The Chain of Life in Geological Time
by Sir John William Dawson · English · Gutenberg #36261 · 421,114 chars · public domain (US)
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Title: The Chain of Life in Geological Time
Author: Sir John William Dawson
Release date: May 30, 2011 [eBook #36261]
Language: English
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| TranscriberaEuro(TM)s note: |
| |
| Genus names are not consistenly italicized in the original book. |
| These have been corrected for consistency according to the |
| modern usage except in the Index where they are consistently |
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+------------------------------------------------------------------+
[Illustration: LIFE IN THE SILURIAN AGE.]
THE CHAIN OF LIFE
IN
GEOLOGICAL TIME
_A SKETCH OF THE ORIGIN AND SUCCESSION OF
ANIMALS AND PLANTS_
BY
SIR J. WILLIAM DAWSON
C.M.G., LL.D., F.R.S., F.G.S., ETC.
AUTHOR OF
aEurooeACADIAN GEOLOGY,aEuroŁ aEurooeTHE STORY OF THE EARTH,aEuroŁ aEurooeEGYPT AND SYRIA; THEIR
PHYSICAL FEATURES IN RELATION TO BIBLE HISTORY,aEuroŁ ETC.
_THIRD AND REVISED EDITION_
WITH NUMEROUS ILLUSTRATIONS
THE RELIGIOUS TRACT SOCIETY,
56 PATERNOSTER ROW; 65 ST. PAULaEuro(TM)S CHURCHYARD;
AND 164 PICCADILLY
1888
RICHARD CLAY AND SONS.
LONDON AND BUNGAY.
PREFACE.
Questions as to the origin and history of life are not at the present
time answered by mere philosophical speculation and poetical imagining.
Such solutions of these questions as science can profess to have
obtained are based on vast accumulations of facts respecting the remains
of animals and plants preserved in the rocky beds of the earthaEuro(TM)s crust,
which have been successively accumulated in the course of its long
geological history. These facts undoubtedly afford the means of
attaining to very certain conclusions on many points relating to the
history of life on the earth. But, on the other hand, they have
furnished the material for hypotheses which, though confidently affirmed
to be indisputable, have no real foundation in nature, and are
indirectly subversive of some of the most sacred beliefs of mankind.
In these circumstances it is most desirable that those who are not
specialists in such matters should be in a position to judge for
themselves; and it does not appear impossible in the actual state of
knowledge, to present, in terms intelligible to the general reader, such
a view of the ascertained sequence of the forms of life as may serve at
once to give exalted and elevating views of the great plan of creation,
and to prevent the deceptions of pseudo-scientists from doing their evil
work. Difficulties, no doubt, attend the attempt. They arise from the
number and variety of the facts, from the uncertainties attending many
important points, from the new views constantly opening up in the
progress of discovery, and from the difficulty of presenting in an
intelligible form the preliminary data in biology and geology necessary
for the understanding of the questions in hand. In order, as far as
possible, to obviate these difficulties, the plan adopted in this work
has been to note the first known appearance of each leading type of
life, and to follow its progress down to the present time or until it
became extinct. This method is at least natural and historical, and has
commended itself to the writer as giving a very clear comprehension of
the actual state of our knowledge, and as presenting some aspects of the
subject which may be novel and suggestive even to those who have studied
it most deeply.
In selecting examples and illustrations, the writer has endeavoured to
avoid, as far as possible, those already familiar to the general reader.
He has carefully sought for the latest facts, while rejecting as
unproved many things that are confidently asserted; and has endeavoured
to avoid all that is irrelevant to the subject in hand, and to abstain
from all technical terms not absolutely essential. In a work at once so
wide in its scope, so popular in its character, and so limited in its
dimensions, a certain amount of hostile criticism on the part of
specialists is to be expected, some portion of it perhaps just, other
portions arising from narrow prejudices due to limited lines of study.
The writer is willing to receive such comments with attention and
gratitude, but he would deprecate the misuse of them in the interest of
those coteries which are at present engaged in the effort to torture
nature into a confession of belief in the doctrines of a materialistic
or agnostic philosophy.
The title of the work was suggested by that of GaudryaEuro(TM)s recent
attractive book, _Les EnchaA(R)nements du Monde animal_. It seemed well
fitted to express the connection and succession of forms of life,
without implying their derivation from one another, while it reminds us
that nature is not a fortuitously tangled skein, and that the links
which connect man himself with the lowest and oldest creatures bind him
also to the throne of the Eternal.
In the few years that have elapsed since the publication of the first
edition of this work, great additions have been made to our knowledge of
fossil animals and plants. Many new species have been described, and
many new facts have been discovered, respecting species previously
known. This rapid progress of discovery has, however, invalidated few of
the statements made in the first edition, and has certainly established
nothing against the general laws of the succession of life as stated in
this work.
Perhaps the most interesting phase of recent discovery is the tracing
back of certain forms of life to earlier periods of the earthaEuro(TM)s
geological history. Some of the most recent facts of this kind are the
finding, by M. Charles Brongniart, of a fossil insect, allied to the
_Blattae_ or cockroaches, in the Silurian of Spain, that of true
Scorpions in the Upper Silurian of Sweden by LindstrAÂm, and in the Upper
Silurian of Scotland by Peach, who has also described fossil Millipedes
from the Lower Devonian. The tendency of such discoveries is to carry
farther back the origin of highly specialised forms of life, and thus to
render less probable their origin by any process of gradual derivation.
Other discoveries serve to fill up blanks in our knowledge, and thus to
render the geological record less imperfect. Of this kind is the close
approximation now worked out in Western America between the end of the
reign of the great MesozoA-c reptiles and the beginning of that of the
mammals of the TertiaryaEuro"a great and abrupt revolution, effected
apparently by a _coup de main_. I have myself had opportunity to show
that a similarly sharp line separates that quaint old MesozoA-c flora of
pines, cycads and ferns, which extends upward into the Lower Cretaceous,
from the rich and luxuriant assemblage of broad-leaved trees of modern
aspect, which takes its place in the middle part of the same formation.
It is not too much to say that these and similar discoveries, while they
serve to bridge over gaps in the succession of organic beings, do not
favour the theory of slow modification of types. They rather point to a
law of rapid development of new forms under special conditions as yet
unknown to science, and this accompanied with the extinction of older
species. Recent discoveries also present many remarkable instances of
the early introduction of highly specialised types, of higher forms
preceding those that are lower in the same class, and of the persistence
of certain types throughout geological time without any important
change.
J. W. D.
MCGILL COLLEGE.
CONTENTS.
CHAP. PAGE
I. PRELIMINARY CONSIDERATIONS AS TO THE EXTENT AND
SOURCES OF OUR KNOWLEDGE 1
II. THE BEGINNING OF LIFE ON THE EARTH 21
III. THE AGE OF INVERTEBRATES OF THE SEA 45
IV. THE ORIGIN OF PLANT LIFE ON THE LAND 89
V. THE APPEARANCE OF VERTEBRATE ANIMALS 117
VI. THE FIRST AIR-BREATHERS 137
VII. THE EMPIRE OF THE GREAT REPTILES 165
VIII. THE FIRST FORESTS OF MODERN TYPE 185
IX. THE REIGN OF MAMMALS 207
X. THE ADVENT OF MAN 233
XI. REVIEW OF THE HISTORY OF LIFE 253
LIST OF ILLUSTRATIONS.
FRONTISPIECE.aEuro"Life in the Silurian Age _To face Title._
FIG. PAGE
1. Bank of stream or coast, showing stratification 4
2. Section at Niagara Falls 4
3. Section obtained by boring, near Goderich, Ontario 5
4. Inclined beds, holding fossil plants 6
5. Ideal section of the Apalachian Mountains 7
6. Generalised section across England from Menai Straits to
the Valley of the Thames 9
7. Generalised section from the Laurentian of Canada to the
coal-field of Michigan 9
8. Unconformable superposition of Devonian Conglomerate on
Silurian slates, at St. AbbaEuro(TM)s Head, Berwickshire 10
9. Section of Trenton limestone, Montreal 14
10. Diagram showing different state of fossilisation of a
cell of a Tabulate Coral 15
11. Cast of erect tree (_Sigillaria_) in Sandstone 16
12. _Protichnites septem-notatus_ 17
12_a_. Footprints of modern _Limulus_, or king-crab 18
13. Current markings on shale, resembling a fossil plant 18
_Frontispiece._ Magnified and restored section of a portion
of _Eozoon canadense_ 20
14. Ideal section, showing the relations of the Laurentian and
Huronian 24
15. Small weathered specimen of _Eozoon_ 28
16. Nature-printed specimen of _Eozoon_ slightly etched with acid 29
17. Magnified group of canals in supplemental skeleton of
_Eozoon_ 31
18. Portion of _Eozoon_ magnified 100 diameters 31
19. Magnified portion of shell of _Calcarina_ 32
20. _AmA"ba_, a fresh-water naked Rhizopod; and _Actinophrys_,
a fresh-water Protozoon 34
21. _Nonionina_, a modern marine Foraminifer 34
22. _Stromatopora concentrica_ 35
23. _Caunopora planulata_ 36
24. _ArchA|ocyathus minganensis_. A Primordial Protozoon 37
25. Receptaculites. Restored 38
26. Section of _Loftusia Persica_. An Eocene Foraminifer 39
27. Foraminiferal Rock Builders, in the Cretaceous and Eocene 41
_Frontispiece._ _Paradoxides Regina_ (Matthew) 44
28. Group of Cambrian Animals 46
29. Portion of skeleton of Hexactinellid Sponge (_CA"loptychium_) 49
30. _Protospongia fenestrata_ (Salter) 50
31. _Astylospongia prA|morsa_ (Roemer) 51
32. Spicules of Lithistid Sponge (_Trichospongia_, Billings) 51
33. _Oldhamia antiqua_ (Forbes) 52
34. _Dictyonema sociale._ Enlarged 52
35. _Dictyonema Websteri_ (Dn.) 53
36. Group of modern Hydroids allied to Graptolites 54
37. Silurian GraptolitidA| 55
38. Central portion of Graptolite, with membrane, or float
(_Dichograpsus octobrachiatus_, Hall) 55
39. _Ptilodictya acuta_ (Hall). Bryozoan 55
39_a_. _Fenestella Lyelli_ (Dn.). A Carboniferous Bryozoan 56
40. _Chaetetes fibrosa._ A Tabulate Coral with microscopic
cells 56
41. _a_, _Stenopora exilis_ (Dn.). _b_, _Chaetetes tumidus_
(Edwards and Haine) 57
42. Living Anthozoan Coral (_AstrA|a_) 58
43. Tabulate Corals (_Halisites_ and _Favosites_) 59
44. Rugose Coral (_Heliophyllum Halli_) 59
44_a_. _Zaphrentis prolifica_ (Billings) 60
45. Rugose Corals (_Zaphrentis Minas_, Dn., and _Cyathophyllum_
_Billingsi_, Dn.) 60
46. Modern Crinoid (_Rhizocrinus Lofotensis_) 61
47. _PalA|aster Niagarensis_ (Hall) 62
48. _PalA|chinus ellipticus_ (McCoy) 62
49. _Pleurocystites squamosus_ 63
50. _Heterocrinus simplex_ (Meek) 63
51. Body of _Glyptocrinus_ 63
52. _Extracrinus Briareus_ 64
53. _Pentacrinus caput-medusA|_ 64
54. _Lingula anatina_ 65
55. Cambrian and Silurian LingulA| 65
56. _Terebratula sacculus_ (Martin) 66
57. Brachiopods; genus _Orthis_ 66
58. _Rhynchonella increbrescens_ (Hall) 66
59. _Spirifer mucronatus_ (Conrad) 67
59_a_. _Athyris subtilita_ (Hall) 67
60. _Productus cora_ (DaEuro(TM)Orbigny) 68
61. Group of Older PalA|ozoic Lamellibranchs 69
62. _Conularia planicostata_ (Dn.). A Carboniferous Pteropod 70
63. Silurian Sea-snails 70
64. Squid (_Loligo_) 72
65. Pearly Nautilus (_Nautilus pompilius_) 72
66. _Orthoceras_ 73
67. _Gomphoceras_ 73
68. _Lituites_ 73
69. _Nautilus Avonensis_ (Dn.) 74
70. _Goniatites crenistria_ (Philips) 74
71. _Ceratites nodosus_ (Schloth) 75
72. _Ammonites Jason_ (Reinecke) 76
72_a_. Suture of _Ammonites componens_ (Meek) 76
73. Cretaceous AmmonitidA| 77
74. _Belemnite_ 78
74_a_. _Belemnoteuthis antiquus_ 78
75. Cambrian Trilobites 79
76. Transverse section of _Calymene_. A Silurian Trilobite 80
76_a_. Burrows of Trilobite and of modern King-crab 81
77. Silurian Trilobites 82
78. Devonian and Carboniferous Trilobites 83
79. PalA|ozoic Ostracod Crustaceans 83
80. _Pterygotus anglicus_ 84
81. _Amphipeltis paradoxus_ (Salter) 85
82. _AnthropalA|mon Hilliana_ (Dn.) 85
_Frontispiece._ _Cordaites_, of the group of _Dory-Cordaites_ 88
83. _Protannularia Harknessii_ (Nicholson) 91
84. American Lower Silurian Plants 92
86. Fragment of outer surface of _Glyptodendron_ of Claypole 93
87. _Psilophyton princeps_ (Dn.) 95
88. Trunk of a Devonian Tree-fern (_Caulopteris Lockwoodi_, Dn.) 97
89. Frond of _ArchA|opteris Jacksoni_ (Dn.) 98
90. Portion of a branch of _Leptophleum rhombicum_ (Dn.) 98
91. _Calamites radiatus_ (Brongniart) 99
92. A Devonian Taxine Conifer (_Dadoxylon ouangondianum_, Dn.) 100
93. Group of Devonian fruits, &c. 101
94. Structures of the oldest-known Angiospermous Exogen
(_Syringoxylon mirabile_, Dn.) 102
95. _Asterophyllites parvula_ (Dn.) and _Sphenophyllum
antiquum_ (Dn.) 103
96. _Calamites_ 104
97. Carboniferous Ferns 105
98. Carboniferous Tree-ferns 107
99. _Lepidodendron corrugatum_ (Dn.) 108
100. _SigillariA|_ of the Carboniferous 109
101. _Trigonocarpum Hookeri_ (Dn.) 111
_Frontispiece._ Pteraspis. Restored 116
102. Siluro-Cambrian Conodonts 118
103. Lower Carboniferous Conodont 119
104. _a_, Head-shield of an Upper Silurian Fish (_Cyathaspis_);
_b_, Spine of a Silurian Shark (_Onchus tenui-striatus_,
Agass.); _c_, _d_, Scales of _Thecodus_ 121
105. _Cephalaspis Dawsoni_ (Lankester) 122
106. Devonian Placoganoid Fishes (_Pterichthys cornutus_,
_Cephalaspis Lyelli_) 123
107. Devonian Lepidoganoid Fishes (_Diplacanthus_ and
_Osteolepis_) 124
108. Modern Dipnoi (_Ceratodus Fosteri_ and _Lepidosiren
annectus_) 124
109. Anterior part of the palate of _Dipterus_ 125
110. Dental plate of _Conchodus plicatus_ (Dn.) 126
111. Dental plate of _Ceratodus Barrandii_ 126
112. Dental plate of _Ceratodus serratus_ 127
113. Jaws of _Dinichthys Hertzeri_ (Newberry) 127
114. Lower Jaw of _Dinichthys Hertzeri_ 128
115. Jaws of _Lepidosiren_ 128
116. A small Carboniferous Ganoid (_PalA|oniscus_
(_Rhadinichthys_) _Modulus_, Dn.) 129
117. Teeth and Spines of Carboniferous Sharks 130
118. Teeth of Cretaceous Sharks (_Otodus_ and _Ptychodus_) 131
119. Tooth of a Tertiary Shark (_Carcharodon_) 132
120. A Liassic Ganoid (_Dapedius_) 132
121. Cretaceous Fishes of the modern or Teleostian type
(_Beryx Lewesiensis_ and _Portheus molossus_, Cope) 133
122. Modern Ganoids (_Polypterus_ and _Lepidosteus_) 134
_Frontispiece._ A Microsaurian of the Carboniferous Period
(_Hylonomus Lyelli_) 136
123. Wings of Devonian Insects 140
124. Land-snail (_Pupa vetusta_, Dn.) 143
125. Land-snail (_Zonites_ (_Conulus_) _priscus_, Carpenter) 143
126. Millipedes (_Xylobius sigillariA|_, Dn.; _Archiulus
Xylobioides_, Scudder; _X. farctus_, Scudder) 145
127. Wings of Cockroaches 146
128. Wing of May-fly (_Haplophlebium Barnesii_, Scudder) 147
129. A Jurassic Sphinx-moth (_Sphinx Snelleri_, Weyenburgh) 148
130. An Eocene Butterfly (_Prodryas persephone_, Scudder) 149
131. Abdominal part of a Carboniferous Scorpion 150
132. Carboniferous Scorpion (_Eoscorpius carbonarius_,
Meek and Worthen) 151
133. Footprints of one of the oldest known Batrachians,
probably a species of _Dendrerpeton_ 152
134. _Archegosaurus Decheni_ 154
135. _Ptyonius_ 154
136. A large Carboniferous Labyrinthodont (_Baphetes
planiceps_, Owen) 155
137. _Baphetes planiceps_ (Owen) 156
138. A lizard-like Amphibian (_Hylonomus aciedentatus_) 157
139. _Stelliosaurus longicostatus_ (Fritsch) 158
140. Section showing the position of an erect _Sigillaria_,
containing remains of land animals 160
140_a_. Section of base of erect _Sigillaria_, containing
remains of land animals 161
_Frontispiece._ Inhabitants of the English Seas in the Age of
Reptiles 164
141. Arm of _Proterosaurus Speneri_ 166
142. Skeleton of _Ichthyosaurus_ 167
142_a_. Head of _Pliosaurus_ 168
142_b_. Paddle of _Plesiosaurus Oxoniensis_ 168
143. Skeleton of _Clidastes_ 170
144. An Anomodont Reptile of the Trias (_Dicynodon
lacerticeps_, Owen) 170
145. A Theriodont Reptile of the Trias (_Lycosaurus_) 170
146. Skeleton of _Pterodochylus crassirostris_ 170
147. Restoration of _Rhamphorhyncus Bucklandi_ 171
148. A Jurassic bird (_ArchA|opteryx macroura_) 172
149. Jaw of a Cretaceous Toothed Bird (_Ichthyornis dispar_) 173
150. Jaw of _Bathygnathus borealis_ (Leidy) 174
151. _Hadrosaurus Foulkii_ (Cope) 175
152. Jaws of _Megalosaurus_ 176
153. Tooth of _Megalosaurus_ 177
154. _Compsognathus_ 179
_Frontispiece._ Lower Cretaceous Leaves 184
155. _Sassafras cretaceum_ (Newberry) 190
156. _Liriodendron primA|vum_ (Newberry) 191
157. _Onoclea sensibilis_ 191
158. _Davallia tenuifolia_ 192
159. Eocene Leaves 194
160. An Ancient Clover (_Trifolium palA|ogA|um_, Saporta) 195
161. An Eocene Maple (_Acer sextianus_, Saporta) 195
162. A European _Magnolia_ of the Eocene (_M. dianA|_, Saporta) 195
163. Flower and Leaf of _Bombax sepultiflorum_ 196
164. Branch and Fruit of _Sequoia CouttsiA|_ (Heer) 197
165. _Cinnamomum Scheuchzeri_ (Heer) 198
_Frontispiece._ _Sivatherium giganteum_ 206
166. Jaw of _Dromatherium sylvestre_ (Emmons) 209
167. _Myrmecobius fasciatus_ 209
168. Jaw and Molar of _Phascolotherium Bucklandi_ 210
169. Jaw and Pre-molar of _Plagiaulax Becklesii_ 210
170. Restoration of _PalA|otherium magnum_ 211
171. Skull of a Lower Eocene Perissodactyl (_Coryphodon
Hamatus_) 214
172. Fore-foot of _Coryphodon_ 215
173. Skull of Upper Eocene Perissodactyl (_Dinoceras
mirabilis_) 216
174. Fore-foot of _Dinoceras_ 217
175. Skull of Miocene Perissodactyl (_Brontotherium ingens_,
Marsh) 217
176. Series of Equine feet 218
177. Skull of generalised Miocene Ruminant (_Oreodon major_) 221
178. Lower Jaw of _Megatherium_ 222
179. Ungual Phalanx and Claw-core of _Megatherium_ 222
180. Tooth of Eocene Whale (_Zeuglodon cetioides_) 223
181. _Mastodon ohioticus_ 225
182. Head of _Dinotherium giganteum_ 226
183. Wing of Eocene Bat (_Vespertilio aquensis_) 226
184. Skull of a Cymetar-toothed Tiger (_Machairodus
cultridens_) 228
185. Lower Jaw of _Dryopithecus Fontani_ 229
_Frontispiece._ Contemporaries of Post-Glacial Man 232
186. _Elephas primigenius_ 241
187. Tooth of _Elasmotherium_ 242
188. Engis Skull 243
189. Outlines of Three Prehistoric European Skulls compared
with an American Skull 244
190. Flint Implement found in KentaEuro(TM)s Cavern, Torquay 245
191. Bone Harpoon (PalA|ocosmic) 246
192. Sketch of a Mammoth carved on a portion of a Tusk of the
same Animal 249
Tabular View of Geological Periods and of Life-Epochs.
+--------------------------------------+---------------+--------------+
| GEOLOGICAL PERIODS. | ANIMAL LIFE. | VEGETABLE |
| | | LIFE. |
+--------------------------------------+---------------+--------------+
| | | |
|C {_Post- {Modern. | Age of _Man_ | |
|A N {Tertiary_ or {Post-Glacial. | and _modern | |
|I E {_Quaternary_. | Mammals_. | Age of |
|N o O { | | _Angiosperms_|
|O r Z {_Tertiary_ {Pleistocene or |Age of _Extinct| and _Palms_. |
|Z O { {Glacial. | Mammals_. | |
|O I { {Pliocene. | (Earliest | |
|I C.{ {Miocene. | Placental | |
|C { {Eocene. | Mammals.) | |
| | | |
| ----------------------------+---------------+--------------+
| | | |
| { {Upper, | | |
| M {_Cretaceous_ {Lower, or | | |
| E { {Neocomian. | | |
| S { | Age of | |
| O {_Jurassic_ {Oolite. | _Reptiles_ and| (Earliest |
| Z { {Lias. | _Birds_. |Modern Trees.)|
| O { | | Age of |
| I { | | _Cycads_ and |
| C. { {Upper, | (Earliest | _Pines_. |
| { {Middle, or | Marsupial | |
| {_Triassic_ {Muschelkalk. | Mammals.) | |
| { {Lower | | |
| | | |
| --------------------------------+---------------+--------------+
| | | |
| {Upper, | | |
| {_Permian_ {Middle, or | | |
| { {Magnesian | (Earliest | |
| { { Limestone. | true | |
| { {Lower. | Reptiles.) | |
| { | | |
| { {Upper | | |
| { { Coal-Formation.| | |
| {_Carbon- {Coal-Formation | | |
| { iferous_ {Carboniferous | | |
| { { Limestone. | | |
| { {Lower | | |
| P { { Coal-Formation.| | |
| A { | | |
| L { {Upper. | Age of | Age of |
| Ać {_Devonian_ {Middle. | _Amphibians_ |_Acrogens_ and|
| O { {Lower. | and _Fishes_.|_Gymnosperms_.|
| Z { | | (Earliest |
| O {_Silurian_ {Upper. | |Land Plants.) |
| I { {Lower. | | Age of |
| C. { | | _AlgA|_. |
| {_Siluro-Cam- | Age of | |
| { brian_ or {Upper. | _Mollusks_, | |
| {_Ordovician_ {Lower. | _Corals_, and | |
| { | _Crustaceans_.| |
| { {Upper. | | |
| {_Cambrian_ {Middle. | | |
| {Lower | | |
| | | |
| --------------------------------+---------------+--------------+
| | | |
| {_Huronian_ {Upper. | | |
| { {Lower. | | |
| E { | Age of | |
| O { | _Protozoa_. | Indications |
| Z { | (First animal | Plants |
| O { {Upper. | remains.) | not |
| I {_Laurentian_ {Middle, | |determinable. |
| C. {Lower, or | | |
| {Bojian. | | |
| | | |
+--------------------------------------+---------------+--------------+
THE CHAIN OF LIFE.
CHAPTER I.
PRELIMINARY CONSIDERATIONS AS TO THE EXTENT AND SOURCES OF OUR
KNOWLEDGE.
It is of the nature of true science to take nothing on trust or on
authority. Every fact must be established by accurate observation,
experiment, or calculation. Every law and principle must rest on
inductive argument. The apostolic motto, aEurooeProve all things, hold fast
that which is good,aEuroŁ is thoroughly scientific. It is true that the mere
reader of popular science must often be content to take that on
testimony which he cannot personally verify; but it is desirable that
even the most cursory reader should fully comprehend the modes in which
facts are ascertained and the reasons on which conclusions are based.
Failing this, he loses all the benefit of his reading in so far as
training is concerned, and cannot have full assurance of that which he
believes. When, therefore, we speak of life-epochs, or of links in a
chain of living beings, the question is at once raisedaEuro"What evidence
have we of the succession of such epochs? This question, with some
accessory points, must engage our attention in the present chapter.
Geology as a practical science consists of three leading parts. The
first and most elementary of these is the study of the different kinds
of rocks which enter into the composition of those parts of the earth
which are accessible to us, and which we are in the habit of calling the
crust of the earth. This is the subject of _Lithology_, which is based
on the knowledge of minerals, and has recently become a much more
precise department of science than heretofore, owing to the successful
employment of the microscope in the investigation of the minute
structure and composition of rocks. The second is the study of the
arrangement of the materials of the earth on the large scale, as beds,
veins, and irregular masses; and inasmuch as the greater part of the
rocks known to us in the earthaEuro(TM)s crust are arranged in beds or strata,
this department may be named _Stratigraphy_. A more general name
sometimes employed is that of _Petrography_. The third division of
geology relates to the remains of animals and plants buried in the rocks
of the earth, and which have lived at the time when those rocks were in
process of formation. These fossil remains introduce us to the history
of life on the earth, and constitute the subject of _PalA|ontology_.
It is plain that in considering what may be learned as to epochs in the
history of life we are chiefly concerned with the last of these
divisions. The second may also be important as a means of determining
the relative ages of the fossils. With the first we have comparatively
little to do.
Previous to observation and inquiry, we might suppose that the kinds of
animals and plants which now inhabit the earth are those which have
always peopled it; but a very little study of fossils suffices to
convince us that vast numbers of creatures once inhabitants of this
world have become extinct, and can be known to us only by their remains
buried in the earth. When we place this in connection with
stratigraphical facts, we further find that these extinct species have
succeeded each other at different times, so as to constitute successive
dynasties of life. On the one hand, when we know the successive ages of
fossil forms, these become to us, like medals or coins to the historian,
evidences of periods in the earthaEuro(TM)s history. On the other hand, we are
obliged in the first instance to ascertain the ages of the medals
themselves by their position in the successive strata which have been
accumulated on the surface. The series of layers which explorers like
Schliemann find on the site of an ancient city, and which hold the works
of successive peoples who have inhabited the place, thus present on a
small scale a faithful picture of the succession of beds and of forms of
life on the great earth itself.
Our leading criterion for estimating the relative ages of rocks is the
superposition of their beds on each other. The beds of sandstone, shale,
limestone, and other rocks which constitute the earthaEuro(TM)s crust have
nearly all been deposited thereon by water, and originally in attitudes
approaching to horizontality. Hence the bed that is the lower is the
older of any two beds. Hence also, when any cutting or section reveals
to us the succession of several beds, we know that fossil remains
contained in the lower beds must be of older date.
We can scarcely walk by the side of a stream which has been cutting into
its banks, or at the foot of a sea-cliff, or through a road-cutting,
without observing illustrations of this. For instance, in the section
represented in Fig. 1, we see at the surface the vegetable soil, below
this layers of gravel and sand, below this a bed of clay, and below this
hard limestone. Of these beds _a_ is the newest, _d_ the oldest; and if,
for example, we should find some marine shells in _d_, some fresh-water
shells in _c_, bones of land animals and flint arrowheads in _b_, and
fragments of modern pottery in _a_, we should be able at once to assign
their relative ages to these fossils, and to form some idea of the
succession of conditions and of life which had occurred in the locality.
On a somewhat larger scale, we have in Fig. 2 a section of the beds cut
through by the great Fall of Niagara. All of these except that marked
_a_ are very ancient marine rocks, holding fossil shells and corals, but
now forming part of the interior of a continent, and cut through by a
fresh-water river.
[Illustration: FIG. 1.aEuro"Bank of stream or coast, showing stratification.
_a_, Vegetable soil.
_b_, Gravel and sand.
_c_, Clays.
_d_, Limestone rock, slightly inclined.]
[Illustration: FIG. 2.aEuro"Section at Niagara Falls, showing the strata cut
through by the action of the Fall. Thickness of beds about 250 feet.
_a_, Boulder clay and gravelaEuro"Post-pliocene.
_b_, Niagara limestone } Upper Silurian, with
_c_, Niagara shale } marine shells and
_d_, Clinton limestone } corals.
_e_, Medina sandstone }]
In deep mines and borings still more profound sections may be laid open,
as in Fig. 3, which represents the sequence of beds ascertained by
boring with the diamond drill in search of rock salt near Goderich in
Canada. Here we have a succession of 1,500 feet of beds, some of which
must have been formed under very peculiar and exceptional conditions.
The beds of rock salt and gypsum must have been formed by the drying up
of sea-water in limited basins. Those of Dolomite imply precipitation
of carbonate of lime and magnesia in the sea-bottom. The marls must have
been formed largely by the driftage of sand and clay, while some of the
limestone was produced by accumulation of corals and shells. Such
deposits must not only have been successive, but must have required a
long time for their formation.
[Illustration: FIG. 3.aEuro"Section obtained by boring with the diamond
drill, near Goderich, Ontario, Canada, in the Salina series of the Upper
Silurian. From a memoir by Dr. Hunt in the Report of the Geological
Survey of Canada for 1876-7.
No. 1, Clay, gravel, and bouldersaEuro"Post-pliocene.
Nos. 2, 4, 7, 9, 13, Dol omite or magnesian limestone, with layers
of marl, limestone, and gypsum.
No. 3, Limestone with coralsaEuro"_Favosites_, etc.
Nos. 5, 11, 15, 17, Marls with layers of Dolomite and anhydrous
gypsum.
Nos. 6, 8, 10, 12, 14, 16, Rock salt.]
[Illustration: FIG. 4.aEuro"Inclined beds, holding fossil plants.
Carboniferous. South Joggins, Nova Scotia.
1. Shale and sandstone. Plants with _Spirorbis_ attached;
rain marks (?).
2. Sandstone and shale, 8 feet. Erect
_Calamites_. } An erect coniferous (?) tree, rooted
3. Gray sandstone, 7 feet. } on the shale, passes up through 15
4. Gray shale, 4 feet. } feet of the sandstones and shale.
5. Gray sandstone, 4 feet.
6. Gray shale, 6 inches. Prostrate and erect trees, with rootlets,
leaves, _Naiadites_, and _Spirorbis_ on the plants.
7. Main coal-seam, 5 feet coal in two beds.
8. Underclay, with rootlets.]
In Fig. 4 we have a bed of coal and its accompaniments. The coal itself
was produced by the slow accumulation of vegetable matter on a
water-soaked soil, and this was buried under successive beds of sand and
clay, now hardened into sandstone and shale, some of the beds holding
trees and reed-like plants, which still stand on the soils on which
they grew, and which must have been buried in sediment deposited in
inundations or after subsidence of the land. In this section we may also
observe that the beds are somewhat inclined; and that this is not their
original position is shown by the posture of the stems of trees, once
erect, but now inclined with the beds. This leads to a consideration
very important with reference to our present subject; namely, that as
our continents are mostly made up of beds deposited under water and
afterwards elevated, these beds have in this process experienced such
disturbances that they rarely retain their horizontal position, but are
tilted at various angles. When we follow such inclined strata over large
areas, we find that they undulate in great waves or folds, forming what
are called anticlinal and synclinal lines, and that the irregularities
of the surface of the land depend to a great extent on these
undulations, along with the projection of hard beds whose edges protrude
at the surface. In point of fact, as shown in Fig. 5, mountain ranges
depend on these crumplings of the earthaEuro(TM)s crust; and the primary cause
of these is probably the shrinkage of the mass of the earth owing to
contraction in cooling. When the disturbances of beds are extreme, they
often cause intricacies of structure difficult to unravel; but when of
moderate extent they very much aid us in penetrating below the surface,
for we can often see a great thickness of beds rising one from beneath
another, and can thus know by mere superficial examination the structure
of the earth to a great depth. It thus happens that geologists reckon
the thickness of the stratified deposits of the crust of the earth at
more than 70,000 feet, though they cannot penetrate it perpendicularly
to more than a fraction of that depth. The two sections, Figs. 6 and 7,
showing the sequence of beds in England and in the northern part of
North America, will serve, if studied by the reader, to show how, by
merely travelling over the surface and measuring the upturned edges of
beds, many thousands of feet of deposits may be observed, and their
relative ages distinctly ascertained.
[Illustration: FIG. 5.aEuro"Ideal section of the Apalachian Mountains
showing folding of the earthaEuro(TM)s crust.
_a_, Anticlinal axes.
_b_, Overturned strata.
_c_, Synclinals.
_d_, Unconformable beds.]
In studying any extensive section of rock we find that its members may
more or less readily be separated into distinct groups. Sometimes these
are distinguished by what is termed unconformability, that is, the lower
series has been disturbed or inclined before the upper has been
deposited upon it. This is seen on a grand scale in the section Fig. 7,
in the case of the Laurentian and Cambrian formations, and on a smaller
scale in Fig. 8 in the unconformable superposition of Devonian
conglomerate on Silurian slates at St. AbbaEuro(TM)s Head. In the last section
it is quite evident that the beds of the lower series have been bent
into abrupt folds and worn away to a considerable extent before the
deposition of the overlying series. In such a case we know not merely
that the upper series is newer than the lower, but that some
considerable time must have elapsed after the deposition of the one
before the other was laid down; and we are not surprised to find that
the fossils in the groups thus unconformable to each other are very
different.
But even when the beds are conformable, they can usually be separated
into groups, depending upon differences of mineral character, or changes
which have occurred in the mode of deposition. One group of beds, for
example, may be largely composed of limestone, another of sandstone or
shale. One group may be distinguished by containing some special
mineral, as, for example, rock salt or coal, while others may be
destitute of such special minerals. One group may show by its fossils
that it was deposited in the sea, others may be estuarine or
lacustrine. Thus we obtain the means of dividing the rocks of the earth
into groups of different ages, known as aEurooeFormations,aEuroŁ and marking
particular periods of geological time. By tracing these formations from
one district or region to another, we learn the further truth that the
succession is not merely local, but that, though liable to variation in
detail, its larger subdivisions hold so extensively that they may be
regarded as world-wide in their distribution.
[Illustration: FIG. 6. Generalised section across England from Menai
Straits to the Valley of the Thames.aEuro"After Ramsay.
0 Huronian? or Laurentian?
1 Cambrian and Lower Silurian.
2 Upper Silurian.
3 Devonian.
6, 7, 8 Trias and lias.
9 and 10 Jurassic.
11 Cretaceous.
12 Eocene.]
[Illustration: FIG. 7.aEuro"Generalised section from the Laurentian of
Canada to the coal-field of Michigan.
0 Laurentian (the Huronian is absent in the line of this section).
1 Cambrian.
2 Lower Silurian.
3 Upper Silurian.
4 Devonian.
5 Carboniferous.]
[Illustration: FIG. 8.aEuro"Unconformable superposition of Devonian
conglomerate on Silurian slates, at St. AbbaEuro(TM)s Head, Berwickshire.aEuro"After
Lyell.]
Putting together the facts thus obtained, we can frame a tabular
arrangement of the earthaEuro(TM)s strata, as in the table prefixed to this
chapter; and when we add the further discovery, very early made by
geologists, that the successive formations differ from each other in
their fossil remains, we have the means of recognising any particular
formation by its fossils, even when the stratigraphical evidence may be
obscure or wanting. Thus our knowledge of Epochs of Life, and indeed of
the whole geological history of the earth, is based on the superposition
of beds in the earthaEuro(TM)s crust, and on the diversity of fossil remains in
the successive beds so superimposed on each other; and it is on these
grounds that we are enabled to construct a Table of Geological
Formations representing the whole series of beds as far as known, with
the characteristic groups of fossils of each period. Here I might close
these preliminary considerations, but there are a few accessory
questions, important to our clear comprehension of the subject, which
may profitably occupy our attention for a short time.
One of these relates to the absolute duration of the time represented by
the geological history of the earth. Such estimates as our present
knowledge enables us to form are very indefinite. Whether we seek for
astronomical or geological data, we find great uncertainty. To such an
extent is this the case, that current estimates of the time necessary to
bring the earth from a state of primitive incandescence to its present
condition have varied from fifteen millions of years to five hundred
millions. Of the various modes proposed, perhaps the most satisfactory
as well as instructive is that based on the rate of denudation of our
present continents, as indicated by the amount of sediment carried down
by great rivers. The Mississippi, draining a vast and varied area in
temperate latitudes, is washing away the American land at the rate of
one foot in 6,000 years. The Ganges, in a tropical climate and draining
many mountain valleys, works at the rate of one foot in 2,358 years. The
mean of these two great rivers would give one foot in 4,179 years, at
which rate our continents would be levelled with the waters in about six
millions of years. But the land has been in process of renewal as well
as of waste in geological time; and a better measure will be afforded by
the amount of beds actually deposited. The entire thickness of all the
stratified rocks of Great Britain has been calculated by Ramsay at
72,000 feet. Now, if we suppose the waste in all geological time to have
been on the average the same as at present, and that this material has
been deposited to the thickness of 72,000 feet on a belt of sea margin
100 miles in width, we shall have about 86 millions of years as the time
required.[1] This has the merit of approximating to Sir William
ThomsonaEuro(TM)s calculation, based on the rate of cooling of the earth, that a
minimum of 100 millions of years may represent the time since a solid
crust first began to form. As it is more likely that the rate of
denudation has on the average been greater in former geological periods
than at present, we may perhaps estimate fifty or sixty millions of
years as the time required for the accumulation of all our formations.
Some geologists object to this as too little, but in this some of them
are influenced by the exigencies of theories of evolution, and others
appear to have no adequate conception of the vast lapse of time
represented by such numbers, in its relation to the actual rates of
denudation and deposition.
It should be mentioned here, however, that, on certain theories now
somewhat generally accepted, respecting the nature and source of solar
heat, the absolute duration of geological time would be much reduced
below the estimate of Sir Wm. Thomson. Prof. Tait has based on such data
an estimate of fifteen millions of years. Prof. Simon Newcomb says that
aEurooeon the only hypothesis science will now allow us to make respecting the
source of the solar heataEuroŁ (the gravitation hypothesis of Helmholtz) aEurooethe
earth was, twenty millions of years ago, enveloped in the fiery
atmosphere of the sun.aEuroŁ Dr. Kirkwood has called attention to these
results in connection with the planetary hypothesis of La Place, in the
_Proceedings of the American Philosophical Society_.[2] Should such
views prove to be well-founded, geological calculations as to the time
required for the successive formations may have to be revised.
If now we attempt to divide this time among the formations known to us,
according to their relative thicknesses, we have, according to an
elaborate estimate of Professor Dana, the time ratios of 12, 3, and 1
for the PalA|ozoic, Mesozoic, and Cainozoic periods respectively. Taking
the whole time since the beginning of the Cambrian as forty-eight
millions of years, we should thus have for the PalA|ozoic thirty-six
millions, for the Mesozoic nine, and for the Tertiary three. Another
calculation, recently made by Professors Hull and Haughton, gives the
following ratios:aEuro"
Azoic 34A.3 per cent.
PalA|ozoic 42A.5 aEuroŁ
Mesozoic and Cainozoic 23A.2 aEuroŁ
This calculation is, however, based on the absolute thickness of the
several series as ascertained in Great Britain, without reference to the
nature of the beds, as indicating different rates of accumulation. Under
either estimate it will be seen that the PalA|ozoic time greatly exceeds
the Mesozoic and Cainozoic together, and consequently that changes of
life seem to have proceeded at an accelerated rate as time wore on.
Another inquiry of some importance relates to the manner of preservation
of fossils, and the extent to which they constitute the material of
rocks. This inquiry is doubly important, as it bears on the genuineness
of fossil remains, and on the means we have of understanding their
nature.
Some rocks are entirely made up of matter that once was alive, or formed
part of living organisms. This is the case with some limestones, which
consist of microscopic shells, or of larger shells, corals, and similar
calcareous organisms, either entire or broken into fragments and
cemented together with pasty or crystalline limestone filling their
interstices. This may be seen in Fig. 9, which represents a magnified
slice of a Silurian limestone. Coal in like manner consists of
carbonised vegetable matter, retaining more or less perfectly its
organic structure, and sometimes even the external forms of its
constituent parts. More frequently, fossils are dispersed more or less
sparsely through the substance of beds composed of earthy matter; and
they have usually been more or less affected by chemical changes, or by
mechanical pressure, or are mineralised by different substances which
have either filled their pores by infiltration or have more or less
completely replaced their substance. Of course, as a rule, the softer
and more putrescible organic matters have perished by decay, and it is
only the harder and more resisting parts that remain. Even these have
often yielded to the enormous pressure to which they have been
subjected, and if at all porous, have been changed by the slow action of
percolating water charged with various kinds of mineral matter in
solution.
[Illustration: FIG. 9.aEuro"Section of Trenton limestone, magnified, showing
that it is composed of fragments of corals, crinoids, and shells.
Montreal.]
[Illustration: FIG. 10.aEuro"Diagram showing different state of
fossilisation of a cell of a tabulate coral (DawsonaEuro(TM)s _Dawn of Life_).
_a_ Natural condition, wall calcite cell empty.
_b_ Wall calcite, cells filled with the same.
_c_ Walls calcite, cells filled with silica or a silicate.
_d_ Wall silicified, cells filled with calcite.
_e_ Wall silicified, cell filled with silica.]
It thus happens that many fossils are infiltrated with mineral matter.
Wood, for example, may have the cavities of its cells and vessels filled
with silica or silicates, with sulphide or carbonate of iron, or with
limestone, while the woody walls of the cells may remain either as coaly
matter or charcoal. I have often seen the microscopic cells of fossil
wood not only filled in this way, but presenting under a high power
successive coats of deposit, like the banded structure of an agate.
In some cases not only are the pores filled with mineral matter, but
the solid parts themselves have been replaced, and the whole mass has
actually become stone, while still retaining its original structure.
Thus silicified wood is often as hard and solid as agate, and under the
microscope we see that the wood has entirely perished, and is
represented by silica or flint, differing merely in colour from that
which fills the cavities. In this case we may imagine the wood to have
been acted on by water holding in solution silica, combined with soda or
potash, in the manner of what is termed soluble glass. The wood, in
decay, would be converted into carbon dioxide, and this as formed would
seize on the potash or soda, leaving the silica in an insoluble state,
to be deposited instead of the carbon. Thus each particle of the carbon
of the wood, as removed by decay, would be replaced by a particle of
silica, till the whole became stone. By similar chemical changes corals
and shells are often represented by silica, or by pyrite, which has
taken the place of the original calcareous matter; and still more
remarkable changes sometimes occur, as when the siliceous spicules of
sponges have been replaced by carbonate of lime. The organic matter
present in the fossils greatly promotes these changes, by the substances
produced in its decay, and thus it often happens that the shells,
corals, etc., contained in limestone have been replaced by flint, while
the inclosing limestone is unchanged. Fig. 10 shows the various
conditions which a coral may assume under these different modes of
treatment.
[Illustration: FIG. 11.aEuro"Cast of erect tree (_Sigillaria_) in sandstone,
standing on a small bed of coal, South Joggins, Nova Scotia (DawsonaEuro(TM)s
_Acadian Geology_).]
The substance of a fossil may be entirely removed by decay or solution,
leaving a mere mould representing its external form, and this may
subsequently be filled with mineral matter, so as to produce a natural
cast of the object. This is very common in the case of fossil plants;
and large trunks of trees may sometimes be found represented, as seen in
Fig. 11, by stony pillars retaining nothing of the original wood except
perhaps a portion of the bark in the state of coal. It sometimes happens
that the substance of fossils has been removed, leaving mere empty
cavities, sometimes containing stony cores representing the internal
chambers of the fossils. Again, calcareous fossils imbedded in hard
rocks are often removed by weathering, leaving very perfect impressions
of their forms. For this reason the fossil remains contained in some
hard resisting rocks can be best seen as impressed moulds on the
weathered surfaces.
[Illustration: FIG. 12.aEuro"_Protichnites septem-notatus._ A supposed
series of crustacean footprints made in sand, now hardened into
sandstone. Cambrian.aEuro"After Logan.]
Lastly, we sometimes have impressions or footprints representing the
locomotion of fossil animals, rather than the fossils themselves. In
this way some extinct creatures are known to us only by their footsteps
on sand or clay, once soft, but now hardened into stone; and in the
case of some of the lower animals the trails thus made are often not
easily interpreted (Figs. 12, 12_a_). It has been found that even
sea-weeds drifted by the tide make impressions of this kind, which, when
they occur in old rocks, are very mysterious. Even rain-drops are
capable of being permanently impressed on rocks, and constitute a kind
of fossils. Besides these we have many kinds of imitative markings which
simulate fossils, as those of concretions or nodules, which are often
very fantastic in shape, those of dendritic crystallisation giving
moss-like forms, and the complicated tracery produced on muddy shores
by the little rills of water which follow the receding tide (Fig. 13).
Such things are often mistaken by the ignorant for fossil remains, but
are easily distinguished by a practised eye.
[Illustration: FIG. 12_a_.aEuro"Footprints of modern _Limulus_, or
king-crab, in the sand, which enable us to interpret those in Fig. 12.]
The reader who has followed these, perhaps somewhat dry, details, will
be rewarded for his patience by having some conception of the conditions
in which we find fossil remains, and of the evidence by which we can
refer these to different periods in the history of the earth.
[Illustration: FIG. 13.aEuro"Current markings on shale, resembling a fossil
plant. Reduced from a photograph (DawsonaEuro(TM)s _Acadian Geology_).]
Carrying this knowledge with us, and at the same time glancing at the
table of successive formations prefixed to this chapter, we shall be
prepared, without any additional geological study, to understand the
statements to be made in the following chapters, and to appreciate the
actual nature of the succession of life in so far as it is at present
known.
[Illustration: MAGNIFIED AND RESTORED SECTION OF A PORTION OF EOZOON
CANADENSE.
The shaded portions show the animal matter of the Chambers, Tubuli,
Canals, and Pseudopodia; the unshaded portions the calcareous
skeleton.]
CHAPTER II.
THE BEGINNING OF LIFE ON THE EARTH.
The day must have been when the first living being appeared for the
first time on our planet. Was it plant or animal? or a generalised
organism uniting in some mysterious way the properties and powers of two
kingdoms of nature, now so distinct, and even contrary to each other in
their manifestations? Did it appear suddenly, or was it slowly evolved
from dead matter by some process in which the albuminous or protoplasmic
matter, which we know forms the basal substance of living beings, was
first produced and then endowed with life? Did the first living being
appear in a mature state, or was it merely a germ from which the mature
individual could be produced? These are questions which science in its
present state has no means of answering. We do not know any process by
which the ingredients of protoplasm can be combined so as to produce
that substance without a previous living being. We do not know what
molecular differences may exist between dead albumen and that which we
see growing and moving and instinct with life; still less do we know how
to set up or establish these differences. We do not know the precise
nature or relation to other forces of the energy which actuates living
organisms. In our experience the simplest creatures that have life
spring from previous germs, themselves the products of previous
generations of living beings. Thus we are in the presence of great
mysteries which it might be impossible for us to solve, even if we were
permitted to visit some new planet on which the dawn of life was
breaking.
Some things, however, we can infer as to the conditions of the
introduction of life.
First, there is every reason to believe that the earth we inhabit was
once a glowing, incandescent mass, condensing from a vaporous condition,
and quite unfit for the abode of living beings, and which, even if in
some previous state its materials had constituted the mass of an
inhabited world, must have lost every trace of any living germ in the
fervent heat to which it had been subjected. There must, therefore, have
been in some way an absolute creation or origination of life and
organisation.
Secondly, we may infer that in the earlier stages of the earth, when it
was perhaps wholly or almost entirely covered with the waters, when it
was still uniformly warmed with its own internal heat, when it was
surrounded with a pall of dense vapours preventing radiation, and
nursing its heat within itself, though in a condition entirely unsuited
to the higher forms of life, it may have presented circumstances more
favourable to the origination and multiplication of living beings of low
organisation than at any subsequent time. This incubation of creative
power in the vaporous mantle over the primA|val ocean was a favourite
imagination of old thinkers, and is not obscurely hinted at in the Book
of Genesis. It has been revived and much insisted on by evolutionists in
our own time, though it has no certain foundation in scientific
observation or experiment.
Thirdly, from the fact that plant-life alone has the power of subsisting
on inorganic matter, and that plants furnish all the nourishment of
animals, we may fairly infer that the life of the plant preceded that of
the animal. It has, indeed, been suggested that some of the humbler
forms of life may combine in a rude and simple way enough of the powers
of the plant and the animal to enable them to bridge over the double
gap between the animal and the plant, and the animal and the mineral, or
that such creatures may in their early stages carry on vegetable
functions, and in their later those of the animal. It is theoretically
possible that life may have begun with such creatures, which some of the
results of microscopical research would lead us to believe still exist.
It is, however, on the whole more probable that simple plants first
existed, and furnished pabulum to animals of low grade introduced almost
contemporaneously.
Fourthly, all our knowledge of the succession of life leads us to
believe that it was not the higher plants and animals that first sprang
into existence from the teeming earth, but creatures of low and humble
organisation, suited to the then immature and unfinished condition of
the planet. It is also in accordance with the amazing fecundity of the
seas in all geological periods in these lower forms of life, to suppose
that the earliest living things originated in the waters, and that the
plants and animals of the land are of later date.
Do we know anything from actual observation of this earliest population
of the world? Such knowledge we can hope to acquire only by studying the
oldest formations known to us; and these, it must be confessed, exist in
a state so highly crystalline, and so much affected by internal heat, by
mechanical pressure, and by movement, as to render it little likely that
organic remains should be preserved in them in a state fit for
recognition.
In many parts of the world, and notably in Canada and Scandinavia, as
well as in Wales, Scotland, and Bavaria, the older PalA|ozoic rocks, the
lowest containing plants in great abundance, rest on still older
crystalline beds, which have become hard and crystalline in
pre-PalA|ozoic times, and have contributed sand and pebbles to the
succeeding very ancient deposits. These old rocksaEuro"the Eozoic series of
our tableaEuro"may be grouped in two great systems, the Laurentian and
Huronian (Fig. 14). The former may be conveniently divided into three
members: First, the Bojian, or Ottawa gneiss, consisting of stratified
granite rocks, usually of a red colour, and of very great thickness.
This contains, so far as known, no limestone, and has afforded as yet no
trace of fossils. Secondly, the Middle Laurentian, the greater part of
which consists of gneiss, but containing important beds of other rocks,
as quartzite, iron ore, and limestone. It is in this series that we have
the first evidence of life, and it is here also that we find the
greatest abundance of carbon, in the form of graphite or plumbago, and
also large quantities of calcium phosphate, or bone earth. Thirdly, the
Upper Laurentian or Norian series. This consists in great part of
Labadorite, or lime feldspar, but has also beds of ordinary gneiss,
limestone, and iron ore.
[Illustration: FIG. 14.aEuro"Ideal section, showing the relations of the
Laurentian and Huronian.
_a_, Lower Laurentian.
_b_, Middle Laurentian.
_c_, Upper Laurentian.
_d_, Huronian.
_e_, Cambrian and Silurian.]
The latter, the Huronian, is much less crystalline, and is divisible
into two seriesaEuro"the Lower Huronian, which includes many beds of
volcanic origin, and the Upper Huronian, which has afforded some obscure
fossils. The Huronian was first recognised by Sir W. E. Logan in Canada,
but corresponding rocks exist in Europe. The Pebidian series of Hicks in
Wales is probably of this age.
It is likely that much of the present appearance and condition of the
most ancient rocks may be attributed to metamorphism, that is, to the
slow baking under the influence of heat, heated water, and pressure, to
which they have been subjected in the lower parts of the earthaEuro(TM)s crust,
when buried deeply under newer deposits. It is also true, however, as
Dr. Sterry Hunt has pointed out in detail, that they present mineral
characters which show a mode of deposition different from that which has
prevailed subsequently, and probably indicating great ejections of
heated mineral matter into the primitive ocean, and comparatively little
of that deposit therein of mere sand and clay which has prevailed in
subsequent geological periods. In short, these rocks have an
unmistakably primitive aspect, distinguishing them from those of later
times, and conveying the impression that they approach at least to the
records of that time when a heated ocean first rested on the thin and
recently solidified crust of our planet. If this is really the case,
then our Lower LaurentianaEuro"hard, compact, destitute of limestone, and
composed of material which may be little else than the _dA(C)bris_ of
products of internal heat merely spread out into bedded forms by
wateraEuro"may represent a time when no living thing as yet tenanted the
waters; and the dawn of life may have appeared in that period when the
Middle Laurentian beds were laid down. Here at least we find two kinds
of evidence pointing to the existence of certain forms of life in the
waters.
The first depends on the mineral character of the beds themselves. This
formation holds several very thick beds of limestone. Now although this
kind of rock may, under certain circumstances, be deposited directly
from solution in water, it is not ordinarily so deposited, but more
usually through the agency of living beings inhabiting the waters, and
forming their skeletons or hard parts of limestone derived from the
water, usually through the medium of humble forms of plant life. In this
way are formed reefs of coral and beds of shells and of chalky ooze, all
composed of material once constituting the skeletons of animals. The
study of limestones of all geological ages shows that this has been the
usual mode of their formation. If the Laurentian limestones had a
similar origin, the seas of that period must have swarmed with animals
having calcareous coverings; and the study of more modern limestones
which have become highly crystalline shows that it is quite possible
that the forms and structures of these organisms may have been
obliterated.
Again, the Middle Laurentian abounds in carbon or coaly matter. True,
this is in the form of graphite or plumbago, but this condition may be a
result of metamorphism; and we know that the carbon of coal-beds and
bituminous shales of much more modern times has been altered into
graphite. Further, the graphite occurs in the way in which we should
expect it to occur if of organic origin. It is found disseminated in the
limestone, just as bituminous matter is found in unaltered rocks of this
kind. It is found interlaminated with gneiss, as carbonaceous and
bituminous matters are found in the shales of the ordinary fossiliferous
rocks, where these substances are known to be of organic origin. The
graphite also occurs in a very pure form in irregular veins, just as in
some bituminous formations the rock oil, oozing into fissures, has been
hardened into asphalt or coaly matter.[3]
To these facts may be added the presence of thick beds and veins of iron
ore and of apatite or calcium phosphate (bone earth). Both of these
substances occur in a disseminated state in nearly all rocks, but they
are concentrated into definite deposits by the action of life. Iron is
usually dissolved out and redeposited by acids produced in the decay of
vegetable matter, as we see in the clay ironstones of the coal formation
and in bog-iron ores. Calcic phosphate is taken up by many animals, and
forms their shells or skeletons, and on their death is deposited in beds
on the sea-bottom, sometimes to a very considerable extent.
The concurrence of all these phenomena in the Middle Laurentian may be
held to afford a strong presumption that, could we discover these rocks
in an unaltered state, we should find the limestones filled with marine
fossils and the graphite showing the forms or structure of plants. The
only startling feature in this conclusion is, that if we admit it, we
must also admit that life was developed in the Laurentian time in an
exuberance not surpassed, if equalled, in any subsequent period. Still,
there is nothing incredible in this, for if the forms of life were few
and low, their increase may have been rapid, because unchecked; and they
no doubt found in the ancient seas a surplusage of material on which to
feed and with which to construct their skeletons. Dr. Hunt has estimated
that the amount of carbon now sealed up as coaly matter would, if
diffused in the atmosphere as carbon dioxide, afford 600 times the
quantity of that gas at present floating in the air. A still more vast
amount is sealed up in the limestone of the several geological
formations. The same chemist has shown that the quantity of lime held in
solution in the ocean must have been much greater in Laurentian times
than at present. These facts at least allow us to suppose that in the
Eozoic times there were great supplies of carbon and of lime available
to such creatures of low organisation as were capable of profiting by
them; and we have no reason to doubt that there may have been plants and
animals so constituted as to flourish in conditions of this kind, in
which perhaps scarcely any modern species could exist.
These probabilities have caused geologists anxiously to search for any
traces of fossil organic remains in the old Laurentian rocks; and they
have been rewarded by the discovery of one species, _Eozoon Canadense_,
still often referred to as only a problematical fossil; but this arises
to a large extent from the prevalent want of knowledge sufficient to
appreciate the evidence for its organic character. This being once
admitted, we have in the existence of _Eozoon_ alone a sufficient cause
for the accumulation of much of the Laurentian limestone, though there
is reason to believe that it was not the only inhabitant of those
ancient seas.
[Illustration: FIG. 15 (Nos. 1 to 4).aEuro"Small weathered specimen of
_Eozoon_. From Petite Nation.
1, Natural size; showing general form, and acervuline portion above and
laminated portion below. 2, Enlarged casts of cells from upper part. 3,
Enlarged casts of cells from the lower part of the acervuline portion.
4, Enlarged casts of sarcode layers from the laminated part.]
The best specimens of _Eozoon_ occur as rounded, flattened, or more or
less irregular lumps or masses in certain layers of the Laurentian
limestone. When weathered on the surface of the rock, these lumps show a
regular concentric lamination, caused by thin fibres of limestone,
alternating with other mineral substances, filling up the spaces between
them. When these intervening layers are composed of such minerals as
Serpentine, Loganite, Pyroxene, or Dolomite, which are more resisting
than the limestone, they project when weathered, or when the limestone
is etched by an acid, so as to show the lamination very distinctly. At
the lower surface of the masses the layers are seen to be thicker than
they are above, and in perfect specimens they are seen toward the
surface to break up into small rounded vesicles of calcite, like little
bubbles, which constitute the so-called acervuline condition of _Eozoon_
(Fig. 15, No. 2). Slices of the fossil etched with an acid show these
appearances very perfectly, and can even be printed from, so as to
present perfect nature-prints of the structure (Fig. 16).
FIG. 16.aEuro"Nature-printed specimen of _Eozoon_ slightly etched with acid.
It shows the lamination, and at one side fragmental _Eozoon_ (_LifeaEuro(TM)s Dawn
on Earth_).
On etching a small fragment or slice with very dilute acid, so as to
dissolve away the calcite slowly, if the specimen be well preserved, we
find that the calcite layers have a very curious structure. This is
indicated by the appearance of little white or transparent threads of
Serpentine, Dolomite, or Pyroxene, which ramify throughout the substance
of the limestone layers, and are left intact when they have been
dissolved. These little processes must originally have been pores in the
limestone layers, which have been filled with the substance which
constitutes the alternate laminA|. In addition to this, if we use a
somewhat high microscopic power, and especially if we study the
structures as seen in thin transparent slices, we can perceive a still
finer tubulation along the sides of the calcite layers, represented by
extremely minute parallel rods of mineral matter (Figs. 17, 18).
Now if we regard these structures as those of an infiltrated fossil, as
described in last chapter, their interpretation will not be difficult.
The original organism was composed of calcareous matter in thin
concentric laminA|, connected with each other by pillars and plates of
similar material. Between these laminA| was lodged the soft, jelly-like
substance of a marine animal, growing by the addition of successive
layers, each protected by a thin calcareous crust. The layers were
originally traversed by very numerous parallel tubuli, permitting the
soft protoplasm to penetrate them; and when, in the progress of growth,
it was necessary to strengthen these layers, they were thickened by a
supplemental deposit traversed by larger and ramifying canals. When the
animal was dead, and its soft parts removed by decay, the chambers
between the laminA|, as well as the minute canals and tubuli, became
infiltrated with mineral matter, in the manner described in the last
chapter, and when so preserved became absolutely imperishable under any
circumstances short of absolute fusion.
[Illustration: FIG. 17.aEuro"Magnified group of canals in supplemental
skeleton of _Eozoon_.
Taken from the specimen in which they were first recognised (_LifeaEuro(TM)s
Dawn on Earth_).]
[Illustration: FIG. 18.aEuro"Portion of _Eozoon_ magnified 100 diameters,
showing the original cell-wall with tubulation, and the supplemental
skeleton with canals.aEuro"After Carpenter.
_a_, Original tubulated wall or aEurooeNummuline layer.aEuroŁ More magnified in
Fig. A. _b_, _c_, Intermediate skeleton, with canals.]
This interpretation leads to the conclusion, at which I arrived from the
study of the first well-preserved specimen ever submitted to microscopic
examination, that the animal which produced the calcareous skeleton of
_Eozoon_ was a member of that lowest grade of Protozoa known as
Foraminifera; and which, after living through the whole of geological
time, still abound in the sea. The main differences are, that _Eozoon_
presents a somewhat generalised structure, intermediate between two
modern types, and that it attained to a gigantic size compared with most
of these organisms in later periods. How near it approaches in structure
to some modern forms may be seen by comparison of the recent species
represented in Fig. 19, in which the parts corresponding to the
chambers, laminA|, tubuli, and canals of _Eozoon_ can be readily
distinguished.
[Illustration: FIG. 19.aEuro"Magnified portion of shell of _Calcarina_.aEuro"After
Carpenter.
_a_, Cells.
_b_, Original cell-wall with tubuli.
_c_, Supplementary skeleton with canals.]
The modern animals of this group are wholly composed of soft gelatinous
protoplasm or sarcode, the outer layer of which is usually somewhat
denser than the inner portion; but both are structureless, except that
the inner layer may present a more or less distinct granular
appearance. Many of them show a distinct spot or cell, called the
nucleus, and some have minute transparent vesicles, which contract and
expand alternately, and appear to be of the nature of circulatory or
excretory organs. They have no proper alimentary canal, but receive
their food into the general mass and digest it in temporary cavities.
Their means of locomotion and prehension are soft thread-like or
finger-like processes, extended at will from the surface of any part of
the body, and known as false feet (pseudopodia). From these processes
the whole group has obtained the name of Rhizopods, or rootfooted
animals. They may be regarded as constituting the simplest and humblest
form of animal life certainly known to us.
The very numerous species of these creatures existing in the waters of
the modern world may be arranged under three principal groups. The first
and highest includes those which have lobate or finger-like pseudopods,
and a well-developed nucleus and pulsating vesicle (Fig. 20, _a_). They
are mostly inhabitants of fresh water, and destitute of a hard crust or
shell. A second group, including many inhabitants of the sea as well as
of fresh waters, has thread-like radiating pseudopodia[4] (Fig. 20 _b_).
Some of these form beautiful silicious skeletons. A third group,
essentially marine, consists of those with reticulated pseudopodia, and
usually destitute of distinct nucleus and pulsating vesicle (Fig. 21).
They produce beautiful calcareous skeletons, often very complex, or
sometimes are content to cover themselves with a crust of agglutinated
grains of sand. It is to this last group that _Eozoon_ belongs, and to the
highest division of itaEuro"that which has the shell perforated with minute
pores, often of two kinds. It is curious that just as we have the
chambers and pores of _Eozoon_ filled with serpentine, so in all
geological formations and in the modern seas it is not uncommon to find
Foraminifera having their cavities filled with glauconite and other
hydrous silicates allied to serpentine.
[Illustration: FIG. 20.aEuro"_a_, _AmA"ba_, a fresh-water naked Rhizopod; and
_b_, _Actinophrys_, a fresh-water Protozoon of the group Radiolaria,
with thread-like pseudopodia.]
[Illustration: FIG. 21.aEuro"_Nonionina_, a modern marine Foraminifer.
Showing its chambered shell and netted pseudopodia.aEuro"After Carpenter.]
If we attempt to trace the Rhizopods onward from the Middle Laurentian,
we are met with a great hiatus in the Upper Laurentian. The species
_Eozoon Bavaricum_ has, however, been found in rocks apparently of
Huronian age; but this is the last known appearance of _Eozoon_, properly
so-called. In the Cambrian or Siluro-Cambrian, however, we meet with
many gigantic Protozoa, more especially those known as _Stromatopora_,
_ArchA|ocyathus_, _Receptaculites_, and _Cryptozoon_.
[Illustration: FIG. 22.aEuro"_Stromatopora concentrica._aEuro"After Hall.
_a_, Section of the same, magnified.
_b_, Small portion highly magnified, showing laminA| and pillars.]
The typical StromatoporA|, or Layer-corals, consist, like _Eozoon_, of
concentric layers, connected by numerous pillars, which are often,
though not always, more definite and regular than in the Laurentian
fossil. The laminA| are perforated, but more coarsely than in _Eozoon_, and
they are often thickened with supplemental deposit which, in some of the
forms, presents canals radiating from vertical tubes or bundles of tubes
penetrating the mass (Figs. 22, 23). The mode of growth of _Stromatopora_
must have closely resembled that of _Eozoon_, and the forms produced are
so similar that it is often quite impossible to distinguish them by the
naked eye. Like _Eozoon_, they form the substance of important limestones,
and single masses are sometimes found as much as three feet in diameter.
The StromatoporA| extend from the Upper Cambrian to the Devonian
inclusive. In the Carboniferous they are continued by smaller and more
regular organisms of the genus _Loftusia_,[5] and this genus seems to
extend without marked change up to the Eocene Tertiary. Recent students
of the StromatoporA| seem disposed to promote them from the province of
Protozoa to that of the Hydroids.[6] The reasons for this seem cogent in
the case of some of the forms, but in my judgment fail in others, more
especially in the older forms. It may ultimately be found that the group
as now held includes very different types of structure. In modern times
I know of no nearer representative than the animal whose skeleton often
adheres in red encrusting patches to our specimens of corals, and which
is known as _Polytrema_. In general structure it is not very far from
being a very degenerate kind of _Stromatopora_.
[Illustration: FIG. 23.aEuro"_Caunopora planulata._ Showing the radiating
canals on a weathered surface. Devonian.aEuro"After Hall.]
It is curious that in the line of succession above stated, the beautiful
tubulated cell-wall of _Eozoon_ disappears; and this structure seems,
after the Laurentian, to be for ever divorced from the great laminated
Protozoans. It reappears in the Carboniferous, in certain smaller
organisms of the type of the _Nummulites_, or Money-stone Foraminifers,
and is continued in this group of smaller and free animals down to the
present time. In the Cretaceous and early Tertiary periods, the
Foraminifera of different types have been nearly as great rock-builders
as they were in the Laurentian. Some of these later rock-builders,
however, have belonged to the lower or imperforate group; others to the
higher or Rotaline and Nummuline groups; and, as a whole, they have been
individually small, making up in numbers what they lacked in size.
Probably the conditions for enabling animals of this type rapidly, and
on a large scale, to collect calcareous matter, were more favourable in
the Laurentian than they have ever been since.
[Illustration: FIG. 24.aEuro"_ArchA|ocyathus minganensis._ A Primordial
Protozoon.aEuro"After Billings.
_a_, Pores of the inner wall.]
In the Siluro-Cambrian age two other forms of gigantic Foraminiferal
Protozoans were introduced, widely different from _Eozoon_, and destined
apparently not to survive the period in which they appeared. These were
_ArchA|ocyathus_, the ancient Cup-corals, and Receptaculites, which may
perhaps be called the Sack-corals. Both are quite remote from _Eozoon_ in
structure, wanting its complexity in the matter of minute tubules, and
having greater regularity and complication on the large scale.
_ArchA|ocyathus_ had the form of a hollow inverted cone with double
perforated walls, connected by radiating irregular plates, also
perforated (Fig. 24). It has been regarded as a sponge, and some species
are certainly accompanied with spicules; but these I have ascertained to
be merely accidental, and will be referred to in the next chapter. The
true structure of _ArchA|ocyathus_ consists of radiating calcareous plates
enclosing chambers connected by pores. ArchA|ocyathus came in with the
Later Cambrian, and seems to have died out in the Siluro-Cambrian. The
only more modern things which at all resemble it are the Foraminifera
called _Dactylopora_, which belong to the Tertiary period.
[Illustration: FIG. 25.aEuro"Receptaculites. Restored.aEuro"After Billings.
_a_, Aperture.
_b_, Inner wall.
_c_, Outer wall.
_n_, Nucleus, or primary chamber.
_v_, Internal cavity.]
Receptaculites is a still more complex organism. It has a sack-like
form, often attaining a large size, and the double walls are composed of
square or rhombic plates, connected with each other by hollow tubes from
which proceed canals perforating the plates (Fig. 25). This curious
structure is confined to the Siluro-Cambrian, and is so dissimilar from
modern forms that its affinities have been subject to grave doubts.
[Illustration: FIG. 26.aEuro"Section of _Loftusia Persica_. An Eocene
Foraminifer. Magnified five diameters.aEuro"After Carpenter and Brady.]
We thus have presented to us the remarkable fact that in the PalA|ozoic
age we have no precise representative of _Eozoon_, but instead three
divergent types, differing from it and from each other, all apparently
specialised to particular uses, all temporary in their duration; while
in later times nature seems to have returned nearer to the type of
_Eozoon_, though on a smaller scale, and separating some characters
conjoined in it. Some portion of this curious result may be due to our
ignorance; and it would be interesting to know, what we may know some
day, how this type of life was represented in the long interval between
the Huronian and the Upper Cambrian, when perhaps there may have been
forms that would at least enable us to connect _Eozoon_ and _Stromatopora_.
Another link in the chain of being remains to be noticed here. In the
Laurentian limestones we meet with numerous minute spherical bodies and
groups of spheres with calcareous tubulated tests.[7] These may either
be small ForaminiferA|, distinct from _Eozoon_, or may be germs or detached
cells from its surface. Similar bodies are found in the lower part of
the Siluro-Cambrian, in the Quebec group at Point Levis; and there they
are filled with a species of glauconite constituting a sort of greensand
rock. Still higher, in the Carboniferous, there are very numerous
species of Foraminifera, presenting forms very similar to those in the
modern seas, so that in the smaller shells of this group we seem to have
evidence of a continuous series all the way from the Laurentian to the
present time. The greater laminated forms co-exist with these up to the
Eocene Tertiary. Throughout the whole of geological timeaEuro"from the
formation of the Laurentian limestones to that of the chalky ooze
accumulating in the modern oceanaEuro"these humble creatures have been among
the chief instruments in seizing on the calcareous matter of the waters
and depositing it in the form of limestone.
[Illustration: FIG. 27.aEuro"Foraminiferal Rock Builders, in the Cretaceous
and Eocene.
_a_, _Nummulites lA|vigata_aEuro"Eocene. _b_, The same, showing chambered
interior. _c_, Milioline limestone, magnifiedaEuro"Eocene, Paris. _d_, Hard
Chalk, section magnifiedaEuro"Cretaceous.]
I have said nothing of the development of higher forms of animal life
from _Eozoon_, simply because I know nothing of it. We shall see in the
next chapter that these are introduced seemingly in an independent
manner. We may be content to trace foraminiferal life along its own line
of development, waxing and waning, but ever confined within the same
general boundaries, from the Laurentian to the present time. It is
likely that if, in any of the ages constituting this vast lapse of time,
a dredge had been dropped into the depths of ocean, it would have
brought up Foraminifera not essentially different in form and structure.
If any one asks to what extent the successive species constituting this
almost endless chain may be descendants one of the other, we have no
absolutely certain information to give. On the one hand, it is not
inconceivable that such forms as _Stromatopora_ or _Nummulina_ may have
descended from _Eozoon_. On the other hand, it is equally conceivable that
the same power which produced _Eozoon_ at first, whether from dead matter
or from some unknown lower form of life, may have repeated the process
in later times with modifications. In any case it is probable that the
Foraminifera have experienced alternations of expansion and shrinkage,
of elevation and decadence, in the lapse of geological time. There were
times in which many new forms swarmed into existence, and times in which
old forms were becoming extinct without being replaced by others. In so
far as the areas of the continents and the adjacent waters are
concerned, those periods when the land was subsiding under the ocean
must have been their times of prosperity, those in which the crust of
the earth shrunk and raised up large areas of land must have been their
times of decay. Still this lowest form of animal life has never
perished, but has always found abundant place for itself, however
pressed by physical change and by the introduction of higher beings.
[Illustration: _Paradoxides Regina_ (Matthews). Lower Cambrian of New
Brunswick.
1/6th Nat. Size.]
CHAPTER III.
THE AGE OF INVERTEBRATES OF THE SEA.
If the middle portion of the Laurentian age was really a time of
exuberant and abounding life, either this met with strange reverses in
succeeding periods, or the conditions of preservation have been such as
to prevent us from tracing its onward history. Certain it is, that
according to present appearances we have a new beginning in the
Cambrian, which introduces the great PalA|ozoic age, and few links of
connection are known between this and the previous Eozoic.
At the beginning of the PalA|ozoic we have reason to believe that our
continents were slowly subsiding under the sea, after a period of
general continental elevation which was consequent on the crumbling of
the earthaEuro(TM)s crust at the close of the Eozoic; and on the new sea-bottoms
formed by this subsidence came in, slowly at first, but in
ever-increasing swarms, the abundant and varied life of the early
PalA|ozoic.
In the oldest portion of the Cambrian series in Wales, Hicks has
catalogued species of no less than seventeen genera, embracing
Crustaceans, the representatives of our crabs and lobsters, bivalve and
univalve shell-fishes of different types, worms, sea-stars, zoophytes,
and sponges. If we could have walked on the shores of the old Cambrian
sea, or cast our dredge or trawl into its depths, we should have found
representatives of most of the humbler forms of sea life still extant,
though of specific forms strange to us. Perhaps the nearest approach to
such experience which we can make is to examine the group of Cambrian
animals delineated in Fig. 28, and to notice, under the guidance of the
geologist above named, the sections seen at St. DavidaEuro(TM)s, in South Wales.
[Illustration: FIG. 28.aEuro"Group of Cambrian Animals (from Nicholson).
_a_, _Arenicolites didymus_, worm tubes. _b_, _Lingulella ferruginea_.
_c_, _Theca Davidii_. _d_, _Modiolopsis solvensis_. _e_, _Orthis
Hicksii_. _f_, _Obolella sagittalis_. _g_, _Hymenocaris vermicauda_.
_h_, Trilobite, _Olenus micrurus_.]
Here we find a nucleus of ancient rocks supposed to be Laurentian,
though in mineral character more nearly akin to the Huronian, but which
have hitherto afforded no trace of fossils. Resting unconformably on
these is a series of partially altered rocks, regarded as Lower
Cambrian, and also destitute of organic remains. These have a thickness
of almost 1,000 feet, and they are succeeded by 3,000 feet more of
similar rocks, still classed as Lower Cambrian, but which have afforded
fossils. The lowest bed which contains indications of life is a red
shale, perhaps a deep-sea bed, and possibly itself partly of organic
origin, by that strange process of decomposition or dissolution of
foraminiferal ooze and volcanic fragments, going on in the depths of the
modern ocean, and described by Dr. Wyville Thomson as occurring over
large areas in the South Pacific. The species are two _LingulellA|_, a
_Discina_ and a _Leperditia_. Supposing these to be all, it is
remarkable that we have no Protozoa or Corals or Echinoderms, and that
the types of Brachiopods and Crustaceans are of comparatively modern
affinities. Passing upward through another 1,000 feet of barren
sandstone, we reach a zone in which no less than five genera of
Trilobites are found, along with Pteropods and a sponge. Thus it is that
life comes in at the base of the Cambrian in Wales, and it may be
regarded as a fair specimen of the facts as they appear in the earlier
fossiliferous beds succeeding the Laurentian. Taking the first of these
groups of fossils, we may recognise in the _Leperditia_ a two-valved
Crustacean closely allied to forms still living in the seas and fresh
waters. The LingulellA|, whether we regard them as molluscoids, or, with
Professor Morse, as singularly specialised worms, represent a peculiar
and distinct type, handed down, through all the vicissitudes of the
geological ages, to the present day. The Pteropods and the sponge are
very similar to forms now living. The Trilobites are an extinct group,
but closely allied to some modern Crustaceans. Had the primordial life
begun with species altogether inscrutable and unexampled in succeeding
ages, this would no doubt have been mysterious; but next to this is the
mystery of the oldest forms of life being also among the newest.
Whatever the origin of these creatures, they represent families which
have endured till now in the struggle for existence without either
elevation or degradation. Yet, though thus vast in their duration, they
seem to have swarmed in together and in great numbers, in the Cambrian,
without any previous preparation. From the Cambrian onward, throughout
the whole PalA|ozoic, there is no decided break in the continuity of
marine life; and already in the Silurian period the sea was tenanted
with all the forms of invertebrate life it yet presents, and these in a
teeming abundance not surpassed in any succeeding age. Let us now, in
accordance with our plan, select some of these ancient inhabitants of
the waters and trace their subsequent history.
Remains of sea-weeds are undoubtedly present in the Cambrian rocks. One
of the lowest beds in Sweden has been named from their abundance the
Fucoidal Sandstone; and wherever fossiliferous Cambrian rocks occur,
some traces, more or less obscure, of these plants may be found. Nearly
all that we can say of them, however, is, that, in so far as their
remains give any information, they are very like the plants of the same
group that now abound in our seas. In the fucoidal sandstone of Sweden
certain striated or ribbed bodies have been found, which have even been
regarded as land plants;[8] but they seem rather to be trails or marks
left by sea-weeds dragged by currents over a muddy bottom. The plants of
the sea thus precede those of the land, and they begin on the same level
as to structure that they have since maintained. I agree with Nathorst,
however, in holding that the Bilobites and many other forms believed by
some to be sea-weeds, are really trails and tracks of animals.[9]
The Foraminifera of the PalA|ozoic we have noticed in the last chapter;
but we now find a new type of ProtozoanaEuro"that of the Sponge. Sponges as
they exist at present may be defined to be composite animals, made up of
a great number of one-celled or gelatinous zoids, provided with
vibrating threads or cilia, and so arranged that currents of water are
driven through passages or canals in the mass, by the action of the
cilia, bringing food and aerated water for respiration. To support these
soft sarcodic sponge-masses, they secrete fibres of horny matter and
needles (spiculA|) of flint or of limestone, forming complicated fibrous
and spicular skeletons, often of great beauty. They abound in all seas,
and some species are found in fresh waters.
[Illustration: FIG. 29.aEuro"Portion of skeleton of Hexactinellid Sponge
(_CA"loptychium_). Magnified. After Zittel.]
With the exception of a very few species destitute of skeleton, and
which we cannot expect to find in a fossil state, the sponges may be
roughly divided into three groups: 1, those with corneous or horny
skeleton, like our common washing sponges; 2, those with skeletons
composed of silicious needles of various forms and arrangement; 3, those
with calcareous spicules. Of these, the second or silicious group has
precedence in point of time, beginning in the Early Cambrian, and
continuing to the present. Two of its subdivisions are especially
interesting in their range. The first is that of the Lattice-sponges
(_HexactinellidA|_), in which the spicules have six rays placed at right
angles, and are attached to each other by their points, so as to form a
very regular network (Fig. 29). The second is that of the Stone-sponges
(_LithistidA|_), in which the spicules are four-rayed or irregular, and
are united by the branching root-like ends of the rays. The most
beautiful of all sponges, the Venus Flower-basket (_Euplectella_), is a
modern Hexactinellid, and the wonderful weaving of its spicules is as
marvellous a triumph of constructive skill as its general form is
graceful. The Lithistids are less beautiful, but are the densest and
most compact of sponges, and are represented by several species in the
modern seas. Both of these types go back to the Early Cambrian, and have
continued side by side to the present day, as representatives of two
distinct geometrical methods for the construction of a spicular
skeleton.
[Illustration: FIG. 30.aEuro"_Protospongia fenestrata_ (Salter). Menevian
group.
_a_, Fragment showing the spicules partially displaced. _b_, Portion
enlarged.]
[Illustration: FIG. 31.aEuro"_Astylospongia prA|morsa_ (Roemer). Niagara
group.aEuro"After Hall.
_a_, Spicules magnified.]
[Illustration: FIG. 32.aEuro"Spicules of Lithistid sponge (_Trichospongia_
of Billings). From the Cambrian of Labrador.]
Many years ago the keen eye of the late lamented Salter detected in a
stain on the surface of a slab of Cambrian slate the remains of a
sponge; and minute examination showed that its spicules crossed each
other, and formed lattice-work on the hexactinellid plan. Salter boldly
named it _Protospongia_ (the first sponge), and it is still the earliest
that we know (Fig. 30). Thus the type whose skeleton is the most perfect
in a mechanical point of view takes the lead. It is continued in the
Silurian in many curious forms, of which the stalkless sponges
(_Astylospongia_) are the most common (Fig. 31). It perhaps attains its
maximum in the Cretaceous, from which the beautiful example in Fig. 29
is taken, and it still flourishes, giving us the most beautiful of all
recent forms. Before the close of the Cambrian there were other sponges
of the Lithistid type. Fig. 32 represents a group of spicules from the
Calciferous (Lowest Silurian or Upper Cambrian) of Mingan,[10] and which
probably belong to a large Lithistid sponge of that early time. The
Lithistids have been recognised in the Upper Silurian and Carboniferous,
and continuing upward to the Cretaceous, there become vastly numerous,
while their modern representatives are by no means few. The silicious
sponges with simple spicules appear to have existed as far back as the
Siluro-Cambrian, and there is believed to be almost as early evidence of
horny or corneous sponges. The calcareous sponges have been recognised
as far back as the Silurian.[11] Thus from the close of the PalA|ozoic
all the types of sponges seem to have existed side by side; and in the
Cretaceous period, when such large areas of our continents were deeply
submerged, they attained a wonderful development, perhaps not equalled
in any other era of the earthaEuro(TM)s history.
[Illustration: FIG. 33.aEuro"_Oldhamia antiqua_ (Forbes).]
[Illustration: FIG. 34.aEuro"_Dictyonema sociale_. Enlarged. _Lingula_
flags.aEuro"After Salter.]
Sponges may be regarded as the highest or most complex of the Protozoa
or the lowest of the Coelenterates. We have no links wherewith to
connect them with the lower Protozoa of the Eozoic period; and through
their long history, though very numerous in genera and species, they
show no closer relationship with the Foraminifera below, and the Corals
above, than do their successors in the modern seas. They thus stand very
much apart; and modern studies of their development and minute
structures do not seem to remove them from this isolation. Though we are
treating here of inhabitants of the sea, it may be proper to mention
that Geinitz has described two species from the Permian which he
believed to be early precursors of the SpongillA|, or fresh-water
sponges; but more recently he seems to regard them as probably AlgA|.
Young has, however, recently found true spicules of _Spongilla_ in the
Purbeck beds.[12]
[Illustration: FIG. 35.aEuro"_Dictyonema Websteri_ (Dn). Niagara formation.
_a_, Enlarged portion (_Acadian Geology_).]
[Illustration: FIG. 36.aEuro"Group of modern Hydroids allied to Graptolites.
Magnified, and natural size.
_a_, _Sertularia_. _b_, _Tubularia_. _c_, _Campanularia._]
[Illustration: FIG. 37.aEuro"Silurian GraptolitidA|.
_a_, _Graptolithus_. _b_, _Diplograpsus_. _c_, _Phyllograpsus_. _d_,
_Tetragrapsus_. _e_, _Didymograpsus_.]
[Illustration: FIG. 38.aEuro"Central portion of Graptolite, with membrane,
or float (_Dichograpsus octobrachiatus_, Hall).]
[Illustration: FIG. 39aEuro"_Ptilodictya acuta_ (Hall). Bryozoan.
Siluro-Cambrian.]
A stage higher than the sponges are those little polyp-like animals with
sac-like bodies and radiating arms or tentacles, which form minute horny
or calcareous cells, and bud out into branching communities, looking to
untrained eyes like delicate sea-weedsaEuro"the sea-firs and sea-mosses of
our coasts (Fig. 36). These belong to a very old group, for in the
oldest Cambrian we have a form referred to this type (Fig. 33), and in
the Upper Cambrian another still more decided example (Fig. 34).[13]
This style of life, once introduced, must have increased in variety and
extended itself with amazing rapidity, for in the Siluro-Cambrian age we
find it already as characteristic as in our modern seas, and so abundant
that vast thicknesses of shale are filled and blackened with the
_dA(C)bris_ of forms allied to the sea-firs, and masses of limestone
largely made up of the more calcareous forms of the sea-mosses. As
examples of the former we may take the _Graptolites_, so named from
their resemblance to lines of writing, and of which several forms are
represented in Fig. 37. The little teeth on the sides of these were
cells, inhabited probably by polyps, like those represented in the
modern _Sertularia_ in Fig. 36. Some of them were probably attached to
the bottom. In others the branches radiated from a central film which
may have been a hollow vesicle or float, enabling them to live at the
surface of the water (Fig. 38). These Graptolites are specially
characteristic of the Upper Cambrian and Lower Silurian. The netted ones
(_Dictyonema_), as may be seen from Figs. 34 and 35, came in before the
close of the Cambrian, and continue unchanged to the Silurian, where
they disappear. The branching forms, seen in Fig. 37, have scarcely so
great a range. They thus form most certain marks of the period to which
they belong, and being oceanic and probably floaters, they diffused
themselves so rapidly that they appear to indicate the same geological
time in countries so widely separated as Europe, North America, and
Australia. It is curious, too, that while the Graptolites thus mark a
definite geological time, and seem to disappear abruptly and without
apparent cause, they are the first link in the long chain of the
Hydroids, which, though under different family forms, continue to this
day, apparently neither better nor worse than their perished PalA|ozoic
relatives. There is a group of little Stony Corals (_MonticuliporidA|_),
which were possibly also the cells of Hydroids, that have a similar
history. They are the only known Corals that date so far back as the
Upper Cambrian; and they continue under very similar forms all through
the PalA|ozic, and are represented by the millepore corals of the present
day. Fig. 40 represents a form found at the base of the Siluro-Cambrian,
and Fig. 41 shows forms characteristic of the Carboniferous Limestone.
[Illustration: FIG. 39_a_.aEuro"_Fenestella Lyelli_ (Dawson). A
Carboniferous Bryozoan.]
[Illustration: FIG. 40.aEuro"_Chaetetes fibrosa_. A tubulate coral with
microscopic cells. Siluro-Cambrian.]
If we turn now to the sea-mosses (Bryozoa), we have a group of minute
polyp-like animals inhabiting cells not unlike those of the Hydroids,
and which form plant-like aggregates. But the animals themselves are so
different in structure that they are considered to be nearer allies of
the bivalve shell-fishes than of the Corals. They are, in short, so
different, that the most ardent evolutionist would scarcely hold a
community of origin between them and such creatures as the Graptolites
and Millepores, though an ordinary observer might readily confound the
one with the other. These animals appear at the beginning of the
Siluro-Cambrian, and such forms as that represented in Fig. 39, very
closely allied to some now living, are large constituents of some of the
limestones of that period. Other forms, like that represented in Fig.
39_a_, are very characteristic of the Carboniferous. These animals,
individually small, though complicated in structure and branching into
communities, scarcely ever of any great magnitude, humble creatures
which have never played any great part in the world, have, nevertheless,
been so persistent that, though specific and generic forms have been
changed, the group may be said to be in the modern seas exactly what it
was in those of the early PalA|ozoic, nor can it be affirmed to have
originated in anything different, or to have produced anything.
[Illustration: FIG. 41.aEuro"_a_, _Stenopora exilis_ (Dawson). _b_,
_Chaetetes tumidus_ (Edwards and Haine). Carboniferous.]
The true Stony Corals (_Anthozoa_) are as yet unknown in the Cambrian.
They entered on the stage in immense abundance in the Siluro-Cambrian,
where considerable limestones are largely composed of their remains,
mixed, however, and sometimes overpowered with those of Bryozoa and
Hydroids. An ordinary coral, such as those of which coral reefs are
builtaEuro"the red coral, used for ornament is not quite similaraEuro"is the
skeleton of an animal constructed on the plan of a sea anemone; with a
central stomach surrounded by radiating chambers, and having above a
crown of tentacles. The stony coral surrounds and protects the soft body
of the animal, and may either be a single cell, for one animal, or an
aggregation of such cells, constituting a rounded or branching mass.
The modern star coral, represented in Fig. 42, is an instance of the
latter condition. It shows nineteen or twenty animals, each with a
central mouth and fringe of short tentacles, aggregated together, and
two of them showing the spontaneous division by which the number of
animals in the mass is progressively increased. The living coral shows
only the soft animals and the animal matter connecting them; but if dead
there would be a white stony mass with a star-like cell or depression
corresponding to each animal.
[Illustration: FIG. 42.aEuro"Living Anthozoan Coral (_AstrA|a_).]
In their general plan, the oldest Corals were precisely of this
character, but they presented some differences in detail, which have
caused them to be divided into two groups, which are eminently
characteristic of the PalA|ozoic ageaEuro"the tabulate or floored corals, and
the rugose or wrinkled corals. In the former (Fig. 43) the cells are
usually small and thin-walled, often hexagonal, like a honeycomb, and
are floored across at intervals with tabulA| or horizontal plates. A few
modern corals present a similar arrangement,[14] but this kind of
structure was far more prevalent in the PalA|ozoic. In the second type
the animals are usually larger and often solitary, the cell has strongly
marked radiating plates, while the horizontal floors are absent or
subordinate, and there is usually a thick external rind or outer coat
(Figs. 44, 45). In general plan, these rugose corals closely resemble
those of our modern reefs; but they differ in their details of
structure, and only a very few modern forms from the deep sea are
regarded as actual modern representatives.[15] One curious point of
difference is that their radiating laminA| begin with four, and increase
by multiples of that number, while in modern corals the numbers are six
and multiples of six; a change of mathematical relation not easily
accounted for, and which assimilates them to Hydroids on the one hand,
and to a higher group, the Alcyonids, on the other, both of which prefer
four and eight to six, or have had these numbers chosen for them. In the
Mesozoic period the tabulate and rugose corals were replaced by others,
the porous and solid corals of the modern seas; but, in so far as we
know, the animals producing these, though differing in some details,
were neither more nor less elevated than their predecessors, and they
took up precisely the same rA'le as reef-builders in the sea, though with
probably more tendency to the accumulation of great masses of coral
limestone in particular spots.
[Illustration: FIG. 43.aEuro"Tabulate Corals.
_a_, _Halisites_, and _b_, _Favosites_. Upper Silurian.]
[Illustration: FIG. 44.aEuro"Rugose Coral (_Heliophyllum Halli_). Devonian.]
[Illustration: FIG. 44_a_.aEuro"_Zaphrentis prolifica_ (Billings). Devonian.]
[Illustration: FIG. 45.aEuro"Rugose Corals.
_a_, _Zaphrentis Minas_ (Dn.), and _b_, _Cyathophyllum Billingsi_ (Dn.).
Carboniferous.]
Leaving the corals, we may turn to the sea-stars and sea-urchins. These
merely put in an appearance in the Early Cambrian, but become vastly
multiplied in the Silurian, where the stalked feather stars (Crinoids)
(Fig. 46) seem to have covered great areas of sea-bottom, and multiplied
so rapidly that thick sheets of limestone are largely made up of the
fragments of their skeletons. The ordinary star-fishes appear first in
the Silurian (Fig. 47). The sea-urchins begin in the Upper Silurian, the
early species having numerous and loosely attached plates, like some of
those now found in the deep sea[16] (Fig. 48).
[Illustration: FIG. 46.aEuro"Modern Crinoid (_Rhisocrinus Lofotensis_).aEuro"After
Sars.]
[Illustration: FIG. 47.aEuro"_PalA|aster Niagarensis_ (Hall). One of the
oldest star fishes.]
[Illustration: FIG. 48.aEuro"_PalA|chinus ellipticus_ (McCoy). One of the
oldest types of sea-urchins.]
The most curious history in this group is that of the feather-stars. In
the Early Cambrian they are represented by a few species known to us
only in fragments, and these belong to a humble group (Cystideans)
resembling the larval or immature condition of the higher Crinoids. Fig.
49 shows one of these animals of somewhat later age. They have few or
rudimentary arms and short stalks, and want the beautiful radial
symmetry of the typical star-fishes. In the Silurian these creatures are
reinforced by a vast number of beautiful and perfect feather-stars
(Figs. 50, 51). These continue to increase in number and beauty, and
apparently culminate in the Mesozoic, where gigantic forms exist, some
of them probably having more complicated skeletons, in so far as number
of distinct parts is concerned, than any other animals. Buckland has
calculated that in a crinoid similar to that in Fig. 52 there are no
less than 150,000 little bones, and 300,000 contractile bundles of
fibres to move them. In the modern seas the feather-stars have somewhat
dwindled both in numbers and complexity, and are mostly confined to the
depths of the ocean. On the other hand, the various types of ordinary
star-fishes and sea-urchins have increased in number and importance. We
thus find in this group a certain advance and improvement from the
Cystideans of the Early PalA|ozoic to the sea-urchins and their allies.
This advance is not, however, along one line for the Cystideans
continue unimproved to the end. The Crinoids culminate in the Mesozoic,
and are not known to give origin to anything higher. The star-fishes and
sea-urchins commence independently, before the culmination of the
Crinoids, and, though greatly increased in number and variety, still
adhere very closely to their original types.
[Illustration: FIG. 49.aEuro"_Pleurocystites squamosus_. Siluro-Cambrian.
After Billings.]
[Illustration: FIG. 50.aEuro"_Heterocrinus simplex_ (Meek). One of the least
complex crinoids of that period. Siluro-Cambrian.]
[Illustration: FIG. 51.aEuro"Body of _Glyptocrinus_. Siluro-Cambrian.]
The great sub-kingdom of the Mollusca, including the bivalve and
univalve shell-fishes, makes its first appearance in the Cambrian, where
its earliest representatives belong to a group, the Arm-bearers or Lamp
shells (Brachiopods), held by some to be allied to worms as much as to
mollusks. The oldest of all these shells are allies of the modern
_LingulA|_ (Fig. 54), some of the earliest of which are shown in Fig.
55. The modern _Lingula_ is protected by a delicate two-valved shell,
composed, unlike that of most other mollusks, of phosphate of lime or
bone earth. It lives on sand-banks, attached by its long flexible stalk,
which it buries like a root in the bottom. Its food consists of
microscopic organisms, drifted to its mouth by cilia placed on two
arm-like processes, from which the group derives its name. In the modern
world about one hundred species of Brachiopods are known, belonging to
about twenty genera, some of which differ considerably from the LingulA|.
The genus _Terebratula_, represented at Fig. 56, is one of the most
common modern as well as fossil forms, and has the valves unequal, with
a round opening in one of them for the stalk, which is attached to some
hard object, and there is an internal shelly loop for supporting the
arms.
[Illustration: FIG. 52.aEuro"_Extracrinus Briareus_. Reduced. Jurassic.]
[Illustration: FIG. 53.aEuro"_Pentacrinus caput-medusA|_. Reduced. Modern.]
[Illustration: FIG. 54.aEuro"_Lingula anatina_. With flexible muscular
stalk. Modern.]
[Illustration: FIG. 55.aEuro"Cambrian and Silurian LingulA|.
_a_, _Lingulella Matthewi_ (Hartt). Acadian group. _b_, _Lingula
quadrata_ (Hall). Siluro-Cambrian. _c_, _Lingulella prima_ (Hall).
Potsdam. _d_, _Lingulella antiqua_ (Hall). Potsdam.]
These curious, and in the modern seas, exceptional shells, were dominant
in the PalA|ozoic period. Upwards of three thousand fossil species are
known, of which a large proportion belong to the Cambrian and Silurian,
nine genera appearing in the Cambrian, and no less than fifty-two in the
Silurian. The history of these creatures is very remarkable. The
LingulA|, which are the first to appear, continue unchanged and with the
same phosphatic shells to the present day. Morse, who has carefully
studied an American species, remarks in illustration of this, that it is
exceedingly tenacious of life, bearing much change of depth,
temperature, etc., without being destroyed. The genus _Discina_, which
is nearly as old, also continues throughout geological time. The genus
_Orthis_ (Fig. 57), which appears at the same time with the last,
becomes vastly abundant in Silurian times, but dies out altogether
before the end of the PalA|ozoic. _Rhynchonella_ (Fig. 58), which comes
in a little later, near the beginning of the Siluro-Cambrian, continues
to this day. _Spirifer_ and _Productus_ (Figs. 59 and 60) appear later, and
die out at the close of the PalA|ozoic. So strange and inscrutable are
the fortunes of these animals, which on the whole have lost in the
battle of life, that their place in nature is vastly less important than
it was. It has been suggested that if any group of creatures could throw
light upon the theory of descent with modification, it would be these;
but Davidson, who has perhaps studied them more thoroughly than any
other naturalist, found them as silent on the subject as the sponges or
the corals. In a series of papers published in the _Geological
Magazine_, a short time before his death, he remarked as follows:
[Illustration: FIG. 56.aEuro"_Terebratula sacculus_ (Martin).
Carboniferous.]
[Illustration: FIG. 57.aEuro"Brachiopods; genus _Orthis_.
_a_, _O. Billingsi_ (Hartt). Lower Cambrian. _b_, _O. pectinella_
(Hall). Siluro-Cambrian. _c_, _O. lynx_ (Eichwald). Siluro-Cambrian.]
[Illustration: FIG. 58.aEuro"_Rhynchonella increbrescens_ (Hall).
Siluro-Cambrian.]
[Illustration: FIG. 59.aEuro"_Spirifer mucronatus_ (Conrad). Devonian.]
[Illustration: FIG. 59_a_.aEuro"_Athyris subtilita_ (Hall). Carboniferous.
_a_, _b_, Exteriors. _c_, Interior, showing spirals.]
aEurooeWe find that the large number of genera made their first appearance
during the PalA|ozoic periods, and since they have been decreasing in
number to the present period. We will leave out of question the species,
for they vary so little that it is often very difficult to trace really
good distinctive characters between them; it is different with the
genera, as they are, or should be, founded on much greater and more
permanent distinctions. Thus, for example, the family _SpiriferidA|_
includes genera which are all characterised by a calcified spiral lamina
for the support of the brachial appendages; and, however varied these
may be, they always retain the distinctive characters of the group from
their first appearance to their extinction. The Brachiopodist labours
under the difficulties of not being able to determine what are the
simplest, or which are the highest families into which either of the two
great groups of his favourite class is divided; so far, then, he is
unable to point out any evidence favouring progressive development in
it. But, confining himself to species, he sees often before him great
varietal changes, so much so as to make it difficult for him to define
the species; and it leads him to the belief that such groups were not of
independent origin, as was universally thought before Darwin published
his great work on the _Origin of Species_. But in this respect the
Brachiopoda reveal nothing more than other groups of the organic
kingdoms.
[Illustration: FIG. 60.aEuro"_Productus cora_ (DaEuro(TM)Orbigny). Carboniferous.]
aEurooeNow, although certain genera, such as _Terebratula_, _Rhynchonella_,
_Crania_, and _Discina_, have enjoyed a very considerable geological
existence, there are genera, such as _Stringocephalus_, _Uncites_,
_Porambonites_, _Koninckina_, and several others, which made their
appearance very suddenly and without any warning; after a while they
disappeared in a similar abrupt manner, having enjoyed a comparatively
short existence. They are all possessed of such marked and distinctive
internal characters that we cannot trace between them and associated or
synchronous genera any evidence of their being either modifications of
one or the other, or of being the result of descent with modification.
Therefore, although far from denying the possibility or probability of
the correctness of the Darwinian theory, I could not conscientiously
affirm that the Brachiopoda, as far as I am at present acquainted with
them, would be of much service in proving it. The subject is worthy of
the continued and serious attention of every well-informed man of
science. The sublime Creator of the universe has bestowed on him a
thinking mind; therefore all that can be discovered is legitimate.
Science has this advantage, that it is continually on the advance, and
is ever ready to correct its errors when fresh light or new discoveries
make such necessary.aEuroŁ The late Joachim Barrande, the great
palA|ontologist of Bohemia, bears similar testimony.
[Illustration: FIG. 61.aEuro"Group of Older PalA|ozoic Lamellibranchs.aEuro"After
Billings.
1, _Cucullea opima_. 2, _Nucula oblonga_. 3, _Nucula lineata_. 4,
_Cypricardia truncata_. 5, _Tellina ovata_. 6, _Nucula bellatula_. 7,
_Modiola concentrica_.]
The ordinary bivalves, like the mussels and cockles, now so very
plentiful on our coasts, are rare in the Cambrian and Silurian, and for
the first time make a somewhat conspicuous appearance in the Upper
Silurian and Devonian. But from the first they resemble very closely
their modern successors, though on the whole neither so large nor so
ornate (Fig. 61). Their fortunes have thus been precisely the opposite
of those of the Brachiopods, though in neither case is there very
marked elevation or deterioration in the individual animals. A very
similar statement may be made as to the sea-snails, whether the curious
winged snails (Pteropods) or the ordinary crawlers (Gastropods). The
former come in early, and are represented by PalA|ozoic forms finer than
any now extant. The genus _Conularia_ (Fig. 62) presents some Silurian
species six inches or more in length, which are giants in comparison
with any now living. The forms of more ordinary Gastropods from the
Silurian represented in Fig. 63 will suffice to show that their styles
are not very dissimilar from those still extant.[17] As in the case of
the ordinary bivalves, however, the modern Gastropods much exceed in
numbers and magnitude those of the PalA|ozoic.
[Illustration: FIG. 62.aEuro"_Conularia planicostata_ (Dn.). A Carboniferous
Pteropod.]
[Illustration: FIG. 63.aEuro"Silurian Sea-snails. Canada.
_a_, _Murchisonia bicincta_ (Hall). _b_, _Pleurotomaria umbilicatula_
(Hall). _c_, _Murchisonia gracilis_ (Hall). _d_, _Bellerophon
sulcatinus_ (Billings).]
The highest group of Mollusks, represented in the modern ocean by the
Nautili and Cuttle-fishes, has a history so strange and eventful, and so
different from what might have been anticipated, that it perhaps
deserves a more detailed notice, more especially as Barrande has
recently directed marked attention to it in his magnificent work on the
PalA|ontology of Bohemia.
The Cuttle-fishes and Squids and their allies are, in the modern seas, a
most important group (Fig. 64). The great numbers in which the smaller
species appear on many coasts, and the immense size and formidable
character of others; their singular apparatus of arms, bearing suckers,
their strange forms, and the inky secretion with which they can darken
the water, have at all times attracted popular attention. The great
complexity of their structures, and the fact that in many points they
stand quite at the head of the invertebrates of the sea, and approach
most nearly to the elevation of the true fishes, have secured to them
the attention of naturalists. Some of these animals have shelly internal
supports, and one genus, that of the Argonauts, or Paper Nautili, has an
external protective shell. Allied, though more distantly, to the
Cuttle-fishes, are the true Nautili, represented in the modern sea
principally by the Pearly Nautilus, though there are two other species,
both of them very rare. The modern pearly nautilus (Fig. 65) may be
regarded as a peculiar kind of cuttle-fish provided with a discoidal
shell for protection, and also for floatage. The shell is divided into a
number of chambers by partitions. Of these the animal inhabits the last
and largest. The others are empty, and are connected with the body of
the animal only by a pipe, or siphuncle, with membranous walls and
filled with fluid. Thus provided, the nautilus, when in the water, has
practically no weight, and can move up or down in the sea with the
greatest facility, using its sucker-bearing arms and horny beak to seize
and devour the animals on which it preys. The buoyancy of the shell
seems exactly adapted to the weight of the animal; and this proportion
is kept up by the addition of new air-chambers as the body increases in
size. In the modern seas this singular little group stands entirely
isolated, and its individuals are so rare that it is difficult to
procure perfect specimens for collections, though its mechanical
structure and advantages for the struggle for existence seem of the
highest order. But in the old world of past geological time the case was
altogether different.
[Illustration: FIG. 64.aEuro"Squid (_Loligo_).]
[Illustration: FIG. 65.aEuro"Pearly Nautilus (_Nautilus pompilius_).
_a_, Mantle. _b_, Its dorsal fold. _c_, Hood. _o_, Eye. _t_, Tentacles.
_f_, Funnel. _g_, Air chambers. _h_, Siphuncle.]
The Nautiloid shell-fishes burst suddenly upon us in the beginning of
the Siluro-Cambrian, or Lower Silurian, BarrandeaEuro(TM)s second fauna; and
this applies to all the countries where they have been studied. In this
formation alone about 450 species are known, and in the Silurian these
increase to 1,200; and here the group culminates. It returns in the
Devonian to about the same number with the Lower Silurian, diminishes in
the Carboniferous to 350, and in the Mesozoic, where the Nautiloid forms
are replaced by others of the type of the Ammonites, becomes largely
reduced. In the Tertiary there are but nineteen species, and, as already
stated, in the modern world _three_. These statements do not, however,
represent the whole truth. In the PalA|ozoic, in addition to the genus
_Nautilus_, we have a great number of other genera, some with perfectly
straight shells, like _Orthoceras_ (Fig. 66), others bent
(_Cyrtoceras_), others differing in the style of siphuncle, or aperture,
or chambers (_Endoceras_, _Gomphoceras_, _Lituites_, Figs. 67 to 69), or
inflated into sac-like forms (_Ascoceras_). There is, besides, the
family of the _GoniatidA|_ (Fig. 70), with the chambers thrown into
angular folds and the siphuncle at the back. Further, some of the early
forms, as the OrthoceratidA|, attain to gigantic dimensions, being six
feet or more in length, and nearly a foot in diameter. Thus the idea
that we should naturally form from the study of the Nautilus, that it
represents a type suited for much more varied and important adaptations
than those that we now see, is more than realised in those PalA|ozoic
ages when these animals seem to have been the lords of the seas.
[Illustration: FIG. 66.aEuro"_Orthoceras_. Siluro-Cambrian. The dotted line
shows the position of the siphuncle.]
[Illustration: FIG. 67.aEuro"_Gomphoceras._]
[Illustration: FIG. 68.aEuro"_Lituites._]
[Illustration: FIG. 69.aEuro"_Nautilus Avonensis_ (Dn.). Carboniferous.
_a_, Shell, reduced. _b_, Section, showing siphuncle.]
[Illustration: FIG. 70.aEuro"_Goniatites crenistria_ (Philips).
Carboniferous.]
[Illustration: FIG. 71.aEuro"_Ceratites nodosus_ (Schloth). Triassic.]
When we leave the PalA|ozoic and enter the Mesozoic, though the Nautiloid
shells still abound, we find them superseded, in great part, by a nobler
form, that of the _AmmonitidA|_ (Figs. 71, 72). These are remarkable for
the ornate markings on the surfaces of their shells, and for the
beautifully waved edges of the partitions (Fig. 72_a_), which, by giving
a much more complete support to the sides of the shell, must have
contributed greatly to the union of lightness and strength so important
to the utility of the shell as a float. This type admits of all the same
variety of straight, bent, and curled forms with the simpler Nautiloid
type, and some of the species are of great size, Ammonites being known
three feet or more in diameter. These animals, unknown in the PalA|ozoic,
appear in numerous species in the Early Mesozoic, culminate in hundreds
of beautiful species in the middle of that era, and disappear for ever
at its close, leaving no modern successors. Many and beautiful species
of Ammonites and their allies have been obtained from the Mesozoic rocks
of British Columbia and other parts of the west coast of North America,
perfectly representing this group as it occurs at the same period in
Europe, and closely resembling the Mesozoic Ammonites of India. These
animals have all perished, yet the Atlantic and the Pacific roll
between, apparently with conditions as favourable for their comfortable
existence as those of any previous time. They perished long ago, at the
dawn of the Tertiary; yet the genus Nautilus, one of the oldest and
least improved of the whole, survived, and still testifies to the
wonderful contrivance embodied in these animals.
[Illustration: FIG. 72.aEuro"_Ammonites Jason_ (Reinecke). Jurassic.]
[Illustration: FIG. 72_a_.aEuro"Suture of _Ammonites componens_ (Meek), of
British Columbia. Showing the complicated folding of the edges of the
chambers to give strength to the shell. Cretaceous.]
[Illustration: FIG. 73.aEuro"Cretaceous AmmonitidA|.
_a_, _Baculites_. _b_, _Ancyloceras_. _c_, _Crioceras_. _d_,
_Turrilites_.]
These are merely general considerations, but Barrande, in his _A%tudes
GA(C)nA(C)rales_, goes much farther. He sums up all the known facts in the
most elaborate manner, considering first the embryonic characters of the
shell in the different genera, then their distribution in space and
time, then all the different parts and characters of the shells in the
different groupsaEuro"the whole with reference to any possible derivation of
the species; and he finds that all leads to the result that in every
respect these shells seem to have been so introduced as to make any
theory of evolution with respect to them altogether untenable. In his
concluding sentence this greatest of PalA|ozoic palA|ontologists affirms
that, aEurooeThe theoretical evolution of the Cephalopods is, like that of the
Trilobites, a mere figment of imagination, without any foundation in
fact.aEuroŁ[18]
[Illustration: FIG. 74.aEuro"_Belemnite._aEuro"After Philips.]
[Illustration: FIG. 74_a_.aEuro"_Belemnoteuthis antiquus._ Supposed to be a
Belemnite, with soft parts preserved.aEuro"Jurassic.aEuro"After Mantell.]
I have reserved no space to notice the geological history of the other
and higher group of Cephalopods, including the true Cuttles and Squids.
This is perhaps less to be regretted, as, from the absence of external
shells, they are likely to be much less perfectly known as fossils. So
far as known, they are vastly younger than the Nautiloids, for no
examples whatever have been found in the PalA|ozoic. They appear
abundantly in the Mesozoic, but are there represented principally by an
extinct group of squids (Belemnites and their allies, Figs. 74, 74_a_),
remarkable for the great and complicated development of their internal
support, which has a chambered float as well as a solid sheath. This
family becomes extinct at the close of the Mesozoic, though the cuttles
as a whole perhaps culminate in the modern.
[Illustration: FIG. 75.aEuro"Cambrian Trilobites.
_a_, _Paradoxides_. _b_, _Dikellocephalus_. _c_, _Conocoryphe_ (head).
_d_, _Agnostus_ (head and tail).]
The remarkable group of the Trilobites had precedence in order of time
of the Nautiloid shell-fishes. No animal structures can well be more
dissimilar than those of the two great groups of aquatic animals which
popular speech confounds under the name of aEurooeshell-fishes.aEuroŁ Take a whelk
and a crab, for example, and compare their general forms, the structure
of their shells, and their organs of motion, and it is scarcely possible
to imagine any two animals more unlike; and when we examine their
anatomy in detail this difference does not diminish. They have, it is
true, corresponding parts, and these parts serve similar uses, but in
plan of structure they are wholly different. Yet both animals may live
in the same pool, and may subsist on nearly the same food. If we attempt
to find some common type which both resemble, we may trace the structure
of the crab back to those of some of the marine worms with which it has
some affinity, and those of the whelk to such creatures as the
_Lingula_, which are supposed to have a resemblance to the worms. But
still the two types, that of the Mollusk and the Articulate, are
distinct even from their first appearance in the egg, nor have either
any close affinities with the Protozoa, the Hydroids, or the Corals.
[Illustration: FIG. 76.aEuro"Transverse section of _Calymene_. A Silurian
Trilobite.aEuro"After Wolcott.
_a_, Dorsal shell. _b_, Visceral cavity. _c_, Legs. _d_,
EpipoditeaEuro"gill-cleaner or palp. _e_, Spiral gills.]
Both types meet us in the Early Cambrian, but while the Mollusk is there
represented only by low forms, the Articulate is then not only in the
humble guise of the worm, but in the complex and highly organised form
of the Trilobite (Figs. 28 and 75). What older phases they may have
passed through we know not; but in the Lower Cambrian we have various
forms of these animals, including some of the largest known as well as
some of the smallest; some of the most complex in number of parts as
well as some of the simplest. These animals, in short, seem to have
appeared at once all over the world fully formed, and in a variety of
generic and specific forms; and nothing short of a very large faith in
the imperfection of the geological record can suffice to account for
their evolution.
[Illustration: FIG. 76_a_.aEuro"Burrows of Trilobite and of modern
King-crab. The Trilobite burrow is known as Ruschinites, and has been
supposed to be a sea-weed of the kind called _Bilobites_.]
A Trilobite is a creature in whose structure the number three is
dominant. Seen from above, it presents three divisions from front to
rear:aEuro"first, a cephalic shield or head-piece; secondly, a thorax,
divided into several segments movable upon each other; and thirdly, a
tail-piece or pygidium, which, when brought against the head by the
rolling up of the body segments, effectually covers the lower parts.
This lower portion was until lately little known; but the discoveries of
Billings and of Wolcott have enabled us to restore the jaws under the
head, the jointed legs and spiral gills under the thorax, and thus to
complete the structure of the animal, and understand better its
relations to modern crabs and shrimps (Fig. 76). Of these it certainly
comes nearest to the King-crabs and Horseshoe-crabs, a somewhat limited
group at present, and one which reaches back in geological time only to
the Upper Silurian, when the Trilobites had perhaps already passed their
culmination.
Constructed as above described, the Trilobite could swim, as is
supposed, usually on its back or side. It could crawl on the bottom.
Using its snout as a shovel, it could burrow like a modern King-crab
(Fig. 76_a_); and when pressed by danger some species could roll
themselves into balls and defy their enemies.
[Illustration: FIG. 77.aEuro"Silurian Trilobites.
_a_, _Isotelus_. _b_, _Homalonotus_. _c_, _Calymene_.]
This type of animal, entering on the stage in full force in the Older
Cambrian, continues under many forms through the whole PalA|ozoic age,
dying out finally in the Carboniferous. Figs. 77 and 78 show a few of
the forms of the Silurian, Devonian, and Carboniferous.
Contemporaneously with the dawn of the Trilobite group, appear some
small shrimp-like forms (Fig. 28),[19] and others with bivalve shells
(Fig. 79), which are closely allied to modern forms,[20] and, like the
_LingulA|_, persist through the succeeding formations with little more
than specific changeaEuro"presenting in this a strange contrast to the
Trilobites. While the latter were still flourishing, about the close of
the Lower Silurian, a remarkable group of large and highly-developed
creatures, allied to the Trilobites, but suited for rapid swimming
rather than creeping, was introduced; and in the Upper Silurian and
Devonian these creatures[21] attained to gigantic sizes, exceeding,
probably, any modern Crustaceans, and were tyrants of the seas.
_Pterygotus anglicus_ (Fig. 80) is supposed to have attained the length
of six feet. Yet these noble representatives of the Crustaceans became
extinct in the Carboniferous. On the other hand, a few small king-crabs
appear in the Upper Silurian, and this type still continues, and seems
to culminate as to size in modern times; so diverse have been the
fortunes of these various groups.
[Illustration: FIG. 78.aEuro"Devonian and Carboniferous Trilobites.
_a_, _Phaceps latifrons_ (Bronn). _b_, _Philipsia Howi_ (Billings)
(tail).]
[Illustration: FIG. 79.aEuro"PalA|ozoic Ostracod Crustaceans. Magnified.
_a_, _Bairdia_. _b_, _Cytherella inflata_ (Jones). _c_, _Cythere_.
Carboniferous. _d_, _Beyrichia Jonesii_ (Dn.). Carboniferous. _e_,
_Beyrichia pustulosa_ (Hall). Silurian.]
The higher, or decapod Crustaceans, now familiar to us in the modern
crabs and lobsters, are first found in a few small species in the
Devonian[22] and Carboniferous, and they are accompanied in the Devonian
by at least one species of the allied group of the Stomapods (Figs. 81,
82).
[Illustration: FIG. 80.aEuro"_Pterygotus anglicus_. Reduced.aEuro"After Page and
Woodward.]
[Illustration: FIG. 81.aEuro"_Amphipeltis paradoxus_ (Salter). A Devonian
Stomapod from New Brunswick.]
[Illustration: FIG. 82.aEuro"_AnthropalA|mon Hilliana_ (Dn). A Carboniferous
Decapod from Nova Scotia. The carapace only.]
The PalA|ozoic age of geology is thus emphatically an age of
invertebrates of the sea. In this period they were dominant in the
waters, and until toward its close almost without rivals. We shall find,
however, that in the Upper Silurian, fishes made their appearance, and
in the Carboniferous amphibian reptiles, and that, before the close of
the PalA|ozoic, vertebrate life in these forms had become predominant.
We shall also see that just as the leading groups of Mollusks and
Crustaceans seem to have had no ancestors, so it is with the groups of
Vertebrates which take their places. It is also interesting to observe
that already in the PalA|ozoic all the types of invertebrate marine life
were as fully represented as at present, and that this swarming marine
life breaks upon us in successive waves as we proceed upward from the
Cambrian. Thus the progress of life is not gradual, but intermittent,
and consists in the sudden and rapid influx of new forms destined to
increase and multiply in the place of those which are becoming effete
and ready to vanish away or to sink to a lower place. Farther, since the
great waves of aquatic life roll in with each great subsidence of the
land, a fact which coincides with their appearance in the limestones of
the successive periods, it follows that it is not struggle for
existence, but expansion under favourable circumstances and the opening
up of new fields of migration that is favourable to the introduction of
new species. The testimony of palA|ontology on this point, which I have
elsewhere adduced at length,[23] in my judgment altogether subverts the
prevalent theory of aEurooesurvival of the fittest,aEuroŁ and shows that the
struggle for existence, so far from being a cause of development and
improvement, has led only to decay and extinction, whereas the advent of
new and favourable conditions, and the removal of severe competition,
are the circumstances favourable to introduction of new and advanced
species. This testimony of the invertebrates of the sea we shall find is
confirmed by other groups of living beings, to be noticed in the
sequel.[24]
* * * * *
NOTE.aEuro"The term aEurooeSiluro-Cambrian,aEuroŁ as used in this and the next chapter,
is synonymous with aEurooeOrdovicianaEuroŁ of Lapworth, which is now coming into
somewhat general use.
[Illustration: CORDAITES, OF THE GROUP OF DORY-CORDAITES. BRANCH
RESTORED.aEuro"After GrandaEuro(TM) Eury.]
CHAPTER IV.
THE ORIGIN OF PLANT LIFE ON THE LAND.
If the graphite of the Laurentian rocks was derived from vegetable
matter, the further question arises, Was this vegetation of the land, or
of the sea? and something may be said on both sides of this question. If
there were land plants in the Laurentian period, they must have grown
either on rocks older than the Laurentian itself, or on such portions of
the beds of the latter as had been raised out of the sea, forming
perhaps swampy flats of newly-made soil. But we know no rocks older than
the Laurentian, and there is no positive evidence that any of the beds
of that formation were other than marine. Still it is not impossible
that some of the beds which are now graphitic gneisses may originally
have been similar to the bituminous shales, coals, or under-clays of the
coal formation. The graphite occurring in veins, if of vegetable origin,
must have been derived from liquid bitumen oozing into fissures; and
veins of this kind occur in later formations, both in marine and
fresh-water beds. The only other positive argument which has been
adduced in favour of the existence of abundant land plants in the
Laurentian is that of Dr. Sterry Hunt, derived from the great beds of
iron ore, which it is difficult to account for chemically except on the
hypothesis of the decay in the air of great quantities of vegetable
matter. The question must remain in doubt till some one is fortunate
enough to find portions of the Laurentian carbon retaining traces of
organic structure. My own observations, though somewhat numerous, allow
me only to say that the graphite sometimes presents fibrous forms, that
it occasionally appears as vermicular threadsaEuro"which, however, I suppose
to be fillings of canals of _Eozoon_aEuro"and that in the graphitic beds there
are occasionally slender root-like bodies of a lighter colour than the
mass; but none of these indications are sufficient to determine anything
as to its vegetable origin, or the nature of the plants from which it
may have been derived.
In any case, the quantity of carbon which has been accumulated in the
Laurentian rocks is very great. I have measured one bed at Buckingham,
on the Ottawa, estimated to contain 20 per cent. of carbon, and which is
at least eight feet in thickness. Sir William Logan has described
another similar bed from ten to twelve feet thick, and more recent
reports of the Geological Survey of Canada mention a bed supposed to be
twenty-five feet thick, in which Mr. Hoffman finds 30 per cent. of
carbon. On the whole the quantity of carbon in the graphitic zone of the
Laurentian is comparable with that in certain productive coal-fields,
and we certainly have in the subsequent geological history no examples
of such accumulations except from remains of the luxuriant vegetation of
swampy flats.
The Upper Laurentian and Huronian have as yet afforded no evidence of
land vegetation. The Cambrian, as already stated, abounds in remains of
sea-weeds; but though the forms which have been named _Eophyton_ have
been regarded as land plants, this claim is, to say the least, very
doubtful; and I have as yet seen nothing of this kind which did not
appear to me to be merely markings made by objects drifted over the
bottom or remains of marine plants. Yet in the Upper Cambrian there are
wide surfaces of littoral sandstone often containing minute carbonised
fragments, and which might be expected to afford indications of land
vegetation, had such existed. I have myself devoted many days of
fruitless labour to the examination of the large areas of Potsdam
sandstone exposed in some parts of Canada. But as these rocks were
evidently formed along the borders of a Laurentian continent capable of
supporting vegetation, we may still hope for some discovery of this
kind, more especially if we could find the point where some fresh-water
stream ran into the Cambrian sea.
[Illustration: FIG. 83.aEuro"_Protannularia Harknessii_ (Nicholson). A
Siluro-Cambrian Plant, from the Skiddaw series.]
The oldest plants, probably higher than AlgA|, known to me by their
external forms, are those described by Nicholson[25] from the
Siluro-Cambrian Skiddaw slates of the north of England (Fig. 83). Their
discoverer has named them _Buthotrephis Harknessii_ and _B.
radiata_,[26] stating, however, that these two species are not
improbably portions of the same plant, and that its form is rather that
of a land plant than of an Alga. The specimens of these plants which I
have seen appear to me to support the conclusion that they represent one
species, and this allied to the _AnnulariA|_ of the Devonian and
Carboniferous periods, which probably grew in shallow water with only
their upper parts in the air, and bore whorls or verticles of narrow
leaves. They were either relatives of the MareaEuro(TM)s-tails, or of the
Rhosocarps, of our modern swamps and ponds.
[Illustration: FIG. 84.aEuro"American Lower Silurian Plants.aEuro"After
Lesquereux.
_a_, _Sphenophyllum primA|vum_. _b_, _Protostigma sigillarioides_.]
Somewhat higher up in the Lower Silurian, in the Cincinnati group of
America, Lesquereux finds objects which he refers to the genus
_Sphenophyllum_, which is closely allied to _Annularia_ (Fig. 84, _a_),
and also a plant which he terms _Protostigma_ (Fig. 84, _b_), and
believes to be the stem of a tree allied to the club-mosses.[27] He also
finds minute branching stems, which he refers to the genus
_Psilophyton_, to be mentioned in the sequel; but as to these I have
some doubts whether they may not be Zoophytes allied to the Graptolites,
rather than plants of that genus. These discoveries tend to show the
probable existence in the Siluro-Cambrian of plants representing two of
the three leading families of the higher cryptogams or flowerless
plants, namely, the Club-mosses and the MareaEuro(TM)s-tails. Thus land
vegetation begins with the highest members of the lower of the two great
series into which botanists divide the vegetable kingdom.
[Illustration: FIG. 86.aEuro"Fragment of outer surface of _Glyptodendron_ of
Claypole. A Silurian Tree.]
If we now turn to the Silurian, further evidence of land vegetation
presents itself. Near the base of this great series, the club-moss
family is represented by a plant discovered by Claypole in the Clinton
group, and referred to a new genus (_Glyptodendron_, Fig. 86). Plants of
this family have also been noticed by Barrande in Bohemia, and by page
in Scotland; and a humble but interesting member of the family,
connecting it with the pillworts, _Psilophyton_ (Fig. 87), though more
characteristic of the Devonian, has been found in the Upper Silurian
both in Canada and the United States. No Ferns or Equiseta have as yet
been found in the Silurian; but in 1870 I recognised in some fragments
of wood from the Ludlow bone-bed, in the Museum of the Geological Survey
of Great Britain, the structure of that curious prototypal tree, to
which I have given the name _Nematophyton_, and which was first
recognised in the Devonian of GaspA(C). Since that time I have found in the
Upper Silurian beds of Cape Bon Ami, in New Brunswick, similar fragments
of fossil wood, associated with round seed-like bodies, having a central
nucleus and a thick wall or test of radiating fibres. These bodies show
a structure similar to that of those found in the Upper Ludlow of
England, and described by Hooker under the name _Pachytheca_. In my
judgment they are certainly true seeds.[28] Seeds of this kind have also
been found by Hicks in the still older Denbighshire grits of North
Wales, along with fragments of the wood of _Nematophyton_, and with
remains of branching stems which have been described under the name
_Berwynia_, though it is not unlikely that they represent the branches
of _Nematophyton_. It is proper to add that these ancient vegetable
fossils are regarded by some English botanists as gigantic algA| or
sea-weeds, but I confess I am unable to adopt this view of their nature.
The supposed fern of the Upper Silurian, figured in the first edition of
this work, has proved on further examination to be merely an imitative
form produced by crystallisation. On the other hand, the recent
discovery of a cockroach and two species of Scorpion in the Silurian,
proves the existence of land animals as well as plants at this period.
[Illustration: FIG. 87.aEuro"_Psilophyton princeps_ (Dn.) Silurian and
Devonian. Restored.
_a_, Fruit, natural size. _b_, Stem, natural size. _c_, Scalariform
tissue of the axis, highly magnified. In the restoration one side is
represented in vernation, and the other in fruit.]
It is probable that these discoveries represent merely a small
proportion of the plants actually existing in the Silurian period. All
the deposits of this age at present known to us are marine; and most of
them were probably formed at a distance from land, so that it is little
likely that land plants could find their way into them. At any time the
discovery of an estuarine or lacustrine deposit of Silurian age might
wonderfully extend our knowledge of this ancient flora.
The Devonian or Erian age, that of the classic Old Red Sandstone of
Scotland, is that in which we find the first great and complete land
flora; and though this is inferior in number of species to that of the
succeeding Carboniferous, and greatly less important with reference to
its practical bearing on our welfare, it is in some respects superior in
that variety which depends on diversity of soil and of station. To
appreciate this, it will be necessary to glance at the range and
subdivisions of the modern flora.
In the modern world we divide all vegetation into two great series, that
of the Flowering Plants (_PhA|nogams_), which also produce true fruits
and seeds, and that of the Flowerless Plants (_Cryptogams_), which
produce minute spores instead of seeds. The latter is in every respect
the lower group. This lower series is again divisible into three
classesaEuro"first and lowest, that of the Seaweeds, Moulds, and Lichens
(Thallophytes). Secondly, that of the Mosses and their allies
(Anophytes). Thirdly, that of the Ferns, Equisetums and Club-mosses
(Acrogens). In like manner the second, or higher series is divisible
into three classes: that of the Pines and Cycads (Gymnosperms), having
naked seeds not covered by true fruits, and woody tissue of simple
structure; that of the Palms and Grasses and their allies (Endogens);
and last and highest, that of the ordinary timber trees and other plants
allied to them, with exogenous stems, netted-veined leaves, and a
two-leaved embryo (Exogens). These last are in every respect the
dominant plants on our present continents. Carrying with us this twofold
division of the vegetable kingdom and its subdivisions, we shall be
prepared to understand the relation of the more ancient floras to that
now living.
[Illustration: FIG. 88.aEuro"Trunk of a Devonian Tree-fern (_Caulopteris
Lockwoodi_, Dn.). Gilboa, New York. One-third natural size.]
[Illustration: FIG. 89.aEuro"Frond of _ArchA|opteris Jacksoni_ (Dn.).
Devonian, of Maine.]
[Illustration: FIG. 90.aEuro"Portion of a branch of _Leptophleum rhombicum_
(Dn.). A Lycopodiaceous tree of the Devonian of Maine. Natural size.]
[Illustration: FIG. 91.aEuro"_Calamites radiatus_ (Brongniart). Middle
Devonian of N. Brunswick.]
In the Devonian age we meet with no land plants of the two lower
classes of the Cryptogams, and with scarcely any that can be referred to
the two higher classes of PhA|nogams, so that the vegetation of this
period presents a remarkable character of mediocrity, being composed
almost entirely of the highest class of the flowerless plants and the
lowest class of those that flower. Of the former there are Tree-ferns
and vast numbers of herbaceous forms (Figs. 88, 89), great
Lycopodiaceous plants, immensely better developed than those now
existing (Fig. 90), and gigantic _Calamites_, allied to the MaresaEuro(TM)-tails
(Fig. 91), along with humbler members of the same group (Fig. 95). Of
the latter there were Pines of great stature, known to us at present
only by their wood (Fig. 92); and that other allied trees existed we
have evidence in numerous seeds which must have belonged to this class
(Fig. 93), and in long flag-like leaves[29] which modern discoveries
would refer to the same group. As yet we know no Devonian Palms or
Grasses; and only a single specimen has been found indicating the
existence of a plant of the highest vegetable class, that of the true
exogens. This unique specimen, found by Hall in the Devonian of the
shores of Lake Erie, is a fragment of mineralised wood, the structures
of which are represented in Fig. 94. The large ducts seen in cross
section in Nos. 1, 2, and 3, and in longitudinal section in Nos. 4 and
5, and the medullary rays, seen in Nos. 1, 4, and 6, testify to the fact
that this chip of wood must have belonged to a tree of the same type
which contains our oaks, maples, and poplars; a type which does not
appear to have become dominant till near the close of the Mesozoic, but
which already existed, though perhaps only in few species, and only in
upland and inland positions, as far back as the Middle Devonian. Perhaps
one of the most interesting discoveries in the Erian or Devonian rocks
has been that of the immense abundance of spores of those humble plants
the Rhizocarps, represented in modern times by the Pillworts and
SalviniA|, &c. To these it is believed that _Sphenophyllum_ and
_Psilophyton_ were allied; but in addition to this there are thick and
vastly extended beds of bituminous shale which owe their inflammable
properties to countless multitudes of Macrospores (_Sporangites_) of the
genus _Protosalvinia_.[30] In Ohio there are beds of this kind 350 feet
thick, and extending across the State. They occur also in Canada, where
these forms were first recognised by the writer in the bituminous shale
of Kettle Point, Lake Huron.
[Illustration: FIG. 92.aEuro"A Devonian Taxine Conifer (_Dadoxylon
ouangondianum_, Dn.). St. John, New Brunswick.
A, Fragment showing _Sternbergia_ pith and wood; _a_, Medullary sheath;
_b_, Pith; _c_, Wood; _d_, Section of pith.
B, Wood cell _a_, and hexagonal areole and pore _b_.
C, Longitudinal section of wood, showing _a_, Areolation, and _b_,
Medullary rays.
D, Transverse section showing _a_, Wood-cells, and _b_, Limit of layer
of growth.]
[Illustration: FIG. 93.aEuro"Group of Devonian Fruits, &c. Middle Devonian,
New Brunswick.
A, _Cardiocarpum cornutum_.
B, _Cardiocarpum acutum_.
C, _Cardiocarpum Crampii_.
D, _Cardiocarpum Baileyi_.
E, _Trigonocarpum racemosum_.
EA╣, EA squared, Fruits enlarged.
F, _Antholithes Devonicus_.
FA╣, Fruit of the same.
G, _Annularia acuminata_.
H, _Asterophyllites acicularis_.
HA╣, Leaf.
K, _Cardiocarpum_. (? young of A.)
L, _Pinnularia dispalans_.
From _Acadian Geology_. ]
[Illustration: FIG. 94.aEuro"Structures of the oldest-known Angiospermous
Exogen (_Syringoxylon mirabile_, Dn.). From Eighteen-mile Creek, Lake
Erie.
1, Transverse section x 100.
2 and 3, Portions of the same x 300.
4, Longitudinal section x 300.
5, Fragment of duct from the same x 600.
6, Wood cells and medullary ray x 600.]
The Devonian flora seems to have been introduced in the northern parts
of the American continent at a time of warm and equable climate, and of
elevation of new land out of the Silurian sea. It spread itself to the
southward, and was finally destroyed in the great subsidences and
disturbances which closed the Devonian age, and which were probably
accompanied with refrigeration of climate. It was succeeded by the more
massive and richer, but more monotonous flora of the Carboniferous, a
period in which large areas of our continents were in the state of
swampy and often submerged flats, and in which the climate was again
warm and uniform.
[Illustration: FIG. 95.aEuro"_Asterophyllites parvula_ (Dn.), and
_Sphenophyllum antiquum_ (Dn.). Middle Devonian, New Brunswick.]
The Carboniferous age was, even more emphatically than the Devonian, an
age of Acrogens and Conifers. A few Carboniferous Fungi have recently
been discovered, but there are no known Lichens or Mosses. There seem to
be a few Endogens, but no true Exogens. The great bulk of the plants
consists of Acrogens and Gymnosperms, as in the previous period. As this
flora is so very important and so much better known than any other of
those belonging to the infancy of the vegetable kingdom, we may notice a
little in detail some of its leading forms.
[Illustration: FIG. 96.aEuro"_Calamites._ Carboniferous.
A, _C. Suckovii_. B, _C. Cistii_ (Bt.). C, Base of _Calamites_. D, E,
Structures.
From _Acadian Geology_.]
[Illustration: FIG. 97.aEuro"Carboniferous Ferns.
A, _Odontopteris subcuneata_ (Bunbury).
B, _Neuropteris cordata_ (Brongniart).
C, _Alethopteris tonchitica_ (Brongniart).]
Beginning with the MaresaEuro(TM)-tails, we find these represented in the
Carboniferous by many gigantic species, attaining to almost tree-like
dimensions (Fig. 96). These are the _Calamites_, which formed dense brakes
and jungles on the margins of the great swampy flats of this period.
Their tall stems, ribbed and jointed, bore whorls of leaves or
branchlets. Sending out horizontal root-stocks and budding out from the
base, they grew in great clumps, and had the capacity to resist the
effects of accumulating sediment by constantly sending out new stems at
higher and higher levels. The larger species assumed a complexity in the
structure of their stems unknown in their modern congeners, and enabling
them to grow to a great height;[31] but their foliage and fructification
were not correspondingly advanced. Thus the family of the EquisetaceA|
culminated in the Carboniferous, and thenceforth descended gradually in
the succeeding ages, leaving the comparatively humble MaresaEuro(TM)-tails and
Scouring Rushes as its present representatives.
The Ferns of the Carboniferous, like those of the Devonian, presented
both gigantic forms like those of the tree-ferns of the modern tropics,
and delicate herbaceous species, and these in great profusion. On the
whole, they do not strike the observer as very dissimilar from those of
modern times. A more critical examination, however, shows that the bulk
of the tree-ferns of the Devonian and Carboniferous are allied not to
the Polypod type, which is the most common at present, but to certain
comparatively rare southern ferns, the _Marattias_ and their allies,
characterised by a peculiar style of fructification, perhaps adapting
them to a moist and warm atmosphere (Fig. 97).[32] Thus the ferns, while
a wonderfully persistent type, were in their grander forms far more
widely distributed in the Carboniferous than at present; and genera now
comparatively rare, and limited to warm and moist climates, were then
abundant, and ranged over those temperate and boreal regions of the
Northern Hemisphere where only a few humble and hardy species can now
subsist. There were also some remarkable and anomalous tree-ferns, of
which that represented in Fig. 98 is an example.
[Illustration: FIG. 98.aEuro"Carboniferous Tree-ferns.
A, _Megaphyton magnificum_ (Dn.).
C, _PalA|opteris Hartii_ (Dn.).
D, _P. Acadica_ (Dn.).]
The family of the Club-mosses, already, even in the Devonian, in advance
of its modern development, experiences in the Carboniferous a remarkable
and portentous extension into great trees of several genera and many
species, constituting apparently extensive forests, and having the woody
tissues of their stems developed to a degree unheard of in their present
representatives (Fig. 99). Further, they become closely linked, in
external form at least, with another and more advanced type, that of the
_SigillariA|_. These remarkable trees were the most abundant of all in
the swamps of the coal-formation, and probably those which most
contributed to the accumulation of coal. They presented tall pillar-like
trunks, often ribbed longitudinally, and with perpendicular rows of
scars of fallen leaves. Dividing at top into a few thick branches, they
were covered with long rigid grass-like foliage. Their fruit was borne
in rings or whorls of spikes surrounding the branches at intervals (Fig.
100). Their roots were strangely symmetrical, spreading out like
underground branches into the soft soil by a regular process of
bifurcation, and were covered with rootlets diverging in every
direction, and so jointed to the main root that when broken off they
left round marks regularly arranged. These roots are the so-called
_StigmariA|_, so abundant in every coal-field, and especially filling the
aEurooeunder-claysaEuroŁ of the coal-beds, which are the soils on which the plants
forming these beds were supported. The true botanical position of the
_SigillariA|_ has been a matter of much controversy. Some of them
undoubtedly have structures akin to those of the tree-like Club-mosses,
as Williamson has well shown, and may have been cryptogamous. Others
have structures of higher character, akin to those of the modern Cycads,
and seem to have borne nutlets allied to those of these plants. Yet the
external forms of these diverse sorts are so similar that no definite
separation of them has yet been made. Either these anomalous trees
constitute a link connecting the two great series of the vegetable
kingdom, or we have been confounding two distinct groups, owing to
imperfect information.
[Illustration: FIG. 99.aEuro"_Lepidodendron corrugatum_ (Dn.). A
characteristic Lycopod of the Lower Carboniferous of America.
A, Restoration.
B, Leaf, natural size.
C, Cone. D, Leafy branch.
E, Forms of leaf-bases.
F, Sporangium.
I, L, M, N, O, Markings on stem and branches, in various states.]
[Illustration: FIG. 100.aEuro"_SigillariA|_ of the Carboniferous.
A, _Sigillaria Brownii_ (Dn.).
B, _S. elegans_ (Brongniart).
BA╣, &c. Leaf and Leaf-scars.]
Another curious, and till recently little understood, group of
Carboniferous trees is that known as _Cordaites_, which existed already
in some of its species in the Devonian. Their leaves are long, and often
broad as well, and with numerous delicate parallel veins, resembling in
this the leaves of grasses. Corda long ago showed that one species at
least has a stem allied to the Club-mosses. More recently GrandaEuro(TM) Eury
has found in the South of France admirably preserved specimens, which
show that others more resembled in their structure the Pines and Yews,
and were probably Gymnosperms, approaching to the Pines, but with very
peculiar and exceptional foliage, of which the only modern examples are
the broad-leaved Pines of the genus _Dammara_ (Frontispiece to Chapter).
Here again we have either two very distinct groups, combined through our
ignorance, or a connecting link between the Lycopods and the Pines.
[Illustration: FIG. 101.aEuro"_Trigonocarpum Hookeri_ (Dn.). A Gymnospermous
seed.
_a_, Testa. _b_, Tegmen. _c_, Nucleus. _d_, Embryo.]
The Yews and their allies among modern trees, while members of the great
Cone-bearing order, bear nut-like seeds in fleshy envelopes, sometimes,
as in the GA-nkgo of Japan, constituting edible fruits. Seeds of this
type seem to have been extremely abundant in the Carboniferous age in
all parts of the world, and were probably produced by trees of several
genera (_Dadoxylon_, _Sigillaria_, _Cordaites_, etc.) (Fig. 101).
Charles Brongniart has recently described no less than seventeen genera
of these seeds from the coal-field of St. A%tienne alone, and it would be
a low estimate to say that we probably know as many as sixty or seventy
species in all, while the trunks of great coniferous trees allied to
TaxineA|, and showing well-preserved structure, are by no means uncommon
in the Devonian and Carboniferous. Had these great Yews appeared for the
first time in the Coal-formation, we might have supposed that they had
been developed from such Lycopods as _Lepidodendra_, and that the
_Cordaites_ are the intermediate forms; but unfortunately the Pines go
almost as far back in geological time as the Lycopods, and it does not
help us, when in search of evidence of evolution, to find the link which
is missing or imperfect in the Early Devonian supplied in the
Coal-formation, where, for this purpose at least, it is no longer
needed.
We have said something of what was in the PalA|ozoic flora; but what of
that which was not? We may answer:aEuro"Nearly all that is characteristic of
our modern forests, whether in the ordinary Exogens, which predominate
so greatly in the trees and shrubs of temperate climates, or in the
Palms and their allies, which figure so conspicuously within the
tropics. The few rare, and to some extent doubtful, representatives of
these types scarcely deserve to be noted as exceptions. Had a botanist
searched the PalA|ozoic forests for precursors of the future, he would
probably have found only a few rare species, while he would have seen
all around him the giant forms and peculiar and monotonous foliage of
tribes now degraded in magnitude and structure, and of small account in
the system of nature.
It must not be supposed that the PalA|ozoic flora remained in undisturbed
possession of the continents during the whole of that long period. In
the successive subsidences of the continental plateaux, in which the
marine limestones were deposited, it was to a great extent swept away,
or was restricted to limited insular areas, and these more especially in
the far north, so that on re-elevation of the land it was always peopled
with northern plants. Thus there were alternate restrictions and
expansions of vegetation, and the latter were always signalised by the
introduction of new species, for here, as elsewhere, it was not
struggle, but opportunity, that favoured improvement.
In the Lower Silurian such plants as existed must have experienced great
restriction at the age of the Niagara or Wenlock limestone. Those of the
Upper Silurian suffered a similar reverse at the time of the Lower
Helderberg or Ludlow limestones. This recurred at the close of the
Devonian and in the time of the Lower Carboniferous limestone; and
finally the PalA|ozoic flora disappeared altogether in the Permian, to be
replaced by new types in the Mesozoic. While, therefore, there is a
great general similarity in the successive PalA|ozoic floras, there are
minor differences, so that the Devonian plants are for the most part
distinct specifically from those of the Lower Carboniferous, those of
the Lower Carboniferous from those of the Coal-formation, and those of
the latter from those of the Permian.
With all these vicissitudes it is to be observed that there is no
apparent elevation of type in all the long ages from the Devonian to the
Permian, that the Acrogens and Gymnosperms of these periods are in some
respects superior, in all respects equal, to their modern successors,
and that their history shows a decadence toward the modern period; that
intermediate forms arrive too late to form connecting links in time,
that several distinct types appear together at the beginning, and that
all utterly and apparently simultaneously perish at the end of the
PalA|ozoic, to make way for the entirely new vegetation of the succeeding
age. Theories of evolution receive no support from facts like these,
though their practical significance, as parts of the one great uniform
scheme of nature, is sufficiently manifest.
Of what use then were these old floras? To the naturalist, vegetable
life, with regard to its modern uses, is the great accumulator of
pabulum for the sustenance of the higher forms of vital energy
manifested in the animal. In the PalA|ozoic this consideration sinks in
importance. In the Coal period we know few land animals, and these not
vegetable feeders, with the exception of some insects, millipedes, and
snails. But the Carboniferous forests did not live in vain, if their
only use was to store up the light and heat of those old summers in the
form of coal, and to remove the excess of carbonic acid from the
atmosphere. In the Devonian period even these utilities fail, for coal
does not seem to have been accumulated to any great extent, and the
petroleum of the Devonian appears to have been produced from aquatic
vegetation. Even with reference to theories of evolution, there seems no
necessity for the long continuance and frequent changes of species of
acrogenous plants without any perceptible elevation. We may have much
yet to learn of the life of the Devonian; but for the present the great
plan of vegetable nature goes beyond our measures of utility; and there
remains only what is perhaps the most wonderful and suggestive
correlation of all, namely, that our minds, made in the image of the
Creator, are able to trace in these perished organisms structures
similar to those of modern plants, and thus to reproduce in imagination
the forms and habits of growth of living things which so long preceded
us on the earth. We may indeed proceed a step further, and hold that,
independently of human appreciation, these primitive plants commended
themselves to the approval of their Maker, and perhaps of higher
intelligences unknown to us; and that in the last resort it was for His
pleasure that they were created.
[Illustration: PTERASPIS. RESTORED.aEuro"AFTER LANKESTER.]
CHAPTER V.
THE APPEARANCE OF VERTEBRATE ANIMALS.
Confessedly the highest style of animal is that which possesses a skull
and backbone, with brain and nerve system to match, and which embodies
the general plan of structure employed in man himself. Yet among the
fishes, which constitute the lowest manifestation of this type, are some
so rudimentary that the brain is scarcely developed, and the skeleton is
merely a cord of gristle. These are represented in the modern world only
by the Lancelot,[33] a creature which has sometimes been mistaken for a
worm, and by a slightly more advanced type, that of the Lampreys.[34] In
these animals the Vertebrates make the nearest approach to the lower
domains of the animal kingdom, collectively known as Invertebrates. We
should naturally expect that since the vertebrates succeed the inferior
animals in time, their lower types should appear first, and that these
should be aquatic rather than terrestrial. On the other hand, as the
oldest fishes that are certainly known are strongly protected with bony
armour, and had to contend against formidable Crustaceans and Cuttles,
we might suppose that the Lancelot and the Lampreys are rather degraded
types belonging to the modern period, than the true precursors of the
other fishes.
[Illustration: FIG. 102.aEuro"Siluro-Cambrian Conodonts. Magnified.aEuro"After
Pander.]
But if fishes like the Lancelot preceded all others, we may never find
in a fossil state any traces of their soft and perishable bodies; and
even the Lampreys have no hard parts except small horny teeth, which
might easily escape observation. But palA|ontologists have sharp eyes,
and it has not escaped them that certain microscopic tooth-like bodies
are somewhat widely distributed in the older rocks. In Russia, Pander
has found in the Upper Cambrian and Lower Silurian, and also in the
Devonian and Carboniferous, minute conical and comb-like teeth, to which
he has given the name of _Conodonts_ (Fig. 102), and which he supposes
to be the teeth of ancient Lampreys. Similar teeth have been found by
Moore and others in the Carboniferous of England, and by Newberry in
Carboniferous shales in Ohio. In point of form, these bodies certainly
resemble the teeth of the humble fishes to which they have been
referred. In the case of the Carboniferous specimens from OhioaEuro"the only
ones I have had an opportunity to examinethe material is calcium
phosphate, and the structures are more like those of teeth of Sharks
than of Lampreys, so that there can be no doubt that they are really
teeth of fishes, and probably of fishes of somewhat higher grade than
the Lampreys.[35] The Cambrian and Silurian specimens are said to be
composed of calcium carbonate, which would render it more probable that,
as has been suggested by Prof. Owen, they may have been teeth of some
species of Sea-snail destitute of shell. It is, however, possible that
they may have originally been horny, and that the animal matter has been
replaced by carbonate of lime. Rohon and Zittel have recently shown that
many of these are more allied to the teeth of worms than of any other
animals.[36]
[Illustration: FIG. 103.aEuro"Lower Carboniferous Conodont.
Magnified.aEuro"After Newberry.]
If these older Conodonts were really teeth of fishes, they carry the
introduction of these nearly as far back as that of the Mollusks and
Crustaceans. If they were not, then the earliest known representatives
of this class belong to a much later age, that of the Silurian. Here we
have undoubted remains of fishes belonging to two of the higher orders
of the class; and in the succeeding Devonian these became multiplied and
extended exceedingly.
Besides the inferior tribes already referred to, the modern seas and
rivers present four leading types of fishes:aEuro"first, the ordinary bony
fishes (Teleostians), such as the Cod, Salmon, and Herring; secondly,
the Ganoid fishes, protected with bony plates on the skin, as the
Bony-pike[37] and Sturgeon; thirdly, the Sharks and their allies, the
Dog-fishes and Rays; fourthly, the peculiar and at present rare group of
semi-reptilian fishes to which the name of _Dipnoi_ has been given, on
account of their capacity for breathing both in air and in water.
Of these four types the first is altogether modern, and includes the
great majority of our present fishes. It does not make its appearance
till the Cretaceous age, and then is at once represented by at least
three of the modern families, those of the Salmon, Herring, and Perch.
The history of the other three groups is precisely the opposite of this.
They abound exceedingly at an early period, and dwindle to a much
smaller number in the modern time. This is especially the case with the
Ganoids and the Dipnoi. It is also remarkable that these groups of
old-fashioned fishes[38] are in some respects the highest members of the
class, approaching the nearest to the reptiles; but this accords with a
well-known palA|ontological law, namely, that the higher members of low
groups give way on the introduction of more elevated types, while the
lower members may continue. Thus the decadence of these higher fish
begins with the incoming of the reptiles, just as the decadence of the
higher Mollusks and predaceous Crustaceans began with the incoming of
the fishes. Further, the modern Ganoids and Dipnoi are mostly
fresh-water animals, though the Sharks are largely pelagic. In the
PalA|ozoic there seem to have been abundance of marine species of all
these types; but though marine, they probably flourished most in bays
and estuaries and on shallow banks; and the existence of these implies
continental masses of land. This explains the curious coincidence that
the introduction of fishes and of an abundant land flora synchronise,
and that the ocean was still dominated by Invertebrates long after the
fishes had become supreme in bays, estuaries, and rivers.
[Illustration: FIG. 104.aEuro"_a_, Head-shield of an Upper Silurian fish
(_Cyathaspis_). _b_, Spine of a Silurian Shark (_Onchus tenui-striatus_,
Agass.). _c_, _d_, Scales of _Thecodus_, enlarged.]
The first fishes that we certainly know are the Ganoids and Sharks,
which appear near the close of the Upper Silurian, in the English Ludlow
for example (Fig. 104). The Ganoids found here all belong to an extinct
group, characterised by the covering of the head and anterior part of
the body with large bony plates. They are mostly small fishes, and
probably fed at the bottom, and used their long or rounded bony snouts
for grubbing in the mud for food. In this respect they present a
singular resemblance to the Trilobites, so that we seem to have here
animals of an entirely new type, the Vertebrate, and with bony instead
of shelly coverings, taking up the _rA'le_ and, to some extent, the
external form of a group about to pass away. Yet I presume that no
derivationist would be hardy enough to affirm that the Trilobites could
have been the ancestors of these fishes. Nor indeed is any ancestry even
hypothetically known for them, for the doubtful Lampreys of the Cambrian
Silurian are too remote and uncertain to be used in that way. The
head-shield copied in outline in Fig. 104, and the restoration after
Lankester in the frontispiece to this chapter, may serve to represent
these curious primitive Ganoids, which are continued in the Devonian
fishes represented in Figs. 105, 106.
[Illustration: FIG. 105.aEuro"_Cephalaspis Dawsoni_ (Lankester). Lower
Devonian of GaspA(C).]
Along with these, and not improbably their enemies, were certain Sharks
(Fig. 104), known to us only by the spines which were attached to their
fins as weapons of defence, and by detached bony tubercles which
protected their skin. These remains are chiefly interesting as
indications that two of the great leading divisions of the class of
fishes originated together.
In the Devonian age the Ganoids and Sharks, thus introduced in the
Silurian, may be said to culminate. The former, more especially, are
represented by a great variety of species, some of them nearly allied to
their Silurian predecessors (Fig. 106), others of forms and structure
not dissimilar to those of the few surviving representatives of the
order, or altogether peculiar to the Devonian (Fig. 107). So numerous
are these fishes, and of so many genera and speciesaEuro"and this not merely
in one region, but in widely separated parts of the worldaEuro"that the
Devonian has not inaptly been called the reign of Ganoids. As an
illustration at once of the very peculiar forms of some of these fishes
and of their wide distribution, I figure here along with the British
species a _Cephalaspis_ (Fig. 105) found in the Lower Devonian of GaspA(C),
in the same beds with some of the antique Devonian plants described in
the last chapter.
[Illustration: FIG. 106.aEuro"Devonian Placoganoid Fishes (_Pterichthys
cornutus_, _Cephalaspis Lyelli_), from Scotland.]
[Illustration: FIG. 107.aEuro"Devonian Lepidoganoid Fishes (_Diplacanthus_
and _Osteolepis_). After Page and Nicholson.]
[Illustration: FIG. 108.aEuro"Modern Dipnoi.
_a_, _Ceratodus Fosteri_. Australia. _b_, _Lepidosiren annectus_.
Africa.]
A new and interesting light has recently been cast upon some of the most
anomalous of the ancient fishes by the study of the now rare and
peculiar species of the group of Dipnoi. Two of these, belonging to the
genus _Lepidosiren_, are the aEurooeMud-fishesaEuroŁ of the rivers of tropical
Africa and America (Fig. 108, _b._) These creatures have an elongated
and elegant form, and the body is covered with overlapping horny scales
like those of ordinary fishes; but the pectoral and ventral fins are
rod-like, and are supported by simple cartilaginous rays, while the
tailfin forms a fringe around the posterior part of the body. Unlike
ordinary fishes, they have lungs as well as gills, and their mouths are
armed with sharp, bony, beak-like teeth (Fig. 115), with which they can
inflict terrible bites on the small fishes and frogs which furnish them
with food. Their most remarkable habit is that of burying themselves in
the mud of dried-up ponds, thus forming a sort of water-chamber or
aEurooecocoon,aEuroŁ in which they remain in a torpid state until the return of the
rainy season sets them free.
Another example of these Dipnoi is the Barramunda, or _Ceratodus_ of the
Australian rivers (Fig. 108_a_). This fish resembles the _Lepidosiren_ in
many essential points of structure; but its fins have lateral rays, and
are consequently of some breadth, though of peculiar form, and its mouth
is armed with flat, pavement-like teeth, wherewith it browses on aquatic
grasses.
[Illustration: FIG. 109.aEuro"Anterior part of the palate of _Dipterus_.
Showing the dental plates at _a_, Devonian.aEuro"After Traquair.]
These modern fishes have enabled us to understand several mysterious
forms met with in the older rocks. In the first place, they show the
meaning of certain flat-toothed fishes, like _Dipterus_ of the Devonian
(Fig. 109), _Conchodus_ of the Carboniferous (Fig. 110), and _Ceratodus_
of the Carboniferous and Trias (Figs. 111, 112), previously of very
doubtful character. These must all have been of similar structure and
habits with the Barramunda, which is thus the sole survivor, perhaps
itself verging on extinction, of a group of herbivorous fishes
introduced, it may be, contemporaneously with the first stream affording
the requisite vegetable food, and which have continued almost without
improvement or deterioration to the present time. These fishes are,
however, very closely connected with the Ganoids, and there are some of
these, with fringed fins and overlapping scales, which, while regarded
as true Ganoids, resemble the Dipnoi very closely.
[Illustration: FIG. 110.aEuro"Dental plate of _Conchodus plicatus_ (Dn.).
Coal-formation of Nova Scotia. _Acadian Geology_.]
[Illustration: FIG. 111.aEuro"Dental plate of _Ceratodus Barrandii_.
Coal-formation of Bohemia. After Fritsch.]
[Illustration: FIG. 112.aEuro"Dental plate of _Ceratodus serratus_. From the
Trias.]
[Illustration: FIG. 113.aEuro"Jaws of _Dinichthys Hertzeri_ (Newberry).
Laterally compressed; one-sixth natural size.]
Again, certain huge fishes, whose remains are found in the Devonian of
Ohio,[39] had jaws on the same plan with those of _Lepidosiren_, but of
enormous size and strength (Figs. 113, 114, 115), so that in this and
some other points of structure they may be regarded as colossal
Mud-fishes, and they must have had the same destructive powers, but on a
far grander scale. They were besides clothed with heavy armour of bony
scales, having some resemblance to that of those mailed fishes of
smaller size already referred to, and indicating that, huge though they
were, and formidable in destructive power, they also had enemies to be
dreaded. These plates serve to ally them with the Ganoids, as their jaws
do with _Lepidosiren_.
[Illustration: FIG. 114.aEuro"Lower Jaw of _Dinichthys Hertzeri_. One-sixth
natural size.]
[Illustration: FIG. 115.aEuro"Jaws of _Lepidosiren_. Natural size.aEuro"After
Newberry.]
We are thus enabled to see in the streams, lakes, and bays of the
PalA|ozoic, harmless fishes, of the type of _Ceratodus_, feeding on plants,
and huge precursors of the Mud-fishes darting from the depths, and
provided with a dental apparatus more formidable than that of any modern
fish, sufficient to pierce the strongest armour of the Ganoids, and to
destroy and devour the largest aquatic animals. These huge fishes, armed
with shears two or three feet in length, and capable of cutting asunder
scale, flesh, and bone, are the _beau idA(C)al_ of destructive monsters of
the deep, far surpassing our modern Sharks; and if, by means of
supplementary lungs, they could breathe in air as well as in water, they
would on that account be all the more vigorous and voracious.
Newberry has well remarked that while in the Devonian the Ganoids and
Dipnoi were the real tyrants of the sea, as well as of the streams, in
the Carboniferous they already diminish in size, though still abundant
as to numbers, and are more limited to estuaries and fresh waters. Thus
their departure from power had already begun, and went on until in
modern times the proportion of Ganoids to ordinary fishes is, according
to GA1/4nther, nine out of 9,000. The Carboniferous, indeed, very specially
abounds in small Ganoids, though there are many large and formidable
species. One of these smaller species, a very beautiful little fish, of
fresh-water ponds and streams in the older part of the Carboniferous
age, is represented of the natural size in Fig. 116, and is not a
restoration, being found preserved entire, though flattened, in a fine
bituminous shale, which has perfectly preserved even the most delicate
sculpturing of its bony scales.
[Illustration: FIG. 116.aEuro"A small Carboniferous Ganoid (_PalA|oniscus
(Rhadinichthys) Modulus_ Dn.). Lower Carboniferous, New Brunswick.
_a_, Outline. _b_, _c_, _d_, Sculpture of scales magnified.]
[Illustration: FIG. 117.aEuro"Teeth and Spines of Carboniferous Sharks. Nova
Scotia.
_a_, _Diplodus penetrans_. _b_, _Psammodus_. _c_, _Ctenoptychius
cristatus_. _d_, Spine, _Gyracanthus magnificus_. One-eighth natural
size.aEuro"_Acadian Geology._]
The Sharks in the Carboniferous increase in number and importance. Fig.
117 shows a few examples of their teeth and spines. In the
Carboniferous, however, there is a great preponderance of those species
with flat, crushing teeth fitted for grinding shells,[40] which in
diminishing numbers continue up to the present time, when they are
represented by the Port Jackson Shark and a few other species. The
increase toward the modern time of the true Sharks[41] with sharp
cutting teeth, is obviously related to the increase of the ordinary
fishes which furnish them with food. Another curious difference,
connected probably with the same circumstance, is the fact that in the
sharp toothed Sharks of the Carboniferous the two side fangs of each
tooth are the largest, or are exclusively developed (Fig. 117, _a_),
while in later periods the central point becomes dominant, or is
developed to the exclusion of the others (Figs. 118, 119).
[Illustration: FIG. 118.aEuro"Teeth of Cretaceous Sharks (_Otodus_ and
_Ptychodus_).aEuro"After Leidy.]
[Illustration: FIG. 119.aEuro"Tooth of a Tertiary Shark (_Carcharodon_).]
[Illustration: FIG. 120.aEuro"A Liassic Ganoid (_Dapedius_).
Restored.aEuro"After Nicholson.]
The Ganoids and Dipnoi still, however, occupy a very important place
through the Mesozoic ages (Fig. 120), and it is only at the close of the
Cretaceous that they finally give place to the Teleosts, or common
fishes, which, though perhaps more fully specialised in purely ichthyic
features, have dropped the reptilian characteristics of their
predecessors (Fig. 121). It is interesting to observe that these
old-fashioned fishes had culminated before the advent of air-breathing
Vertebrates, which appear for the first time in the Carboniferous. It is
further to be observed that groups of fishes furnished with means of
aiding their gills by rudimentary lungs were especially suited to waters
more charged with carbonic acid, and less with free oxygen, than those
of more recent times. This remark especially applies to the mephitic and
sluggish streams and lagoons of the Carboniferous swamps, where, in the
midst of a rank vegetation and reeking masses of decaying organic
matter, the half air-breathing fishes and the amphibious reptilian
animals met with each other and found equally congenial abodes. Thus,
independently of the fact that some of these fishes were probably
vegetable feeders, it is not altogether an accident, but a wise
adaptation, that caused the culmination of the reptilian fishes and
batrachian reptiles to coincide with the enormous development of the
lower forms of land-plants in the Devonian and Carboniferous. Another
curious illustration of the diminishing necessity for air-breathing to
the fishes, is the change of the tail from the unequally-lobed or
heterocercal form, which prevailed in the PalA|ozoic, to the more modern
equally-lobed (homocercal) style in the Mesozoic. The former is better
suited to animals which have to rise rapidly to the surface for air, and
is still continued in some modern fishes, which for other reasons need
to ascend and descend, or to turn themselves in the water; but the
homocercal form is best suited to the ordinary fish, whether Ganoids or
Teleosts (Fig. 122). It is curious also to find the beginning of the
dominancy of the ordinary fish to coincide with that of the broad-leaved
exogenous trees in the later Cretaceous, and to precede immediately the
appearance of the mammals on the land; all these changes being related
to the purer air, the clearer waters, and the more varied continental
profiles of the later geological periods. Thus physical improvement and
the changes of animal and vegetable life are linked together by
correlations which imply not only design, but prescience, whether we
attribute these qualities to a spiritual Creator or to mere atoms and
forces.
[Illustration: FIG. 121.aEuro"Cretaceous Fishes of the modern or Teleostian
type.
_a_, _Beryx Lewesiensis_. English chalk. _b_, _Portheus molossus_
(Cope). A large fish from the American Cretaceous. One twenty-eighth
natural size.]
The history of fishes extends further through geological time than that
of any other Vertebrates, and is perhaps more completely known to us, in
consequence of the greater facilities for the preservation of their
remains in aqueous deposits. If we receive PanderaEuro(TM)s Conodonts as
indicating a low type of cartilaginous fishes, these must have
continued for vast ages without any elevation, and struggling for a bare
existence amidst formidable Cuttle-fishes and Crustaceans, before, under
more favourable conditions, they suddenly expanded into the high and
perfect types of Ganoids and Sharks. If we reject the early Conodonts,
then the two last-mentioned types spring together and suddenly into
existence, like the armed men from the dragonaEuro(TM)s teeth of Cadmus. They
rapidly attain to numbers and grandeur unexampled in later times, and
become the lords of the waters at the time when there was probably no
Vertebrate life on the land. As the reptiles establish themselves on the
land and in the waters, the Ganoids diminish, but the Sharks hold their
own. At length the reign of reptiles is over, but the Ganoids, instead
of resuming their pristine numbers, give place to the Teleosts, and
become reduced to insignificance; while the Sharks, profiting by the
decadence of the great marine reptiles, remain the tyrants of the seas.
This history is strangely unlike a continuous evolution; but we are
anticipating facts which will fall to be discussed in a subsequent
chapter.
[Illustration: FIG. 122.aEuro"Modern Ganoids (_Polypterus._ Africa.
_Lepidosteus._ America).]
[Illustration: A MICROSAURIAN OF THE CARBONIFEROUS PERIOD (_Hylonomus
Lyelli_). RESTORED FROM THE SKELETON AND DERMAL APPENDAGES FOUND IN AN
ERECT SIGILLARIA. HALF NATURAL SIZE.]
CHAPTER VI.
THE FIRST AIR-BREATHERS.
Were our experience limited to the animals whose remains are found in
the earlier PalA|ozoic rocks, we might be unable to conceive the
possibility of an animal capable of living and breathing in the thin and
apparently uncongenial medium of air. More especially would this appear
doubtful if our experience of the atmosphere presented it to us as
loaded with carbonic acid, and less rich in vital air than it is at
present. Even the mechanical difficulties of the case might strike us as
considerable, in our ignorance of the capabilities of limbs. Still, as
time wore on, we should find this problem worked out along three
distinct lines of advancementaEuro"those of the Mollusk, the Arthropod, and
the Vertebrate, and in each of these with different machinery, related
to the previous locomotive and water-breathing apparatus of the type.
Respiration under water depends, not on the water itself, but on the
small percentage of free oxygen which it contains, and this is utilised
for the aA"ration of the blood of animals, by that wonderful and often
extremely beautiful apparatus of delicate fibres or laminA| penetrated
with blood-vessels, which we call a gill. Except those lowest creatures
which aA"rate their blood merely at the general surface of the body, all
animals capable of respiration in water are provided with gills in some
form, though in many of the humbler types, like that of the familiar
Oyster, the gills are used for the double purpose of aA"rating the blood
and, by their minute vibrating threads or cilia, drifting food to the
mouth.
In the great group of radiated animals, the _Protozoa_, _CA"lenterata_,
and _Echinodermata_, no air-breathing creature exists, or, in so far as
is known, has existed, so that this vast group of animals is limited
altogether to the waters; and this is undoubtedly one mark of its
inferiority.
In the sub-kingdom of the Mollusks the highest class, that of the
Cuttle-fishes and Nautili, has been, singularly enough, rejected as
unfit for this promotion, though it was early introduced, and attains to
a high development of muscular energy and nervous power. The group next
in order, that of the Snails and their allies, alone ventures in some of
its families to assume the _rA'le_ of air-breathing. As might be
expected, in creatures of this stamp the simplest means are employed to
effect the result. In the sub-aquatic species the gills are contained in
a chamber, where they are protected and kept supplied with water. In the
air-breathing species, this gill-chamber is merely emptied of its
contents and converted into an air-sac or functional lung. Thus a rude
and imperfect method of air-breathing is contrived, which scarcely
separates the animals that possess it from their aquatic relatives, but
which nevertheless gives to us the beautiful and varied groups of the
Land-snails and of the air-breathing fresh-water Snails.
In the worms and Crustaceans the gills are placed at the sides of the
body, and connected with its several segments. But the Crustaceans, like
the Cuttle-fishes, though the highest aquatic type, never become
air-breathers. It is true some of them, like the Land-crabs, live in the
air; but they retain their gills, and have to carry with them a supply
of water to keep these moist.
But in order to elevate the Annulose type to the true dignity of air
breathing, three new classes had to be introduced, differing altogether
in their details of structure; and all three seem to have been placed on
the earth about the same time. They are: First, the Myriapods, or
Gallyworms and Centipedes; secondly, the Insects; and thirdly, the
Arachnidans, or Spiders and Scorpions.
In the Myriapods a system of air-tubes, kept open by elastic spiral
fibres, penetrates the body by lateral pores, thus retaining the
resemblance to the lateral respiration of the Crustaceans and worms. In
the Insects, where this type of structure rises to its highest
mechanical perfection, and where the animal is enabled to be not merely
an air-breather, but a flier, the same system of lateral pores and
internal air-tubes is adopted, and is so extended and ramified as to
give a very perfect respiration. In the Spiders and Scorpions the system
is the same, except that in the latter and a part of the former the
whole or a part of the tracheal system becomes expanded into
air-chambers simulating true lungs.
Among the Vertebrates, the fishes are breathers by gills attached to
arches at the sides of the neck. But already in the Devonian we have
reason to believe that there were fishes having the swimming-bladder
opening into the back of the mouth to receive air, and divided into
chambers, so as to constitute an imperfect lung. And here we have not,
as in the lower types, an adaptation of the old water-breathing organs,
but an entirely new apparatus. In the next grade of Vertebrates we find,
as in the Frogs, Water-lizards, etc., that the young are aquatic and
breathe by gills, while the adults acquire lungs, sometimes retaining
their gills also, but in the higher forms parting with them. Thus in the
vertebrates alone we have true lungs, distinct structurally from gills;
and these lungs attain to their highest perfection in the birds and
mammals.
[Illustration: FIG. 123.aEuro"Wings of Devonian Insects. Middle Devonian of
New Brunswick.
_a_, _Platephemera antiqua_ (Scudder).
_b_, _Homothetus fossilis_ (Scudder).
_c_, _Lithentomum Harttii_ (Scudder).
_d_, _Xenoneura antiquorum_ (Scudder).]
The oldest air breathers at present known are Scorpions and insects
allied to the modern May-flies, which have been found in the Silurian.
Next to these, and more important in number and variety, are the
insects of the Erian plant beds of New Brunswick. They were discovered
by the late lamented Prof. C. F. Hartt in the plant-bearing shales of
the Middle Devonian (Fig. 123). The beds containing them hold also a
species of _Eurypterus_, an obscure Trilobite, and a Crustacean allied
to the modern Stomapods,[42] besides a shell which may possibly be that
of a Land-snail, to be mentioned in the sequel. They are also
exceedingly rich in beautifully-preserved remains of Devonian plants.
The collection made by Prof. Hartt is limited to a few fragments of
wings; but these, in the skilful hands of Mr. Scudder, have proved to be
rich in geological interest. One is a gigantic _Ephemera_ or May-fly,
which must have been five inches in the expanse of the wings, which are
more complex in their venation than those of its modern allies (Fig.
123, _a_). Another presents peculiarities between those of the May-flies
and Dragon-flies (Fig. 123, _b_). A third is a Neuropter, not belonging
to any known family, but allied to some in the Coal-formation (Fig. 123,
_c_). A fourth (Fig. 123, _d_) is a small and delicate wing, supposed
to have belonged to an animal having some points of resemblance to the
modern crickets. Two others are represented by mere fragments of wings,
insufficient to determine their affinities with certainty. No other
insects of this age have been discovered elsewhere; but it is to be
borne in mind that no other locality rich in Devonian plants has
probably been so thoroughly explored. The hard slaty ridges containing
these fossils are well exposed on the coast near the city of St. John,
and Messrs. Hartt and Matthew of that city, acting, I believe, in
concert with and aided by the Natural History Society of the place, not
only searched superficially, but removed by blasting large portions of
the richest beds, and examined every fragment with the greatest care.
Their pr
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