its whole force from the
280
CHAPTERS ON EVOLUTION.
assumption that its personality, like that of the city, is compound,
and is not simple and single, like that of the element or unit. As we
shall hereafter see more plainly, the sponge must be judged, like
every other living being, not by its appearance or by what it simu-
lates, but by what it originated from. As an apparent collection of
organisms, it might well be regarded as a veritable colony. On
other grounds, the sponge might appear as rightfully entitled to be
considered as single and undivided an animal unit as a man. The
grounds on which these opinions are based need not now be speci-
fied, but the history of how a sponge grows, finds its appropriate
place at this stage of our inquiry. The most typical sponges, as"
already shown, grow each from an egg (Fig. 187, i), which passes
through characteristic stages of development (2, 3, 4), and finally
becomes a cup-shaped body (5), possessing a double wall (<:, ), the
cavity of the cup opening outwardly by a distinct mouth (m). Then
pores or openings (7, /) are formed in the wall of this cup, placing
FIG. 188. — JJfDRJE.
(Young hydrae are represented budding from the parent in each figure.)
its interior in a new fashion in communication with the outside world
The outer wall of the cup, and the inner wall likewise, consist of cells ;
and those of the inner wall finally come to possess cilia, which, by
their constant motion, cause currents of water to flow into the inside
of the cup through the pores, and outwards by its mouth.
The nourishment of a sponge is subserved by these water
currents, bringing food and oxygen to its living cells; and the
simple or cup-shaped sponges (Fig. 187, 7), of which many species
are known, exhibit a history resembling that of which the outlines
have just been sketched. The horny sponges, the skeletons of cer-
tain species of which we use in our domiciles, may and do develop
into organisms of a more complex character than the cup-sponges
present, and they may also originate from buds as well as from
eggs. The common green fresh-water Spongilla, found growing on
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 281
the sides of canal locks and in similar situations, illustrates the former
method of development. This species propagates its kind by verit-
able buds, whilst it also produces eggs ; and another curious fact,
possessing a significant bearing on the individuality of the sponges,
consists in the observation that when two Spongillse are placed in
contact they merge together into one. They may also be divided
artificially, or may separate spontaneously into two or more organisms,
each of which will lead an independent existence. The sponges,
then, may be hereafter referred to as a group of animals which,
whilst originating from eggs, as do higher beings, yet retain much
FIG. i8g. — ZOOPHYTES. 6 and d, magnified portions of a and c respectively.
of that tendency to segregation and separation into distinct and
elementary parts which we may reasonably maintain is probably a
primitive and fundamental character of all living beings.
Nearly allied to the sponges are the little freshwater polypes named
Hydrtz (Fig. 188), and the marine plant-like organisms familiarly
known as "Zoophytes" (Fig. 189). Here we at once enter the
domain of animal "colonies," and find intensified and illustrated in
the plainest fashion those tendencies towards division and segregation
of body which, at the best, are but dimly marked in lower organisms.
The hydra, existing as a little green tubular body — attached by one
extremity to a water weed, and exhibiting at the free end a mouth
and tentacles — at certain seasons exhibits a growth of small projec-
tions on its sides. As these projections increase in size, they grow into
the likeness of young hydras (Fig. 188), each developing a mouth and
tentacles, and possessing, so long as they adhere to the parent body,
free communication with the interior of the latter. These budded
hydrse may in their turn produce buds, and a veritable genealogical
282 CHAPTERS ON EVOLUTION.
tree may thus be viewed, in that three generations of polypes remain
connected as they were produced by the parent stem. The hydra
thus converts itself normally into a compound colony through this
process of budding. But this state of matters is, at the most, transi-
tory and temporary in hydra-existence. The budded individuals,,
sooner or later, break contact with the parent-body, and pass to seek
a new lodgment and to begin life on their own account ; leaving the
parent, single as before, but connected, as we shall presently note,,
to the free offspring by ties which our transcendental philosophy
makes clear and plain. It may lastly be remarked that, in respect of
structural constitution, the closest similarity exists between a cup-
sponge and a hydra. Both possess tubular bodies, and both con-
sist of two cellular layers. Modern zoology has emphasised this
likeness by placing the sponges in the same great group ( Calenterata)
which contains the hydra and zoophytes. It is conceivable enough,
indeed, that a hydra is simply a specialised sponge-form possessing
its compound and colonial nature somewhat disguised beneath an
apparently single personality.
The constitution of a " zoophyte" (Fig. 189) is mere matter of
repetition after the recital of the hydra's peculiarities. The plant-
like Sertularian or "sea-fir" (Fig. 189, a), which we dredge by the
hundred, growing on oyster- shells ; or the Flustra (Fig. 190) or
" sea-mat " — of higher organisation than the " sea-firs," but pre-
senting likewise the aspect of a marine plant — present us each
with a veritable colony of more or less similar beings, united
in the bonds of close relationship. Thus the sea-fir, as the type
of the true zoophyte, bears on its branches hundreds of little
cups (Fig. 189, b, d), each of which contains an animal strictly corre-
sponding in structure to a hydra (Fig. 188). This multitude of animal
bodies is bound together in intimate union. The stem and branches
are hollow, and each little mouth and body, digesting the food its
tentacles have captured, transmits that food to swell the general
stream of nutriment circulating through the tree-like fabric. Thus
we find the principle of co-operation herein illustrated in plainest
guise. Each little animal derives its own share of nutriment from
the general store it has helped to manufacture ; and the exercise of
the principle in question is all the more perfect, in that its practice is
free from those petty jealousies and personal inducements to infringe
the duty of equal and harmonious work which usually beset the
co-operative societies of higher existence. The remaining points
which call for notice in the history of the zoophyte may be shortly
summed up. The little members of the colony are continually dying
off as the result of their life-work, but their place is supplied and the
colonial loss repaired by the production of new buds. As leaves
fall from a tree and are replaced by the growth of new buds, so the
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 283
zoophyte-units wither, fall, and in like fashion are represented anew
in the constitution of the organism. Then, lastly, the origin of the
zoophyte in an egg is worthy of note. Each zoophyte originally
springs from an egg, which, passing through the changes common to
the early development of all ova, produces an embryo which settles
down and attaches itself to some fixed object.
This first embryo next assumes the likeness of a single little
hydra-like unit of the zoophyte colony. Then the process of bud-
ding commences. Bud after bud is produced, each growing into the
likeness of the primary one, and all adhering together as parts of a
connected organism, until we find reproduced before our eyes the
tree-like form with which our research began. Thus a hydra and a
zoophyte are very nearly allied ; the chief difference between these
organisms consisting in the fact that, whilst the buds remain perma-
nently connected together in the latter, they are intended to seek an
independent existence in the former. True, there are buds produced
by the zoophyte which in many cases detach themselves and swim
freely in the sea under the guise of " jelly-fishes," and which, apart
from the zoophyte, mature the eggs from which new generations of
these plant-like animals will spring. But these floating jelly-fishes,
despite their freedom, are in reality buds of the zoophyte. They
are connected by all the ties of blood-relationship with their plant-
like parent, and are essential parts of the zoophyte-colony even
when separated from the parent-organisms by many leagues of sea.
In all zoophytes the component units of the colony belong to one
type. Whatever their function, they are modelled on the type of the
hydra, and on that of the ordinary nutritive members of these animal
trees. Even the jelly-fish buds just mentioned, are but modifications
of the hydra type. This interesting and important feature in their
history is proved by the fact that, when their generative functions
have been discharged, they may revert to the form of the nutritive
members of the colony. We know, lastly, of cases in which a
zoophyte-colony may number no fewer than seven apparently dif-
ferent kinds of members; these units, notwithstanding the diverse
functions they perform, exhibiting a fundamental agreement in type
and structure. There is seen, therefore, a close parallelism between
the repetition and modification of parts in the colonial zoophytes^
and the vegetative repetition of the leaves and buds of the tree.
The Flustras, or "Sea-mats " (Fig. 190), illustrate a slightly different
phase of colonial relationship in animals from that presented by the
zoophytes. We have seen that each member of the zoophyte-colony
exists in intimate structural relationship and connection with every
other unit of the compound organism. But in the " Sea-mats " — each
of which presents us with the appearance of a piece of pale brown
seaweed, bearing on either side its hundreds of little cells (Fig. 190, b\
284
CHAPTERS ON EVOLUTION.
each containing a little tenant — the individual animals of the colony
do not communicate with each other. On the contrary, each
member of the sea-mat colony is perfectly distinct from all its neigh-
bours, and lives enclosed in its separate domicile. But for the union
of its cell-wall with the walls of other cells, each little sea-mat unit
is a thoroughly independent being ; and even the so-called " colonial
nervous system," which was long believed to connect the members
of the fraternity in a common bond of sensitiveness, has been proved
to be non-existent. It is highly interesting, therefore, to find that
compound animals may, like the zoophytes, possess their individual
or component units in close structural harmony and relationship ; or
FIG. 190. — F LUSTRA, OR SEA-MAT.
a, natural tize ; b, magnified, showing the cells.
may, on the other hand, like the sea-mats, exhibit a collection ot
animals each of which is thoroughly independent of its neighbours.
That such differences have not originated in any haphazard fashion,
but that they are a veritable result of the tendencies of development,
is readily enough proved. For, whilst each member of the zoophyte-
stock is in free and full nutritive co-operation with its neighbours, each
" sea-mat " unit preserves within its own cell, not merely a perfect
digestive apparatus, but a nervous system, and reproductive or egg-
producing organs as well. The independence of the " sea-mat's "
members has been accompanied by the development of a much
higher organisation than is found in the interdependent zoophyte-
units ; although, of course, such a statement of fact still leaves the
origin and cause of the independence of the higher " sea-mat " units
an open question. But in its manner of growth, the latter colony
resembles the zoophyte. Each unit has the power of adding to the
colony by the process of budding already described ; whilst each
member of the colony possesses, likewise, the power of giving origin
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 285
to eggs. Each egg, undergoing its full development, produces first
one primitive unit, and thereafter and from this unit develops, by
budding, a whole colony, with its hundreds of component and similar
beings.
There exist in the ranks of that curious class of beings, the in-
ternal parasites, certain interesting examples of the compound animal
form. A tapeworm (Fig. 191), for instance, inhabiting the digestive
tract of some warm-blooded quadrupeds, and attaining a length, it
may be, of many yards, consists of a very minute head (Fig. 191, i),
a slender neck, and many hundreds of so-called "joints." At first
sight, these "joints" might be re-
garded as resembling in their nature
those of the ordinary worms, and
as therefore possessing no distinct
individuality on their own account,
or separate ^rom that of the organism
of which they form part. But the
examination of the joint of a tape-
worm (Fig. 191, 2) shows us that in
reality it preserves a separate and
apparent individuality of its own. In
other words, it is not merely a part of
one animal in the sense that the joint
of a backbone is part of a fish or bird.
It corresponds, on the contrary, with
a member of the zoophyte or " sea-
mat" colony in that it represents a
highly specialised and individualised
unit of an organism, that organism
being of compound nature. Each
"joint" of the tapeworm contains a
complete set of egg-producing organs (0), and presents other indica-
tions of its semi-independent character and constitution. Connected
to its neighbouring joints by water-vessels as well as by the nerve-
cords, the joint is in intimate union with the other units of the colony.
But it is, nevertheless, a distinct unit after all ; and the tapeworm is
not a single animal, but, like the sea-mat and zoophyte, a " colonial
organism."
Amongst other and true "worms," however, we find curious
instances of development, which, in our consideration of the
origin of the conditions we are studying, may serve to elicit some
valuable hints concerning the causation of colonies at large. The
little river-worms known as the Nai'dides (Fig. 192), occasionally ex-
emplify certain peculiar modes of reproduction which deserve careful
study. A nais may be seen to exhibit a slight constriction towards the
FIG. 191. — TAPEWORM.
Head, suckers, neck, and joints ;
2. A single joint (magnified).
286
CHAPTERS ON EVOLUTION.
posterior part of its body. As this constriction deepens, a new head,
eyes, and tentacles are seen to be formed at the spot in question
(Fig. 192), and a second nai's is thus viewed
budding from the hinder extremity of the ori-
ginal individual. This new being, produced
thus by the division of the parent body,
sooner or later becomes detached therefrom,
and seeks an independent existence. Cases
have been observed in which as many as six
new individuals have been produced from a
single nai's. In Cirrhatula> another worm-
genus, Miiller relates that he found three new
individuals adherent " in one length." " The
mother," he remarks,
" had thirty pedate
segments; the young-
est daughter, or that
nearest the mother,
had eleven, but the
head was not yet de-
veloped. The most
remote had seventeen
rings, with both head
and eyes, and, more-
over, the tail of the
mother ; the middle
one had seventeen
segments and a head."
It is matter for re-
mark that' no egg-
producing organs exist in the new individuals
thus budded, which may therefore be named
"neuters," like the "workers" among the
bees. The last-formed individual, however,
in the nai's, develops reproductive organs,
and thus the continuance of the species in
time is duly provided for.
In connection with the production of like FIG. 193.— JOINTS OF LOBSTER.
parts by budding — a process known as that
of "vegetative repetition" of parts, and producing what is known
as the " serial homology " of animals — it is interesting to note that
the twenty joints or so of which an anima like the lobster (Fig.
193) is composed, are constructed, irrespective of size or function,
upon one and the same type. The same remark holds good of an
insect, of a centipede, of a spider, or other articulated animal. Very
FIG. 192. NAJfs, OR FRESH-
WATER WORM.
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 287
striking is it to find that a lobster's " feelers " really correspond in
nature with its legs ; that its eye-stalks agree with part of the append-
ages of its tail-joints, and that its jaws are simply the feet of the
head, so to speak, modified for chewing. These varied organs arise
from a common type, just as the joints which bear them exhibit a
singular uniformity of structure. Hence a lobster, or other Articulate
animal, gains the best possible title to be named colonial, in that it
is not merely composed of visible "units," but also in that these
units are modifications of a common and single plan. In connection
with the curious phases of worm-growth observed in the Nai'dides
(Fig. 192), we may note that the individuals of the centipede-class
increase in size and add new segments to their bodies in a somewhat
FIG. 194. — DEVELOPMENT OF JULUS.
similar fashion. When a young centipede or gally-worm (Julus)
(Fig. 194) is attaining its full growth, new joints are seen to bud out
between the last segment but one (C,/; D, n s) and the joints in
front thereof; so that the last-formed joints (E, 9-14) in a young
centipede are placed towards its tail-extremity. If we could imagine
that some of these last-formed segments developed a head, and
separated themselves from the parent-frame as a new being, we
should possess an exact imitation of the process whereby the young
Nai's (Fig. 192) originates from its parent-form.
An interesting biological speculation has arisen in connection
with the personality of those familiar animals the Starfishes (Fig. 195).
Here we find a central body or disc (Fig. 195, i), with, in the common
species, five rays or arms, containing each an exactly similar arrange-
ment of the organs of the body, diverging therefrom. Haeckel's
288
CHAPTERS ON EVOLUTION.
ingenious speculation that " each arm of the starfish essentially cor-
responds in its organisation with an articulated worm," is objected to
by some naturalists, and amongst others by Huxley, who agrees that
the starfish, or echinus, may have arisen from a worm-stock, but
argues that both the starfish and sea-urchins owe to secondary modi-
fication their characteristic form. Haeckel, however, is supported by
authority so eminent as Gegenbaur, who remarks, that "there is a
certain amount of indepen-
dent organisation in each
arm of a starfish ; its organs
. . . have exactly the same
position as the homologous
organs of an Annulate worm.
If, then, we compare each
of the budding arms with
a worm -like organism, we
must regard the starfish de-
veloped by this process of
gemmation as correspond-
ing to a multiple of such
organisms ; and, further,
we must recognise in this
phenomenon the same pro-
k2 cess of gemmation (or bud-
ding) as that which takes
place in other lower ani-
mals ; for example, in the
compound ascidians (or
sea-squirts) It is a process," says Gegenbaur, "in which several
separate animals are simultaneously budded off; the process does
not go on till these animals are completely separated, but stops in
such a way as to keep them connected together as an individual
of a higher order." We know, as just remarked, of allied cases
amongst the sea-squirts, where several beings are budded in star-
shaped fashion (Botryllus) to form a colony. And when we
reflect that, as every sea-beach shows, a starfish may be deprived
of all its arms, and as one arm (Fig. 195, 3) may not merely live an
independent existence, but will in time reproduce the other four,
Haeckel's idea that a star-fish is really a collection of worm-like
beings, is seen to be so far supported by comparative anatomy and
by the analogies of development as well
The list of animal classes in which a colonial constitution is
developed may appropriately enough be concluded with the brief
recital of the process whereby the Aphides, or plant-lice (Fig. 196),
which devastate our plants, and the bees themselves, propagate their
FIG. 195. — STARFISHES.
FIG. 196. — PLANT-LICE.
a, wingless insect ; b, wingless insect.
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 289
race — the latter forming social colonies which in their essential
nature may be deemed analogous to the zoophyte-stocks of lower
life. The single and undivided personality of a bee or an aphis
would at first sight seem to
admit of no question. Each
presents itself to view as an
active being, possessing no
structural connections with
neighbour - organisms, and
evincing all the apparent
marks and characters of an
ordinary "individual." But
our philosophy relies, as
already remarked, more on
the determination of what
an organism has arisen from,
than upon what its apparent
constitution may be. Hence
the consideration of a bee's origin contains the answer to the question
of its true nature. In the reproduction of the bee race, certain of the
eggs are impregnated or fertilised, whilst others are allowed to develop
without the performance of this process — rightly deemed of essential
nature to the propagation of both animals and plants. Now, those
eggs of a queen-bee which she lays in an unfertilised condition,
invariably develope into drones, or male bees, whilst the fertilised
eggs become females, or queens, or neuters — the latter being merely
imperfect females, on whom devolves the whole work of the hive.
In the plant-lice, the eggs normally produced by both sexes in the
autumn lie dormant all the winter, and then give rise to wingless
female aphides alone. These latter produce, in viviparous fashion, a
winged or wingless progeny, which in turn repeat the fertility of their
parents. As Huxley remarks : " The number of successive viviparous
broods thus produced has no certain limit, but, so far as our present
knowledge goes, is controlled only by temperature and the supply
of food. Aphides kept in a warm room, and well supplied with
nourishment, have continued to propagate viviparously for four
years."
Now, close research has disclosed other cases of this apparent
violation of the ordinary rules of reproduction in the animal world.
We know that in certain saw-flies, some of the female insects will, of
themselves, lay unfertilised eggs, which develop into male saw-flies.
In some insects (Chermes; Coccus] no males have been discovered.
There are also certain caterpillar-like females among the butterflies
and moths (e.g. Psyche and Solenobia) which lay unfertilised eggs giving
origin to female insects like themselves, whilst from fertilised eggs
290
CHAPTERS ON EVOLUTION.
the two sexes are developed in nearly equal numbers. It may be
conceded that in the case of the bees, as insects of specialised type,
we are dealing with insects in which true unfertilised eggs develop
simply into drone or neuter-insects. But in the lower plant-lice,
the process is more nearly related to the budding of the zoophyte.
Each aphis, produced viviparously from the parent-body, grows from
FIG. 197. — COMPARISON OF DEVELOPMENT in (i) a Flowering Plant ; (2) a Zoophyte ; and (3)
a Colony of Plant-Lice {Aphides).
a structure which, whilst it resembles a true egg, does not pass
through the development of that body, and is therefore called a
pseud-ovum. Gradually this "pseud-ovum" grows into the likeness
of the aphis, which, after birth, will develop within itself like
bodies, and thus carry on the work of continuing the species in time.
If we suppose that the aphides remained connected together (Fig.
197, 3), instead of preserving a distinct structural identity, we should
reproduce in this insect-tribe an exact facsimile of the zoophyte-
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 291
colony (Fig. 197, 2), with its budded branches () and its ever-
increasing wealth of members. For plant-lice reproduction is in
reality a process of budding, and the colonial constitution of the
insects is really veiled and masked by their freedom from the parent-
stock. They may, in truth, be compared to those free-swimming
" jelly-fish " buds which the zoophyte develops upon and liberates
from its branches, but which remain, nevertheless, in the gaze of
philosophy, essential parts and constituent units of the animal-tree
which gave them birth. Lastly, let us bear in mind that the egg itself
is merely a reproductive bud; and that there are gradations thus to be
witnessed leading from the true egg, with its normal development,
after fertilisation, to the pseud-ovum with its bud-like career, and
finally to the bud itself, which, as we shall see, never attains, let its
development be what it may, to the rank of a true individual animal.
A glance at Fig. 197 will serve to show the correspondence between
the development of aphides (3), zoophyte (2), and plant (i). In each
case, the bulk of the compound organism is- provided for by a process
of "budding;" whilst, as the colony reaches its higher development,
the production of new and independent individuals, through eggs and
seeds (//, i) respectively, is witnessed.
Rassemblons des faits pour nous donner des idees — taking the
term "ideas" as synonymous with that philosophy the praises of
which have already been sufficiently extolled. From the array of
facts through which we have progressed, what ideas or interences
concerning the origin of animal colonies can be reasonably derived ?
And, firstly, let us inquire what definition biology is prepared to offer
as the criterion of animal or plant individuality. It is perfectly clear
that some such test of an animal's nature is demanded, for instance,
by the very diversity of form and constitution which the animal
kingdom presents. An " individual " animal we may readily define, in
respect of its structural constitution, as one in which all its parts and
organs exist. in such intimate relationship, that interference with one
organ or series means the disorganisation of all. Close and inti-
mately connected structure, forms in reality the plainest criterion of
the " individual " animal viewed from that side of biology which
regards morphology or " structure " as the basis of its philosophy.
The integral constitution of its material parts is thus the plain test
of an animal's " individuality," from the structural point of view. On
such grounds, the man or the dog is obviously a much more typical
" individual " than a " newt," which can part with its tail or legs, and
yet live and develop new members in the place of the injured parts ;
and the newt, in turn, is a truer " individual," judged by its structural
interdependence, than the zoophyte, whose buds as they fall are
replaced without material disorganisation of its constitution.
Professor Asa Gray well sums up the structural view of the
u 2
292 CHAPTERS ON EVOLUTION.
11 individual," when he remarks : " The idea of individuality which
we recognise throughout the animal and vegetable kingdoms, is
derived from ourselves, conscious individuals, and from our corporeal
structure and that of the higher brute animals. This structure is a
whole from which no part can be abstracted without mutilation.
Each individual is an independent organism of which the component
parts are reciprocally means and ends." .
But another method of viewing the personality of the animal
is found in the deductions of physiology. Not "what it is,"
but " from what it has originated," is the test of physiological
individuality. That alone, in physiological eyes, is an " indi-
vidual" animal which is the total result of the full development
of a single egg. Whatever a single egg becomes, in other words,
represents the individual animal or plant. Testing some of the
examples already noted by this criterion, we may readily enough
distinguish the true individuality of the animal races we have
passed in review. With, respect to the personality of the higher
animals, this test is susceptible of the plainest illustration. Each
quadruped, bird, reptile, fish, oyster, &c. springs from a single egg.
When each of the bodies in question has been formed, we know
that the full development of the egg or germ has been attained.
Hence each of the aforesaid animals is an " individual " pure and
simple, when judged by the standard of its representing the total
result of a single germ-development. With the other illustrations, the
case should be equally clear. A zoophyte (Fig. 197, 2) and a sea- mat
(Fig. 190) spring each from a single egg, and the process of budding
gives to each the plant-like form and the colonial organisation familiar
to us in these beings. Hence the whole zoophyte, and the sea- mat
in Mo, are " individuals." What then, it may be asked, are the separate
members of either colony ? Not " individuals " — for they merely repre-
sent parts of a single egg's development — but "zooids " is the biological
reply ; comparable, it may be, to separate-** organs " and " parts " in the
higher animal, but not constituting of themselves "individual" person-
alities. The cases of the gregarina (Fig. 183) and sponge (Fig. 187)
are each resolvable without difficulty on the premisses just indicated.
The single gregarina, arising from a true process of development, is
a single individual, but the divided gregarina represents a compound
personality. The whole sponge, arising as it does from an egg, is an
" individual ;" and if each of its protoplasmic units be held to be not
merely a cell, but a semi- independent and amoeba-like organism, the
sponge is a " compound individual " in addition. So also with a tape-
worm (Fig. 191) or other allied organism. The whole " worm" is one
compound " personality," or one " individual," because it has arisen
from a single egg, and because it represents the full development of that
body. So likewise with the hydra (Fig. 188). Arising from a single
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 293
egg, it gives origin by budding to other hydrse which break away from
the parent-organism, and live an independent existence. But as these
buds, although independent of the parent-body, nevertheless represent
part of the development of the single egg, we see that the " hydra-
individual " is not the parent-hydra alone, but that parent, plus all the
buds or hydrae which are produced by it. The next "individual" exist-
ence begins with the production of an egg. Till that event happens,
all the hydrse, produced by budding or otherwise, are merely parts of
an individual, and have of themselves no distinct personality. With
the zoophyte and the hydra, therefore, the case for the " individual
existence," as represented by the compound animal (i.e. by the single
animal plus its buds) seems clear. Quoting Professor Huxley once
more, we may say that " the multiplication of mouths and stomachs "
in a zoophyte (Fig. 197, 2) — as the result of the budding of new
members of the colony — " no more makes it an aggregation of different
individuals than the multiplication of segments and legs in a centipede
converts that arthropod into a compound animal." " The zoophyte,"
continues the voice of authority, " is a differentiation of a whole into
many parts, and the use of any terminology which implies that it
results from the coalescence of many parts into a whole is to be
deprecated." The plant-lice (Fig. 197, 3) are to be viewed in pre-
cisely the same light. For, as Professor Huxley remarks, " no doubt
it sounds paradoxical to speak of a million of aphides, for example,
as parts of one morphological individual ; but beyond the momentary
shock of the paradox, no harm is done. On the other hand, if the
asexual (i.e. the products of the pseudova) aphides (Fig. 197, 3, ee)
are held to be individuals, it follows as a logical consequence, not
only that all the polypes on a cordylophora (or zoophyte) are ' feeding
individuals,' ...... while the stem must be a ' stump individual,'
but that the eyes and legs of a lobster are ' ocular ' and ' locomotive
individuals.' And this conception is not only somewhat more
paradoxical than the other, but suggests a conception of the origin
of the complexity of animal structure which is wholly inconsistent
with fact."
The point to which our inquiries have led us may be summed up
in the conclusions, firstly, that animals exist either as simple or as
compound " individuals " — the former typified by the higher animals at
large, and the latter by the zoophyte and the tapeworm tribe. A
second inference deducible from our study, is that the personality of an
animal is in reality the direct result of its development, and of the
manner in which its parts and organs are structurally related to each
other. And a third deduction follows from our biological experience,
namely, that the separate parts — or " zooids," as we term them — of a
compound individual, are not necessarily connected by structural ties
to the parent or primitive form. On the contrary, like the detached
294 CHAPTERS ON EVOLUTION.
buds of hydra, the free jelly-fishes of the zoophytes, or the apparently
free and independent members of the plant-lice fraternity, the
" zooids," which make up the personality of the true " individual,"
may be scattered far and wide from the parent organism, and be yet
tied by firm transcendental bonds to the stock of which they are
really intimate parts.
But a further question still besets us, namely, as to the origin and
meaning of the variations which animal individuality thus presents to
view. If the true function of philosophy be that of affording a clue to
the meaning of this world's phenomena by placing facts in their due
relationship to each other, it follows that the higher knowledge of
the varying " individuality " of living beings must resolve itself into
an explanation of the causes through which such personality has
been acquired. Such philosophy is necessarily founded upon that
view of the order of nature which regards the universe as an arena of
constant modification and progressive change, as opposed to the
theory of its originally and inherent stable constitution. It is the
theory of evolution, as opposed to that of specially designed ways
and means in nature, which presents itself as the exponent of animal
" individuality " and its causes. On the former hypothesis alone, is
the question of the individuality of living beings debatable ; since
the idea of stability in living organisms presents a dead wall to the
further discussion of the present as well as most other biological
topics. Hence the data of evolution and progressive descent, with
modification, must, in the present instance, be used as the pathway
along which our explanatory steps are to be pursued.
That every living being begins life in a simpler guise than that
in which it spends its atlult existence, is a kind of home truth in
eyery-day life, as it is a dictum of biological science. The practical
difference between a low and a high animal lies in the fact that the
former does not advance much or anything beyond its primitive
condition, whilst the latter in time exhibits an infinite complexity
on its early structure. A gregarina or an amoeba are lower than an
oyster, because development leaves the two former with bodies but
little more complex at the end of life than at their birth. The oyster,
on the contrary, beginning as an amoeba-like germ, takes farewell of
development as an organism of high complexity, and as one whose
frame exhibits a marked differentiation of its organs, parts, and tissues.
Now, if the body of a higher animal be analysed out into its con-
stituent parts, we may, microscopically, speak of it, with the greatest
possible exactness, as a collection of cells and fibres — or more simply
as a collection of cells, for the fibres arise from and are developed
out of cells. So that even the complex frame of humanity, is truly
resolvable into groups of cells which, however varied in structure
they may be, arise in their turn, at the commencement of develop-
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 295
ment, first, from exactly similar cells, and more primitively from one
and a single cell — the ovum or egg itself. Thus true is it that " all
the higher forms of life are aggregates of such morphological units or
cells, variously modified."
But development teaches us something more. Every animal
above the rank of the amoeba and its kind — and even these latter
may be included in the statement — passes, in the course of its
personal progress towards maturity, through a stage in which the
original substance of the single primitive cell or egg breaks up into
numerous other cells, through the subsequent arrangement of which,
the body of the organism is in due course developed. This epoch,
our developmental studies have familiarised us with under the
designations of " segmentation " and the " morula-stage." In other
words, there is an early tendency on the part of every animal and
plant to depart from the single-celled stage, and to exhibit a com-
pound or collective structure. The egg, at first one cell, thus divides
to form a colony. Nor may the transcendental glance cease at this
stage of matters. If a colonial aggregation of cells at a very early
stage of development be a reality of life, — if some animals, sponge
and hydra for example, are but collections of primitive cells, — a no
less stable fact is expressed in the statement that in the adult body of
the highest animals such colonial aggregations are still to be traced.
Each tissue of the human frame, in its most vital phase, is a colony of
cells — a compound cellular " individual," numbering its units by the
thousands, and possessing the power of growing, and reproducing new
cells, as truly as the zoophyte, by budding, repairs the constant loss
to which its component parts are subject. And there may further
exist in the highest animals, cells or units which exhibit well-nigh
as complete an independence of the frame in which they 'occur
as do the animalcular hosts outside. Thus the white corpuscles of
the blood (Fig. 198) of all animals exactly resemble amoebae in
FIG. 198. — WHITE CORPUSCLES OF THE BLOOD.
Different forms assumed successively by a white blood-corpuscle.
structure, size, and movements. They are known to pass through
the walls of blood-vessels, to roam through the body at will, and
are seen to exhibit an utter and complete independence of all the
tissues of the body. More curious still, these white corpuscles have
been seen to ingest solid particles, exactly as an amoeba or allied
form receives its particles of food. It is not more wonderful, if
we think the matter over, to find in our own bodies many true
296 CHAPTERS ON EVOLUTION.
" colonial " aggregates of cells, than to discover that certain of
the cells thereof have developed an independence and freedom of
motion equal to that of an animalcule living in its native haunts, and
carried out through movements of exactly similar nature to those
performed by the amoeba itself. Thus a first halting-place in our
philosophy of individuality may readily be found in the declaration
that the " colonial " or " compound " body is in reality the normal
constitution of all animals, save the very lowest With the advance
of life there has been exhibited a progress in complexity, and this
progress has found structural expression in the conversion and
multiplication of the original unit of the germ into the colonial and
compound state. We ourselves are "compound" individuals, in
the sense that our physical personality is not single in any sense,
but markedly multiple. Our individuality may be named doubly
" compound," in the sense that, whilst each tissue may be held to
represent a " zooid," or colonial member of the body as a whole, the
tissues are, in their turn, made up of " cells," each of which is a
distinct morphological unit.
If the above deduction be correct, founded as it is upon strict
anatomical detail, it remains to discuss those cases of " colonial
aggregations " in which the separate units are plainly recognisable,
as in zoophyte, sea-mat, and tapeworm. Such cases will be found
to differ not in kind, but in degree only, from the higher colonial
organisations we have just described. The zoophyte and the highest
animal are separated by a gulf not impassable or fixed, when the aid of
broad generalisation in comparative anatomy is invoked. For there are,
firstly, gradations and stepping-stones connecting the two extremes ;
and there exists, moreover, a general principle of development whereby
the differences between the colonial nature of the higher and that of
the lower form may be aptly expressed. Thus the sponge illustrates
a case in which the colonial nature of the highest organisms is plainly
enough foreshadowed. A sponge or a hydra advances but a tithe of
the developmental journey which a bird or quadruped has to pursue ;
and as a result of its early arrest on the developmental pathway, its
component units evince but little elaboration on their primitive and
animalcular state. If a sponge is a mass of amoebae, as to its living
parts, it exists in this simple condition because there was no further
need for a more intimate relationship between its various units. The
fact, already mentioned, that two fresh-water sponges, placed in con-
tact, unite into one, shows the ill-defined nature of the individuality
in a case like the present, where the units are merely placed in appo-
sition, so to speak, and united simply by the common skeleton they
elaborate. In a zoophyte (Fig. 189), which is in reality but little
removed above the sponge in the animal series, development and its
attendant conditions — whatever these latter may have been — have
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 297
together produced units as thoroughly distinct as those of the sponge,
but nevertheless connected in the work of nourishing and repairing
the colony. In the " sea-mats " (Fig. 190 ) we see a stage of colonial
development in an animal form which more nearly approaches
the condition of the higher animals, but which likewise lacks all
the intimate features of connected interests seen therein. The
" sea-mat " colony is an aggregate of units each of which we have
seen to be perfectly independent, save for external connections, of
its neighbour units. There must thus exist a certain and not
distant parallelism between a "sea-mat's" constitution and that of
higher beings ; inasmuch as both are colonial, and in both the
units exist in a relative but by no means corresponding degree of
independence.
Analogies are thus plentiful enough in showing us the stages which
intervene between the dependence and connection of the units in
higher life, and the comparative independence of those in lower life.
But the cases of the Nai's or river- worm (Fig. 192), as well as those of
the plant-lice and bees, show us plainly enough the amazing possibi-
lities of highly organised animals becoming " colonial " organisms,
even with complete separation and detachment of the units of the
colony — which, however, in the case of the bees, as "social"
insects, is again reconstructed in the institution of a co-operative
life and existence. In the Nai's, we see illustrated a tendency
towards repetition of "zooids," which may be viewed as leading
towards an appreciation of the manner in which an originally
jointed animal — itself colonial in one sense — advances towards the
condition of the plant-lice and bees with free and separate units.
It is not more surprising, we may repeat, to find the insect-
individual with its separated and detached units, than to discover
in the higher bird or quadruped the same colonial structure, but
one likewise which is closely combined and intimately related
as to its elementary parts. The possibilities of life, are facts, indeed,
which in the present case cut both ways; demonstrating, even if
leaving the main collateral facts unexplained, how in the higher
spheres of animal society, the independence of an animal colony
may perfectly co-exist with the interdependence of its original units.
But there exists for the biologist a final and authoritative court
of appeal in the matter of the origin of the colonial constitution and
its modifications, in the facts and teachings of development. The
general tendency of any organism undergoing development is, as we
have seen, one leading it towards differentiation and division of its
primitive and originally simple substance. Even in the lowest confines
of life we witness this tendency towards segregation and multiplica-
tion of its parts. The gregarina (Fig. 183, 'a) exhibits such a process,
and the early stages of all living beings are marked by the segmentation
298 CHAPTERS ON EVOLUTION.
and division of the germ. Conversely, as we ascend the scale of
being, we witness as marked a tendency towards concentration and
amalgamation of at least the superficial aspects of the organism.
The higher animal or plant is not so markedly colonial as the lower
organism. Externally, indeed, there may be no trace in the higher
organism of compound nature ; whilst, as we have seen, the intimate
constitution of its tissues fully reflects its colonial constitution. Then,
also, arrest of the process of development seems to increase the
tendency towards the colonial organisation. The tapeworms (Fig. 191)
may, on good authority, be regarded as animals whose development
has been arrested at an early stage of that process. We may readily
enough conceive that, but for such arrest, these animals might have
progressed towards that higher type of worm structure — seen in leech,
nai's, or earthworm — in which the separate joints practically repre-
sent the elements of a colony in close and inseparable union. Thus a
leech or earthworm, like the higher animal, is " colonial." It represents
the transition stage between a low colony with loosely aggregated
units, such as the sponge typifies, and the higher colony in which the
units have become closely merged together, as in the bird or
quadruped. This view of the intermediate place of these animals
is not merely supported by their position in the animal tree, but
likewise by the fact that each apparently closely connected joint of
a true worm accurately represents the structure and functions of
every other joint of the body — save, indeed, the specially modified
segments of head and tail. The worms and their allies thus become
interesting in our eyes, from the fact that they present us with
examples of that degree of development which, whilst leading
towards union of the original units of the organism, yet leaves their
identity sufficiently distinct to permit their ideal separation and the
realisation of their originally colonial nature, through the exercise of
a free philosophy.
Thus we again conclude that the primitive and earliest condition
of structure in the living series is the " colonial " and compound con-
dition. We arrive at this conclusion from a survey of the teachings
of development, which shows us, firstly, that everywhere the germ
in its earliest state tends to division and multiplication. Secondly, we
note that many organisms, such as the lower colonies of protoplasmic
forms, or even the mere primitive sponges themselves, remain per-
manently in a colonial condition, which would naturally enough
represent permanent arrest of development in the early stages of
egg-development. Thirdly, we learn that arrest of development,
even at a later stage, may produce the colonial organisations of higher
types. This latter view meets the case of the tapeworms and of
the true worms likewise. In the latter, as represented by the Nai's
(Fig. 192), we see the hereditary tendency towards colony-making
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 299
reproduced as accurately in the buddings of new individuals from
the parent-body, as in the perpetual budding of the zoophyte.
Last of all, we see in the highest animals the same innate and
fundamental constitution on the basis of the colony. The human
frame, morphologically viewed, is a collection of cell-colonies,
produced by segregation of more primitive collections of units, and
primarily, if the story told by development be true, by the modification
first of one cell, and secondly of one original series of cells.
The fundamental constitution of the living worlds thus appears to
be of colonial nature. It remains for us to discover how the com-
pound constitution has merged into these united and single per-
sonalities we regard as the highest members of the animal and plant
series — in a word, how the " colony " has become the " individual,"
the highest type of which we recognise in ourselves. If varying con-
ditions have operated to produce the diverse constitutions of animals
and plants we see displayed before our waiting eyes to-day, we may
justly assume that a more complex series of causes than we are able
to determine, is responsible for the origin of those higher natures of
which we ourselves form part. Yet here and there clues to the
understanding of the problem are not wanting in the considerations
which the study of even lower grades Of life disclose to view. The
apparently single nature of the germ from which high and low
organisms alike spring may best be explained, perhaps, on grounds
connected with the husbanding of vital power, and on the idea
that the apparent unity and singleness of the germ naturally re-
produce the constitution of the single cells or units of the com-
pound organism from which they spring. The egg or germ, in a word,
reflects in its first stage the constitution of the particular unit from
which it was derived. In its secondary stage it repeats the colonial
condition of which its parent-unit formed part, and the features of
which it is destined in due time to reproduce.
As, however, we survey the fields of animal and plant existence,
we discover plainly-marked tendencies of development which fully
account for the advance from the true colonial constitution of
zoophyte, tapeworm, and social insect, to the marked and apparently
single personality of higher life. The higher we rise in the organic
series, the less marked becomes the tendency to devote the energies
of life to the perpetuation of the species or race ; and the more per-
fectly do the powers which concentrate, ennoble, and advance the
individual interests become developed. It is a self-evident fact that
in lower life much of the bodily energy is occupied with the develop-
ment of new individuals, or, in the case of an animal colony, with
increase of the colonial membership. One has but to glance at the
zoophyte-races to find clear proof of this latter statement. Imitating
the plant-creation in the fulness of their vegetable growth, the tribes of
CHAPTERS ON EVOLUTION.
zoophytes — and the tapeworm-race with its millions of ova, and inde-
finite reproductive power as well — unquestionably possess as their chief
end the perpetuation of the race. How changed is the physiological
prospect in higher existence ! There the energies are devoted to the
improvement, sustenance, and development of the individual. There
is less devotion to tHe species as compared with what obtains in
lower forms ; and the colonial interests, whilst
still represented and conserved, are limited in
their scope and direction to the development
of new tissue-matter. The higher animal, in
short, is not obviously " colonial " in the
sense that a zoophyte or a "sea-mat" is
compound, because the energies and forces,
as well as the material, which in the lower
being reproduces readily the form of the
organism, are devoted to other functions.
Life in the lower and compound organism
is made up of one common interest, namely,
the increase of the colony and species. In
the higher animal, life becomes a far more
personal matter, and its aims are more
distinctly individualised. Existence in the
colonial zoophyte is passed, so to speak, in
marriage and giving in marriage ; and the
interests of the race are bound up in the
work of its own extension. In the higher
organism, individual interests and the life of
the single organism occupy the greater part
of its energies, so that, to use an expressive
dictum, " the organism is like a society in
which everyone is so engrossed by his special business, that he has
neither time nor inclination to marry."
There is abundant illustration at hand of the view that the culti-
vation of individual interests destroys, by concentration of energy,
the colonial organisation. Such an opinion finds its confirmation in
the details of higher animal existence, and in the disappearance of
those powers of bodily separation after injury which characterise
lower life. The organic republic or colony, in which every unit is as
good as its neighbour, is typically and perfectly represented in the
zoophyte. But this thoroughgoing republicanism is as impossible of
continuance in higher physical existence and in spheres biological,
as it is found to be incompatible with the political development of
nations. That is to say, as, in the life political, here and there special
developments cause men to shoot ahead of their neighbours and to
distance their competitors in the struggle for existence by individual
FIG. 199. — DAISY.
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 301
strength and excellence, so in the life biological, there is the same
tendency to development of individual faculties and powers over the
common interests, and the same conversion of the colonial organisa-
tion into the concentrated structure and functions of the individual
organism. In the plant-world there is a similar tendency towards
concentration as the concomitant of higher life. The colonial nature
of many of the lowest plants (e.g. Volvox\ which consist of aggregated
masses of protoplasm, is undoubted. But in the highest plant-life
also (Fig. 197, i), the colonial nature is far more strongly marked
than in many animals of by no means the highest grade. Where
the leaf-type (e e) repeats itself indefinitely, where bud resembles bud,
where there is witnessed
the gradual transforma-
tion of leaf-type into
flower-type (//), and of
flower into the full fru-
ition of plant-life, there
is presented to our men-
tal view an exact picture
of the budding zoophyte
(Fig. 197, 2), with its
series of similar units (ee).
Here and there these
units become modified,
now for this function, now
for that ; but exhibiting
the closest parallelism with the plant, in that its reproductive bodies
(/) are but modifications of the ordinary members of the colony ;
as the flower, in turn, is but the last term in the modification
of the leaf. Thus, as Asa Gray well puts it, " In the as-
cending gradation of the vegetable kingdom, individuality is, so
to say, striven after, but never obtained ; in the lower animals,
it is striven after with greater though incomplete success ; it is
realised only in animals of so high a rank that vegetative mul-
tiplication or offshoots are out of the question — where all parts
are strictly members and nothing else, and all subordinated to
a common nervous centre ; it is fully realised only in a conscious
person."
Yet, whilst the plant-world has not as a whole advanced towards
the higher phases of individuality, we may discern here and there
within its limits, signs of that universal progress which evolution
postulates and which biological research reveals. Here and there we
witness among plants a progression from the prevailing colonial
organisation towards singleness of type. The Composite race of
plants derive their flame from the fact that each flower of the order
FIG. 200. — SECTION OF DAISY.
302
CHAPTERS ON EVOLUTION.
is not a single flower, but a collection of florets. A thistle (Fig.
203) or a daisy-head (Figs. 199 and 200), for example, is not one
flower, in the sense in which a buttercup or lily is single, but is an
aggregation of small stalkless flowers (200, co, co) closely packed
together on one main stalk. If we examine the thistle-head, we shall
find it to consist of numerous little flowers (203 c, c], of similar
appearance, each containing the essential organs and parts seen in
other single flowers. In the Centauries of our
waysides and cornfields, we witness the same
composite structure of the flower-head ; but
here, the outermost florets (202, a) of the
" head " have begun to develop into petal-
like organs, and have lost their stamens and
pistils. The Centaury, in other words, has de-
veloped the beginning of a low individuality ;
it is losing its completely compound nature,
and is advancing towards the singleness of
type of ordinary flowers. Thus, in Centaurea
nigra, these outer florets vary in size; they
may resemble the inner ones in size, or may
^ . J \ / V- be larger, and they may want both stamens
J.W \ and pistils. In another species (C. scabiosa),
stamens and pistils never occur in the outer
florets ; and in Centaurea cyanus (Fig. 202)
likewise, these florets (a) are useless for repro-
duction, and are passing towards the type
and function of ordinary petals. So also in
the familiar dandelions (Fig. 201), we may
witness a stage in advance of the thistle. For
whilst the latter plant has its florets similar
and inconspicuous, the dandelion (Fig. 201)
has added to its similar florets the bright
corollas, which serve to render this wayside
plant so conspicuous to insect eyes as well as
to our own perception. When the dandelion
appears with its outer florets expanded, while
the inner florets have still to unfold, the flower
bears no inconsiderable resemblance to the
ordinary type of single flower. Far more
advanced, however, towards the individuality
of other plants, are the marigolds, daisies (Figs. 199, 200), and their
allies. Here the likeness of the single flower deceives the non-
botanical observer into supposing that each daisy in reality cor-
responds to each buttercup or primrose in its constitution. For the
outer florets of the daisy and marigold have developed, as those of
FIG. 201. — DANDELION.
b, Ripe flower-head.
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 303
the centauries (Fig. 202,0) are developing, into long petal-like organs
(Fig. 200, r\ Moreover, these outer florets are losing the repro-
ductive organs they still possess in the dandelion. The stamens
have disappeared in the outer white and yellow flowers of the daisy
and marigolds respectively, leaving the pistil alone represented (Fig.
200, r, sg); whilst the yellow central florets (dl d*} possess both
stamens and pistil, and are therefore the
true producers of seed. It is foreign to
our present inquiry to notice how this
arrangement of the flower parts, by placing
the brightly coloured parts on the outside,
imparts to these plants their conspicuous
nature, and thus, by attracting insects,
gives them a very marked advantage in
the struggle for existence, through securing
more frequent fertilisation. How or why
this greater attractiveness has been ac-
quired is immaterial. That which is all-
important for us to note is, that concur-
rently with a conspicuous dress, there is
being developed in such flowers as the
daisies and marigolds a return to that
singleness and individuality which was in
all probability once represented in their
race, before the work of aggregating once
separate flowers to form one flower-head
had begun. -The thistles remain types of
a true flower-colony. The dandelions and
centauries lead us from the thistles with
similar florets to an intermediate type,
wherein we see being developed those
features which, along with abortion of
part of the outer florets, are causing the
compound flower to assume the dress of
its simple neighbour ; whilst in the daisies specialisation has advanced
a step further, and has developed a very marked likeness to the simple
flowers around. If these modifications progress in the future as in
the past, we may naturally expect that the " floures white and rede "
of Chaucer, and their allies, will develop a still more marked indi-
viduality, and will leave the lower compound nature of their race
further and further behind.
It may be, lastly, Interesting to note that the crowding together
of flowers on a "flower-head," seen in the daisies and their
neighbours, is susceptible of explanation through a study of the
modifications and gradations witnessed in the arrangement of flowers
FIG. 202. — Centaurea cyanus, or
CORN BLUEBOTTLE.
CHAPTERS ON EVOLUTION.
on their axes. From cases in which we find flowers situated each
on a distinct stalk of its own, as in the Corymb and the Umbel (Fig.
204) of botanists, to the condition of the " flower-head," we can pass
by easy gradations. If we cut short the stalks of the umbel, and
thus crowd the separate flowers on the end of a common stalk, we
obtain a fair idea of the pos-
sible origin of a flower-head
by abortion of the flower-stalks
of an umbel or allied floral
arrangement. The fact that
such crowding of flower-heads
on a common stalk is not limi-
ted to the Composites or Daisy-
tribe, but occurs in other plant-
orders, argues powerfully in
favour of its acquired nature
as the result of common
modifying conditions. Thus
a head of clover essentially
Fro. SOS-HEAD OF TmsTL^sHowiNG NUMEROUS ^^ ^ condkion of ^
thistle or centaury. We can
obtain a fair idea of the effect of modification by the disappearance
of flower-stalks, if we look at a simple umbel, seen in the cherry (Fig.
FIG. 204. a, SIMPLE UMBEL OF CHERRY ; b, COMPOUND UMBEL OF FOOL'S PARSLEY.
204, a), or a compound umbel, seen in fool's parsley (Fig. 204, b],
and, by crowding the flowers together, minus their stalks, imagine their
growth in one stalkless group to represent the " flower-head " of the
daisy or thistle.
Summing up our studies in organic individuality, we may say that,
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 305
firstly, the original and primitive condition of all organic beings is a
colonial condition. This phase is exemplified, primarily, in the
segmentation of the egg and in the cell-multiplication of plant-germs ;
two features of so universal occurrence that we may lawfully claim
for them a great importance in the evolution of the organism and a
high antiquity in the history of living things. It is likewise imitated
in the so-called asexual reproduction of the lowest animals, repre-
sented by the gregarinse and amoebae. A second conclusion that
follows from the teachings of development may be expressed by say-
ing that this tendency to division of substance is most typically seen
in lower organisms, where, as exhibited in the sponges, zoophytes, and
their allies, the constitution of the individual is undeniably compound,
and where its advance is marked merely by the multiplication of
new types of colonial and connected units. We discover, thirdly,
that the tendency to degradation and retrogression may likewise
plainly develop the compound and colonial state. It is highly
probable that the tapeworms', the ordinary worms, and even Articulate
animals themselves, illustrate cases in which a primary development
of like segments or colonial units through arrest of growth, and
through simple bodily division and repetition of like parts, has paved
the way for succeeding modification of the colonial type. If the
evolution of the centipedes, insects, spiders, and crustaceans from a
lower worm type be accepted as proved, or even as probable, the
characteristic features of these animals must have been fundamentally
derived from those colonial tendencies we see exhibited in the worms
of to-day. A fourth conclusion teaches that the plant-world is
markedly colonial even in its highest types. The vegetative repeti-
tion of bud, leaf, and flower is simply a pure indication of colonial
constitution exhibited in all that perfection of detail which has
escaped the more forcible modification of the animal series. A
fifth inference directs attention to the essentially colonial consti-
tution of even the highest animals, as exhibited in their cellular
structure, and more especially in the independent constitution of
many of their component cell-elements. And a sixth and final
conclusion is deducible from our studies — namely, that concentra-
tion of structure and function, and the metamorphosis of the colony into
the true individual, is at once a cause and a result of the progressive
tendency of life at large. The higher we rise in the scale of being,
the more united and specialised do structure and function become.
Such a tendency is represented, as we have seen, even amongst plants,
in which the colonial and compound type tends to resolve itself into
the simpler and more individualised phase. At the same time, we
must recognise that, despite the functional unity of the highest
animals, there remains in their relative cellular independence the
traces of a colonial constitution, once universal, and still linking
x
306 CHATTERS ON EVOLUTION.
them by real as well as by transcendental bonds to lower and ante-
cedent phases of existence.
The topic of the personality of living beings, like most other bio-
logical subjects, relates itself more or less indirectly with matters
personal and ethical which are far beyond the scope of the present
study. But it is permissible, in a closing sentence, to remark that
many of the characteristic traits of the life of the higher animals, in-
cluding man himself, may perchance be traceable to an unconscious
perpetuation of habits and customs which find their beginnings and
germs in the lower colonial organisms whose history has just been
discussed. The nervous acts of man and the higher animals generally,
for instance, convince us that many of the functions of the brain,
and the automatic actions of the body depending on the independent
constitution of our nerve-centres, may be legitimately explained by
referring them, as regards their origin, to an originally colonial con-
stitution, and to a primitively colonial ancestry. Even a glance at
the serial repetition of the bones (or vertebrae) in the spine of man
or other backboned animal, eloquently enough testifies to the appa-
rent colonial constitution of these forms. There is a striking analogy,
which has not escaped biological notice, between the arrangement
of these segments in the Vertebrata and the similar disposition of
parts in the Articulata or worm and centipede-type. However the
Vertebrate's serial arrangement has originated, it may perhaps be
held as legitimate evidence of compound nature ; just, indeed, as the
colonial nature of Vertebrate tissues demonstrates that nature in
another fashion. And so, also, with other phases of human relation-
ship and functions. As the various detached buds of a hydra, or
the free-swimming buds of a zoophyte, are still part and parcel of
the individual constitution, or as the plant-lice and bees, apparently of
distinct personality, are in reality only parts of the connected colony,
so, in the sphere of human relationships, the origin of the tribal
connection or of the family constitution — itself the most expressive
of all human institutions — may perchance be found to exist in germ-,
form in the hidden transcendental bond which the philosophy of the
lower animal individuality discloses. The deep-seated affections and
relationships which, collectively, we term the "family" and "society"
respectively, may have had their first beginnings in the connected series
of interests which even the zoophyte-series discloses to view. In
other words, we are constituted as we are, gregarious, social, and
ethical, because we are physically " colonial " by constitution, and
because in our origin we are essentially of colonial and compound
nature. And if such a thought be regarded as too improbable for
realisation, it should be borne in mind that our structural beginnings
themselves are of the lowliest and simplest description. If the
structural germs of the highest life begin, as they certainly do, under
EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 307
an animalcular guise, is it overstepping the possibilities of natural
facts to suggest that the social traits and characteristics to which that
germ attains may likewise have had a lowly and material beginning ?
Such an idea, so far from possessing any elements of impossibility, is
grounded on a rational basis — namely, on that opinion which teaches
that community of origin may, and often does, entail similarity of
results. Sufficient has been said to show that in human existence
reign many of the colonial traits of lower spheres. And if, perchance,
some dim echoes of such lowly traits may linger in the scientific mind
when contemplating the highest existence of all, the mind will regard
such similarity as founded upon no chance basis, but as having
originated from that continuity of cause and effect which runs un-
broken through the warp and woof of the universe of life.
X 2
3o8 CHAPTERS ON EVOLUTION.
XIV.
THE FERTILISATION OF FLOWERS.
FEW subjects, if any, are better calculated to awaken a lively interest
in the investigation of natural laws and the phenomena of life at
large, than the study of those processes of development whereby
the races of animals and plants retain their hold upon the world,
and maintain a continuous and unbroken round and cycle of exist-
ence. In such studies, more than in any others, we seem to gain
near glimpses of Nature's ways and methods in fashioning the varied
universe of living beings ; whilst the lessons such topics are well
calculated to enforce respecting the order of nature as a whole, form
not the least important result of these investigations. The study of
even the most commonplace object may, under the newer phases of
research, be made to yield an amount of " sweetness and light " for
which we might be wholly unprepared. The day of the Peter Bells,
and of uninquiring moods and tenses, if not altogether a thing of the
past, is happily already in its twilight stage. The schoolboy, with a
primer of botany in hand, understands things at which the previous
generation simply wondered. And even if the results of botanical
study may occasionally be expressed by the hackneyed Words-
worthian idea of thoughts beyond tears, the modern student of
plant-life has ample reason to congratulate himself on having attained
the mastery of many ideas, which in past years were included under
the poetic category of " expressive silence." The primrose still grows
by the " river's brim," in truth, but it is no longer merely a yellow prim-
rose. On the contrary, the flower is in greater part understood, the
mechanism of its life is well-nigh completely within our mental grasp ;
and, best of all, its study has led in the past, as it leads even now, to
the comprehension of wider ideas of nature, and more extensive
views of plant life, than those which formerly met the gaze of the
wayfarer in scientific pastures. The appreciation of what is involved
in part of the life-history of a primrose may thus serve as a starting-
point- for more extensive research into the phenomena of plant-
fertilisation at large ; and this latter topic, in its turn, falls naturally
into its proper niche in teaching us plain lessons respecting the
manner in which the wide domain of life is regulated and governed.
By the " fertilisation " of a plant is meant to be indicated those
actions or processes in virtue of which those little bodies named
THE FERTILISATION OF FLOWERS.
309
"ovules" developed in the seed-vessel (Fig. 205, /) become "seeds,"
and through which they are fitted to develop into new plants.
The unfertilised ovule is incapable of producing a new plant.
When set in the ground it would simply decay, as if it were a leaf
or other detached and dead portion of the plant-economy. When,
FIG. 203. — WALLFLOWER.
on the contrary, it is duly fertilised, the ovule, becoming the " seed,"
has become possessed of the powers and properties in virtue of which
it is capable of evolving the form of the parent-plant from which it
was derived. So much for the very necessary botanical distinction
FIG. 206.— FOXGLOVE.
FIG-. 207. — SNAPDRAGON.
between " ovule" and "seed." The process of fertilisation is thus
seen to be that on which the continuance of plant-existence depends.
More closely regarded, it is known to be that which is capable under
certain conditions of giving origin to new races or varieties of the
plant-species. When the horticulturist, taking the pollen from one
species or variety of plant, applies this fertilising matter to the ovules
3io
CHAPTERS ON EVOLUTION.
of another variety or species, the characters of the two different races
are combined and united in the " hybrid " progeny. Our gardens
and conservatories — and, as we shall strive to show hereafter, the
natural plant-creation at large — have benefited immensely in beauty'
from a knowledge of the changes in colour, form, and size, which
this " cross-fertilisation " may produce. For instance, the finest of
our rhododendrons are crosses in which the characters of Indian
and American species have been thus blended. The union of the
common heartsease with a large-flowered foreign pansy, has pro-
duced a new stock in which the excellences of both species are found.
The pelargoniums of our conservatories represent hybrid stocks
and varieties, which cross- fertilisation and cultivation have together
produced from the small-petaled species of South Africa. Such
results, among countless others, would seem to suggest that beneath
the subject of cross-fertilisation, or even underlying that of ordinary
fertilisation, there lies hid a mine of knowledge respecting the causes
which have wrought out the existing variety of plant- life. For the
plain and unfettered understanding of the subject in its less technical
phases, or to lay the foundations of knowledge respecting an interesting
field of natural-history study, no better subject could be selected than
the history of even the commonest flower — such as a primrose. Rightly
comprehending what is included in
the phases of primrose-life, we may
hope successfully to read some of
the more abstruse problems pre-
sented by the wider aspects of
plant existence at large. "The
ruthe primrose that forsaken dies,"
and the "cowslips wan that hang
the pensive head," afford us delight
even when we are living in all the
simplicity of botanical ignorance.
It is not too much to say that their
systematic study may lead to the
higher delights and more cultured joys included in the knowledge of
some phases of natural law and in an understanding of the hows and
whys of living nature.
The elementary botany of a primrose is a matter of few words.
Like every other perfect flower, it consists of four parts or circles of
organs placed one within the other. Outside, we perceive the circle of
fine green leaves, which we name the calyx, each green leaf of this organ
being named a sepal. In the primrose or campanula (Fig. 214), the
sepals are united, although in many other flowers, (e.g., buttercup and
wall-flower) (Fig. 205, ca\ we should find them free and separate. The
calyx of all flowers is, for the most part, coloured green, its obvious use
A B
FIG. 208.— PRIMROSES.
THE FERTILISATION OF FLOWERS
3"
being to form a protective envelope for the other organs 01 the flower.
Within the calyx, we descry the corolla (Fig. 208, co]. This is the circle
of petals or leaves which, par excellence, we call the " flower," because
it constitutes in the vast majority of flowers the bright and showy portion
thereof. A flower might botanically or physiologically be perfect enough
minus its corolla; al-
though the eye, missing
the bright petals, would
be apt to regard such
a plant as wanting the
first and chiefest ele-
ment of the blossom.
The common nettle,
for instance, appears to
possess no " flowers " in F!G- 209.— NETTLE-FLOWERS.
the popular and accus-
tomed sense of the term ; but when we examine the plant, we readily
discover that it possesses parts corresponding to the flowers (Fig. 209)
of other plants. In the greater nettle, the flowers of one plant are es-
sentially different (in that they possess " stamens " (Fig. 209 s s) alone)
FIG. 210.— FEMALE OR PISTILLATE
FLOWERS OF WILLOW.
FIG. 2ii.— MALE OR STAMINA TE
FLOWERS OF WILLOW.
from those of another plant (which possess " pistils " (/) only). But
in the lesser nettle, or in the oak, these distinct flowers (Figs. 212, 213)
are found on one and the same plant. No vestige of colour appears
in either, however ; and when we study the flowers in question, we
312 CHAPTERS ON EVOLUTION.
find that a corolla is wanting, although a calyx (Fig. 209, c) is present.
Again, in the willow, which, like the greater nettle, has its stamens
and pistils (Figs. 210 and 211) on different plants, there appears to
be no " flower" in the ordinary sense of the term ; and the calyx as "
well as the corolla is found to be wanting in these trees.
The stamens, just mentioned, form the third set of organs proper
to the perfect flower. Looking at buttercup, wall-flower ( Fig. 205, st\
saxifrage (Fig. 215) or campanula (Fig. 214), we readily see the
stamens. They exist as stalked organs (Fig. 214, ss), each con-
FIG. 212. — MALE OR STAMINATE FLOWERS OF OAK.
sisting of a stalk or filament (Figs. 216, 217, ,$•/), and a head called
the anther (a). The head is hollow and contains the fine yellow
dust termed pollen, which, at the time of ripening, is usually found
scattered conspicuously about the interior of most flowers. The
fourth and central set of organs found in the flower constitute the
pistil (Figs. 214, 215, and 218) or seed-producing structure. This
organ is composed of one or more parts called carpels. Each
carpel consists in turn of a lower distended part called the ovary
(Fig. 218, cv\ within which the ovules are produced; of a neck
or filament, the style (sf) ; and of a head borne on the style (sg),
and named the stigma. The style or stigma may be absent ; but in
the great majority of flowers both parts are present, the ovary being
THE FERTILISATION OF FLOWERS.
3'3
however, the essential part of the pistil. In the " head " of a poppy,
for instance, there is no style ; the bulk of the " head " consisting
of the ovary containing "numerous seeds, and the flat cap or lid re-
presenting the "stigma" of the poppy-pistil. As a final observation
concerning the parts of the flower,
it may be noted that the separate
pieces, or " carpels," of which a
pistil is composed, may either be
free and distinct, or closely united
and adherent to each other ; whilst
a second fact of importance in the
general description of flower struc-
ture, consists in the declaration
that the ripe and mature pistil is
the fruit in botanical parlance.
True, there may, as in the straw-
berry (Fig. 219), be found united
to the ripe pistil certain other parts
which constitute the edible and
desirable portion of the plant. The
true pistil in the strawberry consists of the little yellow carpels,
(Fig. 2IQ,/), usually called "seeds," which are .imbedded in the
fleshy mass of the fruit formed by the expanded top of the flower-stalk.
But the aesthetics of taste must be neglected in the strict descriptions
FIG. 213.— PISTILLATE FLOWERS OF OAK.
FIG. 214. — PARTS OF A FLOWER
(CAMPANULA).
FiG. 215. — FLOWER OF SAXIFRAGE
IN SECTION.
of science; and that alone is the " fruit," in the eyes of the botanist,
which is formed by the ripened pistil, or central organ of the flower.
All parts of the flower, it must be observed, are not of equal value
in the eyes of the botanist. Those organs — stamens and pistil — which
produce and elaborate the seed, are physiologically more important
314 CHAPTERS ON EVOLUTION.
than the circlets or whorls of leaves which, in the form of calyx and
corolla, surround and protect them. Yet the latter organs play their
own part in the production of seeds, and in some cases serve as the
actual means whereby special modes of fertilisation are primarily
induced and carried out. As the sequel may show, indeed, the
*g
FIG. 216. — STAMENS
OF IRIS.
FIG. 217.— STAMEN OF
AMARYLLIS.
FIG. 218.— PISTIL OF
CHINESE PRIMROSE.
calyx and corolla — which in previous years were deemed mere
" floral envelopes," being credited, as such, with a merely protective
function — have largely risen in importance in the estimation of the
botanical world ; since on the form, colour, size,
&c., of the corolla especially, largely depend the
working of those mutual relations which have
been formed between the insect-world on the one
hand, and the world of flowers on the other.
Peculiarity of a corolla implies, botanically, as a
rule, peculiarity of fertilisation ; and the impor-
tance of the blossom becomes plainly apparent
FIG. 219.— STRAWBERRY, to us when we discover that in place of the
somewhat limited function formerly assigned to
it by the unscientific philosopher — namely, that of affording delight
to man by its beauty — it subserves the truer and more logical mission
of aiding materially the increase of the race to which it belongs, and
of which it forms such a characteristic part.
Turning to the primrose for practical illustration of the foregoing
THE FERTILISATION OF FLOWERS. 315
precepts, we may readily enough find in its structure plain instruction
in the build of the flower. The circle of green leaves placed outside
the yellow blossom is, of course, the calyx. This greeri cup consists
of five leaves or sepals united in the primrose, but free and easily
separable in the buttercup or wallflower (Fig. 205, A, ca). The blossom
or corolla (Fig. 208, co) of the primrose exhibits similarly a united con-
dition of parts. We can tell that it consists of five petals, or leaves,
by counting its prominent lobes or projections. When we tear the
corolla in two, longwise, we readily perceive the five stamens (a),
which, however, in the primrose, exhibit a somewhat peculiar position,
in that, instead of arising from the end of the flower-stalk, like the
other organs of the flower, they spring from the sides of the united
petals (Fig. 208). If we seize the corolla of a primrose by its upper
portion, and pull it gently upwards, the entire blossom with its
attached stamens will become detached from the flower-stalk,
leaving the calyx and pistil on the latter organ. Then tearing or
cutting away the calyx, we may be favoured with a clear view of the
pistil itself, seated on the extremity of the flower-stalk. In the pistil
(Fig. 2 1 8) we behold a body consisting below of the swelled or rounded
structure already mentioned, and named the ovary (ov). This being
cut across, is seen to contain numerous seeds or ovules, as the case may
be, arranged around a central pillar named the placenta. From the
upper part of the ovary arises a long stalk, the style (st) of the pistil ;
and the style, in its turn, is capped by a flat head, the stigma (sg).
In the pistil of the primrose we therefore see the three typical parts,
already noted as constituting the central organ of the flower. The
pistil in this case, it may be remarked, consists of five carpels, so
closely united that it is only by the aid of the " law of symmetry "
(or that demonstrating the general correspondence of numbers in the
flower-parts) that we can determine its composition. Five is the
ruling number in the calyx, corolla, and stamens. Hence we con-
clude that the pistil of the primrose in its composition will conform
to the type of the other whorls of the flower.
The physiology of the flower naturally follows the consideration
of its structure. Living action, in other words, forms the natural
corollary to living machinery or structure ; hence we may fitly
inquire into the manner in which the work of fertilisation is carried
on in the economy of the primrose. Leaving for after treatment, the
more special features of fertilisation, the general scope of the
function whereby, as we have seen, the immature " ovules " are
converted into " seeds " — each capable of developing, when planted,
into a new primrose — may be readily appreciated. The stamens,
each possessing as its essential part the anther or head (Figs. 216
and 217, a), develop the yellow dust, or pollen, as one of the two
elements concerned in the work of plant-development and repro-
CHAPTERS ON- EVOLUTION.
duction. Sooner or later, the anthers of the stamens open in one
way or another so as to allow the pollen to escape ; and, viewed under
the microscope, the pollen-grains are seen to vary greatly in size and
form in different species of plants. The grains of pollen may be
round (Fig. 220) "or oval in form ; in the evening primrose (Fig. 222)
and fuchsia, they are of triangular shape ; in the hollyhock and
melon (Fig. 223) they are spinous ; and in the orchids they are united
to form masses (Fig. 221) called pollinia.
The pollen-grains being conveyed to the stigma (Fig. 218, sg) of the
pistil, they are there attached by the aid of a glutinous secretion, which
FIG. 220. — POLLEN-GRAINS EMITTING
POLLEN-TUBES.
FIG. 223.— POLLEN-
GRAIN OF MELON
EMITTING CONTENTS.
FlG. 222.
POLLEN-GRAIN OP
EVENING PRIMROSE
(MAGNIFIED).
may likewise be credited with a specific influence on the pollen-
grains, in that it appears to stimulate the curious development they
next evince. This development consists in the rupture of the outer
of the two layers of which each pollen-grain consists. Through the
ruptured outer coat, the inner layer begins to grow in the form of a
long tube — the pollen-tube (Fig. 220) — which penetrates the tissue of
the style (Fig. 224), and grows downwards to reach the ovules con-
tained in the ovary. In some plants, the pollen-tubes emitted from
THE FERTILISATION OF FLOWERS.
317
one pollen-grain may be very numerous, although as a rule only one
tube grows from each grain.
Now, the essence of fertilisation (i.e. the production of a " seed "
fitted to produce a new plant) appears to consist in the contact of the
pollen-tube with the ovule, so that the viscid matter called/cw///^ con-
tained within the pollen-grain, may be applied to the structures of the
ovule. The most important part of the ovule itself is a small cellular
body called the nucleus, enveloped in a couple of coverings. The
hollow interior of the nucleus is named the embryo-sac, and an opening
FIG. 224. — POLLEN-TUBES OF DATURA PENETRATING THE STYLE (MAGNIFIED).
called the micropyle also exists in the coats of the ovule. Through
this opening the pollen-tube passes, gaining admittance thereby to
the nucleus, and thence to its hollow body or embryo-sac, wherein
the fovilla, or contents of the pollen-grain, are discharged.
Such is the work of fertilisation, and such are the processes in virtue
of which the ovule becomes the seed. As the result of these processes,
CHAPTERS ON EVOLUTION.
the " embryo," or young plant, is duly formed within the embryo-sac,
and thus, even before the seed is planted, development has already
proceeded to a certain extent. In the seed of a pea or bean
(Fig. 225), for instance, we readily perceive the rudiment of the
stem (/), the beginning of the root (r), and likewise the first
appendages or " seed leaves (c}" which that stem will develop.
The process of fertilisation, thus described in
its essential nature, involves in the case of
certain plants some curious details, the mere
mention of which may stimulate to an inde-
pendent research into botanical lore. Thus,
often the pollen-tubes may require, from the
length of the style of the pistil, to grow to a
large relative extent. In the crocus, the
pollen-tube requires to grow to a length of
three inches before it can reach the ovules
FIG. 225.— SECTION OF BEAN. jn the ovary. The number of pollen-grains
in flowers may be apparently in excess of all
reasonable proportions — a fact to be accounted for on the well-
founded idea that the pollen of a flower is not usually limited to
that particular flower's wants, but may be destined to serve for the
fertilisation of others of the same species. In the great flowered
cactus ( Cactus grandiflorus), Morren says there are about 500 anthers,
24 stigmas, and 30,000 ovules. Assuming that each anther contains
500 pollen-grains, this will give a total of 250,000 grains to each
flower ; and the interval or space between the stigma and the ovules
of this plant is about 1,150 times the diameter of the pollen-grains.
Nature appears exceedingly lavish in her development of pollen. If
the Tennysonian aphorism that —
Of fifty seeds
She often brings but one to bear,
be true — as it unquestionably is — the apparent over-production of
pollen-grains is even more remarkable, although we have to take into
account the fact just noted, that the development of pollen bears a
relation rather to the species and race, than to the individual necessities
of the plant. Otherwise, Fritz Mullens estimate, that in a single flower
of Maxillaria there are developed 34,000,000 grains of pollen, must
present itself as an inexplicable fact of botanical science. Even the
wheat-plant produces about 50 Ibs. of pollen to the acre. The
pollen of the cone-bearing plants ( Coniferce\ such as the firs, larches,
pines, or that of the catkin-bearers (Amenttfera;), is often borne
through the air as showers of yellow sulphur-like dust. This dust,
falling in regions where the elements of botany are unknown, cause
perturbation amongst the unlearned, and result in the penning of
THE PER TILISA TION OF FL 0 WERS. 3 1 9
epistles to " Mr. Editor " by way of inquiry whether or not the
sulphureous shower is a portent or grave omen of coming disaster or
impending peril.
The phenomena of fertilisation just detailed, take place in our
primrose, as in all ordinary plants ; but whilst there exists a uni-
formity in the details of this process, there is also found a literally
amazing variety in the fashions whereby pollen is conveyed to the
stigma of the pistil. Once placed in the natural position for fertili-
sation, the growth of the pollen-tube follows as a matter of course.
But the means whereby the pollen reaches the stigma, and the
various fashions in which it may gain its ultimate position on the pistil,
constitute features in which are bound up some of the most important
issues of plant existence. To rightly comprehend the bearing of fertili-
sation, a glance at our wallflower (Fig. 205), primrose (Fig. 208), fox-
glove (Fig. 206), or buttercup will suffice as a starting-point for further
investigation. Within the primrose and the buttercup are situated, as
we have seen, the two sets of organs — stamens and pistil — necessary to
secure the production of seed and the continuance of the race. Hence
it might form a very natural and reasonable inference, that the pollen
from the numerous stamens of a buttercup flower should be used to
fertilise the ovules of the pistil of that flower. Such a process — that
in which a flower's own pollen is used to fertilise its own ovules — is
termed " self- fertilisation." Looking at the vast 'majority of our
flowers and plants, which possess each a perfect array of stamens
and pistil, the normal course of things seems strongly suggestive of
self-fertilisation. Hence, in the early days of botany, self-fertilisation
was undoubtedly believed to be the rule of nature. Now, there can
be no question whatever that " self-fertilisation " does occur in nature,
but there is as little doubt that it is the exception, and not — as
botanists from the days of Linnaeus well-nigh to our own day have
maintained — the rule, of plant life. There can be little doubt, for
instance, that many small species of the buttercup order (Ranuncu-
lacecs — e.g. Ranunculus hederaceus) are self-fertilised, because we find
the stamens to arch over the pistil, and to shed their pollen on the
carpels. I n Agrimonia, in the same order, the stamens, at first curved
outwards, curve inwards, so as to bring the pollen within easy reach
of the stigmas. So, also, in a species of Malvaceae (Malva rotundi-
fo/ta), Miiller has demonstrated that this plant is self-fertilised, since
stigmas and anthers actually intertwine, and are thus placed in the
most favourable position for the fertilisation of the ovules. Some
species of Geraniacea (e.g. Geranium pusilluni) are self-fertilising
likewise ; and many flowers belonging to the rose tribe (fiosacea], such
as Potentilla, fertilise themselves.
It is a remarkable fact that in certain plants (e.g. many violets ;
Lamium amplexicaule ; Oxalis, &c.) very small, inconspicuous, and
320 CHAPTERS ON EVOLUTION.
closed flowers are produced, in addition to the ordinary conspicuous
and, as we shall see, " cross " or insect-fertilised flowers. These
closed flowers have been named " cleistogamous " — a term applied
by Kuhn in 1867. They are self-fertilised, and produce numerous,
seeds; and their occurrence in the same plant along with cross-
fertilised blossoms, may perhaps be best explained on the theory that,
whilst the ordinary and less fertile flowers will afford to the- plant
the advantages and benefits which accrue from " cross-fertilisation,"
the " cleistogamous " flowers may be regarded as the normal means
for the ordinary increase of the race. What the flower loses in
variation by the sparing fertility of the cross-fertilised flowers, it may
gain in the number of seeds which the cleistogamous flowers pro-
duce. Cleistogamous flowers likewise tend to economise pollen.
Whilst 400 pollen-grains may serve the purpose of close or self-
fertilisation in Oxalis, or even 100 grains in some violets, three-and-
a-half million grains may be produced in the insect-fertilised flowers
of the peony, and many millions in the case of wind-fertilised
flowers, whose pollen, like that of the firs, has to be distributed over
immense areas of land.
There appears, therefore, to be a proportion of plants in which
the existence of stamens and pistil in the same flower — the normal
condition of matters in ordinary plants — is meant to and does
secure the fertilisation of the ovules by the flower's own pollen.
Why, then, seeing that the presence of correlated stamens and pistils
in each flower appears to be a common condition of plant life, do
we assume that not self-fertilisation but the opposite process — cross-
fertilisation — is the rule of nature ? The reply to this query involves
more than one important consideration. Let us briefly endeavour
to find a convenient starting-point in the familiar flower which Peter
Bell despised, and which, to minds of utilitarian type amongst our-
selves, is but a primrose still, and " nothing more."
If we study the structure of the primroses we may gather in a
bed of these flowers, it will be found that the blossoms obtained from
one set of plants will vary in certain respects from the flowers of the
other and neighbouring plants. There is no difference in appearance
or in outward aspect between the primroses, because the differences
referred to affect chiefly the position of the stamens and the length
of the style (or " neck " of the pistil) in each variety. But we may
readily discover that, selecting any one primrose plant, all the
flowers of that plant will be either long-styled (Fig. 208 A) or short-
styled (Fig. 208 B), and will not exhibit a mixture of the two
varieties. " The two kinds of flowers," says Mr. Darwin, speaking
of the long and short-styled cowslips, which form a closely allied
species to the primroses, " are never found on the same individual
plant ; " and he also remarks that he has never met with any
THE FERTILISATION OF FLOWERS. 321
transitional states between the two forms growing in a state of
nature. The cowslips and other allies of the primrose exhibit a like
disposition of parts. Thus, when we slit one of the primroses
longwise, we see that the stamens (Fig. 208 B, a) are placed high up on
the petals or near the top of the corolla, and the style is comparatively
short. In the other variety (Fig. 208 A, a), the stamens are placed far
down within the tube of the corolla, whilst the style is so long that
the stigma (sf) appears to block up the entrance to that tube, and
reaches to the top of the petals. Thus we speak of " short-styled " (B)
and " long-styled " (A) flowers in primrose and in all other plants in
which these conditions occur; whilst, popularly, the short-styled
forms are called " thrum- eyed," and the long-styled ones "pin-eyed."
Such a disposition of stamens and pistil also occurs in Pulmonaria
offirinalis, in Linum perenne, and in other plants, which are hence
called dimorphic, i.e. having two forms of flower. And in some
plants (e.g. Oxalis and the Spiked Loosestrife or Lythrum Salicaria),
three varieties of flowers are known, and these latter are named
trimorphic in consequence.
Returning to our primroses, we find that the pollen-grains of the two
forms of flower differ in size. Those of the long-styled primroses (A)
are smaller than those of the short-styled flowers. Mr. Darwin remarks
of the pollen-grains of the latter flowers, that " before they were soaked
in water, they were decidedly broader, in proportion to their length,
than those from the long-styled ; after being soaked, they were rela-
tively to those from the long-styled as 100 to 71 in diameter, and more
transparent." Mr. Darwin also compared these two forms of flowers
in other respects. He found that the seeds of the short-styled
flowers " weighed exactly twice as much as those from an equal
number of long-styled plants/' the short-styled being the more pro-
ductive of the two forms. As final facts concerning the differences
between the two varieties, it may be noted that the stigma or head of
the pistil in the long-styled form is more distinctly globular and
roughened on its surface than that of the short-styled primroses-;
whilst the stigma in each form stands nearly, but not quite, on a level
with the anthers of the opposite variety.
What can be affirmed, as a matter of observation, to be the mean-
ing and purpose of this diverse arrangement of stamens and pistils in
these plants ? Meaning it must have, and that one which is closely
bound up with the history of the species. So indeed, it was found,
when, through Mr. Darwin's researches, contributed to the Linnaean
Society's Transactions in 1862, it was clearly demonstrated that the
arrangement in question had. reference to the cross-fertilisation of the
primroses and of all other plants in which a like diversity of structure
was found. Mr. Darwin then pointed out that the structure of the
primrose was eminently adapted to favour the visits of insects as aids in
Y
322
CHAPTERS ON EVOLUTIOtf.
procuring the fertilisation of the long-styled flowers by the pollen of thei
short- styled flowers, and rice versa. Such an interchange of pollen is
accomplished in a manner readily understood. Suppose that an insect
— such as a humblebee — first visits a "long-styled "flower (Fig. 226, 2),^
drawn to the primrose in search of the nectar which this flower, the"
cowslip, and other members of the genus secrete in quantity. The
proboscis of the bee will be thrust into the tube of the corolla, and
in the act of nectar-gathering it will unconsciously dust its proboscis-
with pollen nearer the tip of that organ than the base. Suppose
next that the bee, with its pollen-laden proboscis, flies to another
primrose plant of the " short-styled " variety. The proboscis, inserted
therein as before, will come into contact with the low-lying stigma,
and upon this surface will be left the pollen which was gathered from
the stamens of the long-styled flower. Thus interchange the first is
accomplished. But when visiting the "short-styled" flower (i), the
bee's proboscis, coming in contact with the stamens (placed at the top
of the flower), is dusted near the base with short-styled pollen. Hence
the next visit paid to a " long-styled " flower results in the placing of
pollen from the short-styled flower upon the stigma of the long-styled
primrose. The stigma of the latter is placed, as we have seen, at the
top of the flower (Fig. 226, 2), and it
is thus well calculated to meet the
base of trie bee's " tongue," which has
been dusted with short-styled pollen.
Interchange the second is thus ac-
complished, and the cross-fertilisation
of the primrose race becomes a
matter of well-nigh absolute certainty.
As indicated in the annexed figure,
from Mr. Darwin (Fig. 226), the "le-
gitimate " (A) fertilisation is that which
occurs when pollen from the long-
styled form is placed on the short-
styled pistil, and vice versa. The
" illegitimate " fertilisation (B) is self-
fertilisation in either case; namely, through the pollen of either
flower being placed upon its own stigma. Mr. Darwin's own words,
applying to his observations on the cowslip (Primula veris), may
be read with interest as applying likewise to the similar arrange-
ment in the allied primrose. After noting that humblebees, and
likewise moths, visit these flowers, Mr. Darwin says : " It follows from
the position of the organs (anthers and stigmas) that if the
proboscis of a dead humblebee, or a thick bristle or rough
needle, be pushed down the corolla, first of one form and
then of the other, as an insect would do in visiting the two forms
F;c. 826.
FERTILISATION OF PRIMROSE (Darwin).
THE FERTILISATION OF FLOWERS. 323
growing mingled together, pollen from the long-stamened form,
adheres round the base of the object, and is left with certainty on
the stigma of the long-styled form ; whilst pollen from the short
stamens of the long-styled form adheres a little way above the
extremity of the object, and some is generally left-on the stigma^ of
the other form." Mr. Darwin is also careful to note that " self-fertilisa-
tion " must occasionally occur in these flowers, through " an insect, in
withdrawing its proboscis from the corolla of the long-styled form,"
leaving pollen from the flower on that flower's own stigma. Such a
result will occur most frequently in the case of the short-styled
flowers, as may be experimentally demonstrated, and small insects,
such as those belonging to the genus Thrips, wandering aimlessly
about within the flower may likewise be the means of inducing self-
fertilisation. But, as if in anticipation of such defeat of her clear
intent and purpose, we find a very significant observation brought to
light in the fact that even if a flower's own pollen be placed on its
pistil, cross-fertilisation may yet take place, inasmuch as pollen from
a different form of flower seems to be capable of obliterating the
effect of the flower's own pollen, " even," adds Mr. Darwin, " when
this has been placed on the stigma a considerable time before." An
experiment of very crucial nature supplies an instance of the pre-
potent effect of foreign pollen over a flower's own. On the stigma
of a long-styled cowslip Mr. Darwin placed " plenty of pollen from
the same plant." After a lapse of twenty-four hours he added pollen
" from a short-styled dark-red polyanthus, which is a variety of the
cowslip. From the flowers thus treated thirty seedlings were raised,
and all these, without exception, bore reddish flowers ; so that the
effect of pollen from the same form, though placed on the stigmas
twenty-four hours previously, was quite destroyed by that of pollen
from a plant belonging to the other form."
The philosophy of primrose-existence can hardly be said to be in
any sense comprehended through the mere knowledge of the con-
trivances which exist' in that flower for the prevention of self-fertilisa-
tion and the favouring of the opposite process. On the contrary, the
philosophy which carries with it the understanding and appreciation
of the system and order of nature is only discernible when, firstly,
we step forth more fully " into the light of things," and when,
secondly, we discover, from such wider views of flower-life, the
advantages gained and the ends served b^y the processes under con-
sideration. Hence, for the present, we may turn with profit from the
polity of a primrose to discuss some analogous feature in that wider
realm of flowers to which the primrose and its kind may fitly intro-
duce us. After such survey we may, with additional likelihood of
arriving at just conclusions respecting the philosophy of plant-life,
return to the Primula and its lessons once more.
324 CHAPTERS ON EVOLUTION.
It has been already remarked that "self-fertilisation" is the excep-
tion and " cross-fertilisation " the rule of plant-nature. At any rate,
the cases where " cross-fertilisation " is obviously the process which by
manifold contrivances nature seeks to further and effect, increase in
number year by year. Although self-fertilisation does occur, and
is* a possibility even with normally cross-fertilised plants, yet the
whole drift of modern botanical teaching tends towards the recogni-
tion of the mutual interchange of pollen betwixt related flowers as the
normal way of plant-reproduction. Nor do the comparative results —
tobe hereafter detailed — of cross and self-fertilisation in the leastdegree
vitiate these conclusions. On the other hand, every fact of botany
dealing with ascertained results of the one method of fertilisation, as
compared with those obtained by the other, testifies to the enormous
gain, possible and actual, to the plant- creation through the effects of
cross-fertilisation. The presence of so many different methods
whereby this end is secured, constitutes an eloquent fact in favour of
the supposition that the normal way of plant-life undoubtedly lies in the
direction of pollen-interchange, as a necessity for energetic develop-
ment and for the full fruition of the races and tribes of plants
that people earth's firmament.
Within the limits of the present chapter it would be impossible to
enter into the discussion of those peculiarities of insect structure which
have been developed or modified in turn like the forms of flowers for
the due performance of the work of cross-fertilisation. It may suffice
at present to simply point out that the conformation of the legs of
certain insects, as well as the form of the mouth-parts, and even the
hairiness of body or the reverse conditions, all bear witness to special
adaptation in different insects for the fertilisation of special flowers.
Certain insects are known to confine their visits to special plants —
some to one species of plant only; and probably, when this department
of the subject is more fully and completely studied, the number of
cases in which insect-visitation is of a rigid or exclusive kind, will be
largely increased.
The two chief methods of cross-fertilisation, or, in other words,
of flower-fertilisation at large, are thus : (i) by insects, or more rarely
birds, snails, &c ; (2) by the wind; whilst (3) pollen may be floated
on water from one plant to another, as in the case of Vallisneria
spiralis. Botanists term plants fertilised by insects " entomophilous,"
and those fertilised by the wind " anemophilous." Some plants, e.g.
common rhubarb (Darwin), and some species of Plantago(Delpino and
H. Miiller) exhibit an intermediate condition, in that they may be fer-
tilised in either way. The wind-fertilised plants, as an " invariable
rule," according to Darwin, possess small and inconspicuous flowers,
whilst the insect-fertilised flowers, as might be expected, are conspi-
cuous, or, if not brightly coloured, are strong smelling. Moreover,
THE PER TILISA TION OF FL 0 WERS. 325
there are certain conspicuous differences between the pollen and its
quantity, and between the form of the stigma, &c., in wind-fertilised
and in sect- fertilised flowers.
The pollen of the wind-fertilised plants is produced in far greater
quantity than that of the insect-dependent flowers. Then, also, the
former flowers open before the leaves are in full growth, in order that
the clouds of pollen may gain easy access to the pistils ; whilst their
stigmas are usually branched and bending (e.g., alder, wheat, &c.), so
as the more readily to intercept and detain the pollen in its wind-
flights. Allusion has already been made to the showers of pollen
emitted by coniferous trees, and it may be added here that bucketfuls of
pollen from conifers and grasses are occasionally swept off the decks
of vessels off North American coasts ; whilst North American lakes may
be covered over a considerable area of their surface by the yellow pollen
of the pines. Most of our cereals are presumably wind-fertilised ;
and the importance of light breezes in the early summer may there-
fore be a matter of consideration in respect of the full ears of
autumn. Hooibrenk and Kcernicke, in their practical suggestion,
carried out in Belgium and Germany, of drawing a rope across the
full-flowered ears so as to distribute pollen and cross-fertilise the
plants, seem therefore to have imitated nature's method. The question
of the wind-fertilisation of the cereals, it may be remarked, however, is
at present an open one, since some botanists elect to believe that the
wind-distributed pollen is simply the excess or useless pollen remaining
after fertilisation has been accomplished ; the actual agency in
scattering abroad the fertilising dust being said to be the sudden
extension and elasticity of the stalks of the stamens.
That cross-fertilisation is the rule of nature, is a fact amply
demonstrated by the well-nigh endless contrivances in flower-
structure, form, appearance, and function, through which the inter-
change of pollen is brought about. Let us briefly glance at the out-
lines of such a study. Allusion has already been made to cases in
which a separation of stamens and pistil takes place as a normal
condition of many plants. Such separation may proceed to the
extent of placing stamens in one set of flowers, and pistils in another
set on the same plant ; or it may be illustrated by the more complete
isolation of these organs, so that in the latter case we find all the
flowers on one plant to be " staminate," and all the " pistillate "
flowers to be borne on another plant. The lesser nettle, for instance,
has its stamens and pistils in different flowers on the same plant, as also
have the oak (Figs. 212, 213), melon, cucumber, maize, hop, hazel,
carex, &c. The greater nettle, on the other hand, bears on one plant
none but staminate flowers, and on another plant none but pistil-
bearing flowers ;• whilst hemp, willow (Figs. 210, 211), the variegated
laurel (Attatba Japonica\ palms, &c., also illustrate the complete
326 CHAPTERS ON EVOLUTION.
separation of stamens and pistil. Other conditions, more or less uniting
these dispositions of the stamens and pistil, may be found in flowers.
In a daisy — which is a collection of flowers — we find the outer or white-
florets to possess pistils but no stamens, and the yellow and central
florets to possess both stamens and pistil. We can readily discern
that all such arrangements secure pollen of essentially foreign kind for
fertilisation. Self- fertilisation is, in fact, impossible in such cases as
those just described ; and some very curious facts are found in botanical
archives concerning the difficulties experienced in obtaining " seeds "
where one of the necessary elements — usually the pollen — for fertilisa-
tion was absent. The variegated laurel presents a case in point. The
first specimen of this species introduced from Japan was a pistillate
or female plant, and could produce ovules from its flowers, but no
"seeds" ; inasmuch as, no pollen from another and staminate plant
was forthcoming. The plant was largely reproduced from slips alone
until within comparatively recent years, when staminate plants being
imported, pollen was then forthcoming for the production of seed.
The Egyptians have long been in the habit of bringing palm-branches
bearing stamens from the desert, in order to fertilise the domesticated
pistillate or fruit-bearing palms grown at home. Tliis necessary pro-
cess was frustrated in 1808, when the Frerch occupied Egypt, and
when the stamen-laden branches could not, in consequence of foreign
invasion, be procured. In the well-known Vallisneria spiralis, a
water-plant of Southern Europe, which, like the willow and palm, has
stamens and pistils on separate plants, the pistillate flowers are borne
to the surface at the proper period by the relaxing of a spirally coiled
stalk on which they are supported. The stamen-bearing flowers, on
the contrary, are borne on short stalks, and, becoming detached
therefrom, float to the surface of the water. There they scatter their
pollen, which reaches the pistillate flowers, and the latter being fer-
tilised, are drawn by their stalks once more beneath the water, where
the seeds mature and the fruit in due course ripens.
The present is perhaps a fitting stage of our inquiries to remark
that the tendency towards cross-fertilisation in nature is nowhere
more strongly marked than in cases where a plant is utterly infertile
with its own pollen, but perfectly fertile when impregnated with
pollen from another plant of the same species, or, in some not-
able instances, from an entirely different species of plant. Species
of passion-flowers have been found sterile with their own pollen,
although " slight changes in their conditions, such as being grafted
on another stock, or a change of temperature, rendered them self-
fertile." More extraordinary still, however, is the knowledge of the
fact that the pollen of some orchids actually acts like a poison if
placed in what one would have deemed the most natural position for
it, namely, on their own stigmas. Such facts as these entirely alter
THE FERTILISATION OF F LOWERS.
y-i
the former conceptions of a " species," as a group the members of
which were fertile inter se, but infertile with members even of nearly
allied species ; and such knowledge supplies a wholesome corrective
to the theory that species are separate, independent, and distinct
entities both as to origin and after-relations.
If nature contrives by such means to effect cross-fertilisation, there
exist ample fields for the demonstration of a like result in other and
FIG. 227.— ARUM, OR CUCKOO PINT.
FIG. 228.— CARNATION, SHOWING THE RIPE PISTIL
very varied fashions. In a very large number of flowers, for instance,
the stamens ripen and discharge their pollen before the pistil is ripe, or
the ovules ready for fertilisation. In other cases, but more rarely, the
pistil ripens before the stamens. The former case is illustrated by
most species of geraniums, pelargoniums, by harebells, and other
Camfanulacece (Fig. 214), by many umbelliferous plants, by carnations
328 CHATTERS ON EVOLUTION.
(Fig. 228), sweetwilliam, and allied plants, and by many plants of the
daisy and dandelion type (Composite^). The latter case (of the earlier
ripening of the pistil) is illustrated by the rib-grass (Plantago) of the.,
roadsides, by the cuckoo pint (Arum, Fig. 227), and other plants.
One or two familiar illustrations will suffice to show how clearly and
effectually nature carries out her intention of securing cross -fertilisa-
tion by different periods of ripening in stamens and pistils.
The pink, or carnation (Fig. 228), in its first condition, exhibits the
case of a plant possessing stamens alone. These organs ripen, discharge
their pollen (which is carried by insects to flowers whose pistil may
then be ripe), and then die away. After the stamens, and with them all
chances of self- fertilisation, have disappeared, the pistil matures, and
its style and stigma develop fully (Fig. 228). It is then fertilised by
foreign pollen — that is, by pollen from a pink whose stamens are at
that period in full development. So also is it with thyme, whose stamens
ripen first ; and with the Canterbury bells, harebells, and like flowers.
In the campanulas (Fig. 214) anthers and pistil are closely related
before the flower opens ; and when the anthers discharge their pollen,
it is shed upon the style or stalk of the pistil ; and only after the
stamens have shrivelled up and their pollen has been carried away
by insect agency to other " bell-flowers," does the pistil develop
fully, and its three conspicuous stigmas open out so as to receive
pollen from another and, at that period, pollen-producing flower. In
cases like the preceding, therefore, it is evident nature does not
intend that the flower's own pollen should fertilise its ovules.
The opposite case occurs in the Plantago, where the elegant little
pistils ripen first, and where the stamens do not mature until fertilisa-
tion of their flower has been accomplished by foreign pollen. In the
cuckoo pint (Arum, Fig. 227) there is also witnessed the ripening of
the pistil before the stamens. Every one knows this plant, with its
sheathing leaf (a), and the central stalk (b] bearing the " flowers."
The anthers are placed above the stigmas ; hence it would seem, at
first sight, as if nature intended that their pollen should fall downwards
and fertilise the plant's own ovules. But the pistils ripen long before
the stamens. When the latter are mature, the pistil has been already fer-
tilised. Hence the pollen, it is evident, must be intended for fertilising
other pistils of the species, unless, indeed, we can maintain that
nature, Like Homer, " sometimes nods." The pollen in this case falls
to the bottom of the sheathing leaf, where it might well seem to be
lost entirely to the outer world. Small insects, however, in due
course arrive upon the scene. Entering the cavity of the leaf readily
enough, on the principle Qifacilis deseensus Arerni, they find the reverse
process, revocare gradum, to be impossible. By an arrangement of stiff
hairs, pointing downwards, which they readily enough brush aside on
entering, they are prevented from escaping out of the flower. Hemmed
THE FERTILISATION OF FLOWERS.
329
in by this natural chevaux de frise, as in an eel-trap, we may find
inside an Arum a hundred or two small insects in durance vile.
Here, however, they find nourishment in the honey- secretion, and
here they, in due time, work out nature's will, in that they become
laden with the discharged pollen. So that when the opposing
hairs shrivel and wither away, the insect-crowd disperses itself, and
its units, undeterred by reminiscences of their imprisonment, entering
other Arums in which the stigmas have just ripened, duly cross-fertilise
the latter.
Risks of fertilisation being omitted altogether are not lost
sight of in the economy of nature, and such contingencies are
often duly provided for in re-
markable ways. In Myosotis
versicolor (Fig. 229), for instance,
there is an evident intent to pre-
vent self-fertilisation, from the
fact that the pistil (si) projects
far above the stamens (a) in the
young flower (A), and is therefore
a likely object to be touched by
an insect which has come from
another " Forget-me-not," as this
flower is often named. But
such an arrangement, dependent
on insect visitation, might be rendered futile if no insect happened to
alight on the flower. In due time, however, the corolla is seen to
increase in length ; as it grows upwards, the stamens (a) are carried
upwards (B), until, in due time, they attain the level of the stigma (st),
and by discharging their pollen upon it will fertilise the pistil, if it
has not already undergone that process from a foreign source. Such
a contrivance appears tantamount to the declaration, on the part of
plant-nature, that, although cross-fertilisation is sought and preferred,
yet self-fertilisation is better than none.
Besides the means just noted, there exist a large number of
expedients in flowers for securing fertilisation ; these latter contri-
vances relating to the form and shapes of flowers, to the special posi-
tions of its organs, and to adaptive details of flower structure. The
polity of a primrose, in the peculiar situation of its stamens and pistils,
as adapted to secure cross-fertilisation, falls under this latter division
of floral expedients ; and so, also, would all these peculiarities of sta-
mens whereby the discharge of pollen in a fashion adapted to avoid
self- fertilisation is secured. In some flowers (e.g. Parnassia), as the
five stamens ripen one after the other (before the pistil, as it happens),
each anther is laid back downwards, so to speak, on the stigma or top
of the pistil, so that the pollen escapes by the side farthest from the
FiG. 229. — MVOSOTIS IN ITS EARLY (A) AND
LATER STAGE (B).
330
CHAPTERS ON EVOLUTION.
stigma, and self-fertilisation thus becomes well-nigh an impossibility.
But even the form and shape or colour of a corolla or blossom may
be adapted either of itself, or when associated with other expedients,
to secure cross-fertilisation in especial conjunction with insect aid.
It has been pointed out that every flower of peculiar shape is
cross-fertilised by insects. As notable instances of this fact may be
cited the peas, beans, dead nettles (Fig. 230), sage (Fig. 235), salvia
(Fig. 232), orchids, the peculiar shape of
whose flowers, as well as the special arrange-
ments of stamens and pistil, are correlated
in the most exact positions to compel insects
to visit special parts of the flower, and thus
to ensure the exact performance of the work
of cross-fertilisation. Even the distribution
of colour on a flosver, and the particular
spots or dashes which attract our notice, are
guides and fingerposts directing insects to
the honev. Sprengel, of old, called these
special colour-guides macula indicantcs, and
Mr. Darwin remarks, that Sprengel's ideas
seemed to him "for a long time fanciful."
But the fact that these markings are most
commonly met with on unsymmetrical or
irregular flowers, the entrance into which
would be more likely to puzzle and con-
fuse insects than the apertures of symmetrical flowers, weighs in favour
of dashes and spots of colour being truly directive in function.
Mr. Darwin further remarks that, in the common pelargonium,
the marks in question, borne on the two upper petals, are clearly
related to the position of the " nectary " or honey-store of the flower;
for when the flowers vary so as to become regular, and lose their
nectaries, the marks disappear. When the nectary is in part undeve-
loped, only one of the upper petals loses its characteristic mark. It
is true that humblebees are known to bite through the petals of
flowers, and to surreptitiously suck the honey through the apertures
thus made, and even hive-bees learn to utilise the holes made by
their larger brethren. But, notwithstanding this latter fashion ot
securing stolen sweets — a method indicative of a certain power of
development in bee-intellect — there can be little doubt that originally
to bees, as at present to insects who walk in the trodden paths
of their race, the colour-marks and special hues of flowers are
serviceable, as Mr. Darwin remarks, in guiding insect visitors
rapidly and without loss of time to the store of sweets, and in thus
enabling them to visit a larger number of flowers in a given time
than would otherwise be possible. Sir John Lubbock remarks, that
FIG. 230.— DEAD-NETTLE IN
SECTION.
THE FERTILISATION OF FLOWERS.
331
he did not realise the import of these markings in flowers until he
saw how much time bees lost, if honey which has been put out for
them is even slightly moved from its usual place. Whilst it forms an
allied subject of the most interesting description, to speculate upon
FIG. 231.— FLOWER AND STAMENS OF SALVIA.
the remarkable changes in colour which some plants undergo, and
which, like the times of opening and closing, are doubtless related
to the visits of insects. Thus, we know of some flowers (Hibiscus]
which are white in the morning, of a pale rose hue at mid-day, and
exhibit a bright rosy red colour in the evening. Many flowers
change their hues as the petals wax old and tend to fall off ; and
ti
FIG. 232.— FERTILISATION OF SALVIA.
that such alterations of hue have a reference to insect-visits, and
attract, it may be, insects of different tastes and structure at different
periods, is by no means a far-fetched speculation. The presence of
bright colours in flowers has been shown to bear an important. rela-
tion to fertilisation, and necessarily, through this latter process, to
the development of such species of plants. Bright hues are, as a
332
CHAPTERS ON EVOLUTION.
rule, associated in flowers with a faint development, or even a want
of scent Insects being attracted by one quality or the other, the
presence of scent would be useless where colour is well developed.
If we compare such flowers as pelargoniums, convolvulus, pansy,
fritillary, &c., which are conspicuous and bright tinted, but scentless,
FIG. 233. — SECTION OF FUCHSIA.
FIG. 234.— FUCHSIA.
with the primrose, lily of the valley, rose, and hyacinth, which are
not so conspicuous, but emit powerful odours, we can realise the
principle of nature's economy in avoiding over-lavish provisions for
insect-attraction. The correlation between flower and insect is
even more strongly marked, however, when we discover that flowers
which are fertilised by night-flying moths are usually of white
colour, so as to appear conspicuous at night, and may further emit
their odour only or chiefly at night. Such flowers as Daphne and
Hesperis — obscurely coloured, as it happens — attract insects solely
by their powerful odour. Nageli's experiment of scenting artificial
flowers with essential oils was followed by the attraction of insects
" in an unmistakable manner."
The description of a few of the most typical cases in which
cross-fertilisation is found may fitly conclude the more exact
consideration of the present topic, and preface the abstract philo-
THE FERTILISATION OF FLOWERS.
333
sophy which directs attention to the hearing of the facts of fertilisation
on the constitution and regulation of the world of life at large.
FIG. 235. — FLOWER OF SAGE.
FIG. 236.— FLOWER OF PEA DISSECTED.
A very interesting mechanism for effecting cross-fertilisation is seen
in the case of flowers which, like the peas, beans, and their leguminous
neighbours, present a very characteristic form of 'blossom. These
flowers possess ten stamens (Fig. 237),
nine united to form a bundle, and one
remaining single. The flower is peculiar
in that it consists of one very large petal
(Fig. 236, a), behind or above, two at
the sides or " wings " (b b], and two united
to form the boat-shape " keel " (c) below.
When an insect alights on the side petals
or "wings" (Fig. 237, at], the keel (c)
is thereby pressed downwards. The
pollen of the stamens (st ) and the tip of
the pistil are made to project, so as
respectively to dust the insect's breast
with pollen, and to receive therefrom the
foreign pollen gathered previously from
another flower. Sprengel himself noticed
that the union of the stamens favoured this
conjoined action. When the weight of the
insect's body is removed, the stamens and
pistil resume their normal position. If,
remarks, "the two ends of the wings (in
between the finger and thumb, and pressed down so as to imitate
the effect produced by the pressure of an insect, the keel is
FIG. 237. — SECTION OF PEA.
as Sir John Lubbock
a sweet-pea) be taken
334 CHAPTERS ON EVOLUTION.
depressed with the wings, while the pistil and stamens are thus
partly uncovered." In the bean, when the wings are similarly
pressed down, the stigma of the pistil, and then the " beard " of
the style, laden with pollen, project from the keel, which is of
coiled conformation. When a bee exercises the necessary pressure,
the pistil of the bean will first strike its body and become fertilised
by fresh pollen, whilst the pollen-laden style of the flower will,
secondly, leave fertilising matter on the bee's body for application to
the pistil of another flower.
A dead-nettle (Fig. 230), with its irregular flower, presents a favour-
able and readily understood example of the manner in which a special
form of flower is adapted for the special insect which cross-fertilises
it. A bank of dead-nettles is to humblebees what a country-fair is
to juveniles, in that it presents the insects with a store of sweets spe-
cially intended for their delectation. In shape, the sage (Fig. 235),
or dead-nettle flower, as everyone knows, exhibits a wide mouth,
bounded by a very much arched upper lip, whilst a divided lower lip
is also conspicuous enough. The green cup-like calyx has its sepals
united, whilst the very irregularly shaped corolla is composed of united
petals. There are four stamens — two long and two short — the fifth
stamen of botanical expectation being abortive. The stamens are
peculiar in position, inasmuch as they lie along the arch of the petals
(Fig. 230, st\ instead of surrounding the pistil. The style is very
long, and forked at its tip (jg), and it moreover depends below the
anthers as in fuchsia (Fig. 233). The honey for which the bees visit
the dead-nettle is situated far down within the flower, and if we make
a vertical section of the corolla, we shall find a circle of hairs (Fig.
230, h] placed inside the petals at their lower portion. Now, in
what special fashion is the mechanism, thus described, brought into
play in the fertilisation of the dead-nettle tribe ? The reply may
be found in a simple study of a dead-nettle on a warm summer's day,
when insect-life and the blossoming of flowers together seem to attain
the acme of activity and development. The bee approaches the
flower, and finds in the lower lip of the blossom a convenient door-
mat on which to alight. Here the insect gains a point d'appui for the
movement of the proboscis, which probes the depths of the corolla so
as to reach the nectar, and easily thrusts aside the circlet of stiff hairs
presenting an impassable barrier to a less robust as well as uninvited
insect guest. The acts of the insect, in so far as the work of honey-
getting is concerned, end thus. Meanwhile, however, it has likewise
been performing its unconscious part in the fertilisation of the flower.
The position of the stamens under the hooded petal has been noted.
Such a position assures two results — firstly, that the stamens shall
be brought in contact with the bee's body ; and secondly, that the
pistil shall likewise touch the insect in order that foreign pollen,
THE PER TILISA TION OF FL 0 WERS. 335
obtained from a previously visited dead-nettle, shall be deposited
on the stigma. The stigma, as we have seen, depends below the
anthers. Hence it must be the first object with which the bee
comes in contact. Fertilisation by the foreign pollen is thus secured
before the stamens have dusted the insect with the flower's own
pollen. As Dr. Ogle has pointed out, the position of the stamens
doubtless facilitates in a marked degree the proper placing of pollen
on the insect's body. If the anthers had lain side by side, the bee's
head might have been dusted on parts which do not touch the
stigma as the insect enters the flower ; whilst even the eyes of the
bees might have become disadvantageously covered with pollen.
There is, in short, the closest possible correlation between the structure
of the flower and the form and size of the insect which fertilises it.
Such correlation is exhibited in, if anything, an increased degree in
the genus Salvia, belonging to the dead-nettle order (Labiatce), also
including the sage (Fig. 235). Salvia (Figs. 231, 232) attracted the
notice of Sprengel — Rector at Spandau — who, in his *' Das entdeckte
Geheimniss der Natur im Bau und in der Befruchtung der Blumen"
("The Secrets of Nature in the Structure and Fertilisation of Flowers"),
published in 1793, was one of the first to direct attention to the fact
that nature's law was " cross " and not " self-fertilisation " — or, as he
himself expressed it, " nature does not desire that any complete flower
should be fertilised by its own pollen." It is interesting to note that
a species of Salvia (S. splendens) occurring in the New World
appears to be cross-fertilised through the agency of humming-birds ;
these fairy-like birds thus discharging in this case the functions of
the insects to which some species approach so nearly in size. The
trumpet creepers (Tecoma radicans) and trumpet honeysuckle
(Lonicera sempervivens) are probably fertilised by moths and by
humming-birds as well.
In Salvia officinalis (Fig. 231, A), the general form of which
closely resembles that of the dead-nettle, the stamens ripen before
the pistil ; and as, moreover, the stigma (/) is placed above the
anthers (a), self-fertilisation is an impossibility. When, however, the
stamens have shed their pollen, they shrivel up, and the pistil as
it ripens develops the stigma, so that it elongates, curves down-
wards, and thus assumes a position (Fig. 232, B, st) in which it
cannot escape contact with the back of the bee entering the flower
(Fig. 232, A). The insect's back, it may be noted, is exactly
that region which the ripe stamens in a younger and necessarily
different flower will have dusted with foreign pollen. But the
economy of Salvia includes yet other appliances for more effectually
securing fertilisation by the insect. There are but two well-deve-
loped stamens (Fig. 231, B) in the flower. These organs have
widely separated anther-cells (a, a') ; and when in an undisturbed
336
CHAPTERS ON EVOLUTION.
condition, each stamen is seen to consist of a stalk (the filament)
(/), to which another and movable stalk (the connective) (;;/), bearing
an anther-cell at each end, is attached. Only one of these anther-
cells (a1) is fully developed in each stamen. The connective (///),
like a swing-bar, can be pushed back-
wards on its axis so as to bring the
fully- developed or upper anther-cell (a1)
to a horizontal position (Fig. 231, C).
Such a result is actually brought about
by the bee. Thrusting its head into
the flower in the search for nectar, the
insect pushes before it the lower end
of the. swing-bar, and thus brings the
upper end of the bar with its ripe
anther (a1) in contact with its back
(Fig. 232, A). This latter region is
thus dusted with pollen, and when the
insect flies to another Salvia flower in
which the pistil is ripe, the stigma
(Fig. 232, B j/), as we have seen, will in
due course receive the pollen through
contact with the back of the bee.
A single paragraph only is permissible regarding the curious
details connected with the fertilisation of the Orchids, which possess
FIG. 238.— ORCHID FLOWER.
FIG. 239.— POLLEN MASSES OF ORCHID.
FIG. 240.— SECTION OF ORCHID FLOWER.
flowers (Fig. 238) of markedly irregular shape. The lip (//) in such
a flower as Orchis mascula, a common British species, is very broad ;
whilst the nectary to which bees desire admittance is extremely long
(Fig. 240,;?). The pollen forms two club-shaped masses (Figs. 239,
THE FERTILISATION OF FLOWERS. 337
240 a), each adherent to a disc (Fig. 22 j , //), which in turn lies within
the rostellum or cup (r~). When touched, the rostellum breaks
across, and thus allows the two glutinous discs (12, d} to become
exposed. When a bee visits this peculiar flower, it pushes its pro-
boscis into the nectary («) for the sake of the honey contained
therein. At the same time, the insect comes in contact with the discs
of the pollen-masses (Fig. 239), these masses becoming adherent to
the insect's head. A pencil pushed into an orchis detaches the
pollen-masses after the fashion of the insect's unconscious act.
At first, the pollen-masses remain erect like two abnormal horns
on the insect's head ; but gradually they assume a horizontal position,
so that the insect cannot fail to charge the next orchid-pistil it
enters with the pollen-masses. The stigma, or top of the pistil
(Fig. 238, st, sf), is so placed in these flowers that pollen-masses
borne on a bee's head are certain to strike this surface, and thus
fertilise the contents of the ovary. It is probable that as each
pollen-mass consists of several packets of pollen-grains, one mass
may contain material enough to fertilise several flowers ; each stigma,
through its viscid surface, detaching sufficient pollen from the mass
for its fertilisation. The admirable adaptation of flower to insect
and insect to flower, thus witnessed, is in no detail better exem-
plified than in the fact that the pollen-mass at first retains a vertical
and then assumes a horizontal position in the insect's head. So long
as the pollen- mass is vertical, fertilisation is impossible; and hence
the vertical position persists so long as the bee is engaged in
visiting the flowers of the plant from which it has derived pollen-
masses. Thus self-fertilisation is prevented ; so that, as Sir Joseph
Hooker puts it, by the time the horizontal position of the pollen-
mass is assumed, " the bee has visited all the flowers of the plant
from which it took the pollen, and has gone to another plant."
To enter into further illustration of the contrivances through
which the fertilisation of flowers is secured would be to encroach
on the province of the technical and practical botanist. Such
details are " writ large " in the pages of every botanical text-book.
In the works of Mr. Darwin — and especially in the " Fertilisation
of Orchids" — the reader anxious for further details may find a
perfect encyclopaedia of facts constituting a veritable romance of
botanical science. It, however, remains to us in the present instance
to point out the plain meaning of these virtually marvellous adapta~
tions of the plant-world to the work of cross-fertilisation, and to
note, as far as possible, the bearing of such a study upon the order
of nature regarded as a harmonious whole. It is a perfectly legitimate
supposition that if cross-fertilisation forms, as we have seen, such a
prominent feature of plant-life, that life must, in some very plain and
obvious fashion, benefit therefrom. And further, as plant-life is but
z
338 CHAPTERS ON EVOLUTION.
a part of organic nature, we may feel perfectly justified in supposing
that the conditions and results which cross-fertilisation tends to
evoke and produce, will harmonise in their tendency and direction
with the course of events through which the living universe has
been and is being moulded, developed, and evolved.
To the question, •" Why does cross-fertilisation appear to be
favoured by nature over self-fertilisation? " a plain reply is at hand in
a comparison of ihe results which accrue from each of these
processes. Mr. Darwin's laborious researches on the comparative
fertility of various species of plants when self- fertilised and cross-
fertilised, supply an answer to the foregoing question, by showing that
in every respect the cross-fertilised flowers yield more seeds, and
give rise to a stronger and more numerous progeny than the self-
fertilised. The reader who consults Mr. Darwin's " Forms of
Flowers " will find himself supplied with ample data in proof of the
advantages of cross-fertilisation. In the primrose, for instance, when
short-styled and long-styled flowers were crossed in what Darwin
calls "legitimate union," the result was invariably to produce a larger
number of seeds than when each form was fertilised by its own
pollen. Out of 12 long-styled primroses fertilised by short-styled
poHen, •!! good capsules (or ripe pistils) were produced, with an
average of 66^9 seeds per capsule; whilst 21 long-styled flowers,
fertilised by long-styled pollen, produced only 14 capsules, with an
average of 52^2 seeds per capsule. The cowslip gave a similar result ;
and the tendency towards greater vigour of offspring when cross-
fertilisation is employed, appears to be of the most general kind. In
some plants, indeed, cross-fertilisation is absolutely essential for the
mere continuance of the race ; so that this method of seed-production
is not merely accidental or advantageous, but absolutely necessary
for the continuance of the race. Most of the orchids illustrate this
state of -matters. The presence of humblebees is well-nigh an
absolute necessity for the continuance of the heartsease ( Viola
tricolor]} and the well-known case of the clovers may be cited as
highly characteristic of the benefits developed through cross-
fertilisation. Twenty heads of Dutch clover, protected by Darwin
from bees, yielded no seeds : whilst twenty heads growing exposed as
in a stare of nature, yielded 2,290 seeds. One hundred heads of red
clover protected from bees were absolutely sterile ; a second hundred
exposed yielded 2,700 seeds. The scientific demonstration of the
interdependence of living beings becomes in this fashion perfectly
clear. Carried out to its ultimate results, such demonstration
becomes' sufficiently startling. British brain and sinew depend
(according to a foreign estimate) on home-fed beef; whilst the quality
of that nutriment is said to be dependent upon the clover on which
the ox subsists. But clover owes its continuance to humblebees ;
THE PER TILISA TION OF FL 0 WERS. 339
humblebees in turn are killed by field-mice, whilst cats extirpate the
rodents. As old maids conserve the feline race, it is alleged that
the continuance of the British intellect is dependent upon such
conservation — so that a scientific justification of spinsterhood is thus
rendered possible.
Sprengel laid down the axiom, already mentioned, that " Nature
does not wish any complete flower to be self-fertilised." Darwin in
turn improves upon this dictum in his assertion that " Nature
abhors perpetual self- fertilisation." That " cross-fertilisation is
generally beneficial, and self-fertilisation injurious," is thus a stable
result of botanical investigation. This result may not enable us
fully to comprehend that "law within the law" which regulates the
well-being of the plant-world ; but it may at least lead us plainly
enough to a nearer fact of life — namely, that there exists in nature
an innate tendency to variation and change, and that by furthering
the fullest possible development of seeds, as well as by the cross-
fertilising of plants, there is being illustrated that tendency to evolve
new varieties and species on the existence of which the very idea and
possibility of evolution depends. The tendency to produce a more
numerous offspring gives naturally a larger number of individuals for
the exhibition and operation of the laws of variation. The process of
cross-fertilisation itself produces another tendency to variation ; and
as such variation is the " key-note " of evolution, it is more than
interesting to find the conditions of plant-life in such a marked
manner contributing to the differentiation of the species.
The whole array of features embraced in a study of flower-fertilisa-
tion forms simply a mass of evidence that the production of new races
and varieties, and, through these, of new species, is part and parcel of
nature's constitution. On any other supposition, the extraordinary
array of contrivances favouring cross-fertilisation and the initiation
of variations, is meaningless and utterly inexplicable. The facts of
fertilisation, like the stages of development, present us with un-
impeachable evidence in favour of the evolution of new races by the
modification of the old. Even if a fact here or a detail there may
seem to weigh against the theory of development, it must be borne in
mind, firstly, that defects and gaps in our knowledge are still realities
of biological science ; and, secondly, that the general — and in this
case the immensely overwhelming — probability of nature's and life's
methods testifies to evolution as the true way of creation. Mr. Darwin
succinctly enough says, that his experiments on intercrossing show
that "with animals and plants a cross between different varieties, or
between individuals of the same variety but of another strain, give
vigour and fertility to the offspring ; and, on the other hand, that
close interbreeding diminishes vigour and fertility." And he also adds
that such facts " alone incline me to believe that it is a general law
z 2
340 CHAPTERS ON EVOLUTION.
of nature that no organic being fertilises itself for a perpetuity of
generations ; but that a cross with another individual is occasionally
— perhaps at long intervals of time — indispensable." Remarking
the strange feature of the stamens and pistil of most flowers being
placed closed together, " as if for the very purpose of self-fertilisation,"
and yet being " mutually useless to each other," Mr. Darwin says,
" How simply are these facts explained on the view of an occasional
cross with a distinct individual being advantageous or indispensable ! "
Thus, from the common ground that cross-fertilisation effects the
greatest good in nature — namely, the efficient increase of the race —
we may find many roads and ways for the recognition of further
effects of such action in favouring the operation of the conditions
that increase the species by variation and modification. The full
bearing of the subject may not be completely investigated for years to
come. Sufficient, however, is our present recognition of the fact that
in the work of flower-fertilisation lie the beginnings of those activities
and processes which herald now, as of yore, not merely the increase,
but the variation of species and the evolution of new forms of plants.
Certain matters bearing the same relation to our present subject
that the inevitable moral bears to the fable— albeit that they may
perchance be regarded as of somewhat superfluous nature — may fitly
be touched upon in closing this paper. Our notions of special ends,
aims, and contrivances in nature may in one way be enlarged by the
considerations which the phenomena of flower-fertilisation present to
notice. Under the operation of laws and conditions most of which
are as yet beyond our ken, we see insect acting upon flower, and
flower in turn reacting upon insect, until the interdependence in
some cases proceeds so far that the extinction of the insect means
the disappearance of the flower. But, whilst viewing the beauty of
form and hue exhibited in the plant- world as wrought out by laws of
development, and as accessory, or even primary, conditions in the
evolution of living beings, the new and higher aspects of the subject
bid us regard floral beauty as subserving other and higher uses than
those commonly assigned to it, namely, of ministering to the often
dull and inappreciative senses of man. We may detect a higher
purpose in plant life than is included in the yet too common idea
that man's delight and human interests exclusively determine and
rule — through what some are pleased to call "the beneficence of
providence " — the concerns of nature at large. The utilitarian
cry of " use " and " no use " is by no means extinct, even in these
latter days ; and the consideration of the ways and means in-
volved in the fertilisation of flowers must devolve a strong argument
against the homocentric idea that the beautiful in nature exists solely
for the behalf of man. Darwin says, " Such doctrines, if true, would
be absolutely fatal to my theory." But there is little fear that the
THE PER TILISA TION OF FL 0 WERS. 34 1
hypothesis in question can suffer from arguments familiar in days
when natural theology was strained and wrested to its destruction.
A truer and a higher use for the beauty of plants and of animals as
well is found in the special advantages which such beauty confers
upon the race. In the animal, beauty appears as an aid to the
propagation of the species, as it is in the plant ; and it is by
the action of insects that the beauty of flowers has been extended
and developed. The beauty of the blossom is in truth due to
the visitations of the insect races which' in the past have selected
its petals as a feeding-ground, and which have strengthened and
increased the flower race, thus favoured by a true " natural selection,"
in the universal struggle for existence. The higher ideas of nature
thus implanted, form no mean fruits of a study of the polity of prim-
roses and other common flowers. Such studies correct the idea that
this world is simply a huge workshop, filled with specially contrived
mechanical appliances for man's use and benefit, or a gaudy saloon
decked out with beauty for the indulgence of his senses. Those who
hold such views may not complain if their belief be logically ex-
tended to include the theory that fur-seals were specially created to
afford us seal-skin jackets, and humming-birds designed to trim the
hats of fair wearers in the fleeting fashion of the hour. So that, if
no greater excellence be traceable in the theory of evolution than
is involved in the correction of false notions of the aims and ends
of nature, those who pursue science-studies even to this extent will
reap a rich harvest of rational ideas concerning the true ordering of
this universe, especially within the domain of life itself.
342 CHAPTERS ON EVOLUTION.
XV.
THE EVIDENCE FROM DEGENERATION.
IT cannot be gainsaid that a survey of the fields of life around
us impresses one with the idea that the general tendencies of
living nature gravitate towards progression and improvement, and
are modelled on lines which, as Von Baer long ago remarked, lead
from the general or simple towards the definite, special and complex.
This much is admitted on all hands, and the ordinary courses of life
substantiate the aphorism that progress from low grades and humble
ways is the law of the organic universe that hems us in on every side,
and of which, indeed, we ourselves form part. The growth of plant-
life, which runs concurrently with the changing seasons of the year,
impresses this fact upon us, and the history of animal development
but repeats the tale. In the passage from seed to seed-leaf, from
seed-leaf to stem and leaves, from simple leaves to flower, and from
flower to fruit, there is exhibited a natural progress in plant existence,
which testifies eloquently enough, by analogy at least, to the existence
of like tendencies in all other forms of life. Similarly, in the animal
hosts, progressive change is seen to convert that which is literally
at first " without form and void " into the definite structure of the
organism. A minute speck of protoplasm on the surface of the egg
— a speck that is indistinguishable, in so far as its matter is con-
cerned, from the materies of the animalcule of the pool — is the germ
of the bird of the future. Day by day the forces and powers of
development weave the protoplasm into cells, and the cells, in turn, into
bone and muscle, sinew and nerve, heart and brain. In due season
the form of the higher vertebrate is evolved, and progressive change is
once more illustrated before the waiting eyes of life-science. But the
full meaning of most of the problems which life-science presents to
view is hardly gained by a merely cursory inspection of what may be
called the normal side of things. The by-paths of development —
more frequently, perhaps, than its beaten tracks — reveal guiding clues
and traces of the manner in which the progress in question has come
to pass. So, also, the side avenues of biology open up new phases of,
it may be, the main question at issue, and may reveal, as in the
present instance, an interesting reverse to the aspects we at first
deem of sole and paramount importance. For example, a casual
study of the facts of -animal development is well calculated to show
that life is not all progress, and that it includes retrogression as well
THE EVIDENCE FROM DEGENERATION.
343
as advance. Physiological history can readily be proved to tend in
many cases towards backsliding, instead of reaching forwards and
upwards to higher levels. This latter tendency, beginning now to
be better recognised in biology than of late years, can readily be
shown to exercise no unimportant influence on the fortunes of
animals and plants. In truth, life at large must now be regarded as
existing between two great tendencies — the one progressive and
advancing, the other retrogressive and degenerating. Such a view of
matters may serve to explain many things in living histories which
have hitherto been regarded as somewhat occult and difficult of
solution ; whilst we may likewise discover that the coexistence of
progress and retrogression is a fact perfectly compatible with the
lucid opinions and teachings concerning the origin of living things
which we owe to the genius of Darwin and his disciples.
A fundamental axiom of modern biology declares that in the
development of a
living being we may
discern a panoramic
unfolding, more or
less complete, of its
descent. "Develop-
ment repeats de-
scent" is an aphor-
ism which, as we
have seen, cultured
biology has every-
where writ large
over its portals. Re- ^
jecting this view of
development and its
teachings,the phases^
through which ani- "
mals and plants pass
in the course of their
progress from the
germ to the adult
stage present them-
selves to view as
simply meaningless
facts and useless freaks and vagaries of nature. Accepting the idea —
favoured, one may add, by every circumstance of life-science—much
that was before wholly inexplicable becomes plain and readily under-
stood. And the view that a living being's development is really a
quick and often abbreviated summary of its evolution and descent, both
receives support from and gives countenance to the general conclusion
FIG. 241.— DEVELOPMENT OF FROG.
344 CHAPTERS ON EVOLUTION.
that life's forces tend as a rule towards progress, but likewise exhibit
retrogression and degeneration. If a living being is found to begin its
history, as all animals and plants commence their existence, as a speck
of living jelly, comparable to the adult animalcule of the pool, it is a
fair and logical inference that the organisms in question have descended
from lowly beings, whose simplicity of structure is repeated in the
primitive nature of the germ. If, to quote another illustration, the
placid frog of to-day, after passing through its merely protoplasmic
stage, appears before us in the likeness of a gill-breathing fish (Fig. 241),
the assumption is plain and warrantable that the frog race has descended
from some primitive fish stock, whose likeness is reproduced with
greater or less exactness in the" tadpoles of the ditches. Or if, to cite
yet another example, man and his neighbour quadrupeds (Fig. 242^,
birds, and reptiles, which never breathe by gills at any period of their
5
B
CALF. RABBIT.
FIG. 242. — EMBRYO-VERTEBRATES.
existence, are found in an early stage of development to possess "gill-
arches " (g), such as we naturally expect to see, and such as we find in
the fishes themselves, the deduction that these higher animals are de-
scended from gill-bearing or aquatic ancestors seems to admit of no
reasonable denial. On any other theory, the existence of gill-arches
in the young of an animal which never possesses gills is to be viewed
as an inexplicable freak of nature — a dictum which, it is needless
to remark, belongs to an era one might well term prescientific, in
comparison with the " sweetness and light " of these latter days.
Hanging very closely on the aphorism respecting development
and its meaning, is another biological axiom, well-nigh as important
as the former. If development teaches that life has been and still is
progressive in its ways, and that the simpler stages in an animal's
history represent the conditions of its earliest ancestors, it is a no
less stable proposition that at all stages of their growth living beings
are subject to the action of outward and inward forces. Every living
organism lives under the sway and dominance of forces acting upon
it from without, and which it is enabled to modify and to utilise by
its own inherent capabilities of action. It is, in fact, the old problem
THE EVIDENCE FROM DEGENERATION.
345
of the living being and its surroundings applied to the newer concep-
tions of life and nature which modern biology has revealed. The
living thing is not a stable unit in its universe, however wide or
narrow that sphere may be. On the contrary, it exists in a condition
of continual war, if one may so put it, between its own innate powers
of life and action, of living and being, and the physical powers and
conditions outside. This much is now accepted by all scientists.
Differences of opinion certainly exist as to the share which the
internal constitution of the living being plays in the drama of life
and progress. It seems, however, most reasonable to conclude that
two parties exist to this, as to every other bargain ; and regarding the
FIG. 243.— BRACHIOPODS.
FIG. 244.— KING-CRAB.
animal or plant as plastic in its nature, we may assume such plasticity
to be modified on the one hand by outside forces, and on the other
by internal actions proper to the organism as a living thing.
Examples of such tendencies of life are freely scattered everywhere
in nature's domain. For instance, we know of many organisms which
have continued from the remotest ages to the present time, without
manifest change of form or life, and which appear before us to-day,
the living counterparts of their fossilised representatives of the
Chalk or it may be of Silurian or Cambrian times. The lampshells
(Terebratula) of the Chalk exist in our own seas with well-nigh
inappreciable differences. The Lingnla or Lingulella (Fig. 243, a),
another genus of these animals, has persisted from the Cambrian age
346
CHAPTERS ON EVOLUTION,
(£, c) to our own times, presenting little or no change for the atten-
tion of the geological chronicler. The curious king-crabs or Limuli
(Fig. 244) of the West Indies are likewise presented to our view, with
little or no variation, from very early ages of cosmical history ; and
of the pearly nautilus (Fig. 247) — now remaining as the only existing
four-gilled and externally shelled cuttlefish — the same remark holds
good. The fishes, like-
wise, are not without
their parallel instances
of lack of change and
alteration throughout
long ages of time. The
well-known case of the
genus Beryx presents
us with a fish of high
organisation, found liv-
in§ in the Atlantic and
Pacific Oceans, and
which possesses fossil representatives and facsimiles in the chalk
(Fig. 245). From the latter period to the present day, the genus
Beryx has therefore undergone little modification or change. The
same remark certainly holds good of many of those huge " dragons
FIG. S45-BERYX.
FIG. 246.— ICHTHYOSAURUS (A) AND PLESIOSAURUS (B).
of the prime " (Fig. 246), which revelled in the seas of the trias,
oolite, and chalk epochs — developed in immense numbers in these
eras of earth's history, but disappearing for ever from the lists of
living things at the close of the cretaceous age, and exhibiting little
or no change during their relatively brief history.
Such cases of stability amidst conditions which might well have
THE EVIDENCE FROM DEGENERATION.
347
favoured change, and which saw copious modification and progression
in other groups of animals, might at first sight be regarded as present-
ing a serious obstacle to the doctrine of progressive development on
which the whole theory of evolution depends. As such an obstacle,
the series of facts in question was long regarded. In this light these facts
are sometimes even now advanced, but only by those who imperfectly
appreciate and only partially understand what the doctrine of evolu-
tion teaches and -what its leading idea includes. Even Cuvier him-
self, when advancing the case of the apparently unchanged mummies
of Egyptian animals against Lamarck's doctrine of descent, failed —
possibly through the imperfectly discussed stage in which the whole
question rested in his day — to understand that the very facts of
preservation revealed in the monuments of Egypt testified to the
FIG. 247. — PEARLY NAUTILUS.
absence of those physical changes which could alone have affected
the animals of the Nile land. But the fuller consideration of that
theory of nature which credits progressive change as the usual way of
life, shows us that it is no part of evolution to maintain either that
living beings must needs undergo continual change, or that they
must change and modify at the same rate. On the contrary, Mr.
Darwin, in his classic work, maintains exactly the opposite propo-
sition. There are, in fact, two great factors at work in living nature —
a tendency to vary and change, and the influence of environments or
surroundings. Given the first tendency, which is not at all a matter of
dispute, the influence of the second is plainly enough discernible in
bringing to the front either the original, primitive, or, as it might be
named, the parent form, or the varying forms which are produced by
modification of the parent. As it has well been put : " Granting the
348
CHAPTERS ON EVOLUTION.
existence of the tendency to the production of variations, then,
whether the variations which are produced shall survive and supplant
the parent, or whether the parent form shall survive and supplant the
variations, is a matter which depends entirely on those conditions
which give rise to the struggle for existence. If the surrounding
conditions are such that the parent form is more competent to deal
with them and flourish in them than the derived forms, then in the
struggle for existence the parent form will maintain itself, and the
derived forms will be exterminated. But if, on the contrary, the
conditions are such as to be more favourable to a derived than to the
parent form, the parent form will be extirpated, and the derived form
will take its place. In the first case, there will be no progression, no
change of structure, through any imaginable series of ages ; in the
second place, there will be modification and change of form." To
the same end Darwin himself leads us. In one or two very pregnant
passages, the author of the
"Theory of Natural Selection"
very plainly indicates why pro-
gression should not be universal,
and why certain beings remain
lowly organised whilst others
attain to the summit and pin-
nacle of their respective organi-
sations. " How is it," says
Darwin, " that throughout the
world a multitude of the lowest
forms still exist? and how is it
that in each great class some
forms are far more highly de-
veloped than others ? Why have
not the more highly developed
forms everywhere supplanted
and exterminated the lower?"
Answering his own queries,
Darwin says that natural selec-
tion by no means includes " pro-
gressive development — it only
takes advantage," he remarks,
" of such variations as arise and
are beneficial to each creature
under its complex relations of life. And it may be asked, what
advantage, as far as we can see, would it be to an infusorian
animalcule — to an intestinal worm — or even to an earthworm, to be
highly organised? If it were no advantage, these forms would be
left, by natural selection, unimproved or but little improved, and
FIG. 248.— GLOBIGERINA, ETC.
THE EVIDENCE FROM DEGENERATION. 349
might remain for ages in their present lowly condition. And geology
tells us that some of the lowest forms, as the foraminifera (Fig. 248),
infusoria, and rhizopods, have remained for an enormous period in
nearly their present state. But," adds Darwin, with a characteristically
impartial view of matters, " to suppose that most of the many now
existing low forms have not in the least advanced since the first dawn
of life would be extremely rash; for every naturalist who has dissected
some of the beings now ranked as very low in the scale must have
been struck with their really wondrous and beautiful organisation."
Thus one of the plainest facts of natural history, namely, that in
even one group or class of animals we find forms of exceedingly low
structure included along with animals of high organisation — the
apparently diverse bodies being really modelled on the one and the
same type — is explained by the consideration that with different con-
ditions, or with varied conditions acting differently upon unlike
constitutions, we expect to find extreme differences in the rank to
which the members of a class may attain. In the class of fishes we
find the worm-like clear-bodied lancelet of an inch long, associated
with the ferocious shark, the active dogfish, or the agile food-fishes of
our table. But, as Darwin remarks, the shark would not tend- to sup-
plant the lancelet, their spheres and their conditions of existence
being of diverse nature. The same remark applies to many other
classes of living beings. So that lowly beings still live as such
amongst us, and preserve the primitive simplicity of their race, firstly,
because the conditions of life and their limited numbers may not
have induced any great competition or struggle for existence. On
the " let well alone " principle, we may understand why some
animals, such as the lancelet itself, have lagged behind in the race
after progress. Then, secondly, as Darwin remarks, favourable
variations, by way of beginning the work of progress, may never
have appeared — a result due, probably, as much to hidden causes
within the living being as to outside conditions. We may not fail
to note, lastly, that the simpler and more uniform these latter
conditions are — as represented in the abysses of the ocean, for
example — the less incentive is there for the progress and evolution
of the races which dwell in their midst.
This somewhat lengthy introduction to the subject of degeneration
and its results, is in its way necessary for the full appreciation of the
fashion in which degeneration relates itself to the other conditions of
life. From the preceding reflections it becomes clear that three
possibilities of life await each living being. Either it remains primi-
tive and unchanged, or it progresses towards a higher type, or, last
of all, it backslides and retrogresses. The first condition, that of
stability, is, as already noted, perfectly consistent with the doctrine of
descent; and the two latter conditions also form part and parcel of that
35°
CHAPTERS ON EVOLUTION.
theory. The stable state forces the animal to remain as it now is, or
as it has been in all times past ; the progressive tendency will make
it a more elaborate animal ; and the progress of degeneration will, on
the other hand, tend to simplify its structure. It requires no thought
to perceive that progress is a great fact of nature. The development
of every animal and plant shows the possibilities of nature in this
direction. But the bearings of degeneration and physiological back-
sliding are not, perchance, so clearly seen. Hence, to this latter aspect
of biology we may now specially direct our attention.
That certain animals degenerate or retrogress in their develop-
ment before our eyes to-day, is a statement susceptible of ready and
familiar illustration. No better illustrations of this statement can be
found than those derived from the domain of parasitic existence. When
an animal or plant attaches itself partly or wholly to another living
being, and becomes more or less dependent upon the latter for
support and nourishment, it exhibits, as a rule, retrogression and
degeneration. The parasitic " guest " dependent on its "host" for
lodging alone, or it may be for both
board and lodging, is in a fair way to
become degraded in structure, and,
as a rule, exhibits degradation of a
marked kind, where the association
has persisted sufficiently long. Para-
sitism and servile dependence act
very much in structural lower life as
analogous instances of mental de-
pendence on others act in ourselves.
The destruction of characteristic
individuality and the extinction of
personality are natural results of that
form of association wherein one form
becomes absolutely dependent on
another for all the conditions of life.
A life of mere attachment exhibits
,„, . similar results, and organs of move-
FIG. 249.— COMMON TAPEWORM (Toenta ' o
spiivtn). i. The head extremity, mag- ment disappear by the law ofdisuse. A
nified, showing hooks (a), and suckers Hicrpctivp cvcrpm ic a cnnprfliiitv tn nr\
(b, c); d, the nick, with immature joints. dlgCStlVC *?&> .m IS a SUpCrHUlty tC
2. A joint, largely magnified, showing animal which, like a tapeworm (Fig.
SSLiS^rf^^^SuS 249), obtains its food ready-made in
the very kitchen, so to speak, of its
host. Hence the lack of a digestive apparatus follows the finding of a
free commissariat by the parasite. Organs of sense are not necessary
for an attached and rooted animal; these latter, therefore, go by the
board, and the nervous system itself becomes modified and altered.
Degradation, wholesale and complete, is the penalty the parasite has
7' HE EVIDENCE FROM DEGENERATION.
35»
to pay for its free board and lodging ; and in this fashion Nature may
be said to revenge the host for the pains and troubles wherewith, like
the just of old, he may be tormented.
Numerous life-histories testify clearly enough to the correctness
FIG. 250.— SACCULINA.
FIG. 251.— YOUNG SACCULINA.
of the foregoing observations. Take, as an example, the history ot
Sacculina (Fig. 250), which exists as a bag-like growth attached
to the bodies of hermit crabs,
and sends root-like processes into
the liver of its host. No sign of
life exists in a sacculina beyond
mere pulsation' of the sac-like body,
into and from which water flows by
an aperture. Lay open this sac, and
we shall find the animal to be a bag
of eggs and nothing more. But
trace the development of a single
egg, and one may derive therefrom
lessons concerning living beings at
large, and open out issues which
spread and extend far afield from
sacculina and its kin. Each egg of
the sac-like organism develops into
a little active creature, possessing
three pairs of legs, generally a single
eye, but exhibiting no mouth or
digestive system — parasitism having
affected the larva as well as the
adult. Sooner or later, this larva —
known as the nauplhis (Fig. 251) —
will develop a kind of bivalve shell ;
the two hinder pairs of limbs are cast off and replaced by six pairs
of short swimming feet ; whilst the front pair of limbs develops to
form two elongated organs whereby the young sacculina will shortly
attach itself to a crab "host." When the latter event happens the six
FIG. 252. — BARNACLES.
352 CHAPTERS ON EVOLUTION.
pairs of swimming feet are cast off, the body assumes its sac-like
appearance, and the sacculina sinks into its adult stage — a pure
example of degradation by habit, use, and wont. So also with
certain near neighbours of these crab-parasites, such as the Lerneans,
which adhere to the gills of fishes. Beginning life as a three-legged
"nauplius," the lernean retrogresses and degenerates to become a
mere elongated worm, devoted to the production of eggs, and ex-
hibiting but little advance on the sacculina. There are dozens of
low crustaceans which, like sacculina, afford examples of animals
which are free and locomotive in the days of their youth, but which,
losing eyes, legs, digestive system, and all the ordinary belongings of
animal life, " go to the bad," as a natural result of participating in
what has been well named " the vicious cycle of parasitism."
Plainly marked as are the foregoing cases, there are yet other
familiar crustaceans which, although not parasites, as a rule, never-
theless illustrate animal retrogression in an excellent manner. Such
are the sea -acorns (Balani), which stud the rocks by thousands at low-
water mark, and such are the barnacles (Fig. 252), that adhere to
floating timber and the sides of ships. In the development of sea-
acorns and barnacles, the first stage is essentially like that of the
sacculina. The young barnacle, as our previous studies have shown,
is a " nauplius," three-legged, free- swimming, single-eyed, and possess-
ing a mouth and digestive apparatus. In the next stage we again meet
with the six pairs of swimming feet seen in sacculina, with the enor-
mously developed front pair of legs serving as " feelers," and with two
" magnificent compound eyes," as Darwin describes the organs of vision.
The mouth in this second stage, however, is closed, and feeding is there-
fore impossible. As Darwin remarks, the function of the young bar-
nacles "at this stage is to search out by their well-developed organs of
sense and to reach by their active powers of swimming a proper place
on which to become attached, and to undergo their final metamor-
phosis. When this is completed," adds Darwin, " they are fixed for
life ; their legs are now converted into prehensile organs ; they again
obtain a well-constructed mouth, but they have no antennae, and
their two eyes are now reconverted into a minute, single, simple eye-
spot." A barnacle is thus simply a highly modified crab-like animal
which fixes itself by its head to the floating log, and which " kicks its
food into its mouth with its feet," to use the simile and description of
biological authority. The development of its " shell " and stalk are
matters which do not in the least concern its place in the animal series.
These latter are local and personal features of the barnacle tribe. For
in the " sea-acorns," which pass through an essentially similar develop-
ment, there is no stalk; and the animal, after its free-swimming stage,
simply glues its head, by a kind of marine cement of its own manu-
facture, to the rock, develops its conical shell, and like the barnacle
THE EVIDENCE FROM DEGENERATION.
353
uses its modified feet as means for exercising the commissariat
and nutritive function. It is true that in some respects the adult
barnacle may be regarded as lower than the young, and therefore
as a degenerate being. Thus, it is lower when eyes, feelers, and
movements are taken into account. In other respects the adult may
be considered of higher organisation than the larva. These higher
traits we may logically enough suppose represent the special advances
which adult barnacle life has made on its own account. But, on the
whole, degradation and retrogression, if not so fully exemplified as
in the sacculina, is still plainly enough illustrated in barnacle history.
When we further reflect that even such high crustaceans as prawns
and allied forms begin life each as a "nauplius " or under an allied
guise, we not only merely discover the common origin of all Crustaceans
in some form represented by the " nauplius " of to-day, but we also
witness the possibilities of development which have placed shrimps,
FIG. 253. — STYLOPS.
(Fig. c shows the Stylops, in outline, within the body of the bee ; and Fig. b shows the Stylops
removed from the body of its host.)
prawns, &c., in the foremost rank of the class, and which, conversely,
have left the barnacles and sacculinas, through the action of dege-
nerative changes, amongst the groundlings of the group.
The assumption of a sedentary life, whether parasitic in nature,
like that of sacculina, or whether represented by mere attachment
and fixation to some inorganic thing, as in the case of the barnacles,
is therefore seen to operate in the direction of producing degeneration
of the animal's constitution. The tendency of such habit is towards
A A
354 CHAPTERS ON EVOLUTION.
simplification of structure, and not towards that progressive advance
and evolution which, in the case of the higher crustacean races, have
evolved from the relatively simple " nauplius " of the past the crabs,
lobsters, shrimps, and prawns of to-day.
In groups of the animal series, however, both nearly allied to
the crustacean class, and far removed from it in structure, equally
interesting and often curious examples of degradation may be found.
The class of insects, and the nearly related group (Arachnida), includ-
ing the mites, spiders, and scorpions as its representatives, number in
their ranks instances of degraded and degenerate forms. Amongst the
insects which are parasitic in habits a notable absence of wings is
discernible, and this latter want is seen even in those cases in which
one sex alone of a particular insect species assumes the habit in
question. An excellent illustration of such a fact, and also of the
extreme modification of form which may accompany the degeneracy
of highly organised animals, is found in the history of the insects
collectively known as Strepsiptera, and of which the genus Stylops is
the best-known example. The male Stylops (Fig. 253, a) is an active
insect, possessing a single pair of wings. These wings are the
hinder pair ; the front pair being represented by a pair of twisted
organs (w), which illustrate wing-degeneration, possibly through
disuse. Both males and females, as they leave the egg, are small,
active, six-legged beings (c, d\ which crawl about on the bodies
of bees. Carried into the hive, the young stylops behave like
the proverbial viper, injuring the community which gives them
shelter by boring their way into the bodies of larval or infant
bees. Here the young stylops, casting their skin, become, in
the larval interior, sluggish, footless grubs. Each possesses a
mouth, small jaws, and a digestive system of simple construction.
Meanwhile, bee-development progresses ; and as the larval bee
passes through its chrysalis state with its stylops-lod.ger contained
in its interior, the latter thrusts the front extremity of its body from
between two of the hinder body-segments of the bee. Then the
male stylops, undergoing development in this position, becomes the
winged insect (a) and passes into the world. The female stylops (c),
on the other 'hand, remain in their places on the bees. They undergo
but a slight change of form, persisting as mere sac-like bodies (c)
without legs or digestive system (b), and develop in their interior the
eggs from which succeeding generations of stylops will be produced.
Such a case of absolute degeneracy is all the more remarkable in
view of the facts that it is limited to one sex alone, and that the
free -winged males of stylops are as highly organised as most of their
neighbour insects.
The class of the spiders (Arachnida) offers collective examples of
degeneration and retrogression, which show how large numbers of
THE EVIDENCE FROM DEGENERATION.
355
animals may acquire lower characters, contrasting with the higher
phases to which other members of their class have attained. The
mites and ticks have unquestionably originated from the same root-
stock as the spiders and scorpions. The development of
the two groups proves this much. But whilst the latter
animals have advanced to a high complexity of organisa-
tion, the mites and ticks have degenerated into parasitic
forms — or at least exemplify beings which, first attaining a
respectable rank in their own series, have certainly not
advanced upon that rank. Many of the mites, however,
exhibit well-marked degeneration. Only on the hypothesis
of sweeping retrogression can we account for the singular
and anomalous condition in which a certain harmless
mite, named Demodex follicidorum (Fig. 254), spends its
existence. This mite inhabits the sacs or follicles of the F>G- 254-
human skin at the sides of the nose. It is a minute (magnified),
worm-like animal, possessing eight degenerate rudiments
of legs, and a thoroughly rudimentary structure in other respects.
Here parasitism has denuded the animal of well-nigh every attribute
of its Arachnidan character, and has left it in a condition analogous
in many respects to sacculina itself. Of the equally curious Lingua-
tulina (Fig. 255) inhabiting the "frontal sinuses " or forehead spaces
of dogs, wolves, horses, and sheep,
the same remark holds good. The
body here is thoroughly worm-like
in shape (b, c], and a digestive and
nervous system are to be enume-
rated among the possessions of the
organism. But not even the rudi-
ments of legs are to be perceived,
although the mouth bears certain
apologies for the appendages proper
to that region in the mite and spider
class. Yet the young Linguatulina
(a) exactly resembles the early form
of the mites. It possesses two pairs
of jointed limbs, and certain style-
like organs pertaining to the mouth.
There is thus the clearest evidence
that Linguatulina is a degraded
animal. It is the degenerate descendant of a free living and ap-
parently four-legged — or it may be eight-legged — ancestor ; and its
further history seems to afford a clue to the causes of its retrogression.
For the four-legged larva of Linguatulina escape whilst still within the
egg from the nose of the dog or sheep host which has harboured their
A A 2
FIG. 255. — LINGUATULINA.
356 CHAPTERS ON EVOLUTION.
parents. Received, along with food, into the body of the hare or
rabbit, "the larval being liberates itself. From the rabbit's digestive
system it bores its way through the tissues to the liver, thus reminding
one strongly of the similar migrations of the embryo-tapeworm. In
the liver, further changes ensue. Frequent moultings become the order
of the day; and at length they assume a worm-like aspect, and remain
thus, still imperfect, until, by transference to the body of dog, wolf,
or sheep, and by passage to the frontal sinuses, they acquire perfection
of their life-functions. If the history of these beings teaches us
anything concerning their past, it points to a free and active state
as their original condition, and to the probable acquirement, first, of
a lodgment in the digestive system of one animal as a relatively simple
parasite, and secondly, of a further modification of habit transferring
at once its perfection and completed degradation to the forehead-
cavities of a second host.
But the conditions which make for the degeneracy of an animal
are, as we have seen in the case of the barnacles, not always associated
with a parasitic habit. Mere fixation, as we have
observed, secures the disappearance of useless
organs, such as organs of motion and sense-
organs, which, being possessed by the young
form, clearly indicate that the ancestry of the
animals in question has at any rate been capable
of leading to better things than the descendants
represent in their existent persons. The sea-squirts,
or ascidians, besides serving as a text for ' the
derivation of vertebrates, and for abnormal ways
in the animal chemistry which imitates the plant's
work, have been selected as fruitful objects of
discussion by those biologists who find in the
idea of degeneration an explanation of knotty
points in natural history. For the same voice
FIG. 256.— SEA-SQUIRT, that proclaims the fact that a sea-squirt — which is
a mere rooted bag with a double neck (Fig. 256) — begins life as a
free- swimming, tadpole-like larva (Fig. 257, 5), tells us in the same
breath that there must have been retrogression and degeneration
from an active condition to produce the sac-like adult state. The
assertion that the youthful sea-squirt, moreover, possesses first a
rod-like body — called the notochord (Fig. 257, n) — only found besides
in the young of Vertebrate animals, is also to be taken as implying
the superiority of ascidian infancy to sea-squirt maturity. And
when it is added that the elderly squirt wants the sense-organs and
nervous cord which the larva possesses, it may well be argued that
sheer degeneracy of habit and structure can alone account for the
sweeping transformations which mark the phases of ascidian life-
THE EVIDENCE FROM DEGENERATION.
357
history. Thus it is matter of sober natural-history fact that a
sea-squirt larva, of all invertebrate animals, is the only being that
possesses organs and parts proper to the young vertebrate or to the
adult form of one lower vertebrate in particular. This adult is the
little fish known as the lancelet, which, in the relative simplicity of
its organisation, makes a nearer approach to the poor (or sea-squirt)
relations of the vertebrates than any other fish.
The fact of vertebrate and sea-squirt relationship is worth dwell-
ing upon, because the topic unquestionably presents one with a
common point of view, whence
a survey of the higher develop-
ment, evolution, and progress
of the vertebrates, and a view
of the degeneracy and retro-
gression of the sea-squirts,
may best be obtained. Re-
velling in the freedom of its
early life, the larval sea-
squirt — presenting, as already
noted, a striking resemblance
to the tadpole of the frog, in
its backbone, its nerve-system,
and its breathing-sac, or modi-
fied throat — ultimately settles
down. Like the youthful bar-
nacle somewhat, the young
sea-squirt attaches itself to a
stone or shell by the suckers
with which nature has pro-
vided its head. Then suc-
ceeds the disappearance of
the tail, with its backbone
and its nerve cord, and the
body itself soon assumes the sac-like shape that betokens the
mature ascidian character. The outer skin becomes tough and
leathery, and develops the cellulose which, by biological right, we
might expect to find in plants alone. Then succeeds the fuller
formation of the gill-sac or breathing chamber, and of its neighbour
compartment, which receives the effete water of respiration to be
ejected by the second mouth of the sac-like body. The eye of the
larva likewise disappears, and all that remains to the adult ascidian
is a nerve mass, called by courtesy the " brain," and which serves to
regulate the few acts that mark the placid and rooted existence of
the race.
Attention, has been recently directed in a special manner to
FIG. 257. — DEVELOPMENT OF SEA-SQUIRT.
358 CHAPTERS ON EVOLUTION.
the resemblance which, exists between the eye of the larval sea-
squirt and that of vertebrates — a statement to be taken along with
that which conversely declares the unlikeness of the ascidian eye to
that of all other invertebrate animals. It is matter of fact that the
chief parts of the eye of a vertebrate animal grow inwards as develop-
ments from the skin, and unite with an outgrowth from the brain.
This outgrowth forms the retina, or nervous network of the eye, whereon
the images of things seen are duly received for transmission to brain
and sensorium. Now, in invertebrate animals the retina is formed from
the skin-layer. This latter method of growth, it has been remarked,
is a perfectly natural one. It was to be expected that, as the retina
is to be affected in the discharge of its duty by light rays, it should
form on the surface of the body where the light-rays fall. In the
vertebrate, and in the sea-squirt larva, the retina, on the contrary,
forms away below the skin surface, and grows outwards from the
brain. Why is this so? Professor Ray Lankester maintains that
because the ascidian larva is perfectly transparent, the light-rays pass
through to its brain eye, and thus give rise to sensations of sight.
Hence, if the original and primitive vertebrate animal or rootstock
were like the larval sea-squirt, as we suppose it to have been, its
body would be transparent, and its eye or eyes, situated on its brain,
would receive light-rays through its clear body. But as the evolu-
tion of the vertebrate race proceeded, the tissues became firmer and
denser. By " natural selection " — or, in other words, by the exer-
cise of accommodating power to function — the eyed region of the
brain would tend to grow more and more towards the body's surface,
to receive the rays of light. As development, therefore, advanced,
the mode of growth of the vertebrate eye would be adapted to the
exigencies of its new surroundings. Thus, to-day, the vertebrate
eye grows from without inwards, because light-rays strike naturally
on the outer surface of the body. But it likewise grows from within
outwards as well, because of the ancestral and hereditary tendencies
which cause it to repeat, in the individual growth, the passage to the
surface it had to make in the evolution of the race. If one might
add a suggestion to such an explanation, it would consist in an
endeavour to account for that affinity between brain and outer sur-
face of body which we see to exist. Why the brain should grow
outwards, as it does in eye, ear, and nose likewise, to connect with
the body's surface, and so to form organs of sense, is plain enough.
We must bear in mind that the brain itself is formed from the outer
layer or epiblast of the larva, and from the same layer which develops
into the skin. Brain and skin, to begin with, arise from the same
layer. Hence, before even the matter of eyes falls to be considered,
the affinity of the skin layer aud the nervous system is a fact worth
noting. It is this truest of relationships which may reasonably
THE EVIDENCE FROM DEGENERATION. 359
enough explain, not merely why the sense organs arise from the skin
surface, but also why the brain grows outwards to meet with the
structure to which it is so near akin.
Degeneration of a very pronounced kind, thus accounts for the
peculiarities of sea-squirt structure to-day. The case of ascidian
retrogression is likewise the more interesting, seeing that its reverse
side is that of progressive evolution and development of the highest
forms of life the existing world knows. It is therefore important to
note in passing that the possibilities of development may include
degeneration of a very marked type, along with progressive evolution
of equally pronounced kind. The category of life's extension includes,
in fact, many possibilities which at first sight might appear of most un-
likely kind ; and amongst these possibilities, that of extreme degene-
ration is by no means the least notable as an element in inducing the
material variety of life we behold in the animal and plant worlds of
to-day. The list of causes which lead to the degeneration of living
beings includes, however, other fashions of producing retrogression than
by fixation and parasitic habits, and operates in different ways upon
organisms of varied structure and of different social or biological rank.
Changes in food and feeding may thus accomplish degeneration and
induce physiological backsliding of the most typical description. It is a
familiar fact that the animal organism is of relatively higher nature than
the plant, seeing that the animal frame can, as a rule, feed upon and
build up its tissues from organic or living matter only. Animals, in
other words, demand the substance of other animals or of plants, or
of both combined, as a necessity of their commissariat arrangements.
Plants, on the other hand, are specially constructive and elaborative
in their feeding. They build up from the non-living matters around
them — carbonic acid, water, ammonia, and minerals — the tissues of
their living bodies. They " transubstantiate " this non-living matter
into living tissue ; and the verdant tints of spring, the full glory of the
summer's blossom, or the mellow ruddiness of autumn's fruits, re-
presents, each in its way, the result at once of the plant's constructive
chemistry and of the elaboration into living matter of the inorganic
materials of air and soil around.
The animal frame therefore presents us — amid exceptions to
the above rule in both animal and plant series — with relatively
greater complexity of organs and tissues than the plant body
presents. This statement simply re-echoes what commonplace
observation daily demonstrates. Hence, it may be a natural
enough inference that whatever causes tend to bring the animal
feeding nearer in type to that of the plant will tend to simplify
animal structure, and so to produce retrogression and degenera-
tion of the animal type. Many animals are thus known to develop
chlorophyll^ or the green colour we see characteristically in every
360
CHAPTERS ON EVOLUTION.
leaf. Through the combined operation of this green colour — either
singly or aided by the leaf protoplasm — and the action of light, plants
decompose the carbonic acid of the air, as every schoolboy knows.
They then retain the carbon to aid in the formation of starch, and
set free the oxygen, which thus returns to the atmosphere, and is
•welcomed by the animal hosts. The hydra, or common fresh-water
polype (Fig. 258), many animalcules, and certain worms of a low type
possess this chlorophyll. Like dishonest manufacturers, they seem to
have infringed the patent rights of the plant to elaborate this green
colour. And it is no longer matter of theory, but ascertained fact,
that these green animals are capable, like the plants, of absorbing
carbonic acid — usually a fatal gas to the animal constitution — and
FIG. 258.— HYDR«.
(In both figures young hydrae are represented budding from the side of the parent.)
of elaborating starch therefrom like their plant neighbours. Thus a
simpler mode of feeding, obviating the necessities of animal existence
in the way of digestive apparatus, has apparently led to the simplifica-
tion of structure. Degeneration has followed in the worms just
mentioned, as the result of their imitation and acquirement of
vegetative powers of feeding; and it is probable that other altera-
tions in the way of dietary, of less sweeping character than that
just mentioned, will affect, in like retrogressive fashion, the animal
constitution.
Some of the most curious cases of degeneration known to us
illustrate the total disappearance of digestive apparatus even in
some beings, in which, as in the stylops already mentioned, one
sex becomes retrogressive whilst the other sex remains structurally
fully developed. Such a case is illustrated by the males of
those remarkable organisms, the Rotifera, or " wheel animalcules "
(Fig. 259). These minute creatures, inhabiting our fresh waters, may
be desiccated and dried, and revived, on the application of moisture,
many times in succession. But in their ordinary existence, and in
THE EVIDENCE FROM DEGENERATION.
361
A
the details of their structure, the "wheel animalcules" present
details equally interesting with their exhibition of " potential
vitality." The female animalcules possess a complete digestive
system, a set of water vessels, a nervous ganglion, and other be-
longings ; but their partners are decidedly inferior creatures, since
their digestive system becomes totally abortive ;
whilst in size, the males are likewise far excelled
by the lady rotifers. How this degeneration
and disappearance of digestive apparatus and
the inferiority of size have been produced in
the male rotifers, may be a matter regarding
which difference of opinion will certainly exist
in biological minds. The fact that retrogression
is here illustrated, however,
cannot be questioned. It
may also be added that, in
all probability, the extreme
development of the function
of perpetuating the species,
and the extraordinary fer-
tility of production wit-
nessed in these animalcules,
may satisfactorily account
for the abrogation of diges-
tion in favour of reproduc-
tion. Thus, to the other
causes of degeneration in
animal life and structure,
we may append that which
takes origin from the ex-
treme or excessive develop-
ment of one function over another. Physiological development' in
one direction, overstepping the natural and ordinary limits, runs
concurrently with destruction of life's equilibrium, and naturally
tends to produce degeneration and simplification of other organs
and of other duties of life.
How far the theory of degeneration we have thus briefly discussed
may be applied in explanation of the peculiarities of animal structure,
remains as a task for the future of biology to satisfactorily determine.
Possibly the corrections which the future of every hypothesis carries
with it may be many and sweeping. The deductions and inferences
we extract from a study of degeneration to-day may perchance be
falsified by the higher and newer views of the to-morrow of biological
science. But enough has been said to show that, even in a cursory
review of the doctrine of degeneration and retrogression, many phases
FlG. 259.— ROTIFERA.
362 CHAPTERS ON EVOLUTION.
of living histories become theoretically plain ; and it argues hopefully
for the correctness and value of the doctrine before us that it has, so
far as it has been logically pursued, fitted compactly and harmo-
niously enough with ascertained facts and with received views of the
origin of animals and plants. That higher forms of life than the sea-
squirt and insect race are by no means exempt from the influence ot
retrogressive change, is an observation worth noting at the close of
our researches. We know, for instance, of lowly structures in shell-
fish life appearing in the midst of highly organised frames. A
mussel, a cockle, or an oyster, whose early development, as we have
seen, runs in parallel lines to that of the snail and whelk-class, is
nevertheless esteemed less highly organised than the latter. The
mussel or oyster-tribe possess no head. The snails and their allies,
as every one knows, not merely exhibit a well-developed head, but
have that extremity provided with eyes, tentacles or feelers, and
other addenda of the front region of the animal body. Hence it is
more than probable that the mussel, headless and enclosed in its
shell, and possessing relatively little interest in the affairs of the
outer world, is an example of a degenerate type of molluscs. The
mussels and their relations stand, in fact, at the opposite extreme ot
development in this respect from those well-known molluscs the
cuttlefishes. In these creatures, the tendency to head-development —
or what Professor Dana calls " cephalisation " — reaches its maximum,
as any one may readily enough suppose on looking at an octopus or
squid, with its great head, its enormous eyes, and its nerves massed
together to form a brain enclosed in a kind of skull. Even as com-
pared with the earlier cuttlefishes — whose shells, under the name
of ammonites and the like, we find fossilised in large numbers —
the squids and cuttles of to-day present, in the extreme development
of head, a noteworthy advance.
Thus, whilst the one Molluscan tribe of mussels and their
neighbours has degenerated and gone to its own lowly place
in the series, other groups starting on an equal footing have
advanced, and, through progressive evolution, have produced
those higher manifestations of molluscan life that teem in the
seas of to-day. Even amongst the Vertebrate animals we meet
with examples of degenerative tendency which are not so easily
explicable as the foregoing illustrations. In most snakes only one
lung is fully developed as a rule, the companion organ being rudi-
mentary and degenerate. In birds, the egg-producing organs are
similarly developed on one side only. How degeneration should
be thus partial, and affect one-half of an animal's frame, so to speak,
is very hard to discover. External conditions of life and the in-
fluences of surroundings could apparently possess little effect in
inducing such an unsymmetrical retrogression of parts. Most
THE EVIDENCE FROM DEGENERATION, 363
probably we shall find the solution of such conditions to exist
within the operation of some deep-seated law of the living constitu-
tion, and in the effects of that law in moulding or even contorting
the animal frame.
It constitutes one of the chief glories of biological science, as
pursued amongst us to-day, that its studies are of far-reaching order,
and lead, as the results of their natural extension, to the considera-
tion of fields of thought often widely removed from the original topic
which interests the reader. The present subject of degenerative
changes, regarded as part and parcel of the living constitution, can
readily be shown to possess applications far removed from zoology
and botany, and extending into the most intimate spheres and phases
of human history itself. Degenerative change in human tissues is
medically symptomatic of very many of the ills to which flesh is heir.
Tissues and organs degenerate in individual animals, as animal frames
retrogress in their entirety. Cells retrograde and fibres degenerate
in our bodies, just as the sea-squirt's frame exhibits, as a whole, a
universal physiological backsliding. Nor may, many of our diseases
alone be esteemed mere examples of degeneration affecting our
tissues. The termination and decline of life itself, and the age that
really " melts in unperceived decay," are in reality examples of natural
degeneration also. The decline of existence is largely a retrogression
of structure. There can be no such thing as a really "green old
age," any more than we can speak of " the sere and yellow " of the
auturfmal leaf as imitating the verdant nature of the spring blossom.
Nay, stranger still is it to discern that the full flush of life's vigour is
accompanied by degenerative changes as typical as those which mark
life's decline. For every tissue wastes as it works; and cells degenerate,
die, and are cast off from every surface and tissue of our frames as
the natural result of living and being. " Generally speaking," says
a writer in discussing the degeneration of human tissues, " those
parts which live most slowly are those of which the duration is the
greatest, and in which there is consequently the least frequent change.
Of the exuviation of epidermic structures en masse — a process
altogether comparable to the fall of the leaf — we have striking
examples in the entire desquamation of serpents, the moulting of
the plumage in birds, and the shedding of the hair in mammalia ;
and in the shedding of the antlers of the stag we have an example
of the exuviation of a highly organised and vascular part, which
periodically dies, and which, being external, is cast off entire. ' What
means all this,' says Sir James Pager., ' but that these organs have
their severally appointed tissues, degenerate, die, are cast away, and
in due time are replaced by others, which in their turn are to be
developed to perfection, to live their life in the mature state, and to
be cast off?" And, again, the same high authority remarks that
364 CHAPTERS ON EVOLUTION.
" it is, further, probable that no part of the body is exempt from the
second source of impairment ; that, namely, which consists in the
natural death or deterioration of the parts (independent of the death
and decay of the whole body) after a certain period of their life. It
may be proved, partly by demonstration, and partly by analogy, that
each integral or elemental part of the body is formed for a certain
natural period of existence in the ordinary conditions of active life,
at the end of which period, if not previously destroyed by outward
force or exercise, it degenerates and is absorbed, or dies and is cast
out ; needing, in either case, to be replaced for the maintenance of
health." To these weighty words we may lastly add the opinion of
Dr. Carpenter, who remarks that, " when the adult type has once
been completely attained, every subsequent change is one rather of
degeneration than of development, of retrogression rather than of
advance."
Degeneration is thus an invariable concomitant of life. So far
from being in any way an abnormal phase of living action, it is seen
to be as natural a process for living beings to retrogress — wholly, as
we have seen in some cases, or partly in others — as it is for them to
develop and advance. And what is thus undoubtedly true of the
individual man or other animal, is no less so of the race. " Buried
civilisations " are by no means unknown ; extinct culture is an archaeo-
logical fact ; the decline and fall of nations is matter of history.
May not these things be likewise explained as a part of that wide
theory of life which regards even the highest interests of man as
lying within the operation and sway of causes which mould his
physical organisation ? If this notion be accepted, then is the idea
of degeneration as a normal phase of life rendered still more feasible
and plain. Reaching to the individual and to the species as well ;
extending and including in its scope the lowly organised as well as
the higher being ; affecting one group or class lightly, and influencing
another well-nigh to the complete exclusion of progress, — we find
degeneration and retrogression to be numbered among the stern
realities of existence. And no less clearly and forcibly may we trace
the truly natural place of degeneration in our own physical history :
since, as physiology teaches and daily experience declares, not an
action is wrought or a thought conceived without the presence of
change and decay of tissue — a process this which, limited in early
life by progressive growth and by development, at last comes in our
latter days to assume the reins of government, and in time to dissipate
our energy and substance into the nothingness of physical and
corporate extinction.
The philosophy of biology, however, may, in conclusion, be found
to point out to us that the subject of degeneration, whilst treating of
a powerful factor in modifying the living form, yet possesses a
THE EVIDENCE FROM DEGENERATION. 365
favourable aspect in relation to progress and evolution. High
authority in matters biological may be found for the statement that
degeneration is really a result of progress, that it is dependent on
high development, and that, whilst it simplifies the living being, " it
produces the same effect as differentiation, for it leads to variety in
form." Thus there is a kind of evolution and progress inseparable
even from degeneration itself. For the retrogression may in itself
lead to variety and change, and in due time such variety may be the
starting-point of new and higher developments. So, likewise, we
are reminded that reduction and degeneration of some parts may
proceed contemporaneously with the higher development of others,
with the total result of perfecting the organism, and of evolving a
higher type of structure. The degeneration of a frog's tail is in
reality a feature of its higher type as compared with its tailed friends
the newts and salamanders. The disappearance and reduction of
the tail which the young crab possesses, is a chief reason why we
esteem the crab, whose body is all head and chest, a higher animal
than the lobster or prawn with head, chest, and tail complete. The
degeneration of the " outside" gills of the Alpine salamander's young,
which never have access to water, is not a mark of inferiority but of
superiority ; it is, in reality, the casting-off of the old or larval and
aquatic characters and the putting on of the new and higher features
of the land animal. Even the degeneration of human structures —
the modification of the tail which early human existence exhibits,
and of muscular structures well developed in lower life — are no
proofs of inferiority, but are evidences of superiority in ourselves.
Thus, even in the great work of evolving higher races out of the
lower, to degeneration much is owing for its aid in repressing larval
characters and the structures which belong to lower existences.
Whilst progressive evolution develops the great tree of life, extends
each branch, clothes it with verdure, and expands each blossom, it
is degeneration which lops the worn and aged stems, prunes the
weakly foliage, trims the budding growths, and so directs and moulds
the outlines of the organic whole. It is to evolution and progress
that the world of life largely owes its forward march. But hardly
less is the debt of gratitude due by the living hosts to degenerative
change and retrogression which, though stern and ofttimes cruel in
their ways, nevertheless mark wisely and well the pathways of life,
and prevent the useless and weak from cumbering the ground.
366 CHAPTERS ON EVOLUTION.
XVI.
GEOLOGY AND EVOLUTION.
ALLUSION has frequently been made in preceding chapters to the
fact that the deductions of geology and the theory of evolution
possess many relations in common. The relationship in question
has been demonstrated more than once in the course of our inquiries
into the evidence which living nature presents of the truth of the
development theory. For example, the case for " missing links,"
and for the substantiation or denial of the connected series of beings
which evolution postulates, was seen to be one which must stand or
fall accordingly as the geological evidence revealed the existence of
" links" or not. If the history of the fossil contents of the rock-
formations could be shown to include no examples whatever of the
" transitional forms " between species which the evolutionist demands,
it is clear that the validity of his special conception of the order of
nature would thereby be seriously impugned. The least cultured
critic of the theory of descent would naturally turn to the geological
evidence in search of confirmation or refutation of the views
advanced by the followers of Mr. Darwin. Hence, in the early
days of evolution, when the theory was in -its infancy, the relations of
geology to this conception of nature were foreseen to be of a kind
demanding the most careful attention from both promulgators and
opponents of the hypothesis of descent. In this view of matters,
the chapters on the " Origin of Species," dealing with the " Imper-
fections of the Geological Record," and with the " Geological Suc-
cession of Organic Beings," have always been regarded with special
interest alike by evolutionists and their opponents. Mr. Darwin
had undoubtedly felt the high importance of the geological side of
the great question he had propounded before the world of thought.
The two chapters just mentioned, bear plain evidence of the pains-
taking care with which the venerable author of the classic work
marshalled the evidence at hand, and with which also he reviewed the
conclusions to which that evidence seemed to point A perusal of
the chapters in question will serve to show how consistently the facts
of geology may be placed beside those revealed by the study of the
life that exists to-day, and how the facts of life's past development
often support, whilst they never negative, the ideas on which the
evolutionist bases his belief.
The aspects which the geological evidence offers for the attention
GEOLOGY AND EVOLUTION. 367
of the evolutionist are manifold in number and variety. Our
study of "missing links" has already shown us that one special
phase of geological inquiry relates to the existence in the fossil
record of those forms which may be described as intermediate
in nature between existing species. If we suppose that no such
forms had been known to exist, we can conceive that whilst their
absence would not absolutely have negatived the theory of evolution,
the want of such evidence would have decidedly weakened the
evolutionist's case. The fulness with which the gaps have been
supplied with even a relatively limited search in geological directions,
has more than satisfied the evolutionist of the correctness of his
deductions respecting the existence of the transitional forms he would
expect the life of past seons to have exhibited. A second phase ef
geological inquiry in relation to the theory of descent is found in the
question of past time and its duration. The objection has naturally
been urged, that, if " natural selection " has operated in the past by
the production of minute variations, acting through periods of im-
mense antiquity, the drafts which the- evolutionist is compelled to
make on the bank of time are so great that collapse of the theory is
the result. Physicists of the highest eminence have formulated
opinions, based on apparently stable data, regarding the age of our
planet. It has accordingly been urged that, assuming the most ex-
tended antiquity which such opinions allow, there yet remains a
deficit in the age of the earth, compared with the demands or with
the expectations of the evolutionist. But to all such speculations
and considerations there remains one sufficient answer. So long as
we are comparatively ignorant of the exact factors to which the work
of evolution is due, it is idle to speculate on the time required for
the change and modification of species. That specific change may
have occasionally been more rapid in past ages than at present, is an
idea for which there is considerable justification to be found in the
history of the living beings we are able to study. Again, with im-
perfect knowledge of the progress and succession of climatal and
other changes in the past, and with but shadowy ideas regarding
the possibilities of biological change to-day, we have ample reason
to regard the relations of time past to evolution, as by no means so
simple as certain opinions would appear to indicate. The evidence
of geology itself, whilst rejecting the ancient ideas of" catastrophism "
and of sudden, cataclysmal changes, nevertheless must be held as
proving that cosmical and physical actions have not always presented
the phases we see exemplified before our eyes to-day. In a word,
until we have at hand fuller evidence regarding the manifest inter-
action between cosmical alteration and biological change — between
physical revolutions and their effect upon the life of our globe — all
speculations concerning the duration of time past in its relation to
365 CHAPTERS ON EVOLUTION.
evolution must be held as purely tentative and provisional. The
case for evolution, as indicated in our previous studies, is so over-
whelmingly strong when we regard the evidence from biology alone,
that the evolutionist may well be pardoned if he is inclined to turn a
deaf ear to any arguments against his theory which are derived from
data so manifestly imperfect in their details as those on which specula-
tions concerning past time and life-development are based.
As bearing in the most intimate manner on this very question of
time in relation to evolution, it may be interesting to point out that
vast periods of unrepresented time must be allowed for in all con-
siderations connected with the past history of the earth. If we
tabulate the various fossil-bearing rock- formations in the order in
which they occur in nature, the following table will represent their
succession :
KAINOZOIC Pliocene
PFRTOD nocene.
4 Miocene.
Eocene.
("Recent Life").
Post Tertiary and Recent.
MESOZOIC ( Cretaceous or Chalk.
PERIOD ] Oolite or Jurassic.
(''Middle Life"). 1 Trias.
/ Permian,
p Carboniferous.
I Devonian, or Old Red Sandstone.
x trfKlOL) n c*'i *
(" Ancient Life") I Sllunai?-
'* Cambrian (and Huronian).
\ Laurentian.
This table shows us that the fossil-bearing rocks are arranged in
' a very definite order and succession ; the lowest, and therefore the
oldest, series being the " Palaeozoic " rocks, which in turn possess the
Laurentian group as their most ancient formations, and the Permian
rocks as their newest. Above these, again, lie the " Mesozoic "
rocks, with the Trias, lying above the Permian, as their oldest,
and the Chalk, reposing in turn on the Oolite, as their youngest
beds respectively. The " Kainozoic " rocks form the last and most
recent series of all. They lie upon the Chalk, their oldest strata being
those named the Eocene, and their youngest formations the soils and
gravels of to-day. It is needless to remark that this tabular order is
never anywhere seen in its entirety. Geological revolution dis-
turbing the strata, has produced many and serious breaks in their
continuity. But such gaps do not affect the order of their succes-
sion tabulated by the geologist ; that is to say, the Permian, for
example, wherever found, must overlie the Coal, as the latter in turn
invariably lies above the Devonian. The complete thickness of the
stratified rocks alone — or, in other words, their thickness exclusive
GEOLOGY AND E VOL UT10N. 369
of volcanic or igneous rocks — in Britain is given by Professor
Ramsay as 72,584 feet, or, as Mr. Darwin has put it, "very nearly
thirteen and three-quarters British miles. Some of the formations,"
continues Mr. Darwin, " which are represented in England by these
beds, are thousands of feet in thickness on the continent. Moreover,
between each successive formation we have, in the opinion of most
geologists, blank periods of enormous length."
Now, it is these " blank periods " which certainly affect, in the
plainest fashion, all questions concerning the operation of biological
change in its relations to time past. It is necessary here to bear in
mind a few elementary geological axioms, such as the fact that only
those rocks (the aqueous or stratified rocks) which have been formed
in water contain fossils; and that the igneous, or volcanic, rocks
cannot be expected, from the mere fashion of their formation, to
present any traces of past life. Thus it is clear the aqueous, or
fossil-bearing rocks must have been formed either in the beds of
oceans or in shallow water along coast lines, or in lakes, or at the
mouths of rivers, the materials for these rocks being supplied by the
wear and tear of previously existing formations. The nature of the
included fossils is often the best guide to the exact site wherein the
soft materials were deposited to form the strata of future epochs.
Thus the presence of fossilised corals, star-fishes, and marine forms
of fishes would indicate that the formations containing these fossils
had been deposited in a sea-bed ; just as the discovery of fresh-
water shells and plants in another series of strata would show that
these latter beds were of lake origin ; or, as a mixture of fresh-water
and marine forms would suggest that the strata had been deposited in
brackish water.
The distinctness of any two or more series of strata is inferred
by the geologist when he discovers differences in their included
fossils, whilst he also possesses a criterion of their distinctness in
their mineral characteristics. Now, each group of stratified rocks is
more or less clearly characterised by possessing certain characteristic
fossils, which in some cases may be absolutely confined and limited
to the one group of formations, never passing into any other series ;
or may, on the other hand, extend from one group through another
series, or even through several successive formations. But we are
accustomed to note that certain fossils specially characterise each
formation, and are characteristic of that formation, even when they
may have been slightly developed prior to its period, or have passed
beyond it into the next epoch. The comparative distinctness of the
fossils in the several formations was formerly explained on the
assumption, that at the close of each period the forms of life dwel-
ling therein were extinguished and killed off by some sudden
catastrophe. The life of the succeeding period was supposed to be
B B
370 CHAPTERS ON EVOLUTION.
produced by the special creation of new species; these in turn
becoming extinct at the close of their particular epoch. Creation
and extinction, on this theory, were thus alternating processes ; the
death of the one set of organisms heralded the production of the
new and independent forms. Such a hypothesis, tenable enough, of
course, on the theory of " special creation," is diametrically opposed
to that of evolution. But the case for the latter hypothesis was soon
proved to be overwhelmingly strong when the facts relating to fossils
were more fully investigated. Thus, when it is clear that all the
forms represented as fossils in one series of rocks are not, as a rule,
completely absent from the succeeding epoch, but are often found
represented in the next period, the case for the " special creation "
of each new series of fossils becomes materially weakened. Fur-
thermore, just as lines of genetic relationship connect existing
animals and plants, so like relations may be traced between extinct
and fossil forms. Hence the theory that the fossil animals and plants
of each period must represent the more or less typical descendants
of the life of the preceding epoch, at once rises into the domain of
rational belief. If we can believe that each new period is thus
peopled by the descendants of the preceding epoch, and by the
actual survivals from that period, the case for evolution grows in
strength ; and this belief is exactly that which modern geological
science esteems to have been the true cause of the succession of life
through the changing aeons of the past " If the doctrine of evolu-
tion is sound," says Professor Huxley, " one of its immediate
consequences clearly is, that the present distribution of life upon the
globe is the product of two factors, the one being the distribution
which obtained in the immediately preceding epoch, and the other
the character and extent of the changes which have taken place in
physical geography between the one epoch and the other ; or, to put
the matter in another way, the Fauna and Flora of any given area, in
any given epoch, can consist only of such forms of life as are directly
descended from those which constituted the Fauna and Flora of the
same area in the immediately preceding epoch, unless the physical
geography (under which I include climatal conditions) of the area
has been so altered as to give rise to immigration of living forms
from some other area."
The succession of life thus described offers convincing proofs of
the correctness of the evolutionist's views, and these proofs will be
presently considered. The nature and influence of the breaks or
" blank periods " of Mr. Darwin, which exist between the series,
remain, however, for primary notice. The geologist finds ample
cause in his study of the rock-masses to assume that the periods of
time which have elapsed between the end of one epoch and the
beginning of the next, have been of immense duration. That these
GEOLOGY AND E VOLUTION. 37 1
periods are further "blank," and unrepresented by any tangible
details in the earth's history, is also plain. It is matter of geological
detail, for example, to show how the phenomena known as " uncon-
formability," — wherein one rock system lies athwart another, so to
speak — alone proves the immensity of the period of time which
intervened between their formation. Prior to and during this interval,
migration of the species to be represented as fossils in the epoch
which had just closed, would unquestionably take place. We thus
note how rocks formed in widely separated areas may contain like
fossils, and also how the animals and plants of one period may
have passed onwards to take part in the development of life in the
next. Mr. Darwin's own words are highly succinct and convincing
on the points just mooted. " I have given my reasons," says Mr.
Darwin, "for believing that most of our great formations, rich in
fossils, were deposited during periods of subsidence ; and that blank
intervals of vast duration, as far as fossils are concerned, occurred
during the periods when the bed of the sea was either stationary or
rising, and likewise when sediment was not thrown down quickly
enough to imbed and preserve organic remains. During these long
and blank intervals I suppose that the inhabitants of each region
underwent a considerable amount of modification and extinction,
and that there was much migration from other parts of the world.
As we have reason to believe," continues Mr. Darwin, " that large
areas are affected by the same movement, it is probable that strictly
contemporaneous formations have often been accumulated over
very wide spaces in the same quarter of the world ; but we are very
far from having any right to conclude that this has invariably been
the case, and that large areas have invariably been affected by the
same movements. When two formations have been deposited in two
regions during nearly, but not exactly, the same period, we should
find in both, from the causes explained in the forgegoing paragraphs,
the same general succession in the forms of life ; but the species
would not exactly correspond ; for there will have been a little more
time in the one region than in the other for modification, extinction,
and immigration." It thus seems clear, that on grounds connected with
the lapse of past time in relation to life-development on our globe,
and also for reasons connected with the " blank periods " between the
formations comprising its crust, the evolutionist may be content to
assume, firstly, that questions connected with the duration of time
past need not cause him any perturbation. He may secondly argue,
with reason, that even when the question of unrepresented periods
is alone considered, the evidence at hand tends to prove how
life's succession is in reality continuous beneath the breaks in time,
and in the order of rock-formation ; whilst he also notes how
aptly the theory of descent harmonizes with the full consi eration
BB 2
372 CHAPTERS ON EVOLUTION.
of the facts revealed by a study of the earth's life-history in the
past.
The question of the general succession of life on the earth's
surface presents an important series of considerations to the evolu-
tionist's view. In this latter study, it is interesting to note the theory
of descent finds increased support. Admitting to the full the great
breaks which exist in the continuity of the rock-formations com-
prising the crust of the globe — "blank periods " which in noway
militate against the correctness of evolution — we still possess data of
sufficient accuracy and extent to determine for us the general order
in which life has been developed on the earth's surface. The further
back we pass in the history of the rock-formations, the fewer are the
resemblances we can detect to living animals and plants. Occasion-
ally we meet with certain forms which seem to have persisted from
the earliest times, and without material change, to the present day.
Such are certain of those shellfish named Brachiopods, and also many
of the Foraminifera. These have survived from a period so far back
as the Silurian epoch ; and appear before our eyes to-day as living
forms, in essentially the same guise as their fossil representatives. Such
examples of unaltering life have already been shown, in the chapter
on " Degeneration," to be perfectly consistent with the doctrine of
evolution. They represent species whose surroundings remain the
same to-day as of yore, and whose tendencies to change are, there-
fore, practically non-existent. A glance at the table of rock -forma-
tions already given (page 368) will show that time past has been
divided by the palaeontologist into three great periods, in so far as the
development of life is concerned. Thus the rocks from the Lauren-
tian, or oldest of the fossil-bearing series to the Permian are col-
lectively named " Palaeozoic " (or " Ancient Life ") formations.
Those from the Trias to the Chalk form the "Mesozoic" (or "Middle
Life") period ; whilst the rocks from the Chalk to the soils of to-day
collectively form the " Kainozoic" (or "Recent Life") period.
It is a matter of the most elementary study in geology to discover
that the fossil animals and plants of the Palaeozoic period are most
unlike the existing life of the globe. The fossils of the Mesozoic
rocks approach more nearly in character to the animals and plants
around us, although also including many very divergent forms. In
the Kainozoic rocks, on the other hand, we meet with a far larger
proportion of fossils belonging to living or " recent " species.
Broadly speaking, then, there has been a gradual development of life
.from forms unlike those around us to-day, towards the existing life
of the globe. But geology goes much further in its interpretation of
the past life of our globe. There has been represented, in addition,
a distinct development of higher from lower life as the ages have
progressed. There have been progression and advance, as well as
GEOLOGY AND EVOLUTION. 373
mere succession towards the likeness of existing life. If we turn to
the oldest rocks, we find the fossils to include only those of low or
primitive animals and plants. It is only as we pass to the newer
rocks that we find traces of higher life. If we select any class of
animals or plants, we do not discover, as would be expected if the
theory of " special creation " were true, that all ranks and stages in
that class appear simultaneously in the fossil record. On the con-
trary, the lower forms invariably precede the higher. Fishes, as the
lowest Vertebrates, thus precede other members of their type in time.
The oldest fossil fishes occur in the Upper Silurian rocks ; and it is
not until we arrive at the newer Coal measures that we find the suc-
ceeding and higher class, that of the frogs or amphibians represented.
In the deposits (Permian) above the Coal, the still higher reptiles first
occur ; birds make their first appearance in the Mesozoic rocks, and
the oldest mammals or quadrupeds are also of Mesozoic age. What
holds true of Vertebrate development, applies also to the appearance of
life at large on the earth's surface. Where the fossil evidence that
the lower members of a class appear before the higher seems doubt-
ful, the discrepancy must be referred to that imperfection of the
fossil record which constitutes an impassable barrier to our full and
complete knowledge of the life of the past. But wherever sufficient
materials are to be found, the great law of progression from lower to
higher types of life is seen to be paramount as an expression of the
manner in which the world has attained the fulness of its existing popu
lation. It need hardly be pointed out how powerfully the discovery of
this progression in the past life of the globe supports the evolutionist's
views. Instead of the sudden appearance of whole groups of high
and unaltering organisms in the oldest formations, we are enabled to
trace the gradual development of species. In the case of certain
groups of animals, the evidence for the theory of development be-
comes singularly complete when we discover the exact course in
which the evolution of new species by the modification of the old
has taken place. Amongst the extinct trilobites and ammonites, for
instance, the gradations between the various forms are often traceable
with singular completeness ; the gaps between the different species
being frequently supplied in the most exact, fashion.
A notable feature in the life of the past consists in the observa-
tion that many extinct animals present characters which clearly
belong to the young or embryonic condition of their type, rather
than to the full-grown state, or to that seen in their existing representa-
tives. This fact constitutes in itself a singularly powerful argument
in favour of the evolutionist's views. It shows that the progression
of life in time past has been that which the development of animals
to-day demonstrates. In other words, as the stages in development
we see to-day repeat the ancestry of the developing animal, so these
374 CHAPTERS ON EVOLUTION.
'; embryonic " fossil forms represent the early and lower phases in
the development of the species. The animal of to-day represents
the cumulative advance of its race. It forms a striking confirmation
of the evolution theory that we should discover in the debris of past
epochs, and in the lower life of these epochs the actual evidences of
such advance. The conception of the evolutionist that the
development of life is represented by a huge tree with its innumer-
able diverging branches and twigs, is thus seen to fit the actual
details of which his theory takes cognisance. The species of to-day
represent the topmost or last developed leaves of the branches.
Occasionally, we are able to trace the continuity of past and present
along the branch and backwards to the stem. And even where this
continuity is broken, it is interesting to note how the evidences of its
former existence became clear, when, in the shape of the lower and
earlier fossils, we behold the scars and lopped branches, showing the
lines wherein the development of the past had progressed.
The imperfection of the geological record has already formed
the subject of a previous discussion. We traced the absence of
many of the connecting links between existing groups of animals to
the non-preservation of the forms of the past, as well as to the
wholesale destruction of fossil-histories which changes to the rock
masses undoubtedly involved. Despite the effect of such sweep-
ing physical revolutions, the evidence for the existence of these
" links " between existent forms which evolution postulates, has
been shown in not a few cases to be singularly complete. It is
necessary, however, to refer more exactly in the present instance
to the actual causes which have rendered the fossil record, at the
best, of fragmentary nature. There are thus, firstly, certain groups
of animals which, from the very nature of their habits, we
could not expect to have been well represented by fossil species.
Such forms as insects and birds, for example, could rarely be
expected to become readily fossilised. Their habits lead them
to exist in the majority of cases far from water ; and the lightness of
their bodies would largely preclude the probability of their frequent
deposition amongst the debris of seas and lakes — such deposition
being, of course, the natural prelude to their preservation as fossils.
The comparative scarcity of bird and insect fossils fully attests the
correctness of these deductions. What is true of these two groups
holds equally good in the case of most land animals. By far the
greater proportion of fossils consist of the remains of marine or
aquatic forms, which naturally exist in the most favourable situation
for becoming readily entombed in the soft deposits which form the
rock-formations of the future. Another important consideration
which has had a notable influence in demonstrating why we can
never hope to construct a full and complete history of the past
GEOLOGY AND EVOLUTION. 375
history of life-development, consists in the fact that only the hard
parts of animals and plants are, as a rule, capable of being preserved.
Bones, teeth, and scales, along with shells and corals, are the
structures which most commonly constitute the " fossils " found
in rock-formations. Although in a few instances the footprints of
animals, the tracks of sea- worms, and even the impress of a jelly-
fish have been discovered, the vast majority of fossils consist of the
remains of those animals and plants which possessed hard parts
and structures. If, therefore, we consider the enormous number of
soft-bodied organisms which, in this way, can have left no trace
whatever of their existence, one all-sufficient reason for the
imperfection of the geological record is not difficult to find.
Whilst this record is thus imperfect and at the best fragmentary,
such evidence as we do possess regarding its nature will be found
in no case to negative the conclusions to which evolution would
guide us. The whole of the evidence which geology has to submit
in reference to the life of the past, clearly points to the idea of
progression and modification of living beings as the only hypothesis
which can fully explain and connect the facts of cosmical history. He
who runs may literally read with pleasure and profit the story of life
which is written in the records of the rocks ; and that story is one
of evolution and modification. Of " special creation " the rocks tell
no tale ; and life at large has nothing whatever to say in support
of a tradition which belongs to the pre-scientific era of human
thought.
It may lastly be pointed out how closely and intimately the
phases of geological history parallel those of biological growth.
Time was, when every phase of geological action was regarded as
the result of a sudden catastrophe. Sudden, physical revolution,
was but the counterpart of that special and independent " creation "
of living beings, in which the science of yesterday believed. To-day,
with wider and truer conceptions of both physical and biological
history, both theories have been consigned to oblivion. As the " uni-
formity " of geological action supersedes the catastrophes of the past,
so the evolution of life replaces the idea of its sudden creation. In
each case, the supernatural, unknowable action postulated by ancient
belief, is replaced by an efficient cause, the nature and direction of
which can be investigated by mankind. From a detached, abrupt,
and disconnected theory of nature, we have advanced towards a har-
monious explanation of this world's order, and towards a unity of
cause, outside which, as the growth of science tends to show, no
event of history, cosmical or human, can exist.
INDEX.
Achtheres, 209
Adaptation, 112, 122, 129, 194
Adaptive change 260
/Eolis, 228, 234
/Etiology, 27
Agassiz, 104, 197
Air-bladder, 115^ seq.
Algse, 58
Allman, 74, 79
Alternation of generations, 246
Alytes, 238, 240
Amblystoma, 242 et seq., 248
Amoeba, 22, 46, 64, 65 et seq., 276
Amphibia, 235 et seq.
Amphioxus (see Lancelet)
Analogy, 123
Anchitherium, 94
Ancon sheep, 148
Angrosperms, 56
Animals and plants, constitution of, 36
Annulosa, 42
Anoplotherium, 153
Anted on, 198
Antirrhinum, 83
Antlia, 263
Aphides, 246, 288
Aphysia, 228
Appendicularia, 176
Apple, 141
Apteryx, 87
Apus, 2IO
Aqueous rocks, 369
Arachnida, 354
Archseopteryx, 158 et seq.
Archetype, 128
Architroch, 235
Arenicola, 231
Aristotle, 38, 169
Arius, 239
Arm of man, 90, 109
Arthropoda, 50, 268
Articulata, 40, 42, 45, 51, 55
Ascidians (see Sea-squirts)
Ascula, 174
Auricularia, 200
Australian fauna, 27
Axolotl, 236 et seq. , 242 et seq. , 248
B
Balani (see Sea-acorns)
Balanoglossus, 55, 201
Balfour, F. M., 236, 237
Barnacle, 32
Barnacle, development of, 203, 352
Barramunda (see Ceratodus)
" Beagle," voyage of, i
Bear animalcule, 185, 271
Beetles, 85
Beryx, 346
Biology, study of, 9, 14
Bipinnaria, 196
Birds and reptiles, affinity of, r 54
Birds, development of, 179
Birds fertilizing flowers, 335
Birds, fossil, 157
Blank periods in geology, 369
Blastoderm, 169, 177
Blood corpuscles, 73, 295
Bone, structure of, 71
Bonnet, 170, 183
Botany, 14
Botryllus, 288
Brachiopoda, 230
Bracts, 131, 132
Branchial clefts (see Gill clefts)
Brine shrimp, 209
Browning, 150
Brown-Sequard, 85
Bubble-shells, 228
378
INDEX,
BufFon, 1 6
Bulla, 228
Butterfly, 41,258, 263
c
Cactus, 318
Calyx, 82, 131
Camellia, 132
Campanulacese, 327
Campodea, 269, 270
Carp, 1 16, 117
Carpenter, Dr., 364
Cave crab, 87
Cave rat, 87
Cecidomyia, 246
Cells, 59, 62, 70 et seq., 294
Cellulose, 357
Centaury, 302
Centipedes, 287
Cephalization, 362
Cephalopoda (see Cuttlefishes)
Ceratodus, 30, 113, 114, 116, 165,237
Chara, 68
Chauvin, Fraulein von, 242 et seq.
Chermes, 289
Cherry, 133, 304
Chironomus, 246
Chiton, 222, 225, 233
Chlamydomonas, 278
Chloeon, 257, 267, 270
Chlorophyll, 359
Chondrocanthus, 208
Christol De, 93
Chrysalis, 252, 257
Cicatricula, 169, 179
Cilia, 69, 196, 223
Cimcera, 114
Cirrahatula, 286
Classification, 19, 20, 27, 124
Cleistogamous flowers, 320
Climbing perch, 119
Clio, 222
Clover, fertilization of, 338
Coccus, 289
Coccyx, 99, 101
Cockle, 223
Cockroach, 259
Ccelenterate animals, 46, 55, 184, 282
Collembola, 268
Colonial animals, 273 et seq.
Comatula, 197, 256
Composite plants, 301
Compound animals, 273 et seq.
Compsognathus, 163
Cones, 57
Confervas, 69
Coniferse, 318
Constitution of animal and plant
worlds, 9, 36, 48
Corolla, 82, 131
Corymb, 304
Cosmogony, ancient, 3
Cotyledons, 57
Cowslip, 320
Crabs, 32, 87
Crabs, development of, 212
Crayfish, 211
Cricket, 257, 265
Crinoids, 195, 198
Cross-fertilization, 320 et seq., 324, 325,
339
Cruciferse, 84
Crustaceans, n
Crustaceans, development of, 202 et seq.,
217
Crustaceans, pedigree of, 219
Cryptogams, 56
Cuttlefishes, 222
Cuvier, 32, 38, 40, 48, 93, 153, 347
Cyclops, 49, 208, 215
Cypris, 204, 206, 207
D
Dactylethra, 237
Dana, 362
Dandelion, 302
Daphnia, 207
Darwin, Mr., I, 2, 5, 12, 15, 28, 32,
79, 81, 85, 86, 99, 101, in, 119,
129, 142, 144, 146, 147, 148, 150,
151, 172, 187, 189, 191, 247, 260,
320, 321, 322, 323, 330, 337, 338,
339, 340, 343, 347, 348, 349, 352,
366, 369, 370, 371
Darwinism, 6, 7
Dead-nettle, 334
Decapoda, 210
Degeneration, n, 342 et seq.
Degeneration in man, 363
Demodex, 355
Dentalium, 225, 233
' Descent of Man,' 150
Development, bearing of on evolution,
»72, 343
Development of animals, 5, IO, 19, 32,
167, 191, 220
INDEX.
379
Development theory {see Evolution)
Developmental changes, 260
Diatoms, 58
Dicotyledons, 56, 57
Dinoceras, 152
Dinornis, 88
Dionoea, 22
Distribution, 23 et seq., 28, 29
Dodo, 88
Doris, 228
Dragon-flies, 261
Dujardin, 63
Dumeril, 242
E
Early stages in animal development, 167
Echinoderms, n, 46, 55
Echinoderms, development of, 195 et
seq., 200, 216
Ectoderm, 173
Egg, 255, 275
Egg of sponge, 173
E/nboitement, 170
Embryonic forms as fossils, 373
Encrinites, 198
Endoderm, 173
Eohippus, 95
Ephemera, 267
Epiblast, 179, 180, 186
Epigenesis, 169, 183
Epilepsy, 85
Equisetse, 58
Espada, 239
Etiology (see ^Etiology)
Evolution (see also Natural Selection)
Evolution, theory of, 4, 32, 34, 253
Eye, development of in vertebrates
and invertebrates, 358
Feather star (see Comatula)
Ferns, 56
Fertilization of flowers, n, 308 et seq.
Fertilization, process of, 317
Fig, 141
Fins of fishes, no, 113
Fish-lice, 208
Fish, structure of, 43
Fishes' tails, 102 et seq.
Flies, 258
Flounder, tail of, 104
Flower, Professor, 31, 112
Flowers, II, 56
Flowers, fertilization of, 308
Flow< rs, structure of, 310, 311
Flustra, 230, 282, 283
Food in relation to degeneration, 359
Foot of mollusca, 224
Foraminifera, 46, 277
Fossils, 146
Fossils in relation to evolution, 369
et seq.
Fovilla, 317
Frog, 118, 119, 235
Frog, development of, 119, 193, 344
Fruits, iy)etseq., 313
G
Galen, 169
Ganoid fishes, 106, 116, 118
Gasteropoda. 222, 224, 246
Gastrula, 173, 175, 178
Gegenbaur, 267, 288
Geology and evolution, n, 151, 366
et seq.
Germ (see Egg)
Gill clefts, 101, 181, i%% et seq.
Globe animalcules, 278
Goethe, 125, 126, 130, 133
Gorilla, 150
Grasshopper, 264
Gray, Asa, 291, 301
Gregarina, 275
Gymnosperms, 56, 58
H
Haberlein, 158
Hseckel, 184, 186, 287, 288
Harvey, 169, 183
Haversian canals, Jl
Hawthorn, 138
Heredity, 194
Helmho'ltz, 15
Hesperornis, 160, 161, 162
Heterocercal tail, 106
Hipparion, 93
Hippocampus, 239
Hippocrates, 169
Holothurians, 199
Homocercal tail, 106
Homogeneous structures, 128
Homology, 10, 15, 34, 123 et seq.
Homoplastic structures, 129, 134
TNDEX.
Horse, 33, 90 et seq.
Humming birds, 249
Huxley, Professor, 6, 49, 54, 77, 125,
127, 143, 147. 153. l66» J94, 195,
211,212, 214, 233, 236, 268,279,
288, 289, 293, 370
Hyalaea, 222
Hybrid, 310
Hydra, 44, 281, 360
Hylodes, 238, 240
Hypoblast, 1 80, 186
Ichthyornis, 160, 161, 162
Ichthyosaurus, 346
Iguanodon, 163
Imaginal discs, 258
Individuality of animals, 292
Innervation, 23
Insect fertilization of flowers, 322
Insects, life histories of, 252 et seq.
Intermaxillary gland, 248
Invertebrata, 39, 53
Irritability, 23
Jaws, 135
Jelly-fishes, 45, 283
K
Kangaroo, 26 et seq., 254
King crab, 209, 346
Kirby, 85, 191, 192
Kowalewsky, 54
Lamarck, 2, 16, 32, 39
Lamellibranchiata, 222
Lamprey, 106, 236
Lampshells, 230, 233, 345
Lancelet, 53, 106, 176 et seq.
Lankester, Ray, 129, 226, 234, 358
Larva, 252, 257
Leaf type, 131
Leaves, 131, 137
Leibnitz, 169
Lepidosiren, 106, 117 et seq., 165, 237
Lepidosteus, 1 06, 117
Lepisma, 269
Lernese, 209
Lernceocera, 208
Life, nature of, 75 (see also Protoplasm)
Likenesses, science of, 12 1 et scq.
Lily, 133
Limbs, development of, 182
Limbs of animals, 108 et scq.
Limbs, origin of, 113
Limulus, 209
Lindia, 271
Linguatulina, 355
Links missing (see Missing links)
Linnaeus, 1 6, 38, 319
Liverworts, 58
Living beings, study of, 18
Lobster, 40, 41, 211, 212
Long-styled flowers, 320
Lubbock, 241, 257, 258, 261, 262, 264,
267, 268, 330, 333
Lyell, 151
Lymneus, 226
M
Macaque monkey, 101
Madeira, beetles of, 86
Magosphaera, 184
M aliens, 135, 155
Mallophaga, 269
Malpighi, 169, 183
Mammalia, extinct, 152
Man, colonial nature of, 306
Manx cat, 99
Maraucheuia, 152
Marsh, 152, 160
Marsupials, 27
Marsupials, feet of, III
Medici, 169
Megalopa, 213
Megalosaurus, 164
Mesoblast, 180, 1 86
Mesohippus, 95
Metamorphosis, II
Metamorphosis of insects, 252 et seq.t
257, 262
Metamorphosis of plants, 130
Miohippus, 95
Missing links, 10, 34, 53, 143 et seq.
Mites, 354, 355
Mivart, St., 239
Modification of habits, 241
Molluscs, II, 40, 46, 51
Molluscs, development of, 220 et seq.
Monads, 59
Monboddo, Lord, 101, 144
1XDEX.
Monocotyledons, 56, 57
Morphology, 19
Morren, 318
Morula, 173, 184, 185, 186
Mosses, 56, 58
Mouth of insects, 263
Mulberry stage (see Morula)
Mulder, 63, 64
Miiller, F., 204, 318
Myosotis, 329
Myrmecobius, 29
Mysis, 213, 214
Myxodictyum, 277
N
Nageli, 332
Nais, 285, 297
Natural history, 14, 16
Natural selection, 5, 7, 8, 112, 146
Nauplius, 203, 206 et sey., 211, 268,
351
Nautilus, 347
Nereis, 231
Nerve-cells, 72
Nettles, 311
Neuroptera, 261
Newts, 235 et seq. , 246
Notochord, 53, 104, 177, 178, 181,
356
Nototrema, 238
"Nurses," 261
Nutrition, 22
Oak, 311
Odontopteryx, 161
Ogle, 335
Oken, 125
Olynthus, 172
Ontogenesis, 193
Oolite, 29
Operculum, 232
Ophiocephali, 119
Opisthodelphys, 238
Opossums, 30
Orchids, fertilization of, 336
Origin of species (see Species ; Natural
selection, &c.)
Ornithorhynchus, 165
Orohippus, 95
Orthoptera, 259
Otter sheep, 148
Ovary (see Pistil)
Ovules, 308, 309
Ovum (see Egg)
Owen, 125, 126, 161
Oyster, 223
Packard, 266
Paget, 364
Palaeotherium, 153
Palceozoic rocks, 106
Pallas, 99
Palm, fertilization of, 326
Pander, 170, 183
Parasitism, 50, 350
Parker, W. K., 126, 127
Passion flower, 137
Personality of animals, 289, 292
Petals (see Corolla)
Petiole, 137
Petunia, 133
Penoeus, 213 et seq.
Pentacrinus, 199
Pea, 137, 318, 333
Pear, 141
Peripatus, 268
Permanent larval forms, 176
Phanerogams, 56, 57
Phohippus, 95
Phylogenesis, 193
Physiological division of labour, 49
Physiology, 21
Pigeons, 147
Pipa, 239
Pistil, 82, 131, 312 et seq. '
Placenta, 315
Plantago, 328
Plant-lice (see Aphides)
Plants, industry of, 301
Planula, 173
Plesiosaurus, 346
Pluteus, 197
Pollen (see Stamen)
Pollinia, 316
Polyenma, 267
Polypterus, 103, 116
Polyzoa, 230
Pond snail, 226
Prickles, 138
Primitive groove, 180
Primrose, 310, 315, 320
Proteus, 119, 236
Prothallus, 58
382
INDEX.
Protohippus, 95
Protoplasm, 9, 61 et sey., 34.2
Protozoa, 46, 55
Protozoa, development of, 183
Pseudis, 238
Pseud-ovum, 290
Psyche, 289
Pteridophytes, 56
Pterodactyls, 164
Pteronarcys, 268
Pteropoda, 45, 222, 229, 246
Pupa, 203
Quadrate bone, 135, 155
R
Races, 33, 142
Radiata, 40
Ramsay, 369
Recent rocks, 93
Relation, 23
Remak, 63
Reproduction, 22
Reptiles and birds, 1 54
Reversion, 100, 142, 248
Rhinoderma, 239
Rock formations, table of, 368
Rose, 132
Rose, fruit of, 140
Rotifera, 227, 271, 360
Rudimentary organs, IO, 33, 80 et seq.,
282
Sacculina, 32, 202, 205 et seq., 351
Sacrum, 155
Salamander, 242, 244
Salmon, 103
Salvia, 336
Sarcode (see Protoplasm)
Schultz, 63
Scrophularia, 83
Sea-acorns, 202, 352
Sea-butterflies (see Pteropoda)
Sea-cucumbers, 199
Sea-hares, 228
Sea-lemons, 228
Sea-lilies (see Crinoids)
Sea-mats, 230, 233, 283
Sea-squirts, 54, 174
Sea-squirts, development >f,
356
Sea-urchins, 195, 197
Sea-weeds, 56, 58, 69
Seeds, 82, 309, 317, 318
Segmentation of egg, 173, 184
Selection (see Natural and Sexual
selection)
Self-fertilization, 319
Sensations, 66, 67
Sensitive plant, 68
Serpula, 231
Sexual selection, 8, 9
Shark, IO2
Shipworm, 223
Short-styled flowers, 320
Shrimp, 32
Siredon (see Axolotl)
Sitaris, 247
Sloe, 138
Smilax, 138
Snapdragon, 83
Solenobia, 289
Solitaire, 88
Species, 5, 7, 8, 38
Species, origin of, I, 2, 4, 7, 8, 12
Spence, 191, 192
Spencer, 2, 3, 16, 17, 52, 60
Spider monkey, 98
Spine of man, 99
Splint bones, 92 et seq., 96
Sponge, 278, 279
Sponge, development of, 172
Spore, 58
Sprengel, 330, 333, 339
Springtails, 257
Stability of living beings, 344, 345
Stamen, 82, 131, 312 et seq., 318
Star-fishes, 195, 196, 287
Stipules, 137
Strawberries, 140
Struggle for existence, 7
Stylops, 354
Succession of life, 370
Sunbirds, 249
Surinam toad, 239
Survival of fittest, 8
Swifts, 249
Swimming-bladder (see Air-bladder)
Sympathetic system, 43
Tadpole, history of, 236
Tails of anima's, 98 et seq.
Tapeworm, 285, 350
INDEX.
333
Tardigrade, 185
Taxonomy, 19
Tendrils, 136
Teratology, 96
Terebratulina, 230
Teredo, 223
Tertiary rocks, 93
Thallophytes, 56
Thistle, 302
Thompson, Vaughan, 213
Thomson, Allen, 185, 190
Thorns, 138
Thysanura, 257
Tillodontia, 152
Time past, in relation to evolution, 367
Topshell, 227
Tornaria, 20 1
Tortoise, 134
Toxodonts, 152
Tradescantia, 68
Trias, 29
Trilobites, 209
Triton (see Newt)
Trochus, 227
Tube-worms, 231
Tunicates, 54
Types of animals, 39, 46, 47, 48, 51,
55
Types of plants, 56
u
Umbel, 304
Unio, 224
Urochord, 175
Velum, 223 et seq.> 229, 234
Venus' flytrap, 22, 68
Verbascum, 83
Vertebral theory of skull, 125 et sej,
Vertebrata, 39, 51, 53, 144
Vertebrate embryos, 100
Vetches, 137
Vine, 137
Virginia creeper, 137
Visceral clefts (see Gill clefts)
Vogt, 159
Volvox, 278, 301
Von Baer, 40, 100, 170, 171, 192, 220
w
Wallace, Mr. A. R., i, 6, 249, 250,
251
Wallace's line, 28, 30
Wallflower, 82, 131
Water flea, 32 (see also Cypris,
Cyclops, and Daphnia)
Weismann, 242 et seq., 248, 249
Whale, 20, 21, 88, 102
Whalebone, 88
Wheel animalcules, 227, 360
Wing of bird, 155
Wings of insects, 265 et seq.
Wings, rudimentary, 85 et seq,
Wolff, 130, 183
Worms, 231, 233, 285
Yeast plant, 58
Vallisneria, 68, 326
Variation, 33
Varieties, 2, 4, 142
Vegetative repetition, 286
Veliger stage, 223, 227, 233, 234
Zeuglodon, 89, 153
Zoea, 213, 214, 215
Zoology, 14
Zoophytes, 45, 281
Zoospores, 69
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